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

摘要

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