Issue Date 1993 05 11

Appl. Number 594850 Appl. Date

1990 10 09

Title

Integrity-enhanced thermoelectrics

Inventor(s)

Rolfe; Jonathan L. North Easton, MA

Beaty; John S.

Belmont, MA

Assignee

Thermo Electron Technologies Corporation Waltham, MA

U.S. Class 136/211 136/212 136/225 136/232 IPC H01L 35/28 Field of Srch 136/211 136/212 136/225 136/232 U.S. Refs 2997514 1961 08 00 Roeder 136/4 3713899 1973 01 00 Sebestyen 136/233 4032363 1977 06 00 Raag 136/211 4091673 1978 05 00 Tamura et al. 75/351 4419023 1983 12 00 Hager, Jr. 374/179 4567365 1986 01 00 Degenne 250/338 4614443 1986 09 00 Hamert 374/163 4687879 1987 08 00 Hendricks 136/212 4907060 1990 03 00 Nelson et al. 5051275 1991 09 00 Wong 5057903 1991 10 00 Olla Foreign Refs EPX 1984 04 00 0122121 35 32 Primary Examiner : Nelson; Peter A. Agent Lorusso & Loud Abstract 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. Brief Summary 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. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a diagrammatic representation in perspective of a type of thermoelectric device currently available. FIG. 1a is a view in perspective of a portion of a thermoelectric device similar to that of of FIG. 1. FIG. 2a is a view of a thermoelectric device similar to that of FIG. 1 indicating likely shear failure/fracture points. FIG. 2b is a view of a thermoelectric device similar to that of FIG. 1 indicating likely die and tensile failure/fracture points. FIG. 3 is a cutaway perspective view of a thermoelectric device disclosed according to a preferred embodiment of the invention. FIG. 3a is a magnified cutaway view of the embedded device of FIG. 3. FIG. 3b is a perspective view of a multi-stage or stacked thermoelectric device according to a preferred embodient of the invention. FIG. 4 is a graph of data plotted as temperature difference (delta T) versus applied power in watts for integrity-enhanced and unenhanced thermoelectric devices. FIG. 5 is a graph of data plotted as temperature difference (delta T) versus temperature of integrity-enhanced and unenhanced thermoelectric devices. 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