Solution based freeze cast polymer derived ceramics for isothermal wicking - relationship between pore structure and imbibition

Besides conventional applications for porous monoliths such as filtration, open porosity enables capillary transport. For this application, porous polymer-derived ceramic monoliths (SiOC) with different pore morphology and porosity were obtained by solution-based freeze casting. Methylpolysiloxane and (3-aminopropyl)triethoxysilane were used as a precursor and cross-linking agent, respectively. Tert-butyl alcohol and cyclohexane acted as solvents and created prismatic and dendritic pore morphology. Additionally, differences in solid loading and the addition of preceramic filler particles changed the open porosity from 62% to 79%, the mean pore window diameter from 11 µm to 21 µm and the isotropy. The lateral surface of the monoliths is mainly closed due to the use of a Si-coated film as an intermediate layer which prevents nucleation. Within the parameters characterizing the pore structure, open porosity and pore window diameter were found to be most influencing on wicking tested by the Washburn-Sorption method. The permeability was obtained by constant head experiments and from the viscous-dominated part of the wicking curve. Furthermore, predictions of wicking using the Lucas–Washburn equation with gravity effect which bases on the assumption of capillary bundles were conducted. Wicking experiments showed that describing a real porous structure by macroscopic parameters may not be sufficient for structures deviating strongly from the assumption of capillary bundles. The combination of prediction with the knowledge of main influencing factors allows for tailoring the pore structure of SiOC monoliths prepared by solution-based freeze casting for capillary transport applications.