U.S. patent number 3,762,011 [Application Number 05/208,825] was granted by the patent office on 1973-10-02 for method of fabricating a capillary heat pipe wick.
Invention is credited to Bruce D. Marcus, Karl P. Staudhammer.
United States Patent |
3,762,011 |
Staudhammer , et
al. |
October 2, 1973 |
METHOD OF FABRICATING A CAPILLARY HEAT PIPE WICK
Abstract
A conformal capillary wick for a heat pipe is fabricated by
preparing a slurry composed of an organic solvent containing an
organic binder in solution and powder particles of relatively high
thermal conductivity in suspension; applying a layer of the slurry
to the inner wall surface of the heat pipe casing, evaporating the
solvent from the layer to recover the binder and utilize the
surface tension forces of the binder to draw the particles together
into a highly compacted condition wherein the particles are bonded
to one another and to the casing wall by the binder, and the
particles and binder define a myriad of capillary passages
extending throughout and opening through the surfaces of the layer;
and curing the binder to form a dimensionally stable capillary
structure providing a capillary wick for transporting working fluid
condensate from the condenser section to the evaporator section of
the heat pipe.
Inventors: |
Staudhammer; Karl P. (Gardena,
CA), Marcus; Bruce D. (Los Angeles, CA) |
Family
ID: |
22776203 |
Appl.
No.: |
05/208,825 |
Filed: |
December 16, 1971 |
Current U.S.
Class: |
419/2;
165/104.26; 29/890.032 |
Current CPC
Class: |
B22F
7/002 (20130101); B22F 1/10 (20220101); B22F
3/22 (20130101); F28D 15/046 (20130101); Y10T
29/49353 (20150115) |
Current International
Class: |
F28D
15/04 (20060101); B22F 3/22 (20060101); B22F
7/00 (20060101); B22F 1/00 (20060101); B21d
053/02 (); B21p 015/26 () |
Field of
Search: |
;29/157.3R,157R
;165/105 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lanham; Charles W.
Assistant Examiner: Reiley, III; D. C.
Claims
What is claimed as new in support of Letters Patent is:
1. The method of forming a capillary wick for a heat pipe,
comprising the steps of:
selecting powder particles of relatively high thermal conductivity
and an organic binder both compatible with the working fluid and
operating requirements of the heat pipe and an organic solvent for
the binder;
preparing a slurry composed of said solvent containing said binder
in solution and said particles in suspension;
applying to the inner wall surface of the heat pipe casing a layer
of said slurry extending from the evaporator section to the
condenser section of the casing;
vaporising the solvent from the layer to recover said binder and
utilize the surface tension forces of the binder to draw said
particles together into a highly compacted condition wherein the
particles are bonded to one another and to the casing wall surface
by the binder, and the particles and binder define a myriad of
interconnecting capillary passages extending throughout and opening
through the surface of said layer; and
curing said binder to form a dimensionally stable capillary
structure providing said wick.
2. The method according to claim 1 wherein:
said powder particles are particles of a structural metal selected
from the class consisting of aluminum oxide, silicon carbide,
aluminum, copper, nickel, magnesium, zinc, calcium, silver, gold,
titanium, niobium, tungsten, zirconium, vanadium, chromium, iron,
and cobalt, or an alloy of the same;
said binder is selected from the class consisting of methacylates,
polyester, polyimides, phenolics, and acrylics; and
said solvent is selected from the class consisting of toluene,
methyl ethyl ketone, methyl benzyl ketone, amyl acetate, acetone,
and dimethyl formamide.
Description
BACKGROUND OF THE DISCLOSURE
1. Field of the Disclosure
This disclosure relates generally to the field of heat transfer and
more particularly to a novel heat pipe and method of forming the
capillary wick structure of the heat pipe.
2. Prior Art
Essentially, a heat pipe is a thermal transport device having a
hermetic casing with evaporator and condenser sections and
containing a working fluid and a capillary wick structure extending
between the evaporator and condenser sections. When the evaporator
and condenser sections are placed in heat transfer relation to a
heat source and a heat sink, respectively, thermal energy is
transmitted through the heat pipe from heat source to the heat sink
by a closed thermodynamic cycle of the working fluid. This
thermodynamic cycle involves vaporization of the working fluid
witin the evaporator section by heat inflow from the heat source
and condensation of the fluid within the condenser section by heat
rejection to the heat sink.
Continuous operation of the heat pipe requires continuous flow of
working fluid vapor from the evaporator section to the condenser
section and continuous condensate flow from the condenser section
to the evaporator section. The vapor pressure differential between
the evaporator and condenser sections provides the force for
transporting vapor continuously from the evaporator section to the
condenser section. Capillary force is utilized to transport the
working fluid condensate from the condenser section to the
evaporator section.
To this end, a heat pipe is equipped with an internal capillary
structure extending between the evaporator and condenser sections.
Such capillary structures are commonly referred to as capillary
wicks, or simply wicks, and are constructed of various porous
materials. At the present time, the most widely used wick materials
are woven quartz, wire mesh, and sintered metal powders and fibers.
Listed below are prior art patents of interest in this
connection:
1,938,170 3,111,396 3,413,239 2,996,389 3,285,873 3,485,296
3,090,094 3,288,615 3,563,309
At the present time, heat pipe wicks are generally fabricated
externally of the heat pipes and then mounted or secured within the
pipe casings in various ways, as by bonding, spot welding, brazing,
or sintering or with the aid of suitable positioning means.
The existing wicks and mounting methods possess certain
disadvantages which the present disclosure overcomes. Among the
foremost of these disadvantages are the following. A bonding agent
which is used to secure the wick to the heat pipe wall often
infiltrates and plugs the capillary passages in the wick, thereby
restricting capillary flow of condensate through the wick. It is
difficult and often impossible to apply the existing wicks to
heatpipes of complex surface geometry. Regulation of the wick
porosity, i.e., capillary pore size, is difficult.
SUMMARY OF THE DISCLOSURE
According to one of its aspects, the disclosure provides a heat
pipe with a novel capillary wick structure or wick applied to the
wall of the heat pipe casing. The wick is composed of powder
particles of relatively high thermal conductivity joined to one
another and to the casing wall by a binder. The particles are
highly compacted and define with the binder a myriad of capillary
passages extending throughout and opening through the surfaces of
the wick. A unique feature of the wick resides in the fact that the
binder serves the dual function of joining the particles to one
another and joining the wick to the wall of the heat pipe
casing.
Another aspect of the disclosure is concerned with a unique method
of fabricating the wick. According to this method, a slurry is
prepared composed of an organic solvent containing an organic
binder in solution and powder particles of relatively high thermal
conductivity in suspension. A layer of this slurry is brushed,
sprayed or otherwise applied to the inner wall surface of the heat
pipe casing. This solvent in the layer is evaporated to recover the
binder and utilize the surface tension forces of the binder to draw
the particles together into a highly compacted condition wherin the
particles and binder define a myriad of interconnecting capillary
passages extending throughout and opening through the surface of
the layer. The binder is then cured to form a dimensionally stable
capillary wick structure.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 illustrates a heat pipe with a capillary wick according to
the invention;
FIG. 2 is an enlargement of the capillary wick; and
FIG. 3 is a diagram of the present method of forming the capillary
wick.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The heat pipe 10 shown in FIG. 1 has a hermetic casing 12 with
evaporator and condenser sections 14 and 16. Applied to the inner
wall of the heatpipe casing is a capillary wick structure or wick
18 extending between the evaporator and condenser sections. A
working fluid (not shown) is confined within the casing.
Turning to FIG. 2, the wick 18 is composed of powder particles 20
bonded to one another and to the inner wall surface of the heat
pipe casing 12 by an organic binder 22, which forms fine
interconnecting bonds 24 between the particles. These particles and
bonds define a myriad of interconnecting capillary passages 26
which extend throughout and open through the surfaces of the wick.
A unique feature of the wick resides in the fact that the binder 22
provides both the bonds between the particles 20 and the bond
between wick and the casing wall.
In operation of the heat pipe 10, its evaporator and condenser
sections 14, 16 are placed in heat transfer relation to a heat
source and a heat sink, respectively. Heat inflow into the
evaporator section 14 from the heat source vaporizes the working
fluid within the section. The vapor pressure differential between
the sections 14, 16 causes flow of the resulting fluid vapor to the
condenser section where the vapor is condensed by heat rejection to
the heat sink. The resulting condensate then returns to the
evaporator section to repeat the process by capillary flow through
the wick 18. Thus, the working fluid transports thermal energy from
the heat source to the heat sink by a closed thermodynamic cycle
involving continuous evaporation of the fluid in the evaporator
section and condensation of the fluid in the condenser section.
Reference is now made to FIG. 3 which is a diagram of the present
method of forming or fabricating the capillary wick 18. The initial
step of the method involves preparation of a slurry composed of an
organic solvent containing the powder particles 20 in suspension
and the organic binder 22 in solution. A layer of this slurry is
applied to the inner wall surface of the heat pipe casing 12 by
brushing, spraying or any other convenient method of application.
After application of the layer, the solvent in the layer is
evaporated, leaving the powder particles 20 and the binder 22 in
FIG. 2. During this evaporation, the surface tension forces of the
binder draw the particles together into the highly compacted state
of FIG. 2 to form the capillary passages 26. The binder is then
cured to a dimensionally stable state to provide the final
capillary wick 18. The binder also bonds the wick to the wall of
the heat pipe casing.
A variety of powder particles, binders, and solvents may be
employed in the practice of the disclosure. The preferred powders
are those of aluminum oxide, silicon carbide, aluminum, copper,
magnesium, zinc, calcium, silver, gold, titanium, niobium,
tungsten, zirconium, vanadiam, chromium, iron, and cobalt and
alloys of the listed materials. The preferred binders are
methacylates, polyesters, polyimides, phenolics, acrylics,
collodion, and Duco cement. The preferred solvents are Toulene,
methyl ethyl ketone, methyl benzye ketone, amyl acetate, acetone,
and dimethyl formamide.
It will be understood that the powder particles, binder, and
solvent used in the practice of the invention will be compatible
with one another and with the working fluid and operating
requirements and parameters of the heat pipe. That is to say, the
solvent used must be appropriate for the powder but not dissolve,
degrade, or otherwise adversely affects the binder. Also, the
particles and binder must be chemically inert to the heat pipe
working fluid, and must be immune to degradation and other adverse
affects at temperatures at least up to the maximum operating
temperature of the heat pipe. Moreover, in some applications,
involving the cooling of electrical components, such as
transistors, the particles and binder must produce a wick of high
dielectric strength such that the wick will not short out the
components when the latter are exposed directly to the heat pipe
working fluid. Both thermosetting and thermoplastic binders may be
used in the disclosure, the thermoplastic binders, of course, being
restricted to heat pipes whose operating temperature is below the
softening temperature of the binder. Curing of the thermosetting
binders is accomplished by heating. Curing of thermoplastic binders
is accomplished by drying and cooling, if necessary.
Wicks according to the disclosure have been successfully
fabricated. These wicks were fabricated from powders of aluminum
oxide, silicon carbide, aluminum, copper, and nickel using the
following binder/solvent combinations: collodian/amyl acetate, Duco
cement/acetone, polyimide/dimethyl formamide. In each case, the
slurry prepared from the selected particles, binder, and solvent
was applied to the heat pipe wall after which the solvetn was
allowed to evaporate. The binder wws then cured by the application
of heat. These wicks utilized a binder/solvent solution containing
on the order of 10 percent binder and 90 percent solvent by volume
and powder particles of a mesh size in the range of 50 - 200.
As noted earlier, an important advantage of the disclosure is its
ability to provide a capillary wick conforming to virtually any
heat pipe surface geometry from the most simple to the most
complex. Moreover, the wick thickness may be easily controlled by
controlling the thickness of the slurry layer which is initially
applied to the heat pipe casing. The wick porosity may be regulated
and controlled by varying the particle size and/or the
concentration of solvent to binder used in the slurry.
* * * * *