Probing the mechanical properties and microstructure of WSi2/SixGe1−x multiphase thermoelectric material by nanoindentation, electron and focused ion beam microscopy methods

Abstract

Thermoelectric (TE) materials such as silicon germanium (SiGe) alloys have been traditionally used in radioisotope thermoelectric generators (RTG) NASA applications. Beyond traditional RTG applications, we are exploring other applications in the energy harvesting arena. There is still a need to increment the TE figure of merit (ZT) of SiGe based TE alloys and we have been working on ways to improve it by incorporating tungsten di-silicide (WSi2) phases into the matrix by directional solidification (DS) process. Considerable efforts have been focused until now in microstructural engineering methods that lead to ZT improvement by microstructure optimization of TE materials. Although critical for the previous mentioned applications, work pertinent to the mechanical integrity of this type of WSi2/SiGe based TE materials is lacking. In this work, we explored for the first time the local mechanical properties and microstructure of WSi2/SixGe1−x multiphase thermoelectric material by nanoindentation, scanning electron microscopy (SEM), focused ion beam (FIB) and transmission electron microscopy (TEM) methods. We report hardness (H), modulus (E) and fracture toughness (kc) data for all phases. We obtained average H (and E) values (in GPa) of 12.94 (464.95) for the WSi2 phase, 19.49 (214.52) for the matrix, 14.95 (142.84) for the Si rich phase, and 13.98 (138.56) for the Ge rich phase respectively; while average kc values (in MPam0.5) were 1.37 for theWSi2, 0.52 for the matrix, 0.36 for the Si rich and 0.24 for the Ge rich phases respectively. FIB serial sectioning and cross-sectional TEM analysis is also included which provided insights on the deformation process below the nanoindentation area.