U.S. patent application number 12/317170 was filed with the patent office on 2010-02-04 for high temperature compact thermoelectric module with gapless eggcrate.
Invention is credited to Norbert Elsner, John W. McCoy.
Application Number | 20100024437 12/317170 |
Document ID | / |
Family ID | 41606906 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100024437 |
Kind Code |
A1 |
Elsner; Norbert ; et
al. |
February 4, 2010 |
High temperature compact thermoelectric module with gapless
eggcrate
Abstract
A high-temperature thermoelectric module. The module includes a
two part molded egg-crate for holding in place and providing
insulation and electrical connections for a number of
thermoelectric n-legs and p-legs. A first part of the egg-crate is
comprised of a ceramic material capable of operation at
temperatures in excess of 600.degree. C. and a second part
comprised of a thermoplastic material having very low thermal
conductivity. In preferred embodiments the high temperature ceramic
is zirconia and the thermoplastic material is DuPont Zenite. The
thermoelectric legs are also comprised of high-temperature and
low-temperature material. In preferred embodiments the high
temperature thermoelectric material is lead telluride and the low
temperature material is bismuth telluride. In preferred embodiments
metal felt spacers are provided in each leg to maintain proper
electrical contacts notwithstanding substantial temperature
variations. Preferably the module is sealed in an inert gas filled
insulating capsule.
Inventors: |
Elsner; Norbert; (La Jolla,
CA) ; McCoy; John W.; (San Diego, CA) |
Correspondence
Address: |
ROSS PATENT LAW OFFICE
P.O. BOX 2138
DEL MAR
CA
92014
US
|
Family ID: |
41606906 |
Appl. No.: |
12/317170 |
Filed: |
December 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61137206 |
Jul 29, 2008 |
|
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Current U.S.
Class: |
62/3.2 |
Current CPC
Class: |
H01L 35/30 20130101;
H01L 35/34 20130101; H01L 35/32 20130101; F01N 5/025 20130101 |
Class at
Publication: |
62/3.2 |
International
Class: |
F25B 21/02 20060101
F25B021/02 |
Claims
1. A high-temperature thermoelectric module comprising: A. a
two-part molded egg-crate for holding in place and providing
insulation and electrical connections for a number of
thermoelectric n-legs and p-legs, wherein said egg-crate is
comprised of: 1) a hot side part comprised of a ceramic material
capable of operation at temperatures in excess of 500.degree. C.
and 2) a cold side part comprised of a thermoplastic material
having very low thermal conductivity. B. a plurality of
high-temperature thermoelectric n-legs and p-legs positioned in
said egg-crate, at least a portion of which are electrically
connected in series.
2. The thermoelectric module as in claim 1 wherein said ceramic
material is stabilized zirconium oxide and said thermoplastic
material is in the form of a liquid crystal polymer resin.
3. The thermoelectric module as in claim 2 wherein the cold side
part and the hot side part are joined together at a tab and socket
junction.
4. The thermoelectric module as in claim 3 wherein the module
defines at least four outside walls and a large number of inside
walls and the tab and socket junction includes tabs and sockets in
the outside walls.
5. The thermoelectric module as in claim 4 wherein the tab and
socket junction also includes tabs and sockets in at least a
plurality of the inside walls.
6. The thermoelectric module as in claim 1 wherein the
thermoelectric p-legs and n-legs are comprised of a lead telluride
thermoelectric alloy.
7. The thermoelectric module as in claim 1 wherein the
thermoelectric p-legs and n-legs are segmented legs, comprised of a
high-temperature material and a low-temperature material.
8. The thermoelectric module as in claim 7 wherein high-temperature
thermoelectric material is a lead telluride thermoelectric alloy
and the low-temperature material is a bismuth telluride
thermoelectric alloy.
9. The thermoelectric module as in claim 1 wherein metal felt
spacers are provided in each leg to maintain proper electrical
contacts notwithstanding substantial temperature variations.
10. The thermoelectric module as in claim 9 wherein the metal felt
spacers are impregnated with an elastomer.
11. The thermoelectric module as in claim 10 wherein the elastomer
is silicon rubber.
12. The thermoelectric module as in claim 1 wherein the module is
sealed in an insulating capsule.
13. The thermoelectric module as in claim 1 wherein the module is
combined with other similar modules to provide a thermoelectric
generator.
14. The thermoelectric module as in claim 13 wherein the
thermoelectric generator is adapted to provide electric power from
the waste heat of a motor vehicle.
15. The thermoelectric module as in claim 1 wherein said n-legs are
electrically connected to said p-legs at the hot side of the module
with a lead telluride compatible hot conductor element.
16. The thermoelectric module as in claim 15 wherein the compatible
hot conductor element is comprised of iron.
17. The thermoelectric module as in claim 15 wherein the egg-crate
walls separating the n-legs from the p-legs are adapted to contact
the hot conductor so that tellurium vapor is restrained from
migrating to the n-leg.
18. The thermoelectric module as in claim 7 wherein the p-legs
comprise a thin layer of PbSnMnTe or SnTe at their hot sides.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present invention claims the benefit of Provisional
Patent Application, Ser. No. 61/137,206, filed Jul. 17, 2008.
FIELD OF THE INVENTION
[0002] The present invention relates to thermoelectric modules and
especially to high temperature thermoelectric modules.
BACKGROUND OF THE INVENTION
Thermoelectric Materials
[0003] The Seebeck coefficient of a thermoelectric material is
defined as the open circuit voltage produced between two points on
a conductor, where a uniform temperature difference of 1 K exists
between those points.
[0004] The figure-of-merit of a thermoelectric material is defined
as:
Z = .alpha. 2 .sigma. .lamda. , ##EQU00001##
[0005] where .alpha. is the Seebeck coefficient of the material
(measured in microvolts/K), .sigma. is the electrical conductivity
of the material and .lamda. is the total thermal conductivity of
the material.
[0006] A large number of semiconductor materials were being
investigated by the late 1950's and early 1960's, several of which
emerged with Z values significantly higher than in metals or metal
alloys. No single compound semiconductor evolved that exhibited a
uniform high figure-of-merit over a wide temperature range, so
research focused on developing materials with high figure-of-merit
values over relatively narrow temperature ranges. Of the great
number of materials investigated, those based on bismuth telluride,
lead telluride and silicon-germanium alloys emerged as the best for
operating in various temperature ranges. Much research has been
done to improve the thermoelectric properties of the above three
thermoelectric materials. For example n-type bismuth telluride,
Bi.sub.2Te.sub.3 typically contains 5 to 15 percent Bi.sub.2Se3 and
p-type Bi.sub.2Te.sub.3 typically contains 80 Mol percent
Sb.sub.2Te.sub.3. Lead telluride is typically doped with sodium for
P type and Pbl.sub.2 iodine for N type.
Thermoelectric Modules
[0007] Electric power generating thermoelectric modules are well
known. These modules produce electricity directly from a
temperature differential utilizing the thermoelectric effect. The
effect is that a voltage differential of a few millivolts is
created in the presence of a temperature difference at the two
junctions of p-type thermoelectric semiconductor elements and
n-type thermoelectric semiconductor elements. These thermoelectric
elements are called n-legs and p-legs. Since the voltage
differential is small, many of these elements (such as about 100
elements) are typically positioned in parallel between a hot
surface and a cold surface and are connected electrically in series
to produce potentials of a few volts.
Hi-Z Prior Art Bismuth Telluride Molded Egg-Crate Modules
[0008] For example Hi-Z Technology, Inc. offers a Model HZ-14
thermoelectric bismuth telluride thermoelectric module designed to
produce about 14 watts at a load potential of 1.66 volts with a
200.degree. C. temperature differential. Its open circuit potential
is 3.5 volts. The module contains 49 n-legs and 49 p-legs connected
electrically in series. It is a 0.5 cm thick square module with
6.27 cm sides. The legs are p-type and n-type bismuth telluride
semiconductor legs and are positioned in an egg-crate type
structure that insulates the legs from each other except where they
are intentionally connected in series at the top and bottom
surfaces of the module. That egg-crate structure which has spaces
for 98 legs is described in U.S. Pat. No. 5,875,098 which is hereby
incorporated herein by reference. The egg-crate is injection molded
in a process described in detail in the patent. This egg-crate has
greatly reduced the fabrication cost of these modules and improved
performance for reasons explained in the patent. FIG. 1 is a
drawing of the egg-crate and FIG. 2 is a cross sectional drawing of
a portion of the egg-crate showing how the p-legs and n-legs are
connected in series in the egg-crate. The curved arrows e show the
direction of electron flow through bottom conductors 2, n-legs 4,
top conductors 6, and p-legs 8 in this portion 10 of the module.
Insulating walls 14 keep the electrons flowing in the desired
series circuit. Other Bi.sub.2Te.sub.3 thermoelectric modules that
are available at Hi-Z are designed to produce 2.5 watts, 9 watts,
14 watts and 20 watts at the 200.degree. C. temperature
differential.
Temperature Limitations
[0009] The egg-crates for the above described Bi.sub.2Te.sub.3
modules are injection molded using a thermoplastic supplied by
Dupont under the trade name "Zenite". Zenite melts at a temperature
of about 350.degree. C. The thermoelectric properties of
Bi.sub.2Te.sub.3 peak at about 100.degree. C. and are greatly
reduced at about 250.degree. C. For both of these reasons, uses of
these modules are limited to applications where the hot side
temperatures are lower than about 250.degree. C.
Lead Telluride Modules
[0010] Lead telluride thermoelectric modules are also known in the
prior art. A prior art example is the PbTe thermoelectric module
described in U.S. Pat. No. 4,611,089 issued many years ago to two
of the present inventors. This patent is hereby incorporated herein
by reference. That module utilized lead telluride thermoelectric
alloys with an excess of lead for the n-legs and lead telluride
with an excess of tellurium for the p-legs. The thermoelectric
properties of lead telluride thermoelectric alloys peak in the
range of about 425.degree. C. The egg-crate for the module
described in the above patent was fabricated using a technique
similar to the technique used many years ago for making chicken egg
crates from cardboard spacers. For the thermoelectric egg-crate the
spacers were mica which was selected for its electrical insulating
properties at high temperatures. Mica, however, is marginal in
strength and cracks easily. In addition the walls of the egg-crate
made from the mica spacers all had gaps at the intersections of the
walls that could lead to short circuits. A more rugged high
temperature egg-crate with gapless walls is needed.
[0011] FIG. 3 is a drawing from the U.S. Pat. No. 4,611,089 patent
showing a blow-up of that module. The egg-crate included a first
set of parallel spacers 46a to 46k and a second set of spacers 48a
to 48i. The n-legs are shown at 52 and the p-legs are shown at 54.
The module included hot side conductors 56 and cold side conductors
58 to connect the legs in series as in the Bi.sub.2Te.sub.3 module
described above.
[0012] That lead telluride module was suited for operation in
temperature ranges in excess of 500.degree. C. But the cost of
fabrication of this prior art module is greatly in excess of the
BiTe module described above. Also, after a period of operation of
about 1000 hours some evaporation of the p-legs and the n-legs at
the hot side would produce cross contamination of all of the legs
which would result in degraded performance.
[0013] What is needed is a low cost, compact, high-temperature
thermoelectric module with gapless walls designed for operation at
hot side temperatures in excess of 500.degree. C. preferably with
thermoelectric properties substantially in excess of prior art
high-temperature thermoelectric modules.
SUMMARY OF THE INVENTION
[0014] The present invention provides a high-temperature
thermoelectric module. The module includes a two-part molded
egg-crate for holding in place and providing insulation and
electrical connections for a number of thermoelectric n-legs and
p-legs. A first part of the egg-crate is comprised of a ceramic
material capable of operation at temperatures in excess of
500.degree. C. and a second part comprised of a thermoplastic
material having very low thermal conductivity. In preferred
embodiments the high temperature ceramic is zirconium oxide and the
thermoplastic material is a DuPont Zenite available from DuPont in
the form of a liquid crystal polymer resin. The thermoelectric legs
of preferred embodiments are also comprised of high-temperature and
low-temperature material. In preferred embodiments the
high-temperature thermoelectric material is lead telluride and the
low-temperature material is bismuth telluride thermoelectric alloys
described in the background section. In preferred embodiments metal
felt spacers are provided in each leg to maintain proper electrical
contacts notwithstanding substantial temperature variations.
Preferably the module is sealed in an insulating capsule.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a drawing of a prior art egg-crate for a
thermoelectric module.
[0016] FIG. 2 is a drawing of a portion of a module with the FIG. 1
egg-crate.
[0017] FIG. 3 is a prior art blown-up drawing of a prior art lead
telluride thermoelectric module.
[0018] FIGS. 4 and 4A are drawings showing important features of a
preferred embodiment of the present invention.
[0019] FIG. 5 is a drawing showing an application of the preferred
embodiment used to generate electricity from the exhaust gas of a
truck.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
First Preferred Embodiment
[0020] A first preferred embodiment of the present invention can be
described by reference to FIGS. 4 and 4A. The drawing is similar to
the FIG. 3 drawing, but the module is greatly improved from the
module described in U.S. Pat. No. 4,611,089.
The Egg-Crate
[0021] Egg-crate 70 is injection molded using a technique similar
to that described in U.S. Pat. No. 5,875,098. However, the molding
process in substantially more complicated. The egg-crate comes in
two molded together sections. It includes an upper section (which
will lie adjacent to a hot side) molded from stabilized zirconium
oxide (ZrO.sub.2). ZrO.sub.2 has a very high melting point of
2715.degree. C. and a very low thermal conductivity for an oxide.
The egg-crate also includes a lower side (which will lie adjacent
to a cold side) molded from Zenite Model 7130 available from Dow
Chemical that has a melting point of 350.degree. C. but very low
thermal conductivity.
[0022] The ZrO.sub.2 portion of the egg-crate is fabricated by
injection molding of the ZrO.sub.2 powders with two different
binder materials. Some of the binder material is removed by
leaching prior to sintering. The ZrO.sub.2 portion is then sintered
to remove the second binder and produce a part with good density
and high temperature strength. The ZrO.sub.2 portion will typically
shrink about 20 percent during sintering. This sintered section is
then placed in a second mold and a subsequent injection molding of
the Zenite portion of the egg-crate is then performed, thereby
bonding the Zenite to the ZrO.sub.2. While the thermal conductivity
of the ZrO.sub.2 is among the lowest of any known oxide, its
thermal conductivity of 2 W/mK is much higher than the thermal
conductivity of Zenite which is 0.27 W/mK. An objective of the
present invention is to minimize any loss of heat through the
egg-crate material. Also the Zenite is flexible and will allow the
two-section egg-crate to endure significant rough handling. The
mica of the prior art patent is a relatively weak material that
cracks easily. FIG. 4A shows a preferred technique for assuring a
good bond between the ZrO.sub.2 portion and the Zenite portion. A
tab shown at 30 is molded at the bottom of the ZrO.sub.2 walls 32.
This increases the bonding surface between the ZrO.sub.2 portion 32
and the Zenite portion 34 of the egg-crate walls.
Two Types of Thermoelectric Legs
[0023] The PbTe/Bi.sub.2Te.sub.3 thermoelectric legs of this
preferred embodiment are segmented as shown at 72 and 74 in FIG. 4.
The top portion 72a of n-leg 72 is comprised of lead telluride
thermoelectric material and the bottom portion 72b is comprised of
bismuth telluride thermoelectric material. The top lead telluride
portion 72a is doped with 0.055 Mol percent PbI.sub.2 to create
high temperature n-type material. The bottom bismuth telluride
portion 72b is doped with 0.1 Mol percent iodoform (CHI.sub.3) to
create the lower temperature n-type material. The top portion 74a
of p-leg 74 is comprised of lead telluride material and the bottom
portion 74b is comprised of bismuth telluride material. The top
lead telluride portion 74a is doped with 1.0 atomic percent Na to
create high temperature p-type material. The bottom bismuth
telluride portion 74b is doped with 0.1 parts per million Pb to
create lower temperature p-type material.
Other Module Component
[0024] Egg-crate 70 contains spaces for 80 legs, 40 n-legs and 40
p-legs. The components of these legs are shown blown-up in FIG. 4.
At the top is hot conductor 76 comprised of iron metal. Below hot
conductor 76 in the p-leg 74 is p contact 78 which is an
approximately 0.040 inch thick graphite spacer and is needed to
prevent interaction of the Fe hot shoe and the p-type PbTe. The
lead telluride portion 74a of segmented p-leg 74 is in contact with
contact 78. The lead telluride portion 72a of segmented n-leg 72 is
in direct contact with iron conductor 76. Below both legs as shown
in FIG. 4 is the Pb compatibility foil 80 which prevents any
contamination from cold conductor 82 which is preferably made from
copper sheet material about 0.010 inch thick. Below cold conductor
82 in both legs is insulating sheet of aluminum oxide 84 and below
insulating sheet 84 is fiber metal felt material 86 (discussed in
more detail below) which is made from copper or bronze felt
material.
Technique to Prevent The Evaporation and Contamination
[0025] As indicated in the background section life testing by
Applicants of PbTe modules has shown that some degradation of the
module occurs after approximately 1,000 hours of operation.
Applicants have discovered that the degradation can be attributed
to "cross-talk" between the n-legs and the p-legs near the hot
junction caused by evaporation of tellurium from the p-leg
contaminating the n-leg. (As explained in the background section an
excess of lead in the n-leg is what provides the n-leg with its
thermoelectric doping properties.) The problem is prevented in this
embodiment with two techniques: First, as shown in FIG. 4 the
egg-crate walls separating the n-legs from the p-legs are extended
to contact the hot conductor 76 so that tellurium vapor is
restrained from migrating to the n-leg. Another technique used by
applicants in this embodiment is to add a thin layer of PbSnMnTe at
the top (hot side) of the p-legs (not shown in the drawings).
Applicants have determined that elemental tellurium exhibits little
or no evaporation from PbSnMnTe. While the PbSnMnTe material does
not have as good thermoelectric properties as PbTe, the amount used
is small, only 0.020 inch long out of 0.450 inch overall length.
The PbSnMnTe segments will be cold pressed and sintered with the
p-type PbTe. In some embodiments the PbSnMnTe material may be
substituted for the hot portion of the p-legs.
Good Thermal and Electrical Conductivity
[0026] Compliant Metal Parts
[0027] The thermoelectric module of this preferred embodiment will
typically be placed between a hot surface of about 600.degree. C.
and a cold surface of about 50.degree. C. In many applications
these temperatures may vary widely with temperature differentials
swinging from 0.degree. C. to 550.degree. C. Therefore the module
and its components should be able to withstand these temperatures
and these changes in temperature which will produce huge stresses
on the module and its components. This embodiment is designed to
meet these challenges.
[0028] This preferred embodiment includes at the bottom of each leg
at the cold side a fiber metal felt pad 86 comprised of copper or
bronze wool. These materials provide good thermal conductivity and
are able to deform when the module is placed in compression between
the heat source and heat sink. These materials are resilient to
respond to thermal cycling and to the inevitable warping that the
module will undergo because of the thermal gradient imposed across
it. These compliant materials are also able to cushion the brittle
lead telluride yet maintain intimate contact between it and the
heat sink. The fiber metal felt pads can be impregnated with an
elastomer such as silicone rubber to reduce the risk of creep and
to add deflection and compliance. Silicone rubber can operate at
temperatures up to 300.degree. C.
[0029] The n and p type PbTe legs are creeping or pushing up
towards the Fe hot shoe. A load of 1,000 psi is initially used and
this load can be reduced to 500 psi after the module is operated
for approximately 100 hours and proper seating of the module is
obtained. After this time a lower load of 50 to 100 psi can be used
to maintain the low contact resistance joints.
Overall Module Design
[0030] The module is specifically designed to endure considerable
thermal cycling or steady state behavior. The hot and cold side
joints are free to slide and relieve thermal stresses. The module
needs to be held in compression at approximately 50 to 100 psi
after it reaches its design operating temperatures.
Alternate Bulk Alloys
[0031] Lead telluride based alloys have been used since the 1960s
and the alloys and recommended doping levels are well documented in
the prior art literature. Their thermoelectric properties versus
temperature are given in many publications such as Chapter 10,
"Lead Telluride Alloys and Junctions" of Thermoelectric Materials
and Devices, Cadoff and Miller, published by Reinhold Publishing
Corporation of New York.
[0032] In the past four years newer PbTe based alloys have evolved
that have better properties than the conventional PbTe based alloys
noted above. For example a Jul. 25, 2008 article in Science Daily
reported on a lead telluride material developed at Ohio State
University having substantial improvements in efficiency over prior
art lead telluride materials. This new material is doped with
thallium instead of sodium. The article suggests that efficiency of
the new material may be twice the efficiency of prior art lead
telluride. Applicants are seeking to prepare bulk lead telluride
thermoelectric material with a finer grain size than has previously
been achieved. Fine grain size is expected to lower the thermal
conductivity of the material without significant impact on
resistivity or Seebeck coefficient, thus raising its ZT and
efficiency. In previous attempts others have made to produce a
fine-grained PbTe, the grain size was observed to coarsen rapidly,
even near room temperature, so the benefit of small grain size
could not be retained. In this study, Applicants seek to preserve a
fine grain structure by additions of very fine alumina powder,
which is expected to produce a grain boundary pinning effect, thus
stabilizing the fine grain size. Applicants' recent results
indicate that the PbTe grain size can indeed be held below 2 .mu.m,
even with processing at 800.degree. C.
Lead Telluride Only Modules
[0033] A second preferred embodiment of the present invention is
just like the first preferred embodiment except the entire legs are
comprised of lead telluride thermoelectric alloys. Preferably, the
lead telluride alloy is one of the newer very high efficient
alloys.
Generator Design Using PbTe Type Modules of the Preferred
Embodiment
[0034] The high-temperature module of the preferred embodiment
requires encapsulation to prevent oxidation of the n and p alloys
with an accompanying decrease in thermoelectric properties. An
example of encapsulating PbTe modules would be the 1 kW generator
for diesel trucks shown at 16 in FIG. 5. In this example the
generator is attached to a 5 inch diameter exhaust pipe 18. Lead
telluride thermoelectric modules 20 are mounted on a support
structure 22 which is machined to form an octagon. The inside
surface is generally round with fins (not shown) which protrude
into the gas stream to provide a greater heat transfer area. The
basic design is similar to the design described in U.S. Pat. No.
5,625,245 which is hereby incorporated herein by reference.
[0035] The casting of the support structure has two flanges 24 and
26, one large and one small which are perpendicular to the main
part of the support structure. The large flange 24 is about 10
inches in diameter while the small flange 26 is about 8 inches in
diameter.
[0036] The large flange contains feed-throughs for both the two
electrical connections and four water connections. The two electric
feed-throughs 28 are electrically isolated with alumina insulators
from the support structure. Both the large and small flange will
contain a weld preparation so a metal dust cover 30 can be welded
in position.
[0037] The four water feed-throughs 32 consists of one inch
diameter tubes that are welded into the flange. The inside portion
of the water tubes are welded to a wire reinforced metal bellows
hose with the other end connected to the heat sink by a compression
fitting or a stainless to aluminum bimetallic joint. Two of the
tubes are inlets and the other two tubes are outlets for the
cooling water.
[0038] Once the generator is assembled and the flanges between the
support structure and the dust cover are welded as shown at 34, the
interior volume will be evacuated and back filled with an inert gas
such as Argon through a small 3/8 inch diameter tube 36 in the dust
cover to about 75% of one atmosphere when at normal room
temperature (.about.20 c.). Once filled, the fill tube will be
pinched off and welded.
[0039] In a preferred embodiment nine thermoelectric modules 20 of
the first preferred embodiment are mounted on each of the eight
sides of the hexagonal structure for a total of 96 modules.
Applicants estimate a total electrical output of about 1.1
kilowatts. This estimate is based on prior performance with the
structure described in the U.S. Pat. No. 5,625,245 patent,
utilizing modules of the first preferred embodiment and assuming an
exhaust hot side temperature of about 550.degree. C. and cold side
cooling water temperature of about 100.degree. C.
Variations
[0040] While the above description contains many specificities, the
reader should not construe these as limitations on the scope of the
invention, but merely as exemplifications of preferred embodiments
thereof. For example:
Other High Temperature Thermoelectric Alloys
[0041] Some of the other thermoelectric alloys that are attractive
over high-temperature ranges are: [0042] 20% Si-80% Ge,
LaTe.sub.1.4 [0043] Zintl, (Yb.sub.14MnSb.sub.11) [0044] TAGS
(AgSbTe.sub.2).sub.0.15(GeTe).sub.0.85 [0045] The skutterudites
such as CoSb4 [0046] The half-Huesler alloys [0047] The LAST and
FAST alloys of Michigan State University
[0048] All of these bulk alloys and others under development can be
used in the new ZrO2/Zenite egg-crate design shown in FIG. 4.
Thin Film Quantum Well Modules
[0049] The egg-crate of the present invention could be utilized
with thin film quantum well thermoelectric p and n legs of the type
described in detail in U.S. Pat. No. 5,550,387 which is
incorporated herein by reference. That patent describes n and p
thermoelectric legs that are fabricated using alternating layers 10
nanometers thick of Si/Si.sub.0.8Ge.sub.0.2 layers grown on silicon
substrates. In applications with temperatures above 500.degree. C.
these legs would be used on the cold side. That patent and U.S.
Pat. No. 6,828,579 also disclose high temperature lattices
comprised of thin layers of B.sub.4C/B.sub.9C and Si/SiC can also
be operated at very high temperatures up to about 1100.degree. C.
Details for fabricating B.sub.4C/B.sub.9C thermoelectric legs are
provided in U.S. Pat. No. 6,828,579 (assigned to Applicants'
employer) which is also incorporated herein by reference. See
especially Col. 3 where high temperature performance is discussed.
These B.sub.4C/B.sub.9C and Si/SiC materials could also be used
alone to make thermoelectric legs which could be used in the
egg-crate of the present invention or on the hot side of the legs
along with bismuth telluride, lead telluride or quantum well
Si/SiGe for the cold side.
[0050] A large number of 10 nm quantum well layers are built up on
a compatible substrate that has a low thermal conductivity to
produce quantum well thermoelectric film. Kapton is a good
substitute candidate if the temperature is not too high. For higher
temperature operation silicon is a preferred choice of substrate
material as described in Col. 11 of U.S. Pat. No. 6,828,579. Other
substrate materials are discussed in Col. 7. A good substrate
material not disclosed in the patent is porous silicon. Porous
silicon can survive very high temperatures and has extremely low
thermal conductivity. The pores can be produced in silicon film
such as 5 micron thick film from one side to extend to within a
fraction of a micron of the other side. The pores can be produced
either before the thermoelectric layers are laid down or after they
are laid down.
[0051] The quantum well thermoelectric film is cut and combined to
make n and p type legs of the appropriate size and each leg is
loaded into one opening of the FIG. 4 egg-crate. Before loading the
hot and cold ends of the N and P legs are metallized to yield a low
contact resistance on the cold side. Electrical contacting
materials of Pb foil and Cu connecting straps are then assembled
followed by the Al.sub.2O.sub.3 spacer and Cu felt. The module is
then turned over and the graphite piece is placed against the P
leg, followed by the iron conductor strap which contacts the
graphite and the N type PbTe.
Other Fabrication Techniques
[0052] Cyano Glue
[0053] A technique that can simplify cold side lay-up involves the
use of cyano acrylate glue (called super glue) which can be used to
temporarily hold the parts in position during lay-up but is burned
off at a seating step after the thermoelectric legs are assembled
in the egg-crate.
[0054] Other Compliant Members
[0055] Other compliant members could be substituted for the copper
felt discussed above. These include woven steel felt with a boron
nitride slurry to enhance thermal conductivity. A thin Beliville
spring could be used. Boron nitride powder could be stirred into
the spring material before the spring is cast.
[0056] Other Interfaces
[0057] Instead the tab-socket technique, other techniques for
joining the hot and cold portions of the egg-crate could be used,
such as a roughen surface of the ceramic portion.
[0058] Segmented Legs
[0059] The PbTe portions of the p-legs could be cold pressed and
sintered separately from the BiTe portions. When they are subject
to hot operating conditions they will diffusion bond. The same
applies to the n-legs.
[0060] Hot Pressing of the Legs
[0061] Another option is to hot press the thermoelectric materials
in bulk then slice and dice them into legs.
Other Crate Designs
[0062] Aerogel Crate
[0063] In one variation, an aerogel material is used to fill all
unoccupied spaces in the egg-crate. A module assembly will be sent
to Aspen Aerogels. They will immerse it in silica sol immediately
after dropping the pH of the sol. The sol converts to silica gel
over the next few days. It is then subjected to a supercritical
drying process of tightly controlled temperature and pressure
condition in a bath of supercritical liquid CO.sub.2. This process
removes all the water in the gel and replaces it with gaseous
CO.sub.2. The Aerogel serves multiple functions: (1) helping to
hold the module together, (2) reducing the sublimation rate of the
PbTe and (3) providing thermal and electrical insulation around the
legs.
[0064] In another variation, a portion of the egg-crate could be
made of non-woven refractory oxide fiber material, possibly with a
fugitive polymer binder, and having the consistency of stiff paper
or card stock is used. After assembly, the binder is burned away,
leaving a porous fiber structure that is them infiltrated with
Aerogel. The fiber reinforcement of the aerogel gives added
strength and toughness.
[0065] Those skilled in the art will envision many other possible
variations within its scope. Accordingly, the reader is requested
to determine the scope of the invention by the appended claims and
their legal equivalents, and not by the examples which have been
given.
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