Improved Thomson Coefficient Measurements Using an AC Method
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Improved Thomson Coefficient Measurements Using an AC Method Yasutaka Amagai1, Atsushi Yamamoto2, Megumi Akoshima1, and Hiroyuki Fujiki1 1 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan 2 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ABSTRACT We present an improved AC (Alternating Current) method for the determination of the Thomson coefficient, which can be used for obtaining the absolute Seebeck coefficient. While previous work has focused on DC (Direct Current) methods, we analyze the influence of an AC current in order to derive the Thomson coefficient of a thin wire from measurable quantities. Our expression requires five parameters including AC current, resistance, temperature gradient, and the temperature changes due to the Thomson and Joule effects. Thus, a prior determination of thermal conductivity and sample geometry is not required, unlike DC methods. In order to validate our analysis, the Thomson coefficient of a thin Pt wire has been measured at several frequencies. The results agree with those obtained from a conventional DC method.
INTRODUCTION The Seebeck coefficient is an indispensable physical property in evaluating the potential performance of TE (Thermoelectric) materials [1]. The measured Seebeck coefficient is proportional to the Seebeck coefficient of the TE material under test and that of the reference wire materials used in the measurement such as Pt and Cu. Thus, the absolute Seebeck coefficient of the reference material must be determined in a separate experiment. For this reason, superconducting materials are often used as reference wires [2], since the Seebeck coefficient is zero in the Meissner state. However, this method limits the highest temperature to the superconducting transition temperature Tc, the highest of which is approximately 120 K. To overcome this technical problem, the absolute Seebeck coefficient above room temperature has been determined from the Thomson coefficient by using the Kelvin relationship. Thomson coefficients are obtained from measurements of small temperature changes in thin metallic wires under DC (Direct Current) current in the presence of a temperature gradient [3-5]. Using the DC method [3], the most reliable measurements of Thomson coefficient have been performed for thin metallic wires such as Pb, W, and Pt [6-8]. However, the DC method usually requires accurate prior information including thermal conductivity and the geometrical dimensions. In the absence of such information, the Thomson coefficient can only be determined using complicated circuit temperature balancing procedures. This argument points to the need for developing new measurement concepts. The present work presents an improved method for measuring the Thomson coefficient using an AC (Alternating Current) current. Our method requires five measureable quantities including AC cur
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