Presentation on theme: "Introduction Table 1. Summary of excess energies to be supplied for interstitial occupancy into one of 4b sites or substitutional occupancy of a Mg site."— Presentation transcript:
Introduction Table 1. Summary of excess energies to be supplied for interstitial occupancy into one of 4b sites or substitutional occupancy of a Mg site or a Si site of Mg 8 Si 4 by X (X = Cu, Ag, or Au) to proceed Interstitial Mg 8 Si 4 X 1 Mg substitution Mg 7 X 1 Si 16 Si substitution Mg 8 Si 3 X 1 Cu p-type Au n-type Ag p-type (4 sites) p-type (8 sites) n-type (4 sites) 2GPa Method: CASTEP LDA-GGA MgSiBiTePbCoSb Reserves /ppm 32,000267, (ppb) Price Harmless OOXXXXX High Temp. The reuse of waste heat energy is expected as one of the solutions of the environmental issues. It is necessary for the recovery of the waste heat energy to investigate the high-performance thermoelectric device which consists of N-type and P-type semiconductors. Moreover it is important for the industry and the environment to research the ecological friendly semiconductor which has abundant deposit on the earth and is harmless for the health. The synthesis is difficult, because the boiling point of Mg （ 1363K ） is close to the melting point of Mg 2 Si （ 1358K ）. Mg Thermoelectric properties S:Seebeck coefficient ρ:Electrical conductivity κ:Thermal conductivity Fig.1 Performance of several thermoelectric materials Fig.2 The synthetic condition of Mg 2 Si. Fig.3 Crystal structure of Mg 2 Si. (anti CaF 2 -type: ) Experiment Purpose To find the synthetic condition for Ag-doped Mg 2 Si by high- temperature and high-pressure XRD (Multi Press) To calculate the suitable dopant element for P-type To synthesize the thermoelectric material (Piston Cylinder) and evaluate the thermal property Fig. 12 Electronic densities of states of (1)undoped Mg 54 Si 27 (2)Mg 64 Si 32 Cu 1 (3)Mg 53 Si 27 Ag 1 (4)Mg 54 Si 26 Au 1. Arrows in the Figures show intrinsic energy band gaps. Fermi energies are aligned with 0eV. Fig. 8 Photograph of an electric furnace and automatic control system. Fig. 15 EDX diffraction patterns of Ag-doped Mg 2 Si at 673 K, 1 GPa for various synthesis-time. Diffraction peaks of Mg decreased with increasing synthesis-time, their peaks almost disappeared after 8hrs. Fig. 4 Photographs of the starting materials for the XRD study of Ag-doped Mg 2 Si under high-temperature and high-pressure. The left two material are Mg(1) with particles 150 m in diameter, and Si(2) powder and the right one is pure Mg 2 Si(3) synthesized by Union Material Co., Ltd. The average diameters of the particles of Si were (A)~150mm, (B) ~40mm, (C)~20mm, (D) ~3mm. Fig. 13 EDX diffraction patterns of Mg 2 Si under high-temperature at 1 GPa. The starting material of left figure (1) is Mg and Si powders. Mg 2 Si was synthesized at 573 K which temperature is very low than the melting point of Mg (923 K). Mg peaks disappeared and a broad peak appeared at 973 K. The quenched sample (top of the (1)) does not include Mg and Si. In the right figure (2), the starting material is powdered pure Mg 2 Si synthesized by Union Material Co., Ltd. Some peaks of MgO appeared with increasing temperature. XRD under pressure by synchrotron radiation source at Photon Factory in Tsukuba BeamLine: PF-AR-NE5C Pressure technique: Multi Press (MAX80) Fig. 14 EDX diffraction patterns of Ag-doped Mg 2 Si under high- temperature at 1 GPa. The starting materials of the left (1) and the right (2) are Mg, Si, Ag and Mg 2 Si, Ag, respectively. In the left figure, Mg 2 Si was synthesized at 523 K and Mg peaks disappeared and broad peak appeared at 873 K. There temperatures are lower than the case of undoped sample. Ag peaks disappeared 823 K. However, in the right figure, Ag peaks remains still 873 K. The result means that in the case of Ag-doping, the Mg and Si powders are better than Mg 2 Si powder as the starting material. (2)(1) Summary The first principle calculation expects that Ag is suitable doping element for the P-type conduction of Mg 2 Si and the pressure is effective for the Ag-doping. In the case of mixture of Ag, Mg, Si powders, Mg 2 Si was synthesized at 523 K, and Ag peaks disappeared at 823 K and MgO or SiO 2 peaks did not appeared. This work was supported by MEXT KAKENHI(C) Grant Number , and has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G668, 2012G566). (2) (1) (2)(3) a = nm S.G. = Fm-3m X-ray diffraction studies of Mg 2 Si and Ag-doped Mg 2 Si under pressure Y.Mori, Y.Kaihara, K.Takarabe : Okayama University of Science Fig.6 Particle size distribution of Si powder (D). Fig.5 SEM image of mixture of Mg and Si powders. Fig.7 High-pressure equipment and sample assembly. Fig. 9 Photograph of the sampling plate for the thermoelectric properties. The properties of the small sample such as 5mm in diameter were measured by using this plate. Fig.10 Electrical conductivity of the standard sample of Mg 2 Si was measured. Fig.11 Seebeck coefficients of the standard sample and the synthesized Mg 2 Si under pressure(red).