(a) Schematic of the TE measurement system, not drawn to scale. The sample is pressed between two independently heated BN rods; here the rods are shown separated for viewing the sample placement. The temperature in the BN rods is measured at the four positions indicated in the figure. The Cu sample stage capping the end of each BN rod provides electrical contacts to the sample. The active and passive radiation shields are denoted by dashed lines. (b) Top and side views of the sample stage. Each side of the stage consists of two semi-circular Cu pieces, with a 200 μm gap in between. A thermometer is inserted in the thermometer housing, spanning both semi-circular pieces. Each of the four Cu pieces has an electrical connection for Seebeck and resistivity measurements. (c) Simulation of isothermal lines indicating the temperature offset arising from the contribution of thermal resistance of the Cu body to R 0. Each isotherm represents 1 mk, for a temperature gradient T 1,2 (T 3,4) of 300 mK. The model predicts an offset temperature difference, T 2,3 of 3 mK, or 1% of the net temperature drop along a BN rod. This difference constitutes roughly 1/3 of the measured temperature offset, as the model ignores thermal resistance of the silica-based cement used for assembly.
Temperature correction arising from thermal resistance between the stage thermometer and the sample as a function of temperature, measured using a thin Au foil as a sample. The plot shows the symmetry between both BN rods, and correction ranging from approximately 2% to 5%.
The extracted thermal conductivity of a BN rod, obtained using NIST SRM8424 as a sample as a function of temperature. The polynomial fit is subsequently used to obtain the thermal conductivity of samples.
Measured Seebeck coefficient of NIST SRM 3451 Seebeck standard as a function of temperature. The certified reference data for the SRM is depicted by the solid line, which has an uncertainty of ±2.7% and is shown as dashed lines, in agreement with the measured data.
Measured electrical resistivity of NIST SRM 3451 Seebeck standard as a function of temperature. Due to its multi-valley band structure, Bi2Te3 has a complex temperature dependent resistivity, as the chemical potential level crosses specific bands with changing temperature.
Measured thermal conductivity of NIST SRM 3451 Seebeck standard as a function of temperature. Due to the high electrical conductivity of Bi2Te3, the electronic thermal conductivity dominates at these temperatures.
Extracted figure of merit, ZT, of NIST SRM 3451 Seebeck standard as a function of temperature. This figure is calculated from the measurements of the Seebeck coefficient, electrical resistivity, and thermal conductivity.
Relative uncertainty in the figure of merit of NIST SRM 3451 Seebeck standard as a function of temperature, obtained from propagation of the uncertainties in the individual material parameters. The increase in relative uncertainty primarily arises from the increased uncertainty in the thermal conductivity as the temperature increases.
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