Decelerated crystal growth in a soda-lime-silica glass

Isolated Na ₂O⋅2CaO⋅3SiO ₂ (NC2S3) single crystals grew at temperatures above glass transition in a melt with a slight soda excess. The enrichment of Na in the continuous solid solution Na _4+2zCa _4–z[Si ₆O ₁₈] (with 0 ≤ z ≤ 1) upon crystallisation causes a depletion of soda of the growing interfacial diffusion zone. Therefore its viscosity will change. Experimental data and idealized model calculations of the viscosity show the same trend and indicate that the viscosity of the interfacial zone would be up to several orders of magnitude higher than the viscosity of the melt. This increased viscosity at the crystal growth front and the experimentally observed decrease of the crystallisation velocity with crystal size suggest a linear increase of the effective activation enthalpy of the diffusional transport through the diffusion zone with crystal size, which allows to quantitatively model crystal growth data of the NC2S3 glass investigated.