Carbon content and pyrolysis atmosphere effects on phase development in SiOC systems

In this study, bulk silicon oxycarbides (SiOCs) were fabricated from base polysiloxane (PSO) systems with different carbon content by using Ar or Ar + H₂O pyrolysis atmosphere. Compared to the Ar pyrolysis condition, the SiOC samples pyrolyzed with water vapor plus Ar generally show lower ceramic yield except for the Tospearl (polymethylsilsesquioxane) sample at 1400 °C. The SiOC ceramics contain significantly less SiC and carbon after pyrolysis under Ar + H₂O atmosphere compared to pure Ar atmosphere. The carbon-poor Tospearl sample shows a crystalline SiO₂ structure (cristobalite) after pyrolysis at 1400 °C in Ar + H₂O, which is also confirmed using TEM diffraction pattern analysis. TEM microstructures indicate little change in microstructures for the carbon-rich samples. The fundamentals, such as total Gibbs free energy, the driving force for crystallization, and phase contents at different pyrolysis temperatures can be calculated based on a Gibbs free energy minimization method. The phase content calculations predict considerable decrease in the amounts of SiC and C and significant increase in the percent of SiO₂ after pyrolysis in Ar + H₂O compared to Ar. The thermodynamic calculation results match with our experimental observations. This work provides a guided method to synthesize high temperature SiOCs with desired phases.     

Synthesis of Silicon Carbide Nanocrystals Using Waste Poly(vinyl butyral) Sheet

SiC nanocrystals were prepared using waste poly(vinyl butyral) sheet as a carbon source. SiO₂/poly(vinyl butyral) mixtures are converted to SiO₂/pyrolytic carbon composites via pyrolysis at low temperatures (500°C) in an Ar atmosphere. Subsequently, low‐temperature magnesiothermic reduction and purification processes result in the formation of tiny SiC nanocrystals. The size of the synthesized SiC nanocrystals ranged from 3 to 12 nm, i.e., they are smaller than the SiO₂ precursor offering large specific surface area of 175.76 m²/g and are single phase as 3C–SiC. Hence, 3C–SiC nanocrystals were successfully synthesized using waste poly(vinyl butyral) through this simple, inexpensive, and scalable process, which will be a new application in the recycling industry.     

On the pyrolysis of a methyl-silsesquioxane in reactive CO2 atmosphere: A TG/MS and FT-IR study

Polymer-derived SiOC-C composites are typically obtained through pyrolysis of a polysiloxane precursor in inert atmosphere. Recent studies have shown that novel SiOC microstructures and compositions can be obtained when the pyrolysis is carried out in a reactive environment, as CO₂, which leads to a selective oxidation of the Si─C bonds leaving a microstructure constituted by a nano-dispersed sp² carbon phase within an SiO₂ matrix. However, little is known about the reaction mechanisms between CO₂ and the preceramic polymer to date. In this work, we investigated the pyrolysis of a methyl-silsesquioxane in reactive (CO₂) and inert (Ar or He) atmosphere by combining TG/MS and FT-IR analysis. The results showed that CO₂ starts to react with the preceramic polymer from ≈750°C when the Si─CH₃ groups start to form Si─CH_x-Si units. The reaction breaks the Si─C bond increasing the amount of the free carbon phase and releasing water vapor, detected by MS, even at temperatures exceeding 900°C. At higher temperatures (≈950°C), CO₂ reacts with the free carbon phase leading to a weight loss and the formation of CO.     

Chemical Processes During Energy‐Saving Preparation of Lightweight Ceramics

Lightweight glass‐ceramic material similar to foam glass was obtained at 700°C–800°C directly from alkali‐activated silica clay and zeolitized tuff without preliminary glass preparation. It was characterized by low bulk density of 100–250 kg/m³ and high pore size homogeneity. Chemical processes occurring in alkali‐activated silica clay and zeolitized tuff were studied using X‐ray diffraction, thermal gravimetry, IR‐spectroscopy, and scanning electron microscopy. Pore formation in both compositions is caused by dehydration of hydrated sodium polysilicates (Na₂O·mSiO₂·nH₂O), formed during alkali activation. Additional pore‐forming gas source in alkali‐activated zeolitized tuff is trona, Na₃(CO₃)(HCO₃)·2H₂O, formed during interaction between unbound NaOH and CO₂ and H₂O from air. Influence of mechanical activation of raw materials on chemical processes occurring in alkaline compositions was also studied.     

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO₂ filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO₂ fillers and SiO₂‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO₂ filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m²/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.     

Highly Porous SiOC Bulk Ceramics with Water Vapor Assisted Pyrolysis

Micro/mesoporous SiOC bulk ceramics with the highest surface area and the narrowest pore size distribution were prepared by water‐assisted pyrolysis of polysiloxane in argon atmosphere at controlled temperatures (1100°C–1400°C) followed by etching in hydrofluoric acid (HF) solution. Their pyrolysis behaviors, phase compositions, and microstructures were investigated by DSC, FTIR, XRD, and BET. The Si–O–Si bonds, SiO₂‐rich clusters, and SiO₂ nanocrystals in the pyrolyzed products act as pore‐forming species and could be etched away by HF. Water injection time and pyrolysis temperature have important effects on phase compositions and microstructures of the porous SiOC bulk ceramics, which have a maximum‐specific surface area of 2391.60 m²/g and an average pore size of 2.87 nm. The porous SiOC ceramics consist of free carbon phase, silicon carbide, and silicon oxycarbide.     

Structural Design of Polymer‐Derived SiOC Ceramic Aerogels for High‐Rate Li Ion Storage Applications

SiOC ceramic aerogels with different porosity, pore size, and specific surface area have been synthesized through the polymer‐derived ceramic route by modifying the synthesis parameters and the pyrolysis steps. Preceramic aerogels are prepared by cross‐linking a linear polysiloxane with divinylbenzene (DVB) via hydrosilylation reaction in the presence of a Pt catalyst under highly diluted conditions. Acetone and cyclohexane are used as solvent in our study. Wet gels are subsequently supercritically dried with CO₂ to get the final preceramic aerogels. The SiOC ceramic aerogels are obtained after a pyrolysis treatment at 900°C in two different atmospheres: pure Ar and H₂ (3%)/Ar mixtures. The nature of the solvent has a profound influence of the aerogel microstructure in terms of porosity, pore size, and specific surface area. Synthesized SiOC ceramic aerogels have similar chemical compositions irrespective of processing conditions with ~40 wt% of free carbon distributed within remaining mixed SiOC matrix. The BET surface areas range from 215 m²/g for acetone samples to 80 m²/g for samples derived from cyclohexane solvent. The electrochemical characterization reveals a high specific reversible capacity of more than 900 mAh/g at a charging rate of C (360 mA/g) along with a good cycling stability. Samples pyrolyzed in H₂/Ar atmosphere show a high reversible capacity of 200 mAh/g even at a high charging/discharging rate of 20 C. Initial capacities were recovered after whole cycling procedure indicating their structural stabilities resisting any kind of exfoliations.     

Influence of H2/NH3 mixtures on the composition of SiCNYO and SiCNAl(O) nanopowders

We performed pyrolysis of SiCNAlH and SiCNYOH nanopowder precursors under a reactive atmosphere (Ar/NH₃/H₂) with various compositions of ammonia (NH₃) and dihydrogen (H₂) to diminish C content, which is deleterious for thermal stability and sintering of the powders. This paper continues a previous work on the fabrication of an Si₃N₄/SiC composite without free C by studying the effect of H₂ on the C/N atomic ratio of the powder. We studied the influence of the nature of the gaseous mixture (Ar/NH₃/H₂) on the powder composition. Elemental analysis showed that the introduction of H₂ in the pyrolysis atmosphere limited the decomposition of NH₃ and allowed for control of the C/N ratio. This behaviour can be explained by the structural evolution observed by ²⁹Si NMR spectrometry but also by Fourier transform infrared and Raman spectroscopy. An Si₃N₄/SiC composite, with traces of free C, was obtained after post-pyrolysis heat treatment of the powders synthesized with 10 wt.% of H₂ and 25 wt.% NH₃.     

Mechanical Properties and Microstructural Characterization of Amorphous SiCxNy Thin Films After Annealing Beyond 1100°C

The effect of thermal annealing on structure and mechanical properties of amorphous SiC_xN_y (y ≥ 0) thin films was investigated up to 1500°C in air and Ar. The SiC_xN_y films (2.2–3.4 μm) were deposited by reactive DC magnetron sputtering on Si, Al₂O₃ and α‐SiC substrates without intentional heating and at 600°C. The SiC target with small excess of carbon was sputtered at various N₂/Ar gas flow ratios (0–0.48). The nitrogen content in the films changes in the range 0–43 at.%. Hardness and elastic modulus (nanoindentation), change in film thickness, film composition, and structure (Raman spectroscopy, XRD) were investigated in dependence on annealing temperature and nitrogen content. All SiC_xN_y films preserve their amorphous structure up to 1500°C. The hardness of all as‐deposited and both air‐ and Ar‐annealed SiC_xN_y films decreases with growth of nitrogen content. The annealing in Ar at temperatures of 1100°C–1300°C results in noticeable hardness growth despite the ordering of graphite‐like structure in carbon clusters in nitrogen free films. Unlike the SiC, this graphitization leads to hardness saturation of SiCN films starting above 900°C, especially for films with higher nitrogen content (deposited at higher N₂/Ar). This indicates the practical hardness limit achievable by thermal treatment for SiC_xN_y films deposited on unheated substrates. The ordering in carbon phase is facilitated by the presence of nitrogen in the films and its extent is controlled by the N/C atomic ratio. The suppression of graphitization was observed for N/C ranging between 0.5–0.7. Films deposited at 600°C show higher hardness and oxidation resistance after annealing in comparison with those deposited on unheated substrates. Hardness reaches 40 GPa for SiC and ~28 GPa for SiC_xN_y (35 at.% of nitrogen). Such a high hardness of SiC film stems from its partial crystallization. Annealing of SiC_xN_y film (35 at.% of N) in Ar at 1400°C is accompanied by formation of numerous hillocks (indicating heterogeneous structure of amorphous films) and redistribution of film material.     

Tensile creep properties and damage mechanisms of 2D-woven C/HfC–SiC composites in high temperature

2D-C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO₂ and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high-temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of Hf_xSi_yO_z nanowires in the samples after high-temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.     

Additive and pyrolysis atmosphere effects on polysiloxane-derived porous SiOC ceramics

Porous silicon oxycarbide (SiOC) is emerging as a much superior ultrahigh surface area material that can be stable up to high temperatures with great tailorability through composition and additive modifications. In this study, bulk SiOCs were fabricated from a base polysiloxane (PSO) system by using different organic additives and pyrolysis atmospheres followed by hydrofluoric acid (HF) etching. The additives modify the microstructural evolution by influencing the SiO₂ nanodomain formation. The SiOC ceramics contain significantly less SiC and more SiO₂ with Ar + H₂O atmosphere pyrolysis compared to Ar atmosphere pyrolysis. Water vapor injection during pyrolysis also causes a drastic increase in specific surface areas. The addition of 10 wt% tetraethyl orthosilicate (TEOS) with Ar + H₂O pyrolysis produces a specific surface area of 1953.94 m²/g, compared to 880.09 m²/g for the base PSO pyrolyzed in Ar. The fundamental processes for the composition and phase evolutions are discussed as a novel pathway to creating ultrahigh surface area materials. The ability to drastically increase the specific surface area through the use of pyrolysis atmosphere and organic additives presents a promising processing route for highly porous SiOC ceramics.     

Mechanical and field emission properties of CGed Si(C,N) films synthesized by PECVD from HMDS precursor

Compositionally graded (CGed) Si(C,N) films were prepared by Ar/H₂/N₂ plasma enhanced chemical vapor deposition from liquid injected hexamethyldisiloxane precursor. The films were characterized by scanning/transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Monolithic crystalline SiC and amorphous SiN_x films were produced from Ar/H₂ and Ar/H₂/N₂ thermal plasma, respectively. The CGed SiC–SiN_x film was obtained by changing N₂ flow rate from 2 L/min to zero in Ar/H₂/N₂ during the deposition process, and it was composed of an uppermost crystalline SiC layer, a thin intermediate layer containing nanocomposite c-SiC/a-SiN_x and an innermost layer of amorphous SiN_x. The CGed SiN_x–SiC film, in which SiN_x acts as a top layer with a SiC layer underneath, was fabricated by an inverse change of the plasma gas supply from initial Ar/H₂ to Ar/H₂/N₂. Microhardness increase and promising field emission properties were obtained from these CGed films in comparison with monolithic SiC and SiN_x films.     

Al2O3–SiC composites prepared by infiltration of pre-sintered alumina with a poly(allyl)carbosilane

The Al₂O₃/SiC nanocomposites containing 3–8 vol.% SiC were prepared through infiltration and in situ thermal decomposition of a preceramic polymer SiC precursor (poly(allyl)carbosilane) in pre-sintered alumina matrix. The volume fraction of SiC, and the microstructure of composites were adjusted by concentration of the polymer solution, and by the conditions of pyrolysis and sintering. The specimens were densified by pressureless sintering at temperatures between 1550 and 1850 °C in flowing argon. The use of powder bed producing SiO, CO and other volatile species suppressed decomposition reactions in the composites and was vital for their successful densification. The experimental results are discussed against thermodynamic analysis of the system Al₂O₃/SiC/SiO₂ in an inert Ar atmosphere.     

Conductivity studies of sol-gel prepared BaCe0.85−xZrxY0.15O3−δ solid electrolytes using impedance spectroscopy

A systematic investigation on doped barium cerate perovskites on conductivity was performed by means of ac electrochemical impedance spectroscopy technique. BaCe_0.85?xZr_xY_0.15O_3?δ powders (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by a modified sol-gel Pechini method and sintered at 1,250 °C–1,450 °C, depending on Zr content, to obtain good densities (93–97% of the theoretical ones). The measured total conductivities for these solid solutions in three different atmospheres were reported: in dry oxygen, in dry nitrogen and wet (0.5 bar H₂O) hydrogen (5%H₂/Ar) atmospheres. Arrhenius plots recorded in dry oxygen as well as in dry nitrogen showed some residual hydration which remained in the specimens upon initial heating. The compositions with x = 0.3 and 0.4 gave conductivities close to 10^?2 S/cm in 5%H₂/Ar/H₂O atmosphere at 600 °C. The isothermal conductivities values showed a little variation for x from 0.2 to 0.4 between 500 and 800 °C.     

Microstructural and mechanical characterization of sol gel-derived Si–O–C glasses

The mechanical properties of three silicon oxycarbide glasses pyrolysed under inert (Ar) atmosphere were studied as a function of the pyrolysis temperature. The silicon oxycarbide glasses were prepared from various alkyl substituted alkoxysilanes such as HSi(OEt)₃ and HMeSi(OEt)₂ in different ratios by using the sol-gel method. The Si–O–C-glasses obtained were respectively: (i) silicon oxycarbide network with excess carbon, (ii) stoichometric SiC_xO_2(1−x) where x=0.30 and (iii) silicon oxycarbide matrix with an excess of Si. Si–C bonds introduced in the starting silica gel network can be partially retained in the final glass after pyrolysis under inert atmosphere. After pyrolysis at temperatures between 600–1500 °C, the presence of tetracoordinated C atoms in the silica network results in an improvement of mechanical properties and thermal stability compared with silica glass. By using elemental analysis, density, SEM, BET and XRD (combined with Rietveld-analysis), the glass characterization was performed. Flexural strength (MOR), elastic modulus (E) and Vickers hardness (H_V) were measured and will be discussed in terms of glass composition and microstructure.     

The synthesis of SiCON ceramics through precursor method

A polymeric precursor, polysiloxazane (PSON), for SiCON ceramics has been synthesized by the partial hydrolysis of MeViSiCl₂, MeHSiCl₂, and MeSiCl₃ followed by ammonolysis reaction of the hydrolyzed intermediates with NH₃. The structure and thermal properties of the polymeric precursor were investigated by means of Fourier transfer infrared spectra (FTIR), ¹H‐NMR, ²⁹Si‐NMR, gel permeation chromatography, and thermogravimetric analysis. The structure of the SiCON ceramics derived from the pyrolysis of PSON was characterized by FTIR and X‐ray diffraction. The as‐synthesized PSON produced mainly α‐Si₃N₄ crystalline phase during pyrolysis at 1500°C under N₂ atmosphere, whereas when pyrolyzed at 1500°C under Ar atmosphere, crystalline phases of α/β‐SiC and/or α‐Si₃N₄ were detected. © 2012 Wiley Periodicals, Inc. J fAppl Polym Sci, 2012     

White Si–O–C Ceramic: Structure and Thermodynamic Stability

While pyrolysis of a polysiloxane precursor in argon typically produces a black amorphous Si–O–C ceramic containing “free” carbon (sp² carbon), pyrolyzing the same precursor in hydrogen leads to a white amorphous ceramic with a negligible amount of sp² carbon and a considerable hydrogen content. ²⁹Si magic‐angle‐spinning nuclear magnetic resonance (MAS NMR) spectroscopy confirms the existence of very similar bonding environments of Si atoms in the Si–O–C network for both samples. In addition, ¹H NMR spectroscopic measurements on both samples reveal that the hydrogen atoms are bonded mainly to carbon. For the thermodynamic analysis, the enthalpies of formation with respect to the most stable components (SiO₂, SiC, C) of the black‐and‐white Si–O–C samples obtained after the pyrolysis at 1100°C are determined using high‐temperature oxidative drop‐solution calorimetry in a molten oxide solvent. The white ceramic is 6 kJ/g‐atom more stable in enthalpy than the black one. Although the role of hydrogen in the thermodynamic stability of the white sample remains ambiguous, the thermodynamic findings and structural analysis suggest that the existence of sp²‐bonded carbon in the amorphous network of polymer derived Si–O–C ceramics does not provide additional thermodynamic stability to the ceramic.     

Exploiting pyrolysis protocols on BTDA-TDI/MDI (P84) polyimide/nanocrystalline cellulose carbon membrane for gas separations

Tubular carbon membranes were fabricated by the blending of BTDA-TDI/MDI (P84) polyimide with nanocrystalline cellulose in a controlled pyrolysis process, specifically the pyrolysis environment (He, Ar, and N₂) and the thermal soak time (30–120 min). The carbon membrane layer on a tubular support is converted to carbon matrix at 800 °C with a heating rate of 3 °C min⁻¹. The effects of these controlled pyrolysis conditions on the gas permeation properties have been investigated. The results revealed that the pyrolysis under Ar gas environment at 120 min of thermal soak time have the best gas permeation performance with the highest CO₂/CH₄ selectivity of 68.2 ± 3.3 and CO₂ permeance of 213.6 ± 2.2 GPU. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46901.     

Porous MoxCy/SiO2 Material for CO2 Hydrogenation

The synthesis and characterization of an inexpensive porous Mo_xC_y/SiO₂ material is presented, which was obtained by mixing ammonium hexamolybdate, sucrose, and a mesoporous silica (SBA-15), with a subsequent heat treatment under inert atmosphere. This porous material presented a specific surface area of 170 m²/g. The catalytic behavior in CO₂ hydrogenation was compared with that of Mo₂C and α-MoC_1?x obtained from ammonium hexamolybdate and sucrose, using different Mo/C ratios. CO₂ hydrogenation tests were performed at moderate (100 kPa) and high pressures (2.0 MPa), and it was found that only CO, H₂O and CH₄ are formed at moderate pressures by the three materials, while at higher pressures, methanol and hydrocarbons (C₂H₆, C₃H₈) are also obtained. Differences in selectivity were observed at the high pressure tests. Mo₂C presented higher selectivity to CO and methanol compared with MoC_1?x, which showed preferential selectivity to hydrocarbons (CH₄, C₂H₆). The porous Mo_xC_y/SiO₂ material showed the highest CO₂ hydrogenation activity at high temperatures (270 and 300 °C), being a promising material for the conversion of CO₂ to CO and CH₄.

     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.