Thermodynamic Calculations in New Thermoelectric Materials. Application to Processes
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THERMODYNAMIC CALCULATIONS IN NEW THERMOELECTRIC MATERIALS. APPLICATION TO PROCESSES J.C. TEDENAC, M.C. RECORD, V. IZARD Laboratoire de Physico-Chimie de la Matière Condensée - UMR - CNRS 5617 Université de Montpellier 2 -Sciences et Techniques du Languedoc CC003, Pl. E. Bataillon, 34095 Montpellier, Cedex 5, France Abstract The processing of electronic materials and devices is an important field of application of thermodynamics. Many growth processes of semi-conductors involve vapour-solid or liquid-solid interfaces which can be considered close to the thermodynamic equilibrium. Performance enhancement of thermoelectric modules can be obtained by a good knowledge of the constitution of the materials involved in their fabrication. Its requires a relevant thermodynamic analysis of the material.Making these optimized thermoelectric materials require at least three elements being thermoelectric materials typically multicomponent systems which are usually studied by a CALPHAD analysis of the relevant systems. All these problems are related to the thermodynamic quantities of species involved in the materials such as activities and free energy functions of electrons, holes and components. In this paper we present a contribution to a better understanding of antimony based thermoelectric materials by a CALPHAD approach. INTRODUCTION The improvement of the figure of merit in thermoelectric materials depends primarly on factors related to their microstructural state: crystal structure, microstructure (size and shape of grains), grain boundaries. This structural state can be approach by a good knowledge of the phase diagrams [1]. Tailoring and making optimized materials is a problem relevant of studies of multicomponent systems. Due to that, it is very difficult to check their microstructure. The processes involved during the fabrication of the T.E. component lead to some other problems related mainly to a non equilibrium state. The goal of this contribution is to present the application of thermodynamic calculations based on the CALPHAD method in the field of thermoelectric materials which can help in solving these problems. The CALPHAD approach uses Gibbs energy descriptions in order to calculate phase equilibria as an aid to optimize performance, design and tailor materials [2]. The CALPHAD method has been widely developed for metallic systems, for oxides systems and others, and in the past 10 years applied to some semi-conductors systems as, for example, lead telluride systems [3]. In this last case, it is very important to have the point defects built into the model of the phases in order to map these defects as a function of composition. Due to its difficulty, this characteristic, important for physical applications, was not taken into account in previous assessment of semi-conductor systems. Recently, the defects modelling in semi-conductors has been G1.3.1
studied for some systems such as the Cd-Te binary [4]. These concepts can be applied now to antimony based thermoelectric materials. MODELLING PHASE EQUILIBRIA The
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