Thermal stability and heat capacity changes at the glass transition in K 2O–WO 3–TeO 2 glasses

Abstract

The thermal stability and heat capacity changes in the glass transition region of K 2O–WO 3–TeO 2 glasses (glass formation range 20–90 mol% TeO 2) have been studied to examine the structural relaxation behavior. The glasses with 60–70 mol% TeO 2 and with both K 2O and WO 3 are thermally stable against crystallization. It is proposed from Raman spectral analyses that TeO 4 trigonal bipyramids change to TeO 3 trigonal pyramids with the addition of K 2O and that Te–O–W bonds are formed in the substitution of WO 3 for TeO 2. Heat capacity changes of ΔC p =48–58 J mol −1 K −1 ( ΔC p = C pl − C pg , C pg and C pl are the heat capacities of the glasses and supercooled liquids, respectively), and ratios C pl / C pg = 1.6–1.8 are obtained for xK 2O · xWO 3 · (100 − 2 x)TeO 2 glasses. The ΔC p and C pl / C pg increase with decreasing TeO 2 content, indicating an increase in thermodynamic fragility with decreasing TeO 2 content. But, the kinetic fragility estimated from the activation energy for viscous flow is almost constant irrespective of TeO 2 content. These behaviors have been analyzed using the configurational entropy model proposed by Adam and Gibbs. The results indicate that in K 2O–WO 3–TeO 2 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 ΔC p .