Disclosed are integrity-enhanced thermoelectric devices and methods of their preparation. Such devices have the following characteristics: (1) there is, on average, no greater than about 10% incidence of function loss (failure) of the device on application to the device of a substantial impact or distortion force or corrosion exposure, and (2) the device have at least about 85% of the thermal performance of thermoelectric devices without integrity enhancement (i.e., thermal conductivity across the integrity-enhanced devices is significantly less than 0.0021 Cal-Cm/Cm.sup.2 Sec .degree.C., and is less than or equal to about 0.0015 Cal-Cm/Cm.sup.2 Sec .degree.C.; empirically expressed as maintenance of at least a 40.degree. C. temperature differential over the intra-plate distance which is about 3/16 to about 1/4 of an inch.). Integrity enhancement techniques are described, including the method of embedding components of standard thermoelectric devices in syntactic foam materials, such as those formed of resins and balloon elements.

FIELD OF THE INVENTION 
This invention is directed to integrity-enhanced thermoelectric devices and 
methods of preparation thereof. In particular, it concerns thermoelectric 
devices enhanced to have the following characteristics: (1) there is, on 
average, no greater than about 10% incidence of function loss (failure) of 
the device on application to the device of a substantial impact or 
distortion force or corrosion exposure, and (2) the devices have at least 
about 85% of the thermal performance of thermoelectric devices without 
integrity enhancement. 
BACKGROUND OF THE INVENTION 
Thermoelectric devices for generating electric power from heat or for 
providing heating or cooling upon application of electricity are well 
known. Thermoelectric devices are typically formed from an array of small 
prisms or dies of alternating p-doped and n-doped bismuth telluride 
(BiTe), silicon-germanium alloy or other polycrystalline semiconductor 
materials connected in series by electrical connection (i.e., soldering) 
to metallized pads bonded to thin ceramic plates (e.g., aluminum oxide or 
beryllium oxide plates about 1 mm thick). Silicon-germanium alloys are 
particularly useful at higher temperatures such as about the 600.degree. 
C.-1000.degree. C. range. 
Thermoelectric devices are attractive in many applications because of their 
absence of moving parts, their small size and low weight. Limitations on 
the use of such devices arise from their relative fragility and 
susceptibility to degradation in particular chemical environments, 
specifically corrosive environments. For example, electrical connections 
in thermoelectric devices can fail as a result of mechanical or 
temperature shock or degradation as a result of the operating environment 
(e.g., corrosive chemicals). A single electrical connection failure can 
disable an entire thermoelectric device. A frequent source of failure is 
the differential movement of the ceramic plates on impact or acceleration 
or bending. Stress from such movement or other forces can result in 
failure at the solder joints or the nickel barrier layer. The nickel 
barrier layer is at each end of a die and to which solder bonds. The 
nickel barrier layer prevents poisoning of the die with solder ions. 
Individual parts of thermoelectric devices may also be fragile. The dies 
are brittle and prone to destruction from vibration, flexure and other 
factors. Due to the variety of materials employed in the several 
components of a thermoelectric device, different coefficients of expansion 
can cause bowing, fracture and ultimately failure. In particular 
embodiments thermoelectric devices are built as multiple stage units 
having one thermoelectric stage stacked upon another thermoelectric stage 
to form a unitary thermoelectric device. Due to size and temperature 
differentials, the multiple stage or stacked arrays are particularly prone 
to such damage. 
In addition to failure, partial fracture can result in exfoliation of 
particles which contaminate systems in which thermoelectric devices are 
installed. Another source of failure is operation of a device in a 
corrosive or chemically destructive environment such as ferric chloride 
solution or salt spray etc. A conventional thermoelectric device is 
exquisitely sensitive to corrosion by exposure to ferric chloride and will 
be destroyed in about eight hours (un-powered) or about 2 hours (powered) 
as a result of the metallized pads being dissolved with the formation of 
replacement compounds on the dies. 
Efficiency of a thermoelectric device is limited by heat "leakage" across 
the device nullifying or counteracting the heat differential driving, or 
being established by, the device. It is rarely possible to maintain a 
temperature differential greater than about 65.degree. or 70.degree. C. 
across the plates of a thermoelectric device. In multi-stage 
thermoelectric devices, each successive stage will not produce such a 
large temperature differential, but the individual stages, if driven 
separately, will do so. 
Embedding electrical devices in polymers and other substances is known. 
However, the prior art, in general, teaches the embedding of electrical 
devices only in materials which have relatively high thermal conductivity 
so as to be able to dissipate heat. Surprisingly, the thermoelectric 
devices of the present invention, due to the incorporation or embedding 
therein of syntactic foam (providing "kinematic association" as described 
below) are superior as to shock resistance, yet have little loss as to 
increased heat leakage--less than about 15% when an integrity-enhanced 
device is driven at or near maximum and much less when driven below 
maximum. Moreover, the integrity-enhanced thermoelectric devices are 
resistant to corrosive attack. 
It is an object of this invention to provide a thermoelectric device 
resistant to impact, distortion and corrosion, yet having high thermal 
integrity. 
It is another object of this invention to provide an integrity-enhanced 
thermoelectric device with rapid slew rate. 
SUMMARY OF THE INVENTION 
This invention includes an integrity-enhanced thermoelectric device. In 
particular embodiments the device comprises two or more dies or two or 
more stage units. In particular embodiments, such a device comprises at 
least two thermoelectric dies or thermocouples (such as bismuth 
telluride), which are electrically connected at each of their respective 
ends to a conductive pad bonded to a ceramic plate, and wherein the dies 
and conductive pads are embedded in a syntactic foam in kinematic 
association. This type of device may include two or more stage units. The 
syntactic foam may comprise a resin such as epoxies, polyurethane, 
urea-formaldhyde, silicone or fluorosilicone as well as hollow glass 
spheres. Preferred glass spheres are from about 10 to 250.mu. in diameter. 
Plates of the device include alumina ceramic plates. A preferred syntactic 
foam comprises epoxy resin having glass balloons in a ratio of about 70:30 
(by weight). 
In a specific embodiment of the integrity-enhanced device thermal 
conductivity across the device is equal to or less than about 0.0010 
Cal-Cm/Cm.sup.2 Sec .degree.C., and preferably equal to or less than about 
0.0005 Cal-Cm/Cm.sup.2 Sec .degree.C. Similarly, in a specific embodiment 
of the integrity-enhanced device an empirical temperature differential is 
equal to or greater than about 50.degree. C., and preferably equal to or 
greater than about 60.degree. C. 
In particular embodiments of the invention there is, on average, no greater 
than about a 10% (and preferably no greater than about 5% and most 
preferably no greater than about 1%) incidence of function loss on impact 
of 30 G (i.e., in testing a statistically significant number of units), 
and preferably, no greater than about a 10% (and preferably no greater 
than about 5% and most preferably no greater than about 1%) incidence of 
function loss on 3-axis random vibration of about 5 minutes duration, of 
about 30 DB/octave, 0.04 G.sup.2 /Hz from 20-2,000 Hz. 
The invention includes integrity-enhanced thermoelectric devices having a 
slew rate of at least about 15.degree. C./min, and preferably, a slew rate 
of at least about 40.degree. C./min, and more preferably a slew rate of at 
least about 40.degree. C./20 sec. 
Embodiments of the invention include an integrity-enhanced thermoelectric 
device having syntactic foam of a density equal to or less than about 0.88 
gm/cc and a rigidity of at least about 1 to 2 kg/meter/100 cm.sup.2, and 
preferably, density equal to or less than about 0.44 gm/cc. 
This invention further comprises a method of integrity enhancing a 
thermoelectric device comprising embedding in kinematic association said 
device in syntactic foam to produce an integrity-enhanced thermoelectric 
device. The method includes embedding by injection of syntactic foam under 
pressure, which in one embodiment is at a pressure of least about 5 psi. A 
convenient viscosity of syntactic foam (prior to curing) for embedding is 
from about 1,000 to about 20,000 Cps. In one embodiment, the cured foam 
has a Shore hardness of at least about Shore D45. The method results in a 
device wherein thermal conductivity across the resulting device is equal 
to or less than about 0.0010 Cal-Cm/Cm.sup.2 Sec .degree.C., and 
preferably, equal to or less than about 0.0005 Cal-Cm/Cm.sup.2 Sec 
.degree.C. Otherwise expressed, the method results in a device wherein 
empirical temperature differential of the resulting thermoelectric device 
is equal to or greater than about 50.degree. C., and preferably equal to 
or greater than about 60.degree. C. In a particular embodiment the method 
results in a device comprising at least two thermoelectric dies each end 
of which is electrically connected to a conductive pad bonded to a ceramic 
plate, wherein the dies and conductive pad are embedded in a syntactic 
foam in kinematic association. In specific embodiments, the method entails 
syntactic foam with hollow glass spheres, preferably from about 10 to 
250.mu. in diameter. In particular, the method comprises embedding a 
plurality of bismuth telluride thermocouples and alumina ceramic plates in 
syntactic foam of epoxy resin and glass balloons said resin and balloons 
in a ratio of about 70:30 (by weight). 
In a specific embodiment this invention includes an integrity-enhanced 
thermoelectric device comprising: 
a first plate and a second plate spaced apart from each other; 
a couple including a p-doped leg and an n-doped leg, and means for 
electrically interconnecting a first end of said p-doped leg to a first 
end of said n-doped leg; 
means for connecting opposite ends of said p-doped leg and of said n-doped 
leg to said plates; 
said plates and said legs defining an interspace between said plates; and, 
syntactic foam occupying said interspace and in kinematic association with 
said legs and plates to form an embedded thermoelectric device. In a 
preferred embodiment of this device the plates are formed of ceramic 
material and the means for electrically connecting a p-doped leg to an 
n-doped leg includes a metal pad bonded to the surface of the plate 
defining the interspace and to the first end of said p-doped leg and the 
first end of said n-doped leg. In this embodiment the syntactic foam may 
comprise epoxy resin and also comprise hollow glass spheres. Preferably, 
the device further includes at least two of the couples electrically 
connected in series and having opposed ends connected to the plates. Such 
an embodiment may also comprise a syntactic foam of a vitreous material 
and/or sintered ceramic or glass micro-balloons and the resulting device 
will be ultra-corrosion exposure resistant. Another particular 
integrity-enhanced thermoelectric device of this invention is: 
a first plate and a second plate spaced apart from each other; 
a couple including a p-doped leg and an n-doped leg, and means for 
electrically interconnecting a first end of said p-doped leg to a first 
end of said n-doped leg; 
means for connecting opposite ends of said p-doped leg and of said n-doped 
leg to said plates; 
said plates and said legs defining an interspace between said plates; and, 
syntactic foam upon curing occupying said interspace with said legs and 
plates to form an embedded thermoelectric device, such that: as to the 
formed device; 
(i) there is, on average, no greater than about 10% incidence of function 
loss (failure) of the device on impact or distortion force of acceleration 
forces of up to about 20 G; or 3-axis random vibration of about 5 minutes, 
30 DB/octave, 0.04 G.sup.2 /Hz from 20-2,000 Hz; or bending forces in the 
1-2 Kg/linear meter range; and, 
(ii) in an oxidizing or reducing environment (other than resin dissolving 
environment) sufficient to render an unembedded device inoperative in 
hours or days, said embedded device operates in such an environment at 
least 50 times longer than an unembedded device; and, 
(iii) the device has thermal conductivity across the device significantly 
less than 0.0021 Cal-Cm/Cm.sup.2 Sec .degree.C. being about equal to or 
less than 0.0015 Cal-Cm/Cm.sup.2 Sec .degree.C.; 
(iv) said syntactic foam comprises has not shrunk upon curing; 
(v) said foam upon curing is associated or bonded with said legs and plates 
to resist substantial impact or distortion force while maintaining at 
least about 85% thermal performance and said foam upon curing occupies at 
least a majority of the interspace; and, 
wherein said foam before curing has a viscosity at 65.degree.-70.degree. F. 
(room or ambient temperature) of from about 2,000 to 20,000 Centipoise 
being readily flowable at applied pressures of about 5 psi or greater. 
In a preferred embodiment, the syntactic foam comprises a resin and glass 
balloons, the balloons being from about 45 to about 250.mu. diameter and 
comprising about (30%) by weight of the mixture of balloons and resin. The 
more preferred such devices have a slew rate of at least about 15.degree. 
C./minute, and more preferably a slew rate of at least about 40.degree. 
C./minute, and still more preferably a slew rate of at least about 
40.degree. C./20 seconds, and yet more preferably a slew rate of at least 
about 40.degree. C./10 seconds.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
This invention will be more clearly understood in reference to certain 
terms specifically defined as follows: 
"Integrity-enhanced" in reference to thermoelectric devices means that two 
conditions obtain: (1) there is, on average, no greater than about 10% 
(and preferably no greater than about 5% and most preferably no greater 
than about 1%) incidence of function loss (failure) of the devices on 
substantial impact or distortion force or corrosive exposure, and (2) the 
devices have at least about 85% and preferably 90% of the thermal 
performance of unembedded thermoelectric devices. Reference to a failure 
limitation as "on average" refers to total failure of a device occurring 
in the stated percentage of instances in a statistically significant 
number of tests. 
"Substantial" as determining impact and distortion force means acceleration 
forces of about 20 G; or 3-axis random vibration of about 5 minutes 
duration, 30 DB/octave, 0.04 G.sup.2 /Hz from 20-2,000 Hz. 
"Corrosive exposure" refers to environments sufficient to render an 
unembedded device inoperative within a period of hours or days at most, 
usually oxidizing or reducing environments. Excluded, except where 
specifically noted, are resin dissolving materials such as 
phenol/methylene chloride. However even as to "ultra-corrosive exposure" 
such as resin dissolving materials, with syntactic foam made of a 
non-resin material such as sintered ceramic microballoons in a vitreous 
matrix the thermoelectric device will remain operative and thus 
integrity-enhanced. A device integrity-enhanced operates in such a 
corrosive environment at least about 50 times longer than an unembedded 
device. It will be appreciated that the wide range of corrosive 
environments requires selection of particular syntactic foams (and 
ancillary electrical connector protectant if required) for maximum 
protection in view of anticipated mechanical stress, temperature range of 
operation and cost factors. Based upon the teachings herein the selection 
of resin as to corrosive exposure resistance will be clear to those 
skilled in the art. Based upon the teachings herein the selection of other 
syntactic foam materials as to corrosive/ultra-corrosive exposure 
resistance will be clear to those skilled in the art. 
"Maintained thermal integrity" means thermal conductivity across the device 
significantly less than 0.0021 Cal-Cm/Cm.sup.2 Sec .degree.C., preferably 
equal to or less than about 0.0015 Cal-Cm/Cm.sup.2 Sec .degree.C. so that 
the device has a thermal performance (temperature difference across the 
plates) at least about 85% of the device without integrity enhancement 
when driven at maximum normal operating power. A driven thermoelectric 
device is one operated by either an electric current or heating to 
generate an electric current. The "maximum" reflects the peak heat or 
electrical load that does not reduce efficiency, and is easily empirically 
determined by comparing output efficiency with input energy. Note that 
maintained thermal integrity may also be empirically expressed as the 
maintenance of a temperature difference over the intra-plate distance 
(which is typically about 3/16 to about 1/4 of an inch) of equal to or 
greater than about 40.degree. C. ("empirical temperature differential"). 
In a preferred embodiment thermal conductivity is equal to or less than 
about 0.0010 Cal-Cm/Cm.sup.2 Sec .degree.C., and more preferably equal to 
or less than about 0.0005 Cal-Cm/Cm.sup.2 Sec .degree.C. 
"Thermoelectric device" means a heat-to-power transducing device of p- and 
n-doped semiconductors. In one embodiment a thermoelectric device 
comprises two or more small prisms or legs (commonly called "dies"), often 
of square or rectangular cross-section, of alternating n-doped and p-doped 
bismuth telluride. Dies in multiples (termed "couples") are electrically 
connected in series by affixation to conductive pads bonded to ceramic 
plates. Other embodiments employ dies of bismuth telluride/selenide or 
silicon-germanium alloys or solid solutions or other admixtures known in 
the art. These semiconductors are more widely discussed in Modern 
Thermoelectrics, D. M. Rowe and C. M. Bhandari (Holt, Reinhart and 
Winston, London) (1983), the disclosure of which is herein incorporated by 
reference. In many instances of commercial use a thermoelectric device 
will have dies or legs arranged in 6 or more series junctions designed for 
providing heating or cooling when electric power is applied to the 
interconnected legs or for transducing units of heat to units of current. 
A particular arrangement of a thermoelectric device is the "unijunction" 
comprising two dies connected to a ceramic plate and in electrical 
connection at only one end. As used herein, thermoelectric device is 
understood to be distinct from and exclusive of bimetal thermocouple 
devices or arrays which are characterized in operating at millivolts and 
milliamperes or below and whose uses are typically limited to 
instrumentation such as temperature measurement. Thermoelectric devices of 
the present invention operate at greater than about 0.5 volts and greater 
than about 0.25 amperes. Further distinguishing thermoelectric devices as 
referred to herein from bimetal devices is the high thermal conductivity 
associated with bimetal devices. Bimetal devices have a thermal 
conductivity of at least about 0.058 Cal-Cm/Cm.sup.2 Sec .degree.C. (e.g., 
constantan alloy) and thermoelectric devices as referred to herein have a 
thermal conductivity no greater than about 0.017 Cal-Cm/Cm.sup.2 Sec 
.degree.C.--bimetal thermal conductivity is over 3 time higher than 
thermoelectric devices. 
"Syntactic foam" means (i) a foam or solid polymer or other material having 
a thermal conductivity of no more than about than 0.0015 Cal-Cm/Cm.sup.2 
Sec .degree.C. and preferably less than 0.0010 Cal-Cm/Cm.sup.2 Sec 
.degree.C. and most preferably 0.0005 Cal-Cm/Cm.sup.2 Sec .degree.C. or 
less (or, empirically, will maintain a temperature differential of at 
least about 40.degree. C. over a thickness of about 3/16 to about 1/4 of 
an inch), and (ii) hardness of the cured foam of a Shore hardness of Shore 
D63 or harder. A preferred quality of the syntactic foam employed in the 
integrity-enhanced thermoelectric devices of the present invention is a 
dielectric strength of at least 600 V/mil and preferably 1000 V/mil or 
greater. In one embodiment, the above mentioned parameters are achieved by 
selection of ingredients employed in foam forming--e.g., resin and balloon 
elements. Hardness and thermal insulating and dielectric strength are 
easily determined by methods known to those skilled in the art. 
Nonshrinking during curing is an important characteristic of suitable 
syntactic foams. Shrinking during curing is a characteristic of solvent 
associated resins. It is important to note the expansive nature of the 
term foam to include materials other than polymers and resins. Specific 
reference is made to high temperature materials that are stable well above 
the useful temperature of most polymers. Such materials include vitreous 
frits or ceramics, including sintered ceramics with the above noted 
thermal and hardness characteristics. Microballoons for such foams may be 
made of suitable high temperature material such as alumina ceramic 
material. In particular embodiments such as those requiring high 
temperature resistance and liquid impermeability, ceramic microballoons in 
a vitreous ceramic matrix are employed. Those skilled in the art will 
understand that sintered in place foam will be used with high temperature 
tolerant dies such as silicon-germanium. Alternatively sintering can be 
performed separately and the sintered foam fitted to the dies. 
"Kinematic association" means the association or bonding of cured or 
sintered or otherwise hardened foam with other thermoelectric device 
elements to resist substantial impact or distortion force while 
maintaining at least about 85% thermal performance. Kinematic association 
also provides a barrier to corrosive environments, except in the 
particular case of a sintered ceramic or glass microballoon type frit and 
not in a resin or vitreous matrix. In kinematic association the foam 
occupies at least a majority of the intra-device void volume, and 
preferably at least about 85% void volume and most preferably about 95% or 
more. In particular, the foam is in contact with the structural elements 
(i.e., dies, upper and the lower plates) of the thermoelectric device. 
Foam that shrinks substantially upon curing will not be in kinematic 
association and may induce residual strain. Without being bound by any 
particular theory it is believed that a major protective mode of kinematic 
association is the even distribution of force throughout an embedded 
device. Typically, in an acceleration stress an unembedded device will 
experience lateral movement of one plate, an unsupported plate, relative 
to another of its plates which is typically mounted to a fixed surface. 
This differential acceleration leads to a hinging action at the die/plate 
interface and failure. Kinematic association enhances integrity in 
distributing acceleration force evenly or, similarly, in resisting and 
distributing torsion or vibration. 
"Slew Rate" means the change in temperature per unit time (e.g., 
.degree.C./Sec) of the driven plate of a thermoelectric device. Quick 
adjustment of plate temperature, a desireable characteristic of 
thermoelectric devices in particular applications, is retarded by the mass 
of the embedding material. In certain applications a slew rate of 
15.degree. C./minute is considered fast, and would be well beyond the 
response for thermoelectric devices having solid resins as embedding 
material. In particular embodiments of the present invention a large 
volume of balloon material in the embedding syntactic foam substantially 
reduces its mass, permitting rapid temperature adjustment of the driven 
plate. This permits adjustment or slew on the order of a 40.degree. C. 
change in less than about 20 seconds, or more preferably, in less than 
about 10 seconds. Solid resins could not approach such a rapid slew rate. 
Preferred integrity-enhanced thermoelectric devices maintain temperature 
differentials between plates of at least about 50.degree. C. and more 
preferably at least about 60.degree. C. 
A preferred syntactic foam for the integrity-enhanced devices of the 
invention has low thermal conductivity, high dielectric and mechanical 
strength and good adhesion to the other materials of the thermoelectric 
device. The syntactic foam, prior to polymerization or sintering, should 
have fluid-like properties suitable for its entry into the interstitial 
spaces of the thermoelectric device--generally low enough shear and 
viscosity to readily permit injection. 
A resin suitable for use in making syntactic foam exhibiting appropriate 
characteristics is a low thermal conductivity epoxy resin such as F110.TM. 
to (available from Tra-Con, Inc. of Medford Mass., U.S.A.). To prepare a 
syntactic foam, resin is mixed with a substance such as microballoons of 
glass, phenolic resin, high alumina ceramics or other electrically and 
thermally nonconductive corrosion resistant material available from a 
number of sources (e.g., Emersom & Cuming, Inc. Canton Mass., U.S.A.; 3M, 
Minneapolis Minn.). Depending on the thermal properties of the matrix 
material of the syntactic foam, such as the selected resin, varying 
amounts of balloon material is required to result in suitable thermal 
properties of the resulting syntactic foam. The resulting thermal 
properties are easily determined by calorimetry. (Similarly the dielectric 
strength is determined by appropriate metering, as by a high voltage 
ohmmeter.) If F110 is employed as a resin, glass balloons of 45 to 250.mu. 
diameter and comprising about thirty percent (30%) by weight of the 
mixture of balloons and resin have been found to be effective. At these 
proportions of resin and balloons, the uncured foam has a viscosity at 
65.degree.-70.degree. F. (ambient or room temperature) of about 17,000 
Centipoise (Cps) and is readily flowable at applied pressures of about 5 
psi or greater to fill the inter-die spaces. (Viscosities of from about 
1,000 to 20,000 Cps. are useful.) Upon curing, the syntactic foam formed 
from the above-described resin and balloons exhibits a dielectric strength 
above 600 V/mil, a hardness of Shore D93+, and a tensile strength of about 
2,400 psi. Density of the cured syntactic foam is less than 0.88 gm/cc and 
preferably from about 0.65 to about 0.75 gm/cc. (Shore hardness of from at 
least about Shore D45 is suitable.) 
Systems based upon resins of less adequate (i.e., higher) thermal 
conductivity but suitable rigidity can be used with increased amounts of 
balloon or foam elements. Similarly, systems based upon resins of less 
than adequate rigidity (e.g., silicon elastomers) can be used with 
increased amounts of rigid balloon elements. One such resin is 
urea-formaldehyde. Adhesion of the foam to the dies and other structural 
elements in such a system may require augmenting with a silane coupler 
(e.g., for ceramics) or an isocyanate. In such applications the contiguous 
balloon material may constitute almost the full deminisions of the void 
volume (e.g., from about 70% up to greater than about 90%) and the matrix 
material (e.g., elastomer or resin) merely acts to fix the balloon 
structures in place and provide corrosive environment integrity. 
Particularly in systems where corrosion resistance or temperature aspects 
of integrity enhancement are desired, silicon and fluorosilicon elastomers 
are useful with sufficient rigid element incorporation. High temperature 
resistant integrity-enhanced thermoelectric devices, in a particular 
embodiment, have syntactic foam comprised of microballoons of glass or 
ceramic alumina and a vitreous maitix. Microballoons may be sintered into 
a single unit, fitted to thermoelectric dies and plates, and vitreous 
material introduced subsequently. 
Microballoons are conveniently formed as gas filled spheres. Inert gases 
such as helium, argon and neon are preferred, but relatively inactive 
gases such as nitrogen are also employed in particular applications. 
Mixtures of gas are also included. Where slew rate considerations are 
important, less thermal mass and hence faster slew rate is obtained by 
reduction of the amount of gas or use of gas such as hydrogen, helium or 
argon is contemplated. 
The invention will be better understood with reference to the following 
description of the figures. 
FIG. 1 is a diagrammatic representation of one type of thermoelectric 
device (100) currently available. The illustrated thermoelectric device 
comprises an array of n-doped legs or dies (102) and p-doped legs or dies 
(104) formed of a semiconductor such as bismuth telluride and arranged in 
three files of seven dies each. Opposed plates or supports (106) of 
electrically nonconductive material such as alumina or other ceramic are 
metallized at selected locations on their interior surface (108) with a 
conductor such as copper 0.0005-0.005 inches thick to form pads which 
electrically connect one end of each n-doped die (102) to the adjacent end 
of a p-doped die (104). The end junctions (109) are shown for connection 
to a drive power source or electrical load (111). Solder junctions (110) 
join the dies to the metallized pads to connect the p-doped dies to the 
n-doped dies in series throughout the device. 
FIG. 1a shows the attachment of end junctions (109) to solder junctions 
(110) and metallized portion (114) in a device similar to that of FIG. 1. 
FIG. 2a shows a thermoelectric device similar to that of FIG. 1 indicating 
likely shear failure/fracture points. The thermoelectric device (200) has 
opposed upper (202) and lower (204) ceramic plates and is fastened to a 
fixed surface (206) via the lower ceramic plate (204). Upon application of 
a shearing force, such as acceleration of the surface (206) in one 
direction, the lower ceramic plate (204), moves relative to the 
unsupported upper plate (202). The dies (208) exhibit hinge failure at the 
solder joints or nickel barriers (210). 
FIG. 2b shows a thermoelectric device similar to that of FIG. 1 indicating 
likely die and tensile failure/fracture points upon forces exerted by 
thermal bowing or deflection. Here, the thermoelectric device (240) shows 
flexure of the upper ceramic plate (242) in a direction away from the 
lower ceramic plate (244). Failure aspects include hinge failure at the 
solder joint or nickel barrier (250), plate fracture (252) and die 
fracture (254). 
FIG. 3 and the cutaway perspective view FIG. 3a illustrate a preferred 
embodiment of the integrity-enhanced thermoelectric device (300) of the 
invention. P-doped semiconductor dies (302) (some shown in phantom) and 
n-doped semiconductor dies (304) (some shown in phantom) formed of a 
semiconductor material such as bismuth telluride are arranged in three 
files of seven dies each. Opposed plates or supports (306) of an 
electrically nonconductive material such as alumina or other ceramic are 
metallized at selected locations on the plate interior surface (308) with 
0.0005-0.005 inch thick copper to form pads (309) which electrically 
connect one end of each n-doped die (304) to the adjacent end of a p-doped 
die (302). Solder junctions (310) connect the p-doped dies to the n-doped 
dies in series through the metallized surfaces. The dies (302) and (304) 
and interior surfaces (308) are kinematically associated or embedded in a 
syntactic foam (319) of resin (320) such as epoxy of polyurethane, and 
balloons (322) such as glass, comprising 30% by weight of the uncured 
foam. (For convenience in drawing, microballoons are shown substantially 
enlarged--even as to the scale of the drawing--over the actual size.) In 
particular embodiments the porportion of foam comprising balloons will 
vary from none (in instances where the matrix material has sufficient 
thermal and ridigidity properties) to 100 (in the limiting case of 
sintered balloons and no matrix material) depending on the required 
thermal and ridigidity characteristics. The cured syntactic foam has a 
Shore hardness of at least D63 and the integrity-enhanced thermoelectric 
device pictured has a thermal conductivity of less than 0.0015 
Cal-Cm/Cm.sup.2 Sec .degree.C. (or the ability to maintain a temperature 
differential of at least about 40.degree. C. over an inter-plate 
separation of about 3/16 to 1/4 inch). As in FIG. 1, end junctions (324) 
are shown leading from solder connections (330) for connection to a drive 
power source or electrical load (332). 
FIG. 3b is a perspective view of a two-stage or stacked thermoelectric 
device (350) of the invention showing syntactic foam (352) embedding a 
thermoelectric device. (For convenience in drawing, microballoons are 
shown substantially enlarged--even as to the scale of the drawing--over 
the actual size.) The foam contains a limited amount of resin relative to 
the large volume of balloons or cells (354). The two stages have 
electrically-interconnected dies between three plates (364) (366), and 
(368). 
Properties of the integrity-enhanced thermoelectric device are illustrated 
with reference to Table 1 and 2 which list results of tests conducted on 
multicouple thermoelectric devices both prior to and following embedding 
with syntactic foam. The thermoelectric devices tested were 31 couple, 9 
amp units measuring 32.times.32.times.5 mm, fabricated of 4 rows of 8 
alternating p-doped and n-doped BiTe dies connected electrically in 
series. The individual dies measured 2.5.times.2.5.times.3.5 mm. The 
integrity-enhanced thermoelectric device was embedded in a syntactic foam 
of 30% by weight borosilicate glass balloons in a matrix of epoxy 
(F110.TM., Tra-con Inc., Medford, Mass.) injected by syringe with a PFTE 
mold. 
In Tables 1 and 2 the following abbreviations are used: 
Th=temperature in degrees Centigrade (.degree.C.) or degrees Kelvin 
(.degree.K.) of a heat sink constant or reference block maintained in 
contact with the reference plate of the thermoelectric device. 
I=current maintained through the thermoelectric device (in amperes). 
V=voltage maintained to drive the thermoelectric device to current, I. 
W=power (in watts) employed to drive a thermoelectric device (W=IV). 
Tc=temperature of driven plate (in xC or xK, as indicated). 
delta t=difference in temperature between reference block and driven plate. 
Table 1 shows that, for the test data provided, no degradation in 
performance resulted from kinematic association or embedding of 
thermoelectric devices in that delta t of the embedded device remained at 
the same level as that of the unembedded device at the same levels of 
applied power. 
Table 2 (and FIGS. 4 and 5 (plotted from the Table 2 data)) show that an 
integrity-enhanced (embedded) thermoelectric device provides performance 
levels (delta T) similar to, and only slightly lower than, unembedded 
devices over a wide range of drive power. 
FIG. 4 is a graph of performance in delta T against power in watts for 
integrity-enhanced and unenhanced thermoelectric devices. This figure 
shows that integrity enhancement results in little, if any, performance 
reduction (delta T) below about 20 watts, and above about 20 watts the 
reduction is not substantial (i.e., not more than about 10xC). 
FIG. 5 is a graph of performance in delta T versus driven plate temperature 
for integrity-enhanced and unenhanced thermoelectric devices. Integrity 
enhancement does not reduce performance over the range of temperatures 
tested. 
EXAMPLE 
Preparation 
In one embodiment an integrity-enhanced thermoelectric device was prepared 
by mixing by gentle folding of resin into the balloons taking care to 
avoid breakage of the balloons and to avoid entraining air. The mixture 
was 70% epoxy resin (F110) with 30% (by weight) of nitrogen filled glass 
balloons (MC-37, Emersom & Cuming, Canton, Mass.) of about 45 to 250f 
diameter. The unembedded thermoelectric device, a BiTe, 16 multicouple 
device of about 32.times.32.times.10 mm in size, was placed in a silicon 
rubber mold, having a first injection port or sprue and a second port or 
sprue. Approximately 5 cc of the mixture was then injected into the 
thermoelectric device through the first port or sprue under a pressure of 
5 psi. Injection was continued until excess material exuded from the 
second port of the silicon rubber mold indicating substantial displacement 
of all air within the thermocouple and replacement with the mixture of 
resin and balloons. Filling was to substantially 100% of the void volume 
of the thermoelectric device. Care was taken not to fracture any 
connections or dies. Curing was accomplished at 65xF (ambient or room 
temperature) for 2 hours yielding an integrity-enhanced thermoelectric 
device. 
Evaluation of Integrity 
The integrity-enhanced thermoelectric device was tested in the following 
fashion: 
Drop Test: Dropping the device 10 times from a height of 6 feet onto a 
concrete floor had no adverse effect on its performance. 
Thermal Shock Test: The device was driven at maximum power for 220 cycles, 
a cycle being reaching maximum temperature (Tmax) of about 150xC at one 
plate and then reversing polarity at full power and reaching the minimum 
temperature (T min) of about -50xC at that plate. Several sets of 220 
cycles were run without observable degeneration of performance or thermal 
flexing. 
Corrosion Test: The device was submerged in 6N ferric chloride 
(FeCl.sub.3), 1N saline, and dilute nitric acid, none of which caused any 
apparent deterioration or adverse effect on performance. 
Acceleration/Random Vibration Test: A module retained by embedding in wax 
was positioned in a test fixture mounted to a dynamic shock and 
acceleration tester (Dynatran.TM. model 3100A) operated at 30 DB/octave 
for five minutes in three axis random vibration. 
The resulting integrity-enhanced thermoelectric device showed (1) no 
greater than about 10% (and preferably no greater than about 5% and most 
preferably no greater than about 1%) incidence of function loss (failure) 
of the device on application to the device of a substantial impact or 
distortion force or corrosion exposure, and (2) the device had at least 
about 85% of the thermal performance of thermoelectric device without 
integrity enhancement (i.e., thermal conductivity across the devices was 
significantly less than 0.0021 Cal-Cm/Cm.sup.2 Sec xC, and was less than 
or equal to about 0.0015 Cal-Cm/Cm.sup.2 Sec xC; empirically expressed as 
maintenance of at least a 40xC temperature differential over the 
intra-plate distance which was about 3/16 to about 1/4 of an inch). 
TABLE 1 
______________________________________ 
(31 couple 9 Amp production unit) 
Th I V Tc delta t 
______________________________________ 
PRIOR TO EMBEDDING: 
50xC 9.0 3.79 -16.5 66.5xC 
52 8.0 3.50 -17.5 69.5 
52 9.0 3.80 -15.0 67.0 
52 8.0 3.46 13.7 65.7 
AFTER EMBEDDING: 52 9.0 3.80 -15.0 67.0 
52 8.0 3.46 13.7 65.7 
______________________________________ 
TABLE 2 
______________________________________ 
Unembedded Device Embedded Device 
W (watts) 
Tc (xK) delta T W (watts) 
Tc (xK) 
delta T 
______________________________________ 
0.540 307.000 16.400 0.520 307.300 
16.100 
2.060 293.700 29.700 2.020 295.300 
28.100 
4.380 283.700 39.700 4.290 286.300 
37.100 
7.440 275.500 47.900 7.280 279.300 
44.100 
11.200 268.400 55.000 11.050 273.300 
50.100 
15.600 262.700 60.700 15.420 268.100 
55.300 
20.720 258.300 65.100 20.580 264.000 
59.400 
26.480 255.000 68.400 26.320 261.100 
62.300 
29.665 253.700 69.700 29.580 260.100 
63.300 
33.120 252.500 70.900 33.120 259.200 
64.200 
38.670 251.900 71.500 36.670 258.700 
64.700 
40.600 251.500 71.900 40.600 258.400 
65.000 
44.520 251.200 72.200 44.630 258.300 
65.100 
49.060 251.300 72.100 49.170 258.600 
64.800 
53.700 251.800 71.600 53.930 259.300 
64.100 
58.680 252.600 70.800 59.040 260.300 
63.100 
63.625 253.500 69.900 64.250 261.600 
61.800 
69.680 255.000 68.400 70.330 263.300 
60.100 
81.340 258.700 64.700 82.600 267.700 
55.700 
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