(514a) Fabrication of Silicon-Based Ceramic Membranes Via Pyrolysis of Plasma Deposited Polymer Films

The fabrication of nanoporous asymmetric inorganic membranes is vital to the further development of the clean energy industry, particularly in hydrogen production. Catalytic steam reforming, the principal method of producing hydrogen commercially, is performed at high temperatures, elevated pressures, and corrosive environments, and therefore the membranes utilized must be able to withstand these extreme conditions. Silicon-based membranes, such as those made from silicon carbide (SiC) and silicon oxycarbide (SiOC), have proven to be viable options due to their ability to remain stable in high temperature/pressure and corrosive environments, and also exhibit large separation factors towards hydrogen.

These membranes are commonly prepared by the deposition of pre-ceramic polymer films on underlying supports and their subsequent pyrolysis in controlled atmospheres. Solvent-based techniques for applying such silicon-based precursor polymer films face potential challenges with substrate compatibility and film uniformity, and therefore membrane manufacturing processes must adjust to account for these problems. In this work, we use plasma-enhanced chemical vapor deposition (PECVD) to deposit silicon-based polymers on macroporous SiC supports and study their subsequent pyrolysis. In our previous work, we used initiated chemical vapor deposition (iCVD) to deposit a cross-linked polysiloxane thin film onto a SiC macroporous support, which upon pyrolysis, resulted in a silica microporous membrane capable of sieving hydrogen from bigger molecules such as argon¹. The advantage of the PECVD process is that similarly to iCVD it is a solventless process, but unlike iCVD it avoids the use of initiators, which can potentially affect the final chemical composition of the deposited polymer. In this work, we use vinyl phenyl dimethyl silane (VPDMS) as the monomer and divinyl benzene (DVB) as the cross-linker. VPDMS was chosen due to its physical properties, in particular its vapor pressure, which makes it amenable to deposition in the PECVD chamber. In addition, the structure of VPDMS makes p(VPDMS) pre-disposed to forming SiC-type materials post-pyrolysis. DVB was chosen as the cross-linker because it does not contain oxygen in its structure. The deposited pre-ceramic polymer film of p(VPDMS-co-DVB) was pyrolyzed in an inert environment at temperatures above 700 °C. Its initial chemical composition and that of the film at various times during pyrolysis and post-pyrolysis were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) to understand the processes that take place leading from the original polymer to the final inorganic membrane film. Finally, the permeance and separation factor of the final asymmetric nanoporous membranes were measured.

[1] Nguyen, B., Dabir, S., Tsotsis, T., Gupta, M. Fabrication of Hydrogen-Selective Silica Membranes via Pyrolysis of Vapor Deposited Polymer Films,” Industrial & Chemical Engineering Research, 58, pp. 15190-15198 (2019).