Study of plastic forming in production of thermoelectric bismuth telluride based material
An experimental and theoretical study of the process of the equal - channel angular pressing (ECAP) was performed to obtain a thermoelectric (TE) material based on bismuth telluride. A brief review of the mathematical modeling of the ECAP process is given. The influence of the ECAP design features and temperature modes on the process of plastic forming is considered. The results of calculations of the thermally stressed state of samples at different stages of the ECAP process are presented. The calculations of the ECAP process were carried out by means of Lagrangian finite element mesh, which adjusted adaptively during the process to the die geometry and became finer or coarser depending on the magnitude of the plastic deformation. It was required for the specified calculation accuracy and the convergence of iterative process. The results of an experimental study of the structure and properties of samples obtained by the ECAP using a set of measuring methods (X-ray diffractometry and electron microscopy) are discussed. The thermoelectric characteristics of the obtained materials were measured by Harman method. Comparative methodical calculations of the ECAP process for TE materials based on bismuth telluride have been made by adjusting parameters determining the grain formation (i.e. the critical plastic deformation as a function of temperature and power-law dependence of its rates). It made possible to adjust the ECAP model on the basis of the measured grain sizes for TE materials. The calculation results of grain creation during the plastic forming, which are compared with the measurement data, are presented. The practical result of this research was the improved geometry of the die and the validated technological regimes of plastic deformation, which allowed obtaining samples with the good TE efficiency.