Measuring thermoelectric transport properties of materials

Abstract

In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT.Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed. Measurement guide for authors and reviewersMeasurements should always be repeatable on the same sample, and on new samples produced in the manner described. Thermoelectric effects are steady-state effects so any time dependence or hysteresis is indication that phenomena outside thermoelectric effects are at play. Materials with chemical oxidants/ reductants incorporated are likely to contain unstable internal voltages not due to thermoelectric effects. Unconventional samples or measurement methods deserve reexamination of assumptions. AccuracyTrue accuracy is not represented by a single heating curve from one sample, even with error bars representing instrument precision. Showing heating and cooling data and multiple samples gives a better indication of measurement variability for a typical type of sample. Anisotropy, cracks and inhomogeneities can lead to large variation in measurements. One unusual data point or sample outside the trend, particularly at temperatures just prior to decomposition, usually indicates a problem in sample or measurement. Unusual resultsTypical thermoelectric materials behave like heavily doped semiconductors with thermopower (absolute value of Seebeck coefficient) of less than 300 mV K À1 , resistivity of 0.1-10 mU cm, and are optimized when electronic contribution to the thermal conductivity is about 1/2 the total thermal conductivity. Extraordinary results should be checked by extra means. Unusual results can be caused by bad contacts, thermocouples that have broken, chemically reacted, or simply dried out of calibration. Exceptional resultsReported values of zT > 1 or in unexpected materials receive extra attention from reviewers who may ask for additional conrmation. Convincing measurements may need to be performed on the same sample along the same direction and be repeatable with other samples and measurement methods. There is no official record keeping for claimed or veried zT values. Several papers, patents and press releases have claimed extraordinarily high zT but most have been forgotten over time and likely resul...

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