Abstract:
The present invention relates to a process for the preparation of thermoelectric compositions of the formula In x CO 4 Sb 12  (0&lt;x&lt;1), with a figure of merit greater than 1.0 and a composition made by that process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/527,791, filed on Aug. 12, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for the preparation of thermoelectric compositions of the formula In x CO 4 Sb 12  (0&lt;x&lt;1), with a Figure of Merit greater than 0.2. 
     TECHNICAL BACKGROUND 
     Thermoelectric materials are used in the manufacture of such items as refrigerators, heaters and generators. It is desirable for these thermoelectric materials to have large Seebeck coefficients, as defined herein below, and have high electrical conductivity but low thermal conductivity. The performance of thermoelectric conversion materials is expressed as “Figure of Merit (ZT)”. At present, the best thermoelectric materials have ZT values of about 1.0. 
     Akai et al, Proceedings of the 17 th  International Conference on Thermoelectrics, 1998, 105-108 characterize Indium-doped Cobalt Antimonide produced by a solid phase reaction followed by hot-pressing. 
     In contrast, the process of the present invention uses firing of mixed powder in 1 to 15% hydrogen and 85 to 99% argon followed by a furnace cool. The calcined powder is then reground and pressed into disks which are then sintered at 675° C. for 4 hours in the same hydrogen/argon mixture. This procedure can result in material with a ZT greater than 1.0. 
     SUMMARY OF THE INVENTION 
     The present invention is a process comprising: 
     a) mixing powders of indium, cobalt and antimony to form a mixture of powders such that there is 0.006 to 0.030 atomic percent indium, 0.242 to 0.248 atomic percent cobalt and 0.727 to 0.745 atomic percent antimony in said mixture of powders 
     b) flowing a gas composition through a furnace containing said mixture of powders, said gas composition comprising of 1 to 15 atomic percent hydrogen and 85 to 99 atomic percent argon 
     c) heating said furnace at approximately 1 to 5 C/minute from room temperature to from 590° C. to 620° C. and holding said furnace at 590° C. to 620° C. for between ten and fourteen hours 
     d) further heating said furnace to between 665° C. to 685° C. at approximately 1 to 5 C/minute and holding said furnace at 665° C. to 685° C. for between 30 to 40 hours to form a first solid 
     e) grinding said first solid to form a second powder 
     f) pressing said second powder into a second solid 
     g) flowing a gas composition through said furnace containing said second solid, said gas composition comprising of 1 to 15 atomic percent hydrogen and 85 to 99 atomic percent argon 
     h) heating said furnace at approximately 1 to 5 C/minute from room temperature to between 665° C. to 685° C. and holding said furnace at between 665° C. to 685° C. for between 1 to 8 hours. 
     The present invention is also a composition made by the above-described process. 
     The present invention is further a composition with a figure of merit, ZT, greater than 0.7. 
     The present invention is yet further a refrigerator, heater or generator comprising the above described compositions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the measured Seebeck Coefficient in the temperature range of 300-600 K for various Indium concentration levels. 
         FIG. 2  shows the measured electrical resistivity in the temperature range 300-600 K for various Indium concentration levels. 
         FIG. 3  shows the measured thermal conductivity in the temperature range 300-600 K for various Indium concentration levels. 
         FIG. 4  shows the calculated figure of merit in the temperature range 300-600 K for various Indium concentration levels. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides a method of preparing intermetallic thermoelectric compositions of the formula In x CO 4 Sb 12  (0&lt;x&lt;1). These compounds have a lower electrical resistivity, lower thermal conductivity and higher Seebeck coefficient than CoSb 3  in the temperature range 300 K-600 K. This results in an improvement in the figure of merit from 0.2 (x=0) to 1.2 (x=0.2) at 600 K. 
     Thermoelectrics is the science and technology associated with thermoelectric converters, that is, the generation of electric power by the Seebeck effect and refrigeration by the Peltier effect. The performance of thermoelectric conversion materials is evaluated by ZT(Figure of Merit), which is expressed by the following equation:
 
 ZT=σS   2   T/κ 
 
Where σ, S, κ, and T are the electrical conductivity, Seebeck coefficient, thermal conductivity and absolute temperature, respectively. Materials with large Seebeck coefficient, high electrical conductivity but low thermal conductivity are needed.
 
     At present, the best thermoelectric materials have ZT values close to 1, such as alloys of Bi 2 Te 3 . They operate with poor Carnot efficiency of about 10% when compared to compressor based refrigerators. In a semiconductor structure that conducts heat poorly, such as glass, but conducts electrons (or holes) relatively well like silicon, one can dramatically improve thermoelectric efficiency by reducing theremal conductivity. The reduction of the thermal conductivity can be achieved by preparing ternary or quaternary semiconductors in which one or more of the atoms are weakly bound in oversized “atomic cages”. The “rattling motion” of the caged atoms effectively scatters heat-carrying phonons and markedly reduces the lattice contribution to the thermal conductivity; yet at the same time, the framework atoms maintain good electrical conduction. An example of such structures is the Skutterudites, which have emerged as one of the most promising new thermoelectric materials for power generation applications. 
     The compositions of this invention can be synthesized by the following procedure. High purity powders of Co, Sb, and In are mixed thoroughly in stoichiometric ratio. The mixed powder of starting materials is put into an alumina crucible, which is in turn put into an alumina boat. Another crucible containing pure Sb metal is also put into the boat to compensate for the evaporation of Sb. The boat is then inserted into a quartz reactor with the Sb containing crucible facing the gas inlet. The powder is calcined at about 610° C. for 12 hours, and then 675° C. for 36 hours under a gas mixture of 5% H 2  and 95% Ar. The calcined powder is reground and pressed to 12.8 mm diameter/1-2 mm thick disks. The disks are sintered at 675° C. for 4 hours under the same gas mixture. In both the calcining and sintering steps, the heating rate is about 240° C./hour from room temperature to the calcining or sintering temperature. After the desired reaction time, the samples are furnace cooled to room temperature. Powder X-ray diffraction data showed all the In x CO 4 Sb 12  (0&lt;x&lt;1) phases of this invention crystallize in a cubic Im-3 structure. 
     The electrical resistivity is measured from 300K to 600 K by the Van Der Pauw technique using a commercial apparatus by MMR Technologies of Mountainview, Calif. following manufacturers procedure. Silver paint is used to attach the leads to the pellet. The Seebeck coefficient is measured in the same temperature range. The pellet is placed between silver electrodes which are electrically isolated from each other. One electrode is heated by a resistive heater to develop a thermal gradient across the sample, which varies from 5 to 10 degrees Kelvin at each temperature set point. The testing assemble is placed in a temperature controlled oven under Ar. The voltage developed is measured with a Keithley 181 nanovoltmeter manufactured by Keithley Instruments of Cleveland, Ohio. The measured Seebeck coefficient is negative indicating n-type conduction. The thermal conductivity were determined in Netzsch Laser Microflash with reference material of 1-mm or 2-mm gold-sputtered, graphite-coated Pyrex glass. This instrument is manufactured by Netzsch Instruments Inc. of Burlington, Mass. 
     Thermoelectric materials such as n-type In x CO 4 Sb 12  (0&lt;x&lt;1) can be used to manufacture thermoelectric refrigerators, heaters or generators in conjunction with p-type thermoelectric materials such as CeFe 3 CoSb 12  or LaFe 3 CoSb 12 . In a thermoelectric refrigerator, the thermoelectric material is typically mounted between two plates of materials such as ceramics. One plate is located at the region to be cooled. The other plate is located where the heat is to be rejected. Current of the appropriate polarity is passed through the thermoelectric material, cooling the desired location. If the polarity of the current is reversed, the previously cooled plate will be heated and the plate rejecting the heat will be cooled. To use thermoelectric devices as generators, the thermoelectric material is again mounted between two plates. One plate is exposed to a high temperature source while the second plate is maintained at a lower temperature. Electrical power can be obtained from electrical connections across the sides of the thermoelectric material in the temperature gradient. 
     EXAMPLES 
     Examples 1-7 
     The compositions of In x CO 4 Sb 12  of Examples 1-7 wherein were made using the following procedure. For each Example, appropriate amounts of the starting metals In, Co and Sb were weighed according to the stoichiometric ratios and mixed thoroughly in an agate mortar. The gram amounts for a 2-gram sample size of the starting materials used are shown in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Indium 
                 Cobalt 
                 Antimony 
               
               
                   
                   
                 metal 
                 metal 
                 metal 
               
               
                 Ex 
                 Composition 
                 (gram) 
                 (gram) 
                 (gram) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 CoSb 3   
                 — 
                 0.2779 
                 1.7221 
               
               
                 2 
                 In 0.03 Co 4 Sb 12   
                 0.0041 
                 0.2773 
                 1.7186 
               
               
                 3 
                 In 0.075 Co 4 Sb 12   
                 0.0101 
                 0.2765 
                 1.7134 
               
               
                 4 
                 In 0.1 Co 4 Sb 12   
                 0.0134 
                 0.2760 
                 1.7106 
               
               
                 5 
                 In 0.2 Co 4 Sb 12   
                 0.0267 
                 0.2742 
                 1.6991 
               
               
                 6 
                 In 0.4 Co 4 Sb 12   
                 0.0527 
                 0.2705 
                 1.6768 
               
               
                 7 
                 In 0.5 Co 4 Sb 12   
                 0.0655 
                 0.2688 
                 1.6658 
               
               
                   
               
             
          
         
       
     
     In each Example, the mixed powder was fired at about 610° C. for 12 hours, and then 675° C. for 36 hours under a gas mixture of 5% H 2  and 95% Ar and furnace cooled to room temperature. The calcined powder was reground and pressed to 12.8 mm diameter/1-2 mm thick disks. The disks were sintered at 675° C. for 4 hours under the same gas mixture and used for thermal conductivity measurements. Bars of about 1.5×1.5×7 mm 3  size were cut for resistivity and Seebeck coefficient measurements. 
     X-ray powder diffraction patterns were recorded and the data showed all samples crystallized in a cubic Im-3 structure. The measured Seebeck coefficients, electrical resistivities and thermal conductivities in the temperature range of 300-600 K are shown in  FIGS. 1 ,  2  and  3  respectively. The calculated ZT values are given in  FIG. 4 .