1) Field of the Invention
The present invention relates to a process for preparing a p-type thermoelectric material that is used as a material for thermoelectric devices utilizing Peltier effect or Seebeck effect.
2) Description of the Related Art
A thermoelectric device utilizing Peltier effect or Seebeck effect can be used in a variety of utilities such as elements for heating and cooling and for temperature control, thermoelectric electricity generators and the like.
The thermoelectric device is made from a thermoelectric material. Performance properties of the thermoelectric material are evaluated by the thermoelectric figure of merit Z that is obtained through the following equation,Z=α2/(ρ·κ)in which Z[1/K] is figure of merit, α[μV/K] is Seebeck coefficient, κ [mW/cm·K] is thermal conductivity, and ρ[mΩ·cm] is specific resistance.
If the figure of merit Z is higher, it means that the thermoelectric material has higher performance. To increase the figure of merit Z of the thermoelectric material, the Seebeck coefficient α may be increased and/or the specific resistance ρ and the thermal conductivity κ may be decreased.
Meanwhile, p-type and n-type thermoelectric materials obtained by adding a suitable dopant to an alloy containing at least two elements selected from the group consisting of bismuth (Bi), tellurium (Te), selenium (Se), antimony (Sb), and sulfur (S) are known to have higher figure of merit.
One of the methods for preparing this thermoelectric material is to mix and melt a powder obtained by weighing prescribed amounts of powders of Bi, Te, Se, Sb, S, and a prescribed amount of dopant, to obtain an alloy ingot, to grind the alloy ingot to obtain an alloy powder, and sinter the powder.
The sintering may be hot press sintering, normal pressure sintering, vacuum sintering, gas pressure sintering, plasma sintering, hot isostatic pressing (HIP) and the like. Inter alla, hot press sintering which hardly produces crack due to cleavage and affords a sintered material excellent in the mechanical strength is effective.
The figure of merit Z of a p-type thermoelectric material prepared by hot press sintering is around 3.0×10−3K−1. Generally, it is preferable that the figure of merit Z is high. For example, for use in an electric cooler or use in cooling a central processing unit (CPU) of a personal computer, a p-type thermoelectric material with the performance of 3.0×10−3K−1 may be used without practical problem.
However, a p-type thermoelectric material with the figure of merit Z of 3.0×10−3K−1 is not sufficient in many cases. These cases include an optical communication semiconductor laser requiring temperature control at a precision of 0.1° C. or lower in order to control an oscillation wavelength, use in an electric constant temperature bath necessary for temperature control of solution such as a photoresist solution, a plating solution and various surface treating solutions in a semiconductor manufacturing step, constant temperature control and temperature test for various materials and parts, and temperature control of a culture solution in gene or microorganism culturing, and use in precise temperature control in an ultraprecise air temperature and humidity controlling apparatus at a semiconductor manufacturing step. Thus, a p-type thermoelectric material with higher figure of merit Z is in demand.
However, it is difficult to obtain a p-type thermoelectric material having a higher figure of merit Z with the conventional process. Therefore, a process for manufacturing a p-type thermoelectric material with higher figure of merit Z is in demand.