| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 87 === Abstract
Silicon carbide is a ceramic material of high covalency. It possesses attractive characteristics, such as light, high stability, high mechanical strength, corrosion resistance, and high melting point. The inorganic membrane is targeted on the applications under hostile environment, which the polymeric membrane is not accessible, for instance, high temperature. The inorganic membrane for gas separation is supposed to be sufficiently stable under high temperature and resistant to acid and base. It permeation properties should be enduring for reasonable working time. Therefore, silicon carbide is an ideal candidate for synthesis of the inorganic membrane.
The modification of chemical vapor deposition is carried out in the SiH4/C2H2/Ar reaction system. The modification, by depositing solid in fine pores, often needs to compromise between the permeation rate and selectivity. Reduction in pore size enhances the selectivity of membrane, yet the permeance is sacrificed for pore narrowing. If two porous supports of similar permeance are placed in two environments of different volume/surface V/S ratios, the one in the environment of small V/S suffers less reduction in permeance than the one modified in large V/S for the same pore narrowing effect. Therefore, a better modification is achieved in a chamber of designed V/S ratio. The modification effect of chemical vapor deposition can be predicted by the mathematical model, which calculates the shifting of pore size distribution of support from the experimental gas-phase and surface reaction constants, subsequently the reduction in pore size and the permeance. The flow-average pore radius of one modified specimen 08hd11, is reduced from 297 to 14 nm for a price of 93% reduction in permeance.
The modification of preceramic polymer is carried out on the home-made silicon carbide porous support. The asymmetric support is made by slip casting and sintering of a ceramic tube, then using submicron powder to carry out another casting and sintering to form an inner layer of finer pores. The basic idea behind the preceramic polymer modification is from the fabrication of carbon molecular sieve. Elements of hydrogen and carbon, in form of small molecules, escape from the backbone of polymer, and channel through the polymer matrix. Channeling of these molecular debris creates pores of molecular sizes. These minute pores (< 1nm) are crucial to the gas separation mechanism.
The polydimethylsilane (PMS) coating undergoes thermolytic reaction, which converts part of PMS into polycarbosilane. The subsequent oxygen curing further crosslinks the preceramics. Finally the preceramic is pyrolyzed into a membrane capable of gas separation. The pyrolysis temperatures has a significant influence on membrane permeance and permselectivity. For the membrane pyrolyzed at 573 K, H2 permeance of is 8.9×10-8mol/Pa sec m2 and permselectivity of H2/N2 is 100 at permeation temperature of 473 K. For the membrane pyrolyzed at 873 K, H2 permeance is 4.9×10-9 mol/Pa sec m2, and H2/N2 selectivity is 40 at permeation temperature of 473 K. Thermal treament of preceramics are analyzed, using TGA and DTA. The pore size distribution of the pyrolyzed preceramic is measured by BET.
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