The Fabrication and Thermoelectric Properties of Amorphous Si-Ge-Au Bulk Samples
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The Fabrication and Thermoelectric Properties of Amorphous Si-Ge-Au Bulk Samples Sang Min Lee, Yoichi Okamoto, Toshio Kawahara and Jun Morimoto Department of Materials Science and Engineering, National Defense Academy 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan. ABSTRACT The amorphous Si-Ge-Au bulk samples were fabricated with using the melt spinning method for the practical power supply or cooling devices. X-ray diffraction results showed that our samples were amorphous and the thermoelectric properties were measured by DC method. Although the electrical resistivity of the bulk sample was higher than that of the amorphous thin film, the thermoelectric power of the bulk sample was larger. The thermal conductivity of the amorphous Si-Ge-Au bulk sample was almost the same to the conventional crystalline Si-Ge bulk value. Consequently, non-dimensional figure of merit ZT is around 2 (at 600 K, α=6.5 * 10^-1V/K, ρ =1.9 * 10 ohm-m, κ = 6 W/mK) that is about ten times higher than the conventional crystalline Si-Ge bulk value. INTRODUCTION Recently, thermoelectric materials have attracted a great deal of attentions from the point of the saving energy and the environmental problems. With the development of the scientific technology, it became possible to fabricate new artificial structure materials such as quantum well structures [1] and non-dimensional figure of merit ZT value could be over than 1 by using superlattice structure [2]. Then, we tried to make new thermoelectric materials as an amorphous structure. We were succeeded to get the anomalously large ZT values (about 1000 at 1000 K) with the amorphous superlattice thin film [3]. We have already reported that the amorphous Si-Ge-Au thin film which shows the extremely large non-dimensional figure of merit ZT values [4] and that is mainly caused by the amorphous phase not superlattice structure. The artificial superlattice structure collapsed at around 560 K by the first time thermal annealing [5]. We intended to control "the degree of amorphous" with the substrate temperature that was known to have large effects on amorphous thin films [6]. The power factor of the sample deposited on the substrate at 77 K (liquid nitrogen) increases by the thermal annealing compared to that of the sample deposited at room temperature. In order to make samples transform to other less quasi-stable and/or unstable phase, samples were annealed. We were a great success in enhancing the thermoelectric powers through cooling down the substrate temperature during the deposition to 77 K [5]. Further, an amorphous phase is known to be recrystallized by the thermal annealing [6]. But, there expect lots of quasi-stable amorphous phases in the amorphous Si-Ge-Au thin films and one phase of the sample can be transformed to other quasi-stable amorphous phases by the thermal annealing [5]. However, there are some problems to make devices with the amorphous thin films. Because it will be needed to produce bulky thermoelectric materials which are enough to construct big practical devices. Th
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