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Home , Beryllium telluride, Germanium selenide, Germanium telluride, Lead telluride, Tin telluride

Feb. 1, 1966 I. M. RITCHE 3,232,719 THERMOELECTRIC BONDING MATERIAL Filed Jan. 17, 1962 2 Sheets-Sheet

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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 a bonding material for and con and , 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, or (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 , lead, germanium, tin, manganese, , 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 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 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 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 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- 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, , bond; tin telluride, manganese 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 , strontium selenide, barium selenide, lead erating conditions. selenide, , , manganese In FIG. 1 there is schematically illustrated a bond be selenide, beryllium 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. The Sulphide, barium Sulphide, lead sulphide, germanium Sul contacting electrode 2 preferably may comprise iron or 25 phide, tin sulphide, manganese sulphide, beryllium sul low carbon steel or other material which will not readily phide, Zinc sulphide, ytterbium sulphide, and vanadium diffuse into the bonding material or affect the thermo Sulphide. electric qualities of the device. In addition to the foregoing limitation on selection of The thermoelectric material 1 to which this invention the bonding material it is important to select a bonding is directed, preferably comprises a metal-nonmetal com 30 material which does not have high sensitivity and which position having as its principal constituents at least one will not form high resistivity barriers at the interfaces metal, A, selected from the group consisting of lead, tin of the contacting electrode 2 and the thermoelectric alloy and germanium and at least one nonmetal, B, Selected 1 with the bonding material 3. This may be accomplished, from the group consisting of tellurium, selenium and preferably, by reflecting a bonding material of the unique sulphur. These thermoelectric materials are suitably 35 mixture M.Ry, for the thermoelectric material ABd in doped to form P or N type materials and may consist which x=c and y =d, and in which c, y, c and d are whole of a stoichiometric system having doping agents contained integers representing atomic proportions. Thus, for ex therein, such for example, as the type described in co ample, if the thermoelectric material is lead telluride, pending patent application Serial No. 53,657, filed Sep PbTe, an ideal bonding material would be SnTe. tember 2, 1960, for an invention in "Means and Method 40 If BiTea is the thermoelectric material, AS2Tea may of Making Lead Telluride,' by Beverley A. Shaw, or may preferably be used as a bonding material. alternately consist of thermoelectrically conductive com If InSb is the thermoelectric material, GaSb may pref positions such as described in United States Letters Patent erably be used as a bonding material. Nos. 2,811,440, and 2,811,411, both issued October 29, If AgSbTe, is the thermoelectric material, SnTe may 1957. preferably be used as a bonding material. In addition to the foregoing thermoelectric system, If a bonding material which otherwise may be selected other systems of metals and nonmetals may have utility is inherently P or N and is a type opposite to the thermo in connection with this invention. For example, a thermo electric alloy type, it may create a high resistivity PN or electric system consisting of the metals A, bismuth, anti the bonding alloy NP junction within the thermoelectric mony and arsenic, and the nonmetals, B, Selenium, tellu 50 material. In such an event it should be counterdoped rium and Sulphur in mixtures of metals and nonmetals, to avoid creating a junction at at this interface. For ex may also be used. A system of the metals, A, gallium and ample, if the thermoelectric device consists of an N-type indium, and nonmetals, B, antimony and arsenic, may doped lead telluride thermoelectric material, and it is also be used. Also contemplated are ternary thermo desired to use tin telluride, which is inherently a P-type electric systems, such as silver, antimony, telluride, Ag, 55 material as a bonding material, a quantity of iron which Sb, Te provided they otherwise comply with the limita is an N-type dopant should be added to the tin telluride tions set forth herein. In particular the present inven to neutralize it and thereby avoid creating a P-N junction. tion is useful in bonding to iron or low carbon steel elec Up to 40% by atom weight iron may be substituted for trodes. The bonding material 3 must have several elec the tin. Similarly, germanium telluride which is inher trical, chemical and physical characteristics to be com 60 ently a P-type material would have to be counter-doped patible with the thermoelectric material and electrodes with iron with up to 40% atomic weight of the germanium. referred to above. The thermoelectric material referred It is not necessary oin all cases when counter doping to to above is normally P-type or N-type depending upon alloy an impurity with pure bonding material MRy prior its doping. to its use. When a contacting electrode containing iron With this in mind it has been found that a bonding 65 is used and the bonding alloy is first bonded to the elec material must be either neutral or inherently of the same trode at temperatures sufficiently high to melt both the N- or P-type as the thermoelectric material to which it is bonding alloy and iron, enough iron may diffuse into the to be bonded. A metallic element M, which forms a bonding alloy to alter its P or N type characteristics to a compound M.Ry with the nonmetallic portion R of the satisfactory level. When using this process the iron con thermoelectric material will tend to be an N-type dope tent of the bonding alloy should be limited to about 40% when present as an impurity if x, a whole integer is greater by atomic weight of the metal in the bonding alloy. than y, a whole integer. The compound MRy will also Further, this bond requires adherence to the sequential tend to an N-type dope. Conversely if x is less than y, process hereinafter described for forming the bond. both the element and the compound will tend to act as While the relation of coefficients of expansions of the P-type doping impurities. 5 electrode and thermoelectric material is more critical, the 8,232,719 S 6 bonding material should preferably have a coefficient of nificantly higher than the subsequent temperature cycle expansion close to that of the thermoelectric material. for bonding the bonding alloy to the thermoelectric ma For example, the coefficient of expansion should be in the terial. A temperature cycle of 1000 C. for twenty min range of 18X106 per C. for lead telluride. utes in a reducing atmosphere in a tin telluride-iron con It is of particular importance that the bonding material tacting electrode junction is sufficient to form a suitable have a melting point in the range just below the melting bond. This time and temperature sequence is not criti point of the thermoelectric alloy and above the operating cal, within a wide range, but the temperature should be temperature of the device. However, it is preferable to maintained within a range sufficient to form a suitable select a bonding material which is close to the upper bond. Too low a temperature will not assure an integral limit of this range. It is desired that the bonding ma O formation. Too high a temperature will result in a po terial have a melting point sufficiently below the melting rous bonding alloy. In the case of a tin telluride-iron temperature of the thermoelectric material to avoid prob contacting electrode junction, the process should be main lems of melting the thermoelectric material during manu tained between temperatures of 700° C. and 1100° C. facture, but a melting point sufficiently high to avoid dif Porous bonding alloys are due to vapor formations of fusion problems of impurities in the bond into the ther 5 the bonding alloy at higher temperatures. The time cycle moelectric alloy during operation. The temperature is also somewhat related. For example, five minutes of range cannot be critically defined due to the uncertainty exposure to 800 C. temperature is insufficient to allow in the composition of the bond formed. The alloy formed the alloy to set, while a two hour period of 800 C, is by the bonding material and the thermoelectric material too long because the bonding alloy vaporizes. The re should melt between about 50 C. below the melting 20 ducing atmosphere may be any conventional reducing at temperature of the thermoelectric material and 100 C. mosphere such as hydrogen. above the operating temperature of the device as the Prior to bonding the lead telluride thermoelectric ma lower limit for adequate performance. The bonding ma terial to the tin telluride bonding alloy, the lead telluride terial should, generally speaking, melt in a range of is cleaned to remove oxides from its surface, in accord 600 C. to 875 C. While these ranges may not be pre 25 ance with a heat cleaning cycle described above in con cisely defined they are definitive of a reasonable range. The nection with FIG. 3. absolute melting temperature of the bonding material may The tin telluride bonding alloy with the contacting be varied by the iron content in the contacting electrode, electrode attached is then bonded to the thermoelectric since the iron of the contacting electrode may to some lead telluride at a lower temperature. The bonding alloy extent diffuse into the bonding alloy. In such case, the 30 and thermoelectric material are joined in facing relation bonding material is not a binary System but a ternary under positive pressure in a suitable clamping device, such system and its melting point should be accordingly de as illustrated in FIGS. 4, 5 and 6. As illustrated the termined. Thus, if lead telluride is the thermoelectric thermoelectric elements are supported in a frame having material to be bonded to an iron electrode, a lead upper and lower guide plates 4 and 5 aligned by pins telluride bonding material may be used provided a se 35 6 with the thermoelectric elements positioned within holes quential process of application is utilized as hereinafter in the plates 4 and 5 and compressed by a spring 8. The described. In this process the melting temperature of the arrangement is placed in a reducing atmosphere of hy lead telluride bonding material is lowered at least 50 C. drogen or the like and subjected to heat treatment in a below the lead telluride thermoelectric material as a re temperature cycle such as illustrated in FIG. 2. The sult of the diffusion of iron into the bonding material. AC) temperature should be at least sufficient to melt the bond Of the bonding materials set forth above, germanium ing alloy and the lead telluride at their interface, but telluride and tin telluride have been found particularly not so high as to cause a substantial diffusion of impuri useful in bonding P and N type lead telluride to contact ties into the thermoelectric material. In the case of a ing electrodes of iron and low carbon steel. Tin telluride tin telluride-lead junction, temperatures between 650 C. has a melting point of 790 C., and germanium telluride and 780 C. are sufficient for such purposes. The bond has a melting point of 725 C. which are satisfactorily ing cycle should also be of sufficient length of time as to below the melting point (929 C.) of lead telluride. cause a Satisfactory bond. In the case of a tin telluride In bonding the thermoelectric material to the contact lead telluride junction a bonding time of approximately ing electrode, the process is accomplished in two stages. one hour in the temperature range indicated should be In the first state the bonding alloy or material is first 50 Sufficient. bonded to an oxide free contacting electrode and in the FIG. 7 illustrates a resistance plot across a typical lead second stage the bonding alloy with the attached electrode telluride (sodium doped) P-type thermoelectric material is bonded to the thermoelectric material. These proc tin telluride bond. It will be noted that the contact re esses are carried out in a reducing atmosphere to avoid sistance is substantially negligible. oxidation and consequently high resistivity joints. FIG. 8 illustrates a resistance plot across a lead telluride The following example is the procedure followed to (sodium doped) P-type thermoelectric material tin tellu bond lead telluride to an iron contacting electrode or ride-iron contacting electrode bond. Here again the con cap, but is illustrative of the procedure to follow with tact resistance is negligible. other materials. First, the iron contacting electrode must FIG. 9 shows a resistance plot across a lead telluride be cleaned to remove all traces of oxide films. This may 60 (iodine doped) N-type thermoelectric material-tin iron be accomplished by heating the electrode in a bond in which the iron constitutes 40% of the furnace at a temperature of approximately 1000 C. for tin by atomic per?entage. The contacting resistance is an hour. The temperature and time may be varied pro also negligible in this case. Here it will be noted that vided the temperature is below the melting point of the had pure tin telluride been used it would have formed a iron but sufficiently high to reduce oxides, and the tem 65 high resistance bond, but by using up to 40% of iron perature is maintained for a sufficient time to reduce all in the bonding alloy, the resistivity of the bonding alloy oxides. The bonding alloy should then be fused pref is reduced. While more than 40% iron could be used it erably without removing the electrode from the reducing is not desirable as the melting point of the bond ma atmosphere to the iron cap as soon as possible after the terial becomes increasingly higher with an increased use oxides have been removed. The bonding material com 70 of iron, as does the vapor pressure. prises a slice of tin telluride, or tin iron telluride (if the In FIG. 10 there is a resistivity plot across an N-type lead telluride thermoelectric material is N-type). The lead telluride (iodine doped) thermoelectric material tin iron content of the tin iron telluride may be up to 40% iron telluride bond-iron cap, in which the tin iron telluride by atomic weight of the tin. The temperature for bond has 40% by atomic weight of the tin of iron. The resist ing the bonding alloy to the contacting electrode is sig 75 ance can be seen to be negligible as in the previously 3,232,719 7 8 mentioned cases. Here, the iron content can be reduced bonded to metallic electrodes with the metal of Said or omitted where a bond is to be made to an iron cap electrodes being selected from the group consisting because the iron, soluble in tin telluride, will diffuse into of iron, steel, nickel, molybdenum and chromium, the bonding alloy to satisfactorily counterdope it. said inert material comprising, the composition While the foregoing examples have referred to the use MRy of sodium for P-type doping, and iodine for N-type doping of the thermoelectric elements, low resistivity where bonds can also be formed where the thermoelectric ele M is at least one metal selected from the group consist ment is doped with other materials. For example, low ing of calcium, strontium, barium, germanium, tin, resistivity bonds can be formed to tellurium, selenium manganese, beryllium, vanadium, ytterbium and zinc, or sulphur alloys doped with silver to make them P-type, 10 R is a nonmetal selected from the group consisting of or bismuth to make them N-type. tellurium, selenium and sulphur, said composition What is claimed is: having a coefficient of expansion close to the coeffi 1. In combination with a thermoelectric composition cient of expansion of said semi-conductive material, having as its principal constitutents at least one metal and x and y are whole integers, selected from the group consisting of said composition MRy comprising a unique mixture, in lead, which x=c and y = d and having a melting point at least tin and germanium and at least one metal Selected from 50° C. below the melting point of said semiconductive the group consisting of tellurium, Imaterial. selenium and Sulphur, 20 7. A combination as set forth in claim 6 wherein and a contacting inert metal electrode selected from AB is lead telluride, PhTe, the group consisting of iron, steel, nickel, molybde and M.R. is tin teiluride, SnTe. num, and chromium, 8. A combination as set forth in claim 6 wherein a bonding composition of substantially neutral conduc AB is indium antimide InSb, tivity comprising a stoichiometric mixture of metallic 25 and MR is gallium antimide GaSb. and nonmetallic elements in essentially stoichiometric 9. A combination as set forth in clain 6 wherein proportions wherein said bonding alloy has at least AB is silver , AgSbTe, one metal M, and M.R. is tin telluride. selected from the group consisting of it. A combination as set forth in claim 6 wherein said calcium, 30 MRy is tin iron telluride. strontium, ii. A combination as set forth in claim 5 wherein M barium, is tin and R is selected from the group consisting of germanium, tellurium, selenium and Sulphur. tin, A2. A combination as set forth in claim wherein manganese, said thermoelectric composition has an electrical con beryllium, ductivity selected from the group consisting of P-type Vanadium, conductivity and N-type conductivity and said bonding ytterbium and zinc and at least one non-metal R, Se composition has the same type conductivity as said thermo lected from the group consisting of tellurium, selen electric composition. ium and Sulphur, 40 13. A combination as set forth in claim 5 wherein said to form the unique composition M Rn having a melting bonding composition metal is lead. point at least 50° C. below the melting point of the 4. A method of forming a bond between a thermo thermoelectric composition, said thermoelectric composi electric semiconductive material and a contacting elec tion and said electrode being bonded together by said trode of conductive material, bonding composition to form a bond having negligible 45 Said semiconductive material having as its principal contact resistance. constituents at least one metal from the group con 2. A combination as set forth in claim , wherein the sisting of lead, tin and germanium and at least one melting point of said bonding composition is between element selected from the group consisting of tel substantially 600 C. and 875 C. lurium, Selenium and sulphur, 3. A combination as set forth in claim 2 wherein said 50 Said contacting electrode comprising a metal selected metal electrode is iron. from the group consisting of iron, steel, nickel, no 4. A combination as set forth in claim wherein said lybdenum and chromium, bonding composition is doped to form a low resistivity said method comprising bonding a bonding material to interface between said bonding composition and said Said electrode at a first temperature in excess of the thermoelectric composition. melting point of said bonding material, 5. A combination as set forth in claim wherein said Said bonding material comprising metallic and non bonding composition is doped with iron up to 40% by metailic elements forming a unique mixture M.R., atomic weight of said bonding composition metallic ele with at east one metal M selected from the group ment. consisting of calcium, strontium, barium, lead, germa 6. A combination comprising an inert material form 60 nium, tin, manganese, beryllium, vanadium, ytterbi ing an electrical bond with a compound semiconductive um, and Zinc and at least one non-metal R, Selected material, said bond having negligible contact resistance, from the group consisting of tellurium, selenium and said semiconductive material having the formula Sulphur, Said bonding material being such that when said bond ABd to Said electrode is effected said bonding material where has sufficient thickness to provide a surface spaced A is at least one metal selected from the group consist from said electrode and electrically compatible with ing of lead, tin, germanium, bismuth, antimony, the semiconductor type of said thermoelectric ma gallium and indium, terial for permitting said bonding material to form B is a nonmetal selected from the group consisting of a low resistance bond with said thermoelectric ma tellurium, selenium and Sulphur when A is lead, tin, terial, germanium, bismuth, or antimony, and when A is and thereafter bonding said surface to said thermo gallium or indium, B is selected from the group con electric material at a second temperature lower than sisting of antimony and arsenic, and said first temperature. c and d are whole integers, said inert material being (References or following page) 3,232,719 9 O References Cited by the Examiner 3,037,065 5/1962 Hockings ------136-5 UNITED STATES PATENTS 3,045,057 7/1962 Cornish ------136-5 2,865,794 12/1958 Kroger ------29-472.9 3,082,277 3/1963 Lane ------136-5 2,902,529 9/1959 Busanovich ------136-5 FOREIGN PATENTS 3,017,693 1/1962 Haba ------29-473.1 766,999 1/1957 Great Britain. 3,018,312 1/1962 Cornish ------252-62.3 X 3,031,516 4/1962 Pessel ------136-5 HYLAND BIZOT, Examiner. 3,037,064 5/1962 Rosi ------136-5 DAVID L. RECK, Primary Examiner.