Impact of tellurium on glass transition and crystallization in the Ge-Se-Te-Bi system

Samples with compositions of Ge20Se67−xTexBi13 (x=0, 5, 10, 15 and 20 mol%) were prepared and characterized by XRD, Raman spectroscopy and DSC. XRD patterns and Raman spectra present consistent evidences of glass transition and crystallization in association with the variation of the structure network due to the substitution of Te for Se. Precipitation of thermoelectric (TE) crystals experiences an evolution from Bi2Se2Te (BST) to Bi2SeTe2 (BTS) phases with the increasing Te content. Based on DSC data performed at five heating rates, crystallization kinetics is analyzed by using Kissinger's relation, Ozawa model and Augis-Bennett approximation. Different methods show the identical evaluations of characteristic activation energies (Ec), while additional parameters of frequency factor (K0) and Avrami exponent (n) are available with AB method. In particular, calculated Ec and K0 demonstrate that with the substitution of Se by Te the crystallization process of the target TE BST crystal phase is much enhanced while that of the undesired GeSe2 phase is slightly hindered.