Development of New Solid Solution Zintl Phases for Direct Thermal to Electrical Energy Conversion

Download or read book Development of New Solid Solution Zintl Phases for Direct Thermal to Electrical Energy Conversion written by Nasrin Kazem and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Enhancing the efficiency of the thermoelectric devices is the necessary achievement to guarantee their commercial large-scale application to convert heat to electricity. To compete with the present mechanical energy generators, thermoelectric materials with at least three times more efficiency than today's commercial ones are required. This conversion efficiency corresponds to a thermoelectric figure of merit, zT, of ~ 3, which has been remained elusive for the life time of thermoelectric research. Finding high zT values among Zintl phase compounds such as Yb14MnSb11, EuZn2Sb2, Yb9Mn4.2Sb9, clathrates, and the filled skutterudites in about fifteen years of Zintl studies for thermoelectric applications shows the potential of this family of compounds for discovery of more efficient thermoelectric materials. In this dissertation, two recently discovered family of Zintl antimonide compounds Eu11Cd6Sb12 and Eu9Cd4Sb9 were selected for thermoelectric studies with a focus on their chemistry, crystal structure and transport properties. Zintl phases are a subclass of intermetallic compounds that are traditionally defined by valence precise structures prepared from electropositive and electronegative main group elements. A wide variety of materials with bonding/electronic structure between that of insulators and metals are possible in Zintl phases as a result of the many different choices of elements. The structural and chemical flexibility of Zintl structures enable the engineering of the electronic and chemistry in this family to optimize the structural and electronic properties. In this dissertation, the structural flexibility of Eu11Cd6Sb12 and Eu9Cd4Sb9 Zintl compounds is explored by alloying their crystallographic sites. The resulting solid-solution compounds are extensively studied by conventional and synchrotron X-ray diffraction techniques to carefully characterize their structures in order to relate them to their experimentally observed transport properties. All solid solutions show the "coloring problem" as a result of the non-random distribution of the dopant (solute) in the parent (solvent) crystal structure. In addition, the transport properties are further investigated through the theoretical calculations to relate the bonding/structure to their electronic properties in order to learn the fundamental requirements to designing new materials. All these compounds are p-type materials and their complexity and relatively large unit cells of these classes of compounds result in exceptionally low lattice thermal conductivities (