Gregory P. Meisner

High figure of merit in Ce-filled skutterudites

New thermoelectric materials with superior transport properties at high temperatures have been discovered. These materials are part of the large family of skutterudites, a class of compounds which have shown a good potential for thermoelectric applications. The composition of these novel materials, called filled skutterudites, is derived from the skutterudite crystal structure and can be represented by the formula LnT/sub 4/Pn/sub 12/ (Ln=rare earth, Th; T=Fe, Rn, Os, Co, Rh, Ir; Pn=P, As, Sb). In these compounds, the empty octants of the skutterudite structure which are formed in the TPn/sub 3/ (/spl sim/T/sub 4/Pn/sub 12/) framework are filled with a rare earth element. Some of these compositions, based on CeFe/sub 4/Sb/sub 12/, have been prepared by a combination of melting and powder metallurgy techniques and have shown exceptional thermoelectric properties in the 350-700/spl deg/C temperature range. At room temperature, CeFe/sub 4/Sb/sub 12/ behaves as a p-type semimetal, but with a low thermal conductivity and surprisingly large Seebeck coefficient. These results are consistent with some recent band structure calculations on these compounds. Replacing Fe with Co in CeFe/sub 4/Sb/sub 12/ and increasing the Co:Fe atomic ratio resulted in an increase in the Seebeck coefficient values. The possibility of obtaining n-type conductivity filled skutterudites for Co:Fe values higher than 1:3 is currently being investigated. Measurements on bulk samples with a CeFe/sub 3.5/Co/sub 0.5/Sb/sub 12/ atomic composition and p-type conductivity resulted in dimensionless figure of merit ZT values of 1.4 at 600/spl deg/C.

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.

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.