3,782,927 United States Patent Office Patented Jan. 1, 1974

1 3,782,927 In the past one or two decades, an intensive research MATERIAL FOR DIRECT THERMOELECTRIC effort has been going on in the large industrial countries ENERGY CONVERSION WITH A HIGH FIGURE aiming at the development of new techniques for energy OF MERIT conversion. In these techniques, the reciprocating engine or Michael C. Nicolaou, Toronto, Ontario, Canada (Univer- 5 turbine as well in the electric generator are dispensed with sidad Industrial de Santander, Bucaramanga, Colombia) and the conversion of energy is carried out with the aid of No Drawing. Filed Aug. 24, 1971, Ser. No. 174,531 Int. CI. C22c 23/00, 31/00; HOlv 1/16 solid state devices. These techniques employ various physi- U.S. CI. 75—134 B 25 Claims cal principles or phenomena and are generally labeled: "direct energy conversion." One of these methods is that 10 which utilizes the thermoelectric properties of materials, ABSTRACT OF THE DISCLOSURE namely the Seebeck and Peltier effects. This is, therefore, In devices used hitherto for the direct conversion of heat called direct thermoelectric energy conversion. into electricity, commonly known as "thermoelectric en- In its simplest form, a thermoelectric energy converter ergy converters," the efficiency of conversion is appreci- is basically composed of a hot junction, a cold junction, a ably lower than that of conventional reciprocating or ro- 15 positive branch and a negative branch. The hot junction is tary heat engines. The basic reason for this low efficiency usually a metallic plate attached to one end of each branch is inherent in the physical properties of the materials select- and must be a good conductor of both heat and electricity. ed for the manufacture of these devices. The materials that The electric load is normally connected to the other end of have been and are currently being used for this purpose each branch and thus forms the cold junction of the de- are intermetallic compounds and alloys of silicon and ger- 20 vice. The hot junction is brought into contact with the manium. In this invention an entirely new material is de- source of thermal energy. Now the difference in tempera- veloped. It is composed of an alloy or solid solution of the ture between the hot and cold junctions creates a voltage three intermetallic compounds: magnesium stannide, mag- difference and a flow of electrons from one branch to the nesium germanide and magnesium silicide, and defined other. The electric energy thus developed is utilized by the by the chemical formula: Mg2SixGeySn1_x_y. This mate- 25 electric load. The efficiency of energy conversion of such a rial, when properly doped, possesses a figure of merit and, device depends basically on: consequently, an efficiency of direct conversion of thermal (1) The temperature difference between the hot and cold energy into electrical energy far exceeding that of any junctions other material previously known or used. 30 (2) The physical properties of the materials used for the manufacture of the positive and negative branches of the An application for patent for the same invention de- device. scribed and claimed in the present specification and claims The suitability of any material for thermoelectric energy has been previously filed in Canada. The details of filing the conversion is examined by means of a parameter called prior application are as follows: 35 the "thermoelectric figure of merit Z." The figure of merit Application serial No.: 101,889 is: 2 Filing date: Jan. 4, 1971 Z=S a/k Applicant: Michael C. Nicolaou where Title: Material for Direct Thermoelectric Energy Con- 40 S=the Seebeck coefficient or thermoelectric power version With a High Figure of Merit magnesium chloride (MgCl2), quired by stoichiometry, is provided and incorporated in magnesium iodide (Mgl2) and magnesium bromide the mixture to compensate for any excessive loss of this 55 (MgBr2) may be used as n-type dopants. Alternatively, element by evaporation, that might occur, owing to its for better results, more than one doping element or com- pound may be used. This applies to both n-type and p-type high volatility relative to that of the other three, elements. dopants and becomes all the more important since the For best results, the processes of melting and crystal grow- material is composed of four different elements having ing are carried out in an inert or reducing gas atmosphere 60 widely varying atomic radii and atomic weights. Any other or in vacuum. Preparation in vacuum is preferred. Fur- doping agents (elements or compounds) may be selected thermore, excessive care is exercised to use crucibles or if found to bring about a more effective doping. containers that will not contaminate or react with the basic constituents during melting and crystal growing. The embodiments of the invention in which an exclusive Standard metallurgical techniques are employed during property or privilege is claimed are defined as follows: and after fabrication of the material to ensure that the 65 1. An alloy or solid solution of intermetallic compounds containing magnesium stannide Mg2Sn, magnesium resulting alloy or solid solution Mg2SixGeySn1_x_.y is homogeneous and stoichiometric. These techniques may germanide Mg2Ge, magnesium silicide Mg2Si in any pro- comprise thorough agitation of the ingredients during the portions and optionally containing one or more additional process of melting as well as the well known techniques of doping materials. zone refining or zone melting. 70 2. An alloy or solid solution constituted by the follow- ing three intermetallic compounds: The solid solution or alloy obtained should preferably be a single crystal grown by a modified Bridgman tech- magnesium stannide, Mg2Sn, nique or by any other suitable method. If the Bridgman magnesium germanide, Mg2Ge and or temperature gradient freeze technique is selected, it will 75 magnesium silicide, Mg2Si, 3,7! 52,927 7 8 and defined by the chemical formula: Mg2SixGeySni_x_y, temperature to achieve a homogeneous mixture, and cool- where x and y represent the molecular proportion of each ing the mixture to ambient temperature. of Mg2Si and Mg2Ge in the alloy, respectively. 16. A process for producing an alloy or solid solution 3. An alloy or solid solution according to claim 2, in according to claim 1, which comprises separately prepar- which the molecular proportions of magnesium silicide, 5 ing each of the three compounds: Mg2Sn, Mg2Ge and magnesium germanide and magnesium stannide are equal. Mg2Si by heating their ingredients to at least 830° C., 4. An alloy or solid solution according to claim 2, con- 1165° C. and 1410° C., respectively, intimately mixing taining 20 to 50 mol percent of magnesium silicide, 30 to the three compounds in the required proportions, pref- 40 mol percent of magnesium germanide and 20 to 40 mol erably after granulation or pulverization, optionally adding percent of magnesium stannide. 10 a p-type or n-type doping agent, intimately mixing the dop- 5. An alloy or solid solution as defined in claim 4, in ing agent with the basic constituents, heating the mixture which the molecular proportion of magnesium silicide is to about 1125° C., maintaining the molten ingredients at greater than that of each of magnesium germanide and this temperature to achieve a homogeneous alloy, and magnesium stannide. cooling the alloy to ambient temperature. 6. An alloy or solid solution as claimed in claim 4, in 15 17. A process according to claim 16 which comprises which the molecular proportion of each of magnesium crushing and pulverizing the three intermetallic com- germanide and magnesium stannide is greater than that of pounds into a fine powder, optionally introducing a p-type magnesium silicide. or n-type doping agent, intimately mixing the pulverized 7. An alloy or solid solution according to claim 4, in compounds and the doping agent, and subject- which the molecular proportion of magnesium stannide 20 ing the mixture to hot pressing, or to cold pressing and is greater than that of each o,f magnesium germanide and then sintering. magnesium silicide. 18. A process according to claim 15, wherein an excess 8. An alloy or solid solution as defined in claim 1, of magnesium is added to compensate for the loss of this wherein the alloy or solid solution is a single crystal. element by evaporation. 9. An alloy or solid solution according to claim 1, 25 19. A process according to claim 15, wherein the opera- wherein the additional doping material or materials is tions are carried out in an inert or reducing gas atmosphere selected so as to produce an n-type alloy. or in vacuum. 10. An alloy or solid solution as defined in claim 9, 20. A process according to claim 15, wherein the molten wherein the doping material or materials is/are selected ingredients are subjected to agitation. from the group comprising the elements of Groups V-A, 30 21. A process according to claim 15, wherein at least VI-A, and VII-A of the Periodic Table of the elements part of the doping agent is added during melting. and/or compounds thereof. 22. A process according to claim 16, wherein an excess 11. An alloy or solid solution as claimed in claim 10, of magnesium is added to compensate for the loss of this wherein the compounds of the elements of Groups V-A, element by evaporation. VI-A, and VII-A are magnesium compounds. 35 23. A process according to claim 16, wherein the oper- 12. An alloy or solid solution according to claim 1, ations are carried out in an inert or reducing gas atmos- wherein the additional doping material or materials is phere or in vacuum. selected so as to produce a p-type alloy. 24. A process according to claim 16, wherein the molten 13. An alloy or solid solution as defined in claim 12, ingredients are subjected to agitation. wherein the doping material or materials is/are selected 40 25. A process according to claim 16, wherein at least from the group consisting of lithium, sodium, copper, part of the doping agent is added during melting. silver, cadmium, zinc, aluminum, gallium, indium and gold. References Cited 14. An alloy or solid solution according to claim 1, 45 UNITED STATES PATENTS wherein the doping material or materials is/are selected from the group consisting of magnesium, silicon, ger- 3,279,954 10/1966 Cody et al 136—205 manium and tin. 3,298,777 1/1967 Brixner 23—315 15. A process for producing an alloy or solid solution 3,508,915 4/1970 Paoli 75—135 G according to claim 1, which comprises intimately mixing the four constituents: magnesium, silicon, germanium and 50 L. DEWAYNE RUTLEDGE, Primary Examiner tin in the appropriate stoichiometric amounts, preferably E. L. WEISE, Assistant Examiner after granulation or pulverization, optionally adding a p- type or n-type doping agent, intimately mixing the doping U.S. CI. X.R. agent with the basic constituents, heating the mixture 55 75—134 G, 134 S, 168 R, 175 R; 136—239 to a temperature higher than 1410" C., cooling the molten ingredients to about 1125° C., maintaining them at this