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Suspension- and solution-based freeze casting of polymer-derived ceramics for capillary transportWednesday (07.10.2020) 15:00 - 15:20
The flexibility of freeze casting in terms of pore morphology, pore size and porosity can be used to generate components for capillary transport applications. Preceramic polymers offer the unique feature that they can be processed in suspension- as well as in solution-based freeze casting . Both processes provide distinct characteristics which can be useful in capillary transport applications.
For a water-based process the hydrophobic polysiloxane has to be transformed to hydrophilic preceramic particles at first. For that, a polysiloxane was pre-pyrolyzed to partially decompose the polymer to hydrophilic particles. In a second approach, 3-Aminopropyltriethoxysilane was added to polysiloxanes and introduced hydrophilic aminopropyl groups. After freezing, a hierarchical lamellar micro/meso/macroporous structure is obtained for both approaches. Wicking of these structures revealed a good agreement with a prediction via the Lucas-Washburn equation. While the samples showed promising thermal conductivity for capillary transport at cryogenic conditions, the mechanical strength remains relatively low.
Structures were further improved by using solution-based freeze casting where no pre-treatment of the polysiloxane is necessary. Cyclohexane and tert-butyl alcohol as solvents lead to a dendritic and prismatic pore structure, respectively. Besides altering the surface characteristic, the addition of preceramic and ceramic filler particles significantly improves the mechanical strength. Further adaptions of the pore structure were achieved by changing the freezing conditions: from radial to unidirectional and from constant freezing temperature to constant freezing velocity. This allows to generate a fully aligned pore structures with a constant pore size. Wicking experiments showed that the prismatic structure wicks faster than the dendritic one. Additionally, aligned structures wick considerably faster than clustered structures. Wicking of these complex structures can’t be reliably predicted by the Lucas-Washburn equation.
The comprehensive understanding of the relation between process parameters, pore structure and wicking performance allows for the precise tailoring of components for various capillary transport applications.