| Description |
A polytype of wollastonite, 2M-wollastonite, that breaks into acicular particles under external forces is being synthesized through two different processes, namely a Partial Melting and Recrystallization Process (PMR Process) and a Flux Growth Process (FG Process). The PMR Process combines the advantages of the solid state reaction and liquid phase reaction methods by creating a partially melted phase to maintain the original shape of the compacts of raw materials while melted regions form locally within the compacts, providing a favorable environment for the growth of alpha-wollastonite in the first step. The target crystal, 2M-wollastonite, nucleates and grows during the subsequent recrystallization stage. Using this process, effects of additives on the preparation of acicular wollastonite particles were investigated. It was found that B2O3 is one of the melting point depressing additives that can lower the melting point of the raw mixtures significantly in a small amount. Li2O is one of the catalytic additives that promote the formation of acicular wollastonite particles. The FG Process completely melt the raw mixtures in a suitable crucible at temperatures above 1400oC, and then lets the melt solidify under a favorable cooling rate to allow the nucleation and growth of 2M-wollastotnite during this stage. It takes advantage of the heat transfer properties of the melt-crucible-furnace wall (MCF) system; a vertical temperature gradient is achieved to provide sites for the preferable nucleation of 2M-wollatonite crystals at the top melt surface. The solidified products contain nearly pure 2M-wollastonite crystals as the major component and an amorphous glass phase as the minor part. Both the PMR and FG processes yielded high-aspect-ratio (HAR) particles. The crystals grown by the PMR process were small in size but large in number, and they grew randomly in the final products. The crystals grown by the FG process were aligned and they formed cellular and dendritic patterns. Such a growth behavior offers additional benefits for producing HAR wollastonite particles. When the two processes were compared, the FG process surpasses the PMR process in terms of efficiency. Modeling and simulation work was done on the FG process that presented cellular and dendritic growth. Based on experimental studies, an empirical model was proposed that modified the existing models on predicting cellular and dendritic growth on a SiO2-CaO-B2O3-Li2O ceramic alloy under high temperature unsteady-state heat transfer. Using this new model, the growth rate and primary arm spacing were predicted well compared with experimental observations. Besides, the average growth front, solidification volume fraction, and mean aspect ratio were also simulated. The simulation work helped to understand and predict the growth of wollastonite crystals in a significant way. |
