U.S. patent application number 10/039444 was filed with the patent office on 2003-04-24 for thermal control layer in miniature lhp/cpl wicks.
Invention is credited to Khrustalev, Dmitry, Phillips, Alfred L., Zuo, Jon.
Application Number | 20030075306 10/039444 |
Document ID | / |
Family ID | 21905491 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030075306 |
Kind Code |
A1 |
Zuo, Jon ; et al. |
April 24, 2003 |
Thermal control layer in miniature LHP/CPL wicks
Abstract
A LHP/CPL wick includes an unconsolidated wick portion
positioned within a first consolidated wick portion and a second
consolidated wick portion. This wick provides a large .DELTA.T
across the wick, and sufficient capillary action for miniature
LHP/CPL applications. Furthermore, the unconsolidated wick portion
allows accommodation of mismatched coefficients of thermal
expansion. A LHP/CPL having this wick is particularly applicable to
silicon cooling devices.
Inventors: |
Zuo, Jon; (Lancaster,
PA) ; Khrustalev, Dmitry; (Lancaster, PA) ;
Phillips, Alfred L.; (Pine Grove, PA) |
Correspondence
Address: |
SAMUEL W. APICELLI
DUANE MORRIS LLP
305 NORTH FRONT STREET
P.O. BOX 1003
HARRISBURG
PA
17108-1003
US
|
Family ID: |
21905491 |
Appl. No.: |
10/039444 |
Filed: |
October 19, 2001 |
Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
F28D 15/043 20130101;
F28D 15/046 20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 015/00 |
Claims
What is claimed is:
1. A wick comprising an unconsolidated wick portion positioned
within a consolidated wick portion.
2. A wick in accordance with claim 1, wherein said consolidated
wick portion comprises at least one of a sintered powder, a screen,
and felt.
3. A wick in accordance with claim 1, wherein said consolidated
wick portion is a sintered powder wick comprising a material
selected from the group consisting of nickel, aluminum, silicon,
germanium, a polymer, ceramic, and stainless steel.
4. A wick in accordance with claim 1, wherein said unconsolidated
wick portion comprises at least one of powder and felt.
5. A wick in accordance with claim 1, wherein said unconsolidated
wick portion is a powder wick comprising a material selected from
the group consisting of metal, polymers, ceramics, plastics,
intrinsic semiconductors, and doped semiconductors.
6. A wick in accordance with claim 1 wherein said wick comprises
one of a capillary pumped loop wick and a looped heat pipe
wick.
7. A wick in accordance with claim 1, wherein said unconsolidated
wick portion is contained and constrained within said consolidated
wick portion.
8. A heat pipe comprising: an evaporator section; a condenser
section; a vapor channel interconnecting said evaporator section
and said condenser section in fluid communication; a liquid channel
interconnecting said evaporator section and said condenser section
in fluid communication; and a wick positioned between said liquid
channel and said vapor channel, said wick comprising an
unconsolidated wick portion positioned between a first consolidated
wick portion and a second consolidated wick portion, wherein said
first consolidated wick portion is positioned adjacent to said
vapor channel and said second consolidated wick portion is
positioned adjacent to said liquid channel.
9. A heat pipe in accordance with claim 8, wherein said
consolidated wick portion comprises at least one of a sintered
powder, a screen, and felt.
10. A heat pipe in accordance with claim 8, wherein said
consolidated wick portion is a sintered powder wick comprising a
material selected from the group consisting of metal, polymers,
ceramics, plastics, intrinsic semiconductors, and doped
semiconductors.
11. A heat pipe in accordance with claim 8, wherein said
unconsolidated wick portion comprises at least one of powder and
felt.
12. A heat pipe in accordance with claim 8, wherein said
unconsolidated wick portion is a powder wick comprising a material
selected from the group consisting of metal, polymers, ceramics,
plastics, intrinsic semiconductors, and doped semiconductors.
13. A heat pipe in accordance with claim 8 further comprising an
outer layer, wherein said evaporator section, said condenser
section, said vapor channel, said liquid channel, and said wick are
enclosed within said outer layer.
14. A heat pipe in accordance with claim 11, wherein said outer
layer is directly bonded to a device for cooling said device.
15. A heat pipe in accordance with claim 14, wherein said device
and said outer layer comprise the same material.
16. A heat pipe in accordance with claim 14, wherein said material
comprises at least one of silicon, germanium, a polymer, ceramic,
and metal.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to loop heat pipes and
capillary pumped loops, and specifically to wicks for looped heat
pipes and capillary pumped loops.
BACKGROUND
[0002] The use of heat pipes is well known in the art for cooling
various types of electronic devices and equipment, such as
integrated circuit chips and components. A basic heat pipe
comprises a closed or sealed envelope, or a chamber, containing a
capillary material, such as a liquid-transporting wick, and a
working fluid capable of having both a liquid phase and a vapor
phase within a desired range of operating temperatures. When heat
is applied from a heat source to one end of the heat pipe, the heat
transfer fluid is caused to evaporate from the capillary material
to absorb the latent heat of vaporization. The vapor is moved
toward the other (cooled) end of the heat pipe to condense therein
for the heat of condensation to be transferred to an outer heat
sink through heat conduction. The condensed heat transfer fluid is
absorbed by the capillary material to be moved back by virtue of a
capillary pressure head toward the evaporation zone thereby
completing the working cycle of the heat pipe.
[0003] Because conventional heat pipes transport liquid through the
capillary wick, they incur a large flow pressure drop if they are
made very long. Also, because liquid and vapor flow in opposite
directions, vapor can entrain liquid at high power rates and limit
the operation of the device; this is commonly known as the flooding
limit. To overcome these limitations and transport high thermal
power over long distances, the Loop Heat Pipe (LHP) and Capillary
Pumped Loop (CPL) were developed. (See notably U.S. Pat. No.
4,515,209.)
[0004] In conventional heat pipes, heat typically enters the heat
pipe from the liquid (i.e., convex) side of the meniscus. As is
known in the art, the meniscus is the curved shape of the surface
of a liquid in a container, caused by the cohesive effects of
surface tension (capillary action). Alternatively, in capillary
pumped two phase loop heat pipes, such as loop heat pipes (LHPs)
and capillary pumped loops (CPLs), heat enters the device (e.g.,
LHP, CPL, etc.) from the vapor (i.e., concave) side of the
meniscus. This is known as an inverted meniscus arrangement.
[0005] Because of the `inverted meniscus` arrangement, devices such
as LHPs and CPLs have relatively high thermal resistance in the
evaporator area, and are typically not capable of operating at high
heat fluxes without drying out. Conventional LHPs/CPLs typically
dissipate approximately only 10 W/cm.sup.2. One technique to
improve heat flux is a method for filling the vapor spaces of the
evaporator portion of the LHP/CPL with bidispersed wick in order to
achieve higher heat dissipation figures. LHPs/CPLs with bidispersed
wicks have achieved approximately 100 W/cm.sup.2 of heat
dissipation, however, at the expense of constricting vapor flow and
maximum power capacity, as well as introducing considerable
complexity and cost.
[0006] The nature of a two-phase loop requires that the temperature
difference (often referred to as `delta T` or `.DELTA.T`) from the
vapor to the liquid side of the wick correspond to the capillary
pressure being produced by the wick. If the .DELTA.T is
insufficient, then boiling will occur on the liquid side of the
wick and the loop will deprime (i.e., stop operating). This
.DELTA.T relationship becomes increasingly more difficult to
maintain as the wick dimensions are made smaller, such as in
applications with semiconductor circuits/chips and silicon
compatible LHP/CPLs. Miniature evaporators for two-phase loops are
thus very difficult to design and build. Furthermore, sub-miniature
evaporators of a size that would permit integration with a
semiconductor chip have so far not been feasible.
[0007] Therefore, there is currently a need for means for
maintaining a suitable .DELTA.T across wicks scaled to permit
integration with semiconductor chips.
SUMMARY OF THE INVENTION
[0008] A wick includes an unconsolidated wick portion positioned
between a first consolidated wick portion and a second consolidated
wick portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The above and other advantages and features of the present
invention will be better understood from the following detailed
description of the preferred embodiments of the invention, which is
provided in connection with the accompanying drawings. The various
features of the drawings may not be to scale. Included in the
drawing are the following figures:
[0010] FIG. 1 is a cross sectional view of a LHP/CPL having an
examplary embodiment of a wick in accordance with the present
invention.
DETAILED DESCRIPTION
[0011] This description of preferred embodiments is intended to be
read in connection with the accompanying drawing(s), which are to
be considered part of the entire written description of this
invention. In the description, relative terms such as "horizontal,"
"vertical," "up," "down," "top" and "bottom" as well as derivatives
thereof (e.g., "horizontally," "downwardly," "upwardly," etc.)
should be construed to refer to the orientation as then described
or as shown in the drawing figure under discussion. These relative
terms are for convenience of description and normally are not
intended to require a particular orientation. Terms including
"inwardly" versus "outwardly," "longitudinal" versus "lateral" and
the like are to be interpreted relative to one another or relative
to an axis of elongation, or an axis or center of rotation, as
appropriate. Terms concerning attachments, coupling and the like,
such as "connected" and "interconnected," refer to a relationship
wherein structures are secured or attached to one another either
directly or indirectly through intervening structures, as well as
both movable or rigid attachments or relationships, unless
expressly described otherwise. The term "operatively connected" is
such an attachment, coupling or connection that allows the
pertinent structures to operate as intended by virtue of that
relationship.
[0012] Referring to FIG. 1, a cross sectional view of an exemplary
LHP/CPL, generally designated 100, is illustrated showing outer
layers 4, vapor channel 6, liquid channel 8, first consolidated
wick portion 10, second consolidated wick portion 14, and
unconsolidated wick portion 12. The cross sectional view shown in
FIG. 1 is of an evaporator section of exemplary LHP/CPL 100.
Although not shown in FIG. 1, consolidated wick portions 10 and 14
enclose unconsolidated wick portion 12 such that unconsolidated
wick portion 12 is contained and constrained within consolidated
wick portions 10 and 14. However, for ease of discussion, the
remainder of the unconsolidated and consolidated wick portions, the
condenser section, and the remainder of the LHP/CPL are not shown
in FIG. 1. Note, the term "LHP/CPL" is used herein to refer to a
loop heat pipe, a capillary pumped loop, or both.
[0013] LHP/CPL 100 is filled with a suitable cooling fluid 7, e.g.,
water, freon, ammonia, alcohol, acetone, or some other fluid known
in the art for use in heat transfer devices, and which is capable
of vaporization and condensation within a closed loop environment.
Parameters to be considered when selecting cooling fluid 7 include
the amount of pressure that can be safely applied to the LHP/CPL
100, the operating temperature of the equipment to be cooled (e.g.,
electronic device), the rate of heat transfer, the temperatures
reached within the evaporator, the viscosity of coolant fluid 7,
the boiling point of coolant fluid 7, and chemical compatibility
with other materials used in the LHP/CPL. LHP/CPL 100 is sealed to
the ambient atmosphere so as to form a closed loop system. As will
be explained in detail, cooling fluid 7 exists in various states
throughout the various sections of LHP/CPL 100. These states
include liquid, vapor, and a mixture of liquid and vapor.
[0014] Outer layers 4 are typically good thermal conductors, thus
contributing to efficient heat exchange in the evaporator section
and condenser section of the LHP/CPL 100. Outer layers 4 may
comprise any thermally conductive material. Examples of appropriate
materials for outer layers 4 include metal, ceramics, ceramic
compounds, and intrinsic semiconductors (i.e., undoped
semiconductors) and doped semiconductor materials. Semiconductor
materials may include silicon (Si), germanium (Ge), carbon (C), and
tin (Sn). In an exemplary embodiment of the invention, outer layers
4 comprise silicon. A LHP/CPL 100 having outer layers 4 comprising
silicon may be directly bonded to a silicon device (e.g.,
electronic component) for efficient cooling of the silicon
device.
[0015] Vapor channel 6 and liquid channel 8 may comprise any
structure having the capability to supply liquid and remove vapor
within the evaporator. In an exemplary embodiment of the invention,
vapor channel 6 and liquid channel 8 comprise separate structures
(e.g., plenums, tubes, pipes) for transporting liquid and/or vapor
from/to the condenser. In another exemplary embodiment of the
invention (e.g., a silicon heat pipe), vapor channel 6 and liquid 8
are formed from a single structure, e.g., multiple channels,
etched, molded or cut into a single or multiple blocks.
[0016] Wick 20 comprises first consolidated wick portion 10,
unconsolidated wick portion 12, and second consolidated wick
portion 14. Wick 20 generates capillary pumping sufficient to
maintain proper operation of the LHP/CPL 100. Wick 20 may comprise
any material and structure providing structural support, integrity
(e.g., compatible coefficient of thermal expansion), and sufficient
capillary action. Examples of such materials and structures include
adjacent layers of metal or plastic screens, integrally formed
layer of aluminum-silicon-carbide (AlSiC) or copper-silicon-carbide
(CuSiC), sintered powder, sintered powder with interstices
positioned between the powder particles, polymer powder, felt,
ceramics, ceramic compounds, and intrinsic semiconductors and doped
semiconductors. A sintered powder wick 20 may comprise materials
such as metal, polymers, ceramics, plastics, intrinsic
semiconductors, and doped semiconductors.
[0017] In an examplary embodiment of the invention, thermal energy
(heat) enters outer layer 4 as indicated by arrows 16. The heat is
generated by the device being cooled, such as an electronic
component. As this thermal energy is applied to the surface of
outer layer 4, working fluid 7 in the vapor channel 6 is vaporized.
The working fluid 7, in the form of vapor 7 is at a slightly higher
temperature and pressure than in other areas of the LHP/CPL 100. A
pressure gradient is thus created, which forces the vapor 7 in the
vapor channel 6 to flow to the cooler regions of the LHP/CPL 100
(e.g., condenser section). As the vapor 7 condenses, the latent
heat of vaporization is transferred to the condenser section of the
LHP/CPL 100 (condenser section not shown in FIG. 1). As a result,
liquid forms in liquid channel 8. The liquid in liquid channel 8 is
drawn to vapor channel 6 by the capillary action of wick portions
10, 12, and 14.
[0018] Wick 20 generates a capillary pressure, which is dependent
upon the pore radius of the wick structure and the surface tension
of the working fluid. During proper heat transfer, the capillary
pressure generated by the wick 20 is greater than the sum of the
gravitational losses, liquid flow losses through the wick, and the
vapor flow losses.
[0019] In an examplary embodiment of the invention, dry-out is
avoided by maintaining a temperature differential, .DELTA.T, across
the wick 20 sufficiently large enough to avoid the creation of
nucleation sites within the wick. During normal operation, the
liquid side of the wick is at a lower pressure than the vapor side.
If the .DELTA.T across the wick 20 does not compensate for this
difference in pressure, boiling may occur at the liquid side of the
wick. Liquid will thus be isolated from the wick, and the LHP/CPL
will stop operating. Thus, a wick having good liquid permeability
and providing a sufficient .DELTA.T is desired.
[0020] The inventors have discovered that a wick, as depicted in
FIG. 1, having an unconsolidated wick portion 12 positioned between
first consolidated wick portion 10 and second consolidated wick
portion 14 provides good liquid permeability and maintains a
.DELTA.T across the wick large enough for miniature LHP/CPL
applications (e.g., applications involving electronic components).
As is well understood in the art heat transfer, unconsolidated
powders may exhibit greater than a factor of ten times the thermal
resistance of consolidated powders. Forming unconsolidated wick
portion 12 from an unconsolidated powder, or similar unconsolidated
wick material, provides increased thermal resistance, which
increases the .DELTA.T across unconsolidated wick portion 12, and
thus across wick 20. Further, the junctions between the
consolidated wick portions 10, 14, and the unconsolidated wick
portion 12 exhibit high thermal resistance. The unconsolidated
powder of wick portion 12 also provides good permeability to the
cooling liquid 7. The unconsolidated powder of wick portion 12
provides a pore structure which controls nucleation sites and
prevents boiling of the working fluids between the consolidated
wick portions 10 and 14. Capillary action is provided by first
consolidated wick portion 10 and second consolidated wick portion
14. Consolidated wick portions 10 and 14 comprise small uniform
pores to provide efficient capillary action.
[0021] Unconsolidated wick portion 12 is in direct contact with
consolidated wick portions 10 and 14. The regions at which the
consolidated portions and unconsolidated portion meet (e.g., the
junction of sintered powder and loose powder) exhibit high thermal
resistance, thus preventing all the thermal energy entering the
LPH/CPL 100 via outer layer 4 from entering unconsolidated wick
portion 12. This increases the .DELTA.T across the wick and
inhibits the formation of nucleation sites. Thus, the
unconsolidated wick portion 12, positioned between first
consolidated wick portions 10 and second consolidated wick portion
14, as shown in FIG. 1, provides efficient capillary action and a
.DELTA.T across the wick 20 sufficient to maintain miniature
LHP/CPL 100 performance.
[0022] A further advantage of the wick structure depicted in FIG. 1
is that the detrimental effects upon the LHP/CPL 100 due to
materials having mismatched coefficients of thermal expansion
(CTEs) are reduced. This is of particular interest in semiconductor
(e.g., silicon, germanium) applications in which the cooling device
(e.g., LHP/CPL) is formed from the same material as the device
being cooled. The use of a silicon cooling device, for example,
provides good heat transfer by allowing for direct bonding to the
heat generating silicon integrated circuit. However, if the CTE of
the various portions of the LHP/CPL differ substantially, the
LHP/CPL may be damaged by stresses causes by this mismatch during
temperature fluctuations. The use of unconsolidated powders allows
for a greater mismatch of CTE between various portions of the
LPH/CPL and the device being cooled. Unconsolidated powder (e.g.,
loose powder) can accommodate mismatched CTE, thus preventing
damage to the LHP/CPL.
[0023] In an alternate embodiment of the invention, wick 20
comprises an intrinsic or doped semiconductor, such as silicon.
First consolidate wick portion 10 and second consolidated wick
portion 14 comprises consolidated silicon and unconsolidated wick
portion 12 comprises unconsolidated silicon. Further, outer layer 4
comprises silicon and is directly bonded to a silicon device
requiring cooling, such as an electronic component. In this
alternative embodiment, the first consolidated wick portion 10 may
be formed as an etching on the inner surface of the outer layer 4
(e.g., etched holes). The direct bonding of outer layer 4 to the
silicon device provides efficient heat transfer between outer layer
4 and the silicon device. A temperature differential, .DELTA.T,
across the wick 20, large enough to prevent cooling fluid 7 from
boiling inside of wick 20, is provided by positioning
unconsolidated silicon wick portion 12 between consolidated silicon
wick portions 10 and 14, and by the high thermal resistance
exhibited at the junction between the consolidated wick portions 10
and 14, and the unconsolidated wick portion 12. Efficient capillary
action is provided by the small and uniform pore structure of
consolidated silicon wick portions 10 and 14. Because the wick 20
and outer layer 4 comprise the same material, silicon, and because
wick portion 14 comprises unconsolidated silicon, there is no
mismatch in CTE.
[0024] As is appreciated in the art, the orientation and structure
depicted in FIG. 1 are examplary. In another embodiment, heat
enters from the side opposite the side shown in FIG. 1, with the
functional roles of channels 6 and 8 reversed. Further,
* * * * *