Nucleation and growth behavior of glasses in the TeO2–Bi2O3–ZnO glass system

The kinetics of crystallization of glasses in the (90 − x)TeO2–10Bi2O3–xZnO system with x = 15, 17.5, 20 and 25 have been studied using differential thermal analysis (DTA), hot stage XRD and optical microscopy. Thermal properties, activation energy for crystallization, Johnson–Mehl–Avrami exponent, nucleation and growth regimes and rates were determined and shown to depend on glass composition. These variations were related to the domain of glass formation in the TeO2–Bi2O3–ZnO ternary glass system: the glasses with x = 15 and 25 are at the limit of the glass formation domain while the glasses with x = 17.5 and 20 are in the middle of the glass-forming region. From the Johnson–Mehl–Avrami parameter, which increases slightly from 1.7 to 2.2 when x increases from 15 to 25, and from optical micrographs, the predominant crystallization in the investigated glasses is expected to be 2 dimensional bulk crystallization governed by diffusion. Using XRD, we found that Bi2O3 crystals are the first crystals to form in the glasses when heat treated at their respective temperatures of maximum nucleation. When heat treated at a higher temperature the Bi2O3 crystals were found to transform into Bi2Te4O11. We observed an increase in the crystal size when the glasses were heat treated at their respective temperatures of maximum nucleation revealing a probable overlap between the nucleation and growth regimes in all the glasses, the largest overlap being for the glass with x = 15. Thus, this glass is not an appropriate material candidate for applications which require controlled nucleation and growth as a large distribution of crystal sizes can be obtained in the glass when heat treated at its temperature of maximum nucleation rate. The glass with x = 25 is suspected to be also an inappropriate candidate for this kind of applications as it has a higher tendency to fully crystallize in a small temperature range than the other glasses as evidenced by its high Nq and In.