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.     

The structure evolution of hollow SiOC ceramic microspheres prepared with solvothermal method

Hollow silicon oxycarbide (SiOC) ceramic microspheres were synthesized through solvothermal process of vinyltriethoxysilane in NaOH solution with subsequent pyrolysis at high temperature. Increasing the synthesis temperature not only reduces the Si–C bond and C content in SiOC ceramics, but also transforms the amorphous SiOC ceramics into cristobalite SiO₂ after carbonization. The rearrangement reaction of oxygen-enriched structural units results in the increase of SiO₂C₂ unit. No phase separation occurs at 1400 °C, and SiC nanocrystals are mainly come from the carbothermal reduction reaction of SiO₂ with free C. The size change of SiO₂ nanograins were further investigated by HF etching. The porous carbon is obtained after removal of SiO₂, while HF etching has no effect on the structure of free C. The C content affects the structure evolution of SiOC ceramics significantly. Although the size of SiO₂ grows as increase of pyrolysis temperature, the high C content inhibits the crystallization and growth of SiO₂ during the pyrolysis process.     

The surface carboxyl group of carbonaceous microspheres effects on the synthesis and structure of SiOC ceramics

In this study, C/SiOC and C/SiO₂ composites were prepared by using carbonaceous microspheres with different surface functional groups. Carbonaceous microspheres based on hydrothermal reaction of glucose contains hydroxyl group, while the surface carboxyl group increases after NaOH etching. The hydroxyl group increases the oxygen-enriched structural units of SiOC ceramics, and the C spheres are closely enwrapped in SiOC matrix after pyrolysis at 900 °C. However, the interfacial reaction of surface carboxyl with Si–OH results in the formation of cristobalite SiO₂, and C spheres are not only encased inside the SiOC matrix, but also dispersed outside of SiOC ceramics. After removal of C via calcination at 500 °C for 5 h, C/SiOC and C/SiO₂ composites are transformed into amorphous SiO₂ and cristobalite SiO₂, respectively. The thermogravimetric analysis indicates the oxidation resistance of SiOC is superior to that of C and SiO₂.     

Effect of additive structure and size on SiO2 formation in polymer‐derived SiOC ceramics

Silicon oxycarbide (SiOC) ceramics with highly adjustable properties and microstructures have many promising applications in batteries, catalysis, gas separation, and supercapacitors. In this study, additive structures on the nucleation and growth of SiO₂ within SiOC ceramics are investigated by adding cyclic tetramethyl‐tetravinylcyclotetrasiloxane (TMTVS) or caged octavinyl‐polyhedral oligomeric silsesquioxane (POSS) to a base polysiloxane (PSO) precursor. The effects of the 2 additives on the polymer‐to‐ceramic transformation and the phase formation within the SiOC are discussed. POSS encourages SiO₂ nucleation and leads to more SiO₂ formation with significantly increased ceramic yield, which subsequently leads to higher specific surface of 1557 m²/g with a larger pore size of ～1.8 nm for the porous SiOC. High TMTVS content decreases both the specific surface area and pore volume of the resulting porous SiOCs. This study demonstrates a new approach of using Si‐rich additive POSS to increase the SiOC yield while maintaining or even increasing the specific surface area.     

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.     

Effect of HF and NaOH etching on the composition and structure of SiOC ceramics

Porous SiOC ceramics were prepared with tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) as sol?gel precursors, and followed by etching with HF and NaOH solution. The microstructure evolution and chemical etching as a function of pyrolysis temperature were investigated. The amorphous carbon increases as rising the temperature from 800 ^oC to 1200 ^oC, and the graphitic carbon increases with further etching by HF and NaOH. However, the effect of pyrolysis temperature on the structure of C is more significant. The hydroxylation reaction and phase separation of SiOC ceramics results in the increase of SiO₄ unit, which reacts with HF and NaOH to form micro- and mesopores. The existence of mesopore after HF etching provides more specific surface area and pore volume. However, NaOH etching produces more micropores, and the contribution of micropores to specific surface area and pore volume is higher than that of mesopores. Although HF and NaOH etching increase the specific surface area of SiOC ceramics, the etching effect of NaOH is superior to that of HF etching, and the carbon-enriched SiOC ceramics are obtained after NaOH etching.     

Growth of multi-morphology amorphous silicon oxycarbide nanowires during the laser ablation of polymer-derived silicon carbonitride

Multi-morphology amorphous SiOC nanowires were successfully prepared within the interfacial interstices between the unaffected SiCN ceramic and the bracket during the laser ablation of polymer-derived SiCN ceramic in a low-pressure argon atmosphere. Laser irradiation experiments were performed using a continuous-wave CO₂ laser, and the gas source for the growth of amorphous SiOC nanowires was provided by the laser ablation of the SiCN ceramic. X-ray photoelectron spectroscopy shows that the amorphous SiOC nanowires possess a SiO₂ dominated nanostructure, and the formation of amorphous SiOC nanowires is attributed to the good diffusivity of CO in SiO₂. The morphologies of the amorphous SiOC nanowires include straight nanowires, beaded nanowires, helical nanowires, and branched nanowires, and these are determined by the flowing state of the reactant gases, the laser power, and the surface morphology of the SiCN ceramics. Each amorphous SiOC nanowire with specific morphology can be uniformly distributed in separate regions, which makes it possible to control the growth of amorphous SiOC nanowires in different morphologies.     

Silicon oxycarbide ceramics with reduced carbon by pyrolysis of polysiloxanes in water vapor

SiOC ceramics pyrolyzed from polysiloxanes are usually black because of the formation of excess carbon in the ceramic network. Here we show that the pyrolysis of polysiloxanes in water vapor significantly reduces carbon from SiOC and yields a white SiOC ceramic. Chemical analysis shows the amount of carbon in the white ceramic is only half of that in the black one pyrolyzed in argon. ²⁹Si nuclear magnetic resonance spectral (NMR) analysis indicates the reduction of the carbon-rich [SiC₄] and [SiC₂O₂] units with the enhanced formation of the oxygen-rich [SiO₄] and [SiCO₃] units by the water pyrolysis. Importantly, this water pyrolysis resultant carbon reduction is realized in a bulk polysiloxane, and the white SiOC ceramic is obtained in a bulk body with the retained shape of the precursor body. The water pyrolysis can be adopted as an effective mean to tailor the structure of PDCs via the simple introduction of water vapor in pyrolysis.     

Electrochemical performance of SiOC anodes prepared by a different silicone

SiOC is one of the most promising anodes for lithium-ion batteries, which shows the good structural stability and high capacity comparing to commercial graphite anode. In this paper, different SiOC anodes (SiOC-217, SiOC-H44, and SiOC-MK) were prepared from polymer precursors with different side groups (phenyl, methyl-phenyl, methyl) to investigate the effects of free carbon on the electrochemical performance of SiOC anodes. The results of X-ray photoelectron spectroscopy presented that SiOC was composed by different SiO_xC_4−x units and free carbon phase. The initial discharge capacity of SiOC-217 was 742.67 mA h g⁻¹. After 100 cycles, the reversible capacity of SiOC-217 reached 450.65 mA h g⁻¹ at 0.2 C, indicating a capacity retention rate of 60.68%. After cycling at high current densities, SiOC-217 exhibited a high discharge capacity of 592.88 mA h g⁻¹ at 0.1 C. SiOC-217 exhibited excellent electrochemical performance due to the high content of free carbon phase. Furthermore, the high contents of SiO₂C₂ and SiO₃C units further enhanced the improvement of electrochemical performance.     

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.     

Comparison of traditional and flash pyrolysis of different carbon content silicon oxycarbides

This study is to understand the effect of carbon content on the pyrolysis behaviors and phase contents of silicon oxycarbides (SiOCs). Flash pyrolysis conditions, evolution of different SiOC phases, and free carbon types/amounts are compared for C-rich and less C-rich precursors. The C-rich system experiences the flash event at a much lower pyrolysis temperature with a much higher current density even though the internal temperatures at flash are very similar. SiC formation is more obvious for the C-rich samples along with a much higher carbon content under both flash and traditional pyrolysis conditions. The phase contents of SiO₂, SiC, and other SiOC intermediates can be calculated using a Gibbs energy minimization method, showing that the C-rich sample has more C-rich SiOC intermediates while the less C-rich sample has more Si-rich intermediates. This research provides a general framework in assessing the pyrolysis behaviors of different SiOC materials.     

New insight into SiOC atomic structure evolution during early stage of pyrolysis

This study focuses on the early stage of polymer-derived SiOC ceramic conversion. We demonstrate that the perceived SiOC phase separation is nonexistent. Instead, SiO₂ and free carbon clusters form first and then carbothermal reduction sets in to induce SiOC formation. Such fundamental understanding is supported by both synchrotron X-ray diffraction study and reactive force field simulation. This work for the first time unifies the understanding of atomic evolution process of polysiloxane-based polymer to ceramic conversion.     

Macrostructural design of highly porous SiOC ceramic foams by preceramic polymer viscosity tailoring

Highly porous SiOC ceramic foams with gradient or uniform macrostructures were obtained through polymer derived ceramic routes. Precuring of preceramic polymers and introduction of SiO₂ powders were used to tailor precursor viscosity and hence SiOC foam macrostructure. Effects of polymer viscosity on porosity, pore size, pore distribution were investigated by light microscopy and micro-computed tomography techniques. SiOC ceramic foams. Foams from one unmodified precursor, showed pore size gradient with small pores located at bottom and large pores at the top. To address this non-uniformity, the viscosity of the precursor was increased by pre-curing the preceramic polymer, which resulted in decrease of the average pore size and improvement in pore size uniformity. For a different system with a self-foaming preceramic polymer, because of the simultaneous release of foaming gases and rapid increase in viscosity during crosslinking, the foam had non-uniform macrostructure with large pores and thick struts at the bottom. By addition of SiO₂ fillers, the crosslinking reaction rate was reduced leading to homogeneous pore nucleation and uniform small pore size foams.     

Gas Sensing Behavior of Mesoporous SiOC Glasses

In this study, for the first time, we report the gas sensing behavior of aerogel‐derived silicon oxycarbide (SiOC) glasses. The SiOC glass pyrolyzed at 1400°C has specific surface area of 150 m²/g with pore size in the 2–20 nm range. SiOC sensor shows good response to 5 ppm NO₂ at 300°C. NO₂ response completely disappears at 400°C, and from this temperature SiOC sensor starts respond to H₂. The optimum sensitivity for H₂ is obtained at 500°C. SiOC sensor is very selective; it is not sensitive to other gases such as acetone vapor or CO, even at high concentrations.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

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.     

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.     

Lithium insertion into dense and porous carbon-rich polymer-derived SiOC ceramics

Two polymer-derived SiOC ceramics with different amount of carbon were synthesized either as dense or porous SiOC powders. The dense materials were produced up to a maximum temperature of 1400 °C and show a phase separated nanostructure consisting of SiO₂-rich clusters, nanocrystalline SiC and nanocrystalline carbon phase. The corresponding porous materials were obtained by etching the silica phase of the dense SiOC with 20% HF solution. The electrochemical properties of the dense and porous SiOC ceramics in terms of lithium insertion/extraction were studied. Accordingly, the SiOC materials show a first lithium insertion capacity between 380 and 648 mAh g^?1 followed by significantly lower extraction capacities between 102 and 272 mAh g^?1. We consider the free carbon phase present in the ceramic as the major lithium intercalating agent. The porous samples show a stable electrochemical behavior up to 30 cycles while for the dense materials the efficiency drops to almost zero after 10 cycles.     

Fabrication of polymer-derived ceramics with hierarchical porosities by freeze casting assisted by thiol-ene click chemistry and HF etching

The freeze casting technique assisted with cryo thiol-ene photopolymerization is successfully employed for the fabrication of macroporous polymer-derived silicon oxycarbide with highly aligned porosity. It is demonstrated that the free radical initiated thiol-ene click reaction effectively cross-linked the vinyl-containing liquid polysiloxanes into infusible thermosets even at low temperatures. Furthermore, mixed solution- and suspension-based freeze casting is employed by adding silica nanopowders. SiOC/SiO₂ foams with almost perfect cylindrical shapes are obtained, demonstrating that the presence of nano-SiO₂ does not restrict the complete photoinduced cross-linking. The post-pyrolysis HF acid treatments of produced SiOC monoliths yields hierarchical porosities, with SiOC/SiO₂ nanocomposites after etching demonstrating the highest specific surface area of 494 m²/g and pore sizes across the macro-, meso- and micropores ranges. The newly developed approach gives a versatile solution for the fabrication of bulk polymer-derived ceramics with controlled porosity.     

Effect of Ce addition on Cr/γ-Al2O3 catalysts for methane catalytic combustion

Cerium modified and chromium-based catalysts using nano-γ-Al₂O₃ as the carrier were prepared via incipient wetness impregnation method and investigated for the catalytic combustion of methane (CH₄). The Cr-based catalysts promoted with 3 wt.% Ce displayed the most effective catalytic activity among all catalysts investigated. In addition, Ce significantly improved the catalytic performance of CH₄ combustion by increasing the amount of reaction site [CrO₄]_ads species on the surface of Cr-based catalysts. Introduction of Ce content also restrained the deactivation of catalysts at high calcination temperature. Cr-based catalysts modified with cerium seem to be a promising cheap and low-temperature catalyst for CH₄ combustion.