Sun-Uk Kim

Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg2Si (Mg2Si:Alm, mxa0=xa00, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg2Si was synthesized by solid-state reaction (SSR) at 773xa0K for 6xa0h, and Al-doped Mg2Si powders were obtained by mechanical alloying (MA) for 24xa0h. Mg2Si:Alm were fully consolidated by hot pressing (HP) at 1073xa0K for 1xa0h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 1016 cm−3 to 1019 cm−3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg2Si:Al0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823xa0K.

Doping Effects on Thermoelectric Propertiesin the Mg2Sn System

A weak point of Mg2X thermoelectrics is the absence of a p-type composition, which motivates research into the Mg2Sn system. Mg2Sn thermoelectrics were fabricated by a vacuum melting method and a spark plasma sintering process. As a result, Mg2Sn single phases were acquired in a wide range of Mg-to-Sn atomic ratios (67:33 to 71:29), showing slightly different thermoelectric characteristics. However, the thermoelectric properties of the undoped system were not sufficient for application in commercial production. To maximize the p-type characteristics, many atoms [Ni (VIIIA), Cu (IB), Ag (IB), Zn (IIB), and In (IIIB)] were doped into the Mg2Sn phase. Among them, the power factor values increased only in the Ag-doped case. Ag-doping resulted in a power factor that was more than 10 times larger than the value in the undoped case. This result could be important for developing p-type polycrystalline thermoelectrics in the Mg2X (Xxa0=xa0Si, Sn) system. However, other atoms [Ni (VIIIA), Cu (IB), Zn (IIB), and In (IIIB)] were not determined to act as acceptor atoms. The maximum ZT value for the Ag-doped Mg2Sn thermoelectric was more than 0.18, which is comparable to the value for the n-type Mg2Si system.

Effects of Spark Plasma Sintering Temperature on Thermoelectric Properties of Higher Manganese Silicide

Higher manganese silicide (HMS) is a promising p-type thermoelectric material. HMS samples were synthesized by a vacuum induction melting process and sintered by spark plasma sintering (SPS) at various temperatures to obtain a single phase of HMS and investigate the effect of the SPS temperature on the thermoelectric properties. A single phase of HMS was obtained, and the appearance and the amount of Mn2O3 and MnSi as secondary phases could be controlled via the SPS temperature. The effects of the SPS temperature on the electrical conductivity and Seebeck coefficient of the HMS samples were investigated. The changes in the electrical conductivity and Seebeck coefficient were attributed to changes in the density and the amount of Mn2O3 secondary phase.

Solid-State Synthesis and Thermoelectric Properties of Mg2Si1−xSnx

Mg2Si1−xSnx (0xa0≤xa0xxa0≤xa01) solid solutions have been successfully prepared by mechanical alloying and hot pressing as a solid-state synthesis route. All specimens were identified as phases with antifluorite structure. The electrical conduction changed from n-type to p-type at room temperature for xxa0≥xa00.5 due to the intrinsic properties of Mg2Sn. The absolute value of the Seebeck coefficient decreased with increasing temperature, and the electrical conductivity increased with increasing temperature; this is indicative of nondegenerate semiconducting behavior. The thermal conductivity was reduced by Mg2Si-Mg2Sn solid solution due to phonon scattering by the alloying effect.

The Effect of Microstructure on the Thermoelectric Properties of Polycrystalline Higher Manganese Silicides

In order to obtain single-phase higher manganese silicides (HMS) and investigate the effect of sintering conditions on the thermoelectric properties of a HMS system, HMS compounds were synthesized using a vacuum induction melting method and sintered using spark plasma sintering (SPS) and hot pressing methods. Single-phase HMS with a small amount of second phases was obtained in all of the HMS samples produced. Changes in the electrical conductivity and Seebeck coefficient of the HMS were observed when the sintering temperature was changed, which can be attributed to the presence of the second phases. Similar changes of thermoelectric properties were observed in both the SPS and hot-pressed samples. However, the electrical conductivity and Seebeck coefficient of HMS samples (SPS-HMS) were higher than those of the hot-pressed samples, which can be attributed to SPSs shorter holding time and its ability to control the diffusion rate. The SPS-HMS sample sintered at 1123 K (1123 K SPS-HMS) had a higher figure of merit than any other sample although the sample had a lower power factor. The high value of the figure of merit of the sample can be attributed to its low thermal conductivity. The highest figure of merit value of 0.41 was measured at 850 K in the 1123 K SPS-HMS, which is comparable to the results reported earlier. The results of the present study can be used to optimize the fabrication process of HMS thermoelectric materials.

Solid-state synthesis and thermoelectric properties of N-type Mg2Si

Group BIII (Al, In)-, BV(Bi, Sb) and BVI(Te, Se)-doped Mg2Si compounds were synthesized by solid-state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity and figure-of-merit) were examined. Mg2Si powder was synthesized successfully by solid-state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. It was fully consolidated by hot pressing at 1073 K for 1 h. All doped Mg2Si compounds showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased greatly by doping due to an increase in the carrier concentration. However, the thermal conductivity did not changed significantly by doping, which was due to much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group BV(Bi, Sb) elements were much more effective to...