メカニカルアロイング法で試作した微細結晶粒 BiTe 系材料の熱電特性とキャリアの散乱因子との影響

Abstract

Fine grained samples with a nominal composition of Bi0.5Sb1.5Tex (x=3.0 to 3.4) were prepared by mechanical alloying (MA). Average grain sizes were found to range from 1.0 to 2.1 μm. Tellurium precipitation at x=3.1 was observed by scanning electron microscopy (SEM), X-ray diffraction and differential thermal analysis (DTA). Electrical conductivity as well as Hall and Seebeck coefficients were measured from 300~470 K. Using the relationship between Hall mobility and temperature, carrier scattering factors of the Bi0.5Sb1.5Tex samples were estimated to range from r=−0.9 to −2.0. The thermal conductivities of the samples were found to range from 0.94 to 1.00 Wm−1 K−1 at room temperature. Excess tellurium was recognized to be responsible for the decrease in thermal conductivity. The Lorenz number and the carrier component of thermal conductivity were calculated from the scattering factor and the Fermi integral. The phonon component of thermal conductivity was dominant in the Bi0.5Sb1.5Tex samples. The Seebeck coefficient of the Bi0.5Sb1.5Tex samples was higher than that from the available data which means that the absolute scattering factor of the Bi0.5Sb1.5Tex samples was large. A maximum figure of merit of 7.02×10−3 K−1 was obtained at 310 K for the Bi0.5Sb1.5Te3.1 sample. This high figure of merit results from an enhanced Seebeck coefficient that is due to an increased absolute scattering factor. High thermoelectric performance can, therefore, be established by a reduction in the phonon component of the thermal conductivity and also by an enhanced Seebeck coefficient, which results from an increased absolute scattering factor for finely grained Bi0.5Sb1.5Tex samples that were prepared by MA.