Structural relaxation of lead and barium-free crystal glasses

Abstract

The structural relaxation of Na₂O–K₂O–CaO–ZrO₂–SiO₂ (NKCZ), Na₂O–K₂O–ZnO–ZrO₂–SiO₂ (NKzZ), Na₂O–CaO–ZnO–ZrO₂–SiO₂ (NCzZ), K₂O–CaO–ZnO–ZrO₂–SiO₂ (KCzZ), and Na₂O–K₂O–CaO–ZnO–ZrO₂–SiO₂ (NKCzZ) glasses were studied by thermomechanical analysis. The structural relaxation was described by the Tool–Narayanaswamy–Mazurin model (TNMa). The relaxation function of Kohlrausch, Williams, and Watts (KWW) was used. The parameters of relaxation model were calculated by nonlinear regression analysis of thermodilatometric curves measured under cyclic time–temperature regime by thermomechanical analyzer under the constant load. The values of the exponent b of the KWW equation, modulus K, limit dynamic viscosity η ₀ of the Mazurin’s expression for relaxation time, and constant B of the Vogel–Fulcher–Tammann viscosity equation were optimized. It was found that TNMa relaxation model very well describes the experimental data. A more detailed analysis of the obtained results showed that the equimolar substitution of SiO₂ by ZrO₂ (i.e., the increase of the ZrO₂ content in the glass) decreases the parameter b, therefore the continuous distribution of the relaxation times spectrum is widening. A wider spectrum of relaxation times was obtained even in the case of substitution of ZnO for CaO and K₂O for Na₂O. Substitution of ZrO₂ for SiO₂ decreases the dynamic viscosity limit η ₀ that corresponds to an activation energy increase of temperature dependence of isostructural viscosity. Increased content of ZrO₂ in the glass caused the increase of the value of the modulus K.