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Chemical Vapor Deposition

Chemical vapor deposition (CVD) can convert carbonaceous volatiles, such as methane, ethane, propane, ethylene, benzene and other hydrocarbons, or their mixture, into CMs in a short time on supports or substrates [30, 31]. A typical CVD is performed by exposing volatile precursors with the occurrence of thermal decomposition on the substrate surface to quickly transform a carbon layer. At the same time, the derived by-products are cleared from the system by purging gas flow through the reaction chamber to avoid any side effects. For this technology, the key operation factors include volatile precursors, deposition temperature, working pressure, mass flow rate, substrate size and precursor concentration in gas flow. [32]. The degree of crystallinity and regularity of CMs is expected to be reduced with increased deposition temperature [30].

In order to enhance the deposition efficiency, plasma or remote inductively coupled plasma are sometimes jointly coupled during the CVD process [33, 34]. High-energy ion bombardment can also enhance the gas separation performance, particularly the permeance of CMs [35].

Ion Irradiation

Ion irradiation can also achieve the thermal degradation of polymeric membranes so as to generate CMs. This technique has the merits of a short preparation period and low energy consumption, but is difficult to manipulate the process. Power and irradiation time are the key factors for determining the degree of pyrolysis and the variation of microstructure and separation performance of resultant CMs. Bombardment with high-energy ions alters the physical, chemical or electrical properties of the precursor surface so as to form CMs. However, improper operation might also destroy the structure and the separation performance due to energetic collision cascades. In the preparation of CMs, the operation factors include ion species (such as krypton, xenon, argon and helium), energy, irradiation time and frequency [36, 37]. By tailoring those process parameters, the thickness, pore size, density, length, shape and permeation property of CMs can be controlled well [38, 39].

In the following sections, some typical examples of CMs for the membrane filtration process for liquid mixtures will be introduced.

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