Characterisation of advanced thermoelectric materials and devices

Abstract

A successful method in relation to increasing the thermoelectric figure of merit is by means of employing the lattice thermal conductivity, seeing as this one of the parameters which is not affected by the electronic structure of the material. It can be accomplished by employing the sol-gel method to synthesise the thermoelectric materials. The advantage of the sol-gel method includes minimisation of the lattice vibration and consequently, lower thermal conductivity. In this thesis, the thermoelectric characterisation of nickel nanoparticles embedded in silica aerogel at two different concentrations has studied and compared with pure silica aerogel. The samples exhibit a significant increase in electrical conductivity, while the sample’s thermal conductivity remained almost unchanged. This promising result has motivated the nanostructuring of Zn4Sb3 and embed it in the silica aerogel using the sol-gel method. Thermal conductivity measurement using scanning thermal microscopy proven to be a technique to evaluate the thermal conductivity at submicron scale. However, this technique is limited by several important parameters in relation to extracting thermal conductivity. In this thesis, thermal conductivity measurements using scanning thermal microscopy were studied and all relevant parameters measured experimentally with the aim of improving the accuracy of the thermal conductivity measurements. Likewise, a method has created that will measure the seebeck coefficient of the material using SThM. The advantage of this technique compared with existing methods are direct measurement, no sample preparation and no need to fabricate seebeck coefficient test structures. In this thesis, the thermoelectric materials are characterised using different techniques to evaluate the performance of the material from 300K to 420K. The measurements system used in the thesis are thermal conductivity measurements using three specific techniques: scanning thermal conductivity, HotDisk analyser and Raman spectroscopy temperature dependence. The electrical resistivity of the materials was studied using the Keithley 4200- SCS four probe station. The material seebeck coefficient was measured using scanning thermal microscopy.