U.S. patent application number 12/812556 was filed with the patent office on 2010-11-18 for metal-based microchannel heat exchangers made by molding replication and assembly.
Invention is credited to Fanghua Mei, Wen Jin Meng.
Application Number | 20100288479 12/812556 |
Document ID | / |
Family ID | 41162478 |
Filed Date | 2010-11-18 |
United States Patent
Application |
20100288479 |
Kind Code |
A1 |
Meng; Wen Jin ; et
al. |
November 18, 2010 |
Metal-Based Microchannel Heat Exchangers Made by Molding
Replication and Assembly
Abstract
Compression molding of metals is used to make microchannel heat
exchangers. Heat transfer can be improved by employing controlled
microchannel surface roughness. Flux-free bonding is achieved using
a eutectic thin-film intermediate layer. Seals are leak-tight,
mechanically strong, and uniform across multiple contact areas. The
metal heat exchangers may be mass-produced inexpensively, and are
useful for applications including the cooling of computer chips and
other high-power electronic devices, air conditioning,
refrigeration, condenser plates, radiators, fuel cell heat
management, and instant water heating.
Inventors: |
Meng; Wen Jin; (Baton Rouge,
LA) ; Mei; Fanghua; (Baton Rouge, LA) |
Correspondence
Address: |
PATENT DEPARTMENT;TAYLOR, PORTER, BROOKS & PHILLIPS, L.L.P
P.O. BOX 2471
BATON ROUGE
LA
70821-2471
US
|
Family ID: |
41162478 |
Appl. No.: |
12/812556 |
Filed: |
January 13, 2009 |
PCT Filed: |
January 13, 2009 |
PCT NO: |
PCT/US2009/030834 |
371 Date: |
August 3, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61020789 |
Jan 14, 2008 |
|
|
|
Current U.S.
Class: |
165/177 ;
228/165 |
Current CPC
Class: |
F28F 3/12 20130101; B81C
2203/038 20130101; F28F 1/00 20130101; F28F 21/08 20130101; H01L
23/473 20130101; H01L 21/4882 20130101; B81C 99/0085 20130101; H01L
23/3675 20130101; H01L 2924/0002 20130101; F28F 2260/02 20130101;
B81B 2201/058 20130101; H01L 2924/00 20130101; H01L 2924/0002
20130101 |
Class at
Publication: |
165/177 ;
228/165 |
International
Class: |
F28F 1/00 20060101
F28F001/00; B23K 1/20 20060101 B23K001/20; B23K 31/02 20060101
B23K031/02 |
Goverment Interests
[0001] The development of this invention was partially funded by
the United States Government under grant number CMMI-0556100
awarded by the National Science Foundation. The United States
Government has certain rights in this invention.
Claims
1. A process for making a metal microchannel heat exchanger, said
process comprising the steps of: (a) forming one or more open
microchannels on a surface of a first homogeneous metal piece,
wherein at least one of the microchannels has a width between about
30 .mu.m and about 1000 .mu.m, and a depth between about 30 .mu.m
and about 1000 .mu.m; (b) providing a second homogeneous metal
piece that, when bonded to the first metal piece, will convert one
or more open microchannels on the first metal piece into one or
more closed microchannels, wherein the one or more closed
microchannels are adapted to transport liquid without substantial
leakage; (c) providing a eutectic layer or eutectic precursor layer
at one or more of the following locations: a surface of the first
metal piece, a surface of the second metal piece, or between the
first and second metal pieces; (d) simultaneously applying pressure
to and heating the first and second metal pieces, wherein: (i) the
pressure pushes the first and second metal pieces toward each
other, with the eutectic layer or eutectic precursor layer between
the first and second metal pieces; (ii) the pieces are heated to a
temperature at which the eutectic layer or eutectic precursor layer
melts, or at which the eutectic layer or eutectic precursor layer
interacts with the metal pieces to form a molten eutectic
composition between the first and second metal pieces; and (iii)
the temperature to which the metal pieces are heated is
sufficiently below the melting temperature of the first and second
metal pieces that no substantial deformation of the one or more
microchannels occurs; (e) cooling the first and second metal pieces
to a temperature substantially below the eutectic melting
temperature, while maintaining the pressure during at least a
portion of said cooling; such that the first and second metal
pieces fuse together; such that the one or more open microchannels
are converted into one or more closed microchannels, wherein the
one or more closed microchannels are adapted to transport liquid
without substantial leakage; and wherein no substantial blockage of
the one or more closed microchannels occurs as a result of said
heating, applying pressure, and cooling; and wherein: (f) the one
or more closed microchannels are enclosed entirely by the fused
first and second metal pieces and eutectic layer; whereby the fused
first and second homogeneous pieces and eutectic layer, together
with the enclosed one or more closed microchannels, form a
microchannel heat exchanger.
2. A process as in claim 1, wherein the heat exchanger is capable
of withstanding an internal pressure in the one or more closed
microchannels of 100 atmospheres or greater.
3. A process as in claim 1, wherein said microchannel-forming step
comprises compression molding of one or both metal pieces with a
refractory metal mold insert.
4. A process as in claim 1, wherein at least one of the closed
microchannels has a surface roughness between about 3 .mu.m and
about 15 .mu.m.
5. A metal microchannel heat exchanger produced by the process of
claim 1.
6. A metal microchannel heat exchanger comprising one or more
closed microchannels; wherein at least one of said microchannels:
(a) is enclosed entirely by a first homogeneous metal piece, a
second homogeneous metal piece, and a eutectic layer; wherein said
first and second homogeneous metal pieces are brazed to one another
by said eutectic layer; (b) has a width between about 30 .mu.m and
about 1000 .mu.m, and a depth between about 30 .mu.m and about 1000
.mu.m; and (c) is adapted to transport liquid without substantial
leakage.
7. A heat exchanger as in claim 6, wherein said heat exchanger is
capable of withstanding an internal pressure in said one or more
closed microchannels of 100 atmospheres or greater.
8. A heat exchanger as in claim 6, wherein at least one of said
closed microchannels has a surface roughness between about 3 .mu.m
and about 15 .mu.m.
Description
[0002] (In countries other than the United States:) The benefit of
the 14 Jan. 2008 filing date of U.S. provisional patent application
61/020,789 is claimed under applicable treaties and conventions.
(In the United States:) The benefit of the 14 Jan. 2008 filing date
of U.S. provisional patent application 61/020,789 is claimed under
35 U.S.C. .sctn.119(e).
TECHNICAL FIELD
[0003] This invention pertains to microscale structures made by
compression molding of high melting-temperature metals,
particularly to metal-based microchannel heat exchangers.
BACKGROUND ART
[0004] The nearly exponential growth in the heat generated by
miniaturized electronic devices in recent years demands significant
improvements in cooling technology. Existing fan-assisted air
cooling methods will be insufficient for the next generation of
microprocessors. Only liquid-cooled heat exchangers will be able to
absorb and dissipate heat rapidly enough to maintain safe
microprocessor operating temperatures. A stringent requirement of
high efficiency is imposed on such a heat exchanger. The cooling
system must be small and must be a closed loop, so that it may: a)
fit within a desktop or laptop computer; and b) not require
external cooling water.
Microchannel Heat Exchangers
[0005] Decreasing the liquid cooling channel dimensions to the
micron scale in a solid-liquid heat exchanger leads to high heat
transfer rates. Convective heat transfer from the channel surface
to water is fast, but diffusional heat transfer from the liquid at
the interface to liquid in bulk is slow. By reducing the liquid
cooling channel dimensions, the interface area-to-bulk volume ratio
increases, thereby reducing rate-limiting diffusional heat
transport.
[0006] There have been prior demonstrations of high solid-fluid
heat transfer from microchannels, primarily in silicon-based
microchannel heat exchangers. The use of silicon in such devices in
these studies was not so much because silicon has desirable heat
transfer properties, but rather because fabrication techniques for
Si-based, high-aspect-ratio microscale structures (HARMS) are
relatively mature and widely available. Indeed, Si possesses a
substantially lower bulk thermal conductivity than that of the
metals that would otherwise be preferred in larger-scale heat
exchangers, such as Cu and Al. Further, Si is relatively brittle,
and consequently Si-based devices tend to be fragile and easily
damaged.
[0007] Si microfabrication techniques typically involve a
photolithography process in which a uniform, polymerizable resist
layer is deposited onto a Si substrate, and a desired pattern is
photoexposed into the resist layer. Unpolymerized resist is removed
chemically or by solvation, and the Si substrate is etched through
the developed resist pattern either by wet chemical etching (WCE)
or reactive ion etching (RIE). Additional deposition and etching of
thin metal films may be required for the RIE process.
Photolithography and etching are required for each Si microscale
device, and to enjoy an economy of scale, a substantial investment
in large clean room and thin film deposition facilities is
required.
[0008] Microchannel heat exchangers have also been fabricated in
materials other than Si by the LiGA process. LiGA combines deep
X-ray/UV lithography (Lithographie) of a polymeric resist, followed
by metal electrodeposition (Galvanoformung) into the developed
resist recesses to form durable, primary HARMS. Replication of
secondary HARMS from the primary HARMS via molding (Abformung) then
follows. For example, U.S. Pat. No. 6,415,860 discloses Ni
electrodeposition to make microscale Ni mold inserts that are then
used to mold microchannel heat exchangers in polymethylmethacrylate
(PMMA). Metal-based crossflow heat exchangers, such as those made
from NiP alloys, were also made, by an additional electroless
deposition onto LiGA-fabricated polymer templates. F. Arias et al.,
"Fabrication of metallic heat exchangers using sacrificial polymer
mandrils," JMEMS vol. 10, p. 107 (2001) reported the fabrication of
Ni-based heat exchangers by electrodeposition of nickel onto
sacrificial polymer mandrels.
[0009] There are unfilled needs in existing heat exchangers. For
example, the thermal conductivity of PMMA is poor, and PMMA-based
microchannel heat exchangers cannot endure temperatures higher than
about 100.degree. C. While Ni-based and NiP-based heat exchangers
can function at higher temperatures, their heat conductivities are
still less than optimal. Furthermore, the electrode-based and
electroless deposition techniques used to make them are slow, and
require close monitoring and control. Their cost of fabrication is
high and is expected to remain high because of the extra deposition
steps involved in these "lost-mold" processes.
[0010] Existing Si microfabrication techniques do not work for
making metal-based microstructures. For example, the structural and
chemical isotropy of polycrystalline metals leads to removing
material in a somewhat isotropic manner in a WCE process,
broadening features from those defined lithographically. RIE
techniques are also inappropriate for metallic substrates. Because
metal-based microchannel devices are highly desirable for heat
transfer applications, there is an unfilled need for improved
fabrication techniques to mass-produce metal-based microchannel
devices rapidly and inexpensively.
Microchannel Fabrication by Compression Molding
[0011] Microscale compression molding, or hot embossing, of
polymeric plastic materials is an established technique. First, a
primary HARMS mold insert is produced, typically through a sequence
of lithography, etching, deposition steps, with optional additional
steps. Second, the mold insert is impressed into a substrate, and
polymer fills voids in the mold insert through viscous or plastic
flow to form the negative of the insert pattern. A large number of
negative HARMS replicas can be reproduced from a single primary
HARMS. In principle, under favorable conditions one primary mold
insert may be used to produce hundreds or even thousands of
replicas rapidly and at low cost.
[0012] The quality of the replica depends upon, among other
factors, the mechanical yield strength of the mold insert at
elevated molding temperatures. An important problem confronting
compression metal microstructure molding is the lack of
microstructure mold insert materials that retain high mechanical
yield strengths at the molding temperatures required for metals. An
electrodeposited Ni mold insert, for example, suffers permanent
shape deformation when used to mold a higher-melting temperature
metal, such as Cu.
[0013] Another problem can arise from chemical reactivity between
the mold insert and the metal substrate. During compression,
chemical bonds can form between the insert and the substrate. These
bonds can cause the insert to break and can damage the molded
structure as it is withdrawn from the substrate. These surface
chemistry problems had restricted the metals that could be used as
mold inserts and as substrates, until the development of a
conformal ceramic surface coating to inhibit chemical bond
formation. Using ceramic conformal coatings, secondary HARMS have
been successfully reproduced in previously problematic, chemically
reactive metals, such as Zn and Al, with LiGA-fabricated Ni mold
inserts. See generally D. Cao et al., "Amorphous hydrocarbon based
thin films for high-aspect-ratio MEMS applications," Thin Solid
Films 398-399 (2001)553-559; and D. Cao et al., "Conformal
deposition of Ti--C:H coatings over high-aspect-ratio micro-scale
structures and tribological characteristics," Thin Solid Films 429
(2003)46-54.
Bonding the Cover Plate
[0014] Once a microchannel has been fabricated in a substrate,
whether Si or metal, a leak-tight cover plate must be affixed
before it can be used as a practical heat exchanger. Several
bonding methods have been reported for Si-based microsystems,
including anodic bonding and direct bonding. However, these
techniques are not well-suited for bonding metal-based HARMS.
Eutectic Bonding
[0015] Braze-bonding of bulk metal pieces has previously been used
in different applications. Brazing is a joining process in which a
non-ferrous filler metal or alloy is heated to its melting
temperature and distributed between two (or more) close-fitting
metal parts by capillary action. The filler metal can optionally be
a eutectic mixture. A "eutectic" mixture is a mixture whose
proportions are such that the melting point is as low as possible;
and such that the constituents of the mixture all crystallize
simultaneously at this temperature from molten liquid solution, a
temperature that is called the eutectic point. For example, it has
been reported that thin films of Si, Si--Al, and Zn--Al have been
deposited onto bulk Al pieces by electron beam evaporation or
sputtering. These Al pieces were then braze-bonded to one another
by heating to 578-595.degree. C., with flux introduced to remove
surface aluminum oxides. This technique would be unsuitable for use
with microchannels, however, because flux residue would tend to
block the microchannels.
[0016] D. Tuckerman et al., "High performance heat sinking for
VLSI," IEEE Elect. Dev. Lett. 2, 126-129 (1981) discloses a
water-cooled, integral heat sink fabricated in silicon with a Pyrex
cover plate.
[0017] A. Tiensuu et al., "Assembling three-dimensional
microstructures using gold-silicon eutectic bonding," Sensor Actuat
A 45, 227-236 (1994) discloses the use of gold-silicon eutectic
bonding to join silicon microelements to one another.
[0018] B. Vu et al., "Patterned eutectic bonding with Al/Ge thin
films for microelectromechanical systems," J Vac Sci Technol B
14(4):2588-2594 (1996) discloses the use of an aluminum/germanium
eutectic to bond silicon dice to one another.
[0019] P. Lee et al., "Investigation of heat transfer in
rectangular microchannels," Int. J. Heat Mass Transf, vol. 48, no.
9, pp. 1688-1704 (2005) discloses measurements and numerical
modeling of heat transfer in rectangular microchannels. Test pieces
were made of copper, with ten microchannels in parallel, and a
polymeric cover plate.
[0020] D. Cao et al., "Microscale compression molding of Al with
surface engineered LiGA inserts," Microsyst Technol. 10 (2004)
662-670 discloses the use of high-temperature compression molding
of aluminum plates with high-aspect ratio microscale mold inserts
made of nickel conformally coated with a titanium-containing
hydrocarbon. See also W. Meng et al., "Stresses during micromolding
of metals at elevated temperatures: pilot experiments and a simple
model," J. Mater. Res. 20 (2005) 161-175; J. Jiang et al., "Further
experiments and modeling for microscale compression molding of
metals at elevated temperatures," J. Mater. Res. 22 (2007)
1839-1848; U.S. Pat. No. 7,114,361; and U.S. published patent
application 2005/0056074.
[0021] F. Mei et al., "Eutectic bonding of Al-based high aspect
ratio microscale structures," Microsyst Technol. 13: 723-730
(published online 16 Jan. 2007) reports work from our research
group concerning the eutectic bonding of Al-based high aspect ratio
microscale structures with Al--Ge intermediate layers. See also F.
Mei et al., "Evaluation of eutectic bond strength and assembly of
Al-based microfluidic structures, Microsyst Technol. 14: 99-107
(published online 3 Apr. 2007); F. Mei et al., "Fabrication,
assembly, and testing of Cu- and Al-based microchannel heat
exchangers, J. Microelectromechanical Systems 17(4): 869-881
(published online Jun. 27, 2008); and F. Mei et al., "Evaluation of
bond quality and heat transfer of Cu-based microchannel heat
exchange devices," J Vac Sci Technol A 26(4):798-804 (published
online Jun. 30, 2008).
[0022] U.S. Patent Application 2006/0142401 discloses the use of
partial boiling in a minichannel or microchannel to remove heat
from an exothermic process. Surface roughness was said to enhance
nucleation for boiling. See, e.g., Example 11.
[0023] U.S. Patent Application 2006/0157234 discloses a
microchannel heat exchanger, and briefly mentions surface
roughness.
[0024] U.S. Pat. No. 5,727,618 discloses a modular microchannel
heat exchanger formed from a stack of multiple thin copper sheets
etched with rows of elongated holes, coated with silver and held
together with the holes aligned, e.g. with pins. The stack is
heated, and the copper and silver form a fused or eutectic alloy
brazing the sheets together. The holes through the multiple sheets
then form a microchannel.
SUMMARY OF THE INVENTION
[0025] We have discovered novel metal-based microscale structures,
and methods of making those microstructures, including but not
limited to structures that incorporate microchannels. The preferred
manufacturing method employs compression molding of high
melting-temperature metals, for example copper, copper-based
alloys, aluminum, and other metals. Molding replication of such
metals requires mold inserts with high yield strengths at elevated
molding temperatures. We have developed a process to fabricate mold
inserts made of refractory metals that possess heretofore
unattainable microscale detail.
[0026] Another aspect of the invention is the joining of microscale
structures made of similar or dissimilar metals via a bonding
process involving thin film eutectic or near-eutectic mixtures
comprising micro- or nano-domains that are substantially smaller
than the contact areas of the microstructures to be joined.
[0027] Another aspect of the invention is the connection of
compression-molded metal microchannels, substantially free of
obstructions, to larger channels/plena for fluidic connection to
tubes, pipes, valves and other external fluidic elements, where the
external elements typically (but not necessarily) have larger
dimensions than the microchannels.
[0028] Yet another aspect of the invention is the modulation of
microchannel wall roughness. Enhanced wall roughness is a
consequence of the preferred manufacturing methods disclosed here,
but does not generally result from other microchannel fabrication
methods. The enhanced roughness causes a surprising and significant
enhancement in heat transfer from the microchannel surface to a
fluid flowing through it. Without wishing to be bound by this
hypothesis, we believe that the improved heat transfer is a
consequence of hydrodynamic effects induced by the microchannel
surface roughness.
[0029] We have developed improved methods for bonding metal
microchannel plates to plain metal plates, other metal microchannel
plates, or other metal structures. Flux-free bonding is achieved
using a eutectic thin-film intermediate layer. The novel technique
produces seals that are leak-tight, mechanically strong, and
uniform across multiple microscale contact areas. The thin film
intermediate layers are fabricated, for example, by vapor phase
deposition onto the metallic HARMS or other metallic layers to be
bonded, such as by physical or chemical vapor deposition. An
alternate method is to sandwich free-standing thin films of metals
or alloys between the metallic HARMS or other metallic layers to be
bonded. Metal heat exchangers in accordance with the present
invention may be mass-produced inexpensively, and are useful for
numerous applications, including for example the cooling of
computer chips and other high-power electronic devices, air
conditioning, refrigeration, condenser plates, radiators, fuel cell
heat management, and instant water heating. The heat exchangers are
capable of handling high internal pressures, e.g. 100, 200 or 300
atmospheres or even higher. Because they are made of metal, they
are better able to withstand external mechanical stress and shock
than devices made from silica or from polymers. They can have a
very low-profile construction; for example, the distance between
the heat source and the fluid medium may be as small as 500 .mu.m
or even closer.
[0030] Using the novel technique, it is not necessary to use a
large number of thin layers to build up microchannels. Rather, the
microchannels may be formed from only two metal layers, either or
both of which can include open channels prior to their bonding
together; and an intermediate layer of eutectic or eutectic
precursor to braze the two metal layers to one another. The open
channels in the metal layer or layers are preferably formed by
molding, but may also be formed by techniques otherwise known in
the art, such as milling. Depending on the geometry of the
particular device, precise alignment between the two metal layers
can sometimes be important, while in other instances such precise
alignment may not be needed (for example, if all open channels are
in one of the two metal pieces, and if the second piece acts as a
cover plate to convert the open channels into closed channels).
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIGS. 1(a) and 1(b) depict SEM micrographs of an example of
Al molding replication, and the typical sub-micron sidewall
roughness in the replicated Al HARMS.
[0032] FIG. 2 depicts an SEM high magnification view of one section
of a molded microchannel in Al, showing elevated surface roughness
on the microchannel sidewalls.
[0033] FIG. 3 depicts an SEM micrograph of an amorphous silicon
nitride-coated Inconel mold insert.
[0034] FIG. 4 depicts a high magnification SEM micrograph of an
amorphous silicon nitride coated Inconel mold insert, showing a
typical rectangular microprotrusion.
[0035] FIG. 5 depicts an EDS spectrum of an Al--Ge film that was
deposited with an Al cathode current of 0.5 A and a Ge cathode
current of 0.4 A.
[0036] FIG. 6 depicts the Ge to Al composition ratios for a series
of Al--Ge films that were deposited at a fixed Al cathode current
of 0.5 A, as a function of the Ge cathode current. The dashed line
is an interpolation between the measured points.
[0037] FIG. 7 depicts a cross-sectional view of an embodiment of a
typical, one-layer, Al-based microchannel structure bonded with
Al--Ge nanocomposite thin film intermediate layers.
[0038] FIG. 8 depicts a cross-sectional view of an embodiment of a
two-layer, Al-based, microchannel device bonded with Al--Ge
nanocomposite thin film intermediate layers.
[0039] FIG. 9 depicts a high magnification view of a typical bonded
Al microchannel from the device depicted in FIG. 8.
[0040] FIG. 10 depicts a cross-sectional view of a portion of a
Cu-based microchannel structure bonded with Al--Ge nanocomposite
thin film intermediate layers.
[0041] FIG. 11 depicts a cross-sectional view of a portion of a
Cu-based microchannel structure bonded with a 10 .mu.m
free-standing Al thin film intermediate layer.
[0042] FIG. 12 depicts measured Al--Al interface bond strength as a
function of bonding temperature.
[0043] FIG. 13 depicts measured Al--Al interface bond strength as a
function of applied pressure during bonding.
[0044] FIG. 14 depicts measured Al--Al interface bond strength as a
function of the thickness of the nanocomposite Al--Ge thin film
intermediate layer deposited on bonding surfaces.
[0045] FIG. 15 depicts measured Cu--Cu interface tensile bond
strength as a function of the average applied pressure.
[0046] FIG. 16 depicts measured Cu--Cu interface tensile bond
strength as a function of the thickness of the free-standing Al
thin film intermediate layer.
[0047] FIG. 17 depicts three X-ray diffraction patterns obtained
from three fractured Cu interfaces that had been bonded with
free-standing Al thin film intermediate layers, 10, 25, and 38
.mu.m thick.
[0048] FIG. 18 depicts one embodiment showing a transition from a
long meandering microchannel to a broadened outlet leading to a
larger fluid plenum.
[0049] FIG. 19 depicts a molded Cu coupon containing an array of
rectangular microchannels, a fluid supply channel, and a fluid
drain plenum. Numbers shown on the ruler in the foreground are in
mm.
[0050] FIG. 20 depicts a detailed view of a transition from a
microchannel array to a fluid supply channel formed by a single cut
with .mu.EDM.
[0051] FIG. 21 depicts a detailed view of a typical Cu-based
microchannel array fabricated by molding replication.
[0052] FIG. 22 depicts Cu device surface temperature versus time as
water flowed through the microchannel array.
[0053] FIG. 23 depicts the fast time constants associated with
surface temperature drop for Cu and Al microchannel devices.
[0054] FIG. 24 depicts a pre-assembly, two-layer, Cu-based, instant
water heater prototype. Numbers shown on the ruler in the
foreground are in mm.
[0055] FIG. 25 depicts the difference in water temperature between
the outlet and inlet of the Cu instant water heater prototype as a
function of the water flow rate.
[0056] FIG. 26 depicts overall heat transfer efficiency of the Cu
instant water heater prototype as a function of the water flow
rate.
MODES FOR CARRYING OUT THE INVENTION
Microchannel Fluid Flow
[0057] X-ray/UV lithography LiGA processes can produce very smooth
sidewalls in the developed polymeric resist recesses, and the high
fidelity of electrodeposition and molding processes conveys this
smoothness to the molded metal. FIG. 1 depicts an example of Al
molding replication and the typical sub-micron sidewall roughness
in the replicated Al HARMS. The novel methods allow one to control
surface roughness, in particular to increase surface roughness, as
compared to the surface smoothness that is more typically achieved
by the LiGA process. We have discovered that, surprisingly,
modulation of surface roughness can produce significant
improvements in microchannel heat exchange.
[0058] Microchannel cover plate bonding should satisfy several
requirements. First, bonding across the mating surfaces for each
individual microchannel should be complete to prevent
cross-communication between different microchannels. Second,
because microchannels generally have high internal flow resistance,
they will often require significant driving pressures, which in
turn means that bond strengths should be high. Finally, to reduce
the likelihood of microchannel deformation a relatively low bonding
temperature is desirable, a temperature that depends on the melting
temperature of the particular metal. For example, with an
aluminum-based device, a bonding temperature lower than about
578-595.degree. C. is desirable, since the melting temperature of
aluminum is 660.degree. C. (or 933.degree. K).
Fabrication of Metal-Based Microchannels by Compression Molding
[0059] One aspect of the invention pertains to compression molding
of metal-based microchannels. In a preferred embodiment, mold
inserts are fabricated from a refractory metal or alloy, for
example Ta or the superalloy Inconel X750.RTM. (a
nickel-chromium-iron alloy, with additions of aluminum, titanium,
and niobium). The insert may be fabricated, for example, by using
micro electrical discharge machining (.mu.EDM) to create microscale
recesses in a refractory metal or alloy, followed by
electrochemical polishing (ECP) to remove surface layers damaged
during .mu.EDM. Next is an optional, but preferred conformal
deposition of a bond-inhibitor coating, for example as described in
U.S. Pat. No. 7,114,361, using, for example, a ceramic or ceramic
mixture coating, such as amorphous carbon or silicon nitride. The
coated, refractory metal mold inserts may then be used to make
HARMS in a softer metal, such as Cu or Ni, by molding replication
with little or no apparent damage to the mold inserts.
[0060] Significant cost benefits are achieved when a refractory
mold insert can be used to produce a large number of metal
replicas. We have found that various refractory metals, including
for example Ta, W, Mo; refractory alloys such as Ni-based
superalloys and Fe-based tool steels; and engineering ceramics such
as transition metal carbides and transition metal nitrides, possess
sufficient mechanical yield strengths to repeatedly produce
high-fidelity replicas without substantial damage or degradation.
Because these refractory metals and alloys cannot conveniently be
electrodeposited using known techniques, we developed a .mu.EDM/ECP
process to fabricate mold inserts with intricate, microscale
features.
[0061] Specifically, we have used .mu.EDM to create microscale
trenches in refractory metals and alloys, thereby achieving desired
mold insert geometries. The .mu.EDM process is preferably followed
by ECP to remove surface damage from the .mu.EDM step. Importantly,
control of ECP operating parameters allows one not only to minimize
surface roughness, but to also impart roughness to the mold insert
surfaces when desired. We have found that a conformal coating
deposition following ECP, which is used to prevent chemical
reactions and bonding between the mold insert and the molded
metals, generally does not substantially change the mold insert's
surface roughness. Control of the ECP parameters allows one to
impart surface roughness on the order of tens of microns, measured
as the mean peak-to-valley roughness Rz. The microchannel surface
roughness is significant. We unexpectedly discovered that increased
surface roughness can substantially enhance the exchange of heat
from the microchannel wall to fluid flowing through the
microchannel.
[0062] In one embodiment, .mu.EDM combined with LiGA-fabricated
electrodes, ECP, and conformal coating deposition can be used to
create mold inserts with complex geometries, such as branched,
serpentine, or meandering microchannels, and microchannels with
asymmetric cross-sections, unequal depths, and other arbitrary
profiles.
[0063] Because the molding replication process faithfully
reproduces a negative of the mold insert onto the molded metals,
the resulting microchannels have a surface roughness similar to
that of the mold insert. FIG. 2 depicts a portion of a single
meandering microchannel in Al, created by replication from a Ta
mold insert that was fabricated by .mu.EDM and ECP, followed by a
conformal coating with Ti-containing, amorphous, hydrogenated
carbon. In contrast to the smooth surfaces of the LiGA-fabricated
Ni inserts, as shown in FIG. 1, the microchannel roughness depicted
in FIG. 2 was five microns or higher, reflecting that of the Ta
insert.
EXAMPLE 1
[0064] Insert fabrication. Microscale mold inserts were fabricated
in Ni-based superalloy Inconel X750.RTM. plates in three steps: 1)
.mu.EDM of the active area; 2) electrochemical polishing (ECP) of
the machined microscale Inconel features; and 3) deposition of a
conformal, amorphous silicon nitride (a-Si:N) coating over the
electrochemically polished microscale features. As-received Inconel
plates were machined to square insert blanks, with an active area
of .about.15000 .mu.m.times..about.15000 .mu.m, .about.3200 .mu.m
in height. The top surface of the blank was mechanically polished
with SiC abrasive papers down to 1200 grit size. A SARIX high
precision micro erosion machine, model SR-HPM-B, was used for
insert .mu.EDM. Flat molybdenum (Mo) sheets with a thickness of 500
.mu.m were used as blade electrodes. A series of parallel cuts was
made on the insert blank. Erosion of the insert blank by .mu.EDM
produced a trench under the Mo electrode. Sequential cuts led to
the formation of an array of trenches, or an array of parallel
rectangular microprotrusions between trenches. As-machined Inconel
blanks were electrochemically polished for 10 min in
"current-controlled" mode in a mixed acid solution of HClO4 (70%)
and CH3COOH (80%) at a volume ratio of 1:1. Following ECP, a
conformal a-Si:N coating was deposited over the Inconel inserts in
a radio frequency (rf), inductively coupled plasma (ICP)-assisted,
hybrid chemical/physical vapor deposition system, generally
following the procedures of W. J. Meng et al., "Temperature
dependence of inductively coupled plasma assisted deposition of
titanium nitride coatings," Surf. Coat. Technol. 120/121, 206
(1999).
[0065] Surface morphology was examined both with a Hitachi S3600N
scanning electron microscope (SEM), and with a Veeco Wyko3100
optical profilometer (OP). An SEM overview of one a-Si:N coated
Inconel insert is shown in FIG. 3. The thickness of the a-Si:N
coating was .about.600 nm. The insert contained 19 parallel
rectangular microprotrusions, with center-to-center spacings of
.about.750 .mu.m. The sequential cutting process led to variations
in the widths of the microprotrusions. The average width of all
microprotrusions, as measured from the SEM micrographs, was 154
.mu.m. The trench bottoms were somewhat rounded. The height of the
straight section of the microprotrusions was .about.400 .mu.m. FIG.
4 depicts a close-up SEM image of a typical microprotrusion. The
numerous micron-scale features on the microprotrusion surface
resulted from preferential etching during the ECP process, probably
delineated by the grain structure of the underlying Inconel
substrate. The a-Si:N coating presumably adheres to the substrate
morphology conformally without significantly modifying it.
EXAMPLES 2 AND 3
[0066] Microchannel compression molding in Cu and Al using
refractory mold inserts with enhanced surface roughness. Cu 110
(99.9+wt. % Cu) and Al 6061 (1.0 wt. % Mg, 0.6 wt. % Si, 0.27 wt. %
Cu, 0.2 wt. % Cr, balance Al) coupons, with the same geometry of
35.5 mm.times.35.5 mm.times.6.4 mm, were molded at high
temperatures with the a-Si:N coated Inconel insert shown in FIGS. 3
and 4. Before molding, the top surfaces of the Cu and Al coupons
were mechanically polished with SiC abrasive papers of different
sizes and finished with a 1 .mu.m diamond particle suspension.
Compression molding was carried out in a MTS858 single-axis testing
system interfaced to a high-vacuum molding chamber, which housed
two heating stations. One heating station was mechanically attached
to the bottom of the vacuum chamber. A second heating station was
mechanically attached to the top linear actuator of the MTS858
testing system through a bellow-sealed motion feed-through. The two
heating stations were heated separately by resistive heating
cartridges, and temperatures were monitored by two separate K-type
thermocouples. The coupon to be molded was fastened to the top
surface of the bottom heating station, while the insert was
mechanically attached to the bottom surface of the top heating
station, which in turn was attached to the linear actuator. Total
axial force and axial displacement of the insert were measured
continuously during the entire molding and demolding process.
Molding of Cu coupons occurred with both the coupon and the insert
heated to .about.500.degree. C., while molding of Al coupons
occurred with both the coupon and the insert heated to
.about.400.degree. C. The molding process faithfully replicated the
array of 19 parallel rectangular microprotrusions into parallel
rectangular microchannels in both the Cu and the Al coupons. The
average width of microchannels on the Cu coupon was measured as 150
.mu.m. The average width of microchannels on the Al coupon was
measured as 148 .mu.m. The final width of the molded microchannel
tended to be somewhat smaller than that of the corresponding
microprotrusion on the mold insert, by .about.3% for Cu, and
.about.4% for Al. The depths of the molded microchannels were
greater than 400 .mu.m.
[0067] Optical profilometry (OP) images from the bottoms of
replicated Cu microchannels were used to assess the surface
roughness within molded microchannels, the peak-to-valley roughness
Rz. (Data not shown; see FIG. 5 of priority application
61/020,789.) Four independent OP images were obtained from the
bottom surfaces of four different microchannels in the Cu and Al
coupons. Values of Rz obtained from the four OP images were
averaged. The average Rz values measured for the bottom surfaces of
the microchannels were 11.8 .mu.m for Cu, and 8.2 .mu.m for Al. Due
to difficulties of optical access, values of Rz for the
microchannel sidewalls were not obtained. From qualitative SEM
observations, it is surmised that surface roughness of the
microchannel sidewalls was somewhat smaller, about 5 .mu.m.
[0068] As expected from the replication process, surface roughness
on the microchannel bottoms mimicked the roughness of the top
surfaces of the insert microprotrusions. The surface roughness on
the mold insert is believed to result from preferential etching
during the ECP step of insert fabrication. The observed Rz values,
about 10 .mu.m, were larger than what is typically obtained from
mechanical micromilling of metals, and greatly exceeded what is
typically obtained from electrodeposited metallic structures formed
by LiGA processing.
[0069] Eutectic Bonding of Metal-Based Microchannel Structures
Using Thin Film Intermediate Layers
[0070] Another aspect of this invention pertains to joining two or
more metallic structural components, one or both of which may
contain microchannel features, using one or more thin film
intermediate layers. The problem solved was to create uniform,
mechanically strong bonds across numerous mating surfaces, of
microscale dimensions, of possibly dissimilar metals, without
requiring flux.
[0071] A preferred, novel technique employs flux-less, eutectic
bonding. The metal components to be bonded were first subjected to
plasma etching in vacuum, followed by deposition of an Al--Ge
eutectic nanocomposite thin film. The plasma etch removed surface
metals oxide, removing any need for flux. With small channel
dimensions, the thickness of the eutectic nanocomposite
intermediate layer should also be small to avoid microchannel
blockage, typically ranging from below 1 micron to about 10
microns. The eutectic intermediate layer is used to ensure that the
bonding interface will melt at a substantially lower temperature
than the melting temperature of the metal components to be joined,
thereby minimizing mechanical deformation at the temperature and
pressure used for bonding. For example, we have successfully bonded
Al-based microchannel structures in the temperature range
450.degree. C. to 550.degree. C., considerably below the
660.degree. C. melting point of elemental aluminum.
[0072] Eutectic mixtures are generally not homogeneous, but are
instead typified by a collection of domains, each of which consists
primarily of one of the eutectic's elemental components. The domain
size effectively dictates a lower limit on the size of areas to be
joined by eutectic bonding. Indeed, it is preferred that the
eutectic domain size should be substantially smaller than the
dimensions of the areas to be bonded. For example, with an Al--Ge
eutectic nanocomposite thin film, typical domain sizes are on the
order of 100-200 nm. These small domains not only ensure uniform,
strong bonding across areas that can have relatively small
dimensions, but also ensure that melting across the entire bonding
interface occurs at or near the lowest possible temperature (i.e.,
at or near the eutectic point), reducing the potential for
mechanical deformation of the microchannels or other structures
during bonding. The eutectic thin film intermediate layers may be
fabricated, for example, by vapor phase deposition methods, such as
physical or chemical vapor deposition onto metallic HARMS, or by
sandwiching free-standing thin films of metals or alloys between
the metallic pieces to be bonded. The melting point of the eutectic
or near-eutectic thin film should be substantially below the
melting point of the components to be bonded, preferably at least
about 50.degree. C.-100.degree. C. lower.
[0073] The nanodomain eutectic bonding technique is not limited to
plate-to-plate bonding. It may be used for other metal-to-metal
bonding, for example to elaborate a plain or molded metal surface
by serial addition of semi-layers, rods, alignment pins, and the
like.
Bonding of Al-Based Microchannel Structures with Al--Ge
Nanocomposite Thin Film Intermediate Layers.
EXAMPLE 4
[0074] Synthesis of Al--Ge nanocomposite thin films, and bonding of
one-layer devices. Al--Ge composite thin films were deposited with
a radio frequency (rf) inductively coupled plasma (ICP)-assisted
hybrid tool, which combined a 13.56 MHz ICP with direct current
(dc) balanced magnetron sputtering. One Al (99.99%) cathode and one
Ge (99.99%) cathode, each 75 mm in diameter, were placed facing
each other and were sputtered in current-controlled mode. The
distance between the Al and Ge cathodes was about 250 mm.
Substrates for Al--Ge deposition included Si wafers with diameter
50 mm, polished Al coupons (Al 1100, 99%+) with diameter 35 mm, and
replicated Al HARMS (Al 1100, 99%+). The substrates were
ultrasonically cleaned in acetone and methanol before being mounted
on a rotatable holder between the two targets. The ultimate base
pressure of the deposition system was less than 1.0.times.10.sup.-8
Torr. Typical background pressures prior to deposition runs were
about 1.times.10.sup.-7 Torr.
[0075] The entire deposition sequence was carried out in an Ar
(99.999%) atmosphere, with a total pressure of about 1.3 mTorr. The
deposition sequence comprised a substrate surface etch, followed by
codeposition of Al and Ge. Substrate etching occurred in a pure Ar
ICP with a total input rf power of 1000 W, a substrate bias of -200
V, and an etch duration of 3 min. Sputtering of Al and Ge cathodes
commenced immediately after substrate surface etch.
[0076] One series of Al--Ge composite films was deposited on
Si(100) substrates. For this series of specimens, a fixed Al
cathode current of 0.5 A was used, and the Ge cathode current was
varied from 0.1 A to 0.5 A to alter the Ge composition within the
film. The deposition duration was fixed at 30 min for this series
of specimens. Additional depositions on flat Al coupons and
replicated Al HARMS were carried out with a fixed Al cathode
current of 1.0 A and a fixed Ge cathode current of 0.45 A for 60
min. The substrate bias was fixed at -100 V for all depositions.
Substrates were rotated continuously in the center of the
deposition zone at about 12 rpm during both etching and deposition.
No intentional substrate heating or cooling was applied during the
deposition process.
[0077] Bonding experiments were carried out using a MTS858
single-axis testing system interfaced to a custom-built high-vacuum
chamber. A turbomolecular pump system produced an ultimate molding
chamber base pressure of about 2.times.10.sup.-9 Torr Typical
background pressures during bonding experiments were about
1.times.10.sup.-6 Torr. Two heating stations were installed in the
vacuum chamber. The lower heating station was mechanically attached
to the bottom of the vacuum chamber. The upper heating station was
connected to the linear actuator through a bellow-sealed motion
feedthrough. The two heating stations were separately heated by
resistive heating cartridges, and the temperatures were measured by
two separate K-type thermocouples. Careful machining of the heating
stations ensured that the surfaces of the two heating stations were
parallel to each other and perpendicular to the actuator axis. The
linear actuator could be programmed to move either according to
prescribed load forces in a force-controlled mode, or according to
prescribed actuator displacements in a displacement-controlled
mode. The total axial force was measured by a 25 kN load cell, and
the total axial displacement of the actuator was measured by a
linear variable displacement transducer. Flat Al coupons and Al
HARMS with Al--Ge composite films deposited on the bonding surfaces
were placed face-to-face on the lower heating station. The chamber
was evacuated, and both heating stations were heated to about
400.degree. C. The upper heating station was then contacted with
the Al assembly to be bonded, and an increasing compression force
was applied to the assembly at a constant loading rate of 100
N/min. A constant force was held for 12 min after the compression
force reached the desired level. The temperatures of both heating
stations were increased during the compression force increase, so
that the specimen temperature reached about 500.degree. C. during
the constant force hold. After the constant force hold, the linear
actuator was withdrawn from the Al assembly and the system was
allowed to cool down.
EXAMPLES 5 AND 6
[0078] Characterization of Al--Ge nanocomposite thin films and
bonding of two-layer devices. A similar bonding process was used to
assemble an Al-based, two-layer, microchannel device. Al--Ge
composite thin films, with an approximate thickness of .about.2
.mu.m, were deposited on both sides of a polished Al foil and the
feature surfaces of two replicated Al HARMS. After Al--Ge film
deposition, the two Al HARMS were placed face to face on the bottom
heating station, with the Al foil inserted in the middle. Both
heating stations were heated to slightly above 500.degree. C., and
the upper heating station was then placed in contact with the
assembly. An increasing compression force was applied to the
assembly at a constant loading of 300 N/min. A constant force was
held for 10 min after the compression force reached .about.1000 N,
corresponding to an applied pressure of .about.1.5 MPa. After the
constant force hold, the linear actuator was withdrawn from the Al
assembly and the system was cooled down.
[0079] A JEOL2010 transmission electron microscope (TEM) was used
to characterize the micro- and nano-scale structure of the Al--Ge
composite films. Cross-sectional TEM specimens of Al--Ge films
deposited on Si(100) substrates were prepared with standard
face-to-face gluing, mechanical thinning, dimple grinding, and ion
milling using 4 kV Ar.sup.+ ions at a 4.degree. take-off angle on a
Gatan Precision system. Compositional analysis of Al--Ge composite
thin films was performed by energy dispersive X-ray spectroscopy
(EDS). EDS measurements were made using a EDAX system equipped with
an ultra-thin window detector, attached to a Hitachi S3600N
scanning electron microscope (SEM). EDS spectra were collected at
an electron beam energy of 15 keV and a detector take-off angle of
36.degree.. SEM examinations of bonded Al assemblies were made on a
Hitachi S3600N SEM.
[0080] For eutectic bonding with an Al--Ge interlayer, the
composition of Al--Ge films was measured and correlated to the
deposition conditions. In addition, the micro- and nano-scale
structure of the Al--Ge films was characterized. FIG. 5 depicts an
EDS spectrum collected from one Al--Ge film, deposited at Al and Ge
cathode currents of 0.5 A and 0.4 A, respectively. Similar EDS
spectra were collected from Al--Ge films deposited at different Al
and Ge cathode currents, at the same spectrometer settings, the
same magnification, and from an identical area about 10
.mu.m.times.10 .mu.m. The manufacturer-supplied EDS data analysis
software, EDAX Genesis (version 3.6), was used for automatic
background fitting and removal. A standardless quantification
routine employing ZAF corrections was included as a part of the
data analysis software, and was used to obtain atomic percentages
from raw EDS spectra. Spectra collected from Al--Ge films contained
signals from Al and Ge within the film, as well as signal from the
Si substrate. No oxygen signal was seen above the noise level.
Because there was a strong signal from the Si substrate, only the
ratio of Ge at. % composition to Al at. % composition was
considered. FIG. 6 depicts the Ge to Al composition ratio for a
series of Al--Ge films as a function of Ge cathode current,
deposited at a fixed Al cathode current of 0.5 A. The thickness of
this series of Al--Ge films, as measured by cross-sectional SEM,
was about 360 nm. The deposition rate for this series of Al--Ge
films was thus about 12 nm/min. As expected, the Ge composition
increased with increasing Ge cathode current. The Ge to Al
composition ratio for the film deposited at an Al cathode current
of 0.5 A and a Ge cathode current of 0.2 A was close to the
eutectic Al.sub.70Ge.sub.30 composition.
[0081] Cross-sectional TEM bright-field (BF) micrographs were taken
of a film deposited at an Al cathode current of 0.5 A and a Ge
cathode current of 0.3 A. The nanoscale structure of the Al--Ge
films could be seen, from regions close to the Si(100) substrate to
regions close to the top surface of the film. (Data not shown; see
FIG. 8 of priority application 61/020,789.) High-resolution imaging
showed that the region immediately adjacent to the Si(100)
substrate, with dark contrast, was crystalline Ge, while regions
with light contrast above the Ge layer were crystalline Al. These
imaging results were also confirmed with a series of EDS spectra
collected from the different regions. The bright-field micrographs
provided strong evidence of phase separation within the
co-deposited Al--Ge film. In companion dark-field (DF) micrographs,
the Ge regions exhibited light contrast while the Al regions showed
dark contrast. The dark-field micrographs again showed that a
crystalline Ge layer formed next to the Si(100) substrate. (Data
not shown; see FIG. 9 of priority application 61/020,789.)
Immediately above the Ge layer, lateral separation occurred between
the crystalline Ge and crystalline Al grains, roughly in a plane
parallel to the substrate surface. As the Al--Ge film deposition
continued, some Al grains began to spread laterally and cover the
Ge region underneath, showing evidence of transverse separation
between crystalline Ge and crystalline Al grains in the direction
perpendicular to the substrate surface. Thus we saw evidence of
both lateral and transverse separations between Ge and Al within
the co-deposited Al--Ge film. A high-resolution micrograph of a
co-deposited Al--Ge film with a composition close to eutectic
clearly showed phase-separated Al-rich and Ge domains, typically
several tens of nm in size (data not shown).
[0082] Without wishing to be bound by this hypothesis, it is
believed that nanoscale separation between the crystalline Ge and
crystalline Al domains within codeposited Al--Ge films resulted
from competition between the thermodynamic driving force for Al--Ge
phase separation and the growth kinetics dictated by the film
deposition rate. For the purpose of using Al--Ge films for
intermediate layer bonding, the Al and Ge domains in such
codeposited films are on the order of 100 nm or smaller. This
intimate mixing promotes eutectic melting of the entire Al--Ge film
once the eutectic point T.sub.E is reached, and is beneficial for
bonding of mating surfaces with microscale dimensions. Codeposition
of Al--Ge films is a preferred route for making nanoscale
phase-separated Al--Ge eutectic mixtures since it can produce an
intimate mixture of Al and Ge domains with controlled phase
separation. The film composition may be controlled by adjusting the
individual Al and Ge sputter erosion rates.
[0083] In one embodiment, nanocomposite Al--Ge intermediate layers
were codeposited onto a replicated Al HARMS with a parallel array
of straight microchannels, and one flat Al coupon. To test the
feasibility of simultaneously bonding features of different sizes,
the microchannel width was varied between 120 .mu.m and 180 .mu.m.
The Al--Ge intermediate layer was deposited at a fixed Al cathode
current of 1.0 A and a fixed Ge cathode current of 0.45 A, making
the film's composition close to eutectic. After the Al--Ge
intermediate layers had been deposited, the replicated Al HARMS and
the flat Al coupon were bonded at a final holding temperature of
510.degree. C. The applied pressure during the final hold stage of
bonding was about 1.5 MPa. FIG. 7 depicts a cross-sectional view of
a typical, one-layer, Al-based microchannel structure following
such bonding. The replicated Al microchannel structure and the flat
Al coupon were bonded tightly together, with no evident gaps.
[0084] In another embodiment, two Al HARMS pieces were used to
assemble a two-layer structure with closed microchannels. In each
Al piece, a set of parallel rectangular microchannels, .about.1 cm
long and .about.330 .mu.m deep, was replicated in the Al bulk from
surface-engineered Inconel X750 inserts by compression molding. Two
plena were machined into the Al bulk and connected to the two ends
of the microchannel array. To test the feasibility of
simultaneously bonding microfeatures of different sizes, the widths
of microchannels were varied from less than 80 .mu.m to more than
250 .mu.m. A polished Al foil, with an Al--Ge film deposited on
both sides, was inserted between the two Al HARMS pieces, and the
bonding process as described above was used to produce a
three-piece Al assembly, containing two layers of parallel
microchannels. Holes were drilled through the entire bonded
specimen at the plenum regions on each side of the microchannels.
Threaded holes were tapped into the Al bulk for external fluid
connections. FIG. 8 depicts a cross-sectional view of a portion of
the assembled two-layer microchannel structure, obtained by
mechanical cutting. The structure contained two layers, with 20
microchannels in each layer. FIG. 9 depicts a close-up view of a
typical bonded microchannel. The bonding interface is not even
discernable in this high-magnification view, indicating the quality
of bonding achieved.
[0085] Water was fed into one plenum, and it then flowed freely out
of the cut cross-section as individual jets. (Not shown; see FIG.
13 of priority application 61/020,789.)
[0086] The replicated Cu and Al microchannel arrays each contained
19 rectangular microchannels. Our observations showed exit water
jets from all 19 microchannels in the assembled Cu MHE, while exit
water jets were observed from only 18 microchannels in the
assembled prototype Al MHE. It appeared that one microchannel in
the assembled Al MHE had been blocked during the bonding process;
the reason for the blockage is not understood. Our measurements
indicated that the average width of microchannels in the Cu
prototype device exhibited little change from that on the
as-replicated Cu coupon, while the average microchannel width of
the Al device had decreased somewhat from that on the as-replicated
Al coupon. Considering that Al bonding occurred at about 83% of the
Al melting temperature, we speculate that the observed narrowing of
the microchannels may have resulted from plastic deformation during
the bonding process.
EXAMPLES 7 AND 8
[0087] Bonding of Cu-based microchannel structures with Al--Ge
nanocomposite thin film intermediate layers and free-standing Al
thin films. To prepare prototype embodiments of one-layer,
enclosed, Cu-based microchannel structures, Cu coupons containing
replicated microchannels were bonded onto a flat Cu plate. The
thickness of both the flat plate and the Cu coupons was 6.4 mm. The
bonding surfaces of the coupons and flat plates were mechanically
polished with SiC abrasive papers, and finished with a 1 .mu.m
diamond particle suspension. Al--Ge composite thin film
intermediate layers were deposited onto the polished surfaces by
sputter co-deposition in a pure Ar (99.999%) atmosphere, at a
pressure of .about.1.3 mTorr. Two separate sputter targets were
used, one for pure Al (99.99%) and the other for pure Ge (99.99%).
The polished Cu coupons and plates were ultrasonically cleaned in
acetone and methanol before being mounted on a rotatable holder in
the middle of the deposition zone. The deposition sequence
comprised a radio frequency (rf) inductively coupled plasma (ICP)
substrate surface etch, followed by Al and Ge co-deposition.
Substrate etching occurred in a pure Ar ICP with a total rf input
power of 1000 W, a substrate bias of -100 V, and an etch duration
of 20 min. Sputtering of Al and Ge targets commenced immediately
after the substrate surface etch. Substrates were rotated
continuously at .about.12 rpm during both etching and deposition.
All Al--Ge depositions were carried out using fixed target
currents: 1.0 A for Al, and 0.45 A for Ge. The substrate bias
during deposition was held at -50 V. These deposition parameters
resulted in a composition ratio close to the Al.sub.70Ge.sub.30
eutectic. The deposition duration was 60 min, producing an Al--Ge
film thickness of .about.2 .mu.m.
[0088] Bonding experiments were carried out using the MTS858
single-axis testing system interfaced to a high-vacuum chamber
containing two heating stations. Cu coupons containing microscale
features and flat Cu plates, with Al--Ge composite films deposited
on the bonding surfaces, were placed face-to-face on the lower
heating station. The chamber was evacuated, and both heating
stations were heated. Bonding of the Cu coupon and plate occurred
at a temperature about 540.degree. C. with an applied pressure
about 3 MPa. FIG. 10 depicts a cross-sectional view of a portion of
a prototype Cu microchannel structure bonded with Al--Ge
nanocomposite thin film intermediate layers.
[0089] In an alternative embodiment, Cu microchannel structures
were bonded with a single free-standing Al thin film as the
intermediate layer. To form Cu-based, single-layer, microchannel
structures, one Cu coupon containing a parallel array of replicated
microchannels and one blank Cu coupon were placed face to face on
the bottom heating station, with a 10 .mu.m thick Al free-standing
thin film inserted in the middle. Surfaces of Cu coupons were
polished with 1200-grit silicon carbide papers prior to bonding.
The entire assembly was placed on top of the bottom heating
station. After evacuation, both heating stations were heated above
500.degree. C., and the upper heating station was then put into
contact with the assembly. An increasing compression force was
applied to the assembly at a constant loading rate of 300 N/min.
The force was held constant for 15 min after the compression force
had reached the desired level of .about.3000 N, corresponding to an
average applied pressure of about 3 MPa. The final bonding
temperature for the coupon/block/coupon assemblies was held at
about 580.degree. C. After the constant force hold, the linear
actuator was withdrawn from the assembly and the system was cooled
down. FIG. 11 depicts a cross-sectional view of a portion of one Cu
microchannel structure bonded with one free-standing Al thin film
intermediate layer. Comparing FIGS. 10 and 11 shows that both
bonding approaches produced clean, enclosed microchannel structures
without blockages.
EXAMPLES 9 AND 10
[0090] Evaluation of the strength of bonded Al--Al and Cu--Cu
interfaces. Tensile testing specimens were prepared to evaluate the
strength of Al--Al interfaces bonded with Al--Ge eutectic
nanocomposite thin film intermediate layers. Al--Ge composite films
were deposited onto the bonding surfaces of two cuboid Al coupons,
.about.22 mm.times..about.22 mm in area. The two Al coupons were
placed face-to-face on the bottom heating station, forming an
assembly .about.36 mm long in the direction perpendicular to the
bonding interface. A small hole was drilled at the corner of the
bottom Al coupon close to the bonding interface. A K-type
thermocouple was inserted into the hole to measure the temperature
of the interface during bonding. After the chamber was evacuated,
both heating stations were heated to temperatures about 10 degrees
higher than the target bonding temperatures, which were 450, 500,
and 550.degree. C. for different tests. During the heating process,
the top heating station was held close to the top surface of the Al
coupon assembly but was not in contact with it. The temperature of
the bonding interface during this initial heating process, as
measured by the K-type thermocouple, was always less than
390.degree. C., below the Al--Ge eutectic temperature of
424.degree. C. Once the bottom and top heating stations had reached
steady state temperatures, the upper heating station was put into
contact with the Al coupon assembly using the linear actuator. A
linearly increasing compression force was applied to the Al coupon
assembly such that loading force levels of 250, 500, and 750 N were
reached after a constant duration of 8 min. These loading forces
corresponded to applied pressures of .about.0.5, .about.1.0, and
.about.1.5 MPa, respectively. A constant force was held for 10 min
after the compression force reached the desired levels. During the
compression force increase, the temperature of the bonding
interface increased to close to the target temperature of 450, 500,
or 550.degree. C. Further temperature increases were registered on
the K-type thermocouple during the constant force hold. For all
bonding runs, the total temperature change measured by the K-type
thermocouple during the 10 min constant force hold was less than
12.degree. C. The nominal bonding temperature was taken to be the
average value of the K-type thermocouple reading during the 10 min
constant force hold. After this hold period, the heaters were
turned off to cool the bonded specimen, with the same force still
applied. After the temperature of the bonding interface had
decreased below 400.degree. C., the linear actuator was withdrawn
from the Al assembly, and the system was cooled down to room
temperature.
[0091] We prepared a series of two Al-coupon assemblies bonded at
different temperatures, different applied pressures, and different
thicknesses of Al--Ge intermediate layers. Using a Struers Accutom5
cutting machine, the four outermost sections along the axial
direction, each .about.3-4 mm thick, were removed from the bonded
Al specimen. From the remaining specimen, which was .about.14
mm.times..about.14 mm.times..about.36 mm, four tensile testing
specimens were obtained with two perpendicular, bisecting cuts
parallel to the axial direction, resulting in final tensile
specimens .about.7 mm.times..about.7 mm.times..about.36 mm. After
cutting, the four surfaces of the tensile specimen parallel to the
axial direction were mechanically polished with 600 grit silicon
carbide papers. Tensile testing was conducted along the specimen's
axial direction to evaluate the tensile strength of the bonding
interface, using a MTS810 system with hydraulic grips. Surface
morphologies of the fractured bonding surfaces were examined on a
Hitachi S3600N scanning electron microscope (SEM), as well as with
an X-ray dispersive spectroscopy (EDS) system (EDAX) equipped with
an ultra-thin window detector.
[0092] FIG. 12 summarizes the results of our measurements of bond
strength as a function of bonding temperature. For this series of
measurements, all bonding runs were conducted at an applied
pressure of .about.1.5 MPa, with a .about.2 .mu.m Al--Ge
intermediate layer deposited on each bonding surface (i.e., total
thickness of both Al--Ge intermediate layers at the bonding
interface was .about.4 .mu.m). Of 15 tensile tests total, the four
performed on specimens corresponding to a bonding temperature of
.about.450.degree. C. all resulted in clean breaks at the bonding
interface (denoted "break" in the figure). Of the seven tests
performed on specimens corresponding to a bonding temperature of
.about.500.degree. C., only one resulted in breaking at the bonding
interface. The other six tests resulted in deformations at the
gripped sections without any break at the bonding interface
(denoted "no break" in the figure). All four tests performed on
specimens corresponding to a bonding temperature of
.about.550.degree. C. resulted in deformations at the gripped
sections only (denoted "no break"). It is evident from FIG. 12 that
the measured values of bond strengths exhibited significant
scatter. At a bonding temperature of .about.450.degree. C.,
measured bond strength varied from .about.85 to .about.156 MPa; and
at a bonding temperature of .about.500.degree. C., measured bond
strength varied from .about.77 to greater than .about.167 MPa.
[0093] FIG. 13 summarizes results of bond strength measurements as
a function of the pressure that was applied during bonding. For
this series of measurements, all bonding runs were performed at a
bonding temperature of .about.500.degree. C., with a .about.2 .mu.m
Al--Ge intermediate layer deposited on each bonding surface (i.e.,
total thickness of both Al--Ge intermediate layers at the bonding
interface was .about.4 .mu.m). At the lowest applied pressure of
.about.0.5 MPa, all four tensile tests resulted in clean breaks at
the bonding interface, yielding measured values of bond strength
from .about.84 to .about.140 MPa. Of 11 tensile tests performed on
specimens bonded at applied pressures of .about.1.0 or .about.1.5
MPa, only two tests resulted in breaks at the bonding interface.
The remaining specimens failed at the gripped sections during
testing. Of these tests, the highest strength values obtained were
higher than 175 and 167 MPa, at applied pressures of .about.1.0 and
.about.1.5 MPa, respectively.
[0094] FIG. 14 summarizes results of bond strength measurements as
a function of the thickness of the Al--Ge intermediate layer. For
this series of measurements, all bonding runs were performed at a
bonding temperature of .about.500.degree. C. and an applied
pressure of .about.1.5 MPa. At the smallest Al--Ge film thickness
of .about.0.5 .mu.m, measured bond strengths ranged from 89 to over
152 MPa. No gross differences were discerned in tests performed on
specimens bonded at Al--Ge film thicknesses of .about.1.0 or
.about.2.0 .mu.m, with the highest measured bond strengths
exceeding 167 MPa in both cases. The maximum observed strength was
obtained at an Al--Ge film thickness of .about.1.0 .mu.m, exceeding
.about.188 MPa. At the three Al--Ge film thicknesses of .about.0.5,
.about.1.0, and .about.2.0 .mu.m, a similar scatter in bond
strengths was observed. The data shown in FIGS. 12-14 indicated
that, notwithstanding the scatter in the measurements, interfacial
strength values exceeded .about.77 MPa, or .about.770 atmospheres,
under all bonding conditions tested.
[0095] The structures of the Al--Al interfaces within the bonded Al
microchannel structures were examined by combining SEM with focused
ion beam (FIB) images (data not shown). The imaged area,
approximately 250 .mu.m.times.190 .mu.m, was close to one corner of
a microchannel. Prior to SE image and EDS spectrum acquisition, the
entire area was lightly etched with a Ga.sup.+ ion beam. The
secondary electron (SE) image showed a band with speckled contrast
around the location of the original bonding surfaces, with a width
of .about.100 .mu.m. The speckling apparently arose from Ge
precipitates, as it was generally in the same locations
corresponding to a Ge-L X-ray intensity image (data not shown). The
Ge precipitates ranged in size from <1 to .about.4 .mu.m. A band
of Ge precipitates also surrounded the sidewall of the Al
microchannel, where the mating Al plate was not present. The Al--Ge
thin film had been deposited onto the microchannel sidewall during
sputter-codeposition, which likely accounts for the presence of Ge
precipitates around the sidewall of the Al microchannel.
[0096] To confirm that the Ge precipitate band was not an artifact
of the mechanical polishing process, a perpendicular cut, .about.38
.mu.m long and .about.20 .mu.m deep, was made with a focused
Ga.sup.+ ion beam into the cross section surface. This cut
straddled the location of the original bonding surfaces. Ge
precipitates, ranging from .about.0.7 .mu.m to .about.4 .mu.m, were
seen dispersed within Al grains 10 .mu.m and larger in an
apparently random fashion (data not shown). The presence of Ge
precipitates was also confirmed by corresponding Al--K and Ge-L
X-ray intensity maps (data not shown). Other than the presence of
the random Ge precipitates, no clear demarcation could be seen to
indicate the location of the bonding surfaces of the two original
Al pieces. Our observations indicated that the Ge precipitates were
distributed in an approximately uniform manner across an extended
interface region .about.100 .mu.m wide. SE images, and Al--K and
Ge-L X-ray intensity maps also showed the presence of a band of Ge
precipitates surrounding the location of the original bonding
surfaces, with a width of .about.100 .mu.m.
[0097] Consistent observations were made in tensile fracture
surfaces of bonded Al--Al specimens. For example, in an SE image of
a tensile fracture surface of a specimen bonded at
.about.500.degree. C., faceted regions with sizes ranging from
<1 to .about.4 .mu.m were interdispersed with regions containing
numerous micron and submicron sized dimples (data not shown). The
chemical compositions of the faceted and dimpled regions were
probed by EDS mapping. Faceted regions yielded low Al--K and high
Ge-L counts, representing exposed Ge crystallites. Dimpled regions
had low Ge-L counts, and represented the Al matrix. Our SEM
fractography measurements confirmed Al--Ge phase separation within
the bonding interface region. The presence of faceted Ge
crystallites on the fracture surface suggested that separations
occurred either at interfaces between Ge crystallites and the Al
matrix, or across Ge crystallites in a brittle manner; while the
observation of micro/nano scale dimples on the fracture surface
suggested that separation of the Al regions involved ductile
fracture.
[0098] Without wishing to be bound by this hypothesis, we believe
that these observations support our proposed explanation for the
high average tensile strength measured at the bonded Al--Al
interface. The Al--Ge intermediate layer bonding process
effectively joined the Al coupon with the flat Al plate across an
extended interface region, with no demarcation at the location of
the original bonding surfaces. The average tensile bond strength
was dominated by ductile fracture of the Al matrix within this
extended interface region. The approximately uniform distribution
of Ge precipitates, <4 .mu.m in size, within this extended
interface region .about.100 .mu.m wide meant that the region of Ge
precipitates across the cross section was lower than would be the
case where all Ge atoms concentrated within a narrow interface
region. This dispersal of the Ge precipitates increased the
fractional area of Al--Al bonds, and thus the average tensile
strength.
[0099] Without wishing to be bound by this hypothesis, we believe
that certain mechanisms likely predominated throughout the extended
bonding interface region. During bonding, the two solid Al pieces
were in contact with a layer of Al--Ge eutectic liquid, .about.4
.mu.m thick. Because bonding occurred at .about.500.degree. C.,
above T.sub.E=424.degree. C., there was likely some broadening of
the liquid layer by dissolution of solid Al into the Al--Ge
eutectic liquid. Simple calculations suggested that the composition
shift likely broadened the Al--Ge liquid layer from .about.4 to
.about.6 .mu.m. Furthermore, the solidus reaction and Ge diffusion
into the solid aluminum broadened the bonding interface region to
.about.50 .mu.m. Additional cross-sectional metallographic
observations around the bonding interface region showed Al grains
spanning the entire bonding interface region, suggesting the
possibility of epitaxial re-growth from un-melted Al grains.
[0100] Without wishing to be bound by this hypothesis, we believe
that the nanoscale domain size of the Al-rich and Ge regions within
the Al--Ge intermediate layer, together with eutectic melting, aids
the broadening of the bonding interface region, and improves the
bonding of microscale Al-based structures. Generally similar
results are expected using other eutectic bonding
intermediates.
[0101] Tensile testing specimens were prepared to evaluate the
strength of Cu--Cu interfaces bonded with an Al thin film
intermediate layer. Two rectangular Cu coupons (.about.25
mm.times..about.16 mm.times..about.16 mm), with surfaces
mechanically polished to less than 1 .mu.m roughness, were bonded
with one Al thin film intermediate layer. The two Cu coupons were
placed face to face on the bottom heating station with a thin Al
film (Al 1100, 99%+) inserted in the middle, forming an assembly
.about.32 mm long in the axial direction, perpendicular to the
bonding interface. A small hole was drilled at the corner of the
bottom Cu coupon close to the bonding interface, into which a
K-type thermocouple was inserted. After the chamber was evacuated,
both heating stations were heated. The top heating station was
close to the top surface of the Cu coupon assembly, but not in
contact with it. After the bottom and top heating stations reached
500.degree. C., the upper heating station was placed in contact
with the Cu coupon assembly using the linear actuator. A linearly
increasing compression force was applied to the Cu coupon assembly,
so that loading force levels of 425, 850, and 1700 N were reached
after a constant duration of 10 min. These compressive loading
forces corresponded to average applied pressures of .about.1,
.about.2, and .about.4 MPa, respectively. A constant force hold was
executed after the compression force reached the desired level.
During the compression force increase, the temperatures of the
bottom and top heaters were raised, leading to further temperature
increase at the bonding interface. The interface temperature was
recorded continuously during the constant force hold, 12 min after
the interface temperature reached .about.550.degree. C.,
corresponding to the Al--Cu eutectic temperature of 548.degree. C.
During this 12 min hold, the interface temperature increased
further and reached steady state. This steady state temperature, as
measured by the K-type thermocouple, was taken as the nominal
bonding temperature. After the 12 min hold, the heaters were turned
off with the same force still applied. After the temperature of the
bonding interface decreased to <450.degree. C., the linear
actuator was withdrawn from the top surface of the Cu assembly, and
the system was cooled to room temperature.
[0102] A series of Cu two-coupon assemblies was bonded at different
applied pressures and with different thicknesses of Al films. Using
a Struers Accutom5 precision cutting machine, the four outermost
sections along the axial direction, .about.2-3 mm thick, were
removed from the bonded Cu specimen. From the remaining specimen,
.about.20 mm.times..about.12 mm.times..about.32 mm, four tensile
testing specimens were obtained by making two perpendicular
bisecting cuts parallel to the axial direction, resulting in a
final tensile specimens .about.10 mm.times..about.6
mm.times..about.32 mm. After cutting, the four surfaces of the
tensile specimen parallel to the axial direction were mechanically
polished with 600-grit silicon carbide papers to remove
irregularities. Tensile testing along the specimen axial direction
was performed to evaluate the tensile strength of the bonding
interface, using an MTS810 system with hydraulic grips. X-ray
diffraction (XRD) patterns from fractured bonding surfaces were
collected on a Rigaku MiniFlex X-ray diffractometer using Cu
K.alpha. radiation.
[0103] Tensile testing was performed on bonded Cu two-coupon
assemblies. During tensile testing, the two ends of the specimen
were gripped by the hydraulic grips. The gripped sections were
.about.10 mm long, leaving an un-gripped section .about.12 mm long,
with the bonding interface in the middle. As the specimen extension
increased, the tensile stress increased to a maximum, followed by
breaking at the bonding interface and a rapid drop in stress. The
maximum tensile stress observed on the stress-extension curve is a
measure of the tensile bond strength.
[0104] FIG. 15 depicts measured bond strength as a function of the
applied bonding pressure. For this series of measurements, all
bonding runs were performed at .about.580.degree. C. with an Al
film thickness of .about.25 .mu.m. In all cases, clean breaks
occurred at the bonding interface. At the lowest pressure of
.about.1 MPa, measured bond strength ranged from 33 to 45 MPa. No
significant difference was seen in tests performed on specimens
bonded instead at pressures of .about.2 or .about.4 MPa. The
maximum strength measured was .about.48 MPa.
[0105] FIG. 16 depicts measured bond strength as a function of the
thickness of the Al film at the bonding interface. For this series
of measurements, all bonding runs were performed at
.about.580.degree. C. at an applied pressure of .about.4 MPa. At
the lowest Al film thickness of .about.10 .mu.m, measured bond
strength ranged from 39 to 52 MPa. No gross differences were
evident in tests performed on specimens bonded with Al films with
thickness of .about.25 or .about.38 .mu.m. Measured bond strength
values ranged from 33 to 46 MPa. Data shown in FIGS. 15 and 16
indicated little variation in bond quality within the range of
applied pressure and Al film thickness tested.
[0106] FIG. 17 depicts three XRD patterns obtained from fracture
surfaces bonded at Al film thicknesses of 10, 25, and 38 .mu.m,
respectively. Major diffraction peaks within all three patterns can
be assigned to either an fcc Cu phase, or to a fcc Al phase,
indicating the presence of these phases in the interface region
after bonding. The lattice parameters for the Cu phase were 3.663
.ANG., 3.658 .ANG., and 3.667 .ANG. at Al foil thicknesses of 10,
25, and 38 .mu.m, respectively. In comparison, the lattice
parameter for the starting Cu coupon was measured at 3.618 .ANG..
The corresponding lattice parameters for the Al phase were 4.102
.ANG., 4.103 .ANG., and 4.111 .ANG., respectively. In comparison,
the lattice parameter for elemental Al is 4.05 .ANG.. The
respective lattice parameter increases over those of elemental Cu
and Al suggest the possibility that there may have been some
dissolution of Al into Cu and vice versa in the interface region.
Besides those indexed to fcc Cu and fcc Al phases, other
diffraction peaks were seen in all three XRD patterns, indicating
the formation of additional Al--Cu compounds within the interface
region during bonding. According to the standard Al--Cu phase
diagram, a single eutectic should exist between the fcc Al phase
and the .theta.-Al.sub.2Cu phase. If bonding occurred solely via
the eutectic mechanism, only the .theta.-Al.sub.2Cu phase would be
expected to be present in the interface region. Not all the
additional diffraction peaks present within the three XRD patterns
can be assigned to .theta.-Al.sub.2Cu, however, suggesting the
presence of Al--Cu compounds other than .theta.-Al.sub.2Cu in the
interface region and suggesting that bonding may have occurred via
a combination of both eutectic and diffusional mechanisms.
Assembly of Metal-Based Microchannel Devices
[0107] Another aspect of this invention pertains to the fluidic
transitions from the microchannel arrays to larger scale plena. To
create functional metal-based microchannel devices, for example for
heat exchanger applications, it is desirable to have techniques to
fabricate unobstructed fluidic transitions from microchannel arrays
to larger scale plena. The larger scale plena are used to provide
fluidic inlet and outlet connections to the "outside world."
[0108] Factors in designing microchannel-to-large-plenum
transitions are ease of operation, and the parallel creation of
many transitions at once. Serial subtractive fabrication techniques
are not well-suited for this purpose. For example, micromilling
would involve contact of a milling tool with formed microchannels,
and could cause deformation at the microchannel-to-plenum
transitions, leading to partial or complete blockage. This
mechanical machining also demands small-scale tooling, perhaps on
the order of 100 micron or even smaller. Furthermore, this kind of
serial operation creates only one transition at a time, and is very
time consuming for a large number of connections for microchannel
arrays. It is therefore preferred that fabrication protocols should
involve parallel forming or machining. Non-contact machining
methods are also preferred.
EXAMPLE 11
[0109] Creating microchannel-to-large-plenum fluidic transitions by
molding replication. FIG. 18 depicts an example of a mold insert
design suitable for creating a microchannel-to-large-plenum fluidic
transition by molding. In this embodiment, a long meandering
microchannel terminates at a triangular opening, which can widen
into a large scale plenum. The microchannel structure shown in FIG.
18 can be created by molding replication.
EXAMPLE 12
[0110] Creating microchannel-to-large-plenum fluidic transitions by
non-contact machining methods. FIGS. 19 and 20 depict an example of
multiple-microchannels-to-large-plenum fluidic transitions created
in Cu by .mu.EDM. An array of straight microchannels created by
molding replication was connected to one supply channel and to one
drain plenum using .mu.EDM. Flat stainless steel sheets with a
thickness of 1100 .mu.m were used as blade electrodes for .mu.EDM.
A single cut with the steel blade electrode formed the fluid supply
channel, while six consecutive cuts with partial overlaps formed
the fluid drain plenum. All cuts were perpendicular to the
microchannel array. The depth of the cuts was the same for all cuts
on both the supply and drain sides.
[0111] As shown in FIG. 19 for one Cu coupon, mechanical drilling
was used to make two through holes to connect to the supply
channel, and three through holes within the drain plenum. The
through holes in the drain plenum were placed symmetrically with
respect to the microchannel array. All through holes were
mechanically tapped from the coupon's far side, away from the
microchannel array, to allow external fluid connections using
plastic adapters. The 6.4 mm coupon thickness sufficed to
accommodate the taps. FIG. 20 provides a more detailed view of the
fluidic supply connections on the Cu coupon from one tapped hole to
the microchannels. The supply channels had the mottled surface
morphology typical of structures cut by .mu.EDM. The entrances were
unobstructed.
[0112] Alternatively, the microchannel-to-plenum transitions shown
in FIGS. 19 and 20 can be formed by molding with an appropriate
insert geometry design.
Improving Heat Exchange Efficiency with Metal-Based Microchannels
Having Substantial Surface Roughness
[0113] A further aspect of the invention pertains to the creation
of microchannel heat exchangers with engineered surface roughness
within the microchannels. Surprisingly, we found that surface
roughness within microchannels, for example on the order of a few
microns to several tens of microns, substantially increases
convective heat transfer performance of the entire device as
compared to an otherwise similar device with smoother surfaces,
i.e., one having a roughness less than a few microns. We further
found that molding techniques are well adapted to replicate such
surface roughness in metal-based microchannels. For example, the
surface roughness of the refractory metal or alloy mold insert can
be altered through the .mu.EDM and ECP process control, for example
by controlling the current density or etch time. Such surface
roughness on the mold insert is conveyed with high fidelity through
the molding process onto the metal substrate being molded. By
contrast, such surface roughness would usually not be seen in
microchannel structures made by conventional
"semiconductor/IC-type" processing methods. With the novel molding
replication technique, reproducible roughness within metal-based
microchannels becomes fast and inexpensive.
[0114] We have discovered, quite unexpectedly, that this surface
roughness substantially enhances microchannel heat exchanger
performance. Our data suggested that surface roughness within the
microchannels promotes fluid flow mixing to a surprising degree,
which consequently increases the convective heat transfer
coefficient as compared to similar microchannels with smooth
surfaces. Such heat transfer enhancements exist over a large range
of flow rates or Reynolds numbers.
EXAMPLES 13 AND 14
[0115] Enhancing heat transfer performance in Cu- and Al-based,
single-layer, microchannel devices with increased surface
roughness. We prepared bonded Cu and Al microchannel devices, and
attached polymer external fluid adaptors. (Not shown; see FIG. 26
of priority application 61/020,789.) The internal microchannel
array configurations within these devices was the same as that
shown in FIGS. 19 and 20. FIG. 21 depicts a high-magnification
scanning electron micrograph of a portion of the replicated Cu
microchannel array. The Cu microchannels have vertical sidewalls
and sharp sidewall-to-bottom transitions. Elevated surface
roughness is clearly visible on the sidewall and bottom of all
microchannels. Generally similar observations were made in the Al
microchannel device (data not shown).
[0116] The surface roughness within the molded microchannels was
quantitatively evaluated by optical profilometry, expressed as
peak-to-valley roughness Rz. The average Rz values were 11.8 .mu.m
and 8.2 .mu.m for the bottom surfaces of Cu and Al microchannels,
respectively. Surface roughness of the microchannel sidewalls was
somewhat smaller, on the order of 5 .mu.m. The observed Rz values,
on the order of 10 .mu.m, substantially exceeded what is typically
obtained from micromilling (typically 1 .mu.m or less).
[0117] We compared our measured heat transfer rates with some that
have been reported in the literature. (Data not shown; see FIG. 28
of priority application 61/020,789.) Measured heat transfer
coefficients, h, were converted to dimensionless Nusselt numbers,
Nu, based on the average hydraulic diameter of the microchannels,
D.sub.h,
Nu = hD h K f , ##EQU00001##
where K.sub.f is the thermal conductivity of the fluid. Values of
Nu are plotted versus the Reynolds number, Re, which represents a
dimensionless average fluid velocity through the microchannels, V.
The value of Re is defined as
Re = .rho. V _ D h .mu. . ##EQU00002##
Both the fluid density .rho. and the viscosity .mu. (in this case
for liquid water) may depend on temperature.
[0118] In the range 500<Re<2250, good agreement existed
between Nusselt numbers measured from the Cu and the Al MHE
specimens. For Re>2500, data from the Cu and the Al MHE
specimens began to diverge, with Nu values from the Cu specimen
exceeding those from the Al specimen.
[0119] We compared our measurements to data taken from Lee et al.,
Int. J. Heat Mass Transfer 48(9), 1688-1704 (2005) for machined Cu
microchannels with smooth surfaces; the data set of Lee et al. were
taken from microchannels with D.sub.h=318 .mu.m, and extended over
a smaller range of Re, from .about.500 to .about.2500. Our measured
Nu values were substantially and significantly higher than those of
Lee et al. over much of the range 500<Re<2500. We also
compared our measured values to some conventional Nusselt number
correlations. Correlations for fully developed laminar flow and the
Sieder-Tate correlation for simultaneously developing laminar flow
were calculated using dimensions corresponding to the Cu
microchannel device. The Gnielinski correlation for transitional
and fully developed turbulent flows was also calculated. Our data
from the novel Cu and Al microchannel devices exhibited trends
generally similar to those predicted by the conventional
correlations, but with higher values of Nu. (Data not shown; see
FIG. 28 of priority application 61/020,789.)
[0120] The data obtained from the Cu and Al microchannel devices,
as well as the data of Lee et al., showed Nusselt number increasing
with increasing Reynolds number. At the same Re values, our data
showed higher Nu values as compared to those of Lee et al. Direct
visualization of flow within the microchannel arrays would be
impractical for the assembled, opaque Cu and Al microchannel
devices. Nonetheless, it appears that the surface roughness within
the microchannel arrays resulting from the molding replication
process is responsible for the observed higher Nu values. The
increased surface roughness within the microchannel arrays may, for
example, lead to increased cross-wise flow mixing, resulting in
higher heat transfer as compared to that in smoother channels.
[0121] Accurate measurements of the solid-to-fluid heat transfer
rate require an accurate estimate of the solid wall temperature of
the microchannel array. Relatively large temperature gradients can
be induced within the body of the metal-based MHEs during constant
heat flux testing, making the estimate of solid wall temperature
less reliable. Therefore, an alternative, constant solid surface
temperature testing configuration was adopted to measure heat
transfer rates more accurately. Higher Nu values were obtained as
compared to the uncorrected results, approaching 40 at Re of
.about.3000. The maximum uncertainty for Nu values was
.about.16%.
[0122] We compared known Nusselt number correlations to our
experimental data. The experimental Nu values significantly
exceeded the Hausen and Sieder-Tate correlation values at
250<Re<1500.
[0123] For turbulent flow, the Dittus-Boelter correlation and the
Petukhov correlation were used in the Reynolds number range
2000<Re<3000. These two correlations yielded nearly identical
Nu values over this Reynolds number range. Our measured Nu values
substantially exceeded the Dittus-Boelter and Petukhov correlation
values. When corrections were made to account for a surface
roughness of .about.5 .mu.m, the correlations better matched the
observed Nu data. These trends illustrate the need to consider
surface roughness and entrance length effects in analyzing flow and
heat transfer data, and further demonstrate that engineering
surface roughness into microchannel surfaces can be an effective
means to increase heat transfer efficiency.
[0124] The usefulness of heat transfer devices in accordance with
the present invention is illustrated in FIG. 22. Initially, heaters
attached to the prototype Cu microchannel device were turned on and
the device was allowed to reach a temperature of about 100.degree.
C. Keeping the same power input, about 20 W, in the heaters, room
temperature water (.about.28.degree. C.) was suddenly introduced
into the Cu microchannel device with a fixed pressure drop .DELTA.P
from the outlet to the inlet. The device's surface temperature was
monitored in real time using an infrared (IR) camera. FIG. 22 shows
the change in surface temperature of the Cu microchannel device as
a function of time, both before and after introduction of water
into the specimen at time t=10 sec. The surface temperature dropped
more than 50.degree. C. within the first 5 seconds, followed by a
more gradual decrease to .about.30.degree. C. over the next 60
seconds. The initial, faster drop in temperature was attributed to
drawing heat from the Cu. The second and more gradual drop in
temperature was attributed to drawing heat from the surrounding
insulation material. A fit was made of the measured surface
temperature versus time, assuming a compound exponential decay with
two distinct time constants .tau..sub.1 and .tau..sub.2. Consistent
with expectation, .tau..sub.1 was on the order of a few seconds
while .tau..sub.2 was on the order of a few tens of seconds. FIG.
23 shows values of the fitted time constant .tau..sub.1 for both
the Cu and Al microchannel devices, obtained at different values of
.DELTA.P and initial specimen temperature T.sub.1. As T.sub.1
varied from 70 to 100.degree. C., the values of .tau..sub.1 showed
little variation. However, .tau..sub.1 decreased with increasing
.DELTA.P or water flow rate. .tau..sub.1 was in the range of 1 to 2
s for the Cu device, and 2 to 2.5 s for the Al device. This very
rapid cooling demonstrated the advantages of incorporating high
thermal conductivity metals, such as Cu and Al, into heat
exchangers.
EXAMPLE 15
[0125] Two-layer, Cu-based, microchannel devices incorporating
heating cartridges: instant water heater prototype. A process
generally similar to that described above was used to manufacture
Cu-based, two-layer microchannel, "instant" water heater
prototypes. Cu blocks containing holes for accommodating
cylindrical, electric cartridge heaters, and holes/plena for
fluidic connections were made by conventional machining. Arrays of
parallel microchannels were replicated in Cu coupons by molding.
Surfaces of Cu blocks and coupons were polished with 1200-grit
silicon carbide papers prior to bonding. The prototype assembly
comprised one such Cu block placed between two Cu coupons, each
containing a parallel array of replicated microchannels. One 10
.mu.m-thick, free-standing Al thin film was inserted at each
coupon/block interface. The entire coupon/block/coupon assembly was
placed on top of the bottom heating station. After evacuation, both
heating stations were heated above 500.degree. C., and the upper
heating station was placed in contact with the assembly. An
increasing compression force was applied to the assembly at a
constant loading rate of 300 N/min. The force was held constant for
15 min once the compression force reached .about.3000 N,
corresponding to an average applied pressure of .about.3 MPa. The
final bonding temperature for the coupon/block/coupon assemblies
was held at .about.580.degree. C. After the constant force hold,
the linear actuator was withdrawn from the assembly and the system
was cooled down.
[0126] A photograph of a breakdown of a prototype instant water
heater assembly is shown in FIG. 24. There were two Cu coupons,
each containing one set of parallel rectangular microchannels
.about.15 mm long, .about.150 .mu.m wide, .about.400 .mu.m deep,
covering a total area of .about.15 mm.times.15 mm, together with a
custom-made Cu heater block with overall dimensions of 43
mm.times.43 mm.times.15 mm. On the top heater block surface, two
plena were machined to connect to the two ends of the microchannel
array on the Cu coupon. The distance between the two plena was
.about.12.8 mm. The bottom heater block surface had an identical
configuration (not visible in FIG. 24). A hole in each plenum,
.about.7.5 mm diameter (labeled "A" in FIG. 24), was machined
through the entire heater block to connect the two plena on the top
and bottom heater block surfaces. An additional hole on each side
of the heater block was machined perpendicular to hole A (labeled B
in FIG. 24), for external water connections. Four parallel holes
(labeled C in FIG. 24), each with a diameter .about.6.4 mm and a
length .about.41 mm, were drilled parallel to hole B, and were used
to house four cylindrical cartridge, 180 W, electrical-resistance
heaters.
[0127] A testing apparatus was designed and built to evaluate the
heat transfer characteristics of assembled Cu instant water heater
prototype. The apparatus comprised three principal sections: water
supply section, test section, and data acquisition section. The
water supply section comprised a pressure-regulated water storage
tank, which supplied water to the specimen at a constant pressure
for a smooth and stable flow through the microchannels at low flow
rates. A valve downstream of the tank exit was used for fine
adjustments to the flow rate. An Instrunet data acquisition system
interfaced to a PC was used to collect thermocouple readings.
[0128] Water flow through the assembled Cu instant water heater
prototype, and heat transfer from the cartridge heaters to water
were measured. The total pressure drop across the inlet and exit
fluid connections was measured with a Dywer digital manometer with
a minimum reading of 690 Pa (.about.0.1 psi). The rates of water
flow through the microchannel arrays in the prototype were measured
as a function of the associated pressure drop. The flow rate
increased monotonically with increasing pressure drop across the
prototype, and reached 1.5 liter/min at a pressure drop of
.about.0.48 MPa (.about.70 psi).
[0129] Thermocouples were inserted into both the inlet and outlet
tubes with T-fittings, and were sealed with epoxy cement.
Additional thermocouples were placed on the top and bottom surfaces
of the prototype, as well as on the side surface closest to the
heaters. Because none of the thermocouples were placed within the
microchannels, whether the water flow within the microchannels is
laminar or turbulent should not affect the temperature
measurements. The entire assembly was then encased within PVC
insulation, with holes drilled into the PVC to allow for the fluid
inlet, outlet, and pressure meter tube connections. FIG. 25 depicts
the (outlet-inlet) difference in water temperature as a function of
water flow rate through the prototype, measured at two different
heater input powers. At the higher input power of .about.710 W, the
water temperature increased >7.degree. C. at a flow rate of
.about.1.4 liter/min and >25.degree. C. at a flow rate of
.about.0.4 liter/min. The efficiency of heat transfer, defined as
the ratio of power gained by water to total heater input power, is
shown in FIG. 26 as a function of the water flow rate. The
efficiency was very high in all cases, ranging from just under 98%,
to a maximum of 99.7% at higher flow rates. At higher flow rates,
the lower prototype body temperature resulted in lower heat loss to
the PVC insulation and therefore higher overall heat transfer
efficiency.
Miscellaneous
[0130] Those of skill in the art will recognize that various
modifications may be made to the embodiments described above, while
staying within the scope of the present inventions. Among the
possible modifications and alternative embodiments are those
described above and below.
[0131] A metal-based microchannel heat exchange device may be
formed of a variety of metals, including aluminum, aluminum-based
alloys, copper, copper-based alloys, nickel, or nickel-based
alloys, such as nickel-titanium alloys.
[0132] The microchannel arrays may be straight, curved, or
profiled, in any or all dimensions.
[0133] The microchannel arrays may be formed from multiple metal
sheets, with multiple bonding interfaces, or multiple connections
from microchannel arrays to fluid inlet and outlet plena. The
microchannels preferably have elevated surface roughness to improve
heat exchange, for example in the range about 1
.mu.m.ltoreq.Rz.ltoreq.20 .mu.m, preferably the range about 3
.mu.m.ltoreq.Rz.ltoreq.15 .mu.m, most preferably about 10 .mu.m.
The insert surface roughness may be controlled, for example, by
altering electrochemical polishing conditions.
[0134] The microchannel arrays are preferably made by microscale
compression molding using a microscale mold insert made of a
refractory metal or alloy. Among the materials that may be used for
the refractory mold insert are the following: Ta, W, Mo, Nb; their
respective binary, ternary, and quaternary alloys, with or without
metalloid element additions such as C, B, Si; transition metals and
alloys, such as Hf, Zr, Ti, V, Cr; their respective binary,
ternary, and quaternary alloys, with or without metalloid element
additions such as C, B, Si; all classes of Fe-based tool steels,
including M-series, T-series, and H-series tool steels; a Ni-based
alloy or superalloy, for example one of the Inconel series of Ni
alloys; or a refractory ceramic, including a metal carbide such as
TaC, WC, MoC, TiC, NbC, pseudo-binary alloys of metal carbides,
such as TaC--WC, or a metal nitride such as TaN, WN, MoN, TiN, NbN,
or pseudo-binary alloys of metal nitrides, such as TaN--WN; or
diamond. The inserts may be made, for example, by electrical
discharge machining, or by micro-electrical discharge machining
using lithographically patterned electrodes. The inserts may
optionally be conformally coated with a suitable ceramic, carbon,
or hydrocarbon coating.
[0135] The bonding interfaces may occur between metal sheets
containing microchannel arrays, or between metal sheets containing
microchannel arrays and solid plates, or between metal sheets
containing microchannel arrays and perforated plates. The thin film
intermediate bonding layer may itself have a eutectic or
near-eutectic composition; or it may be a eutectic precursor that
will form a eutectic or near-eutectic composition when heated in
contact with the adjacent metal piece(s). The intermediate bonding
layer may, for example, comprise a free-standing thin film
(eutectic precursors), e.g., Cu, Zn, Al, Mg, Sn, Ga, In, or Ni; or
it may comprise a eutectic or near-eutectic nanocomposite layer,
e.g., Al--Ge, Al--Si, Al--Mg, Al--Sn, Al--Ga, Au--Si, Cu--Al,
Al--Zn, Sn--In, Cu--In, Au--In, Ag--In, Ag--Sn, Cr--Sn, Cu--Sn,
Au--Sn, and binary, ternary, or quaternary mixtures of any of the
above. The individual domains within the eutectic bonding layer are
preferably primarily in the range from about 100 nm or smaller to
about 400 nm. Alternatively, a free standing thin metal film may be
used as a eutectic precursor, including for example Al, Sn, Zn, Cu,
Ni, or their alloys. The individual bonding layer thickness is
preferably from about 0.3 to about 10 .mu.m. The thin film eutectic
or near-eutectic bonding layer may, for example be fabricated by
direct physical or chemical vapor phase deposition onto the pieces
to be bonded; or a free-standing thin film may be fabricated by
metallurgical means and inserted between the pieces to be bonded;
or other techniques otherwise known in the art may be used, such as
sputter deposition or co-deposition, co-evaporation, or e-beam
co-evaporation. The pieces are heated to an appropriate temperature
to ensure proper quality of bonding, without excessive deformation,
by heating to a temperature at or just above the eutectic point;
for example from about 450.degree. C. to about 550.degree. C. in
the case of aluminum or aluminum-containing alloys, or from about
500.degree. C. to about 600.degree. C. in the case of copper or
copper-containing alloys. Preferably pressure is applied during the
heating step to promote bonding between the thin film layer and the
metal workpiece, for example, from about 0.5 to about 5 MPa, or
higher.
[0136] Among the materials that may be used for the low-melting
metal to form the heat exchanger are the following: Aluminum and
aluminum-based alloys, such as the 1 xxx series of Al alloys, 2xxx
series of Al alloys, 3xxx series of Al alloys, 5xxx series of Al
alloys, 6xxx series of Al alloys, and 7xxx series of Al alloys;
Copper and copper-based alloys, e.g. the C1xxxx series of Cu
alloys, C2xxxx series of Cu alloys, C5xxxx series of Cu alloys, and
C7xxxx series of Cu alloys; Nickel and nickel-based alloys, e.g.
Ni--Ti alloys, Ni--Cu alloys, Ni--Al alloys; Zinc and zinc-based
alloys; and Magnesium and magnesium-based alloys.
[0137] Preferred ranges for various dimensions and other numerical
values in novel heat exchangers in accordance with the present
invention are: (a) Microchannel length: from about 100 .mu.m
upwards (no upper limit in length; for example, a meandering
channel could be several meters in total length). (b) Microchannel
width: from about 30 .mu.m minimum to about 2000 .mu.m, preferably
from about 30 .mu.m to about 1000 .mu.m. (c) Microchannel depth:
from about 30 .mu.m to about 2000 .mu.m, preferably from about 30
.mu.m to about 1000 .mu.m. (d) Microchannel cross-sectional aspect
ratio: from about 0.03 to about 35. (e) Number of microchannels in
heat exchanger: from 1 (e.g., one long meandering channel) to 1000
or 10,000 (e.g., a total device width of about one meter). (f)
Overall dimensions of metal-based heat exchangers: from about 5
mm.times.5 mm.times.1.5 mm (e.g., a one-microchannel-layer device
for cooling a single "hot-spot"); to about 1000 mm.times.100
mm.times.100 mm (e.g., a multiple-microchannel-layer device).
[0138] Definitions. The "length" of a microchannel is defined as
its total distance measured along the direction in which fluid will
generally tend to flow through the microchannel, measured along the
path of that fluid flow. The "width" and "depth" of a microchannel
are distances measured perpendicular (or approximately
perpendicular) both to each other and to the length of the
microchannel. There is no preferred direction defined as "width,"
nor as "depth," but each may be taken in a convenient direction,
consistent with the preceding definitions. The use of the terms
"length," "width," and "depth" should not be construed to imply
that the microchannel must assume any particular shape. As a few of
many possible examples, a cross-section of a microchannel may be
square, circular, rectangular, or elliptical; and the microchannel
itself may be straight, curved, spiral, sinusoidal, serpentine, a
racetrack, etc. A "homogeneous" metal layer or metal component is
one that is essentially uniform throughout, except perhaps at a
surface or boundary where it may be brazed or joined to another
layer or component. More specifically, a component that contains
separate, multiple, internal layers of metal interspersed (or
brazed together) with separate, multiple, internal eutectic layers
is not considered to be "homogeneous" within the scope of this
definition. Different "homogenous" components in the same device
need not necessarily have the same composition, although in many
cases it will be preferred that their compositions should be the
same. A microchannel is considered to be "enclosed entirely" by
specified components (such as by two homogeneous metal layers and a
eutectic layer) if it is enclosed and bounded by the specified
components--and by no other components--along essentially the
entire length of the microchannel; with possible exceptions at (and
only at) the microchannel's fluid inlet(s) and fluid outlet(s). At
the inlet(s) and outlet(s), the microchannel may optionally be open
rather than closed; and at the inlet(s) and outlet(s) the
microchannel may optionally connect to or be bounded by other
component(s).
[0139] The complete disclosures of all references cited in this
specification are hereby incorporated by reference; including, by
way of example and not limitation, the entire disclosure of
priority U.S. provisional application 61/020,789, filed 14 Jan.
2008. In the event of an otherwise irreconcilable conflict,
however, the present specification shall control.
* * * * *