This review presents the synthesis, characterization techniques, processing and potential applications of silicon-based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based polymers provides materials which are denoted as polymer-derived ceramics (PDCs). In particular, this procedure is a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO. There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs route, common preceramic polymers are poly(organosilazanes), poly(organosilylcarbodiimides) and poly(organosiloxanes). One basic advantage of the PDC route is that the materials can be easily shaped in form of fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry. The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approximately 1500°C. Recent investigations have shown that in some cases the high temperature stability in terms of decomposition and/or crystallization can be increased even up to 2000°C if the preceramic polymer contains some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely related to the variation of composition and solid state structure of these materials.

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Several types of Si-based ceramic fibers have been prepared using an organic-to-inorganic conversion process. In this article, the preparation, microstructure, and high-temperature stability of SiC-based ceramic fibers prepared from polycarbosilane of an organosilicon polymer are reviewed. The first fiber produced was an amorphous Si-C-O fiber, next was a low-oxygen content Si-C fiber, and finally, a nearly stoichiometric SiC fiber was developed. These fibers are continuous fine fibers that have high tensile strength with high heat resistance. The pyrolytic behavior and active-passive oxidation of these fibers at high temperatures are described here. The heat resistance of these fibers increases in the order of the Si-C-O fiber, the Si-C fiber, and the SiC fiber. The high-temperature properties are dependent on the atomic arrangement and the quantity of amorphous SiC_xO_y and glassy carbon in the fibers. Possible applications for these fibers are as refractory materials, and as reinforcement fibers for plastics, metals, and ceramics.

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Low-oxygen-content Si-C fibers (Hi-Nicalon) with 0.5 wt% oxygen were prepared from polycarbosilane with electron beam irradiation curing and pyrolysis. The thermal stability of the Hi-Nicalon fibers was significantly improved compared to Si-C-O fiber (Nicalon) with 12 wt% oxygen. However, creep deformation occurred in the Hi-Nicalon fiber at high temperatures, caused by SiC micro crystals and amorphous carbon. Then, stoichiometric and highly crystalline SiC fiber (Hi-Nicalon type S) was prepared from EB irradiation cured fiber by pyrolysis in a hydrogen gas flow. Type S fibers had high tensile modulus, excellent thermal stability, and creep resistance at high temperature. Hi-Nicalon and Hi-Nicalon type S fiber appear to be the best candidates for the reinforcement of ceramic matrix composites.

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Pyrolytic “organic-to-inorganic” conversion of precursors (preceramic materials) into aluminum nitride (AlN) has been investigated for the past few decades. AlN precursors can be classified based on their starting aluminum compounds (organoaluminum compounds, inorganic compounds containing aluminum, and metallic aluminum). The preparation of various AlN precursors and their conversion mechanisms during pyrolysis are reviewed.

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Processes for fabricating ceramic microstructures are in demand for use in areas such as microreactors and MEMS that can be used in harsh environments requiring a high temperatures tolerance, corrosion resistance, as well as tribological properties. This review describes the manufacture of tailored porous SiC ceramic microstructures using sacrificial templates, the feasible applications of microporous structures and microchannels as a microreactor, and microfabrication of preceramic polymers using soft lithography and sterolithography. Macroporous and mesoporous SiC ceramic structures have been fabricated using various templates including packed silica sphere assemblies, porous carbon templates, and nanoporous silica structures. In addition, SiC nanotubes have also been obtained from an alumina membrane used as a template. These porous structures were generally obtained using a series of infiltration, curing and pyrolysis, and chemical or oxidative etching steps. Complicated SiC ceramic microfeatures were fabricated using near-net shape lithographic techniques to preceramic polymers. Line patterns and porous channels were produced by soft lithography, whereas the 3D woodpile structures were fabricated using stereolithography. These novel structures show great promises for use in high temperature micro-reactors for the on-demand reforming of higher hydrocarbons into hydrogen in portable power sources. It is expected that the integration of preceramic polymers into new economic manufacturing strategies such as precursor casting in templates and lithography utilization will be a break-through technology for the creation of complex ceramic nanostructures.

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Polyorganoborosilazanes with a wide range of B/Si/C ratios were synthesized by co-ammonolysis of various organochlorosilanes and borontrichloride. By pyrolysis of SiMe₂Cl₂-based or SiHC₂H₅Cl₂-based polyorganoborosilazane with limited B/Si/C ratios in an inert atmosphere, amorphous borosilicon carbonitrides stable at 1880°C were prepared. The amorphous structure of borosilicon carbonitride was closely related to distributions of B-N bonds, substituent groups on Si atoms and contents of B and C atoms in the preceramic polymers. Three-dimensional BCN networks in the ceramics maintained amorphous structures up to higher temperatures. These prepared ceramics were not only thermally stable, but the oxidation resistance is also excellent at 1700°C in dry air and at 1500°C under reduced pressures.

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Polycarbosilane (PCS), a well known preceramic polymer which is frequently used as an SiC precursor, was synthesized from the thermal decomposition of polydimethylsilane (PDMS) in the presence of various types of zeolites as catalysts. The PDMS with different types and amounts of the zeolite catalysts was heated at 350°C for 6 h, followed by heating at 400°C for an additional 6 h in an autoclave. The use of the zeolite catalysts allowed the PCS to be synthesized under milder conditions and with a higher efficiency than those observed in the Yajima process, by accelerating the thermal decomposition of the PDMS. The PCS product was separated into three fractions, viz. the insoluble fraction, the solid and soluble fraction, and the low molecular weight liquid product. The solid and soluble PCS was characterized by TGA, FT-IR, GPC and NMR analysis, in order to study the effect of the catalysts on the chemical properties of the polymer.

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Cyclic and bicyclic oligosiloxazanes were synthesized by the reaction of 1,3-diamino-1,3-di-t-butoxy-1,3-dimethyldisiloxane with 1,1,3,3-tetraisocyanato-1,3-dimethyl-1,3-disiloxane (M-TIDS) in 1 : 1 and 1 : 2 molar ratios, respectively. Based on these results, polysiloxazane M-PSZ was prepared by the ammonolysis of M-TIDS in tetrahydrofuran (THF) at 213 K. The structure and stability against the gelation of M-PSZ were found to be dependent on the temperature and polarity of the solvents. The molecular weight of M-PSZ was increased by aging a THF solution of M-PSZ at room temperature. A maximum weight average molecular weight of 70000 was observed for M-PSZ, for which a dilute solution was stable over 6 months. Silicon oxynitride was prepared by the pyrolysis of M-PSZ under an ammonia atmosphere. The ceramization process was monitored by infrared and nuclear magnetic resonance spectra, which showed the replacement of a methyl group with an amino group following a condensation reaction to provide silicon oxynitride at 1173 K.

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Si-Al-N-C precursors have been prepared from perhydropolysilazane (PHPS) and cage-type poly(ethyliminoalane) [(HAlNEt)_n; PEIA, n is mainly 8] with different Si/Al molar ratios (Si/Al=3, 1, 1/3). Spectroscopic analysis indicates that essentially no Si-N-Al linkages form, indicating that the precursors are blended polymers. TG analysis reveals that the ceramic yields of the blended precursors up to 900°C are higher than those of PHPS and (HAlNEt)_n, suggesting that cross-linking reactions occur during pyrolysis. Possible reactions are dehydrocoupling and radical reactions, as suggested by the results of solid-state nuclear magnetic resonance (NMR) spectroscopy. The results of solid-state NMR spectroscopy and X-ray diffraction (XRD) analysis indicate that the residue pyrolyzed at 1600°C contains mainly crystalline AlN, 2H-SiC and β-SiC.

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Polytitanasilazanes with Ti/Si ratios ranging from 0.001 to 0.1 were synthesized by chemical modification of commercially available perhydropolysilazane (PHPS) with Ti(N(CH₃)₂)₄. Amorphous Si-Ti-N ceramics were synthesized by pyrolysis of the polytitanasilazanes in Ar. The results of XRD analysis showed that the crystallization of α-Si₃N₄ from the Si-Ti-N materials with the Ti/Si ratios ranging from 0.001 to 0.01 started at 1273 K. This crystallization temperature was 200 K lower than that of the PHPS-derived Ti-free amorphous Si-N material. FT-IR and ²⁹Si MAS NMR spectroscopic analyses revealed that, during the polymer/ceramics conversion process, the dehydrogenation reactions of the PHPS were apparently accelerated by the small amount of Ti-doping using Ti(N(CH₃)₂)₄. This could lead to enhancing the lower temperature formation of SiN₄ tetrahedral units necessary for the crystallization of α-Si₃N₄ at 1273 K.

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Liquid processing to form fine polycarbosilane nanoparticles based on the supersaturation in mixed solvents was investigated, in order to synthesize fine precursor powders for silicon carbide. Polycarbosilane solids were dissolved in n-hexane, followed by two kinds of mixing methods with ethanol. By the method of gradual mixing the polycarbosilane solution into stirred ethanol, fine precipitates were formed. On the other hand, precipitates gathered and formed a sticky lump by the method of gradual mixing ethanol into the polycarbosilane solution. The fine precipitation by the former method is attributed to the drastic reduction in solubility of the polymer in scattered droplets. The precipitates had fine primary particle size of less than 100 nm, although these particles were partially bonded. The results of FT-IR indicated that PCS can be dissolved and precipitated during the processing without change of the molecular structure. The dried precipitates were converted to amorphous Si-C(-O) powders by oxidative cure treatment and pyrolysis at 1000°C.

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A melt-spinnable precursor for SiC based fibers was prepared from blend polymers of polycarbosilane (PCS) and polymethylsilane (PMS). After melt spinning, thermal oxidation curing, and pyrolysis, Si-C-O fibers were obtained. The addition of a small amount of PMS (0.2 or 0.5 mass%) to PCS was effective in increasing the resulting ceramic yield at 1273 K. Fiber decomposition temperature of the obtained Si-C-O fibers shifted to higher temperature (~100 K) by the PMS addition. For high PMS content (20 mass%), however, cross-linking of the blend polymers made the spinning process impossible. Doping of a radical trapping agent in the starting blend polymers was effective in preventing the cross-linking process. Si-C-O fibers with high oxygen content and reduced C/Si ratio were obtained in this case.

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Perhydropolysilazane (PHPS) films ca. 0.2 μm in thickness were deposited by spin-coating on Si(100) and silica glass substrates. The PHPS films obtained were suspended over aqueous ammonia of various concentrations for various periods of time at room temperature. The Si-H and N-H infrared absorption peaks decreased, while the Si-O-Si peaks increased, and the nitrogen content decreased from 33.6 to 0.5 atomic%, suggesting that PHPS-to-silica conversion occurs on the exposure treatment. The refractive index decreased from 1.56 to 1.48, and the contact angle decreased from 94 to 67° on the exposure treatment. The changes in infrared absorption spectra and in refractive index were more significant when the as-deposited PHPS film was suspended over aqueous ammonia of higher concentrations, indicating that the use of aqueous ammonia of higher concentrations for exposure treatment is more effective in PHPS-to-silica conversion. As the PHPS-to-silica conversion proceeded, the durability of the films in hot water increased, evidenced in both series of films subjected to the exposure for various periods of time and those suspended over aqueous ammonia of various concentrations. When the exposed film was fired at 900°C, the nitrogen content further decreased from 0.5 to 0.01 atomic%, the refractive index decreased from 1.48 to 1.45, and the contact angle decreased from 67 to 29°, all of which approached those of pure and dense silica glass. These changes observed on firing suggest that the silica thin films obtained by the exposure treatment at room temperature are not identical to pure and dense silica glass.

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A novel preceramic polymer suitable to form a SiBCN ceramic was synthesized by hydroboration reaction of 1,3,5-trivinyl-1,3,5-trimethyl-cyclotrisilazane with borane dimethylsulphide. The obtained polymer denoted as poly(borosilazane) was characterised by FT-IR and NMR spectroscopy and its thermal stability was studied by thermal gravimetric analysis in combination with in situ mass spectrometry (TG/MS). The polymer-to-ceramic transformation was achieved at 1050°C in inert argon atmosphere yielding black and X-ray amorphous SiBCN ceramics thermally stable up to 1800°C. Using the dip-coating technique, a SiBCN ceramic thin film was formed on a porous alumina substrate. N₂ sorption isotherm analysis revealed that the thin film contained a small amount of micropores of about 0.6 nm in diameter, as well as mesopores between 2.7 and 6 nm in size. The total pore volume was found to be about three orders of magnitude smaller than that of a hydrogen permselective amorphous silica membrane derived from polysilazane. These results indicated potential application of the SiBCN thin film as a molecular sieve membrane suitable for high-temperature separation of small gas molecules like hydrogen below 0.3 nm in size.

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Poly(silylene-p-phenylene)s bearing cyanophenyl groups were prepared and their application to patternable precursors of conducting ceramic films was studied. When polymer films were irradiated in air through a photomask, then heated at high temperature, only the non-irradiated area was converted to conducting ceramic films with the conductivities of 1.1-2.2×10² S/cm, while the irradiated area became an insulator.

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Amorphous silicon carbide (SiC) membranes with molecular sieving properties were successfully synthesized on the outer surface of a γ-alumina (γ-Al₂O₃)-coated α-alumina (α-Al₂O₃) porous support through pyrolysis of polycarbosilane without oxygen-curing process. The amorphous SiC membrane pyrolyzed at 1073 K after dip coating with capillary force possessed the hydrogen (H₂) permeance of 8.1×10^-7 mol•m^-2•s^-1•Pa^-1 and the H₂/carbon dioxide (CO₂) permselectivity of 13 at 873 K. On the other hand, the amorphous SiC membrane pyrolyzed at 1073 K a total of four times after dip coating without capillary force possessed a helium (He) permeance of 2.4×10^-7 mol•m^-2•s^-1•Pa^-1, a He/H₂ permselectivity of 1.9 and a He/CO₂ permselectivity of 19 and at 873 K. Gas permeance properties were depended on coating method, densification of the Si-C network by heat history and film thickness.

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Silicon carbide-based microporous ceramic membranes for hydrogen separation were prepared via pyrolysis of polycarbosilane as a polymeric precursor. To prepare membranes with excellent hydrogen separation performance, effects of hydrosililation cross linking, addition of thermally-labile polymer as well as pyrolysis conditions on the structural evolution during conversion of the polycarbosilane precursor into the silicon carbide-based microporous membranes, have been investigated. The results of the activation energy for possible reactions, change in functional groups, and nitrogen adsorption and gas permeance measurements suggested that the cross-linking reaction between triple bond and Si-H bond gave three dimensional networks, resulting in smaller micropore and larger surface area. The size of the gases evolved during pyrolysis was also suggested to have relationship with the micropore structure.

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A silicon boron imide gel BSi₃(NH)_x(NH₂)_y(NMe₂)_z was prepared by an acid catalyzed ammonolysis of [tris(dimethylamino)silylamino] boron B [HNSi(NMe₂)₃]₃. Pyrolysis of the gel at 1000°C under NH₃ flow leads to the formation of amorphous silicon boron nitride materials. All the gel and pyrolyzed products exhibit a mesoporous structure with a high-surface-area and narrow-pore-size distribution. The effective surface area of the pyrolyzed silicon boron nitride residues decreases with increasing temperature whereas the pore size of the residues increases as the temperature increases. The pyrolysis process was investigated by use of Fourier transform infrared spectra (FTIR), ²⁹Si and ¹¹B solid-state NMR and nitrogen adsorption analyses.

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A refined processing route for producing highly porous, open-cell, microcellular SiC ceramics by the expansion method using expandable microspheres has been demonstrated. The strategy adopted for making microcellular SiC ceramics involved the following steps: (i) fabricating preceramic foams by heating a mixture of polysiloxane, carbon black or phenol resin (used as a carbon source), Al₂O₃-Y₂O₃ (used as a sintering additive), expandable microspheres (used as sacrificial templates), and SiC (an optional inert filler); (ii) cross-linking the polysiloxane in the foamed body; (iii) transforming the polysiloxane and phenol resin by pyrolysis into silicon oxycarbide and carbon, respectively; and (iv) synthesizing SiC by carbothermal reduction. Effects of the carbon source, expandable microsphere content, and inert filler content on microstructure, porosity, and cell density were investigated in this study. By controlling the expandable microsphere and inert filler contents, it was possible to adjust the porosity within a range of 77% to 94%.

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In this study, the synthesis of bulky FSM materials was attempted by a hydrothermal hot-pressing (HHP) method at relatively low temperatures. The obtained FSM bulks had dense microstructures without large voids and extremely high values of specific surface areas of over 1000 m²/g. TEM observations showed that the bulks synthesized by this HHP method possessed a regular hexagonal arrangement of uniform mesopores and no amorphous phase at the interface between FSM grains, leading to bulky FSM bodies with interconnected mesopore-networks. As a result, dense bulky bodies with hexagonal structured mesopores and high values of specific surface areas could be obtained by this HHP method. These bulky FSM bodies are expected to be applicable as catalysts, adsorbents, sensors, electronics materials, and so on.

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Polymethylsilane (PMS) modified with titanium chloride (TiCl₄) or molybdenum chloride (MoCl₅) was pyrolyzed at various temperatures to enable a detailed investigation of stable phases formed during pyrolysis. The existence of TiSi₂ in PMS-TiCl₄ precursor and β-MoSi₂ in the PMS-MoCl₅ precursor below 1273 K suggested a strong interaction between Si-H and the metal chlorides during mixing. During pyrolysis of the PMS-10 mass% TiCl₄ precursor, a silicon rich Si-Ti-C state was obtained below 973 K, which simultaneously formed crystalline Si, TiSi₂, and SiC phases at relatively low temperatures below 1273 K. During pyrolysis of the PMS-MoCl₅ precursors, however, formation of crystalline Si and SiC phases were not accelerated, while the crystalline β-MoSi₂ or α-MoSi₂ phase was formed rapidly.

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The organic-to-inorganic conversion process of cage-type poly(ethyliminoalane), (HAlNEt)_n (n is mainly 8), was investigated by analyzing the gases evolved during pyrolysis. The conversion process can be divided into three temperature ranges: up to 300°C, 300-600°C, and above 600°C. Below 300°C, ethylene and ethylamine were formed probably via Al-N bond cleavage to form nitrogen terminal (≡N:), and polymerization of cages could occur via two possible reactions involving nitrogen terminals. The conversion process between 300 and 600°C involved the evolution of ethylene, ethane and methane, whose formation can be attributed to radical reactions. Ethylamine was also formed in this temperature range. Methane formation was also observed in the temperature range above 600°C, and appears to be ascribable to the rearrangement of the mineralized network.

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A novel sol-gel-derived preceramic polymer, namely poly(organosilyl)carbodiimide, was synthesized for fabrication of thermally stable amorphous Si-C-N membranes. The gelation process as well as the viscosity of the poly(organosilyl)carbodiimide was adjusted for spin coating by reacting a mixture of alkyl and dialkyl chlorosilane monomers with bis(trimethylsilyl)carbodiimide. The polymeric precursor film was successfully fabricated on a porous substrate by solvent-free spin coating, subsequently converted into a mesoporous amorphous Si-C-N thin film by pyrolysis at 1000°C in Ar. SEM observation as well as N₂ sorption isotherm analysis exhibited the formation of meso-pore channels within the amorphous thin layer. These results indicate that the novel poly(organosilyl)carbodiimide developed in this study is suitable for fabricating meso-porous amorphous membranes in the ternary Si-C-N system and with high thermal and chemical stability.

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Mesoporous silicon carbide (SiC) ceramics were produced via the pyrolysis of polycarbosilane (PCS) with and without SiC nano sized particle filler, and their pore characteristics such as size and BET surface area were investigated. The amount of filler and the pyrolysis temperature significantly affected the surface area and pore size. The surface area of PCS reached around 0.5 m²/g by the pyrolysis at 800°C and pore also was found to almost disappear. In contrast, the specimen with filler showed high surface area above 200 m²/g after the pyrolysis at 800°C and the retention of mesopore even after the pyrolysis at 1200°C.

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Consolidation of non-crystalline Si-C-N powders has been tried by hot isostatic pressing without sintering additives, in order to fabricate dense non-oxide ceramic monoliths without the crystallization. A polyvinylsilazane polymer was thermally cross-linked at 250°C and pyrolyzed at 1050°C under Ar atmosphere, which resulted in the formation of the ceramic state with a chemical composition of Si_1.0C_1.6N_1.3 in atomic ratio. The pyrolyzed powders were die-pressed into rectangular bars at room temperature and consolidated by hot isostatic pressing at 1400°C-900 MPa. A ceramic monolith with a level of almost full density was obtained by the consolidation, although a few pores of about 1 μm still remained in the cross-sectional structure. The amorphous phase was significantly maintained in the consolidated sample. In the compression tests of the sample, only slight plastic deformation was observed at 1400°C and 1500°C in spite of high compressive stress over 1600 MPa. On the other hand, the sample showed plastic deformation at 1600°C.

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The oil wettability and sliding property of polymethylsilsesquioxane (PMSQ) based organic inorganic hybrid coating films prepared from methyltriethoxysilane and various metal alkoxide were investigated. The wettability of oil increased in the order of PMSQ<Ta-<Ti-<Al-PMSQ, and was strongly affected by the surface roughness. The friction coefficient showed that Ti-PMSQ among hybrid coating films was the lowest value, which was found to be due to the presence and maintenance of the lubricating oil film during the sliding tests.

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