Jihui Yang

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.

High temperature thermoelectric properties of MNiSn (M=Zr, Hf)

The high temperature transport properties in a series of intermetallic half-Heusler alloys of the form MNiSn, where M=Zr, Hf, have been examined. The semiconducting nature of these materials due to the formation of a pseudo-gap in the density of states make them promising candidates for intermediate temperature thermoelectric applications. Samples of pure and Sb-doped ZrNiSn, HfNiSn, and (Zr-Hf)NiSn were prepared by arc melting and homogenized by heat treatment. Phase purity was determined by X-ray diffraction and the microstructures were examined by scanning electron microscopy. The temperature dependence of the electrical resistivity and Seebeck coefficient of these samples was characterized between 300 K and 1050 K. At room temperature, the data match closely with the results recently reported by us. The thermopower initially increases with temperature, exhibits a broad maximum between 400 K and 600 K, and decreases to a common value, characteristic of the magnitude of the forbidden gap. The electrical resistivity decreases with temperature following a T/sup -1/ dependence. A correlation between the magnitude of the thermopower and the Hf/Zr ratio was observed. An estimate of the magnitude of the gap was made from a plot of 1n(/spl sigma/) versus reciprocal temperature, giving a value of 0.21 eV which is in good agreement with previous estimates. The effects of antimony and bismuth doping on the electrical properties are discussed.

Thermoelectric properties of Bi-doped half-Heusler alloys

Based on our preliminary promising results for Sb-doped ZrNiSn-type half-Heusler intermetallics, we made a new series of Bi-doped samples Zr/sub 0.5/Hf/sub 0.5/NiSn/sub 1-x/Bi/sub x/ with concentrations x in the range 0/spl les/x/spl les/0.02. While the isoelectronic alloying of Zr and Hf reduces the lattice thermal conductivity, doping on the Sn site with Hf controls the carrier density and thus the nature of transport. This confirms the trend we reported previously for Sb, namely, that a small amount of the group V semimetal substituted on the Sn-site has a spectacular effect on all transport properties. Specifically, the electrical resistivity is much reduced due to an order of magnitude higher carrier density, and the thermopower is exceptionally large (/spl sim/-250 /spl mu/V/K for x=0.01), comparable to the thermopower of the parent (undoped) compound. Doping on the Sn site with Sb or Hi is thus an effective way to achieve high power factors in these half-Heusler intermetallics. The optimal operational range of the intermetallics is above room temperature.

Cerium filling and lattice thermal conductivity of skutterudites

We have investigated the structural and transport properties of filled skutterudites of the general form Ce/sub y/Fe/sub 4-x/Co/sub x/Sb/sub 12/. We show that the Ce occupancy of the cages is a strong function of the presence of cobalt, varying from an almost 100% filling for x=0 down to no more than 10% filling in pure Ce/sub y/Co/sub 4/Sb/sub 12/, i.e., for x=4. We demonstrate a strong suppression of the lattice thermal conductivity by both the rattling motion of Ce, and by alloying Fe with Co. We propose that the partially filled skutterudites can be viewed as solid solutions of fully filled CeFe/sub 4/Sb/sub 12/ and /spl square/Co/sub 4/Sb/sub 12/, where /spl square/ stands for a vacancy.

Transport and magnetic properties of Co/sub 1-x/Ni/sub x/Sb/sub 3/ with x less than 0.01

The filled skutterudite compounds based on the binary skutterudite CoSb/sub 3/ are currently being investigated for their potential application as thermoelectric materials. One route to optimization of these compounds is by doping on the Co site. An obvious candidate for an n-type dopant is Ni, because it has one more electron in its valence shell than Co. We present electrical resistivity, thermopower, Hall effect and magnetic susceptibility measurements on polycrystalline, n-type Co/sub 1-x/Ni/sub x/Sb/sub 3/ with x less than 0.01. A model which takes into account conduction of electrons residing in the conduction band in addition to hopping conduction within an impurity band formed by the Ni atoms is consistent with the observed behavior. The doping dependence suggests Ni acts as an electron donor thus taking the tetravalent state Ni/sup 4+/ and assuming the d/sup 6/ electronic configuration. The measured magnetic susceptibility suggests a paramagnetic state implying that Ni may carry a moment at least at low temperatures. We attempt to tie together the transport and magnetic susceptibility data to provide a consistent picture of the overall behavior.

Transport properties of Co/sub 1-x/Fe/sub x/Sb/sub 3/

The role of iron on the cobalt site of the filled skutterudites has yet to be firmly ascertained. To address this question, we prepared a series of samples of the form Co/sub 1-x/Fe/sub x/Sb/sub 3/ with x=0, 0.5, 1, 2, 5 and 10 at% by induction melting. We performed measurements of thermal conductivity, thermopower, resistivity, Hall effect, X-ray diffraction, and magnetization. The presence of iron reduces dramatically the lattice thermal conductivity. Iron doping increases the hole concentration resulting in a concomitant reduction of the room temperature Seebeck coefficient. We also observed phonon drag effects at low temperature. Magnetization studies indicate the development of a paramagnetic state. The subtle role of iron in creating lattice defects will also be discussed.