The alumina – zirconia – silica ternary system (AZS) provides relevant compositions for refractory materials dedicated to glass melting furnaces. The slow cooling of our selected composition mixture results in a multi-phase and polycrystalline material as predicted by thermodynamic calculation. It was observed that, depending on the cooling rate, pure glass could also form despite the high amount of zirconia well known as a typical nucleating agent. Moreover, adding sodium oxide in the raw material as done for industrial electrofused refractory materials causes new solidification routes and formation of different ceramics far from thermodynamics equilibrium. The objective of this study was to understand the impact of cooling rates and composition on the solidification path of the material with a dynamic way that is to say while taking the kinetics in account. To do so in situ methods were used allowing us to follow the formation of out of equilibrium phases.
Two AZS melts with same composition (48.6 wt.% Al2O3, 36 wt.% ZrO2, 14 wt.% SiO2) were prepared without and with sodium oxide (1.4 wt.%) using an aerodynamic levitation set up coupled with laser heating. The use of such containerless synthesis methods allows the monitoring of endogenous crystallization phenomena with pyrometers acting as optical probes. Depending on the cooling rate, the sodium oxide free compositions formed a peraluminous glass and a ceramic, whereas the sodium oxide - containing material formed several kinds of ceramic. The structural changes of the local environments of 27Al were followed in real time by in situ nuclear magnetic resonance (NMR) in the molten liquids during the cooling. In addition, the final materials were characterized with 27Al and 23Na NMR, X-Ray Diffraction, and Scanning Electron Microscopy to achieve complete description of the materials structure. With adapted compositions and innovative instrumentation (High Temperature NMR), this study brought the opportunity to observe the structure of melts before crystallization and thus to understand why the liquid crystallized or not during solidification.