Mitigation of anomalous expansion of carbon xerogels and controllability of mean-pore-size by changes in mold geometry

Abstract Distribution of pore sizes in xerogels is a very relevant property of materials when it comes to applied research and commercial implementations requiring precise pore control at the meso–macro scales (10–1000nm radii). Resorcinol–formaldehyde (RF) xerogels with mean pore radii in the elusive intermediate range of few to several hundreds of nanometers are rarely reported. One of the reasons is that controlling pore size with chemical concentrations becomes very sensitive in that range. We report the manipulation of a less sensitive, and controllable variable, mold geometry, as a way to synthesize resorcinol–formaldehyde xerogels with mean pore radii ranging from 300 to 700 nm. Squared molds with a depth-to-side ratio of 1:3.33, for example, led to a final RF substrate with pore radii of 321±48 nm. Similarly, a squared mold with a depth-to-side ratio of 1:3.55 resulted in samples with radii of 704±50. This variation in pore size due to mold geometry was found to be related to the RF skin creation process. Additionally, since thermal expansion characteristics between heating and cooling phases have a noticeable discrepancy in RF materials, thermal expansion properties and mitigation of anomalous coefficients of thermal expansion were also studied in order to avoid fracture of substrates under applications that require dimensional stability under thermal loads.