No 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/mK2 at temperatures in the 700 to 800 K range, and zT 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.

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.

Multilayer ytterbium-hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC-based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb 2 Si 2 O 7 and Yb 4 Hf 3 O 12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb 2 SiO 5 in contact with Yb 4 Hf 3 O 12 do not experience similar degradation. The outer Yb 4 Hf 3 O 12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine-type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.

The interaction of molten calcium–magnesium alumino‐silicate (CMAS) with yttrium monosilicate (YMS), a candidate environmental barrier coating (EBC) for SiC‐based ceramic matrix composites, was investigated. YMS monoliths were exposed to CMAS melts at 1300°C for times ranging from 1 to 100 h and characterized by scanning electron microscopy, transmission electron microscopy, energy‐dispersive spectroscopy/electron microprobe analysis, and electron back‐scattered diffraction. YMS is dissolved into CMAS and reprecipitates as a Ca2Y8(SiO4)6O2 oxyapatite phase. A nominally continuous layer of apatite is readily established at the interface, but its effectiveness as a diffusion barrier is compromised by the presence of thin amorphous films along the grain boundaries that enhance chemical transport between the bulk melt and the YMS. Moreover, the growth mechanism involves continuous renucleation that eventually leads to separation of the crystals formed previously from the interface and their subsequent coarsening. The latter results in widening of the amorphous boundary films, enhancing interdiffusion. The significance of these findings on the viability of YMS as a durable EBC is discussed.