Hydride synthesis of thermoelectric skutterudites (Ba,Sr)Fe3±xNi1±xSb12 optimized via design of experiments†

Reactive metal hydrides, SrH₂ and BaH₂, were used to make the novel thermoelectric skutterudite Sr_0.92Fe_2.98±xNi_1.00±xSb₁₂ and the previously studied skutterudite Ba_0.9Fe_3.00±xNi_1.00±xSb₁₂, demonstrating the advantages of the diffusion-enhanced hydride synthesis. High-temperature in situ powder X-ray diffraction showed that BaH₂, Fe, Ni, and Sb react to form binary and pseudo-binary intermediate products, which further react with one another and the remaining Sb to form BaFe₃NiSb₁₂ in a narrow temperature range around 855 K, the optimal synthesis temperature of BaFe₃NiSb₁₂. Statistical analysis, the fundamental basis of design of experiments (DoE) methodology, was tested using 12 synthesis attempts, and indicated 861 K was the optimal annealing temperature for BaFe₃NiSb₁₂ synthesis, validating the utility of statistical methods for the optimization of synthesis conditions in multicomponent solid-state systems. Synthesis optimization for SrFe₃NiSb₁₂ was performed entirely using DoE tools, demonstrating the efficacy of DoE to narrow down the set of conditions needed to form single-phase products using the fast hydride route. Synthetic insensitivity to the ratio of Fe to Ni enabled tuning of electronic transport properties, resulting in a peak thermoelectric figure of merit (zT) of 0.54 at 673 K for Sr_0.92Fe_3.28Ni_0.70Sb₁₂.