Manipulating Ice Nucleation and Growth During Freeze-Casting

Abstract

Freeze-casting is a method of fabricating porous materials with aligned microstructures. The method employs directional freezing of aqueous slurries to control the growth of ice crystals, which template pores in the resulting material. Such materials have potential use in applications such as energy storage and tissue engineering. However, supercooling of the lower region of the slurry before ice nucleation leads to rapid ice growth, and the material will consequently suffer from poor pore alignment in this disordered 'transitional zone'. Furthermore, the stochastic nature of ice nucleation means that the nucleation temperature, degree of supercooling and therefore the height of the transitional zone can vary between samples. This variation is problematic for commercialisation of the process and is rarely addressed in the literature. The goal of this project was to develop a scalable method of controlling ice nucleation during unidirectional freeze-casting, with the aim of limiting the variation of the transitional zone height in the microstructure of the resulting materials. The strategy for achieving this was to alter the substrate frozen upon, so that ice nucleation became more favourable at higher temperatures. A new 'perforated substrate' method was developed that had a significant impact on the ice nucleation behaviour. This was first demonstrated when freezing water alone, so that the effect on the temperature and location of ice nucleation could be assessed. When freeze-casting a graphene oxide dispersion, the perforated substrate method reduced the temperature range of ice nucleation from 18.8 to 5.2 degrees Celsius, compared to a pristine control substrate. Consequently, the number of samples with a visible transitional zone was lower for the altered substrate in a group of 12 samples. Potential routes for further optimisation of this method are discussed, along with possible applications for which the method would be beneficial.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor), David Lewis (Supervisor) & Suelen Barg (Supervisor)