United States Patent (19) 11) 4,211,889 Kortier et al. 45) Jul. 8, 1980 (54) THERMOELECTRIC MODULE 3,279,954 10/1966 Cody et al. ...... ... 136/205 3,296,033 1/1967 Scuro et al. .......................... 136/205 75) Inventors: William E. Kortier; John J. Mueller; 3,351,498 11/1967 Shinn et al. .......................... 136/205 Philip E. Eggers, all of Columbus, 3,382,109 5/1968 Kendall, Jr. et al. ................ 136/237 Ohio 3,41,955 11/1968 Weiss ................... ... 136/205 73 Assignee: The United States of America as 3,444,005 5/1969 Avis et al. ....... ... 36/205 represented by the Department of 3,510,362 5/1970 Charland et al. ................ 136/205 X Energy, Washington, D.C. Primary Examiner-Harvey E. Behrend Attorney, Agent, or Firm-R. V. Lupo; Frank H. 21 Appl. No.: 761,894 Jackson 22 Filed: Sep. 16, 1968 57 ABSTRACT 51 Int. Cl’............................................... HOW 1/5 A thermoelectric module containing lead telluride as 52 U.S. C. ..................................... 136/237; 136/238 the thermoelectric material is encapsulated as tightly as 58 Field of Search ............... 136/205, 203, 237, 204, possible in a stainless steel canister to provide minimum 136/238,223, 208, 228 void volume in the canister. The lead telluride thermo 56) References Cited electric elements are pressure-contacted to a tungsten U.S. PATENT DOCUMENTS hot strap and metallurgically bonded at the cold junc 2,997,514 8/1961 Roeder, Jr. .......................... 136/204 tion to iron shoes with a barrier layer of tin telluride 3,051,767 8/1962 Fredricket al.... ... 136/205 X between the iron shoe and the p-type lead telluride 3,076,051 1/1963 Haba .................................... 136/204 element. 3,082,276 3/1963 Corry et al. ......................... 136/205 3,232,719 2/1966 Ritchie ............................. 136/237 X 1 Claim, 1 Drawing Figure R N S2 ZZéza NN 2É %2/ N 2. 3 2-/6 N M2 s 23N 2SS26%-ys a NN22 2Z2 2 I 6, 75ZZ == S. 4,211,889 1. 2 reaction of the excess tellurium with the material of the THERMOELECTRIC MODULE hot contact or shoe or by sublimation of the tellurium. It appears that the presence of oxygen in even very CONTRACTUAL ORIGIN OF THE INVENTION small amounts in the atmosphere surrounding the ther The invention described herein was made in the 5 moelectric element promotes the sublimation of tellu course of, or under, a contract with the UNITED U. STATES ATOMIC ENERGY COMMISSION. We have found that we can reduce degradation of the power developed by the thermoelectric element by (a) BACKGROUND OF THE INVENTION employing a tungsten contact at the hot junction of the This invention relates to a thermoelectric module. In 10 p-type element and an iron shoe separated from the more detail the invention relates to a nondegrading lead thermoelectric material by a layer of tin telluride at the telluride thermoelectric module with low contact resis cold junction of the p-type element and by (b) hermeti tance junctions. cally sealing the thermoelectric module within a casing The principle of thermoelectric conversion has been which fits as tightly as possible around the thermoelec known for many years but until recent years has found 15 tric material to provide minimum voids within the cas little other application than in temperature measure ing. ment and control. Recently, however, development of While it is clear that each of these expedients taken power systems for use at remote locations-such as in individually produces improved results, it is also clear space-has become necessary and thermoelectric con that it is only by employing all of the expedients simul version has been utilized for this purpose. 20 taneously that maximum benefits from the invention are Desirable properties for thermoelectric materials attained. include (1) proper free-electron generation characteris tics, i.e., the number of free electrons must be sufficient BRIEF DESCRIPTION OF THE DRAWING to overcome internal electrical resistance, yet not so The drawing is a sketch of a thermoelectric module great as to introduce scattering effects, (2) low thermal 25 constructed in accordance with the present invention. conductivity so that heat cannot flow through the mate Although the sketch is of a module constructed specifi rial without performing some useful work, (3) ability to cally for experimental purposes, an operational module operate at high temperatures, and (3) various additional property requirements depending on the specific appli would not differ from the sketch in details material to cation for the material. 30 the present invention. Because of (1) and (2) above, semiconductor materi DESCRIPTION OF THE PREFERRED als are more efficient thermoelectric materials than EMBODIMENT ordinary metals. One material that has found practical application is lead telluride (PbTe), a binary compound As shown in the drawing, a rectangular thermoelec of lead and tellurium capable of operation at tempera 35 tric module 10 constructed in accordance with the pres tures up to 1100-1200' F. By addition of an impurity (a ent invention includes an n-type lead telluride (PbTe) process referred to as doping) the lead telluride can be thermoelectric element 11 separated from a p-type ther made to operate as an n- or p-thermoelectric material. moelectric element 12 by a mica barrier sheet 13. The Although lead telluride has been used more widely than n-type PbTe element is doped with iodine and excess any other thermoelectric material up to this date, ther lead and the p-type PbTe element is doped with sodium moelectric modules made from lead telluride have not and excess tellurium. These elements are enclosed performed to the levels theoretically available from the within a mica sleeve 14 and the entire unit is hermeti material. An additional problem to that of achieving cally sealed within a stainless steel casing 15 which high-level performance is that of performance degrada tightly encloses the module to provide minimum voids tion during operation of the module. Postoperative 45 within the casing. examinations of elements from modules have identified Thermoelectric module 10 is provided with a tung the p-type elements as being the primary source of the sten hot strap 16 which contacts the hot end of both the trouble; specifically the resistance increases in the hot n-type thermoelectric element 11 and the p-type ther zone of the p-type leg of the thermocouple. This degra moelectric element 12 and with iron shoes 17 and 18 on dation of the module substantially reduces module life 50 the cold ends of the n-type and p-type thermoelectric from that theoretically attainable. elements, respectively. Shoe 17 is brazed directly to A substantial effort has, of course, been made to iden n-type thermoelectric element 11, but shoe 18 is sepa tify the cause of and to suggest means for reducing the rated from p-type thermoelectric element 12 by a bar degradation of lead telluride thermoelectric modules. rier layer 19 of tin telluride. The unit including shoe 18, We feel that we have identified this cause and have 55 barrier layer 19 and element 12 is formed by cold press suggested means whereby degradation of such modules ing and sintering under axial pressure. can be substantially reduced, thereby providing longer Contact between elements 11 and 12 and tungsten hot life for the modules. strap 16 is obtained by the pressure of Belleville springs 20 against copper spring followers 21 which force the SUMMARY OF THE INVENTION n-type and p-type elements against tungsten hot strap We have found that degradation of p-type lead tellu 16. Hot platen 22, which is welded to the top of stainless ride thermoelectric elements can be attributed primarily steel casing 15, resists this pressure. Spring followers 21 to very slight changes in the proportion of tellurium are soldered to iron shoes 17 and 18 using an In-Sn present in the element. P-type lead telluride elements solder. are doped with excess tellurium as well as sodium and 65 A cold sink 23 formed of copper includes a cavity 24 reduction in the amount of tellurium present adversely in which Belleville springs 20 and spring followers 21 affects the performance of the element. This reduction are disposed, a coolant tube 25 extending through the in the amount of tellurium present can occur through cold sink, and a rectangular well 26 which receives the 4,211,889 3 4. free end of casing 15, the well being filled with an epoxy composite of PbTe, SnTe and iron and sintering in resin which serves as a seal 27. Cold sink 23 is electri hydrogen at 1200' F. with an axial load applied. cally divided by electrical insulation 28 to prevent a Elements prepared by both procedures showed an short circuit between the n-type and the p-type thermo acceptable resistance across the bond and a generally electric elements. Electrical contacts, not shown, are, of 5 high level of stability through a cycling period which course, also provided. involved six to eight cycles from a hot-junction temper Thus, it is evident that the design of the containers ature of 980 F. to near room temperature. However, features (1) a welded stainless steel enclosure at the hot during subsequent life testing over an extended period junction and a high-density epoxy seal at the cold junc of time (1000 hours), a significant degradation of power tion of the couple to assure isolation from the environ 10 occurred. Resistance traverses of the elements after ment, (2) an internally contained spring-loading assem testing disclosed a significant increase in material resis bly to maintain the required level of axial pressure on tance in the zone of the hot junction.