Complex growth structures of mullite after electrochemically induced nucleation

Abstract

Glasses with the mol% composition 52.6 SiO₂·18.7 Al₂O₃·14.3 MgO·7.7 TiO₂·4.7 B₂O₃·2.0 CaO were crystallized using electrochemically induced nucleation. During this process, a voltage is applied between a platinum wire and a platinum crucible containing the melt heated to 1330 °C. At the platinum wire, which acts as the cathode, Ti⁴⁺ is reduced to Ti³⁺, leading to a local nucleation of mullite crystals which grow in the glass melt. The square needles of 1–50 μm diameter exhibit complex growth structures including very fine tips surrounded by halos and various stages of needle fragmentation. They are surrounded by an Al/Si-diffusion barrier of up to 7 μm thickness and a dark frame where SiO₂ is enriched. It is proposed that the stepwise diameter increase of the needles is mainly initiated by dendritic growth localized to tips formed at the edges of the square needles. The presented results provide visual examples on how crystals manipulate their environment during growth which in turn affects the growth of the crystals.