US3601887A

US3601887A - Fabrication of thermoelectric elements

Info

Publication number: US3601887A
Application number: US806585A
Authority: US
Inventors: Phillip V Mitchell; William P Blankenship
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1969-03-12
Filing date: 1969-03-12
Publication date: 1971-08-31
Anticipated expiration: 1988-08-31

Abstract

An isostatic hot compressing process step is employed to exert a pressure of from 5,000 p.s.i. to 50,000 p.s.i. on the exterior surfaces only of a hollow cylindrical thermoelectric element having thermoelectric material disposed between exterior and interior cylindrical shell members to plastically deform the exterior surfaces of the element and reducing the annular crosssectional area of the element from 1 percent to 15 percent to provide at least an intimate physical contact between the body of thermoelectric material contained therein and the inner and outer members of the element. The inner member of the element is in compression after the hot isostatic compression is removed. Isostatic cold compressing may be applied to the element prior to the isostatic hot compressing process step.

Description

' UnitedStates Patent Continuation-impart of application Ser. No. 7 593,528, Nov. 10, 1966, now abandoned.

FABRICATION 0F THERMOELECTRIC ELEMENTS 13 Claims, 6 Drawing Figs.

1m. 01 B01 j 17/00, 11011 15/00, 110111 49/00 Field of Search Primary ExaminerJohn F. Campbell Assistant Examiner-D. M. Heist Attorneys-F. Shapoe and C. L. Menzemer ABSTRACT: An isostatic hot compressing process step is employed to exert a pressure of from 5,000 p.s.i. to 50,000 p.s.i. on the exterior surfaces only of a hollow cylindrical thermoelectric element having thermoelectric'material disposed between exterior and interior cylindrical shell members to plastically deform the exterior surfaces of the element and reducing the annular cross-sectional area of the element from 1 percent to 15 percent to provide at least an intimate physical contact between the body of thermoelectric material contained therein and the inner and outer members of the element. The inner member of the element is in compression after the hot isostatic compression is removed. Isostatic cold compressing may be applied to the element prior to the isostatic hot compressing process step. 1

PATENTEB M1831 I971 sum 1 or 2 FIG.|.

FIG.2.

FIGS.

FIG.4.

NORMALIZED EFFICIENCY NORMALI ZED THERMAL CONDUCTANCE ENTEU AUGS] IBH 3501.8 7

sum 2 [IF 2 O-TYPICAL POINTS FROM (6) ELEMENTS FABRICATED BY PRIQR ART PROCESS (EXAMPLE I) x -:l "I

g ELEMENTS FABRICATED ,5 EXAMPLE I AJ4 BY NEW PROCESS A V J5 EXAMPLE m g I I 6 EXAMPLE m I00 200 300 400 soo e00 "/00 B00 9'00 OPERATING TIME- HOURS FIG.6.

O- TYPICAL POINTS FROM (6) ELEMENTS FABRICATED OPERATING TlME HOURS FABRICATION OF THERMOELECTRIC ELEMENTS This application is a continuation-in-part of our application Ser. No. 593,528, filed Nov. 10, 1966, now abandoned, the assignee of which is the same as that of the present application.

The present invention relates to a process for preparing thermoelectric elements.

A prior process employed to fabricate hollow cylindrical thermoelectric elements which comprise an outer cylinder member and a smaller inner cylindrical member disposed therein with the thermoelectric material proper being placed in the space between them, utilizes apparatus and techniques exerting a high pressure upon both the outer surface of the outer cylindrical member and the inner surface of the inner cylindrical member of the elements. The application of the pressure at a high temperature upon both outer and inner cylindrical surfaces reduces the diameter of the outer cylindrical member and increases the diameter of the inner cylindrical member. It should be noted that the inner cylindrical member is stretched or elongated in the process. This process eliminates radial assembly gaps between the components of the element, densifies the thermoelectric material and establishes a good thermal contact at each radial interface.

However, when an element processed in this manner is operated at an elevated temperature, the inner cylindrical member has reduced efficiency. This has been found to result from the fact that the innermost member relaxes inwardly, thereby reducing its contact pressure upon the adjoining components of the element. The resulting reduction in contact pressure increases the thermal contact resistances within the element, thereby causing a reduction in the efficiency of the element.

An object of this invention is to provide a process for producing hollow cylindrical thermoelectric elements having the innermost portions in compression whereby the innermost cylindrical portions do not undergo relaxation when the elements are operated at a high temperature.

Another object of this invention is to provide a process for manufacturing a hollow cylindrical thermoelectric element by applying isostatic pressure only to the outer surface whereby the element is collapsed onto a heat source within the hollow member, thereby assuring a good thermal conductivity relationship between the cylindrical element and heat source, therebypreventing inward relaxation of the thin wall member when the element is operated at a high temperature.

A further object of this invention is to provide a process for manufacturing a hollow cylindrical thermoelectric element embodying a thick wall inner cylindrical member .to develop a compressive stress on the innermost portions adjacent the thick wall whereby when the element operates at a high temperature the thick wall inner cylindrical member maintains good thermal contact with the adjoining components of the element.

Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of this invention, reference should be had to the following detailed description and drawings, in which:

FIGS. l and 2 are perspective views, partly in cross section, of thermoelectric elements made in accordance with the teachings of this invention;

FIGS. 3 and 4 are views, partly in cross section, of ther moelectric elements made in accordance with the teachings of this invention;

FIG. 5 is a graph comparing the normalized overall efficiency of thermoelectric elements made by a prior art process and compared with thermoelectric elements made in accordance with the teachings of this invention; and

FIG. 6 is a graph comparing the normalized thermal conductance of thermoelectric elements made by a prior art process and compared with thermoelectric elements made in accordance with the teachings of this invention.

In accordance with the'present invention and in attainment of the foregoing objects there is provided a process for producing an integral thermoelectric element, the steps comprising disposing a thermoelectric material, or a plurality of layers of thermoelectric material, within a hollow compartment defined by the outer wall of an inner metal member and the inner wall of a larger diameter outer metal member, the metal members being concentric with each other, sealing the ends of the compartment to retain the thermoelectric material therein and to prevent substantial pressure developing in the hollow, preferably evacuating the compartment, and hot pressing the entire assembly at a temperature from about 250 C. to slightly below the melting temperature of the lowest melting temperature component in the assembly by the application of an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. or higher to a selected surface area of only the outer metal member to effect a reduction of from 1 percent to 15 percent in the cross-sectional area of the compartment to provide at least an intimate physicalcontact, and in some instances a metallurgical bond between the thermoelectric material and the walls of the metal members and increasing the density of the thermoelectric material by 4 percent to ID percent. The process results in a compressive stress being imparted to the innermost layers after the pressure is released.

In one preferred embodiment of the invention, a plurality of compressed bodies of powdered thermoelectric material are assembled within the space between an inner and outer cylindrical metal member. Electrical insulation is interposed between portions of the bodies. Bridging electrical contacts are disposed between certain portions of the bodies of thermoelectric material with the insulation being disposed between the cylindrical members and the bridging members, to provide a closelypacked assembly. The inner cylindrical metal member is hollow and has a thin wall and this hollow is completely filled with a solid heat source such, for example, as a radioisotope, or a nuclear reactor fuel, for example U0 and is sealed to prevent fluid pressure developing therein during subsequent processing.

The entire assembly is hot pressed at an isostatic pressure of from about 5,000 p.s.i. to 50,000 p.s.i. at a temperature of from about 250 degree C. to the melting temperature of the lowest melting temperature component of the assembly until the space between the inner and outer cylindrical members has a reduction in cross-sectional area of from about 1 percent to 15 percent to provide a metallurgical bond between the thermoelectric bodies, the metal members and the bridging electrical contacts and to increase the density of the thermoelectric material by about 1 percent to 10 percent. The thin wall of the inner cylindrical member will be collapsed upon the surface of the solid heat source so that the two are in an intimate physical contact and a good thermal conductivity relationship with each other.

In an alternate and preferred embodiment of the invention, the components are the same as in the first preferred embodiment except that the inner cylindrical member is a thick walled hollow tube. Prior to processing, the bore of the thick wall hollow tube is hermetically sealed to exclude autoclave gas. One method of hermetically sealing the tube is to fill the tube with a closely fitting solid metal bar which is then seal welded to the ends of the inner tube. Another suitable method of hermetically sealing the bore of the tube is to insert an end plug into each end of the tube and seal each plug therein. The tube bore may be evacuated, filled with air, or filled with an inert gas. Additionally, various solid fillers such, for example, as solid cylindrical punchings of metal, metal bars or metal rods may be stacked or otherwise disposed within the bore of the tube prior to inserting the end plugs and welding them to the tube. The solid filler must physically support the inner wall of the tube bore and be nonreactive with the material of the cylindrical member during processing of the device.

In each instance of the alternate and preferred embodiment of this invention, the assembled components are hot pressed by applying an isostatic pressure to the exterior surface of the assembled components only thereby reducing the diameter of each cylindrical component layer, and radially compressing the inner cylindrical member rather than stretching it as in prior art devices.

The thermoelectric materials employed herein may comprise metallic and nonmetallic substances such, for example, as solid or powdered metals, ceramics or semiconductors or mixtures of two or more. The thermoelectric members may be preliminarily prepared by compressing powdered thermoelectric materials in a suitable die to a density of about 85 percent and higher, or by casting in a manner known to those skilled in the art. The insulating materials employed as partition members and the like between thermoelectric bodies may comprise any good inorganic electrical insulators such, for example, as silica, mica, alumina, boron nitride, beryllium oxide and inorganic silicates such as boron silicate and lime glasses and those materials comprising the reaction product of mica lead borate glass sold under the trade name of Mycalex and materials comprising magnesium silicates sold under the trade name Lavite. The cylindrical metal members may comprise a good electrically and thermally conductive material, such as silver, aluminum, nickel, stainless steel, pure iron and copper or base alloys thereof.

In the preferred embodiments of the invention, there is provided an integral, elongated, thermoelectric element comprising a series of washer-shaped P- and N-type thermoelectric bodies electrically joined in series within a compartment provided between an outer cylindrical metal member and an inner cylindrical metal member electrically insulated therefrom but in good thermal conducting relation therewith. The thermoelectric element includes a plurality of inner and outer bridging metal ring members on an insulated cylindrical metal member disposed between successive thermoelectric washers. The bridging ring members may be electrically insulated from the cylindrical members by disposing a relatively thin insulating member therebetween either prepared separately as when employing a material such a'boron nitride or mica, or produced in situ by plasma jet spraying an insulating material, such as alumina, on the walls of the cylindrical members. When employing a series of adjacent bridging ring members, they will be electrically insulated from each other. The bridging ring members may comprise any good electrically conductive metal such, for example, as nickel, copper, aluminum, iron or base alloys thereof.

A plurality of compressed washers of powdered thermoelectric material are disposed with one inner diameter face on the inner bridging ring members and an outer bridging metal ring member disposed on the outer periphery of the compressed washer of thermoelectric material so that the thermoelectric washer members are electrically connected at each peripheral face or diameter thereof by either the inner or outer concentric bridging ring members while successive washers are electrically insulated from each other at the other diameter or peripheral face. The insulating materials employed between the thermoelectric washers may comprise any of the electrical insulators described previously with respect to the partition members of the less complicated configurations.

The number of thermoelectric washer members employed will determine the number of metal bridging ring members needed to provide electrical contact therebetween. It is preferred that each pair of thermoelectric washers electrically contacting each other consist of a P-type thermoelectric material and an N-type thermoelectric material. The components of the assembly are as closely packed as is possible so that the total free or gap space in the assembly is not above about 1 percent of the diameter of the outer cylindrical member.

The resulting elongated thermoelectric member after being fully processed by isostatic compressing has electrical leads attached to the end thermoelectric washers so that they may be connected in an electrical circuit.

In certain embodiments of the invention a complete thermoelectric device is not produced in a single operation. Thus,

a single P or N-type thermoelectric body is bonded metallurgically to the walls of two concentrically disposed cylindrical metal members. In this case the resulting isostatically processed member having its inner walls under compressive stress may be severed into a plurality of cylindrical units of any desired length which may be further machined to shape or size, or the member may be severed into relatively small individual thermoelectric pellets of desired shape. The cylindrical units or pellets may be joined to other thermoelectric pellets of opposite type to produce a stack of composite thermoelectric elements and assemblies which may be electrically connected, and suitably insulated both electrically and thermally, into thermoelectric power generators or cooling devices.

It is particularly desirable in all embodiments of the invention that the interior of the unit being deformed or hot pressed be evacuated prior to deformation to remove all reactive gases.

The terms isostatic pressure," isostatic deformation, isostatic cold pressing or isostatic hot pressing as used herein refer to a method involving the application of high enough pressures for reducing the cross-sectional area of a member to a sufficient degree to plastically deform and urge adjacent components into an intimate physical contact with each other, and even bond a component to the mating face of another adjacent contacting component, the pressures being uniformly applied or induced to all the exterior surfaces by using gases and/or liquids as a compressing medium. Although the preferred pressure range is from 5,000 to 50,000 psi, it should be appreciated that higher and, in some instances, slightly lower pressures may be employed depending upon the materials involved in the assembly, the temperature, time of application of temperature and pressure, and other factors. lt should be understood also, that an intimate physical contact, and when required, good metallurgical bonds between mating faces should be obtained, and when initial close tolerances between components are present, the amount of deformation required to provide the intimate physical contact and good bonds may be 1 percent or even less.

The temperatures employed in the process are selected by reference to the materials and their properties. In assemblies where some of the components may have large differences in thermal expansivity, in order to minimize joint stresses on cooling the joining temperatures should be selected at the lowest possible temperature which will effect a satisfactory bond of all component. Generally, the temperature chosen by the above consideration will be best for bonding any assembly whether there is expansivity mismatchbetween components or not. Other considerations are modulus of elasticity of the various materials and quality of the bonds in order to achieve the best bonded assembly with the minimum of internal stresses.

The period of time of application of temperature and pressure is selected by the consideration of allowing all parts of the assembly to achieve thermal equilibrium, and that of allowing solid-state diffusion to occur to effect the metallurgical bonds between adjacent contacting components of the assembly. If necessary, this latter requirement is readily determined by experiment in each case. Experimental data has indicated that many metallurgical bonds can be formed in as little as 15 minutes of heating, but periods of 2 hours appear to be more reliable. Bonding may be speeded up, or accomplished at lower temperatures or pressure, if desired, by adding joint bond promoters of various kinds, such as rapidly diffusing elements as is well known in the art.

By the isostatic process disclosed herein, all the bonds between all the components of the thermoelectric element may be formed in a single operation. That is, the complete assemblies are disposed in a suitable pressure vessel, such as an autoclave containing a heating coil and the desired temperatures, pressures and times for the particular assembly are imposed. After removal from the pressure vessel all the necessary bonds have been provided so that the only processing necessary thereafter is of a mechanical nature, suchas cleaning and applying leads, so that the completed unit may be integrated in some type of an electrical circuit.

Referring to FIG. 1, there is shown an isostatically hot pressed thermoelectric element 10, after one end of the element is removed as by machining, consisting of an inner thick wall hollow cylindricalmetal contact member 12 and an outer cylindrical concentric metal contact member 14 with a body 16 of compressed powdered thermoelectric material disposed therebetween and metallurgically bonded to the walls of the

metal members

12 and 14. Surprisingly, good bonding is effected between the

metal walls

12 and 14 with the body 16. The thermoelectric material body 16 may consist of any one of the P orN-type thermoelectric materials or two or more suitable layers in any desired arrangement or sequence.

The metals used in forming the

members

12 and 14 are selected on the basis of their compatibility with the thermoelectric material, desired electrical and thermal characteristics and resistance to the corrosive atmospheres for a given application.

The wall thickness of tubular member 12 is sufficient so that the tube will not collapse when subjected to the autoclave temperature and pressure. The entire assembly is evacuated, the materiall4 sealed off with a cap and the ends of member 12 are capped to prevent any appreciable pressure developing therein. When subject to the pressure of the autoclave of up to 50,000 p.s.i. or more at the desired temperature, the inner tube 12, as well as the adjacent layers of material 14, are compressed rather than stretched as in prior art devices. in subsequent service upon being heated to a high temperature, the member 12 expands partly due to the retained compressive stress, whereas prior art devices contracted. The ability of the member 12 to expand at the high temperature enables it to remain in a good thermal conductivity relationship with the thermoelectric component 16 of the element thereby preventing degradation of efficiency of element 10 due to loss of contact pressure between adjacent components.

When employing the isostatically hot pressed thermoelectric element 10 or a section thereof in an operational device, it is often desirable to connect two or more of either P or N-type or alternate P-N-type elements in a particular type of arrange ment and circuitry.

If it comprises the hot junction of a refrigerating device, the inner hollow contact member12 not only serves to carry electrical current, but enables a cooling. fluid such as water or air to be conveyed to dissipate heat. If the element 10 is employed as part of an electrical generator, hot gases, liquid or other heat source may be disposed in or passed through the hollow contact member 12. The outer contact member 14 may coola space or it may dissipate heat to a cold sink in either of these cases. The functions of the outer contact member 14 and the inner contact member 12 can be reversed.

With reference to FIG. 2 there is shown a thermoelectric element 20 which is another desirable embodiment of this invention.

The element 20 comprises two concentric cylindrical metal members, an outer member 22 and an inner member 24. A body 26 of thermoelectric material is disposed in the compartment defined by the outer wall of the inner member 24 and the inner wall of the outer member 22. A solid heat source 28 such, for example, as a radioisotope or a nuclear reactor fuel such as U0 disposed in a refractory, is disposed within and closely fills the hollow inner member 24. The assembly is evacuated and plugs seal up and ends 22 and 24 so that no hydrostatic pressure enters interiorly. I

The inner member 24 has a thin wall capable of being collapsed upon the solid heat source when subjected to the temperature and pressure of the autoclave during the isostatic compression of the element 20. Further, the wall 24 is put into compression by the pressure transmitted from wall 22 through the consolidation of material 26. A dense unitary element 20 is produced wherein the heat source'28 is in intimate contact tact with material 26 and so on. The heat source 28 is also in compression after the isostatic compression process step so that good heat flow occurs.

Referring to FIG. 3, there is shown a thermoelectric device 30 comprising an isostatically deformed complete thermoelectric element 40 embodying the teachings of this invention. The element 40 comprises a thick wall inner cylindrical metal member 42 and a concentric outer cylindrical metal member 44.

An insulating hollow cylindrical layer 46 is disposed about and joined to member42. The layer 46 comprises a material such, for example, as alumina, porcelain, mica and boron nitride. However, the insulating material may be plasma jet sprayed on the outer surfaces of the inner cylindrical member 42.

A plurality of inner bridging ring members 48 are disposed about and joined to the insulating layer 46. The ring members 48 are electrically insulated from each other by means of insulating washers 50 comprising materials, such as mica, or those selling under the trade name of Lavite or Mycalex (the latter being a glass-mica reaction product).

A plurality, of N-type thermoelectric material washer members 52 and a plurality of .P-type thermoelectric material, preferably previously preformed under pressure, washer members 54 are alternately disposed'on and joined to the bridging metal ring members 48.

A plurality of outer bridging metal ring members 56 are disposed on, and joined to, the thermoelectric washer members 52 and. 54, the ring members 56 each contacting a pair of adjacent P and N-type

thermoelectric washer members

52 and 54. The ring members 56 are electrically insulated from each other by means of insulating washers 58. The insulating washers 58 may comprise the same material as the insulating washer 50. The washers 58 have a larger outside diameter and a larger inside diameter than the corresponding diameters of washers 50.

A hollow concentric insulating cylindrical layer 60 comprising a material such as that employed for layer'46, is disposed about, and joined to, the outer ring members 56. The outer cylindrical metal member 44 is disposed about and joined to the insulating cylindrical layer 60.

The components of element 30, such as the

thermoelectric washer members

52 and 54, bridging

members

48 and 56, and the insulating

washers

50 and 58 are slipped into the space between

cylindrical members

40 and 42 with their applied in sulating layers 46 and 60 already applied. The smallest possible clearance is provided. The ends of the assembly are capped by welding a disk at each end and the interior is evacuatedthrough a hole or tube left in place, which opening is then sealed off. The assembly is then heated and compressed in an autoclave at pressures of up to 50,000 p.s.i. to consolidate the whole into a bonded element.

The isostatic hot pressing operation provides an intimate and effective metallurgical bond between the bridging

metal ring members

48 and 56 and the adjoining

thermoelectric washers

52 and 54 so as to provide a good electrical contact to each

thermoelectric washer member

52 and 54 whereby the

thermoelectric washer members

52 and 54 are electrically connected in series. A good thermal conductivity relationship allowing good heat flow is formed between the outer cylindrical metal member 44 and the insulating cylindrical layer 60 and between the insulating layer 60 and the metal ring members 56. Similarly, a good bond is formed between the inner hollow cylindrical member 42 and the inner cylindrical insulating layer 46 and between the insulating layer 46 and the inner metal ring members 48. Member 42 is in compressive stress.

After removal of the welded ends, disks, electrical connector clamps 62 and 64 may be then attached to form the thermoelectric device 30. The device 30 may then be connected to'a load 66 by .means of

electrical leads

68 and 70 attached to terminals 63 and 65 of

clamps

62 and 64.

The inner cylindrical metal member 42 of the device 30 is particularly suited to serve for passing high temperature gases and liquids so as to make this the hot side, and the outer cylindrical metal member of the device can be exposed to the cooling medium to serve as the cold side of a thermocouple. The inner cylindrical member may be conveniently heated by passing hot water, steam, a flame or the like therethrough. The outer cylindrical member may be cooled by flowing water or cold gases or air thereover. The difference in temperature between the hot side and cold sidewall cause an electrical current to be generated in the thermoelectric device by the phenomenon which is known in the art as the Seebeck effect. However, it should be understood that the inner metal member may serve as the cold side and the outer metal member as the hot side.

Furthermore, the Peltier efiect may be employed on the device 30 to produce refrigeration or cooling devices by passing electrical current through the

leads

68 and 70.

The preferred embodiment of this device 30 anticipates that in use the member 42 is the high temperature side. Thick wall inner metal member 42 is in compression as a result of processing of the element 40 in autoclave. The thickness of the member 42 is selected so as to enable the member 42 to withstand the temperature and pressure of the autoclave without failure or collapse. The hot pressing of the components of the element 40 at the high temperature and pressure of the autoclave leaves the metal of the thick wall inner member 42 in-a state of high compression.

Therefore, when the member 42 is the hot side of the thermoelectric element 40, the elevated temperature of the high temperature media passing through the hollow causes the member 42 to expand thereby enabling the member 42 to endeavor to remain in a good thermal conductivity relationship with adjoining components of the element 30.

With reference to FIG. 4 there is shown a thermoelectric element 80 utilizing an alternate embodiment of the teachings of this invention.

The element 80 comprises a thin wall hollow inner cylindrical metal member 32 and a concentric outer cylindrical metal member 84. Disposed within the hollow inner cylindrical metal member 82 is a suitable solid fuel source 86.

An insulating hollow cylindrical member 88 is disposed about and joined to the member 82. The member 88 comprises a material such, for example, as alumina, porcelain, mica and boron nitride. However, the insulating material may be plasma jet sprayed on the outer surface of the inner cylindrical member 82.

A plurality of inner bridging ring members 90 are disposed about and joined to the insulating member 88. The ring members 90 are electrically insulated from each other by means of insulating

washers

92 and 94 comprising materials such, for example, as mica, or those selling under the trade name of Lavite or Mycalex are alternately disposed between the ring members 90. The only difference between the

washers

92 and 94 is the inside and outside diameter measurements.

A plurality of a first N-type thermoelectric material washer members 96 and a plurality of first P-type thermoelectric material washer members 98 are alternately disposed on and joined to the bridging metal ring member 90.

A plurality of second N-type thermoelectric material washer members 100 are disposed on, and joined to, the first N-type washer members 96. A plurality of second P-type thermoelectric material washer members 102 are disposed on, and joined to, the first P-type washer member 98. The thermoelectric materials selected for comprising the

members

96, 98, 100 and 102 are chosen to provide the most efficient source of thermoelectricity for the expected temperature gradient across each

member

96, 98, 100 and 102 to be experienced during operation of the element 80,

The employment of the plurality of two different N-type thermoelectric materials and the plurality of two different P- type thermoelectric materials is a technique variously known as thermal cascading" or segmenting." As a specific example of this technique to illustrate this particular embodiment of this invention, the thermoelectric materials may be as follows:

Washer members 96-an N-type lead telluride alloy embodying a small amount of lead iodide as an additive and commercially available under the trade mark TEC-S-BN."

Washer members an N-type lead telluride alloy embodying a small amount of lead iodide as an additive and commercially available under the trade mark TEGS-ZN."

Washer members 98-a P-type lead telluride-tin telluride alloy commercially available under the trade mark TEGS3 P."

Washer members 102a P-type lead telluride alloy commercially available under the trade mark TEGS2P."

Electrically insulating the members 100 from the members 102 from each other is a plurality of insulating

washers

104 and 106, alternately disposed on, and joined to, the corresponding

insulation washers

92 and 94. The

washers

104 and 106 comprise the same materials as the

washers

92 and 94.

A plurality of outer bridging metal ring members 108 are disposed on, and joined to, the

thermoelectric washer members

100 and 102, the ring members 108 each contacting a pair of composite P-type thermoelectric washer members 96-100 and P-type thermoelectric washer members 98402. The ring members 108 are electrically insulated from each other means of the insulating washer 106.

A hollow concentric insulating cylindrical member 110 comprising a material such as that employed for reference numerals 88 is disposed about and joined to the outer ring members 108. The outer cylindrical metal member 84 is disposed about and joined to the insulating cylindrical member 1 10.

This alternate embodiment of the teachings of this invention also enables the thin wall of the inner member 82 to be collapsed under the influence of the temperature and the pressure of the autoclave onto the surface of the solid fuel source 86. The resulting structure therefore enables a good thermal conductivity relationship to be retained between all components of the element 80 during operation of the element 80.

Although hot isostatic pressing alone will produce a satisfactory thermoelectric device in accordance with the teachings of this invention, it is desirable in many instances to interpose a cold working or a cold compaction processing step between the assembly of the components and the hot isostatic pressing of the components of the device. The purpose of this cold working or compaction process step is to remove substantially all of the assembly clearances and voids from the internal circuit structure of the element prior to exposing the components of the element to the elevated temperatures of the hot isostatic pressing process. This precautionary cold working or compaction step effectively limits the possibility of sublimation of any of the component materials of the element, particularly the thermoelectric materials thereby preventing the resulting transport of the resultant vapors of the sublimed material through clearances and voids in the internal circuit structure and subsequent condensation of the sublimed material vapors at undesirable locations within the structure of the element.

Swagging, rocking-roll tube reducing and isostatic cold compressing are suitable processes for effecting this intermediate process step on the thermoelectric elements. Usually, a specific thermoelectric element is designed mechanically to be compatible with a specific cold working or a cold compaction process. Swagging and rocking-roll tube reducing will yield uniform overall diametral changes at the expense of a slight distortion and/or elongation of the element in an axial direction. Isostatic cold compressing, performed in a liquid or a gas autoclave at about room temperature and applied pressures of from 20,000 p.s.i. to 100,000 p.s.i., will yield uniform diametral changes proportional to the elastic/plastic properties of the material or materials at any given cross-sectional plane along the axis of the element with substantially no attendant axial displacement. The temperatures of this cold The reduction in the annular area between the inner and the inner tube surfaces of the assembly. The assemblies were initially under a 10,000 p.s.i. pressure at room temperature, and then heated to a temperature of 650 C. while maintaining the same pressure and they were held at that temperature and 2 Cold isostatic compressing.

this invention:

Example I Six thermoelectric elements similar to that shown in FIG. 3 were assembled and fabricated by employing a prior art method as follows:

The inner cylindrical member employed was a stainless steel hollow tube having a 0.376 in. 1D. and 0.426 in. OD. and having a 0.015 in. thick tube of boron nitride disposed thereon. The bridging contact ring members consisted of low carbon. steel and were of two different sizes. The inner contacts measured 0.460 in. [.D. and 0.514 in. OD. The outer contacts measured 0.76 in. ID. and 0.79 in O.D. The thermoelectric washers employed consisted of P and N-type lead telluride. having a density of 90 percent of theoretical and measuring 0.764 in. O.D. and 0.516 in. [.D. The insulating material between alternate thermoelectric washers and bridging contacts consisted of mica washers and were of two sizes. The inner insulating washers measured 0.764 in. O.D. and 0.460 in. ID. The outer insulating washers measured 0.788 in. O.D. and 0.516 in. ID. The outer bridging contacts had a 0.015 in. thick boron nitride tube disposed thereon. The outer cylindrical member consisted of a stainless steel tube 10 in. long and measuring 0.870 in. O.D. and 0.821 in. ID. The total gap space of the assembly in the radial direction was 0.006 in.

Annular end plugs consisting of sheet stainless steel were then inserted and welded at the ends of the assembly to the inner and outer tubes and the annular area between the inner and outer tubes of the assembly was evacuated through a tube which was then sealed off. The assemblies were each disposed in a separate autoclave and treated in a similar manner. However, autoclave pressure was imposed on both the outer and outer cylindrical members which results from any of the 5 pressure for 2 hours. The assemblies were then cooled and the above-mentioned intermediate cold working process steps is pressure decreased to about 4000 or 5000 p.s.i., the decrease generally less than percent. The percentage reduction of in pressure being linear with the decrease in temperature. The the annular area produced by this cold compacting step gas pressure maintained in the autoclave was through the use renders a small reduction during the hot isostatic pressing of OfhBhUm g the annular area fully adequate to secure metallurgical bond- The deVlCeS e t st d by ln rllng a r h a er nd thering, so thatthe larger reductions in the 1 percent to percent mocouples each bore. A temperature differen e of 1 8 C. range are unnecessary Thus, 1 Percent m 6 Del-cam hot was maintained between the outer and inner cylindrical mem- Static reduction in b f ll adequate, and in some cases even bers when the outer side was cooled with water. The elements less than 1 percent is sufficient. l 5 were evaluated for non'nalized thermal conductance and nor- Referring now to Table I thereisshown a compilation of [hauled overall emcwncy when Operate}! over 3 Period of data obtained from the fabrication of each of six different tlme- The data Pbtamed these tests 15 Plotted FIGS- 5 types of tubular thermoelectric elements in accordance with and 6 and ldentlfied as Prlor elements- I the teachings of this invention. Particularly, the data of Table I Example 11 illustrates the magnitudes of the reduct1on 1n the annular cross-sectional area of each element achieved as a result of an Three thin wall thermoelectric elements hereinafter intermediate cold working process step, as well as the addiidentified as No. l, 2 and 3, were prepared and assembled. tional reduction in the annular cross-sectional area achieved The basic construction of each element was the same as the as a result of hot isostatic pressing. The cold isostatic pressing l m nt h wn in FIG. 3 except that the inner member of each and the hot isostatic pressing applied pressure only to the ex- 0f the elements was f h hm W ll h ll w tube type and a terior surfaces of the elements. simulated solid fuel source was employed. Each of the ele- TABLE I 52335 Cold processing Ilot isostatic pressing IEJEHEuJII W 4 Itcductibn O.D. LD. O.D. in annulur 'lump. Pressure 0.1). in annular Element type (in.) (in.) Method (in.) area 1 C.) (p.s.i.) (in.) area 1 (Example I) 0. 870 0.375 Swaging 750 10, 000 0. 840 8.3 2 (Example II). 1.625 0.500 do 750 10,000 1.570 7.3 3 (Example IV). 1.700 0 500 Gold compactl 650 20, 000 1.660 5. 1 4 1. 700 650 15, 000 1. 663 4. 7 5..- 2. 714 650 20, 000 2. 632 7. 1 6 1. 685 650 20, 000 1. 642 6. 3

1 Percent of initial area.

ments was exactly the same as the prior art elements fabricated and tested in Example I except that

elements

1, 2 and 3 were fabricated in accordance with the teachings of this invention.

The inner cylindrical member employed was a stainless steel hollow tube having a 0.376 in. ID. and 0.426 in. O.D. and having a 0.015 in. thick tube of boron nitride disposed thereon. The bridging contact ring members consisted of low carbon iron and were of two different sizes. The inner contacts measured 0.460 in. LB. and 0.514 in. O.D. The outer contacts measured 0.760 in. ID. and 0.790 in O.D. The thermoelectric washers employed consisted of P- and N-type lead telluride having a density percent of theoretical and measuring 0.516 in. ID. and 0.764 in. O.D. The insulating material between alternate thermoelectric washers and bridging contacts consisted of mica washers and were of two sizes. The inner insulating washers measured 0.460 in. ID. and 0.764 in. O.D. The outer insulating washers measured 0.516 in. ID. and

0.788 in. O.D. The outer bridging contacts had a 0.015 in.

thick boron nitride tube disposed thereon. The outer cylindrical member consisted of three sections of stainless steel tube totaling 10 in. in length and measuring 0.821 in. ID. and 0.870 in. O.D. The total gap space of the assembly in the radial d rsst 2.919% i The outer bridging contact at each end contained a flange into which four steel conductor pins were inserted. These pins passed through tubular blocks of electrically insulating material comprising aluminum magnesium silicate and a stainless steel retaining ring. The pins were insulate from the retaining ring with small tubular washers of boron nitride. At each end of the devices a stainless steel end ring containing an evacuation tube was welded to the inner and outer cylindrical member s The joints between the three sections of the outer cylindrical member were welded together such that joints were also formed between the outer cylindrical member and the retaining rings. The annular region between the inner and outer cylindrical members was then evacuated through the evacuation tubes and the tubes were then sealed by welding.

Rod-type electrical resistance heaters measuring 0.375 in. OD. and 3.25 in. long were inserted in the bores of the assembled elements such that the heater length was adjacent to the active thermoelectric circuit. The remainder of the bore was filled with solid plugs of insulating material comprising aluminum magnesium silicate. A stainless steel solid plug was then welded into each end of the bore of the inner cylindrical member.

The elements so sealed were then passed through the dies of a swaging machine, reducing the OD. of the element from 0.870 to 0.860 in. This represents a reduction in annular cross-sectional area of about 2.8 percent, and effectively placed all of the layers in physical contact with one another.

The elements were then disposed in an autoclave. The autoclave was heated with an internal electric furnace and pressurized by employing helium gas. A pressure of7500 p.s.i. was imposed on the elements at room temperature. They were then simultaneously heated and additionally pressurized until a temperature of 650 C. and a pressure 10,000 p.s.i. were reached. These conditions of temperature and pressure were maintained for 2 hours, after which the temperature and pressure were simultaneously reduced until room temperature and a pressure of about 5000 p.s.i. were reached. The remaining pressure was then removed and the elements were taken out of the autoclave. The CD. of the elements had been reduced to 0.840 inch, which represents an annular area reduction of about 5.7 percent from the preautoclave condition and a total annular area reduction of about 8.3 percent from the preswaging condition.

The ends of the devices were machined to expose the conductor pins of the thermoelectric circuit and the terminals of the rod heater. Testing was accomplished using the rod heater and an external blower. A temperature difference of 168 C.

was maintained across the radius of each device by heating the inner cylindrical member with the rod heater and cooling the outer cylindrical member with the forced air form the blower.

The elements were evaluated for normalized thermal conductance and normalized overall efficiency when operated over a period of time. The data obtained are graphed as shown in FIGS. and 6. The superiority in the performance of

elements

1, 2 and 3 is clearly evident and can be attributed solely to the novel fabrication process of this invention.

Example 111 Two thick wall thermoelectric elements, hereinafter identified as elements No. 4 and 5, were assembled. The basic construction of each element was similar to that shown in FIG. 3.

The inner cylindrical member was a Hastelloy B hollow tube having a 0.500 in. ID. and 0.650 in. OD. and having a 0.018 in. thick tube of boron nitride disposed thereon. The bridging contact ring members consisted of low carbon and and were of two different sizes. The inner contacts measured 0.690 in. [.D. and 0.791 in. OD. The outer contacts measured 1.291 in. ID. and 1.330 in. OD. The thermoelectric washers employed consisted of P- and N-type lead telluride having a density 93 percent of theoretical and measuring 0.792 in. ID. and 1.290 in. 0.1). The insulating material between alternate thermoelectric washers and bridging contacts consisted of mica washers and were of two sizes. The inner insulating washers measured 0.690 in. ID. and 1.289 in. OD. The outer insulating washers measured 0.793 in. ID. and 1.330-in. OD. The outer bridging contacts had a 0.020 in. thick boron nitride tube disposed thereon. The outer cylindrical member consisted of three sections of stainless steel tube totaling 14.4 in. in length and measuring 1.374 in. ID. and 1.625 in. OD. The total gap space of the assembly in the radial direction was 0.006 in.

The outer bridging contact at each end contains a flange into which four steel conductor pins are inserted. These pins pass through tubular blocks of insulating material comprising magnesium aluminum silicate and a stainless steel retaining member. The retaining member was welded to the inner cylindrical member. The pins were insulated from the retaining member with small tubular washers of boron nitride. At each end of the elements a stainless steel end ring containing an evacuation tube was welded to the inner and outer cylindrical members. The joints between the three sections of the outer cylindrical member were welded together such that joints were also formed between the outer cylindrical member and the retaining members. The annular region between the inner and outer cylindrical members was then evacuated through the evacuation tubes and the tubes were then sealed by welding.

The elements were then passed through the dies of a swaging machine, reducing the OD. from 1.625 to 1.590 in. This represents a reduction in annular cross-sectional area of about 4.7 percent and effectively eliminated internal clearances and placed all the layers in close contact with one another. A stainless steel of solidplug was then welded into each end of the bore of the inner cylindrical member of each device.

The devices were then disposed in an autoclave which could be pressurized with helium gas and heated with internal coils. A pressure of 7500 p.s.i. was imposed on the elements at room temperature. They were then simultaneously heated and additionally pressurized until 750 C. and 10,000 p.s.i. were reached. These conditions were maintained for 2 hours, after which the temperature and pressure were simultaneously reduced until room temperature and about 7500 p.s.i. were reached. Then the remaining pressure was removed and the elements were taken out of the autoclave. The outside diameter of the elements had been reduced to 1.570 inch, which represents an area reduction of about 2.8 percent from the preautoclaving condition and a total area reduction of about 7.3 percent from the preswaging condition.

The ends of the elements'were machined to expose the conductor pins of the thermoelectric circuit and to remove the plugs from the bores of the inner cylindrical members.

Testing was accomplished by inserting a rod heater in the bore to heat the inner surface and using forced air from a blower to cool the outer surface. A temperature difference of about 450 C. was maintained across the radius of the devices in this manner.

The elements were evaluated for normalized thermal conductance and normalized overall efficiency when operated over a period of time. The data obtained are plotted on a graph as shown in FIGS. 5 and 6, as elements No. 4 and 5.

Example IV A thick wall thermoelectric element identical to those elements fabricated and described in Example 111, except for having an inner cylindrical member made of Inconel X-750 and an outer cylindrical member measuring 1.700 in. OD, was assembled, evacuated, and the ends hermetically sealed by welding. This element is described hereinafter as element No. 6.

Along, close-fitting solid stainless steel center rod and two short solid stainless steel end plugs, all closely fitting the bore were inserted as a solid column into the bore of the inner cylindrical member and the end plugs were seal welded to the ends of the member. The element was then cold isostatic compressed in a liquid autoclave at room temperature, with an iso static pressure of 50,000 p.s.i. being applied to the exterior surfaces of the element only. This cold isostatic compression process reduced the outer diameter from 1.700 to 1.675 inches, which represented a reduction in annular cross-sectional area of about 3.2 percent, and effectively placed all the layers of the element in an intimate physical contact with one another.

The thermoelectric element was then disposed within an autoclave which could be pressurized with helium gas. The autoclave was heated internally with electrical coils. While at room temperature helium at a pressure of 15,000 p.s.i. was imposed on the exterior surfaces of the element only. The element was then simultaneously heated and the pressurized increased until the conditions of temperature of 650 C. and a pressure of 20,000 p.s.i. were reached. The elevated temperature and higher pressure were maintained on the element for 2 hours, at which time the temperature and the pressure were simultaneously reduced until the helium was at room temperature and a pressure of 15,000 p.s.i. was achieved. The helium gas was released from the autoclave and the processed element removed from the autoclave.

The outside diameter of the element had been reduced to 1.660 inches which represented an annular area reduction of about 2.0 percent from the prehot isostatically compressed condition, there being a total annular area reduction of about 5.1 percent from the precold compaction condition;

The element was prepared for testing, tested and the results evaluated in a similar manner as the elements of Example III. The data obtained from the tests is plotted in FIGS. 5 and 6 and denoted as element No. 6.

A review of the data plotted in FIG. 5 reveals that the element utilizing and fabricated by means of the teachings of this invention are far superior to those elements manufactured by the prior art method.

It is to be noted that the elements embodying the teachings of this invention still have a normalized thermal conductance of 0.95 of the original values or better after 900 hours of operating time. In the elements embodying the teachings of the prior art the normalized thermal conductancedeteriorated to below 0.7 after less than 600 hours of operating time.

In reviewing the collected data plotted in FIG. 6, it will be seen that the elements embodying the teachings of this invention have a higher normalized efficiency for a greater operating time than the elements embodying the teachings of the prior art. After 600 hours of operating time the elements embodying the teachings of this invention dropped to a normalized overall efficiency of approximately 0.9 of the initial values and remained at this efficiency for more than 300 hours more. By comparison the prior art processed elements decreased to an efficiency of from approximately 0.6 to 0.85 in 500 hours of operating time or less.

While the invention has been described with reference to particular embodiments and examples, it will be understood, of course, that modifications, substitutions and the like may be made therein without departing from its scope.

We claim as our invention:

1. In a process for producing an integral thermoelectric element comprising a hollow inner cylindrical member, a concentric outer cylindrical member and a body of thermoelectric material disposed in the compartment defined by the outer surface of the inner member and the inner surface of the outer member, the steps comprising:

1. disposing the body of thermoelectric material in the compartment between the inner and the outer member,

2. sealing the ends of the compartment and the hollow of the inner member so that no fluid may enter therein,

3. evacuating the sealed body of thermoelectric material in the compartment,

4. compressing the members and the thermoelectric material at a temperature from about 250 C. to slightly below the melting temperature of the lowest melting material comprising the element by applying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. only to exterior surfaces of the element until the element has been plastically deformed at its exterior surfaces and reduced in crosssectional area by a value of from about 1 percent to percent to provide at least an intimate physical contact between the body of thermoelectric material and the inner and outer members, and the inner cylindrical -member is in compression after the isostatic pressure is removed.

2..The process of claim 1 in which compressing the members results in a metallurgical bond between the body of thermoelectric material and the inner and outer members of the element.

3. The process of claim 1 in which the hollow inner cylindrical member has a thin wall, sealing and evacuating a solid heat source disposed within the hollow of the inner member, and collapsing the thin wall upon thesolid heat source by the application of the isostatic pressure at the elevated temperature.

4. The process of claim Lincluding cold'working the components comprising the element after assembly to reduce internal clearances and close gross voids prior to being subjected to the isostatic pressure at an elevated temperature.

5. The process of claim 4 in which the cold working is performed by isostatic pressure applied only to the exterior surface of the assembled components at room temperature.

6. The process of claim 1 in which the hollow of the inner member is filled with an inert gas and sealed prior to the assembled element being subjected to the isostatic pressure at an elevated temperature. V

7. In a process for producing a thermoelectric element, the steps comprising: I

. disposing a plurality of inner bridging metal ring members on an insulated hollow cylindrical metal member, the ring members being electrically insulated from each other,

2. disposing a plurality of washers of thermoelectric material on the ring members,

3. disposing a plurality of outer bridging metal ring members on said thermoelectric washers, the ring members being electrically insulated from each other, disposing an insulated outer cylindrical metal member on the ring members, the assembly having a gap space of not above about 1 percent of the diameter of the outer cylindrical member,

4. sealing the ends of the cylindrical metal members to provide a sealed enclosure for the components therein,

5. sealing the end of the hollow cylindrical metal member so that no fluid can enter therein, 7

6. evacuating the internal spaces within the sealed assembly,

and

7. compressing the members and the components contained therebetween at a temperature of from 250 C. to slightly below the melting temperature of the lowest melting temperature component in the assembly by applying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. to the exterior surface only of the assembly until plastic deformation occurs at the outer portions and until there is a reduction in cross-sectional area of the space between the inner and outer cylindrical members of from 1 percent to 15 percent whereby to provide a metallurgical bond between the bridging metal ring members and the thermoelectric washers so that an applied or induced electric current will meet alow electrical resistance flow between said washers, and the element in use at elevated temperatures maintains a high electrical and thermal conductivi- 8. "l he process of claim 7 including:

cold working the assembled members and components of the element prior to compressing the members and the components isostatically at an elevated temperature to reduce internal clearances within the element.

9. The process of claim 8 in which cold working is performed by isostatic pressure on exterior surfaces only of the element at about room temperature.

10. The process of claim 7 in which the hollow inner cylindrical member has a thin wall, sealing and disposing a solid heat source disposed within the hollow of the inner member, and collapsing the thin wall upon the solid heat source by the application of the isostatic pressure at the elevated temperature.

11. The process of claim 7 in which the hollow of the inner portion of the washer.

13. The process of claim 9 in which the washers of thermoelectric comprise at least two varieties for each type of thermoelectric material each variety having of same type thermoelectric material having an optimum range of thermoelectric efficiency different from each of the other varieties, one variety of one type thermoelectric material comprising one radial portion of the washer and a second variety of said one type thermoelectric material comprising the second radial portion of the washer.

Claims

1. In a process for producing an integral thermoelectric element comprising a hollow inner cylindrical member, a concentric outer cylindrical member and a body of thermoelectric material disposed in the compartment defined by the outer surface of the inner member and the inner surface of the outer member, the steps comprising: 1. disposing the body of thermoelectric material in the compartment between the inner and the outer member, 2. sealing the ends of the compartment and the hollow of the inner member so that no fluid may enter therein, 3. evacuating the sealed body of thermoelectric material in the compartment, 4. compressing the members and the thermoelectric material at a temperature from about 250* C. to slightly below the melting temperature of the lowest melting material comprising the element by applying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. only to exterior surfaces of the element until the element has been plastically deformed at its exterior surfaces and reduced in cross-sectional area by a value of from about 1 percent to 15 percent to provide at least an intimate physical contact between the body of thermoelectric material and the inner and outer members, and the inner cylindrical member is in compression after the isostatic pressure is removed.

2. The process of claim 1 in which compressing the members results in a metallurgical bond between the body of thermoelectric material and the inner and outer members of the element.

3. The process of claim 1 in which the hollow inner cylindrical member has a thin wall, sealing and evacuating a solid heat source disposed within the hollow of the inner member, and collapsing the thin wall upon the solid heat source by the application of the isostatic pressure at the elevated temperature.

3. evacuating the sealed body of thermoelectric material in the compartment,

4. The process of claim 1 including cold working the components comprising the element after assembly to reduce internal clearances and close gross voids prior to being subjected to the isostatic pressure at an elevated temperature.

4. compressing the members and the thermoelectric material at a temperature from about 250* C. to slightly below the melting temperature of the lowest melting material comprising the element by applying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. only to exterior surfaces of the element until the element has been plastically deformed at its exterior surfaces and reduced in cross-sectional area by a value of from about 1 percent to 15 percent to provide at least an intimate physical contact between the body of thermoelectric material and the inner and outer members, and the inner cylindrical member is in compression after the isostatic pressure is removed.

5. sealing the end of the hollow cylindrical metal member so that no fluid can enter therein,

6. The process of claim 1 in which the hollow of the inner member is filled with an inert gas and sealed prior to the assembled element being subjected to the isostatic pressure at an elevated temperature.

6. evacuating the internal spaces within the sealed assembly, and

7. In a process for producing a thermoelectric element, the steps comprising:

7. compressing the members and the components contained therebetween at a temperature of from 250* C. to slightly below the melting temperature of the lowest melting temperature component in the assembly by applying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. to the exterior surface only of the assembly until plastic deformation occurs at the outer portions and until there is a reduction in cross-sectional area of the space between the inner and outer cylindrical members of from 1 percent to 15 percent whereby to provide a metallurgical bond between the bridging metal ring members and the thermoelectric washers so that an applied or induced electric current will meet a low electrical resistance flow between said washers, and the element in use at elevated temperatures maintains a high electrical and thermal conductivity.

8. The process of claim 7 including: cold working the assembled members and components of the element prior to compressing the members and the components isostatically at an elevated temperature to reduce internal clearances within the element.

11. The process of claim 7 in which the hollow of the inner member is filled with an inert gas and sealed prior to the assembled element being subjected to the isostatic pressure at an elevated temperature.

12. The process of claim 7 in which each of the washers of thermoelectric material comprise at least two different varieties of the same type thermoelectric material each variety of thermoelectric material having an optimum range of thermoelectric efficiency different from each of the other varieties, one variety of one type thermoelectric material comprising one radial portion of the washer and second variety said one type thermoelectric material comprising the second radial portion of the washer.