Thermal Stability of Lead-Free Transparent Cloisonne Glazes

Glazes of (70 - x) ZnO-30 B2O3-x Bi2O3 with different Bi2O3 contents were prepared by the conventional melt quench technique. Differential scanning calorimetry (DSC) curves were obtained to determine the glass transition temperature (T-g) and crystallization temperature (T-c) of the glazes. The activation energy of the glass transition (E-g) and crystallization (E-c) were calculated using the Moynihan and Kissinger models, respectively. The glass transition temperature (T-g) decreased linearly with increasing Bi2O3 content. This is because the larger Bi3+ ions reduced network connectivity and opened up the structure. The T-g increased gradually with increasing heating rate (& beta;). This is because the higher heating rate provided more energy for the glass to transition to the liquid state. The activation energy of the glass transition (E-g) decreased with increasing Bi2O3 content. This indicates that the glass-forming ability of the system increased with increasing Bi2O3 content. The energy corresponding to the amorphous-to-crystalline transformation during nucleation and crystal growth (E-c) increased with increasing Bi content to about 30%, and then decreased above 40%. This suggests that higher E-c values have an advantage in preventing crystallization in the crystallization danger region. It can be seen that the addition of Bi2O3 in (70 - x) ZnO-30 B2O3-x Bi2O3 glazes affects the density and distribution of oxygen atoms in the glass structure. It can also be seen that the increased Bi content promotes the formation of Bi-O-Bi bonds, which act as network modifiers to reduce the number of non-cross-linked oxygen atoms and increase network connectivity.