Thermoelectric Properties of ReSi_(1.75) Based Silicides ―Property Improvements Through Defect Engineering

摘要

(1) The crystal structure of ReSi_(1.75) is determined to be monoclinic with the space group Cm (mc44) due to the ordered arrangement of vacancies on Si sites in the underlying C11_b lattice. The crystal contains four differently oriented domains; two domains related with each other by the 90°-rotation about the c-axis of the underlying C11_b lattice and twin domains for each of the two domains. (2) Upon alloying ReSi_(1.75) with Mo, an incommensurate microstructure, in which two block layers with different chemical compositions and vacancy concentrations stack randomly along in the parent C11_b lattice, appears. (3) Upon alloying ReSi_(1.75) with Ru, a shear-type microstructure, in which two neighboring domains are sheared (shifted) with each other on (101) by the vector of 1/8 in order to increase the Si vacancy concentration, appears. (4) The thermoelectric properties of ReSi_(1.75) are highly anisotropic. The value of electrical resistivity is higher for along than for. Its electrical conduction is of n-type when measure along while it is of p-type when measured along. Such anisotropy may come from anisotropy in the electronic structure due to its rather complicated crystal structure as well as from a complex microstructure. (5) Although the value of Seebeck coefficient along is moderately high (150-200 μV/K), it is very high along (250-300 μV/K). The value of thermal conductivity is rather isotropic and reasonably small. As a result, a very high value of dimensionless figure of merit (ZT) of 0.7 is achieved at 1073 K when measured along. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition.