Kinetics of growth of spinel crystals in a borosilicate glass

Abstract

Three aspects of the kinetics of spinel crystallization in a high-level waste (HLW) glass were studied: (1) the effect of nucleation agents on the number density ( n s ) of spinel crystals, (2) crystallization kinetics in a crushed glass, and (3) crystallization kinetics in a glass preheated at T> T L (liquidus temperature). In glass lacking in nucleation agents, n s was a strong function of temperature. In glasses with noble metals (Rh, Ru, Pd, and Pt), n s increased by up to four orders of magnitude and was nearly independent of temperature. The kinetics of spinel crystallization in crushed glass lacking nucleation agents was dominated by surface crystallization and was described by the Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation with the Avrami exponent n≅0.5. For application to HLW glass melter processing, it was necessary to preheat glass at T> T L to eliminate the impact of temperature history and surface crystallization on crystal nucleation and growth. In the temperature range of glass processing, crystals descend under gravity when they reach a critical size. Below this critical size, crystallization kinetics is described by the KJMA equation and above the critical size by the Hixson–Crowell equation. At low temperatures, at which glass viscosity is high and diffusion is slow, the KJMA equation represents crystal growth from nucleation to equilibrium. As n s increases, the temperature interval of the transition from the KJMA to Hixson–Crowell regime shifts to a higher temperature.