title: Improving the thermoelectric properties of half-Heusler TiNiSn through inclusion of a second full-Heusler phase: microwave preparation and spark plasma sintering of TiNi(1+x)Sn.
abstract: Half-Heusler thermoelectrics offer the possibility to choose from a variety of non-toxic and earth-abundant elements. TiNiSn is of particular interest and - with its relatively high electrical conductivity and Seebeck coefficient - allows for optimization of its thermoelectric figure of merit, reaching values of up to 1 in heavily-doped and/or phase-segregated systems. In this contribution, we used an energy- and time-efficient process involving solid-state preparation in a commercial microwave oven and a fast consolidation technique, Spark Plasma Sintering, to prepare a series of Ni-rich TiNi1+xSn with small deviations from the half-Heusler composition. Spark Plasma Sintering plays an important role in the process by being a part of the synthesis of the material rather than solely a densification technique. Synchrotron powder X-ray diffraction and microprobe data confirm the presence of a secondary TiNi2Sn full-Heusler phase within the half-Heusler matrix. We observe a clear correlation between the amount of full-Heusler phase and the lattice thermal conductivity of the samples, resulting in decreasing total thermal conductivity with increasing TiNi2Sn fraction. This trend shows that phonons are scattered effectively as a result of the microstructure of the materials with full-Heusler inclusions in the size range of microns to tens of microns. The best performing samples with around 5% of TiNi2Sn phase exhibit maximum figures of merit of almost 0.6 between 750 K and 800 K which is an increase of ca. 35% compared to the zT of the parent compound TiNiSn.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA.
authors.forename: Christina S
authors.initials: CS
authors.lastname: Birkel
authors.forename: Jason E
authors.initials: JE
authors.lastname: Douglas
authors.forename: Bethany R
authors.initials: BR
authors.lastname: Lettiere
authors.forename: Gareth
authors.initials: G
authors.lastname: Seward
authors.forename: Nisha
authors.initials: N
authors.lastname: Verma
authors.forename: Yichi
authors.initials: Y
authors.lastname: Zhang
authors.forename: Tresa M
authors.initials: TM
authors.lastname: Pollock
authors.forename: Ram
authors.initials: R
authors.lastname: Seshadri
authors.forename: Galen D
authors.initials: GD
authors.lastname: Stucky