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A7: Developments in measurement techniques and preparation methods
Development of thermoelectric materials is being vigorously carried out throughout the world now, utilizing state-of-the-art nanotechnology and nanomaterials [1]. Recent notable advancements have been made for example by applying nanostructuring and spark plasma sintering (SPS) to lead to selective scattering of phonons. We will present several novel routes to control thermal conductivity which can lead to enhancement of ZT. First, in contrast to the ball-milling method, we used a quick, inexpensive, bottom-up wet process to fabricate nanosheets of telluride thermoelectric materials; CuTe and recently also Bi2Te3. These nanosheets are a few nanometers in thickness and their consolidated bulk exhibits highly reduced thermal conductivity while maintaining electrical conductivity and hence leading to enhancement of ZT [2]. Secondly, ubiquitous building (or “tiling”) defects have previously been discovered in layered AlB2-type borides. From counterintuitive crystal growth experiments it was receently found to be possible to control the defect formation [3]. Time-domain thermoreflectance (TDTR) measurements on microcrystals revealed that the building defects of the tiling arrangement in α-TmAlB4 significantly lower the thermal conductivity by 30% [4]. Coupled with the ubiquitous nature of these building defects in layered compounds, and the recent synthesis method of control of defect formation, this is shown to be another interesting method to control thermal conductivity. A newly developed method at NIMS to evaluate thermal conductivity on the nanoscale will also be presented.
[1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, Springer Series in Materials Science (Springer, Heidelberg, 2013).
[2] C. Nethravathi et al., J. Mat. Chem. A 2, 985 (2014).
[3] T. Mori et al., J. Appl. Phys., 111, 07E127 (2012).
[4] X. J. Wang, et al., submitted to APL Materials.