The scale of research for developing and applying silicon carbide (SiC) membranes for gas separation has rapidly expanded over the last few decades. The precursor-derived ceramic approaches for preparing SiC membranes include chemical vapor deposition (CVD)/chemical vapor infiltration (CVI) deposition and pyrolysis of polymeric precursor. Generally, SiC membranes formed using the CVD/CVI deposition route have dense structures, making such membranes suitable for small-molecule gas separation. On the contrary, pyrolysis of a polymeric precursor is the most common and promising route for preparing SiC membranes, which includes the steps of precursor selection, coating/shaping, curing for cross-linking, and pyrolysis. Among these steps, the precursor, curing method, and pyrolysis temperature significantly impact the final microstructures and separation performance of membranes. Ref.
Membrane Material | Amorphous Structure | Advantages | Disadvantages |
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Silica (SiO2) | ![]() |
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Carbon molecular sieve (CMS) or carbon membranes |
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Silicon carbide (SiC) | ![]() |
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Shape and Pore Size of Supports | Raw Materials of Layers on Supports | Layer Deposition Method | Top Layer Thickness [μm] | Thermal Treatment | Pore Size (Type: MF, UF) | Applications and Other Remarks | Ref. | |
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Transition Layer | Top Layer | |||||||
Disk, 15 μm | na. | α-SiC powder (10 μm), SiC whisker, methylcellulose (MC) 2, CaO 5, ZrO2 5, mullite 5, TL-56NQ 4, water 1 | Spray coating | 125 | 1150–1250 °C, 2 h in air; then 1350–1500 °C, 4 h in Ar | 2.31 μm (MF) |
Membranes | Precursor | Supports | Deposition Temperature | Ref. | ||||||||
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SiCN | SiH4/C2H2/NH3 | α-Al2O3; disk | 1050 °C in Ar | [60][37] |
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[48][25] | ||||||
Flat tube, 1.8 μm | ||||||||||||
SiCO | na. | SiH2Cl2/C2H2/H2SiC powder (0.55 μm), IPA 1, PVA 3, PEG 3, Darvan-CN 2, water | γ-Al21 | ODip-coating | 12–30 | 3900–1300 °C, 1 h | 75–155 nm (MF) | /α-Al2O3; tube
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[ | 800–900 °C in H2 | [61,62][38][49][26] | |
39 | ] | Flat tube, 34.92 μm | na. | SiC powder (22 µm), B4C 5, PVA 3, TMAOH 2, water | Dip-coating | ~100 | 2200–2250 °C | 9.93 μm (MF) | ||||
SiC | Triisopropylsilane (TPS) | SiC; disk; and tube | 760–800 °C in Ar/He | [59][36] |
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[50][27] | ||||||
Tube, 15 μm | na. | α-SiC powder (0.4 μm and 0.6 μm), Al(NO3)3·9H2O 5, Optapix CS-76 3, polysaccharide dicarbonic acid polymer 3, water 1 | ||||||||||
SiC | Dip-coating | 27.3–29.4 | 1600–1900 °C | 0.35 μm (MF) | Silacyclobutane (SCB) | Ni-γ-Al2O3/α-Al2O3; tube | 515 °C in Ar | [63][40] |
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[51][28] | ||
Flat sheet, 5.6–14.1 μm | na. | SiC powders (0.5 µm and 3 µm), PAA 2, CMC 3, water 1 | Dip-coating | 60 | 1900–2000 °C in vacuum | 0.5 μm (1900 °C); 1.4 μm (2000 °C) (MF) |
Dip-coating | 10–19 | 200 °C, 1 h; 400 °C, 1 h; and then 750 °C, 2 h | <50 nm (UF) |
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[53][30] |
Tube/na. | SiC powder (0.6 µm), acetone; pore diameter =130 nm | PS 7, toluene 1, AHPCS 6, hexane 1 | Slip-casting + dip-coating |
7 | 200 °C, 1 h, 400 °C, 1 h, and then 750 °C, 2 h in Ar; 450 °C, 2 h in air | Nanoporous SiC membranes |
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[54][31] |
SiC | ||||||||
1,3-disilabutane (DSB); TPS | ||||||||
α-Al | 2 | O3; γ-Al2O3/α-Al2O3; tube | TPS: 700–800 °C in He, and annealed at 1000 °C; DSB: 650–750 °C in He |
[64,65][41][42 |
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[52][29] | ||
] | Flat disk, <100 μm | Consisting of several SiC layers; pore diameter <300 nm |
α-SiC powder (0.4 µm), AHPCS 6, hexane 1, hexane/tetradecane 1 |
The fabrication of SiC ceramics by the pyrolysis of a polymeric precursor process is strongly influenced by the chemistry and architecture of the Si-based preceramic precursors, their processing routes, and the parameters used for their pyrolysis. To be suitable for producing SiC ceramic materials, Si-based polymers must meet the following important requirements: (i) the polymers should have a sufficiently high molecular weight to avoid volatilization of components with low molecular weights during subsequent curing and pyrolysis processes; (ii) they should have appropriate rheological characteristics, as well as solubility for the coating/shaping process; (iii) they should have latent reactivity, provided by the presence of specific functional groups (e.g., Si–H, N–H, Si–OH, and Si–CH=CH2), which induce cross-linking during the curing process upon exposure to thermal stimuli, chemical stimuli, or irradiation (e.g., UV, electro-beam, and γ-ray).
The quality of the coating/shaping depends on the deposition techniques and the solution properties of the precursors. Precursor coating methods mainly include dip-coating [31][44], wipe-coating [55][32], casting [54][31], spin-coating [80][45], spray-coating [48][25], and even three-dimensional (3D) printing [81][46]. Dip-coating and wipe-coating are the most frequently used methods for fabricating membranes because of their relative simplicity. An important factor affecting the coating layers is the viscosity (a rheological characteristic) of the coating solutions, which should be controlled carefully by changing the molecular architectures of the precursors, concentration, and solvent species to achieve coating layers with high quality. This is because the viscosity of the coating solutions could affect the thickness and uniformity of the membrane layers.
The coated precursor typically requires to be cured at low temperatures (up to 300–400 °C) prior to pyrolysis [71[47][48],83], which plays an important role in determining the final quality of SiC ceramic materials, including their microstructural properties. The curing process of polymeric precursors for cross-linking converts the thermoplastic polymers into thermosetting polymers via a series of reactions, such as dehydrogenation and oxidation, which prevents the coating layers/shapes from fusing together during pyrolysis [84][49]. Additionally, curing is known to promote high ceramic yields of polymeric precursors because cross-linking prevents the volatilization of precursor components with low molecular weight at high temperatures. To date, several curing techniques have been carried out to produce SiC membranes, such as ultraviolet (UV) radiation, electron beam (EB)/γ-ray irradiation [85[50][51],86], and conventional thermal treatment under an oxidizing or inert atmosphere [73,87][52][53]. These curing techniques induce various condensation (dehydrogenation or demethanization) and addition reactions converting linear polymer networks to 3D polymer networks.
After curing for cross-linking, further thermal treatment at elevated temperatures (usually ≥300 °C) and under an inert atmosphere, i.e., pyrolysis, results in an organic-to-inorganic conversion (from thermoset polymers to amorphous SiC ceramics) [66,73][43][52]. This conversion is caused mainly by radicals, condensation (dehydrogenation and demethanization), and rearrangement reactions, which lead to the cleavage of chemical bonds and the formation of new bonds accompanied by the elimination of organic groups and the release of gases, such as H2, CH4, and C6H6 [55][32]. For most polymeric precursors, the conversion from polymeric precursor to amorphous ceramic is complete at <900 °C, followed by crystallization from the amorphous phase at higher temperatures (>1100 °C) and resulting in phase separation [34,55,71,72,73,82][32][47][52][54][55][56].
During the organic-to-inorganic ceramic transformation, the microstructure of polymeric precursors undergoes dramatic changes with increasing pyrolysis temperature [55][32]. As shown in Figure 76, the microporous properties and pore structure parameters (BET and micropore volume) of the pyrolytic precursor (polytitanocarbosilane, TiPCS) powders were analyzed by N2 adsorption–desorption isotherms [55][32]. The adsorption capacity, micropore volume, and BET surface area increased by firing from 500 °C to 650 °C and then decreased with increasing pyrolysis temperature from 650 °C to 1000 °C. It is worth noting that this is a general conclusion because PCS [72,96][55][57], polydimethylsilane (PMS) [97][58], and AHPCS [73][52] precursors follow the same trends within a similar pyrolysis temperature range of 300–850 °C. These trends suggest that the decomposition of organic groups generates many micropores at moderate temperatures and the micropores are then narrowed gradually as the pyrolysis temperature increases because of densification and rearrangement reactions.
Figure 86. (a) N2 adsorption–desorption isotherms at 77 K, (b) BET surface area and micropore volume (at a relative pressure of P/P0 = 0.01, pore size ≤ 1 nm) of pyrolyzed precursor (TiPCS) powders. Adapted from [55][32] with permission from Elsevier.