טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBoris Nijikovsky
SubjectDesign of Thermoelectric Properties of Ag-Sb-Te-Based
Compounds for Energy Conversion
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Amouyal Yaron
Full Thesis textFull thesis text - English Version


Abstract

Generating electromotive force (emf) solely via temperature difference was firstly reported in 1821 by T. J. Seebeck (1770-1831), attracting keen interest to the phenomenon. This is called the Seebeck effect and it occurs in metals and more so in semiconductors. The continuous study of underlying physics and the development of novel materials using advanced technology have created the modern field of thermoelectrics. Nowadays applications include spacecraft thermoelectric (TE) modules and power generators for oil pipeline monitoring, while recovery of automotive waste heat and powering implanted medical devices represent emerging implementations.

Application of TE devices in daily life requires improvement of their conversion efficiency, which currently does not exceed about 15 %. The TE figure of merit, zT, is the fundamental parameter grading thermal-to-electrical energy conversion efficiency of TE materials. However, zT includes mutually dependent material properties such as thermal and electrical conductivities, yet it dictates the need for minimizing the former and maximizing the latter for the best material performance. Moreover, these properties are highly sensitive to microstructure. Addressing these demands represent the principal issue of the field and poses a great challenge for the development of TE materials.

In this study we focus on the Ag-Sb-Te (AST) system and investigate the temporal evolution of microstructure of the ternary δ-phase - a semiconducting compound with promising TE properties. This is a prerequisite for the development of high-performance thermoelectric materials. The phase diagram of the system indicates the possibility of controllable precipitation of nanoscale Sb2Te3 particles. Nano-precipitates are expected to reduce lattice thermal conductivity without significantly deteriorating electrical conductivity. Noticeably, however, the AST system involves open questions on fundamental issues. The rates of Sb2Te3 precipitation, as well as, knowledge of the microstructure-property relationships in δ-based materials are currently unavailable. We address these issues in this study.

We observe formation of the metastable Sb8Te3 phase due to aging of the δ-phase matrix at 380°C. The resulting phase morphology comprises elongated lamellas of the Sb8Te3 phase residing with their basal planes parallel to {111} of δ. The Sb8Te3 precipitates nucleate very fast and are coherent with the δ-phase. The precipitates’ thickness grows parabolically upon aging up to 8 h, yielding an interdiffusion coefficient of (3.36 ± 0.64)•10-18 m2s-1. Interestingly, elevating the aging temperature to 400 °C initiates dissolution of Sb8Te3  in favor of formation of the equilibrium Sb2Te3 phase. For both of the heat treatments, the microstructure undergoes coarsening starting from 32 h aging.

The precipitation of both antimony telluride phases profoundly influences the TE properties of AST. The evolution of  Sb2Te3 is found to improve electrical conductivity by a factor of four with respect to the as-quenched state, whereas the formation of  Sb8Te3 yields only a two-fold enhancement. This is, however, accompanied by about 18 % decrease of the Seebeck coefficient. We, eventually, apply the Debye-Callaway model and a general effective medium approach to explain these trends on the basis of microstructure-property relationship. This leads us to the derivation of processing guidelines for optimizing the TE properties of AST-based materials.