July 16, 1968 G. SONNENSCHEN 3,392,439 METHOD AND MATER ALS FCR OBTAINING LOW-RESISTANCE BONDS TO TELLURIDE THERMOELECTRIC BODIES Fied Oct. 28, 1963 9 29 INVENTOR. GEORGE SONNENSCHEIN ATTORNEY 3,392,439 United States Patent Office Patented July 16, 1968 2 3,392,439 METHOD AND MATERALS FOR OBTAINING the joining of thermoelectric semi-conductor elements LOW-RESISTANCE BONDS TO TELLURIDE into arrays of Suitable voltage and power output. This THERMOELECTRICBODES difficulty has been particularly pronounced in forming a George Sonnenschein, Los Angeles, Calif., assignor to satisfactory bond between the thermoelectric element and North American Rockwell Corporation, a corporation the conductive material at the hot junction, particularly of Delaware where this hot junction is operated at an elevated tem Filed Oct 28, 1963, Ser. No. 319,301 perature Such as found in a nuclear reactor. The conduc 7 Claims. (CI. 29-573) tive material to be bonded to the semiconductor material This invention relates to improved thermoelectric de must satisfy a varied set of stringent requirements, namely, vices and to methods of fabricating such devices. More 0. low electrical resistivity, high thermal conductivity, particularly, the invention relates to improved materials thermal expansivity closely matching that of the semi and methods for obtaining mechanically strong, thermally conductor, low vapor pressure, melting point well above stable, low-resistance contacts to thermoelectric bodies. the maximum operating temperature of the device, and, Still more particularly, the invention relates to a method particularly, chemical and atomic or electronic compati for bonding aluminum to lead telluride. 5 bility with the semiconductor. By chemical compatibility Thermoelectric components or circuit members are or stability, I refer to the fact that the conductive ma made of semiconducting bodies of thermoelectric ma terial and the thermoelement being joined do not form terials such as lead telluride, bismuth telluride, antimony an intermetallic compound of higher resistivity than either telluride, germanium telluride, lead tin telluride, silver material, thereby resulting in a high-resistance contact. indium telluride, silver gallium telluride, copper gallium. 20 Chemical instability may also occur in other forms. For telluride, silver antimony telluride, sodium manganese example, the electrode material may alloy with the thermo telluride, and the like. Small amounts of various additives element in a eutectic reaction which lowers the melting or doping agents may be incorporated in the thermo point of the alloyed layer; or the conductive electrode electric composition to modify the thermal conductivity, material may diffuse into the thermoelement forming ???????ical conductivity, or electrical polarity of the ma 25 second phase highly conductive material which causes terial. local short circuiting of the thermoelectric element; or Thermoelectric devices which convert heat energy the electrode material may react directly with the thermo directly into electrical energy do so by means of the electric alloy to destroy its molecular form; or the elec Seebeck effect. That is, when heat is applied to one ?unc trode material may dissolve a doping agent to effectively tion of a thermoelectric device, while the other junction 30 leach it out of the thermoelement. is cooled, an electrical potential is produced proportional By atomic or electronic compatibility, I refer to the to the thermoelectric power of the thermoelements em fact that the conductive material does not "poison' the ployed and to the temperature difference between the semiconductor thermoelement; that is, no deterioration junctions. occurs in the thermoelectric power of the thermoelement Generally two thermoelectric circuit members or com 35 by the transfer of charge carriers between the thermo ponents are bonded to a block of metal, which may, for element and the conductive material. Thus, the electrode example, be aluminum, copper, or iron, to form a thermo material may diffuse into the thermoelement where it may electric junction. The two members are of thermoelec form donor or acceptor sites to alter the local carrier con trically complementary types: one member is made of centration. For example, a conductive material containing P-type thermoelectric material and the other of N-type 40 arsenic would ordinarily be unsatisfactory for use with thermoelectric material. Whether a particular thermo a semiconductor such as germanium telluride because electric material is designated N-type or P-type depends pentavalent arsenic would act as a donor of charge car upon the direction of conventional current flow across riers to the germanium, which could deleteriously affect the cold junction of a thermocouple formed by the thermo the thermoelectric properties of the germanium telluride. electric material in question and a metal, such as copper 45 Because of this multiplicity of varying and often con or lead, when the thermocouple is operating as a thermo flicting requirements, it is frequently necessary to match electric generator according to the Seebeck effect. If the the conductive material and the semiconductor in accord conventional current in the external circuit flows from the ance with the more stringent of the requirements, and thermoelectric material, then the material is designated compromise with regard to those of secondary importance, as "P-type'; if the current in the external circuit flows 50 such as thermal and electrical conductivities. Aside from toward the thermoelectric material, then the material is the melting-point consideration, the fundamental require designated as "N-type.” The present invention relates to ments to be met by a satisfactory ohmic bond relate to both P-type and N-type thermoelectric materials. These the chemical and atomic compatibilities, as well as a materials consist of the binary and ternary semi-conduct matching of the coefficient of thermal expansion. These conditions severely restrict the choice of conductive mate ing alloys of tellurium. Preferably the binary telluride rials for forming a juncture with a given semiconductor. alloys such as lead telluride, bismuth telluride, antimony Other difficulties arise in that intermediate layers of telluride, and germanium telluride are employed as the high resistivity are encountered in many junctions where thermoelectric materials. Particularly preferred because oxidized surfaces are brought together without adequate of their desirable thermoelectric and physical properties 60 are lead telluride and bismuth telluride. removal of the oxide layer. Most of the thermoelements A good thermoelectric material should have a high of practical use today form thin surface oxide layers im electrical conductivity and a low thermal conductivity mediately upon exposure to air and must be properly since the electromotive force generated in energy con treated to remove such oxides before a good contact can verters of this type utilizing the Seebeck effect is de be formed. pendent upon the temperature difference between the 65 Accordingly, it is an object of the present invention to hot and cold junctions. The generation of Joulean heat provide improved thermoelectric devices. in the thermoelectric device due to the electrical resistance Another object of the invention is to provide improved of either the thermoelectric members, the auxiliary com methods for obtaining mechanically strong, low-resistance ponents, or the electrical contacts to the two members electrical connections to thermoelectric bodies. will reduce the efficiency of the device. 70 A further object of the invention is to provide improved Heretofore, there has been considerable difficulty in methods for obtaining mechanically strong, thermally 3,392,439 3 4. stable, low-resistance electrical bonds between a thermo thermoelectric bodies may be used in accordance with electric body and a metal body. this invention, preferably the thermoelectric bodies are Still another object of the invention is to permit the binary telluride alloys, particularly lead telluride. It will utilization of a variety of conductive materials solely on be understood that the conductivity types of thermoelec the basis of their thermomechanical and electrical prop tric bodies 1 and 12 and those of bodies 13 and 14 may erties without regard to their chemical or atomic com be reversed. One end of thermoelectric bodies 11 and 13 patibility with the semiconductor. and one end of bodies 12 and 14, which pairs are of In accordance with the invention, a barrier layer and opposite conductivity type, are bonded respectively to conductive tungsten granules are provided between the conductive bodies 15 and 16, which preferably are metal thermoelectric body and the conductive body, which are plates of aluminum, copper, or stainless steel. Inasmuch both compatible with the tungsten granules. Then the fac 0. as conductive bodies 15 and 16 need not be chemically or ing surfaces of these bodies are contacted under pressure atomically compatible with the thermoelectric bodies, so that the particles or granules consisting principally of because of the manner of bonding in accordance with tungsten penetrate the barrier layer, forming low-resist this invention, these metal plates are selected primarily ance ohmic conductive paths for the conduction of an 5 on the basis of having a melting point above that of the electric current between the telluride thermoelectric body temperature of operation of the thermoelectric