U.S. patent application number 10/829104 was filed with the patent office on 2005-02-03 for heat transfer device and method of making same.
Invention is credited to Bilski, W. John, Ernst, Donald M., Lindemuth, James E., Rosenfeld, John H..
Application Number | 20050022976 10/829104 |
Document ID | / |
Family ID | 35320832 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050022976 |
Kind Code |
A1 |
Rosenfeld, John H. ; et
al. |
February 3, 2005 |
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 including a first
species having a first diameter and a second species having a
second diameter that are joined together to form a capillary
structure having homogenous layers of particles.
Inventors: |
Rosenfeld, John H.;
(Lancaster, PA) ; Bilski, W. John; (Mohnton,
PA) ; Lindemuth, James E.; (Lititz, PA) ;
Ernst, Donald M.; (Lancaster, PA) |
Correspondence
Address: |
DUANE MORRIS LLP
P. O. BOX 1003
305 NORTH FRONT STREET, 5TH FLOOR
HARRISBURG
PA
17108-1003
US
|
Family ID: |
35320832 |
Appl. No.: |
10/829104 |
Filed: |
April 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10829104 |
Apr 21, 2004 |
|
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10607337 |
Jun 26, 2003 |
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Current U.S.
Class: |
165/104.11 |
Current CPC
Class: |
H01L 2924/00 20130101;
F28F 2275/04 20130101; F28D 15/046 20130101; H01L 2924/0002
20130101; F28D 15/0233 20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
165/104.11 |
International
Class: |
F28D 015/00 |
Claims
What is claimed is:
1. A capillary structure for a heat transfer device comprising: a
plurality of particles comprising a first species of particle and a
second species of particle, said plurality of particles being
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; wherein said first species of particle and
said second species of particle are each disposed within said
capillary structure in homogenous layers.
2. A capillary structure for a heat transfer device according to
claim 1 comprising a plurality of homogeneous layers.
3. A capillary structure for a heat transfer device according to
claim 1 comprising at least three homogeneous layers.
4. A capillary structure for a heat transfer device according to
claim 1 comprising at least three species of particle.
5. A capillary structure for a heat transfer device according to
claim 4 comprising at least three homogenous layers.
6. 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.
7. A capillary structure according to claim 1 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.
8. A capillary structure according to claim 1 wherein said fillets
are formed by capillary action of said braze compound when in a
molten state.
9. A capillary structure according to claim 1 wherein said metal
particles are selected from the group consisting of carbon,
tungsten, copper, aluminum, magnesium, nickel, gold, silver,
aluminum oxide, and beryllium oxide.
10. A capillary structure according to claim 1 wherein said metal
particles comprise a shape selected from the group consisting of
spherical, oblate spheroid, prolate spheroid, ellipsoid, polygonal,
and filament.
11. A capillary structure according to claim 1 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.
12. A capillary structure according to claim 6 wherein said brazing
compound comprises six percent by weight of a finely divided
copper/gold brazing compound.
13. A capillary structure according to claim 6 wherein said brazing
compound is present in the range from about two percent to about
ten percent.
14. A capillary structure according to claim 6 wherein said metal
particles comprise copper powder comprising particles sized in a
range from about twenty mesh to about two-hundred mesh.
15. A capillary structure according to claim 6 wherein said braze
compound particles comprise about minus three hundred and
twenty-five mesh.
16. A capillary structure according to claim 1 wherein said metal
particles that are a constituent portion of said braze compound
comprise a smaller size than said metal particles.
17. 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.
18. A capillary structure according to claim 1 wherein said
plurality of metal particles comprise aluminum and magnesium and
said brazing compound comprises an aluminum/magnesium intermetallic
alloy.
19. A wick for a heat pipe comprising: a plurality of particles
comprising a first diameter and a second diameter, said plurality
of particles being joined together so as to form a network of
capillary passageways between said particles; wherein said first
diameter particles are disposed within a first substantially
homogenous layer and said second diameter particles are disposed
within a second substantially homogenous layer.
20. A wick according to claim 19 comprising a plurality of
homogenous layers.
21. A heat pipe comprising: a sealed enclosure having an interior
surface; a working fluid disposed within said enclosure; and a
wicking structure disposed upon said interior surface and
comprising a plurality of particles including a first species of
particle and a second species of particle, said plurality of
particles being joined together so as to form a network of
capillary passageways between said particles; wherein said first
species of particle and said second species of particle are each
disposed within said wicking structure in substantially homogenous
layers.
22. A capillary structure for a heat transfer device comprising: a
plurality of particles comprising a first species of particle
having a first size and a second species of particle having a
second size, said plurality of particles being 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;
wherein said first species of particle and said second species of
particle are each disposed within said capillary structure in
substantially homogenous layers, wherein a plurality of vapor vents
are defined through said capillary structure.
23. A capillary structure according to claim 22 wherein said vapor
vents comprise a cross-sectional profile selected from the group
consisting of cylindrical, conical, frustoconical, triangular,
pyramidal, rectangular, rhomboidal, pentagonal, hexagonal,
octagonal, polygonal and curved.
24. A capillary structure for a heat transfer device comprising: a
plurality of particles comprising a first species of particle
having a first average particle diameter and a second species of
particle having a second average particle diameter wherein said
plurality of particles are joined together so as to form a network
of capillary passageways between said particles, and further
wherein said first species of particle and said second species of
particle are each disposed within said capillary structure in
substantially homogeneous layers; and a plurality of blind bores
are defined through said capillary structure such that each blind
bore has a closed end comprising a particle layer that comprises at
least one dimension that is no more than about six average particle
diameters of at least one of said first species and said second
species of particle.
25. A capillary structure for a heat transfer device comprising: a
plurality of particles comprising a first species of particle
having a first diameter, and a second species of particle having a
second diameter wherein said plurality of particles are joined
together in substantially homogenous layers so as to form a network
of capillary passageways between said particles, wherein a
plurality of blind bores are defined through said homogenous layers
of particles such that each blind bore has a closed end comprising
a vent-wick layer that comprises at least one dimension that is no
more than about six average particle diameters of at least one of
said first species and said second species of particle.
26. 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 comprising a first species of
particle having a first size and a second species of particle
having a second size, said plurality of particles being joined
together so as to form a network of capillary passageways between
said particles; wherein said first species of particle and said
second species of particle are each disposed within said wick in
substantially homogenous layers; and a two-phase fluid at least
partially disposed within a portion of said wick.
27. A heat pipe according to claim 26 comprising graded
substantially homogeneous layers.
28. A heat pipe according to claim 26 comprising transversely
graded substantially homogeneous layers.
29. 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 comprising a first species of
particle having a first diameter and a second species of particle
having a second diameter, said plurality of particles being joined
together so as to form a network of capillary passageways between
said particles; wherein said first species of particle and said
second species of particle are each disposed within said wick in
substantially homogenous layers and further wherein a plurality of
blind bores are defined through said capillary structure such that
each blind bore has a closed end comprising a vent-wick layer that
comprises at least one dimension that is no more than about six
average particle diameters of at least one of said first species
and said second species of particle; and a two-phase fluid at least
partially disposed within a portion of said wick.
30. A heat pipe according to claim 29 wherein said vent-wick layer
at the closed end of each of said blind bores comprises said first
species of particle.
31. A heat pipe according to claim 29 wherein said vent-wick layer
at the closed end of each of said blind bores comprises said second
species of particle.
32. A heat pipe according to claim 29 comprising a plurality of
homogeneous layers formed by those portions of said first species
of particles and said second species of particles disposed around
said vent-wick layer at said closed end of each of said plurality
of blind-bores.
33. A heat pipe according to claim 29 comprising at least three
homogeneous layers formed by those portions of said first species
of particles and said second species of particles disposed around
said vent-wick layer at said closed end of each of said plurality
of blind-bores.
34. 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 comprising a first species of
particle having a first size and a second species of particle
having a second size, said plurality of particles being joined
together so as to form a network of capillary passageways between
said particles; wherein said first species of particle and said
second species of particle are each disposed within said wick in
graded homogenous layers, and further wherein at least one vapor
vent is defined through said wick; and a two-phase fluid at least
partially disposed within a portion of said wick.
35. A heat pipe comprising a sealed and partially evacuated tubular
enclosure having an internal surface covered by a brazed wick
comprising a plurality of copper particles comprising a first
species of particle having a first diameter and a second species of
particle having a second diameter, and joined together by a brazing
compound comprising about sixty-five percent weight copper and
thirty-five percent weight gold 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 wherein said first species of particle and said
second species of particle are each disposed within said wick in
substantially homogenous layers; and a working fluid disposed
within said tubular enclosure.
36. A heat pipe according to claim 35 wherein said metal particles
are selected from the group consisting of carbon, tungsten, copper,
aluminum, magnesium, nickel, gold, silver, aluminum oxide, and
beryllium oxide.
37. A heat pipe according to claim 36 wherein said metal particles
comprise a shape selected from the group consisting of spherical,
oblate spheroid, prolate spheroid, polygonal, and filament.
38. A heat pipe comprising a sealed and partially evacuated tubular
enclosure having an internal surface covered by a brazed wick
comprising a plurality of copper particles comprising a first
species of particle having a first diameter and a second species of
particle having a second diameter, and joined together by a brazing
compound comprising about sixty-five percent weight copper and
thirty-five percent weight gold 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 wherein said first species of particle and said
second species of particle are each disposed within said wick in a
plurality of substantially homogenous layers, and including a
plurality of vapor vents defined through said wick; and a working
fluid disposed within said tubular enclosure.
39. A heat pipe comprising: a sealed and partially evacuated
enclosure having an internal surface; a wick disposed upon said
internal surface comprising a plurality of sintered particles
comprising a first species of particle, a second species of
particle, and a third species of particle, wherein said first
species of particle, said second species of particle, and said
third species of particle are each disposed within said wick in
substantially homogenous layers; and a working fluid disposed
within said enclosure.
40. A heat pipe comprising: a sealed and partially evacuated
enclosure having an internal surface; a wick disposed upon said
internal surface comprising a plurality of sintered particles
comprising a first species of particle, a second species of
particle, and a third species of particle, wherein said first
species of particle, said second species of particle, and said
third species of particle are each disposed within said wick in
substantially homogenous layers, and further including at least one
vapor vent that is defined through a portion of said wick; and a
working fluid disposed within said enclosure.
41. A heat pipe comprising: a sealed and partially evacuated
tubular enclosure being sealed at a first end and having an
internal surface covered by a brazed wick comprising a plurality of
particles comprising a first species of particle and a second
species of particle, said first species and said second species of
particle being joined together by a brazing compound such that
fillets of said brazing compound are formed between adjacent ones
of said particles so as to form a network of capillary passageways
between said particles; a base sealingly fixed to a second end of
said enclosure so as to form an internal surface within said
enclosure wherein said wick is formed on said base including said
first species of particles and said second species of particles
each disposed within said wick in substantially homogenous layers;
a working fluid disposed within said enclosure; and at least one
fin projecting radially outwardly from an outer surface of said
tubular enclosure.
42. A heat pipe comprising: a sealed and partially evacuated
tubular enclosure being sealed at a first end and having an
internal surface covered by a brazed wick comprising a plurality of
particles comprising a first species of particle and a second
species of particle, said first species and said second species of
particle being joined together so as to form a network of capillary
passageways between said particles; a base sealingly fixed to a
second end of said enclosure so as to form an internal surface
within said enclosure wherein said wick is formed on said base
including said first species of particles and said second species
of particles each disposed within said wick in substantially
homogenous layers, and further including at least one vapor vent
that is defined through a portion of said wick; a working fluid
disposed within said enclosure; and at least one fin projecting
radially outwardly from an outer surface of said tubular enclosure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 10/607,337, filed Jun. 26, 2003.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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).
[0007] 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.
[0008] 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
[0009] The present invention provides a capillary structure for a
heat transfer device comprising a plurality of particles including
a first species of particle and a second species of particle. The
plurality of particles are 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. The first species
of particle and the second species of particle are each disposed
within the capillary structure in homogenous layers.
[0010] In one alternative embodiment, a wick for a heat pipe is
provided including a plurality of particles comprising a first
diameter and a second diameter. The plurality of particles are
joined together by, e.g., sintering or brazing, so as to form a
network of capillary passageways between the particles. The first
diameter particles are disposed within a first homogenous layer and
the second diameter particles are disposed within a second
homogenous layer so as to enhance the thermal transfer properties
of the wick.
[0011] In another alternative embodiment, a heat pipe is provided
including a sealed enclosure having an interior surface and a
working fluid disposed within the enclosure. A wicking structure is
disposed upon the interior surface and comprises a plurality of
particles including a first species of particle and a second
species of particle. The plurality of particles are joined together
by a brazing compound such that fillets of the brazing compound are
formed between adjacent ones of the plurality of particles thereby
forming a network of capillary passageways between the particles.
The first species of particle and the second species of particle
are each disposed within the wick structure in homogenous
layers.
[0012] In a further embodiment, a capillary structure for a heat
transfer device is provided including a plurality of particles
comprising a first species of particle and a second species of
particle. The plurality of particles are joined together so as to
form a network of capillary passageways between the particles. The
first species of particle and the second species of particle are
each disposed within the capillary structure in homogenous layers,
and a plurality of vapor vents are defined through the capillary
structure.
[0013] In another embodiment, a capillary structure for a heat
transfer device is provided including a plurality of particles
comprising a first species of particle having a first diameter, and
a second species of particle having a second diameter. The
plurality of particles are 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. A plurality of
blind bores are defined through the capillary structure such that
each blind bore has a closed end defined by a particle layer that
comprises at least one dimension that is no more than about six
average particle diameters of at least one of the first species and
the second species of particle.
[0014] In a further embodiment, a capillary structure for a heat
transfer device is provided that includes a plurality of particles
comprising a first species of particle having a first diameter and
a second species of particle having a second diameter. The
plurality of particles are joined together so as to form a network
of capillary passageways between the particles. A plurality of
blind bores are defined through the capillary structure such that
each blind bore has a closed end formed by a particle layer that
comprises at least one dimension that is no more than about six
average particle diameters of at least one of the first species and
the second species of particle.
[0015] In yet a further embodiment, a heat pipe is provided that is
formed from a hermetically sealed and partially evacuated enclosure
that includes internal surfaces. A wick is disposed on at least one
of the internal surfaces and includes a plurality of particles
comprising a first species of particle and a second species of
particle. The plurality of particles are joined together so as to
form a network of capillary passageways between the particles. The
first species of particle and the second species of particle are
each disposed within the wick in homogenous layers. A two-phase
fluid is at least partially disposed within a portion of the
wick.
[0016] In another embodiment, a heat pipe is provided that is
formed by a hermetically sealed and partially evacuated enclosure
having internal surfaces. A wick is disposed on at least one of the
internal surfaces and includes a plurality of particles comprising
a first species of particle having a first diameter and a second
species of particle having a second diameter. The plurality of
particles are joined together so as to form a network of capillary
passageways between the particles. The first species of particle
and the second species of particle are each disposed within the
wick in homogenous layers, and a plurality of blind bores are
defined through the capillary structure such that each blind bore
has a closed end comprising a particle layer that comprises at
least one dimension that is no more than about six average particle
diameters of at least one of the first species and the second
species of particle. A two-phase fluid is at least partially
disposed within a portion of the wick.
[0017] In an alternative embodiment, a heat pipe is provided that
is formed by a hermetically sealed and partially evacuated
enclosure having internal surfaces. A wick is disposed on at least
one of the internal surfaces and includes a plurality of particles
comprising a first species of particle and a second species of
particle. The plurality of particles are joined together so as to
form a network of capillary passageways between the particles. The
first species of particle and the second species of particle are
each disposed within the wick in homogenous layers, and at least
one vapor vent is defined through the wick. A two-phase fluid is at
least partially disposed within a portion of the wick.
[0018] In a further alternative embodiment, a heat pipe is provided
that is formed in a sealed and partially evacuated tubular
enclosure having an internal surface covered by a brazed wick. The
brazed wick comprises a plurality of copper particles including a
first species of particle having a first diameter and a second
species of particle having a second diameter. The particles are
joined together by a brazing compound comprising about sixty-five
percent weight copper and thirty-five percent weight gold 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. The first species of particle
and the second species of particle are each disposed within the
wick in homogenous layers. A working fluid is disposed within the
tubular enclosure to effect heart pipe activity.
[0019] In another embodiment, a heat pipe is provided that is
formed in a sealed and partially evacuated tubular enclosure having
an internal surface covered by a brazed wick. The brazed wick
includes a plurality of copper particles comprising a first species
of particle having a first diameter and a second species of
particle having a second diameter. The particles are joined
together by a brazing compound comprising about sixty-five percent
weight copper and thirty-five percent weight gold 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. The first species of particle
and the second species of particle are each disposed within the
wick in homogenous layers, and a plurality of vapor vents are
defined through the wick. A working fluid is disposed within the
tubular enclosure to effect heart pipe activity.
[0020] In a further embodiment, a heat pipe is provided that is
formed in a sealed and partially evacuated enclosure having an
internal surface. A wick is disposed upon the internal surface
including a plurality of particles comprising a first species of
particle, a second species of particle, and a third species of
particle. The first species of particle, the second species of
particle, and the third species of particle are each disposed
within the wick in homogenous layers, and a working fluid is
disposed within the enclosure to effect heart pipe activity.
[0021] In another alternative embodiment, a heat pipe is provided
that is formed in a sealed and partially evacuated enclosure having
an internal surface. A wick is disposed upon the internal surface
including a plurality of particles comprising a first species of
particle, a second species of particle, and a third species of
particle. The first species of particle, the second species of
particle, and the third species of particle are each disposed
within the wick in homogenous layers. At least one vapor vent is
defined through a portion of the wick, and a working fluid is
disposed within the enclosure to effect heart pipe activity.
[0022] In an additional embodiment, a heat pipe is provided that is
formed in a sealed and partially evacuated tubular enclosure having
an internal surface covered by a brazed wick. The brazed wick
includes a plurality of particles comprising a first species of
particle having a first diameter and a second species of particle
having a second diameter. The first species and the second species
of particle are joined together by a brazing compound such that
fillets of the brazing compound are formed between adjacent ones of
the particles so as to form a network of capillary passageways
between the particles. The enclosure is sealed at a first end. A
base is sealingly fixed to a second end of the enclosure so as to
form an internal surface within the enclosure. The wick is formed
on the base so as to include the first species particles and the
second species of particles each disposed within the wick in
homogenous layers. A working fluid is disposed within the
enclosure, and at least one fin projects radially outwardly from an
outer surface of the tubular enclosure.
[0023] In another embodiment, a heat pipe is provided that is
formed in a sealed and partially evacuated tubular enclosure having
an internal surface covered by a wick. The wick includes a
plurality of particles comprising a first species of particle and a
second species of particle. The first species and the second
species of particle are joined together so as to form a network of
capillary passageways between the particles. The enclosure is
sealed at a first end and a base is sealingly fixed to a second end
of the enclosure so as to form an internal surface within the
enclosure. The wick is formed on the base, and includes the first
species particles and the second species of particles each disposed
within the wick in homogenous layers, with at least one vapor vent
defined through a portion of the wick. A working fluid is disposed
within the enclosure, and at least one fin projects radially
outwardly from an outer surface of the tubular enclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] 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:
[0025] FIG. 1 is an exploded perspective view of a typical heat
pipe enclosure of the type used in connection with the present
invention;
[0026] FIG. 2 is a perspective view of the heat pipe enclosure
shown in FIG. 1;
[0027] FIG. 3 is a cross-sectional view of the heat pipe shown in
FIG. 2;
[0028] 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;
[0029] 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;
[0030] 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;
[0031] 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;
[0032] FIG. 8 is a cross-sectional view, as taken along lines 8-8
in FIG. 7;
[0033] FIG. 9 is a further alternative embodiment of heat pipe
enclosure formed in accordance with the present invention;
[0034] 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;
[0035] 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;
[0036] FIG. 12 is a perspective cross-sectional view of a tower
heat pipe having a brazed wick formed in accordance with the
present invention;
[0037] 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;
[0038] FIG. 14 is an alternative embodiment of tower heat pipe
having grooved base wick formed in accordance with the present
invention;
[0039] FIG. 15 is a highly enlarged surface view of a brazed wick
formed in accordance with the present invention;
[0040] FIG. 16 is a broken-way cross-sectional view of the
groove-wick shown in FIGS. 7, 8, and 14;
[0041] FIG. 17 is a highly enlarged cross-sectional view of a
portion of the groove brazed wick shown in FIGS. 7, 8, 14, and
16;
[0042] FIG. 18 is an end view of a mandrel used in manufacturing a
grooved brazed wick in accordance with the present invention.
[0043] FIG. 19 is a further alternative embodiment of tower heat
pipe having vapor vents formed in a wick structure in accordance
with the present invention;
[0044] FIGS. 20-30 comprise a group of top elevational views and
perspective cross-sectional views of a variety of possible wick
structures having vapor vents formed in accordance with the present
invention;
[0045] FIG. 31 is an exploded perspective view of a heat pipe heat
spreader including a wick structure having vapor vents formed in
accordance with the present invention;
[0046] FIG. 32 is a perspective view of the heat pipe heat spreader
shown in FIG. 31, as assembled;
[0047] FIGS. 33-35 are top elevational views of a further variety
of patterns of vapor vents that may be employed with wick
structures formed in accordance with the present invention;
[0048] FIG. 36 is a broken-way, cross-sectional view of an
alternative embodiment of wick structure comprising a graded,
brazed wick formed in accordance with the present invention;
[0049] FIG. 37 is a broken-way, cross-sectional perspective view
similar to FIG. 36, showing a graded sintered wick structure;
[0050] FIG. 38 is a broken-way, cross-sectional view of a
alternatively graded wick structure;
[0051] FIG. 39 is a broken-way, cross-sectional perspective view of
a wick structure comprising a plurality of cylindrical
particles;
[0052] FIG. 40 is a broken-way, cross-sectional view of a further
alternative embodiment of wick structure comprising a transversely
graded wick structure;
[0053] FIG. 41 is a broken-way, cross-sectional perspective view of
a further alternative embodiment of graded wick structure;
[0054] FIGS. 42-43 are broken-way, cross-sectional views of a
portion of a heat pipe heat spreader having a multiple layer graded
wick structure; and
[0055] FIGS. 44-51 comprise a group of top elevational views and
perspective cross-sectional views of a variety of possible wick
structures having vapor vents formed in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0056] 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.
[0057] 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, 14 and 19). Regardless of outer profile, each
enclosure structure defines an evaporator section 5, a condenser
section 7, and an internal void space or vapor space 10 (FIG. 3).
For example, in a planar rectangular heat pipe 2, vapor space 10 is
defined between a bottom wall 12 and a top wall 14. In a tubular or
tower heat pipe 2, vapor space 10 extends longitudinally from one
end of the tube to the other (FIGS. 9, 12, 14, and 19).
[0058] 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 space 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 space 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 space 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 fill tube through which the
working fluid is injected (fill tube is not shown).
[0059] Referring to FIGS. 3-6, in order for heat pipe operation to
be initiated within the enclosure of heat pipe 2, a capillary
structure must be present within vapor space 10 that will pump
condensed liquid from condenser section 7 back to evaporator
sections, substantially unaided by gravity. In one embodiment of
the present invention, a brazed wick 25 is located on inner surface
18 which defines the boundaries of vapor space 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.
[0060] 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 tends 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.
[0061] 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 of wick material. 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.
[0062] 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.
[0063] 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 has
minimal impact on 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.
[0064] 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.
[0065] 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) 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 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.
[0066] 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, with mandrel 40
previously in placed on top to form a grooved brazed wick structure
38.
[0067] 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.
[0068] In yet a further embodiment of the present invention,
groove-wick 55 may be replaced by a wick structure defining a
plurality of vapor-vents 60 that are defined as blind-bores
throughout the evaporator wick structure (FIGS. 19-38). Vapor-vents
60 are defined through a wick structure 62 that comprises either a
uniformly brazed wick 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, Alternatively, a plurality of sintered particles may
also be used to form wick structure 62. In one embodiment,
vapor-vents 60 extend through wick structure 62 so as to expose a
portion of the underlying base structure, e.g., inner surface 18 of
bottom wall 12, onto which the wick is brazed or sintered. Wick
structure 62 may be employed in either a circular or elliptically
shaped portion of a tower-type heat pipe (FIG. 19) or a
rectangularly or polygonally shaped heat spreader configuration
(FIG. 39). The actual shape will of course normally be determined
by the shape of the heat source and the evaporator.
[0069] The cross-sectional profile of vapor-vents 60, and their
grouping and location in wick structure 62, may vary significantly
from device to device or within the same device (FIGS. 20-38). The
cross-sectional profile of vapor-vents 60 may include cylindrical,
conical, frustoconical, triangular, pyramidal, rectangular,
rhomboidal, pentagonal, hexagonal, octagonal, and other less
commonly occurring polygonal or curved shapes. Each vapor vent 60
defines an opening 65 in the upper surface of wick structure 62 and
a blind-bore 67 that may extend downwardly toward, e.g., inner
surface 18 of bottom wall 12, or equivalent structures in a
tower-type heat pipe. Openings 65 and blind-bores 67 are sized,
shaped, and positioned relative to an evaporator portion of heat
pipe 2 dependent upon local heat flux, wick thickness, wick pore
radius, and wick permeability, such that the pressure drop required
to get the vapor out of the evaporator portion is minimized and
therefore the .DELTA.T may be minimized.
[0070] In addition to the various shapes, sizes and positions of
vapor vents 60, the powdered material that forms wick structure 62
may also vary in size and shape. For example, wick structure 62 may
be formed from powdered metal particles 27 that are spherical,
spheroidal, polygonal, or even chopped pieces of fine wire. In
addition, wick structure 62 may be formed from powdered metal
particles 27 comprising a mixture of particles including two or
more distinct species of particles, e.g., a first species of
particle 71 having a first diameter, a second species of particle
73 having a second diameter, or even a third species of particle 76
having a different diameter as particles 71 and 73. Other inherent
characteristics of the particles may also be used to define
distinct species of particle, e.g., length, width, chemical
composition, number of sides, etc. For example, multiple wire
diameters 80 of varying lengths may form multiple species of
particles. Each species of particle is segregated from one another
in homogeneous layers. For example, relatively larger diameter
particles may be located in lower heat flux regions of wick
structure 62, while smaller diameter particles may be located in
higher heat flux regions of wick structure 62 (FIGS. 35, 38, 40,
and 41). In this way, a variety of pore sizes may be created within
each variety of wick structure 62. Typically, the variation in
particle or wire diameters may range from several microns to
several millimeters. In the case where the diameter of the
particles defines the distinguishing characteristic of a species,
proper adjustment of particle sizes, and thus the pore sizes,
allows for vapor to vent through larger pores while liquid remains
in smaller pores thus increasing the critical heat flux limit.
[0071] In addition to having a mixture of two or more powder
particle sizes, a graded wick structure 90 may be employed in the
present invention (FIGS. 36, 37, 42, and 43). In one embodiment of
the invention, a graded wick 90 is formed by layering particles 71,
such that coarse (i.e., relatively large) particles are located in
a homogeneous first layer 92 near the surface of wick structure 90,
with fine particles 73 (i.e., relatively small) located in a second
homogeneous layer 94 deposited in underlying relation to first
layer 92. It will be understood that multiple homogeneous layers of
varying species of particle may be employed in the present
invention as well, such that grading can be done with as many
layers as is needed, with particle and pore sizes varying up to two
orders of magnitude across wick structure 62 (FIGS. 42 and 43).
[0072] Referring to FIGS. 38, 40, and 41, powder particles 27 may
also be arranged in a transversely graded array such that one
species of particle is arranged transversely adjacent to another
species of particle. Alternatively, wick structure 62 may have a
step-wise grading in which powder particles 27 are arranged at
different thicknesses.
[0073] As disclosed hereinabove, powder particles 27 forming wick
structures 62 or 90 may either be brazed or sintered to each other
and the evaporator plate in accordance with the methods herein
disclosed and according to the present invention. Sintering
temperatures vary for each different metal powder as well as being
a function of the size and distribution of powder particles 27
within wick structure 62. In addition, when sintering, an
appropriate protective atmosphere such as hydrogen, forming gas,
vacuum or an inert gas such as helium, nitrogen or argon should be
employed for adequate results.
[0074] Alternatively, a relatively thin layer of either brazed or
sintered metal particles 27 may be deposited upon inner surface 18
of bottom wall 12 so as to form a vent-wick 80 at the bottom of
each vapor-vent 60 (FIGS. 44-51). For example, brazed or sintered
copper powder particles 27 are deposited on inner surface 18 (not
shown in FIGS. 44-51, but identified in FIGS. 20-27) at the closed
end of each vapor-vent 60 such that vent-wick 80 comprises an
average thickness in the range from 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. Often, vent-wick 80 comprises
an average thickness in the range from about one to three average
copper particle diameters.
[0075] 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.
* * * * *