Silicon Oxycarbide Research Articles

Polymer-derived microporous SiOC ceramic coated gallium nitride sensor for selective H2/CO detection

In this work enhancement of H2/CO selectivity of GaN sensors coated with amorphous microporous silicon oxycarbide (SiOC) filter layer has been studied. Amorphous SiOC ceramic has been synthesized by pyrolysis of vinyl-functionalized polysiloxane at 700 °C under argon. N2-adsorption measurement confirms formation of microporous SiOC ceramic. SiOC layer with a thickness of about 6 µm was coated on GaN sensor substrate after two-fold coating/pyrolysis steps. Transient response characteristics and sensor signals of uncoated GaN and SiOC-coated GaN sensors were measured towards H2 (50, 500 and 1000 ppm) and CO (50, 70 and 100 ppm) in nitrogen at 400 °C. Uncoated GaN showed high sensor signal towards both H2 and CO whereas for SiOC-coated GaN almost no sensing towards CO was found.

Excellent oxidation behavior of the spin-coated SiCO layers on the austenitic steel

Silicon oxycarbide (SiCO) coated AISI 301 steel was fabricated by pyrolysis of spin-coated three polysiloxane precursors (C1, C2, C3). In each of these precursors, the V3 polymer had a different molar mass (5 000, 10 000, 20 000 g/mol). The oxidation behavior of AISI 301 substrates covered with SiCO coatings was studied in air atmosphere under isothermal conditions at 900 °C for 100 h. It was determined that SiCO coated steel exhibits better performance (kp ∼ 10-13 g2cm-4s−1) than the uncoated AISI 301 steel (kp ∼ 10-12 g2cm-4s−1). This is due to the formation of dense and well-adherent to the substrate SiCO coatings. The molecular weight of the polymer did not affect the thickness or protective properties of the layers. An attempt was made to investigate the mechanism of SiCO layer oxidation by means of a marker experiment. The samples used in this experiment were examined via Scanning Electron Microscopy (SEM) and Raman spectroscopy that revealed inhibition of reactants transport.

Mechanical response of silicon oxycarbide materials processed by spark plasma sintering

Nano- and micro-indentation experiments have been used to evaluate the mechanical properties of dense silicon oxycarbide (SiOC) derived materials obtained by spark plasma sintering (SPS). The sintering temperature varied from 1300 to 1700 °C, with a pressure of 40 MPa. The relationship between the mechanical properties and densification, porosity, evolution of the composition, structure and crystallization depending on the SPS sintering temperature has been determined. The SiOC sample obtained at 1400 °C shows the highest values of hardness (12 GPa) and elastic modulus (95 MPa) in both nano- and micro-indentation tests. The SPS treatment produces a neat densification of the material without promoting the phase separation of the Si–O–C matrix. In terms of the elasto-plastic behaviour, hf/hmax >0.5 and Wel/Wtot <0.7, in conjunction with AFM images, highlight the negligible formation of pile-ups. The elastic recovery is quite high, and is probably due to the flexible segregated carbon phase. The Vickers indentation patterns show the Hertzian cones typical of anomalous glasses. Some minor radial cracks can also be seen, and can be attributed to the presence of both stiffer Si–C bonds which reduce the propagation of cracks, and to carbon, which reduces the brittleness of the SiOC derived materials.

SnS nanoplate coated with crystalline silicon-oxy carbide as composite anode for lithium-ion storage applications

In this work, SiOC, and SnS@SiOC composite have been prepared by thermal pyrolysis method, whereas SnS is prepared by CTAB-assisted hydrothermal method. The presence of a crystalline carbon phase in a SiOC is identified by high-magnification TEM images. The nanoplate-like morphology of SnS and the nanoparticle nature of silicon-oxy carbide are established by SEM and TEM images. The Chemical state of elements in the SnS@SiOC composite is validated by X-ray photoelectron spectroscopy. CV graphs exposed SiOC behaving non-faradic process; SnS and SnS@SiOC composite behaves combined lithium-ion storage mechanism. The rate capability nature of SnS@SiOC composite is superior to SnS and SiOC electrode materials. The initial discharge capacity of SiOC, SnS, and SnS@SiOC is 1191 mAh g−1, 1007 mAh g−1, and 1315 mAh g−1 @ 0.1 A g−1 respectively. At a current density of 1 A g−1, it provided the 1000th cycle discharge capacity of 196.8, 437.5, and 570 mAh g−1 for SiOC, SnS, and SnS@SiOC composite, respectively. Even though, it delivered a discharge capacity of 448.7 and 491.8 mAh g−1 for SnS and SnS@SiOC composite after 800 cycles @ 5 A g−1. It has a capacity retention of 94 % and 90 %, respectively. Silicon oxy carbide and SnS work synergistically to increase lithium-ion storage capacity and cycle stability for long-term energy storage and conversion applications.

WS2 Nanosheet Loaded Silicon-Oxycarbide Electrode for Sodium and Potassium Batteries

Transition metal dichalcogenides (TMDs) such as the WS2 have been widely studied as potential electrode materials for lithium-ion batteries (LIB) owing to TMDs' layered morphology and reversible conversion reaction with the alkali metals between 0 to 2 V (v/s Li/Li+) potentials. However, works involving TMD materials as electrodes for sodium- (NIBs) and potassium-ion batteries (KIBs) are relatively few, mainly due to poor electrode performance arising from significant volume changes and pulverization by the larger size alkali-metal ions. Here, we show that Na+ and K+ cyclability in WS2 TMD is improved by introducing WS2 nanosheets in a chemically and mechanically robust matrix comprising precursor-derived ceramic (PDC) silicon oxycarbide (SiOC) material. The WS2/SiOC composite in fibermat morphology was achieved via electrospinning followed by thermolysis of a polymer solution consisting of a polysiloxane (precursor to SiOC) dispersed with exfoliated WS2 nanosheets. The composite electrode was successfully tested in Na-ion and K-ion half-cells as a working electrode, which rendered the first cycle charge capacity of 474.88 mAh g-1 and 218.91 mAh g-1, respectively. The synergistic effect of the composite electrode leads to higher capacity and improved coulombic efficiency compared to the neat WS2 and neat SiOC materials in these cells.

Preparation and structure of silicon oxycarbide aerogels fabricated by polymer derived ceramics method

Preparation and structure of silicon oxycarbide aerogels fabricated by polymer derived ceramics method

Analyzing the effect of composition, density, and the morphology of the “free” carbon phase on elastic moduli in silicon oxycarbide ceramics

We perform atomistic simulations to model structures and calculate elastic properties of silicon oxycarbide ceramics. We explore individual parameters – composition, density, carbon content – to disentangle mutual dependencies that are difficult to separate in experimental studies. Each parameter is studied through dynamic simulations at finite temperatures for a wide range of temperatures. With multi-million atom models in simulation boxes as large as 40 nm, we reveal a hitherto “hidden” parameter: the morphology of the “free” carbon phase. Embedding, distribution, and interconnection of the carbon phase inside the amorphous matrix of SiCO severely impact the material's mechanical properties. As a consequence, we call for the development of new characterization techniques that will quantify the morphology of carbon in this and similar systems.

SiOC Phase Control and Carbon Nanoribbon Growth by Introducing Oxygen at Atom Level for Lithium-Ion Batteries.

Poor intrinsic conductivity and the presence of irreversible lithiation phase affect the electrochemical performance of silicon oxycarbide anode materials. Even though it can be improved by increasing free carbon content or composition, scarification of reversible capacity and initial Coulombic efficiency (ICE) remain as challenge. Here, polycarbosilane (PCS) with alternating distribution of silicon and carbon atoms is employed as precursor of SiOC ceramics. Air oxidation cross-linking is used to regulate the content of oxygen and carbon elements in PCS at atom level, so as to explore a solution to improve the intrinsic conductivity and reversible lithium phase content of SiOC ceramics. This strategy provides extremely excellent rate capability, areal/volumetric capacity, and ICE. This is also the first concept for feasible precursor structure design to control the SiOC glass phase and regulate the growth of C nanoribbon that can improve the intrinsic conductivity and reversible capacity of SiOC ceramic anode materials.

Hydrogen storage in graphene nanoplatelet incorporated silicon oxycarbides

AbstractHighly porous materials having high specific surface areas are strong candidate materials for hydrogen storage by physisorption. However, the development of suitable adsorbents for hydrogen storage having chemical stability is still a challenge. Micro and mesoporous silicon oxycarbide (SiOC) ceramics due to their excellent chemical stability can overcome this constraint. Contrary with our expectations, the addition of graphene nanoplatelets (GNP) to SiOC ceramics resulted in a lower gravimetric density. A maximum gravimetric storage density of 1.58 wt.% was observed at 6 bar after adding 0.3 wt.% of GNP, whereas the pristine SiOC ceramic showed a gravimetric capacity of 2.07 wt.% at 6 bar. We have proposed a theoretical framework to quantify these phenomena in line with the existing structural model. We show that the addition of GNP results in an increase in the size of the silica nanodomain leading to an overall reduction in hydrogen uptake. Furthermore, we hypothesize that the addition of GNP beyond 3 wt.% could lead to an increase in the coordination of mixed bonds in the interface and subsequent loss of porosity in the composite. Our study confirms that the pore sizes are crucial in determining the hydrogen uptake in these composites.

Equilibrium model for the ablation response of silicone-coated PICA

A novel equilibrium ablation and thermal response model for NASA’s Phenolic Impregnated Carbon Ablator (PICA) is presented. For the first time, the model accounts for the effect of NuSil, a silicone overcoat applied as contamination control during assembly, test, and launch operations for both the Mars Science Laboratory and the Mars 2020 heatshields. The protective effect of NuSil against atomic oxygen is discussed based on experimental results collected under arc jet environment. A detailed mass and heat transfer analysis for PICA-NuSil was implemented in the Porous material Analysis Toolbox based on OpenFOAM, PATO. The model includes specific boundary conditions for the coating removal and surface equilibrium processes using representative elements of the NuSil-environment system. The charred NuSil surface was modeled as SiO2 based on the observation that a silicon oxycarbide layer, formed during the decomposition of NuSil, transforms into domains of pure silica and charred PICA. Two-dimensional material response simulations were performed to compare PICA and PICA-NuSil using boundary conditions calibrated with arc jet data. The present work constitutes the first demonstration of a multi-dimensional ablation response of a silicone-coated carbon-phenolic ablator. It is shown that simulations agree with experiments, and the model reproduces measured temperature and recession. Differences between the PICA-NuSil response under Earth and Mars atmospheres are discussed.

Rational design of carbon-rich silicon oxycarbide nanospheres for high-performance microwave absorbers

Polymer-derived-ceramics (PDCs) are considered as a promising candidate for excellent microwave absorption under harsh environments due to their controllable microstructure at the molecular level. Herein, a series of silicon oxycarbide (SiOC) nanospheres are successfully fabricated through a PDC method and then pyrolyzed under argon atmosphere at different temperatures and the evolutions of microstructure and their microwave absorption properties are investigated simultaneously. It is found that the microwave absorption properties mainly depend on carbon content. With a relative carbon content of 38.13%, the carbon-rich SiOC nanospheres-1 pyrolyzed at 1400 °C possess excellent microwave absorption properties with a minimum reflection loss (RL) value of −56.28 dB at 6.8 GHz with a thickness of 2.93 mm. Electromagnetic analysis demonstrates that proper impedance matching, intrinsic attenuation ability, and the multiple reflection are responsible for the excellent RL. This work will provide a novel approach to obtain the PDCs with excellent microwave absorption properties.

Area selective atmospheric pressure PECVD of organosilicon precursors: Role of vinyl and ethoxy groups on silicon oxycarbide deposition patterns - A case study

An atmospheric pressure Dielectric Barrier Discharge plasma torch was used to perform selective area plasma enhanced chemical vapour deposition of four organosilicon precurors: Triethylvinylsilane (VTES), Vinyltriethoxysilane (VTEOS), Tetraethoxysilane (TEOS) and Tetraethylsilane (TES). The precursors were chosen in such a way that they differ only by the presence or absence of vinyl and/or ethoxy groups. Experimentally, major differences have been observed on the deposition speed, coating patterns and thin film composition. Precursor with vinyl group has confined deposition at the central region of plasma torch, in contrast precursor with only ethoxy groups has circular deposition in the periphery region of plasma torch, and precursors with both the vinyl and ethoxy groups has deposition at both locations. A detailed chemical analysis revealed the film composition to be silicon oxycarbide (SiOxCyH) with high content of carbon for the central region's coating whereas the peripherical coating is close to silica like (SiOx) with a minuscule carbon content. Coatings patterns and composition have been investigated in the light of gaseous flows mixing obtained from computational Fluid Dynamic (CFD) simulation. A strong correlation is evidenced for central SiOxCyH deposition of vinyl containing precursors, to the high concentration of precursor. Whereas the peripheral SiOx deposition of ethoxy containing precursors corresponds to high concentration of plasma species. Furthermore, the role of reactive plasma species on the underlying deposition mechanisms has been identified as crucial and has been purposed as the driving factor.

Single source precursor‐derived SiOC/TiOxCy as an anode component for Li‐ion batteries

AbstractAmorphous silicon oxycarbides are known to be an effective anode material for lithium‐ion batteries. Despite their exceptional properties and high charge capacities, however, their practical uses are limited by their significant first‐cycle loss, considerable hysteresis, and low cyclic ability. Comparatively, SiOC/metal oxide materials have demonstrated increased rate capability and cyclic stability. This study utilized a liquid precursor‐derived ceramic method to modify SiOC with titanium (IV) butoxide precursor to synthesize SiOC/TiOxCy. X‐ray diffractograms confirmed the amorphous nature of SiOC/TiOxCy. The elemental composition and bonding properties were investigated using X‐ray photoelectron spectroscopy, and electron microscopy was used to explore morphological features. In the first cycle, the reversible capacity of pyrolyzed SiOC/TiOxCy was 520 mAh g−1, which then increased to 736 mAh g−1 for the 1200°C annealed SiOC/TiOxCy due to the increased free carbon network and TiC conductive phases. The irreversible capacity of the first cycle was 568 mAh g−1, which was lower than the annealed SiOC irreversible capacity of 695 mAh g−1. Interestingly, the rate stability of the pyrolyzed SiOC/TiOxCy performed more stability than the annealed sample. Localized carbothermal reactions between amorphous SiOC/TiOxCy and free carbon at annealing temperatures resulted in loss of structure stability.

Study on nano-graphitic carbon coating on Si mold insert for precision glass molding

Precision glass molding is an efficient and promising technique for large-volume micro-optical manufacturing. Even though significant progress in precision glass molding has been achieved on the anti-adhesive coating, technical challenges and barriers still exist towards tailoring the coating layer to ensure durability and long-term processing consistency in numerous molding cycles. This manuscript presents a novel molding technique utilizing micro-patterned Si mold inserts coated with a newly developed nano-graphitic carbon coating as an isolation layer in high-precision glass molding. The coating is prepared by atmospheric pressure chemical vapor deposition (APCVD), in which the liquid benzene and silicone rubber are used as the carbon source and silicon oxycarbide (SiOC) source, respectively. The nano-morphologies of the nano-graphitic carbon coating with different conditions are characterized and analyzed, illustrating notable differences in surface features with the addition of silicone rubber and various growth durations. In the demonstration, the nano-graphitic carbon-coated Si mold inserts exhibit unique properties, including anti-adhesive and hydrophobic at elevated temperatures. Two kinds of micro-patterned Si mold inserts, a micro-pillar matrix and a micro-lens array, generated by UV lithography and diamond turning, are utilized to demonstrate the feasibility of the anti-adhesive coating in precision glass molding. The obtained results indicate that the micro-optical features on the coated Si mold inserts are successfully transferred to glass substrates with high replication fidelity without notable signs of adhesion or contamination in repetitive molding cycles. This emerging technology makes Si an alternative mold material with low surface adhesion, enhances micro-optical machining efficiency, and replicates high-precision, low-cost micro-optics on large scales that were previously unavailable.

Passive Filler-Loaded Silicon Oxycarbide Coating on Nickle Alloy with High Thermal Shocking Behavior and Oxidation Resistance.

Polymer-derived ceramic (PDC) coatings of considerable thickness can offer promising protection for metallic and superalloy substrates against oxidation and corrosion, yet the preparation remains challenging. Here, a SiOC/Al2O3/YSZ coating was prepared on a nickel alloy with a spraying method using Al2O3 and yttria-stabilized zirconia (YSZ) as passive fillers. The thickness can reach up to 97 μm with the optimal mass fraction and particle sizes of the passive fillers. A small or isolated SiOC phase is formed in the coating, which can effectively alleviate the shrinkage and cracking during the pyrolysis. The SiOC/Al2O3/YSZ coating exhibits low thermal conductivity and high bonding strength with the substrate. Moreover, the coating shows good thermal shock resistance between 800 °C-room temperature cycles and oxidation resistance at 1000 °C for 36 h. This work provides an effective guide for the design of thick PDC coatings to further promote their application in the thermal protective field.

Impact of blending with polystyrene on the microstructural and electrochemical properties of SiOC ceramic

AbstractIn this work, we present the electrochemical behavior and microstructural analysis of silicon oxycarbide (SiOC) ceramics influenced by an addition of polystyrene (PS). Polymer‐derived ceramics were obtained by pyrolysis (1000°C, Ar atmosphere) of different polysiloxanes prepared by sol–gel synthesis. This method is very effective to obtain desired composition of final ceramic. Two alkoxysilanes phenylthriethoxysilane and diphenyldimethoxysilane were used as precursors. Before pyrolysis polysiloxanes were mixed with PS using toluene as a solvent. Blending with PS affects the microstructure and free carbon content in the final ceramic material. Free carbon phase has been confirmed to be a major lithium storage host. Nevertheless, we demonstrate here that capacity does not increase linearly with increasing carbon content. We show that the amount of SiO4 units in the SiOC microstructure increases the initial capacity but decreases the cycling stability and rate capability of the material. Furthermore, the microstructure of the free carbon influences the electrochemical performance of the ceramic: More ordered graphitic clusters favor better rate capability performance.

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.

Revealing Nanodomain Structures of Bottom-Up-Fabricated Graphene-Embedded Silicon Oxycarbide Ceramics.

Dispersing graphene nanosheets in polymer-derived ceramics (PDCs) has become a promising route to produce exceptional mechanical and functional properties. To reveal the complex nanodomain structures of graphene–PDC composites, a novel reduced graphene oxide aerogel embedded silicon oxycarbide (RGOA-SiOC) nanocomposite was fabricated bottom-up using a 3D reduced graphene oxide aerogel as a skeleton followed by infiltration of a ceramic precursor and high-temperature pyrolysis. The reduced graphene oxide played a critical role in not only the form of the free carbon phase but also the distribution of SiOxC4−x structural units in SiOC. Long-ordered and continuous graphene layers were then embedded into the amorphous SiOC phase. The oxygen-rich SiOxC4−x units were more prone to forming than carbon-rich SiOxC4−x units in SiOC after the introduction of reduced graphene oxide, which we attributed to the bonding of Si atoms in SiOC with O atoms in reduced graphene oxide during the pyrolysis process.

Silicon oxycarbide (SiOC) foam from methylphenylpoly(silsesquioxane)(PS) by direct foaming technique

Silicon oxycarbide (SiOC) foam from methylphenylpoly(silsesquioxane)(PS) by direct foaming technique

In-situ formation of titanium carbide in carbon-rich silicon oxycarbide ceramic for enhanced thermal stability

Silicon oxycarbide (SiOC) ceramic has attracted great attention as fascinating candidate of high-temperature material, however, its thermal stability is significantly limited by the phase separation at high temperature. Here, a TiC/SiOC ceramic was prepared by pyrolysis of a tetrabutyl titanate modified carbon-rich polysiloxane (TBT/PSO) precursor. The TiC phase is in-situ formed by the carbothermal reaction of TBT-derived amorphous TiO2 phase with excess free-carbon phase during pyrolysis, and its size and amount increase with the pyrolysis temperature. The SiC phase appears at a higher temperature than the TiC phase and is hindered by the increased Ti content in the TBT/PSO precursor. Thus, the TiC/SiOC ceramic exhibits better thermal stability and crystallization resistance than the TiC-free SiOC ceramic under the thermal treatment (1500 °C) in argon atmosphere. The in-situ formation of metal carbide into the carbon-rich SiOC ceramic would further expand its application at high temperature environments.