U.S. patent application number 11/128453 was filed with the patent office on 2005-10-06 for integrated circuit heat pipe heat spreader with through mounting holes.
Invention is credited to Dussinger, Peter M., Minnerly, Kenneth L., Myers, Thomas L., Rosenfeld, John H..
Application Number | 20050217826 11/128453 |
Document ID | / |
Family ID | 35428471 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050217826 |
Kind Code |
A1 |
Dussinger, Peter M. ; et
al. |
October 6, 2005 |
Integrated circuit heat pipe heat spreader with through mounting
holes
Abstract
A heat pipe with superior heat transfer between the heat pipe
and the heat source and heat sink is provided. The heat pipe is
held tightly against the heat source by mounting holes which
penetrate the structure of the heat pipe but are sealed off from
the vapor chamber because they each are located within a sealed
structure such as a pillar or the solid layers of the casing
surrounding the vapor chamber. Another feature of the heat pipe is
the use of 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 of the
wick.
Inventors: |
Dussinger, Peter M.;
(Lititz, PA) ; Myers, Thomas L.; (Lititz, PA)
; Rosenfeld, John H.; (Lancaster, PA) ; Minnerly,
Kenneth L.; (Lititz, PA) |
Correspondence
Address: |
DUANE MORRIS LLP
P. O. BOX 1003
305 NORTH FRONT STREET, 5TH FLOOR
HARRISBURG
PA
17108-1003
US
|
Family ID: |
35428471 |
Appl. No.: |
11/128453 |
Filed: |
May 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11128453 |
May 13, 2005 |
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10841784 |
May 7, 2004 |
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6896039 |
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10841784 |
May 7, 2004 |
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09852322 |
May 9, 2001 |
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09852322 |
May 9, 2001 |
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09310397 |
May 12, 1999 |
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6302192 |
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Current U.S.
Class: |
165/104.26 ;
257/E23.088 |
Current CPC
Class: |
F28D 15/046 20130101;
H01L 23/427 20130101; B22F 3/11 20130101; B22F 2999/00 20130101;
H01L 23/4006 20130101; B22F 3/11 20130101; B22F 3/1035 20130101;
F28D 15/0233 20130101; H01L 2924/00 20130101; B22F 2999/00
20130101; H01L 2924/0002 20130101; H01L 2924/0002 20130101; F28F
2275/04 20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 015/00 |
Claims
1-41. (canceled)
42. A heat pipe for spreading heat comprising: a boundary structure
including spaced-apart first and second plates that define an
enclosed vapor chamber having a capillary structure 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; at least one opening
defined by a surrounding portion of said first plate which is
bonded to said second plate so that said opening is isolated from
said vapor chamber.
43. A heat pipe for spreading heat according to claim 42 comprising
at least one spacer positioned within said vapor chamber and
extending between and contacting said first and second plates.
44. A heat pipe for spreading heat according to claim 42 wherein
said spaced-apart first and second plates include confronting
interior surfaces; and a wick positioned upon said confronting
interior surfaces including that portion of the interior surface of
said first plate that forms said surrounding portion.
45. A heat pipe for spreading heat according to claim 44 wherein
said wick is constructed with at least two separate sections of
different materials, with one section being located on said first
plate interior surface and being formed of a material with higher
heat conductivity than sections located on said second plate
interior surface.
46. A heat pipe for spreading heat according to claim 42 wherein
said first and second plates each include a peripheral lip located
at an edge of said boundary structure which are bonded
together.
47. A heat pipe for spreading heat according to claim 42 wherein
said brazing compound comprises about sixty-five percent weight
copper and thirty-five percent weight gold particles such that said
fillets of said brazing compound are formed between adjacent ones
of said plurality of particles so as to create a network of
capillary passageways between said particles.
48. A heat pipe for spreading heat according to claim 42 wherein
said fillets are formed by capillary action of said braze compound
when in a molten state.
49. A heat pipe for spreading heat according to claim 42 wherein
said metal particles are selected from the group consisting of
carbon, tungsten, copper, aluminum, magnesium, nickel, gold,
silver, aluminum oxide, and beryllium oxide.
50. A heat pipe for spreading heat according to claim 42 wherein
said metal particles comprise a shape selected from the group
consisting of spherical, oblate spheroid, prolate spheroid,
ellipsoid, polygonal, and filament.
51. A heat pipe for spreading heat according to claim 42 wherein
said metal particles comprise at least one of copper spheres and
oblate copper spheroids having a melting point of about one
thousand eighty-three .degree. C.
52. A heat pipe for spreading heat according to claim 42 wherein
said brazing compound comprises six percent by weight of a finely
divided copper/gold brazing compound.
53. A heat pipe for spreading heat according to claim 42 wherein
said brazing compound is present in the range from about two
percent to about ten percent by weight.
54. A heat pipe for spreading heat according to claim 42 wherein
said braze compound is selected from the group consisting of
nickel-based Nicrobrazes, silver/copper brazes, tin/silver,
lead/tin, and polymers.
55. A heat pipe for spreading heat according to claim 42 wherein
said plurality of particles comprise aluminum and magnesium and
said brazing compound comprises an aluminum/magnesium intermetallic
alloy.
56. A heat pipe for spreading heat comprising: a boundary structure
including spaced-apart first and second plates that define an
enclosed vapor chamber having a capillary structure 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; at least one opening
defined by a surrounding portion of said first plate which is
bonded to said second plate so that said opening is isolated from
said vapor chamber; and at least one post formed on said second
plate which projects into said vapor chamber and is bonded to said
first plate.
57. A heat pipe for spreading heat according to claim 56 wherein
said at least one post formed on said second plate comprises a flat
portion that is in contact with an inner surface of said first
plate.
58. A heat pipe for spreading heat according to claim 56 wherein
said spaced-apart first and second plates include confronting
interior surfaces; and a wick positioned upon said confronting
interior surfaces including that portion of the interior surface of
said first plate that forms said surrounding portion.
59. A heat pipe for spreading heat according to claim 56 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.
60. A heat pipe for spreading heat according to claim 56 wherein
said brazing compound comprises about sixty-five percent weight
copper and thirty-five percent weight gold particles such that said
fillets of said brazing compound are formed between adjacent ones
of said plurality of particles so as to create a network of
capillary passageways between said particles.
61. A heat pipe for spreading heat according to claim 56 wherein
said fillets are formed by capillary action of said braze compound
when in a molten state.
62. A heat pipe for spreading heat according to claim 56 wherein
said metal particles are selected from the group consisting of
carbon, tungsten, copper, aluminum, magnesium, nickel, gold,
silver, aluminum oxide, and beryllium oxide.
63. A heat pipe for spreading heat according to claim 56 wherein
said particles comprise a shape selected from the group consisting
of spherical, oblate spheroid, prolate spheroid, ellipsoid,
polygonal, and filament.
64. A heat pipe for spreading heat according to claim 56 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.
65. A heat pipe for spreading heat according to claim 60 wherein
said brazing compound comprises six percent by weight of a finely
divided copper/gold brazing compound.
66. A heat pipe for spreading heat according to claim 60 wherein
said brazing compound is present in the range from about two
percent to about ten percent by weight.
67. A heat pipe for spreading heat according to claim 60 wherein
said particles comprise copper powder comprising particles sized in
a range from about twenty mesh to about two-hundred mesh.
68. A heat pipe for spreading heat according to claim 56 wherein
said braze compound is selected from the group consisting of
nickel-based Nicrobrazes, silver/copper brazes, tin/silver,
lead/tin, and polymers.
69. A heat pipe for spreading heat according to claim 56 wherein
said plurality of particles comprise aluminum and magnesium and
said brazing compound comprises an aluminum/magnesium intermetallic
alloy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/852,322, filed May 9, 2001, which is a
continuation of U.S. patent application Ser. No. 09/310,397, filed
May 12, 1999, now U.S. Pat. No. 6,302,192.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to active solid state
devices, and more specifically to a heat pipe for cooling an
integrated circuit chip, with the heat pipe designed to be held in
direct contact with the integrated circuit.
[0003] As integrated circuit chips decrease in size and increase in
power, the required heat sinks and heat spreaders have grown to be
larger than the chips. Heat sinks are most effective when there is
a uniform heat flux applied over the entire heat input surface.
When a heat sink with a large heat input surface is attached to a
heat source of much smaller contact area, there is significant
resistance to the flow of heat along the heat input surface of the
heat sink to the other portions of the heat sink surface which are
not in direct contact with the contact area of the integrated
circuit chip. Higher power and smaller heat sources, or heat
sources which are off center from the heat sink, increase the
resistance to heat flow to the balance of the heat sink. This
phenomenon can cause great differences in the effectiveness of heat
transfer from various parts of a heat sink. The effect of this
unbalanced heat transfer is reduced performance of the integrated
circuit chip and decreased reliability due to high operating
temperatures.
[0004] The brute force approach to overcoming the resistance to
heat flow within heat sinks which are larger than the device being
cooled is to increase the size of the heat sink, increase the
thickness of the heat sink surface which contacts the device to be
cooled, increase the air flow which cools the heat sink, or reduce
the temperature of the cooling air. However, these approaches
increase weight, noise, system complexity, and expense.
[0005] It would be a great advantage to have a simple, light weight
heat sink for an integrated circuit chip which includes an
essentially isothermal surface even though only a part of the
surface is in contact with the chip, and also includes a simple
means for assuring intimate contact with the integrated circuit
chip to provide good heat transfer between the chip and the heat
sink.
SUMMARY OF THE INVENTION
[0006] The present invention is an inexpensive heat pipe heat
spreader for integrated circuit chips which is of simple, light
weight construction. It is easily manufactured, requires little
additional space, and provides additional surface area for cooling
the integrated circuit and for attachment to heat transfer devices
for moving the heat away from the integrated circuit chip to a
location from which the heat can be more easily disposed of.
Furthermore, the heat pipe heat spreader is constructed to assure
precise flatness and to maximize heat transfer from the heat source
and to the heat sink, and has holes through its body to facilitate
mounting.
[0007] The heat spreader of the present invention is a heat pipe
which requires no significant modification of the circuit board or
socket because it is held in intimate contact with the integrated
circuit chip by conventional screws attached to the integrated
mounting board. This means that the invention uses a very minimum
number of simple parts. Furthermore, the same screws which hold the
heat spreader against the chip can also be used to clamp a finned
heat sink to the opposite surface of the heat spreader.
[0008] The internal structure of the heat pipe is an evacuated
vapor chamber with a limited amount of liquid and includes a
pattern of spacers extending between and contacting the two plates
or any other boundary structure forming the vapor chamber. The
spacers prevent the plates from bowing inward, and therefore
maintain the vital flat surface for contact with the integrated
circuit chip. These spacers can be solid columns, embossed
depressions formed in one of the plates, or a mixture of the two.
Porous capillary wick material also covers the inside surfaces of
the heat pipe and has a substantial thickness surrounding the
surfaces of the spacers within the heat pipe, thus forming pillars
of porous wick surrounding the supporting spacers. The wick
therefore spans the space between the plates in multiple locations,
and 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.
[0009] The spacers thus serve important purposes. They support the
flat plates and prevent them from deflecting inward and distorting
the plates to deform the flat surfaces which are required for good
heat transfer. The spacers also serve as critical support for the
portions of the capillary wick pillars which span the space between
the plates provide a gravity independent characteristic to the heat
spreader, and the spacers around which the wick pillars are located
assure that the capillary wick is not subjected to destructive
compression forces.
[0010] The spacers also make it possible to provide holes into and
through the vapor chamber, an apparent inconsistency since the heat
pipe vacuum chamber is supposed to be vacuum tight. This is
accomplished by bonding the spacers, if they are solid, to both
plates of the heat pipe, or, if they are embossed in one plate,
bonding the portions of the depressions which contact the opposite
plate to that opposite plate. With the spacer bonded to one or both
plates, a through hole can be formed within the spacer and it has
no effect on the vacuum integrity of the heat pipe vapor chamber,
from which the hole is completely isolated.
[0011] An alternate embodiment of the invention provides the same
provision for mounting the heat pipe spreader with simple screws
even when the heat pipe is constructed without internal spacers.
This embodiment forms the through holes in the solid boundary
structure around the outside edges of the two plates. This region
of the heat pipe is by its basic function already sealed off from
the vapor chamber by the bond between the two plates, and the only
additional requirement for forming a through hole within it is that
the width of the bonded region be larger than the diameter of the
hole. Clearly, with the holes located in the peripheral lips, the
heat pipe boundary structure can be any shape.
[0012] Another alternative embodiment of the invention provides for
improved heat transfer between the integrated circuit chip and the
heat pipe heat spreader. This is accomplished by using a different
capillary wick material within the heat pipe at the location which
is directly in contact with the chip. Instead of using the same
sintered copper powder wick which is used throughout the rest of
the heat pipe, the part of the wick which is on the region of the
heat pipe surface which is in contact with the chip is constructed
of higher thermal conductivity sintered powder. Such powder can be
silver, diamond, or many other materials well known in the art.
This provides for significantly better heat transfer in the most
critical heat transfer area, right at the integrated circuit
chip.
[0013] The present invention thereby provides a heat pipe superior
heat transfer characteristics, and the simplest of all mounting
devices, just several standard screws.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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:
[0015] FIG. 1 is a cross-sectional view of one embodiment of a flat
plate heat pipe with through holes through its vapor chamber and in
contact with a finned heat sink;
[0016] FIG. 2 is a cross-sectional view of the flat plate heat pipe
shown in FIG. 1, with the finned heat sink removed for clarity of
illustration;
[0017] FIG. 3 is a plan view of the flat plate heat pipe shown in
FIGS. 1 and 2;
[0018] FIG. 4 is an exploded and enlarged view of a portion of the
wick structure formed in accordance with the present invention;
[0019] FIG. 5 is a representation of a brazed wick formed in
accordance with one embodiment of the present invention; and
[0020] FIG. 6 is a representation of another brazed wick formed in
accordance with a further embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Heat pipe 10 is constructed by forming a boundary structure
by sealing together two formed plates, contact plate 18 and cover
plate 20. Contact plate 18 and cover plate 20 are sealed together
at their peripheral lips 22 and 24 by conventional means, such as
soldering or brazing, to form heat pipe 10. Heat pipe 10 is then
evacuated to remove all non-condensible gases and a suitable
quantity of heat transfer fluid is placed within it. This is the
conventional method of constructing a heat pipe, and is well
understood in the art of heat pipes.
[0022] The interior of heat pipe 10 is, however, constructed
unconventionally. While contact plate 18 is essentially flat with
the exception of peripheral lip 24, cover plate 20 includes
multiple depressions 26. Depressions 26 are formed and dimensioned
so that, when contact plate 18 and cover plate 20 are joined, the
flat portions of depressions 26 are in contact with inner surface
28 of contact plate 18. Depressions 26 thereby assure that the
spacing between contact plate 18 and cover plate 20 will be
maintained even through pressure differentials between the inside
volume of heat pipe 10 and the surrounding environment might
otherwise cause the plates to deflect toward each other.
[0023] Heat pipe 10 also includes internal sintered metal capillary
wick 30 which covers the entire inside surface of contact plate 18.
As is well understood in the art of heat pipes, a capillary wick
provides the mechanism by which liquid condensed at the cooler
condenser of a heat pipe is transported back to the hotter
evaporator where it is evaporated. The vapor produced at the
evaporator then moves to the condenser where it again condenses.
The two changes of state, evaporation at the hotter locale and
condensation at the cooler site, are what transport heat from the
evaporator to the condenser.
[0024] In the present invention, heat pipe 10 also has capillary
wick pillars 32 which bridge the space between contact plate 18 and
cover plate 20. Pillars 32 thereby interconnect cover plate 16 and
contact plate 14 with continuous capillary wick. This geometry
assures that, even if heat pipe 10 is oriented so that cover plate
16 is lower than contact plate 14, liquid condensed upon inner
surface 34 of cover plate 20 will still be in contact with
capillary pillars 32. The liquid will therefore be moved back to
raised surface 28 which functions as the evaporator because it is
in contact with a heat generating integrated circuit (not shown).
Capillary pillars 32 are wrapped around and supported by
depressions 26, which prevents the structurally weaker capillary
pillars 32 from suffering any damage.
[0025] FIG. 1 also shows frame 36 which is typically used to
surround and protect heat pipe 10. Frame 34 completely surrounds
heat pipe 10 and contacts lip 24 of contact plate 18. When heat
pipe 10 is used to cool an integrated circuit chip (not shown)
which is held against contact plate 18, cover plate 20 is held in
intimate contact with fin plate 38, to which fins 16 are connected.
The entire assembly of heat pipe 10, frame 34, and fin plate 38 is
held together and contact plate 18 is held against an integrated
circuit chip by conventional screws 40, shown in dashed lines,
which are placed in holes 42 in fin plate 38 and through holes 12
in heat pipe 10, and are threaded into the mounting plate (not
shown) for the integrated circuit chip.
[0026] Holes 12 penetrate heat pipe 10 without destroying its
vacuum integrity because of their unique location. Holes 12 are
located within sealed structures such as solid columns 44, and
since columns 44 are bonded to cover plate 20 at locations 46,
holes 12 passing through the interior of columns 44 have no affect
on the interior of heat pipe 10.
[0027] The preferred embodiment of the invention has been
constructed as heat pipe 10 as shown in FIG. 1. This heat pipe is
approximately 3.0 inches by 3.5 inches with a total thickness of
0.200 inch. Cover plate 20 and contact plate 18 are constructed of
OFHC copper 0.035 inch thick, and depressions 26 span the 0.100
inch height of the internal volume of heat pipe 10. The flat
portions of depressions 26 are 0.060 inch in diameter. Capillary
wick 30 is constructed of sintered copper powder and averages 0.040
inch thick. Columns 44 have a 0.250 inch outer diameter, and holes
12 are 0.210 in diameter.
[0028] FIG. 2 is a cross section view of an alternate embodiment of
the flat plate heat pipe 11 of the invention with through holes 48
located within peripheral lips 22 and 24 of the heat pipe and hole
50 shown in another sealed structure, one of the depressions 26.
The only requirement for forming hole 50 within a depression 26 is
that the bottom of depression 26 must be bonded to inner surface 28
of contact plate 18 to prevent loss of vacuum within the heat pipe.
Of course, the region of the peripheral edges is also a sealed
structure since bonding between lips 22 and 24 is inherent because
heat pipe 11 must be sealed at its edges to isolate the interior
from the outside atmosphere.
[0029] The only differences between heat pipe 11 of FIG. 2 and heat
pipe 10 of FIG. 1 are that finned heat sink 16 is not shown in FIG.
2, lips 22 and 24 are slightly longer in FIG. 2 to accommodate
holes 48, and hole 50 is shown. In fact, through holes 12 shown in
FIG. 12 are also included in FIG. 2. Although it is unlikely that
holes 12, holes 48, and hole 50 would be used in the same assembly,
manufacturing economies may make it desirable to produce all the
holes in every heat pipe so that the same heat pipe heat spreader
can be used with different configurations of finned heat sinks. The
unused sets of holes have no effect on the operation or benefits of
the invention.
[0030] FIG. 3 is a plan view of the internal surface of the contact
plate 18 of the heat pipe 10 of the invention showing region 31 of
capillary wick 30. Region 31 is constructed of sintered silver
powder. While the balance of capillary wick 30 is conventional
sintered metal such as copper, region 31 of capillary wick 30,
which is on the opposite surface of contact plate 18 from the
integrated circuit chip (not shown), is formed of powdered silver.
The higher thermal conductivity of silver yields significantly
better heat conduction through region 31 of the wick 30, and
thereby reduces the temperature difference between the integrated
circuit chip and the vapor within heat pipe 10. This reduction of
temperature difference directly affects the operation of heat pipe
10, and essentially results in a similar reduction in the operating
temperature of the chip.
[0031] In one embodiment of the present invention, a brazed wick 65
is located on the inner surface of contact 18. Brazed wick 65
comprises a plurality of metal particles 67 combined with a filler
metal or combination of metals that is often referred to as a
"braze" or brazing compound 70. It will be understood that
"brazing" is the joining of metals through the use of heat and a
filler metal, i.e., brazing compound 70. Brazing compound 70 very
often comprises a melting temperature that is above 450.degree.
C.-1000 C but below the melting point of metal particles 67 that
are being joined to form brazed wick 65.
[0032] In general, to form brazed wick 65 according to the present
invention, a plurality of metal particles 67 and brazing compound
70 are heated together to a brazing temperature that melts brazing
compound 70, but does not melt plurality of metal particles 67.
Significantly, during brazing metal particles 67 are not fused
together as with sintering, but instead are joined together by
creating a metallurgical bond between brazing compound 70 and the
surfaces of adjacent metal particles 67 through the creation of
fillets of re-solidified brazing compound (identified by reference
numeral 73 in FIGS. 5 and 6). Advantageously, the principle by
which brazing compound 70 is drawn through the porous mixture of
metal particles 67 to create fillets 73 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 70 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 67 tends to draw the molten braze toward
each location where adjacent metal particles 67 are in contact with
one another. Fillets 73 are formed at each such location as the
molten braze metals re-solidify.
[0033] In the present invention, brazing compound 70 and fillets 73
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, e.g., region 31, is subjected
to, the conductance of brazed wick 65 has been found to increase
between directly proportional to and the square root of the thermal
conductivity increase. Importantly, material components of brazing
compound 70 must be selected so as not to introduce chemical
incompatibility into the materials system comprising flat plate
heat pipe 10.
[0034] Metal particles 67 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 67 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 flat plate heat pipe 10
will be about 1000 C. By varying the percentage brazing compound 70
within the mix of metal particles 67 or, by using a more "sluggish"
alloy for brazing compound 70, a wide range of heat-conduction
characteristics may be provided between metal particles 67 and
fillets 73.
[0035] 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 67). More
or less braze is also possible, although too little braze reduces
the thermal conductivity of brazed wick 65, 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 67, a preferred shape of
particle is spherical or spheroidal. Metal particles 67 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 70 should
often be several times smaller in size than metal particles 67 so
as to create a uniformly brazed wick 65 with uniform
properties.
[0036] Other brazes can also be used for brazing copper wicks,
including nickel-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.
[0037] Brazing compound 70 should often be well distributed over
each metal particle surface. This distribution of brazing compound
70 may be accomplished by mixing brazing compound 70 with an
organic liquid binder, e.g., ethyl cellulose, that creates an
adhesive quality on the surface of each metal particle 67 (i.e.,
the surface of each sphere or spheroid of metal) for brazing
compound 70 to adhere to. In one embodiment of the invention, one
and two tenths grams by weight of copper powder (metal particles
67) 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 67 (i.e.,
the surface of each sphere or spheroid of metal) for braze compound
70 to adhere to. A finely divided (e.g., -325 mesh) of braze
compound 70 is mixed into the liquid binder coated copper powder
particles 67 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 67 and brazing compound 70 are
applied to the internal surfaces of flat plate heat pipe 10, for
example the inner surface contact plate 18 and heated evenly so
that brazing compound 70 is melted by heating metal particles 67.
Molten brazing compound 70 that is drawn by capillary action, forms
fillets 73 as it solidifies within the mixture of metal particles
67. For example, vacuum brazing or hydrogen brazing at about 1020 C
for between two to 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 about 5.times.10.sup.-5 torr or lower.
[0038] It is to be understood that the form of this invention as
shown is merely a preferred embodiment. Various changes may be made
in the function and arrangement of parts; equivalent means may be
substituted for those illustrated and described; and certain
features may be used independently from others without departing
from the spirit and scope of the invention as defined in the
following claims. For example, through holes could also penetrate
heat pipe boundary structures with curved surfaces or heat pipe
boundary structures with offset planes which create several
different levels for contact with heat sources or heat sinks.
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