Observation of spatial distribution of dimensionless figure of merit in powder-sintered Bi2Te3-based bulk thermoelectric materials via time-domain impedance spectroscopy

The time-domain impedance spectroscopy (TDIS) method enables direct determination of the dimensionless figure of merit (zT) from a single thermoelectric specimen, eliminating uncertainties associated with sample-to-sample variations inherent in conventional measurements. This work demonstrates direct, spatially resolved zT measurements within a powder-sintered bulk ruthenium-doped Bi2Te3 specimen, allowing quantitative mapping of its spatial distribution. Three 3 × 3 × 5 mm3 specimens, equipped with thin-film Ti/Cu electrodes, were randomly selected from the same bulk. Impedance and transient-resistance responses were measured at 300 K under high-vacuum and temperature-stabilized conditions. The TDIS method achieved a zT uncertainty below 1.5% and quantitatively resolved spatial variations ranging from 0.413 ± 0.005 to 0.475 ± 0.007 over a distance of 5 mm, providing experimental evidence that intrinsic transport-defined zT inhomogeneity is manifested as spatial zT variations in powder-sintered bulk materials. Distinct local variations in thermoelectric properties (zT and resistivity) were detected even within nominally homogeneous powder-sintered bulk materials. Electrode geometry also affected the observed distribution, with point electrodes exhibiting larger fluctuations (approximately 7%) than strip electrodes (<3%). These results demonstrate the presence of measurable spatial zT variation within a single specimen and highlight the importance of accounting for material inhomogeneity when evaluating thermoelectric performance.