Abstract:
There are sintered Bi 2  Te 3  -based thermoelectric materials containing Ag 2  S, which prevent p- to n-type transition of Bi 2  Te 3  -based thermoelectric materials during polycrystalline sintering and have improved thermoelectric properties.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to sintered Bi 2  Te 3  -based thermoelectric materials containing Ag 2  S which prevent p- to n-type transition of Bi 2  Te 3  -based thermoelectric materials during polycrystalline sintering and have improved thermoelectric properties. 
     2. Description of the Prior Art 
     Thermoelectric materials based on Bi 2  Te 3  have high figures-of-merit (Z) at room temperature (Z˜1.5×10 -3  K) and, thus, have been drawn much attention as one of the most promising thermoelectric materials for low temperature applications. The materials are usually employed in the form of a single crystal for thermoelectric devices. Single crystalline Bi 2  Te 3 , however, is extremely weak mechanically due to easy cleavage (P. D. Delavignette, &#34;Dislocation nets in Bi 2  Te 3  and Sb 2  Te 3  &#34; Phil. Mag., 5, 729-744 (1960)) and is normally subject to difficulties with mass production. In order to circumvent these drawbacks associated with the Bi 2  Te 3  -based single crystalline material, attempts have been made to prepare polycrystalline Bi 2  Te 3  via sintering. However, polycrystalline Bi 2  Te 3  -based materials go through p- to n-type transition during sintering (W. R. George, &#34;The Sintering of Bi 2  Te 3  &#34; Proc. Phys. Soc., 74, 768-770 (1959)). Such a change of electrical condition behavior from a p-type (holes) to an n-type (electrons) during processing causes thermoelectric power and conductivity of the material to be decreased. It is believed that p- to n-type transition occurs due to a defect of donors in sintering. One always ends up with n-type material after sintering even if started with p-type powder, which have prevented p-type thermoelectric material based on Bi 2  Te 3  from being fabricated via the powder metallurgical technique. It is thus necessary to prevent, while maintaining the high figure-of-merit, the p-n transition of the Bi 2  Te 3  -based thermoelectric materials in sintering. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide Bi 2  Te 3  -based thermoelectric materials which prevent p- to n-type transition of a thermoelectric material in sintering, while keeping excellent thermoelectric properties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows thermoelectric power of sintered Bi 2  Te 3  -based thermoelectric materials of the present invention depending on the change of temperature and content of Ag 2  S. 
     FIG. 2 shows the conductivity of sintered Bi 2  Te 3  -based thermoelectric materials of the present invention depending on the change of temperature and content of Ag 2  S. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The basic composition of the thermoelectric material of the present invention is a Bi 2  Te 3  -based material, particularly quasi-binary 69% Bi 2  Te 3  -33% Sb 2  Te 3 , containing Ag 2  S up to 20 mol %. If the content of Ag 2  S is larger than 20 mol %, quantity of the second phase in the obtained sintered body after sintering the thermoelectric material increases and electric conductivity rapidly decreases. 
     For the preparation of the composition according to the present invention, the raw powders of Bi, Sb and Te are put into a quartz tube in an appropriate ratio. The tube is then evacuated and sealed to keep the raw materials from oxidation. The sealed tube is heated in order to melt the raw materials, rocked for homogeneous mixing, and cooled. The p-type ingot is crushed into particles of 45-75 μm size. To the obtained powder, Ag 2  S powder is added, and the mixture containing Ag 2  S is ball-milled for several hours and formed. In order to form the n-fixture, cold-pressing into pellets is possible. The formed mixture is sealed within a quartz tube and sintered at 500° C. for 5 hours. 
     Thermoelectric powder (α) and thermal conductivity (κ) are measured to characterize the thermoelectric figure-of-merit, Z(=σα 2  /κ). Electric conductivity (σ) is measured by a D.C. 4 probe technique. Thermoelectric power is measured by a heat pulse technique (P. C. Eklund and A. K. Mabatah, &#34;Thermoelectric Power Measurement Using Analog Substraction&#34;, Rev. Sci. Instrum., 48[7], 775-777 (1977)), and the thermal conductivity (κ) by a technique introduced by Harman (T. C. Harman, &#34;Special Technique for Measurement of Thermoelectric Properties&#34;, J. Appl. Phys., 29, 1373-1374 (1958)). 
     As shown in FIG. 1, it is clear that any of those systems containing Ag 2  S do not undertake a p-to-n transition. Thermoelectric power for the systems of sintered Bi 1 .333 Sb 0 .667 Te 3  in various contents of Ag 2  S are similar, but the specimen with 1 mol % Ag 2  S exhibits a little lower thermoelectric power. The thermoelectric powers of all the samples first increase with temperature up to ca. 150° C. and then decrease rather rapidly. The conductivities of the samples added with 3 mol % of Ag 2  S and 1 mol % of Ag 2  S are higher than the others. Thermoelectric power amounts to 120-160 μV/K at room temperature and the maximum is 160-180 μV/K. 
     As shown in FIG. 2, the conductivities of all the samples decrease with increasing temperature and display a minimum at about 250° C. The conductivities of these samples are as high as about 1000Ω -1  cm -1  at room temperature. 
     Thermal conductivities of all the samples are about the same at room temperature in the range of 1.3-1.4×10 -2  W/Kcm. 
     The value for the figure-of-merit at room temperature is about Z=1.8×10 -3  /K for the sample added with 3 mol % Ag 2  S. It is as good as that of the single crystalline specimen. 
     EXAMPLE 
     The raw powders of Bi, Sb and Te in the purity of 99.999% were weighed to 69% Bi 2  Te 3  -33% Sb 2  Te 3 , and put into a quartz tube. The tube was evacuated and vacuum-sealed. The sealed tube was heated to 700° C., for 5 hours, rocked for homogeneous mixing, and cooled to room temperature. The p-type ingot was crushed into particles of 45-75 μm size. To the obtained powder, Ag 2  S powder of 1 mol %, 3 mol %, 10 mol % and 20 mol % was added, respectively. Each mixture containing Ag 2  S was homogenized with a ball-mill for 3 hours and cold-pressed into pellets. The pellets were sealed within a quartz tube and sintered at 500° C. for 5 hours. 
     Electric conductivity (σ), thermoelectric power (α) and thermal conductivity (κ) were measured to characterize the thermoelectric figure of merit, Z(=σα 2  /κ).