U.S. patent application number 11/120238 was filed with the patent office on 2005-09-22 for brazed wick for a heat transfer device and method of making same.
Invention is credited to Minnerly, Kenneth G., Rosenfeld, John H..
Application Number | 20050205243 11/120238 |
Document ID | / |
Family ID | 34062287 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050205243 |
Kind Code |
A1 |
Rosenfeld, John H. ; et
al. |
September 22, 2005 |
Brazed wick for a heat transfer device and method of making
same
Abstract
A capillary structure for a heat transfer device, such as a heat
pipe is provided having a plurality of particles joined together by
a brazing compound such that fillets of the brazing compound are
formed between adjacent ones of the plurality of particles. In this
way, a network of capillary passageways are formed between the
particles to aid in the transfer of working fluid by capillary
action, while the plurality of fillets provide enhanced thermal
transfer properties between the plurality of particles so as to
greatly improve over all heat transfer efficiency of the device. A
method of making the capillary structure according to the invention
is also presented.
Inventors: |
Rosenfeld, John H.;
(Lancaster, PA) ; Minnerly, Kenneth G.; (Lititz,
PA) |
Correspondence
Address: |
DUANE MORRIS LLP
P. O. BOX 1003
305 NORTH FRONT STREET, 5TH FLOOR
HARRISBURG
PA
17108-1003
US
|
Family ID: |
34062287 |
Appl. No.: |
11/120238 |
Filed: |
May 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11120238 |
May 2, 2005 |
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10607337 |
Jun 26, 2003 |
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Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
H01L 2924/0002 20130101;
B22F 2998/00 20130101; B22F 3/1103 20130101; B22F 2998/00 20130101;
F28D 15/0233 20130101; F28F 2275/04 20130101; F28D 15/046 20130101;
H01L 2924/0002 20130101; B22F 2998/00 20130101; H01L 23/427
20130101; Y10T 29/49353 20150115; B22F 2201/20 20130101; H01L
2924/00 20130101; B22F 2201/013 20130101; B22F 3/1103 20130101;
B22F 3/1103 20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 015/00 |
Claims
1. A capillary structure for a heat transfer device comprising: a
plurality of particles joined together by a brazing compound such
that fillets of said brazing compound are formed between adjacent
ones of said plurality of particles so as to form a network of
capillary passageways between said particles.
2. A capillary structure according to claim 1 wherein said
plurality of particles comprise a first melting temperature and
said brazing compound comprises a second melting temperature that
is lower than said first melting temperature.
3. (canceled)
4. A capillary structure according to claim 1 wherein said fillets
are formed by capillary action of said braze compound when in a
molten state.
5. A capillary structure according to claim 1 wherein said
particles are selected from the group consisting of carbon,
tungsten, copper, aluminum, magnesium, nickel, gold, silver,
aluminum oxide, and beryllium oxide.
6. A capillary structure according to claim 1 wherein said
particles comprise a shape selected from the group consisting of
spherical, oblate spheroid, prolate spheroid, ellipsoid, polygonal,
and filament.
7. A capillary structure according to claim 1 wherein said
particles comprise at least one of copper spheres and oblate copper
spheroids having a melting point of about one thousand eighty-three
.degree. C.
8-11. (canceled)
12. A capillary structure according to claim 2 wherein metal
particles are a constituent portion of said braze compound prior to
reaching said second melting temperature and comprise a smaller
size than said plurality of particles.
13. A capillary structure according to claim 1 wherein said braze
compound is selected from the group consisting of nickel-based
Nicrobrazes, silver/copper brazes, tin/silver, lead/tin, and
polymers.
14. A capillary structure according to claim 1 wherein said
plurality of particles comprise aluminum and magnesium and said
brazing compound comprises an aluminum/magnesium intermetallic
alloy.
15. A heat pipe comprising: a hermetically sealed and partially
evacuated enclosure, said enclosure comprising internal surfaces; a
wick disposed on at least one of said internal surfaces and
comprising a plurality of particles joined together by a brazing
compound such that fillets of said brazing compound are formed
between adjacent ones of said plurality of particles so as to form
a network of capillary passageways between said particles; and a
two-phase fluid at least partially disposed within a portion of
said wick.
16. A heat pipe according to claim 15 wherein said plurality of
particles comprise a first melting temperature and said brazing
compound comprises a second melting temperature that is lower than
said first melting temperature.
17. (canceled)
18. A heat pipe according to claim 15 wherein said fillets are
formed by capillary action of said braze compound when in a molten
state.
19. A heat pipe according to claim 15 wherein said particles are
selected from the group consisting of carbon, tungsten, copper,
aluminum, magnesium, nickel, gold, silver, aluminum oxide, and
beryllium oxide.
20. A heat pipe according to claim 15 wherein said particles
comprise a shape selected from the group consisting of spherical,
oblate spheroid, prolate spheroid, polygonal, and filament.
21. A heat pipe according to claim 14 wherein said particles
comprise at least one of copper spheres and oblate copper spheroids
having a melting point of about 1083.degree. C.
22. (canceled)
23. A heat pipe according to claim 16 wherein said brazing compound
is present in the range from about two percent to about ten
percent.
24. A heat pipe according to claim 16 wherein said particles
comprise copper powder comprising particles size in a range from
about twenty mesh to about two-hundred mesh.
25. A heat pipe according to claim 16 wherein said braze compound
comprises particles that comprise about minus three hundred and
twenty-five mesh prior to melting.
26. A heat pipe according to claim 16 wherein particles are a
constituent portion of said braze compound prior to reaching said
second melting temperature and comprise a smaller size than said
plurality of particles.
27. A heat pipe according to claim 16 wherein said braze compound
is selected from the group consisting of nickel-based Nicrobrazes,
silver/copper brazes, tin/silver, lead/tin, and polymers.
28. A heat pipe according to claim 16 wherein said plurality of
particles comprise aluminum and magnesium and said brazing compound
comprises an aluminum/magnesium intermetallic alloy.
29-43. (canceled)
44. A heat pipe comprising: a sealed and partially evacuated
enclosure having an internal surface; a grooved brazed wick
disposed upon said internal surface comprising a plurality of
individual particles and a brazing compound such that fillets of
said brazing compound are formed between adjacent ones of said
plurality of particles, and further including at least two lands
that are in fluid communication with one another wherein said
particles are thermally engaged with one another by a plurality of
said fillets; and a working fluid disposed within said
enclosure.
45-58. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to heat transfer
devices that rely upon capillary action as a transport mechanism
and, more particularly, to wicking materials for such devices.
BACKGROUND OF THE INVENTION
[0002] It has been suggested that a computer is a thermodynamic
engine that sucks entropy out of data, turns that entropy into
heat, and dumps the heat into the environment. The ability of prior
art thermal management technology to get that waste heat out of
semiconductor circuits and into the environment, at a reasonable
cost, limits the density and clock speed of electronic systems.
[0003] A typical characteristic of heat transfer devices for
electronic systems is that the atmosphere is the final heat sink of
choice. Air cooling gives manufacturers access to the broadest
market of applications. Another typical characteristic of heat
transfer devices for electronics today is that the semiconductor
chip thermally contacts a passive spreader or active thermal
transport device, which conducts the heat from the chip to one of
several types of fins. These fins convect heat to the atmosphere
with natural or forced convection.
[0004] As the power to be dissipated from semiconductor devices
increases with time, a problem arises: over time the thermal
conductivity of the available materials becomes too low to conduct
the heat from the semiconductor device to the fins with an
acceptably low temperature drop. The thermal power density emerging
from the semiconductor devices will be so high that copper, silver,
or even gold based spreader technology will not be adequate.
[0005] One technology that has proven beneficial to this effort is
the heat pipe. A heat pipe includes a sealed envelope that defines
an internal chamber containing a capillary wick and a working fluid
capable of having both a liquid phase and a vapor phase within a
desired range of operating temperatures. When one portion of the
chamber is exposed to relatively high temperature it functions as
an evaporator section. The working fluid is vaporized in the
evaporator section causing a slight pressure increase forcing the
vapor to a relatively lower temperature section of the chamber,
which functions as a condenser section. The vapor is condensed in
the condenser section and returns through the capillary wick to the
evaporator section by capillary pumping action. Because a heat pipe
operates on the principle of phase changes rather than on the
principles of conduction or convection, a heat pipe is
theoretically capable of transferring heat at a much higher rate
than conventional heat transfer systems. Consequently, heat pipes
have been utilized to cool various types of high heat-producing
apparatus, such as electronic equipment (See, e.g., U.S. Pat. Nos.
3,613,778; 4,046,190; 4,058,299; 4,109,709; 4,116,266; 4,118,756;
4,186,796; 4,231,423; 4,274,479; 4,366,526; 4,503,483; 4,697,205;
4,777,561; 4,880,052; 4,912,548; 4,921,041; 4,931,905; 4,982,274;
5,219,020; 5,253,702; 5,268,812; 5,283,729; 5,331,510; 5,333,470;
5,349,237; 5,409,055; 5,880,524; 5,884,693; 5,890,371; 6,055,297;
6,076,595; and 6,148,906).
[0006] The flow of the vapor and the capillary flow of liquid
within the system are both produced by pressure gradients that are
created by the interaction between naturally-occurring pressure
differentials within the heat pipe. These pressure gradients
eliminate the need for external pumping of the system liquid. In
addition, the existence of liquid and vapor in equilibrium, under
vacuum conditions, results in higher thermal efficiencies. In order
to increase the efficiency of heat pipes, various wicking
structures have been developed in the prior art to promote liquid
transfer between the condenser and evaporator sections as well as
to enhance the thermal transfer performance between the wick and
its surroundings. They have included longitudinally disposed
parallel grooves and the random scoring of the internal pipe
surface. In addition, the prior art also discloses the use of a
wick structure which is fixedly attached to the internal pipe wall.
The compositions and geometries of these wicks have included, a
uniform fine wire mesh and sintered metals. Sintered metal wicks
generally comprise a mixture of metal particles that have been
heated to a temperature sufficient to cause fusing or welding of
adjacent particles at their respective points of contact. The
sintered metal powder then forms a porous structure with capillary
characteristics. Although sintered wicks have demonstrated adequate
heat transfer characteristics in the prior art, the minute
metal-to-metal fused interfaces between particles tend to constrict
thermal energy conduction through the wick. This has limited the
usefulness of sintered wicks in the art.
[0007] Prior art devices, while adequate for their intended
purpose, suffer from the common deficiency, in that they do not
fully realize the optimum inherent heat transfer potential
available from a given heat pipe. To date, no one has devised a
wick structure for a heat pipe, which is sufficiently simple to
produce, and yet provides optimum heat transfer characteristics for
the heat pipe in which it is utilized.
SUMMARY OF THE INVENTION
[0008] The present invention provides a capillary structure for a
heat transfer device that comprises a plurality of particles joined
together by a brazing compound such that fillets of the brazing
compound are formed between adjacent ones of the plurality of
particles. In this way, a network of capillary passageways are
formed between the particles to aid in the transfer of working
fluid by capillary action, while the plurality of fillets provide
enhanced thermal conduction properties between the plurality of
particles so as to greatly improve over all heat transfer
efficiency of the device.
[0009] In one embodiment, a heat pipe is provided that includes a
hermetically sealed and partially evacuated enclosure, where the
enclosure has internal surfaces and is at least partially drenched
with a two-phase vaporizable fluid. A wick is disposed on at least
one of the internal surfaces of the enclosure. The wick
advantageously comprises a plurality of particles joined together
by a brazing compound such that fillets of the brazing compound are
formed between adjacent ones of the plurality of particles so as to
form a network of capillary passageways between the particles.
[0010] In a further embodiment of the present invention, a heat
pipe is provided comprising a sealed and partially evacuated
enclosure having an internal surface and a working fluid disposed
within a portion of the enclosure. A grooved brazed wick is
disposed upon the internal surface of the heat pipe. The grooved
brazed wick comprises a plurality of individual particles which
together yield an average particle diameter and a brazing compound
such that fillets of the brazing compound are formed between
adjacent ones of the plurality of particles. At least two lands are
provided that are in fluid communication with one another through a
particle layer disposed between the at least two lands wherein the
particle layer comprises at least one dimension that is no more
than about six average particle diameters wherein the particles in
the particle layer are thermally engaged with one another by a
plurality of the fillets.
[0011] A method is also provided for making a heat pipe wick on an
inside surface of a heat pipe container comprising the steps of
providing a slurry of metal particles that are mixed with a brazing
compound. The metal particles have a first melting temperature and
the brazing compound has a second melting temperature that is lower
than the first melting temperature. At least a portion of the
inside surface of the container is coated with the slurry, and
dried to form a green wick. The green wick is then heated to a
temperature that is no less than the second melting temperature and
below the first melting temperature so that the brazing compound is
drawn by capillary action toward adjacent ones of the metal
particles so as to form heat-distribution fillets between the
adjacent metal particles thereby to yield a brazed wick.
[0012] In an alternative embodiment of the method of the invention,
a mandrel having a grooved contour and a plurality of recesses is
positioned within a portion of a heat pipe container. A slurry of
metal particles having an average particle diameter and that are
mixed with a brazing compound is introduced into the container. The
metal particles comprise a first melting temperature and the
brazing compound comprises a second melting temperature that is
lower than the first melting temperature. At least a portion of the
inside surface of the container is coated with the slurry so that
the slurry conforms to the grooved contour of the mandrel and forms
a layer of slurry between adjacent grooves that comprises no more
than about six average particle diameters. The slurry is then dried
to form a green wick. The green wick is then heated to a
temperature that is no less than the second melting temperature and
below the first melting temperature so that the brazing compound is
drawn by capillary action toward adjacent ones of the metal
particles so as to form heat-distribution fillets between the
adjacent metal particles thereby to yield a brazed wick.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other features and advantages of the present
invention will be more fully disclosed in, or rendered obvious by,
the following detailed description of the preferred embodiments of
the invention, which are to be considered together with the
accompanying drawings wherein like numbers refer to like parts and
further wherein:
[0014] FIG. 1 is an exploded perspective view of a typical heat
pipe enclosure of the type used in connection with the present
invention;
[0015] FIG. 2 is a perspective view of the heat pipe enclosure
shown in FIG. 1;
[0016] FIG. 3 is a cross-sectional view of the heat pipe shown in
FIG. 2;
[0017] FIG. 4 is a significantly enlarged cross-sectional view of a
portion of a brazed wick formed in accordance with one embodiment
of the present invention;
[0018] FIG. 5 is a broken-way perspective view that has been highly
enlarged to clearly represent metal particles and fillets that
comprise one embodiment of the present invention;
[0019] FIG. 6 is a highly enlarged view, similar to FIG. 5, of an
alternative embodiment of brazed wick formed in accordance with the
present invention;
[0020] FIG. 7 is an exploded perspective view of a heat pipe
enclosure having an alternative embodiment of brazed wick in
accordance with the present invention;
[0021] FIG. 8 is a cross-sectional view, as taken along lines 8-8
in FIG. 7;
[0022] FIG. 9 is a further alternative embodiment of heat pipe
enclosure formed in accordance with the present invention;
[0023] FIG. 10 is a cross-sectional view of the tubular heat pipe
enclosure shown in FIG. 9, as taken along lines 10-10 in FIG.
9;
[0024] FIG. 11 is a highly enlarged view of a portion of a brazed
wick disposed on the wall of the heat pipe shown in FIG. 10;
[0025] FIG. 12 is a perspective cross-sectional view of a tower
heat pipe having a brazed wick formed in accordance with the
present invention;
[0026] FIG. 13 is a highly enlarged surface view of a brazed wick
coating the anterior surfaces of the tower heat pipe shown in FIG.
12;
[0027] FIG. 14 is an alternative embodiment of tower heat pipe
having grooved base wick formed in accordance with the present
invention;
[0028] FIG. 15 is a highly enlarged surface view of a brazed wick
formed in accordance with the present invention;
[0029] FIG. 16 is a broken-way cross-sectional view of the
groove-wick shown in FIGS. 7, 8, and 13;
[0030] FIG. 17 is a highly enlarged cross-sectional view of a
portion of the groove brazed wick shown in FIGS. 7, 8, 13, and 15;
and
[0031] FIG. 18 is an end view of a mandrel used in manufacturing a
grooved brazed wick in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] This description of preferred embodiments is intended to be
read in connection with the accompanying drawings, which are to be
considered part of the entire written description of this
invention. The drawing figures are not necessarily to scale and
certain features of the invention may be shown exaggerated in scale
or in somewhat schematic form in the interest of clarity and
conciseness. 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. In the claims,
means-plus-function clauses are intended to cover the structures
described, suggested, or rendered obvious by the written
description or drawings for performing the recited function,
including not only structural equivalents but also equivalent
structures.
[0033] Referring to FIGS. 1-6, the present invention comprises a
wick structure for a heat pipe or heat spreader 2, hereinafter
referred to as simply a heat pipe. Such heat pipes 2 are often
sized and shaped to transfer and/or spread the thermal energy
generated by at least one thermal energy source, e.g., a
semiconductor device (not shown), that is thermally engaged between
a portion of the heat pipe and a heat sink (not shown). Heat pipes
2 generally comprise a hermetically sealed enclosure such as a
flat, hollow plate-like structure (FIG. 2) or a tubular structure
(FIGS. 9, 12 and 14). Regardless of outer profile, each enclosure
structure defines an evaporator section 5, a condenser section 7,
and an internal void space or vapor chamber 10. For example, in a
planar rectangular heat pipe 2, vapor chamber 10 is defined between
a bottom wall 12 and a top wall 14. In a tubular or tower heat pipe
2, vapor chamber 10 extends longitudinally from one end of the tube
to the other (FIGS. 9, 12, and 14).
[0034] In one preferred embodiment of a rectilinear enclosure,
bottom wall 12 and a top wall 14 comprise substantially uniform
thickness sheets of a thermally conductive material, e.g., copper,
steel, aluminum, or any of their respective alloys, and are
spaced-apart by about 2.0 (mm) to about 4.0 (mm) so as to form
vapor chamber 10 within heat pipe 2. Top wall 14 of heat pipe 2 is
often substantially planar, and is complementary in shape to bottom
wall 12. Bottom wall 12 preferably comprises a substantially planer
inner surface 18 and a peripheral edge wall 20. Peripheral edge
wall 20 projects outwardly from the peripheral edge of inner
surface 18 so as to circumscribe inner surface 18. Vapor chamber 10
is created within heat pipe 2 by the attachment of bottom wall 12
and a top wall 14, along their common edges which are then
hermetically sealed at their joining interface 24. A vaporizable
fluid (e.g., water, ammonia or freon not shown) resides within
vapor chamber 10, and serves as the working fluid for heat pipe 2.
For example, heat pipe 2 may be made of copper or copper silicon
carbide with water, ammonia, or freon generally chosen as the
working fluid. Heat pipe 2 is completed by drawing a partial vacuum
within the vapor chamber after injecting the working fluid just
prior to final hermetic sealing of the common edges of bottom wall
12 and the top wall 14.
[0035] Referring to FIGS. 3-6, in order for heat pipe operation to
be initiated within the enclosure of heat pipe 2, a capillary must
be present within vapor chamber 10 that will pump condensed liquid
from condenser section 7 back to evaporator sections, substantially
unaided by gravity. In the present invention, a brazed wick 25 is
located on inner surface 18 which defines the boundaries of vapor
chamber 10. Brazed wick 25 comprises a plurality of metal particles
27 combined with a filler metal or combination of metals that is
often referred to as a "braze" or brazing compound 30. It will be
understood that "brazing" is the joining of metals through the use
of heat and a filler metal, i.e., brazing compound 30. Brazing
compound 30 very often comprises a melting temperature that is
above 450.degree. C.-1000.degree. C. but below the melting point of
metal particles 27 that are being joined to form brazed wick
25.
[0036] In general, to form brazed wick 25 according to the present
invention, a plurality of metal particles 27 and brazing compound
30 are heated together to a brazing temperature that melts brazing
compound 30, but does not melt plurality of metal particles 27.
Significantly, during brazing metal particles 27 are not fused
together as with sintering, but instead are joined together by
creating a metallurgical bond between brazing compound 30 and the
surfaces of adjacent metal particles 27 through the creation of
fillets of re-solidified brazing compound (identified by reference
numeral 33 in FIGS. 5 and 6). Advantageously, the principle by
which brazing compound 30 is drawn through the porous mixture of
metal particles 27 to create fillets 33 is "capillary action",
i.e., the movement of a liquid within the spaces of a porous
material due to the inherent attraction of molecules to each other
on a liquid's surface. Thus, as brazing compound 30 liquefies, the
molecules of molten brazing metals attract one another as the
surface tension between the molten braze and the surfaces of
individual metal particles 27 tend to draw the molten braze toward
each location where adjacent metal particles 27 are in contact with
one another. Fillets 33 are formed at each such location as the
molten braze metals re-solidify.
[0037] In the present invention, brazing compound 30 and fillets 33
create a higher thermal conductivity wick than, e.g., sintering or
fusing techniques. This higher thermal conductivity wick directly
improves the thermal conductance of the heat transfer device in
which it is formed, e.g., heat pipe, loop heat pipe, etc. Depending
upon the regime of heat flux that evaporator 5 is subjected to, the
conductance of brazed wick 25 has been found to increase between
directly proportional to and the square root of the thermal
conductivity increase. Importantly, material components of brazing
compound 30 must be selected so as not to introduce chemical
incompatibility into the materials system comprising heat pipe
2.
[0038] Metal particles 27 may be selected from any of the materials
having high thermal conductivity, that are suitable for fabrication
into brazed porous structures, e.g., carbon, tungsten, copper,
aluminum, magnesium, nickel, gold, silver, aluminum oxide,
beryllium oxide, or the like, and may comprise either substantially
spherical, oblate or prolate spheroids, ellipsoid, or less
preferably, arbitrary or regular polygonal, or filament-shaped
particles of varying cross-sectional shape. For example, when metal
particles 27 are formed from copper spheres (FIG. 5) or oblate
spheroids (FIG. 6) whose melting point is about 1083.degree. C.,
the overall wick brazing temperature for heat pipe 2 will be about
1000.degree. C. By varying the percentage brazing compound 30
within the mix of metal particles 27 or, by using a more "sluggish"
alloy for brazing compound 30, a wide range of heat-conduction
characteristics may be provided between metal particles 27 and
fillets 33.
[0039] For example, in a copper/water heat pipe, any ratio of
copper/gold braze could be used, although brazes with more gold are
more expensive. A satisfactory combination for brazing compound 30
has been found to be about six percent (6)% by weight of a finely
divided (-325 mesh), 65%/35% copper/gold brazing compound, that has
been well mixed with the copper powder (metal particles 27). More
or less braze is also possible, although too little braze reduces
the thermal conductivity of brazed wick 25, while too much braze
will start to fill the wick pores with solidified braze metal. One
optimal range has been found to be between about 2% and about 10%
braze compound, depending upon the braze recipe used. When
employing copper powder as metal particles 27, a preferred shape of
particle is spherical or spheroidal. Metal particles 27 should
often be coarser than about 200 mesh, but finer than about 20 mesh.
Finer wick powder particles often require use of a finer braze
powder particle. The braze powder of brazing compound 30 should
often be several times smaller in size than metal particles 27 so
as to create a uniformly brazed wick 25 with uniform
properties.
[0040] Other brazes can also be used for brazing copper wicks,
including nickelnickel-based Nicrobrazes, silver/copper brazes,
tin/silver, lead/tin, and even polymers. The invention is also not
limited to copper/water heat pipes. For example, aluminum and
magnesium porous brazed wicks can be produced by using a braze that
is an aluminum/magnesium intermetallic alloy.
[0041] Brazing compound 30 should often be well distributed over
each metal particle surface. This distribution of brazing compound
30 may be accomplished by mixing brazing compound 30 with an
organic liquid binder, e.g., ethyl cellulose, that creates an
adhesive quality on the surface of each metal particle 27 (i.e.,
the surface of each sphere or spheroid of metal) for brazing
compound 30 to adhere to. In one embodiment of the invention, one
and two tenths grams by weight of copper powder (metal particles
27) is mixed with two drops from an eye dropper of an organic
liquid binder, e.g., ISOBUTYL METHACRYLATE LACQUER to create an
adhesive quality on the surface of each metal particle 27 (i.e.,
the surface of each sphere or spheroid of metal) for braze compound
30 to adhere to. A finely divided (e.g., -325 mesh) of braze
compound 30 is mixed into the liquid binder coated copper powder
particles 27 and allowed to thoroughly air dry. About 0.072 grams,
about 6% by weight of copper/gold in a ratio of 65%/35% copper/gold
brazing compound, has been found to provide adequate results. The
foregoing mixture of metal particles 27 and brazing compound 30 are
applied to the internal surfaces of heat pipe 2, for example inner
surface 18 of bottom wall 12, and heated evenly so that brazing
compound 30 is melted by heating metal particles 27. Molten brazing
compound 30 that is drawn by capillary action, forms fillets 33 as
it solidifies within the mixture of metal particles 27. For
example, vacuum brazing or hydrogen brazing at about 1020.degree.
C. for between two and eight minutes, and preferably about five
minutes, has been found to provide adequate fillet formation within
a brazed wick. A vacuum of at least 10.sup.-5 torr or lower has
been found to be sufficient, and if hydrogen furnaces are to be
used, the hydrogen furnace should use wet hydrogen. In one
embodiment, the assembly is vacuum fired at 1020.degree. C. for 5
minutes, in a vacuum of is 5.times.10.sup.-5 torr or lower.
[0042] Referring to FIGS. 7, 8, 14, and 16-17, grooved brazed wick
structure 38 may also be advantageously formed from metal particles
27 combined with brazing compound 30. More particularly, a mandrel
40 (FIG. 18) is used to create grooved wick structure 38 that
comprises a plurality of parallel lands 45 that are spaced apart by
parallel grooves 47. Lands 45 of mandrel 40 form grooves 50 of
finished brazed grooved wick structure 38, and grooves 47 of
mandrel 40 form lands 52 finished brazed grooved wick structure 38.
Each land 52 is formed as an inverted, substantially "V"-shaped or
pyramidal protrusion having sloped side walls 54a, 54b, and is
spaced-apart from adjacent lands. Grooves 50 separate lands 52 and
are arranged in substantially parallel, longitudinally (or
transversely) oriented rows that extend at least through evaporator
section 5. The terminal portions of grooves 50, adjacent to, e.g.,
a peripheral edge wall 20, may be unbounded by further porous
structures. In one embodiment, a relatively thin layer of brazed
metal particles is deposited upon inner surface 18 of bottom wall
12 so as to form a groove-wick 55 at the bottom of each groove 50
and between spaced-apart lands 52. For example, brazed copper
powder particles 27 are deposited between lands 52 such that
groove-wick 55 comprises an average thickness of about one to six
average copper particle diameters (approximately 0.005 millimeters
to 0.5 millimeters, preferably, in the range from about 0.05
millimeters to about 0.25 millimeters) when deposited over
substantially all of inner surface 18 of bottom wall 12, and
between sloped side walls 54a, 54b of lands 52. Advantageously,
metal particles 27 in groove-wick 55 are thermally and mechanically
engaged with one another by a plurality of fillets 33 (FIG. 17).
When forming grooved brazed wick structure 38, inner surface 18 of
bottom wall 12 (often a copper surface) is lightly coated with
organic binder ISOBUTYL METHACRYLATE LACQUER and the surface is
"sprinkle coated" with braze compound copper/gold in a ratio of
65%/35%, with the excess shaken off. Between 1.250 and 1.300 grams
(often about 1.272 grams) of braze coated copper powder 27 is then
placed on the braze coated copper surface and mandrel 40 is placed
on top to form a grooved brazed wick structure 38.
[0043] Significantly groove-wick 55 is formed so as to be thin
enough that the conduction delta-T is small enough to prevent
boiling from initiating at the interface between inner surface 18
of bottom wall 12 and the brazed powder forming the wick. The
formation of fillets 33 further enhances the thermal conductance of
groove-wick 55. Groove-wick 55 is an extremely thin wick structure
that is fed liquid by spaced lands 52 which provide the required
cross-sectional area to maintain effective working fluid flow. In
cross-section, groove-wick 55 comprises an optimum design when it
comprises the largest possible (limited by capillary limitations)
flat area between lands 52. This area should have a thickness of,
e.g., only one to six copper powder particles. The thinner
groove-wick 55 is, the better performance within realistic
fabrication constraints, as long as the surface area of inner
surface 18 has at least one layer of copper particles that are
thermally and mechanically joined together by a plurality of
fillets 33. This thin wick area takes advantage of the enhanced
evaporative surface area of the groove-wick layer, by limiting the
thickness of groove-wick 55 to no more than a few powder particles
while at the same time having a significantly increased thermal
conductance due to the presence of fillets 33 joining metal
particle 27. This structure has been found to circumvent the
thermal conduction limitations associated with the prior art.
[0044] It is to be understood that the present invention is by no
means limited only to the particular constructions herein disclosed
and shown in the drawings, but also comprises any modifications or
equivalents within the scope of the claims.
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