Development of a non-contact scanning temperature sensing method and characterization of ZT in pnictogen-chalcogen alloys

Abstract

As an environment-friendly energy source, thermoelectric materials have attracted increasing interests. Due to the characteristics of long lasting, stability and no moving parts, they have been widely used in cooling and powering micro-devices. Development of the characterization technique and the understanding of materials are two faces of a coin. In this thesis, we will include both the development of a characterization technique and the characterization and analysis of three series of Pnictogen-Chalcogen based thermoelectric alloyed materials.

There are two general ways of enhancing the thermoelectric property. One route is to increase the power factor and the other way is to reduce the phonon contribution to the thermal conductivity. Bismuth telluride based materials have the highest ZT values around the room temperature. Since bismuth selenide, bismuth telluride and antimony telluride belong to the same space group and have similar chemical properties, it is easy for them to form isomorphous solid solutions. Bismuth telluride and antimony telluride are usually added into bismuth telluride to enhance the thermoelectric property. In this work, we characterized and analyzed the transport properties of two ternary and one quaternary systems of thermoelectric alloys, each of which were fabricated by mixing and sintering two of the bismuth telluride, antimony telluride and bismuth selenide starting compounds. We performed a point defect based analysis of the transport properties of the alloys. Although alloying is typically used as a route to reduce the lattice thermal conductivity, our study of alloys with different compound ratios showed a wide range of electrical property tuning, which provides useful information and the possibility of using point defects as a tool of improving thermoelectric properties. And the systematic study of the alloy effect in the quaternary system was performed for the first time.

The thermoelectric performance is described by the figure of merit zT value which is the product of the z value and the temperature. The three parameters that determine the z value, the Seebeck coefficient, the thermal conductivity and the electrical conductivity are all functions of temperature. And the performance of a thermoelectric micro-device depends on the temperature gradient and its working temperature. Therefore, an in-situ micro-scale temperature characterization method is crucial for the development of thermoelectric materials as well as for evaluating the performance of thermoelectric micro-devices. Scanning thermal microscopy has been proved to be an effective tool for temperature mapping and requires minimal sample preparation. However, quantitative temperature sensing with a thermal probe is still challenging due to the complex heat transfer mechanisms. In this work, we developed a scanning probe technique which is able to perform micro-scale quantitative temperature scanning in the noncontact mode. The heater-on and heater-off effect related to the global heat transfer effect has been observed and discussed. Our study showed that the global heat transfer effect is counted in the thermal contact parameters calibrated with the micro-heater on, by using a one dimensional heat transfer model of the probe. With a two dimensional heat transfer model of the sample, quantitative temperature sensing has been successfully performed on film-on-substrate samples.