Solubility Limit and Annealing Effects on the Microstructure & Thermoelectricproperties of Fe 2V 1-XTa xAl 1-YSi y Heusler Compounds

Abstract

Full-Heusler compounds with the composition Fe₂V_1-XTa_xAl_1-YSi_y have recently shown to exhibit some of thehighest power factors reported so far for bulk materials due to the band convergence and band gap opening causedby the V/Ta substitution. Therefore, the solubility limit of Ta and Si regarding the stability of the L2₁ phase isinvestigated in this study. The crystal structure and microstructure of a large number of samples is probed by X-raydiffraction as well as scanning electron microscopy and energy dispersive X-ray analysis. The results show that the Al/Si substitution significantly hampers the solubility of Ta within the Heusler structure. Furthermore, Fe₂V_0.9Ta_0.1Al and Fe₂V_0.95Ta_0.05Al_0.9Si_0.1 reveal nanoscale impurity precipitates in the microstructure, together with diffuse contrasts that indicate a non-equilibrium metastable state. For that reason, different annealing conditions, varying the temperature and time, have been applied to the latter and the effect on the microstructure and thermoelectric properties is investigated. It is found that additional annealing leads to further phase segregation and grain growth of the impurity precipitates, which due to their metallic-like nature have a detrimental effect on the Seebeck coefficient. They can however effectively reduce the lattice thermal conductivity if the average grain size remains below the phonon mean free path. The thermoelectric efficiency in terms of the dimensionless gure of merit ZT is increased up to ZT = 0.3 - 0.34 at 300K which is beyond the values previously reported for Fe2VAl-based bulk materials.