Crystal structure and thermoelectric properties of Al-containing Re silicides

The microstructure, defect structure and thermoelectric properties of Al-containing ReSi1.75 based silicides have been investigated. All the Al-containing alloys investigated contain four differently oriented domains accompanied by the twinned microstructure, as the binary alloy does. However, thin defect layers containing a kind of shear structure are locally and sporadically formed at some of twin boundaries. In the defect layer, shear occurs by the vector of [100] on either ( 109) or (107) planes. Binary ReSi1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 degrees C when measured along [001], although the ZT value along [100] is just moderately high. Al-containing Re silicides considerably increase the ZT value along [100] so that the maximum value of 0.95 is achieved at 150 degrees C for the ReSi1.75Al0.02 alloy. The temperature dependence of electrical resistivity changes from of semiconductor for the binary alloy to of metal for the M-added alloys and the value of electrical resistivity is significantly reduced when compared to the binary counterpart.