The thermal stability and heat capacity changes in the glass transition region of K2O-WO3-TeO2 glasses (glass formation range 20-90 mol% TeO2) have been studied to examine the structural relaxation behavior. The glasses with 60-70 mol% TeO2 and with both K2O and WO3 are thermally stable against crystallization. It is proposed from Raman spectral analyses that TeO4 trigonal bipyramids change to TeO3 trigonal pyramids with the addition of K2O and that Te-O-W bonds are formed in the substitution of WO3 for TeO2. Heat capacity changes of ΔCp = 48-58 J mol-1 K-1 (ΔCp = Cp1 - Cpg, Cpg and Cp1 are the heat capacities of the glasses and supercooled liquids, respectively), and ratios Cp1//Cpg = 1.6-1.8 are obtained for xK2O·xWO3·(100 - 2x)TeO2 glasses. The ΔCp and Cp1/Cpg increase with decreasing TeO2 content, indicating an increase in thermodynamic fragility with decreasing TeO2 content. But, the kinetic fragility estimated from the activation energy for viscous flow is almost constant irrespective of TeO2 content. These behaviors have been analyzed using the configurational entropy model proposed by Adam and Gibbs. The results indicate that in K2O-WO3-TeO2 glasses, Te-O-Te bonds are weak and bond breakings occur easily in the glass transition region, leading to large configurational entropy changes and thus large ΔCp.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Materials Chemistry