Improvement in thermoelectric properties of FeSi2 prepared from mechanically alloyed powder

Mechanical alloying (MA) is a non-equilibrium processing technique in which we put energy into the system. One can visualize the system using part of this energy for alloying, phase change, compound and metastable phase formation. The remaining energy can be thought of as being used in the creation of defects and distortions. The first part is termed as chemical energy and the second part is termed as non-chemical energy. Mechanical alloying is an ideal technique to synthesize metastable phases. Equilibrium phases synthesized by mechanical alloying are found to exhibit improved properties due to the refinement of grain size. The mechanism behind the metastable phase formation and the grain size refinement are discussed. Mechanical alloying technique has been applied for the synthesis of FeSi2 to improve the thermoeletric properties. The evolution of beta-FeSi2 phase with milling time has been studied starting from elemental powder mixture of Fe and Si near the composition FeSi2, using a horizontal ball mill. The short time milled powder consists of Fe and Si. beta-FeSi2 phase was synthesized in the as milled condition after substantially long time milling. beta-phase is produced in samples milled for shorter durations by heat treating in the beta-phase stable temperature range. p-type FeSi2 powder doped with Al and Mn were prepared by MA and the powder was hot press sintered. The sintered specimens showed smaller grain size and lower thermal conductivity than those prepared by ingot metallurgy. The sintered specimen also showed higher figure of merit and hence, higher conversion efficiency.