U.S. patent application number 12/063226 was filed with the patent office on 2009-03-05 for nanostructured micro heat pipes.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Rohit Karnik, Woochul Kim, Arun Majumdar.
Application Number | 20090056917 12/063226 |
Document ID | / |
Family ID | 37728028 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090056917 |
Kind Code |
A1 |
Majumdar; Arun ; et
al. |
March 5, 2009 |
NANOSTRUCTURED MICRO HEAT PIPES
Abstract
A heat pipe comprising a chamber; a wick in the chamber, and a
heat sink, which is adjacent to a first portion of the wick. A heat
source adjacent to a second portion of the wick, wherein the wick
is configured such that a gas condenses at the first portion of the
wick and a liquid evaporates at the second portion of the wick. The
fluid moves from the first portion of the wick to the second
portion of the wick, and wherein the wick comprises nanostructures
having a differentially-spaced apart gradient along the length of
the wick so as to promote capillary fluid flow therealong.
Inventors: |
Majumdar; Arun; (Orinda,
CA) ; Karnik; Rohit; (Cambridge, MA) ; Kim;
Woochul; (Seoul, KR) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
The Regents of the University of
California
Oakland
CA
|
Family ID: |
37728028 |
Appl. No.: |
12/063226 |
Filed: |
August 9, 2006 |
PCT Filed: |
August 9, 2006 |
PCT NO: |
PCT/US06/31196 |
371 Date: |
September 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60706578 |
Aug 9, 2005 |
|
|
|
Current U.S.
Class: |
165/104.26 ;
165/133 |
Current CPC
Class: |
H01L 2924/00 20130101;
F28D 2015/0225 20130101; H01L 23/427 20130101; F28D 15/046
20130101; H01L 2924/0002 20130101; H01L 2924/0002 20130101; F28D
15/0233 20130101 |
Class at
Publication: |
165/104.26 ;
165/133 |
International
Class: |
F28F 13/02 20060101
F28F013/02; F28F 13/18 20060101 F28F013/18; F28D 15/02 20060101
F28D015/02 |
Goverment Interests
GOVERNMENT INTEREST
[0002] The present invention was made with Government support under
Grant (Contract) No. R21 CA103071 awarded by the National
Institutes of Health/National Cancer Institute. The United States
Government has certain rights to this invention.
Claims
1. A heat pipe, comprising: a chamber; a wick in the chamber, a
heat sink adjacent to a first portion of the wick; and a heat
source adjacent to a second portion of the wick, wherein the wick
is configured such that a gas condenses at the first portion of the
wick and a liquid evaporates at the second portion of the wick,
wherein fluid moves from the first portion of the wick to the
second portion of the wick, and wherein the wick comprises
nanostructures having a differentially-spaced apart gradient along
the length of the wick so as to promote capillary fluid flow
therealong.
2. The heat pipe of claim 1, further comprising: a substance in the
chamber, the substance changing from gas phase to liquid phase at
the first portion of the wick and from liquid phase to gas phase at
the second portion of the wick.
3. The heat pipe of claim 1, wherein the nanostructures comprise:
an array of nanowires, nanopores, nanotubes, or any nanoscale
protrusions.
4. The heat pipe of claim 1, wherein the nanostructures are formed
by one or a combination of the following processes: directly
depositing a nanoporous film on a substrate; depositing a solid
film on a substrate and then electrochemically etching the solid
film forming arrays of nano-pores; electrochemically growing
nanowires or nanotubes in the nano-pores; annealing the nanowires
to form polycrystalline wires; etching the film away leaving a
nanowire array; and/or direct deposition of nanowires on a
substrate.
5. The heat pipe of claim 4, wherein the nanostructure size and
density are determined by the manufacturing process.
6. The heat pipe of claim 1, wherein the nanostructures are spaced
between 20 nm to 500 nm apart from one another.
7. The heat pump of claim 1, wherein the nanostructures are spaced
farther apart at the first portion of the wick and closer together
at the second portion of the wick.
8. A heat dissipation system, comprising: a chamber; a heat sink; a
heat source; and a nanostructure array extending from the heat
source.
9. The heat dissipation system of claim 8, further comprising: a
plurality of channels in the heat sink.
10. The heat pipe of claim 1, wherein the wick has a capillary
pressure between 0.01 MPa to 100 MPa.
11. A nanostructured composite wick comprising: a channel; and a
plurality of nanostructures, wherein the nanostructures have a
differentially-spaced apart gradient along the length of the
channel so as to promote capillary fluid flow therealong.
12. The wick of claim 11, wherein the nanostructure size and
density are determined by the manufacturing process.
13. The wick of claim 11, wherein the nanostructures are spaced
between 20 nm to 500 nm apart from one another.
14. The wick of claim 11, further comprising a heat source adjacent
to a second portion of the wick, wherein the wick is configured
such that a gas condenses at the first portion of the wick and a
liquid evaporates at the second portion of the wick, wherein fluid
moves from the first portion of the wick to the second portion of
the wick, and wherein the nanostructures are spaced farther apart
at a first portion of the wick and closer together at a second
portion of the wick.
15. The heat pipe of claim 14, further comprising: a substance in
the chamber, the substance changing from gas phase to liquid phase
at the first portion of the wick and from liquid phase to gas phase
at the second portion of the wick.
16. The heat pipe of claim 11, wherein the nanostructures comprise:
an array of nanowires, nanopores, nanotubes, or any nanoscale
protrusions.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a 371 application of International Application No.
PCT/US2006/031196 filed Aug. 9, 2006, which claims priority to U.S.
Provisional Application Ser. No. 60/706,578, filed Aug. 9, 2005,
all of which are incorporated herein by reference.
FIELD OF INVENTION
[0003] This invention relates to a nanostructured micro heat pipe,
and more particularly to a method and system for designing and
fabricating a nanostructured wick that is made using low-cost
manufacturing techniques in the field of heat pipe thermal
manufacturing.
BACKGROUND
[0004] Thermal management is one of the critical issues in
packaging of modern microelectronic processors. The increasing
integration of logic and memory onto a single processor poses two
challenges, namely: (1) the total power dissipation from a single
processor is about 100 W, which produces an average heat flux of
about 100 W/cm.sup.2; and (2) the peak power densities can increase
up to 500-1000 W/cm.sup.2 in the future. To dissipate this power
and power density with a resistance at less than 0.5 K/W requires
innovative solutions. The use of large heat sinks are precluded in
many applications that rely on small volume and footprints for
packaging, especially in devices having computer processors, and
portable devices having microprocessors, such as personal digital
assistants (usually abbreviated to PDAs), or other suitable
handheld devices, and cell phones. In addition, for such
applications, the demand performance-cost ratio of a thermal
management solution is steadily increasing.
[0005] It has also becoming increasingly clear that single phase
gas convective heat transfer is unlikely to be an adequate solution
for such higher heat fluxes, especially when the requirement is for
small volume solutions. In addition, single-phase liquid
microchannel cooling is a potential solution for the average heat
flux of 100 W/cm2, but cannot address the peak fluxes of 500-1000
W/cm2. Furthermore, pumping remains a major bottleneck for
reliability of microchannel cooling.
[0006] Alternatively, the latent heat of vaporization makes phase
heat transfer an ideal choice for dissipating such high fluxes.
However, two-phase convective cooling in microchannels in fraught
with difficulties due to vapor-liquid instabilities. The only
likely candidate that utilizes the latent heat of vaporization and
requires no external power is the heat pipe. It is, therefore, not
surprising that heat pipes have found use in most laptop thermal
management.
SUMMARY
[0007] In accordance with one embodiment, a heat pipe, comprises: a
chamber; a wick in the chamber, a heat sink adjacent to a first
portion of the wick; and a heat source adjacent to a second portion
of the wick, wherein the wick is configured such that a gas
condenses at the first portion of the wick and a liquid evaporates
at the second portion of the wick, wherein fluid moves from the
first portion of the wick to the second portion of the wick, and
wherein the wick comprises nanostructures having a
differentially-spaced apart gradient along the length of the wick
so as to promote capillary fluid flow therealong.
[0008] In accordance with another embodiment, a heat dissipation
system comprises: a chamber; a heat sink; a heat source; and a
nanostructure array extending from the heat source.
[0009] In accordance with a further embodiment, a nanostructured
composite wick comprises: a channel; and a plurality of
nanostructures, wherein the nanostructures have a
differentially-spaced apart gradient along the length of the
channel so as to promote capillary fluid flow therealong.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a schematic diagram of a heat pipe in
accordance with one embodiment.
[0011] FIG. 2 shows a schematic diagram of a thermal resistance
network for the heat pipe of FIG. 1.
[0012] FIGS. 3A-3D show a schematic diagram illustrating synthesis
of Cu (or other types of) nanowire arrays with controlled density
and wire diameter.
[0013] FIGS. 4A and 4B show a schematic diagram illustrating
synthesis of Cu (or other types of) nanowire arrays with controlled
density and wire diameter as shown in FIG. 3
[0014] FIG. 5 shows a fabrication scheme for a heat pipe using
nanostructures and microchannels in accordance with one
embodiment.
[0015] FIG. 6 shows a fabrication scheme for a heat pipe using
nanostructures and microchannels of FIG. 5.
[0016] FIG. 7 shows a fabrication scheme for a heat pipe using
nanostructures and microchannels of FIGS. 5 and 6.
[0017] FIG. 8 shows a fabrication scheme for a heat pipe using
nanostructures and microchannels.
[0018] FIG. 9 shows a schematic diagram of another embodiment of a
heat pipe in the form of a hotspot adaptive thermal spreader.
[0019] FIG. 10 shows a schematic diagram of a nanostructured
composite wick and a microchannel having a plurality of
nanostructures.
[0020] FIG. 11 shows a cross sectional view of a nanostructured
composite wick having a microchannel with a plurality of
nanostructures.
[0021] FIG. 12 shows a schematic diagram of thermal resistance
diagram of a hotspot adaptive thermal spreader.
[0022] FIG. 13 shows a cross sectional view of the glancing angle
deposition (GLAD) technique for fabrication of nanowires on a
substrate.
DETAILED DESCRIPTION
[0023] FIG. 1 shows a schematic diagram of a heat pipe 10 in
accordance with one embodiment using a nanostructured wick 30. The
heat pipe 10 includes a chamber 20; a wick 30 in the chamber 20, a
heat sink 40 adjacent to a first portion 32 of the wick 30, and a
heat source 50 adjacent to a second portion 34 of the wick 30. The
wick 30 is configured such that a vapor or gas 36 condenses at the
first portion 32 of the wick 30 and a liquid or fluid 38 evaporates
at the second portion 34 of the wick 30, wherein the liquid or
fluid 38 moves from the first portion 32 of the wick 30 to the
second portion 34 of the wick 30, and wherein the wick 30 comprises
nanostructures 70 (FIG. 6) having a differentially-spaced apart
gradient along the length of the wick 30 so as to promote capillary
fluid flow therealong. It can be appreciated that the chamber 20
includes a substance 60 in the chamber 20, the substance 60
changing from gas phase 36 to liquid phase 38 at the first portion
32 of the wick 30 and from liquid phase 38 to gas phase 36 at the
second portion 34 of the wick 30.
[0024] FIG. 2 shows a schematic diagram of a thermal resistance
network for the heat pipe 10 of FIG. 1. As shown in FIG. 2, the
thermal resistance network comprises a heat source 50
(processor)--heat pipe 10 interface (R1), a wick 30 (R2), a vapor
36 (R3), the wick 30 (R4), the heat pipe 10--heat sink 40 interface
(R5), and the axial connection (R6). It can be appreciated that the
power dissipated is equal to Q=mh.sub.fg, where m is the mass flow
rate and h.sub.fg is the latent heat of vaporization. It can also
be appreciated that the mass flow rate will typically depend on two
factors: (1) the pressure drop in the vapor flow 36; and (2) the
capillary pressure gradient in the wick 30 that drives the liquid
flow 38. However, the capillary pressure gradient in the wick 30
that drives the liquid flow 38 is often the main bottleneck in
improving the performance of a heat pipe 10. Hence, the design of
the wick 30 can be critical for determination of the mass flow
rate. The design of the wick 30 also controls the thermal
resistances (R2 and R4) in the resistance network. Thus, the lower
resistance is clearly preferable.
[0025] Typically, heat pipes 10 use a porous wick 30 made by
sintering copper (Cu) particles, wherein, the pore sizes, (r) are
generally 10-20 .mu.m in size. In addition, assuming water is the
working substance or fluid 60, and given that the surface tension
of water is 0.07 N/m, the capillary pressure that such pore sizes
(r) can generate is about Pc=2.sigma./r=14 kPa. However, the
difference in capillary pressure between the wick region and the
condensing region drives the flow. Therefore, one would grade the
pore sizes in a heat pipe 10, i.e., smallest pores near the
evaporator (heat source 50) and the largest pores near the
condenser (heat sink 40). However, most heat pipes 10 usually do
not employ graded pore sizes along the length of the heat pipes 10,
although companies, such as Thermacore, have proposed such
designs.
[0026] In addition, despite the use of copper (Cu), the sintered
porous heat pipes 10 pose significant thermal resistance due to the
presence of multiple interfaces in the heat flow path. It can also
be appreciated that a reduction of the thermal resistance due to
the presence of multiple interfaces can play a significant role in
the overall thermal performance.
[0027] FIGS. 3A-3D show the schematic diagram of a method and
process for the manufacturing of a nanostructure 70 in the form of
a nanowire 100. It can be appreciated that the heat pipe 10 having
a nanostructure 70 can be formed by one or a combination of the
following processes: (i) grow nanowires on a porous template, e.g.,
electrochemical deposition of nanowires in alumina template; or
(ii) direct deposition of nanowires on a substrate, e.g., GLAD
technique (FIG. 13).
[0028] FIGS. 3A-3D show a method of growing nanowires on a porous
template, which can be formed by one or a combination of the
following processes directly depositing a nanoporous film on a
substrate; depositing a solid film on a substrate and then
electrochemically etching the solid film forming arrays of
nano-pores 92; and/or electrochemically growing nanowires or
nanotubes in the nano-pores; annealing the nanowires 100 to form
polycrystalline wires; and etching the film away leaving a nanowire
array 110.
[0029] As shown in FIG. 3A, an aluminum (Al) film 80 of 1-10 mm in
thickness 82 is deposited onto a silicon (Si) or copper (Cu)
substrate 84.
[0030] FIGS. 3B and 4A show a highly ordered hexagonal array 90 of
alumina nanopores 92, which are obtained by anodic oxidation of the
Al film 70. It can be appreciated that the pore size and
periodicity can be controlled by the electrolyte used in the
anodization process. As shown in FIGS. 3B and 4A, the hexagonal
array 90 of nanopores 92 shows a high degree of ordering over area
of approximately 1 cm.sup.2.
[0031] FIGS. 3C and 4B show that using the porous alumina 80 as a
template, copper (Cu) nanowires 100 can be electrochemically grown
in the nanopores 92 and then annealed to form a polycrystalline
wire.
[0032] As shown in FIG. 3D, the alumina 80 can then be etched away,
leaving the nanowires array 110. It can be appreciated that the
method as described, the nanostructures 70 can be an array of
nanowires 100, nanopores 92, nanotubes, or any nanoscale
protrusions.
[0033] It can be appreciated that the copper (Cu) nanowires 100
serve two purposes, namely: (1) as fins for efficient heat
conduction with low thermal resistance between the heat pipe
surface to liquid, and (2) for creating high capillary pressure
gradients for increased mass flow. In addition, the capillary
pressure difference can be generated by modulating the
inter-nanowire 100 spacing along the length of the heat pipe 10,
and by controlling the anodization conditions. For example,
nanowire 100 spacing of 20-500 nm can be designed, and assuming
water as the working fluid, the corresponding capillary pressure
can range from about 0.1 to 1 Mpa. It can be appreciated that the
method as described above can produce a low-cost manufacturing
process, which allows one to create a capillary pressure gradient
from 0.1 to 1 Mpa over a 1 cm length, which is several orders of
magnitude higher than what is currently used.
[0034] Alternatively, in another embodiment, microchannels 120 can
be fabricated using a transport liquid 130. FIGS. 5-7 show a
fabrication process in which the region to be cooled has copper
(Cu) nanowires 100, and wherein the heat sink 40 has microchannels
120. As shown in FIG. 5, a silicon wafer 130 is etched forming a
plurality of partial chambers 20. As shown in FIG. 6, fabricated
nanostructures 70 are then added to the chambers 20, which is then
filled and bonded with a silicon wafer 130 with microchannels 120
as shown in FIG. 7. It can be appreciated that since the mass flow
rate is proportional to D.sup.4 for pressure-driven flow, where D
is pore/channel size, a plurality of microchannels 120 will reduce
the pressure drop. At the same time, the plurality of nanowires 70
will increase capillary pressure difference, considerably
increasing the mass flow rate.
[0035] FIG. 8 shows a heat source 50 with a plurality of
nanostructures 100. As shown in FIG. 8, the heat pipe 10 includes a
plurality of microchannels 120 with a capillary pressure gradient.
It can be appreciated that by incorporating nanostructures 70 in
the form of nanowires 100, nanopores 92, nanotubes, or any
nanoscale protrusions can enable the heat pipe 10 to be
miniaturized, offering the promising prospects of high performance
thermal management at low cost for portable and handheld
devices.
[0036] FIG. 9 shows another embodiment of a heat pipe 10 in the
form of a hotspot adaptive thermal spreader (HATS) 200, which is
designed to remove hotspots and make a compact heat sink. As shown
in FIG. 9, the thermal spreader 200 includes a nanostructured vapor
chamber 210, which is comprised of a plurality of sub-vapor
chambers 220, and a hotspot or heat source 230, such as a
microprocessor. Each of the sub-vapor chambers 220 includes a
plurality of channels or microchannels 240, which are configured to
converging mass flow to the hotspot 230. The microchannels 240 are
preferably comprised of a nanostructured composite wick 30 as shown
in FIG. 11. It can be appreciated that by providing nanostructures
70 within the microchannels 240, the capillary pressure scales as
approximately 1/r, which increases the capillary limit of the heat
pipe 10. In addition to increasing capillary pressures, the
embedded nanostructures 70, cause the mass flow rate to be
predominantly determined by the microchannel 240 size.
[0037] FIG. 10 shows a coordinate for the analysis on a
nanostructured composite wick 30 (FIG. 11). As shown in FIG. 10,
the hotspot 250 is located in the center of the coordinate, where L
is a length of the channel 240 and/is the radius of the hotspot
230. In accordance with one embodiment, it can be appreciated that
the nanostructured composite wick 30 can be only on the channel 240
and preferably includes embed nanowires 100 (FIG. 3C), which are
located only on the hotspot area 250. It can be appreciated that
the wick 30 is preferably a composite wick since it is a
combination of a channel or microchannel 240 with an angle of
approximately 2.alpha., and nanowire arrays 110 on the channel 240
approximated as a `small-angle-channel` with angle of 2.beta.. For
a steady viscous flow in a convergent channel, i.e. Jeffrey-Hamel
flow, the fluid velocity is a function of r only.
[0038] It can be appreciated that to enhance the performance of the
heat pipe 10, an increase in mass flow is critical since the heat
flow rate is related to the mass flow as {dot over (Q)}={dot over
(m)}.sub.maxh.sub.fg. Considering limitations due to wicking, the
pressure drop due to a capillary force can be expressed is:
.DELTA. P capillary = .DELTA. P gravity neglect + .DELTA. P liguid
+ .DELTA. P vapor neglect ##EQU00001##
[0039] wherein an average pressure drop in the big channel based on
the Jeffrey-Hamel flow is,
p .alpha. z = 2 .mu. m .alpha. .rho. .alpha. L 3 H ( tan ( 2
.alpha. ) / 2 .alpha. tan ( 2 .alpha. ) 2 .alpha. - 1 ) = 2 .mu. m
.alpha. C .alpha. .rho. .alpha. L 3 H ##EQU00002##
where .rho., .mu. are density and viscosity respectively. H is
height of the channel. Pressure drop in the nanowire arrays 110,
i.e. the small-angle-channel, is,
p .beta. z = 2 .mu. m .beta. C .beta. .rho. .beta. L 3 h
##EQU00003##
where h is height of the nanowires 100. Assuming the pressure drop
in the channel is the same as the one in the nanowire arrays
110,
p .beta. z = p .alpha. z ##EQU00004## m .beta. m .alpha. = C
.alpha. C .beta. .beta. h .alpha. H ##EQU00004.2##
[0040] Let .PHI. be a filling factor and n be the number of
small-angle-channels, i.e. nanowire arrays 110,
.alpha..phi.=.beta.n
[0041] Total mass flow rate in the nanostructured composite wick,
m.sub.c, is
m c = m .alpha. + n m .beta. = m .beta. ( .alpha. .phi. .beta. + m
.alpha. m .beta. ) ##EQU00005##
[0042] Now, the capillary limitation, can be written as
2 .sigma. l .beta. = .mu. C .beta. .rho. .beta. h m c ( .alpha.
.phi. .beta. + m .alpha. m .beta. ) [ 1 l 2 - 1 ( l + L ) 2 ]
##EQU00006## m c .cndot. ( .alpha. .phi. .beta. + m .alpha. m
.beta. ) h C .beta. ##EQU00006.2##
where .sigma. is surface tension. The capillary limitation of a
homogenous channel can be written as,
2 .sigma. l .alpha. = .mu. C .alpha. .rho. .alpha. ( h + H ) m
.alpha. ' [ 1 l 2 - 1 ( l + L ) 2 ] ##EQU00007## m .alpha. '
.cndot. ( h + H ) C .alpha. ##EQU00007.2##
[0043] Finally, ratio between mass flow of the nanostructured
composite wick 30 and that of the homogenous channel is:
m c m .alpha. ' = ( .alpha. .phi. .beta. + m .alpha. m .beta. ) h (
h + H ) C .alpha. C .beta. = .alpha. .phi. .beta. h ( h + H ) C
.alpha. C .beta. > 0 + H ( h + H ) .alpha. .beta. > 1 , if h
.cndot. H > 1 ##EQU00008##
[0044] As long as height of the nanowires 100 is much shorter than
that of the channel (h<<H), the mass flow of the
nanostructured composite wick 30 is always greater than that of the
homogeneous channel. In other words, height, H, and width, .alpha.,
of the channel 240 should be large for a small liquid pressure drop
and distance between nanowires 100 should be small (small .beta.)
for a large capillary force. For example, if the angle, .alpha., is
20 degrees and a mean spacing between nanowires 100, 2.beta.A is
approximately 200 nm, where A is around 0.5 mm (a radius of the
hotspot 230), then, .beta. is approximately 0.01 degrees, so the
resulting enhancement is around 200.
[0045] FIG. 11 shows a nanostructured composite wick 30 having a
channel or microchannel 240 with a plurality of nanostructures 70,
preferably in the form of a nanowire 100. As shown in FIG. 11, the
diameter of the nanowire 100 is a, and accordingly, it can be
appreciated that the surface roughness can enhance the
hydrophilicity of the hydrophilic surfaces. More specifically, a
contact angle, .theta., can be expressed as:
cos .theta. = { 1 + 4 ( h a ) [ 1 ( 2 .beta. A a + 1 ) 2 ] } cos
.theta. e ##EQU00009##
where .theta..sub.e is the equilibrium contact angle of the liquid
drop on a flat surface made of the surface material. To enhance the
contact angle, it can be appreciated that it is preferably that the
microchannel 240 includes:
[0046] 1. a large h/a
[0047] 2. a small 2.beta.A/a
[0048] In addition, as our previous calculation suggests:
[0049] 3. a small 2.beta.A is preferred for large capillary
force.
[0050] 4. a small h is preferential based on the equation for the
ratio between mass flow of the nanostructured composite wick 30 and
the that of the homogeneous channel 240.
[0051] In addition, it can be appreciated that in order to have a
large h/a with a small h, the diameter of the nanostructure 70 or
nanowire 100 in FIG. 11 should be small. At the same time, the
diameter of the nanostructure 70 or nanowire 100 should be larger
or at least comparable to the spacing between the nanostructure 70
or nanowires 100, 2.beta.A, to meet the requirements 2 and 3. It
can also be appreciated that silicon (Si) nanowires 100 can also be
grown using a Vapor-Liquid-Solid mechanism.
[0052] FIG. 12 shows a schematic diagram of thermal resistance of a
hotspot adaptive thermal spreader 200. The thermal spreader 200 is
comprised of a vapor chamber 210 for heat spreading and a plurality
of microchannels 240. As shown in FIG. 12, the thermal spreader 200
includes a silicon (Si) (or Cu or any other) substrate 260
(R.sub.1), a nanowire 100 with a liquid (R.sub.2), a chamber 210
(R.sub.3), and a silicon (or Cu or any other) substrate 260
(R.sub.4) having a plurality of microchannels 240 (R.sub.5). As set
forth below, each of the thermal resistances have a unit of
cm.sup.2K/W.
TABLE-US-00001 TABLE 1 R.sub.1 [cm.sup.2 K/W] Thickness [.mu.m] k
of Si [W/mK] R [cm.sup.2 K/W] 400 140 0.0286
[0053] Usually, the thickness of the silicon (Si) substrate 260 is
around 500 .mu.m, however, it can be appreciated that about 100
.mu.m is typically etched away for vapor flow.
TABLE-US-00002 TABLE 2 R.sub.2 [cm.sup.2 K/W] Liquid level Nanowire
Effective k k of water R [.mu.m] filling factor [W/mK] [W/mK]
[cm.sup.2 K/W] 20 0.3 42.42 0.6 0.0047
[0054] This assumes steady-state operation. The liquid level is
preferably thinner, which provides a small thermal resistance. At
t=0, the water level is expected to be higher than 20 .mu.m.
TABLE-US-00003 TABLE 3 R.sub.4 Thickness [.mu.m] R [cm.sup.2 K/W]
100 0.0071
[0055] R.sub.4 represents thickness of the Si layer. Here, we
assume that channel height of the microchannel is around 400 .mu.m.
So, the thickness of the Si layer is around 100 .mu.m.
TABLE-US-00004 TABLE 4 R.sub.5 h [W/m.sup.2K] R [cm.sup.2 K/W]
1587450.787 0.0063
[0056] This heat transfer coefficient is based on our calculation.
The thickness of channel 240 and wall is set to 10 .mu.m and 10
.mu.m respectively.
TABLE-US-00005 TABLE 5 Overall thermal resistance based on Si-HATS.
R [cm.sup.2 K/W] R.sub.1 0.0286 R.sub.2 0.0047 R.sub.4 0.0071
R.sub.5 0.0063 R.sub.total 0.0467
[0057] As shown in Table 5, the biggest resistance comes from the
silicon (Si) substrate 260. Therefore, it can be appreciated that
the silicon (Si) substrate 260 can be replaced with a copper (Cu)
plate or substrate. The overall thermal resistance calculation for
a copper plate or substrate and a hotspot adaptive thermal spreader
(HATS) is shown in Table 6.
TABLE-US-00006 TABLE 6 Overall thermal resistance based on Cu-HATS
R [cm.sup.2 K/W] R.sub.1 0.0100 R.sub.2 0.0017 R.sub.4 0.0071
R.sub.5 0.0063 R.sub.total 0.0251
[0058] As shown in Table 5 and Table 6, it can be appreciated that
a reduction in the thermal resistance can be achieved by replacing
the silicon (Si) HATS 200 with a copper (Cu) HATS 200. It can be
appreciated that in order to achieve a copper (Cu) HATS, it is
necessary to fabricate the growing copper (Cu) nanowires on a
copper (Cu) plate. In accordance with one embodiment, a glancing
angle deposition (GLAD) technique 300 as shown in FIG. 13 can be
used. Alternatively, an electrochemical deposition of copper (Cu)
on a porous alumina template as shown in FIGS. 3A-3D and 4A-4B can
be used.
[0059] FIG. 13 shows a cross sectional view of the glancing angle
deposition (GLAD) technique 300 for fabrication of nanowires 100 on
a substrate 260. As shown in FIG. 13, a substrate 260 is positioned
at an oblique angle relative to the incident vapor flux. If the
incident flux does not have enough mobility, the flux would stay on
the point where it sits on the substrate 260. Due to the oblique
angle, this leads to an effect called atomic shadowing. Therefore,
a columnar structure can be obtained in this way. It can be
appreciated that the substrate is preferable a silicon (Si)
substrate, for epitaxial growth of the copper (Cu) nanowire 100.
However, it can be appreciated that a single-crystal silicon
substrate 260 is not necessary, and any suitable substrate material
can be used, including fabricating of copper (Cu) nanowires on a
copper (Cu) plate or substrate 260.
[0060] While this invention has been described with reference to
the preferred embodiment described above, it will be appreciated
that the configuration of this invention can be varied and that the
scope of this invention is defined by the following claims.
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