日本金属学会誌

J. Japan Inst. Metals, Vol. 67, No. 10 (2003),
pp. 555-561

The Effects of Heat Treatment on the β-FeSi2 Transformation in the Fabrication of FeSi Thermoelectric Element Utilizing the MA/SPS Process

Jun-ichi Yamamoto1, Yasuo Shimizu2, Hajime Ohtani3, Keisuke Shiozaki3 and Kazuhiko Sakaki2

1Industrial Research Institute of Nagano Prefecture, Nagano 380-0928
2Department of Mechanical System Engineering, Faculty of Engineering, Shinshu University, Nagano 380-8553

3Graduate School of Science and Technology, Shinshu University, Nagano 380-8553

Abstract:

P-type and n-type α-FeSi2 produced by utilizing mechanical alloying (MA) process and spark plasma sintering (SPS) technique were heat-treated to transform their structure to the p-type and n-type β-FeSi2, respectively. Raw powders of iron(99.5%), silicon(99.9%), manganese(99.5%) and cobalt(99.9%) were mixed and mechanically alloyed for 72 ks using a planetary ball mill. Mixing fractions of the powders for preparing p-type and n-type α-FeSi2 were in accord with the mole fractions of the phases of Fe0.92Mn0.08Si2 and that of Fe0.98Co0.02Si2, respectively. The SPS process was carried out at 1293 K for 0.3 ks.

The power factor of the p-type β-FeSi2 heat-treated at 973~}1073 K for 7.2 ks was improved significantly as compared to that of the β-FeSi2 prepared by only the SPS process without heat treatment. Maximum power factor was measured to be 0.46 mW/mK2 at 850 K for the p-type β-FeSi2 heat-treated at 1073 K for 7.2 ks. The power factor of the n-type β-FeSi2 heat-treated either at 973{~}1073 K for 3.6{~}7.2 ks or at 1173 K for 1.8{~3.6 ks was improved as compared to that of the β-FeSi2 prepared by only the SPS process without heat treatment. Maximum power factor was measured to be 0.15 mW/mK2 at 650 K for the n-type β-FeSi2 heat-treated at 1073 K for 7.2 ks. However the maximum power factors obtained were not improved sufficiently, because the ε-FeSi phase partially existed in the specimen and also the p-type and n-type β-FeSi2 were contaminated with Fe, Cr and Ni originated from the MA pot and balls.


(Received May 6, 2003)

Keywords:

mechanical alloying, spark plasma sintering, thermoelectric materials, iron-silicon semiconductor, β-FeSi2, power factor


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