Effect of locally-gradient Ni@NiTe2 inclusions on the Seebeck coefficient of Bi2Te3 + xNi composites

Abstract

The Bi2Te3 + xNi composites with x = 0.00, 0.50, 0.85, 1.00, 1.25 and 1.50 wt% were prepared by spark plasma sintering method. Owing to high-temperature diffuse redistribution of matrix and filler atoms, filler inclusions in Bi2Te3 matrix are forming as locally-gradient Ni@NiTe2 inclusions. Since electrical conductivity of Ni is much higher as compared to that for Bi2Te3, the composites are electrically-inhomogeneous semiconductors. With increasing x, fraction of NiTe2 shell increases, and fraction of Ni core decreases, although all the composites with different x were sintered at the same temperature. This feature is originated from local overheating of the filler inclusions due to flowing pulse electrical current through the electrically-inhomogeneous medium. Diffusion coefficient of Ni inside Bi2Te3, estimated from analysis of Ni concentration profiles near the Ni@NiTe2 inclusions, enhances with increasing x. By using energy activation of the diffusion, local overheating temperatures of the filler inclusions were extracted. The x-dependent changes in electron concentration of the composites are related to competition of increasing in number of the filler inclusions and decreasing in fraction of Ni cores inside the inclusions. Effect of the filler content on the Seebeck coefficient of the composites was analysed by the Pisarenko relationship, plotted for simple parabolic band model. Magnetic electron scattering by ferromagnetic Ni cores in the Ni@NiTe2 inclusions can contribute to the Seebeck coefficient via relevant changes in scattering factor.