Mechanical Fabrication of Si-Ge-B Bulk Samples with Superior Thermoelectric Power
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1145-MM04-07
Mechanical Fabrication of Si-Ge-B Bulk Samples with Superior Thermoelectric Power Makoto Abe, Hiroaki Takiguchi, Yoichi Okamoto, Hisashi Miyazaki and Jun Morimoto. Department of Materials Science and Engineering, National Defense Academy, Japan. ABSTRACT Si-Ge-B alloy fine particles were prepared by mechanical alloying. Large amounts of defect and strain were introduced into particles. Then, the prepared fine particles were pressed into 2 5 15 mm3 bulk samples. Thermoelectric power and electric resistivity of the samples were measured by four terminals method with temperature range from room temperature to 923 K in N2 flow at 1.0 105 Pa. The best thermoelectric power was found to be -6 mVK-1. XRD measurements, SEM observations and BET measurements were also carried out. It is invoked that the sub-micron particles have a large contribution to the superior thermoelectric power. INTRODUCTION Thermoelectrics is a technique which converts thermal energy and electric energy directly. Even if temperature difference is very small, the thermoelectrics can generate electrical power from thermal energy, such as wasted heat from engine. Generally, the thermoelectric conversion system is maintenance free, because there are no moving parts. In the thermoelectric conversion process, the system does not produce another wasted heat and pollutions. Therefore, thermoelectrics is one of the techniques which is effective to solve global warming or energy problem. In general, the performance of the thermoelectric materials is evaluated by the figure of merit Z. Z is expressed as Z = α2ρ-1κ-1,
(1)
where, α, ρ and κ are thermoelectric power, electrical resistivity and thermal conductivity, respectively. We have already reported that Au or B doped Si-Ge amorphous thin film showed superior thermoelectric power [1, 2]. The maximum value of the thermoelectric power reached around 30 mVK-1, which is 2 orders larger than that of conventional crystal Si-Ge materials. And the amorphous phase was responsible for the superior thermoelectric power, MQW (multi quantum well) effect was not [1]. For the practical power generation, the bulk materials are required because the power generation of thin film is too small. We also succeeded in Si-Ge-Au bulk sample fabrication with superior thermoelectric power by melt spinning method [3]. The samples have thermoelectric property (ρ ~ 10 Ωm, α ~ 10-1 VK-1, κ ~ 10 Wm-1K-1 and ZT ~ 10) which was around 1 order superior than the conventional crystal Si-Ge materials. Unfortunately, the reproducibility of the superior thermoelectric power was less than 1 %. This series of study in our laboratory is quite original in the world. There are three major techniques for the amorphous fabrication of amorphous materials such as vapor phase deposition technique (quench from vapor phase), quench from liquid phase technique and mechanical alloying technique. In the paper mentioned above [3], melt spinning method (quench from liquid phase technique) was used. It is practically impossible to fabricate
bulk mate
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