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[16a-P6-7] Enhancement of thermoelectric properties in amorphous Si1-xGex synthesized by
mechanical alloying process
キーワード：Si-Ge alloy, Amorphous, Thermoelectric properties
In order to improve the performance of thermoelectric Si1-xGex, we tried, in this study, to synthesize the amorphous bulk samples involving nano-sized crystalline particles by means of simple mechanical alloying process.
The stoichiometric ratio of Si (99.99%), and Ge (99.99%) powders were sealed in a stainless-steel container together with stainless steel balls (f 10 mm) under the pressurized Ar atmosphere. The alloying was conducted in a planetary ball mill (Fritsch P7) rotating at 400 rpm for long durations up to 300 h. The structure, morphology, composition, crystallinity, and thermal stability of synthesized powders were investigated by means of powder x-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive x-ray spectrometry (EDX), and differential thermal analysis coupled with thermo gravimetric analysis (DTA-TG). We also employed hot pressing technique at 550 ºC for 24 hours under the high pressure of 400 MPa, and succeeded in preparing bulk samples consisting almost solely of amorphous phase with a rather large density of
88 %. We used these samples for the measurement of thermoelectric properties.
Fig. 1 shows the XRD patterns of ball milled powder-sample. It revealed that Si-Ge alloy was formed after 50 h ball milling. The lattice constant was calculated to be 5.5 Å, which showed good consistency with that of bulk Si0.65Ge0.35. After 125 h ball milling, the powder-samples became to consist mainly of amorphous phase together with a tiny amount of Si0.65 Ge0.35 nano-crystals with ~2 nm in diameter, which was estimated using the Scherrer’s equation. We also found, as a consequence of EDX, that the samples after 300 hours milling contain Fe contamination of ~0.8 at.%. DTA analysis with heating rate of 0.16 ºC /sec. suggested that the crystallization takes places at 742 ºC. Thermal conductivity of Si0.65Ge0.35 bulk sample measured using laser flash method were shown in Fig.2. Much smaller value of thermal conductivity than 5.5 Wm-1K-1 of bulk crystal [1] was observed over the whole temperature of measurement. This small magnitude in thermal conductivity below 1.4 Wm-1K-1 should be attributed partly to the amorphous phase and partly to the low density. In the presentation, their electrical resistivity and lattice thermal conductivity will be presented in detail together the resulting values of dimension less figure of merit ZT.
The stoichiometric ratio of Si (99.99%), and Ge (99.99%) powders were sealed in a stainless-steel container together with stainless steel balls (f 10 mm) under the pressurized Ar atmosphere. The alloying was conducted in a planetary ball mill (Fritsch P7) rotating at 400 rpm for long durations up to 300 h. The structure, morphology, composition, crystallinity, and thermal stability of synthesized powders were investigated by means of powder x-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive x-ray spectrometry (EDX), and differential thermal analysis coupled with thermo gravimetric analysis (DTA-TG). We also employed hot pressing technique at 550 ºC for 24 hours under the high pressure of 400 MPa, and succeeded in preparing bulk samples consisting almost solely of amorphous phase with a rather large density of
88 %. We used these samples for the measurement of thermoelectric properties.
Fig. 1 shows the XRD patterns of ball milled powder-sample. It revealed that Si-Ge alloy was formed after 50 h ball milling. The lattice constant was calculated to be 5.5 Å, which showed good consistency with that of bulk Si0.65Ge0.35. After 125 h ball milling, the powder-samples became to consist mainly of amorphous phase together with a tiny amount of Si0.65 Ge0.35 nano-crystals with ~2 nm in diameter, which was estimated using the Scherrer’s equation. We also found, as a consequence of EDX, that the samples after 300 hours milling contain Fe contamination of ~0.8 at.%. DTA analysis with heating rate of 0.16 ºC /sec. suggested that the crystallization takes places at 742 ºC. Thermal conductivity of Si0.65Ge0.35 bulk sample measured using laser flash method were shown in Fig.2. Much smaller value of thermal conductivity than 5.5 Wm-1K-1 of bulk crystal [1] was observed over the whole temperature of measurement. This small magnitude in thermal conductivity below 1.4 Wm-1K-1 should be attributed partly to the amorphous phase and partly to the low density. In the presentation, their electrical resistivity and lattice thermal conductivity will be presented in detail together the resulting values of dimension less figure of merit ZT.