Method: CASTEP LDA-GGA

1. Mg Mg Mg Mg • Fine Powder of Sample • Thermal properties • XRD under HP • Experiment • Purpose • Summary • Calculation • XRD @ High Temp. M1.00064 X-ray diffraction studies of Mg2Si and Ag-doped Mg2Si under pressureY.Mori , Y.Kaihara, K.Takarabe : Okayama University of Science 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. a = 0.6340 nm S.G. = Fm-3m Fig.3 Crystal structure of Mg2Si. (anti CaF2-type:) Fig.2 The synthetic condition of Mg2Si. Fig.1 Performance of several thermoelectric materials S:Seebeck coefficient ρ:Electrical conductivity κ:Thermal conductivity The synthesis is difficult, because theboiling point of Mg（1363K） is close to the melting point of Mg2Si（1358K）. Thermoelectric properties • To find the synthetic condition for Ag-doped Mg2Si by high-temperature and high-pressure XRD (Multi Press) • To calculate the suitable dopant element for P-type • To synthesizethe thermoelectric material (Piston Cylinder) and evaluatethe thermal property Method: CASTEP LDA-GGA 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 Mg8Si4 by X (X =Cu, Ag, or Au) to proceed (1) (2) (3) Fig. 12 Electronic densities of states of (1)undoped Mg54Si27 (2)Mg64Si32Cu1 (3)Mg53Si27Ag1 (4)Mg54Si26Au1. Arrows in the Figures show intrinsic energy band gaps. Fermi energies are aligned with 0eV. Fig.5 SEM image of mixture of Mg and Si powders. Fig. 4 Photographs of the starting materials for the XRD study of Ag-doped Mg 2Si under high-temperature and high-pressure. The left two material are Mg(1) with particles 150 mm in diameter, and Si(2) powder and the right one is pure Mg2Si(3) synthesized by Union Material Co., Ltd.The average diameters of the particles of Si were (A)~150mm, (B) ~40mm, (C)~20mm, (D) ~3mm. 3.1mm Si:(D) Fig. 13 EDX diffraction patterns of Mg2Si under high-temperature at 1 GPa. The starting material of left figure (1) is Mg and Si powders. Mg2Si 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 Mg2Si synthesized by Union Material Co., Ltd. Some peaks of MgO appeared with increasing temperature. (2) (1) Fig.6 Particle size distribution of Si powder (D). • XRD under pressure by synchrotron radiation source at Photon Factory in Tsukuba • BeamLine: PF-AR-NE5C • Pressure technique: Multi Press (MAX80) Fig.7 High-pressure equipment and sample assembly. Fig. 14 EDX diffraction patterns of Ag-doped Mg2Si under high-temperature at 1 GPa. The starting materials of the left (1) and the right (2) are Mg, Si, Ag and Mg2Si, Ag , respectively. In the left figure, Mg2Si 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 Mg2Si powder as the starting material. (1) (2) Fig. 8 Photograph of an electric furnace and automatic control system. Fig.10 Electrical conductivity of the standard sample of Mg2Si was measured. 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.11 Seebeck coefficients of the standard sampleand the synthesized Mg2Si under pressure(red). • The first principle calculation expects that Ag is suitable doping element for the P-type conduction of Mg2Si and the pressure is effective for the Ag-doping. • In the case of mixture of Ag, Mg, Si powders, Mg2Si was synthesized at 523 K, and Ag peaks disappeared at 823 K and MgO or SiO2 peaks did not appeared. • This work was supported by MEXT KAKENHI(C) Grant Number 11013342, and has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G668, 2012G566). Fig. 15 EDX diffraction patterns of Ag-doped Mg2Si at 673 K, 1 GPa for various synthesis-time. Diffraction peaks of Mg decreased with increasing synthesis-time, their peaks almost disappeared after 8hrs.