Characterization of Thin-film Thermoelectric Micro-modules using Transient Harman ZT Measurement and Near-IR Thermorefle
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1044-U10-01
Characterization of Thin-film Thermoelectric Micro-modules using Transient Harman ZT Measurement and Near IR Thermoreflectance Rajeev Singh1, James Christofferson1, Zhixi Bian1, Joachim Nurnus2, Axel Schubert2, and Ali Shakouri1 1 Electrical Engineering Department, University of California, Santa Cruz, CA, 95064 2 Micropelt GmbH, Freiburg, Germany ABSTRACT We characterize several thin-film thermoelectric micro-modules composed of 20 µm-thick elements and designed for cooling applications to identify factors that may limit device performance. Thermoelectric figure-of-merit measurements using the transient Harman technique are compared with maximum cooling data under no heat load. Correlation between the two measurements depending on the location of the parasitic joule heating in the module is analyzed. Near-infrared thermoreflectance is used to examine temperature non-uniformity in the module. The temperature distribution on the metal contacts due to the Peltier and Joule effects is obtained non-destructively through the silicon substrate of an active module. INTRODUCTION Thermoelectric devices with micrometer-scale element lengths are of great interest for use in cooling applications because of their potentially high cooling power densities on the order of 100 W cm-2. Additionally, microscale thermoelectric devices are useful in applications requiring high frequency cooling due to their fast response time on the order of 10-3 s. As the length of the thermoelectric element decreases into the micrometer regime, electrical and thermal device parasitics begin to significantly decrease the cooling performance of the device. In particular, electrical contact and thermal interface resistances can become a significant percentage of the thermoelectric element values as the element length is decreased. These parasitic electrical and thermal resistances will decrease the coefficient-of-performance (COP) of the thermoelectric device and limit the heat flux through the device. Electrical and thermal device parasitics must be minimized in micrometer-scale thermoelectric device design and fabrication in order to more closely achieve the intrinsic cooling potential of a thermoelectric material. The parasitic of contact resistivity alone can significantly reduce thermoelectric device COP for micrometer-scale element lengths. Fig. 1 below is a plot of thermoelectric device COP versus element length for a n-element Bi2Te3-based thermoelectric module for cold- and hot-side temperatures of 275 K and 300 K, respectively, and for contact resistivities ranging from 10-5 Ω cm2 to 10-8 Ω cm2 (the ideal case of zero contact resistivity is also plotted) [1]. Contact resistivity is the only parasitic considered in the calculation and the n- and p-type material properties are assumed identical other than opposite Seebeck coefficient polarities. It can be seen that contact resistivity can significantly degrade thermoelectric device COP for element lengths in the micrometer regime. In effort to empirically quantify the effect of conta
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