Coarsening of dendrites in solution-based freeze-cast ceramic systems

Abstract

The morphologies of freeze-cast materials are typically controlled and tuned by adjusting the freezing front velocity and temperature gradient. Recently it has been demonstrated that coarsening, similar to that commonly practiced in alloy systems, is also effective for morphological control in freeze-cast materials. However, the underlying coarsening mechanisms and their effect on microstructure evolution are largely unknown. In this study, frozen preceramic polymer/cyclohexane solutions were coarsened at 2 °C and 4 °C for up to 5 h, and the resulting morphologies were characterized using scanning electron microscopy, mercury intrusion porosimetry, and X-ray computed tomography. During coarsening the microstructure evolved from dendritic (primary and secondary pores) to honeycomb-like (large open channels with flat walls). The size of both primary and secondary pores increased linearly with the cube root of coarsening time, consistent with dendritic coarsening in alloy systems. Other important metrics such as primary dendrite spacing, dendrite growth directionality, and the effect of coarsening on the pore-ceramic interface area are reported. These findings provide novel insights into coarsening of freeze-cast systems and can lead to new avenues for microstructure tailorability.