1. Field of the Invention
The present invention relates to thermoelectric conversion elements applied to thermoelectric power generation and to thermoelectric cooling, and thermoelectric materials used for these thermoelectric conversion elements, and particularly relates to thermoelectric materials and thermoelectric conversion elements having improved performance indices.
2. Description of the Related Art
In general, there are two types of the thermoelectric materials, one is a one-directionally solidified material and another is a sintered material. The one directionally solidified materials are produced by the following procedure. The raw materials are placed in a fused silica tube, and the fused silica tube containing the raw materials is sealed by fusing the end of the tube. The sealed tube is inserted in a tubular furnace and the raw materials are melted and the melt is agitated by shaking the tubular furnace. Subsequently, by applying a temperature gradient to the tubular furnace, the melt is solidified while the crystal orientation is maintained. Thereby, the one directionally solidified material, in which the solidified crystal structure grows in one direction, is obtained.
In contrast, the sintered material is obtained by pulverizing coagulated materials and by hot pressing the pulverized material into a solid form. In this case, a crystal direction (a-axis) having low electrical resistance grows in a direction perpendicular to the pressing direction at the time of press forming, so that the thermoelectric elements and a thermoelectric module constituted of a plurality of thermoelectric elements are assembled by attaching electrodes such that the electric current flows in the direction of the a-axis.
FIG. 21 is a schematic diagram showing an arrangement of the crystal grains of the thermoelectric material and the direction of hot-pressing (shown by an arrow). When the thermoelectric material 1 is formed by hot-pressing, the a-axis of the crystal grain 2 grows in the direction perpendicular to the hot pressing direction and the c-axis of the crystal grain grows in the direction parallel to that of the hot-pressing. Since the thermoelectric materials 1 generally have such a structural anisotropy, the a-axis of the crystal grain 2 grows faster than the c-axis. Thereby, the crystal grain 2 grows until a size of a few millimeter, and its aspect ratio becomes more than 5.
However, problems are encountered in the conventional thermoelectric material in that the material has low resistance to mechanical impacts because the crystals constituting the thermoelectric material are weak due to the large crystal size of more than a few milimeters and the tendency to cleavage. In addition, conventional thermoelectric materials generally show high thermal conductivity. The performance index Z of the thermoelectric materials is expressed by the following equation (1), when the Seebeck coefficient is represented as .alpha.(.mu..multidot.V/K), the specific resistance as .rho. (.OMEGA..multidot.mm), and the thermal conductivity as .kappa. (W/m.multidot.K). EQU Z=.alpha..sup.2 /.rho..times.K; (1)
As shown by the equation (1), the performance index Z becomes low when the thermal conductivity is high. That is, the improvement of the performance is limited when the thermal conductivity is high.
In the case of conventional thermoelectric materials, the powder size is the same as that of the crystal grain. In general, the larger the crystal grain size, the higher the thermal conductivity K and the lower the specific resistance .rho.. The smaller the crystal size, the lower the thermal conductivity .kappa., and the larger the specific resistance p. Since the effect of the grain size is smaller in the specific resistance than in the thermal conductivity, it is effective to reduce the grain size of the crystal in order to improve the performance index Z. Conventionally, since the powder size and the grain size of the crystal thermoelectric materials are the same, pulverization of the crystal grains is limited. Moreover, when further pulverization of the crystal grains is executed, the crystal grain surface will be oxidized, and the crystal grain surface is contaminated, which may cause an increase of the specific resistance and a degradation of the performance index.