Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb Half-Heusler Compounds

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.

Structure and Failure Mechanism of the Thermoelectric CoSb3/TiCoSb Interface

“…23 Our optimized lattice 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 parameter for TiCoSb is a = 5.855 Å, which is only 0.48% smaller than the experimental value of 5.883 Å. 24 These calculated values are in good agreement with the previously published theoretical value of 9.14 Å for CoSb 3 and 5.89 Å for TiCoSb. [25][26][27] To determine the favorable surface slab, we calculated the surface energy, γ , from the following expression, 28 A is the surface area.…”

High Efficiency Half‐Heusler Thermoelectric Materials for Energy Harvesting

“…[ 57 ] Birkel et al used microwave heating to prepare TiNiSn and TiCoSb, reducing the reaction time signifi cantly from more than a week to one minute. [ 58 ] A combined ball milling and hot pressing approach was adopted to obtain nanostructured HH alloys by Yan et al [ 59 ] Nevertheless, impurity phase is still an obstacle for these methods to rapidly obtain high quality products. A summary of processing method, microstructure and TE performance of typical HH compounds is presented in Table 1 .…”

Micro- and Macromechanical Properties of Thermoelectric Lead Chalcogenides

“…1 Indeed the past two decades has seen dramatic enhancements in the TE figure of merit, zT = α 2 σ/κ, of various materials, achieved through simultaneously optimizing the power factor (α 2 σ) and reducing the thermal conductivity 2 for such materials as PbTe, [3][4][5] skutterudite CoSb 3 , [6][7][8] Bi 2 Te 3 , 9-11 Mg 2 Si, [12][13] and half-Heusler alloys. 14,15 However, despite the improved efficiencies, applications of these TE materials usually impose a severe cyclic temperature gradients that can generate cracks in the microstructure. 16,17 Such cracks lead to deterioration in the material performance, accelerating failure processes of these TE devices upon cycling.…”