Precise measurement of absolute Seebeck coefficient from Thomson effect using ac-dc technique

Y Amagai,N.-H Kaneko,T Shimazaki,K Okawa,T Kawae,H Fujiki

doi:10.1063/1.5095485

Y Amagai, N.-H Kaneko + Show 4 more

Open Access

https://doi.org/10.1063/1.5095485

Copy DOI

Abstract

The Seebeck coefficient is the most widely measured property specific to thermoelectric materials. The absolute Seebeck coefficient S determined from the Thomson effect is highly sensitive to systematic errors incurred in the determination of the material thermal conductivity and geometry and heat loss from the sample to surroundings caused by temperature differences. Here, we report a technique for the precision measurement of S based on the Thomson effect using an ac-dc technique. This technique utilizes accurate equivalent-amplitude ac and dc currents, which can eliminate the need for acquiring accurate thermal conductivity and geometry values. These parameters can be replaced by the precisely and readily measurable parameters of electrical resistance and temperature changes caused by the Joule effect. The correction term of the heat loss owing to heat transfer via the thermocouple vanishes upon calculating the ratio of the measured temperature changes for both ac and dc excitations. We obtain an S value of -4.8 μV/K ± 0.2 μV/K at a temperature of 300 K for platinum, which is most widely used as a reference, with an expanded relative uncertainty of 4% (2σ). The obtained S value of Pt is closely consistent with that obtained from the conventional method using the Thomson effect within the uncertainty, and importantly, the measurement uncertainty improves to an acceptable level, which is four times more precise.

Highlights

The Seebeck coefficient is an essential indicator of the potential performance of thermoelectric materials used for thermoelectric power generators.1 The results of a recent international roundrobin study of thermoelectric materials suggest that it is important to ensure absolute data accuracy when comparing the evaluated values acquired by several different laboratories.2 The determination of the Seebeck coefficient is challenging because it is obtained by means of inherently relative measurements
The sample thermal conductivity and geometry can be replaced by the precisely and readily measurable parameters of electrical resistance and temperature change caused by the Joule effect in the ac-dc technique
This is because the ac-dc technique compensates for the linear heat loss owing to heat transfer via the thermocouple via calculation of the ratio of the measured temperature changes for both ac and dc excitations as shown in (10)

Summary

Introduction

The Seebeck coefficient is an essential indicator of the potential performance of thermoelectric materials used for thermoelectric power generators. The results of a recent international roundrobin study of thermoelectric materials suggest that it is important to ensure absolute data accuracy when comparing the evaluated values acquired by several different laboratories. The determination of the Seebeck coefficient is challenging because it is obtained by means of inherently relative measurements. The determination of the Seebeck coefficient is challenging because it is obtained by means of inherently relative measurements. The. Seebeck coefficient of a thermoelectric material, Ssample, is evaluated via measuring the output of a thermocouple circuit, and is expressed as Ssample = Sreference + ∆V ∆T (1). Where Sreference denotes the Seebeck coefficient of the reference material and ∆V the voltage induced by temperature difference ∆T across the sample. The Seebeck coefficient of reference materials such as Pb, Pt, Cu, and W4–8 must be determined through separate experiments for the entire temperature range of interest. Two techniques are commonly used to measure the absolute Seebeck coefficient: at low temperatures, a superconducting material is used as the reference because the Seebeck coefficient is zero in the Meissner state. The main drawback of this method is that the highest measurable temperature is currently limited by the transition temperature Tc of the reference material, which is approximately 90 K for YBa2Cu3O7-x (YBCO). At higher temperatures, the absolute Seebeck coefficient can be calculated by measuring the Thomson coefficient μ directly and using the Kelvin relation for the Seebeck coefficient:

Methods

Results

Conclusion