Rapid Microwave Preparation
of Thermoelectric TiNiSn
and TiCoSb Half-Heusler Compounds
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Abstract
The 18-electron ternary intermetallic systems TiNiSn
and TiCoSb
are promising for applications as high-temperature thermoelectrics
and comprise earth-abundant, and relatively nontoxic elements. Heusler
and half-Heusler compounds are usually prepared by conventional solid
state methods involving arc-melting and annealing at high temperatures
for an extended period of time. Here, we report an energy-saving preparation
route using a domestic microwave oven, reducing the reaction time
significantly from more than a week to one minute. A microwave susceptor
material rapidly heats the elemental starting materials inside an
evacuated quartz tube resulting in near single phase compounds. The
initial preparation is followed by a densification step involving
hot-pressing, which reduces the amount of secondary phases, as verified
by synchrotron X-ray diffraction, leading to the desired half-Heusler
compounds, demonstrating that hot-pressing should be treated as part
of the preparative process. For TiNiSn, high thermoelectric power
factors of 2 mW/mK<sup>2</sup> at temperatures in the 700 to 800 K
range, and <i>zT</i> values of around 0.4 are found, with
the microwave-prepared sample displaying somewhat superior properties
to conventionally prepared half-Heuslers due to lower thermal conductivity.
The TiCoSb sample shows a lower thermoelectric figure of merit when
prepared using microwave methods because of a metallic second phase