Nano-scale effects in bulk nanostructured thermoelectrics

The technique of energy harvesting via thermoelectric (TE) materials is one of the favorable directions towards manifesting sustainable energy resources. The ability of TE materials to directly convert heat energy to electricity facilitates the reduction in consumption of natural resources for power generation. The requirements of high electrical conductivity and Seebeck coefficient while maintaining a low thermal conductivity for attaining higher TE performance introduced newer material processing techniques. Several efficient techniques for nano-scale structural modifications such as alloying, point defects, nanostructuring etc. were implemented for improvement in the figure-of-merit. Quantum confinement techniques based on nanostructuring of compounds gained prominence due to the resulting reduction of the lattice thermal conductivity.;In this dissertation, various aspects of theoretical and experimental techniques pertaining to the nano-scale effects in TE materials were investigated. As a first step, in order to better understand the advantages and disadvantages of nanostructuring, TE characteristics of silicide based materials such as Mg2Si and Si1-xGex were theoretically modeled. A comprehensive comparison of effects of nanostructuring in both the materials was deduced. The fact that nanostructuring may not always be beneficial was highlighted through estimation of phonon mean free path in nanostructured compounds. In the second phase of this dissertation, a novel technique through mixing of a conductive glass-frit for improving the mechanical stability of Mg2Si was successfully developed. The studies were followed up by investigations on the benefits of combinatorial effects of nano-inclusions, nanostructuring and long duration annealing based on Bi2Te 3. In the final phase of this dissertation work, the technique of rapid decrystallization of single crystal silicon by high energy microwaves was introduced and the beneficial effects of rapid decrystallization were experimentally deduced. It was shown that a significant reduction in room temperature thermal conductivity of single crystal silicon could be achieved by means of grain size reduction via microwave energy. The advantages of nanostructuring in thermoelectric materials combined with techniques such as nano-inclusions, long duration annealing and rapid decrystallization have been explored comprehensively in this dissertation work. Such combinatorial techniques could be beneficially used to further enhance the efficiencies of thermoelectric materials.