Freeze Casting of Ceramics: Pore Design from Solidification Principles

Citation

Arai, Noriaki (2021) Freeze Casting of Ceramics: Pore Design from Solidification Principles. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3rmr-cz93. https://resolver.caltech.edu/CaltechTHESIS:11062020-163041829

Abstract

Freeze casting is a porous material processing method which allows the creation of directionally aligned pores by the solidification process. Pores are generated by sublimation of solidified crystals which reject suspending particles or dissolved solutes during freezing. Although freeze-cast ceramics have been identified for applications such as filtration and bioceramics, the lack of understanding of the process often results in a discrepancy between the desired pore structure and the fabricated structures.

Since solidification is the foundation upon which freeze casting is built, this work seeks to understand the solidification process, especially the growth and time evolution of dendrites. To understand the dendritic growth process, two solidification parameters, freezing front velocity and temperature gradient, are independently controlled to investigate the effects of each parameter. Dendritic pore size changes with solidification parameters and shows good agreement with dendrite growth theory. The theory of constitutional supercooling serves as a guide to control pore morphology between dendritic pores and cellular pores. Furthermore, dendrite growth under the effects of the gravitational force is investigated by changing the solidification direction with respect to the gravity direction. Convection changes the degree of constitutional supercooling, and results in different pore sizes as well as pore morphology.

Time evolution of dendrites through isothermal coarsening is investigated. During the coarsening of dendrites, they are transformed to cylinder-like crystals, which yield honeycomb-like structures. Moreover, dendrite size changes linearly with the cube root of coarsening time. Both findings are well-established phenomena in alloy solidification. Further comparison with alloy systems are achieved with tomography-based analysis where similar microstructural evolution with alloy system is demonstrated.

Based upon the understanding of underlying solidification principles in freeze casting, three applications are explored. First, the freeze-cast structure is designed to improve shape-memory properties. Processing variables are controlled such that shape-memory porous zirconia can enable martensitic phase transformations and shape deformation without fracture. Other applications utilize unique pore space. Dendritic pores are investigated for size-based filtration to preferentially capture small particles. Flow-through experiments and in-situ observation by confocal microscopy confirm that pores created by secondary dendrites capture small particles. Finally, honeycomb-like structures are filled with functional microgels to create a ceramic/polymer composite as an application for membrane chromatography. The fabricated composite demonstrates advantages such as mechanical stability during the fluid flow.

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