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This work correlates the charge carrier transport mechanism of silicon oxycarbide-based thin films with their morphology and thermal stress. Segregation of highly-graphitized carbon-rich, oxygen-depleted C/SiC areas homogeneously dispersed within an oxygen-rich C/SiOC matrix was seen on the 500 nm-SiOC thin films. Compressive biaxial stress induced by the mismatch with the Si-substrate thermal expansion coefficient was calculated at 109 MPa. Through Hall measurements, p-type carriers were shown dominating the SiOC film similar to monolithic samples. Thin films and monoliths have comparable carrier concentrations while the carrier mobility in SiOC thin films was 2 magnitudes higher than that of monolithic samples and is considered a consequence of the compressive thermal stress acting on the film. Improved conductivity of 16 S cm -1 is measured for the SiOC thin film sample which is assumed considering the enhanced carrier mobility alongside the reduced percolation threshold ascribed to the phase-separated morphology of the thin film.

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 °C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp2-hybridized boron-containing carbon (i.e. ByC). High temperature annealing of ByC/SiHfC(N,O) leads to the precipitation of HfCxN1-x nanoparticles as well as to β-SiC crystallization. After annealing at temperatures beyond 1900 °C, HfB2 formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 ± 5.3 and 344.7 ± 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfCxN1-x/HfB2/C ceramic nano/micro composite.

The present work investigates the influence of pyrolysis temperature on the microstructure and thermal conductivity ( λ ) of polymer-derived SiC ceramics. For the study, both monolithic thin ceramic disks and highly porous reticulated foams were considered. The λ of the polymer-derived SiC disks were measured, in the temperature range 100 °C–1000 °C, to amount between 0.5 W.m−1. K−1 and 47 W.m−1. K−1, depending from the maximum pyrolysis temperature. While, the λ measured at RT on the SiC foams (porosity 79.5 %–99.0 %), ranges between 0.03 W.m−1. K−1 and 0.19 W.m−1. K−1. The thermal transport in polymer-derived SiC ceramics was observed to be more significantly governed by the nanocrystalline to crystalline transition which occurs at pyrolysis temperatures above 1200 °C as compared to mesopore formations or the nature and amount of free carbon.

Mesoporous SiVN(O) ceramics were prepared from a mixture consisting of VO(acac)2-modified perhydropolysilazane and polystyrene. The resulting amorphous single-phase SiVN(O) ceramics remained amorphous in nitrogen atmosphere up to 1400 °C. The as-prepared materials consist of nanoscaled vanadium nitride dispersed in amorphous Si3N4; exposure to 1600 °C leads to the crystallization of VN and Si3N4. The specific surface area (SSA) and the pore size of the SiVN(O)-based ceramics can be easily controlled by the temperature of thermal treatment and by the amount of polystyrene. The average pore size of the prepared SiVN(O) ceramics was 4–10 nm and their largest SSA values, 642 and 506 m2/g, were achieved upon ammonolysis at 800 and 1000 °C, respectively. The combination of metal-modified single-source precursors and encapsulated porogens provides a convenient one-pot synthesis process to prepare mesoporous ceramic nanocomposites with controllable phase compositions and morphology.

Oxidation behavior of boron-containing HfC/SiC nanocomposites (SHBC) at temperatures up to 1500 °C and with exposure time up to 100 h was investigated. Two strategies to improve the oxidation resistance of the HfC/SiC ceramics are proposed. First concept involves the incorporation of a small amount of boron (ca. 0.6 wt.%) into the nanocomposite via a single-source-precursor approach, which contributes significantly to the enhancement of its oxidation resistance. Parabolic oxidation rate constants of 10−3 to 10-4 mg2/(cm4 h) at 1300−1500 °C were measured for SHBC and were several orders of magnitude lower than those recorded for boron-free HfC/SiC. The second improvement concept is realized via passivation of the samples upon short-term oxidation at 1400 °C, providing an excellent oxidation resistance over a wide temperature range. This is a crucial step especially when considering the poor oxidation behavior of HfC and the sluggish formation of protective silica scale at moderate temperatures.

International audience; Silicon oxycarbides with varying compositions were investigated concerning their elastic and plastic properties. Additionally, the impact of thermal annealing on their elastic properties was assessed. Phase separation of SiOC seems to have no significant impact on Young's modulus (high values of β-SiC compensate the low values of the vitreous silica matrix) and hardness. However, it leads to an increase in Poisson's ratio, indicating an increase in the atomic packing density. The phase composition of SiOC significantly influences Young's modulus, hardness, brittleness and strain-rate sensitivity the amount of both β-SiC and segregated carbon governs Young's modulus and hardness, whereas the fraction of free carbon determines brittleness and strain-rate sensitivity. Thermal annealing of SiOC glass-ceramics leads to an increase in Young's modulus. However, the temperature sensitivity of Young's modulus and Poisson's ratio is not affected, indicating the glassy matrix being stable during thermal annealing. A slightly improved ordering of the segregated carbon and the β-SiC nanoparticles upon thermal annealing was observed. It is suggested that this is responsible for the increase in Young's modulus.

Laser ablation test of SiHfC-based ceramic nanocomposites as well as ceramic matrix composites (CMCs) was conducted by exposure to a CO2 laser beam in air. Laser ablation behavior and possible degradation mechanisms of dense monolithic HfC/SiC ceramic nanocomposites as well as of Cf/SiHfC CMCs were investigated. Dense SiC monoliths and Cf/SiC CMCs were exposed to same laser ablation conditions and considered as reference materials. The evolution of microstructure and chemical/phase composition of the studied ceramics was addressed by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) as well as by X-ray diffraction. The results reveal that from the center to the edge of the damaged region of the materials three sections with different surface morphologies and ablation mechanisms are identified. The comparation between the SiC-based monoliths and CMCs with and without Hf demonstrates the positive effects of Hf-incorporation on their laser ablation resistance.

Mg and Ca substituted silicon oxycarbides are investigated with respect to their network architecture, enthalpy of formation and bioactivity. The synthesized materials have highly connected structural network with an open architecture and a minor but critical fraction of depolymerized Q-species. This combination of structural features allows for providing bioactivity at the same time with enhanced thermo-mechanical robustness and crystallization resistance. It is argued that silicon oxycarbide amorphous ceramics may consequently serve as model compounds for developing rational design concepts for advanced bioactive glasses.

Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 ° C/min.) and high pressures (≥100 MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure. The hardness and the elastic modulus of dense SiHfCN were found to be 26.8 and 367 GPa, respectively. Whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa. The investigation of the oxidation of the prepared dense ceramic nanocomposites at high temperature revealed that the parabolic oxidation rates of SiHfCN were comparable to those of ultra-high temperature ceramics (UHTCs, e.g. HfC-20 vol% SiC); whereas the parabolic oxidation rates of SiHfBCN were several orders of magnitude lower than those. The results obtained within this study indicate the feasibility of SPS for rapid preparation of dense though nano-scaled Hf-containing ceramic nanocomposites that are promising candidates for high-temperature applications in harsh environments.

Environmental barrier coatings are required to protect Si3N4 against hot gas corrosion and enable its application in gas turbines, among which yttrium and ytterbium silicate-coatings stand out. Thus, the polymer-derived ceramic route was used to synthesize these silicates for basic investigations regarding their intrinsic properties from a mixture of Y2O3 or Yb2O3 powders and the oligosilazane Durazane 1800. After pyrolysis above 1200 °C in air, the silicates are predominant phases. The corrosion behaviour of the resulting composites was tested at 1400 °C for 80 h in moist environments. The material containing x2-Yb2SiO5 and Yb2Si2O7 undergoes the lowest corrosion rate (−1.8 μg cm−2 h−1). Finally, the processing of Y2O3/Durazane 1800 as well-adherent, crack-free and thick (40 μm) coatings for Si3N4 was achieved after pyrolysis at 1400 °C in air. The coating consisted of an Y2O3/Y2SiO5 top-layer and an Y2O3/Y2Si2O7 interlayer due to the interaction of the coating system with the substrate.

In the present work, novel V8C7/SiC(O) ceramic nanocomposites were synthesized upon thermal transformation of a polymer-derived single-source-precursor, which was obtained by the chemical modification of a polycarbosilane with vanadyl acetylacetonate. High-temperature treatment of the precursor in argon atmosphere first leads to an amorphous SiVOC single-phase ceramic which subsequently undergoes phase-separation, crystallization and finally converts into V8C7/SiC(O) ceramic nanocomposites. Interestingly, the high-temperature stability of V8C7/SiC(O) was shown to strongly depend on the oxygen content present either in the SiC(O) matrix or in the atmosphere during the annealing process. Thus, larger oxygen contents induce a conversion of the V8C7 phase into V5Si3. The specific surface area (SSA) of the obtained nanocomposite powders depends on the processing temperature: The SSA decreases from 64 to 4 m2/g as the pyrolysis temperature increases from 600 to 1300 °C, respectively. Whereas it increases again to ca. 50 m2/g as the sample is exposed to 1700 °C (6 h annealing), due to the evolution of CO. Preliminary results of the catalytic activity of the V8C7/SiC(O)-based materials show that they are active for the decomposition of the ammonia. The maximum ammonia conversion efficiency was found to be 35% at around 650 °C, which is higher than that of the pure vanadium carbide reported in the literature (ca. 13%).

The behavior of Cf/SiHfBCN composites upon laser ablation was investigated. The evolution of microstructure and phase composition of the composite was studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the ablation mechanism and the microstructure evolution are mainly dependent on the local temperature which directly relies on the distance from the laser spot and the thermal conduction of the carbon fibers. Three regions with different ablation behavior are identified. In the center region, bubble-like structures consisting of HfO2, and SiHfxOy-based layers, cover the ends of the carbon fibers which retain their original shape despite their corrosion. Sheets consisting of HfCxOy and SiO2 as well as porous HfO2 and liquid SiO2 are generated in the transition region. The carbon fibers are well protected from oxidation in the two regions. SiO2 is produced and suppresses the further oxidation of the matrix in the fringe region.

In this study we present the high-temperature creep behavior of a dense SiOC glass ceramic free of segregated carbon. Solid-state NMR spectroscopy, XRD and TEM investigations indicate that the sample consists of β-SiC nanoparticles homogeneously dispersed in an amorphous silica matrix. Compression creep experiments were performed at 1100–1300 °C and stresses of 50–100 MPa. The calculated creep viscosity of SiOC is two orders of magnitude higher than that of pure silica. Whereas the activation energy for creep (696 kJ/mol) is close to that determined in pure silica glass. However, a stress exponent of 1.7 was calculated, suggesting that other mechanisms might contribute to the creep in addition to the Newtonian viscous flow. The strong difference in the creep rates and creep mechanism of the SiOC glass ceramic and amorphous silica is discussed in terms of possible contributions of the interface between the silica matrix and the β-SiC nanoparticles.

Within this study, the oxidation behavior of SiHfBCN ceramic powders and monoliths was studied at temperatures from 1200 to 1400 °C. Both powder and monolithic samples exhibited parabolic oxidation behavior characterized by very low rates (10−9–10−8 mg2 cm−4 h−1). The activation energy of 112.9 kJ mol−1, which was determined for the SiHfBCN powder, is comparable to that of other silica formers such as silicon or SiC and relates to the diffusion of molecular oxygen through silica scale. Whereas, the values determined for the SiHfBCN ceramic monoliths (174 and 140 kJ mol−1, depending on the Hf content) indicate the complex nature of their oxidation process, leading at temperatures below 1300 °C to a continuous oxide scale consisting of borosilicate, silica, m-and t-HfO2. At higher temperatures, the oxide scale consists of silica, HfSiO4 as well as m-and t-HfO2 and becomes discontinuous, probably due to the evaporation of boria.

Novel silica-containing nanocomposites with a controlled carbon phase based on high quality reduced graphene oxide (rGO) were prepared via a cheap and facile sol–gel method, followed by pyrolysis in inert gas atmosphere. The resulting nanocomposite materials have an intimate bonding between rGO of low defect density and a partially crystalline silica matrix, as shown by Raman, FTIR and HRTEM studies. This processing route allows a straight-forward control of the content of the carbon phase and an excellent dispersion thereof within the silica matrix.

Within the present work, boron-containing silicon carbide (SiBC) powders and monoliths were prepared from a polyborocarbosilane. The main aspect addressed here relates to the fate of boron in the prepared SiBC ceramics, which has not been clarified unambiguously so far. X-ray diffraction data, corroborated with XPS, FTIR and Raman spectroscopic results indicate that boron preferably gets incorporated within the segregated carbon phase present in polycarbosilane-derived SiC; thus, the prepared SiBC samples show dispersed boron-containing carbon phases with boron contents from ca. 9 to 18 at%. Interestingly, the B-containing carbon phase does not convert into crystalline boron carbide, even upon annealing at very high temperatures and despite the high boron content. Moreover, processing details concerning the pressureless preparation of dense Si(B)C monoliths are considered in the present paper and their high temperature evolution is analyzed.

Herein we report on the preparation and high-temperature evolution of polysilylcarbodiimides-derived SiCN ceramic coatings. Two polysilylcarbodiimide-based preceramic polymers with different molecular structures (i.e., a rather linear and a highly-branched polymer) were synthesized and their use for the preparation of sub-μm-thick SiCN ceramic coatings was assessed. The different molecular architecture of the polysilylcarbodiimide has been shown to significantly affect the composition (e.g., the content of segregated carbon) and properties of the resulting ceramic coatings. Thus, the high-temperature crystallization behavior and the ordering of the segregated carbon phase depend on the carbon content and consequently on the preceramic polymer structure. Moreover, the content of the segregated carbon phase present within the microstructure of the prepared SiCN coatings determines their hardness and elastic properties. The low-carbon-content coating has been shown to be stiffer and have higher hardness than the carbon-rich SiCN sample.

SrMo(O,N)(3) and SrW(O,N)(3) oxynitride powders were synthesized via a solvothermal route followed by ammonolysis. Their rapid densification by spark plasma sintering with a heating rate of 300-400 ...

Cf/SiCN and Cf/SiHfBCN-based ceramic matrix composites (CMC) were fabricated using the precursor infiltration and pyrolysis technique (PIP). Their behavior in subcritical hydrothermal conditions was investigated at 150–250 °C using exposition times of 48, 96 and 240 h and shown to rely on active corrosion. The data of the mass loss as a function of the corrosion time and temperature at S/V of 0.075 were used to rationalize the corrosion kinetics of the CMCs. Both materials have been shown to exhibit excellent stability in subcritical hydrothermal conditions. The corrosion rates of Cf/SiHfBCN were lower than those determined for Cf/SiCN; furthermore, SEM investigation indicates that spallation occurred in the Cf/SiCN samples; whereas the ceramic matrix was still attached on the individual carbon fibers in Cf/SiHfBCN. The results indicate that the incorporation of Hf and B into SiCN matrix leads to a significant improvement of its hydrothermal corrosion performance.

Within the present study, atomic-scale electron microscopy investigation on the crystallization behavior of polysilylcarbodiimide-derived SiCN was performed. The as-prepared SiCN sample was found to be homogeneous and consisted of amorphous silicon nitride nano-domains dispersed within an amorphous, highly entangled graphene-like carbon matrix. Annealing of the sample at 1400 ◦ C induced a slight increase of the ordering of the carbon phase. Additionally, the crystallization onset of the silicon nitride has been observed for the first time. In the sample annealed at 1400 ◦ C small nano-clusters with average sizes of 0.8, 1.5 and 2.1 nm (consisting of 30, 180 and 570 atoms, respectively; corresponding to Si12N18, Si72N108 and Si228N342) were imaged and assigned to -silicon nitride. The crystallization of the amorphous silicon nitride phase into -Si3N4 is thought to occur via diffusion of Si and N, which rely on the presence of large number of dangling bonds in the amorphous SiCN sample. © 2015 Elsevier Ltd. All rights reserved.

Novel amorphous polymer-derived Si–Hf–N ceramics were prepared via ammonolysis of single-source precursors which were synthesized by the chemical reaction of a commercially available perhydropolysilazane (PHPS) with different amounts of tetrakis(dimethylamido) hafnium(IV) (TDMAH). The prepared Si–Hf–N materials were annealed in nitrogen atmosphere at different temperatures ranging from 1100 °C to 1800 °C and investigated with respect to their chemical composition, crystallization behavior and phase evolution. The single-source precursors converted upon heat treatment in ammonia environment at 1000 °C into amorphous single-phase ceramics and remained amorphous even after annealing at 1400 °C in nitrogen atmosphere. Electron microscopy investigation showed that the annealing of the highly homogeneous single-phase Si–Hf–N induces local enrichment (clustering) of Hf, leading to amorphous HfN/SiNx nanocomposites. Annealing in nitrogen at 1600 °C leads to the crystallization of HfN and Si3N4. The results of the present work emphasize a convenient preparative access to silicon–metal–nitride nanocomposites from suitable single-source precursors.

This paper presents the pressureless preparation of dense and crack-free near stoichiometric SiC monoliths via cross-linking and pyrolysis of a polycarbosilane, followed by polymer-infiltration-pyrolysis cycles. The composition and the porosity of the samples strongly depend on the processing temperature. Thus, at 1050–1100 °C, the SiC monoliths are X-ray amorphous and exhibit low amounts of oxygen and excess carbon; their porosity was rather high (>10%). Higher processing temperatures induced the crystallization of β-SiC. The removal of oxygen and excess carbon due to CO release allowed for obtaining near-stoichiometric compositions at 1700 °C. However, the residual porosity of the samples increased. The use of the PIP technique led already after six cycles to dense monoliths (residual porosity ca. 0.5%). The present study emphasizes the potential of the polymer processing technique for the fabrication of near stoichiometric and dense SiC monoliths, which might be used for structural applications in harsh conditions.

We present here the single-source-precursor synthesis of Fe3Si and Fe5Si3-containing SiOC ceramic nanocomposites and investigation of their magnetic properties. The materials were prepared upon chemical modification of a hydroxy- and ethoxy-substituted polymethylsilsesquioxane with iron (III) acetylacetonate (Fe(acac)3) in different amounts (5, 15, 30 and 50 wt%), followed by cross-linking at 180 °C and pyrolysis in argon at temperatures ranging from 1000 °C to 1500 °C. The polymer-to-ceramic transformation of the iron-modified polysilsesquioxane and the evolution at high temperatures of the synthesized SiFeOC-based nanocomposite were studied by means of thermogravimetric analysis (TGA) coupled with evolved gas analysis (EGA) as well as X-ray diffraction (XRD). Upon pyrolysis at 1100 °C, the non-modified polysilsesquioxane converts into an amorphous SiOC ceramic; whereas the iron-modified precursors lead to Fe3Si/SiOC nanocomposites. Annealing of Fe3Si/SiOC at temperatures exceeding 1300 °C induced the crystallization of Fe5Si3 and β-SiC. The crystallization of the different iron-containing phases at different temperatures is considered to be a consequence of the in situ generation of a Fe–C–Si alloy within the materials during pyrolysis. Depending on the Fe and Si content in the alloy, either Fe3Si and graphitic carbon (at 1000–1200 °C) or Fe5Si3 and β-SiC (at T > 1300 °C) crystallize. All SiFeOC-based ceramic samples were found to exhibit soft magnetic properties. Magnetization versus applied field measurements of the samples show a saturation magnetization up to 26.0 emu/g, depending on the Fe content within the SiFeOC-based samples as well as on the crystalline iron silicide phases formed during pyrolysis.

The polymer-to-ceramic transformation of a hafnium alkoxide-modified polysilazane was investigated via thermogravimetric analysis coupled with in situ mass spectrometry (TG/MS), nuclear magnetic resonance (MAS NMR) and transmission electron microscopy (TEM). The results indicate that the structural evolution of the polysilazane upon ceramization is strongly affected by the modification with hafnium alkoxide. Thus, the content of carbon in the ceramic backbone was relatively low, whereas a large amount of SiN4 sites and a segregated carbon phase was present in the sample. Furthermore, this study revealed the formation of a SiHfCNO amorphous single phase ceramic via pyrolysis of the polymer at 700 °C, whereas at higher pyrolysis temperatures precipitation of hafnia was observed, leading to an amorphous hafnia/silicon carbonitride ceramic nanocomposite. The precipitation of hafnia was shown to not rely on decomposition processes, but to be a result of rearrangement reactions occurring within the ceramic material.

This paper presents the pressureless preparation of fully dense and crack-free SiOC ceramics via direct photo-crosslinking and pyrolysis of a polysiloxane. Elemental analysis revealed the presence of high levels of carbon in the SiOC ceramics. Thus, the samples showed the highest content (78–86 mol%) of segregated “free” carbon reported so far. XRD investigations indicated that the materials prepared at 1100 °C were X-ray amorphous, whereas the sample prepared at 1400 °C contained a turbostratic graphite-like phase and silicon carbide as crystalline phases, as additionally confirmed by TEM and Raman spectroscopy. Vickers hardness was measured to be 5.5–8.6 GPa. The dc resistivity of the prepared material at 1100 °C was 0.35 Ω m, whereas the ceramic pyrolyzed at 1400 °C showed a value of 0.14 Ω m; both values are much lower than those of other known SiOC materials. This latter feature was attributed to the presence of a percolating carbon network in the ceramic.