Feb. 1, 1966 I. M. RITCHE 3,232,719 THERMOELECTRIC BONDING MATERIAL Filed Jan. 17, 1962 2 Sheets-Sheet 9 OO 8OO 3. 7OO H 1 , ; 6OO 2 FG. 2 500 4OO is 3OO - 2OO OO V / O 2 4 6 8 IO 2 4 6 1820 22 24, 26 TIME (MINUTES) 8OO FIG.2 7OO 6OO l, SP 5OO 71 4OO 4003.5 f 2 2Oo32 6/ 5. 3OO L Cl EY 20o O 2 3 4 H DISTANCE ALONG BAR N mm FIG.O O 2 4 6 8 IO 2 4 6 82O 222426 TIME (MINUTES) FIG.3 INVENTOR. (36,747/7%a?eIAN M. RITCHIE ATTORNEYS Feb. 1, 1966 I. M. RITCHE 3,232,719 THERMOELECTRIC BONDING MATERIAL Filed Jan. 17, 1962 2 Sheets-Sheet 2 IEE 3 DISTANCE ALONG BAR INVENTOR. (IN mm) IAN M. RITCHE FIG.8 ATTORNEYS 3,232,719 United States Patent Office Patented Feb. 1, 1966 1. 2 rial to impair or prevent the formation of a satisfactory 3,232,719 bond. This problem is partially solved by carrying out THERMOELECTRIC BONDING MATERAL Ian M. Ritchie, Wakefield, Mass., assignor to Transitron all operations in a reducing atmosphere, since such ther Electronic Corporation, Wakefield, Mass., a corpora moelectric material oxidizes very quickly. tion of Delaware Another object of the present invention is to provide a Filed Jan. 17, 1962, Ser. No. 166,896 bonding material for a thermoelectric material and con 14 Claims. (C. 29-195) tact electrode which is particularly adapted for com merical manufacture. The invention relates to means for electrically contact One other object of the present invention is to provide ing Semimetallic, thermoelectric compositions such as lead a bonding material for thermoelectric materials and con and Selenium, tellurium and/or sulphur systems. As tacting electrodes which minimizes problems of cracking stated in United States Letters Patent No. 2,811,569, due to differential thermal expansions or thermal expan issued to Fritts et al. on October 29, 1957, a major ob sion mismatches. stacle in using electrical conductors of the semimetallic A further object of the present invention is to provide alloy type referred to above for thermoelectric purposes 15 a material for bonding thermoelectric devices and con has been "the difficulty of making electrical contact to tact electrodes which will operate over a wide range of the conductors without encountering an alloying or solu temperatures without affecting the thermal or electrical tion of the electrode in the conductors. Such alloying or characteristics of the thermoelectric device. Solution between the electrical conductor and the elec The present invention overcomes the foregoing prob trode causes a change in composition of the electrical con 20 lems by providing a bonding material or alloy for use in ductor which generally results in the reduction of the connection with both P and N-type thermoelectric mate high thermoelectric power, hence, such alloying or solu rials of various thermoelectric systems. For example, tion must be controllably restricted if uniformity of the metal-nonmetal thermoelectric alloys which have as their electrical properties and long life of the electrical con principal constituents at least one element selected from ductor are desired.” It is further suggested that material 25 the group consisting of lead, tin or germanium (hereafter used in contacting the electrodes and the thermoelectric defined as the metal group), and at least one element material must not dissolve one in the other at any tem selected from the group consisting of tellurium, selenium perature within the operating range of the device. and sulphur (hereafter defined as the nonmetal group), Unfortunately, most metals commonly considered to be are primarily useful in this invention. In the present in electrode materials will readily alloy, and thereby poison 30 vention there is provided a bonding material or alloy con Selenium, tellurium and sulphur compounds. Fritts et al. sisting of a unique mixture of metal and nonmetal ele Supra, offers to solve the problems partially referred to ments in selected atomic proportions, with the metal se above by providing a contact electrode of iron. However, lected from the group consisting of calcium, strontium, iron as a contact electrode is not an altogether satisfactory barium, lead, germanium, tin, manganese, beryllium, va Solution. Bonds to iron must be made at temperatures in 35 nadium, ytterbium and zinc and the nonmetal selected the range of 900 C., to 1000° C. Since this temperature from the group consisting of tellurium, selenium and Sul range exceeds the melting point of such thermoelectric phur. As defined in this connection, "unique mixture' materials the bond must be effected by a localized melting refers to a composition of the aforesaid metals and non process which requires extreme care and control to avoid metals which has but a single stable stoichiometric ar melting the entire thermoelectric device. In addition, rangement. The bonding material should have a melting bonding in a temperature range of 900 C. to 1000° C. point lower than, but close to the melting point of the Substantially increases the problems relating to oxidation thermoelectric material to which it is bonded. If the in which oxide films tenaciously adhere to the contact melting point is too close to the melting point of the electrodes. thermoelectric material, bonding must take place by lo In addition to avoiding problems inherent in the use of ... calized melting of the thermoelectric material which is iron as a contacting electrode, it is also desirable to pro not a satisfactory process. If the melting point is too low vide a very low resistivity bonding material for bonding it may coincide with the operating temperature of the the thermoelectric device to an electrode. In order to device itself and thereby be unstable. Moreover diffusion attain a low resistivity bond it is important to provide a of bonding material impurities into the thermoelectric bonding material which does not contain impurities capa 50 material is greatly enhanced near the melting point. It ble of diffusing into and poisoning the thermoelectric ma is preferable to select a bonding material which may be terial, and which will not deteriorate either by degrada secured to the thermoelectric composition at a tempera tion of electrical properties or by mechanical cracking. ture approximately 50° below the melting point of the Nor should the material forming the contacting electrode thermoelectric material. 55 In the case of lead telluride thermoelectrics, a melting diffuse into the bonding alloy in sufficient quantity to point for the bonding material in the range of approxi deleteriously affect the bond to the thermoelectric mate mately 500 C. to 875 C. is preferred, and in the case rial. Consequently, the bonding material should not ma of tin telluride a melting point in the range of 500 C. to terially change the Seebeck coefficient of the thermoelec 740 C. is preferred. tric material and should not decrease the overall thermo 60 These and other objects and advantages of the present electric figure of merit. invention will be more clearly understood when consid Of particular difficulty is the bonding of a P-type lead ered in conjunction with the accompanying drawings, in telluride, usually doped with sodium, to a contact elec which: trode. Oxides readily form on such thermoelectric mate FIG. 1 is a schematic diagram of a thermoelectric ma 3,232,719 3. 4. terial bonded to a contact electrode by means of a bond With the foregoing in mind, it is postulated that if an ing material or alloy; alloy, for bonding purposes, consists of the formula MRy, FIG. 2 is a graph of a bonding temperature cycle; and where M.Ry is a unique mixture, the metal-nonmetal FIG. 3 is a graph of a cleaning temperature cycle; compounds defined thereby will include all of those com FIGS. 4, 5 and 6 are respectively, side, top and end pounds which are suitable as bonding materials or alloys. views of an arrangement used during the bonding of a By unique mixture we mean a metal-nonmetal system of thermoelectric element to a contacting electrode; the formula MRy which has but a single stoichiometric FIG. 7 is a resistance plot across a P-type lead telluride arrangement. The formula MR is further limited in the (sodium doped)-tin telluride bonding material junction; present invention to a system in which M is selected from FIG. 8 is a resistance plot across a P-type lead telluride the group consisting of calcium, strontium, barium, lead, (sodium doped)-tin telluride bonding material-iron con O germanium, tin, manganese, beryllium, vanadium, ytter tacting electrode bonds; bium and zinc, and R is selected from the group consist FIG. 9 is a resistance plot across an N-type lead tellu ing of tellurium, selenium and Sulphur. ride (iodine doped)-tin telluride bonding material junc The mono-tellurides, mono-Selenides and mono-Sulphur tion; 5 compounds which may come within the definitions set FIG. 10 is a resistance plot across an N-type lead tellu forth above, include: calcium telluride, strontium tellu ride (iodine doped)-tin telluride-iron contacting electrode ride, barium telluride, lead telluride, germanium telluride, bond; tin telluride, manganese telluride, beryllium telluride, zinc FIG. 11 illustrates graphically the life test of an N-type telluride, ytterbium telluride and vanadium telluride; cal (iodine doped) thermoelectric material under typical op 20 cium selenide, strontium selenide, barium selenide, lead erating conditions. selenide, germanium Selenide, tin Selenide, manganese In FIG. 1 there is schematically illustrated a bond be selenide, beryllium selenide, Zinc selenide, ytterbium sele tween a thermoelectric material or alloy 1 and a contact nide and vanadium selenide; calcium sulphide, strontium ing electrode 2 using a bonding material or alloy 3.