Siqing Hu

Materials With Open Crystal Structure as Prospective Novel Thermoelectrics

A useful approach to identify materials with high thermoelectric figure of merit is to search for solids that offer great flexibility to modify and tailor the structure so as to achieve the optimal transport behavior. Among the most promising novel thermoelectric materials are solids with “open crystal structure”. They may be typified by structures with unfilled cages, crystals with an empty atomic sublattice, and by a network of polyhedral cages enclosing guest species. In this paper we present our latest results concerning transport properties in the above classes of solids. Specifically, we focus on the filled skutterudites, half-Heusler alloys, and clathrates.

Transport properties of ZrNiSn-based intermetallics

Intermetallic compounds of the form MNiSn, where M is titanium, zirconium, or hafnium, are semiconductors with band gaps on the order of 0.2 electron volts. Due to their large conduction band masses, they show promise as thermoelectric materials with high figure of merit. In order to assess this potential, we have studied the thermoelectric properties of pure and doped ZrNiSn and Zr/sub 0.5/Hf/sub 0.5/NiSn mixed crystals. Upon doping with Sb the resistivity can be reduced by a factor of 20-40 with a reduction in the Seebeck coefficient of only a factor of two. The resulting power factors of doped samples at room temperature are comparable to those of state of the art thermoelectric materials. The thermal conductivity of Zr/sub 0.5/Hf/sub 0.5/NiSn is reduced strongly relative to ZrNiSn but must be reduced further to obtain a large figure of merit. The transport properties of these materials are very sensitive to annealing conditions.

Magnetic and Electronic Transport Properties of Single Crystal La0.64Pb0.36MnO3

We studied the magnetic and electronic transport properties of a single crystal sample of La 0.64 Pb 0.36 MnO 3 in the temperature range 5 K to 350 K and magnetic field up to 5.5 T. A magnetic transition is found at 210 K. The single crystal sample is ferromagnetic below the transition temperature (T c ) and becomes paramagnetic at temperatures T > T c . Magnetization measurements along three different orthorhombic crystal axes show no significant difference. The magnetoresistance approaches a maximum value of about -60% at T c in 5 T magnetic field strength and has qualitatively different field dependence below and above T c . The scaling behavior between resistivity and magnetic moment is examined for temperatures both below and above the transition. A low temperature (T