Oxidation behaviour of ceramic fibres from the Si–C–N–O system and related sub-systems

The oxidation of Si–C–N–O fibres has been investigated. The oxidation rates and the activation energies for the Si–C–O system are similar to those for crystalline SiC. The oxygen and the free carbon concentrations in the ceramics have a limited influence on the oxidation behaviour. As long as the formed silica scale is protective, oxidation kinetics are essentially controlled by the diffusion of oxygen through SiO₂. The parabolic rates in the Si–C–N–O and Si–N–O systems are lower and their activation energies higher than those for SiC. Their values strongly depend on the ratios of C and N bonds to Si and continuously vary from those for SiC (E_a=110−140kJ mol⁻¹) to Si₃N₄ (E_a=330–490 kJ mol⁻¹). The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon-oxynitride (like for Si₃N₄) or silicon-oxycarbonitride layer. The oxidation behaviour of such complex systems is not significantly influenced by the oxygen nor the free carbon contents. It might be governed by the C/Si and N/Si ratios, limiting the nitrogen concentration gradient of the silicon-oxy(carbo)nitride sub-layer and therefore affecting the diffusion/reaction rates.     

Preparation of interpenetrating ceramic–metal composites

Ceramic reinforced metals are attractive because of their enhanced elastic modulus, high strength, tribological properties and low thermal expansion. Most work in this sector has focused on particle- or fiber-reinforced composites where the ceramic phase is not continuous. This work presents aluminium–alumina composites where both phases are interpenetrating throughout the microstructure. Ceramic preforms were produced with sacrificial pore forming agents leading to porosities between 50% and 67%. Pore wall microstructure was varied by changing the sintering temperature. Permeability and strength was measured for the porous preforms and infiltration results were compared with theoretical predictions based on capillary law and Darcian flow. A direct squeeze-casting process was used to infiltrate the preforms with aluminium resulting in an interpenetrating microstructure on both macropore and micropore scale.     

Synthesis and characterization of precursor derived TiN@Si–Al–C–N ceramic nanocomposites for oxygen reduction reaction

The development of efficient and durable catalysts is critical for the commercialization of fuel cells, as the catalysts’ durability and reactivity dictate their ultimate lifetime and activity. In this work, amorphous silicon-based ceramics (Si–C–N and Si–Al–C–N) and TiN@Si–Al–C–N nanocomposites were developed using a precursor derived ceramics approach. In TiN@Si–Al–C–N nanocomposites, TiN nanocrystals (with sizes in the range of 5–12 nm) were effectively anchored on an amorphous Si–Al–C–N support. The nanocomposites were found to be mesoporous in nature and exhibited a surface area as high as 132 m²/g. The average pore size of the nanocomposites was found to increase with an increase in the pyrolysis temperature, and a subsequent graphitization of free carbon was observed as revealed from the Raman spectra. The ceramics were investigated for electrocatalytic activity toward the oxygen reduction reaction using the rotating disk electrode method. The TiN@Si–Al–C–N nanocomposites showed an onset potential of 0.7 V versus reversible hydrogen electrode for oxygen reduction, which seems to indicate a 4-electron pathway at the pyrolysis temperature of 1000°C in contrast to a 2-electron pathway exhibited by the nanocomposites pyrolyzed at 750°C via the Koutecky–Levich plot.     

A novel polyborosilazane for high-temperature amorphous Si–B–N–C ceramic fibres

Polyborosilazane synthesised from BCl₃, HMeSiCl₂, and Me₃SiNHSiMe₃ is easy to cross-link for dehydrogenation of Si–H and N–H, which limits its practical applications for Si–B–N–C fibres on an industrial scale. Therefore, in this context, MeSiCl₃ was used instead of HMeSiCl₂ to synthesise a novel polyborosilazane with limited cross-linking density to fabricate Si–B–N–C fibres. The polyborosilazane synthesised from BCl₃, MeSiCl₃, and Me₃SiNHSiMe₃ exhibits good melt-processability and 1 km long polyborosilazane fibre can be obtained by melt spinning. Prior to pyrolysis, chemical curing with vapour HSiCl₃ at 80 °C was utilised to make the λ green fibres infusible. The as-cured fibres were subsequently pyrolyzed at 1200 °C in nitrogen atmospheres to provide Si–B–N–C ceramic fibres with ca. 1.5 GPa in tensile strength, ca. 160 GPa in Young's modulus, ca. 12 μm in diameter and keeping amorphous up to 1700 °C, which makes them to be promising reinforcements in ceramic matrix composites for high temperature applications.     

Preparation and characterisation of diopside-based glass–ceramic foams

Foaming and crystallisation behaviours of compacted glass powders based on a diopside glass–ceramic composition were investigated using the sintering route. The foaming agent was 2 wt.% SiC particles. The effect of PbO on the foaming ability of glasses was investigated. The results showed that the addition of PbO not only improved the foaming ability, by improving the wettability of the glass–SiC particles but also increased the crystallisation temperature and widened the temperature interval between the dilatometric softening point and the onset of crystallisation. The glass–SiC wetting angle was decreased from 85° for the lead-free glass, to 55° for the glass that contains 15 wt.% PbO.     

High-temperature oxidation induced layering process for Si–C–B–N ceramic fibers with SiC nanograins

High-temperature oxidation resistance is important for Si–C–B–N ceramic fibers when reinforcing ceramic matrix composites with superior reliability and faulting tolerance. At present, few studies have investigated on the high-temperature oxidation behavior of Si–C–B–N fibers, limiting their further applications. In this work, we analyzed the high-temperature oxidation process of Si–C–B–N ceramic fibers with SiC nanograins (SiBCN-SiCn fibers) at 1000–1500 °C in air. SiBCN-SiCn fibers stated to be oxidized at 1000 °C, with the formation of thin oxide layer. After oxidizing at 1300 °C, obvious oxide layer that mainly consisted of amorphous SiO₂ could be detected. Further oxidizing at 1500 °C caused the thickness increment of oxide layer, which could inhibit the oxidation products (CO, N₂) to release away from the fibers. The remained CO and N₂ may react with SiC nanograins to form SiO₂ and graphite-like g-C₃N₄, causing the formation of additional transition layer. Our finding may support useful information for the applications of SiBCN-SiCn fibers under harsh environment.     

Quantitative EFTEM study of precursor-derived Si–B–C–N ceramics

Ceramic materials derived from a boron modified polysilazane were investigated by means of energy-filtering transmission electron microscopy (EFTEM). After cross-linking of the polymer and subsequent thermolysis, a coarse powder with average composition Si_24.0B_8.0C_44.0N_24.0 is obtained. For further investigation, monolithic particles with sizes of several millimeters were heat treated in crucibles under a flowing nitrogen atmosphere at 1800 °C for 10 h. During thermolysis, the particles developed internal cracks on the macroscopic scale. At the crack surfaces, a layer of pure carbon was found. In the crack-free region, the material is composed of Si₃N₄ and SiC nano crystallites which are embedded in a turbostratic BNC-matrix. Quantitative electron spectroscopic imaging (ESI) shows an atomic ratio of the elements B:C:N of 1.0:4.0:1.1 in this matrix. In the vicinity of the cracks, silicon nitride locally decomposes with formation of silicon carbide because of its reaction with excess carbon. A detailed EFTEM study of the phase distribution near the crack surfaces showed that the first Si₃N₄ crystallites occur at a distance of approx. 5 μm from the carbon covered crack surface. In additional experiments the composition of the BNC-layers as a function of the distance from the crack surface was investigated.     

Processing of ZrO2 scaffolds coated by glass–ceramic derived from 45S5 bioglass

Three dimensional, highly porous, ZrO₂ scaffolds coated by glass–ceramic derived from 45S5 bioglass were fabricated. The surface reactivity of 45S5 in aqueous solution was investigated as a function of the immersion time. The influence of the solid loading on the rheological behavior of 45S5 aqueous slips with ammonium polyacrylate (NH₄PA) was studied; besides the effect of poly(vinyl)alcohol (PVA) on the relative viscosity was determined. The structure and microstructure of uncoated and coated ZrO₂ scaffolds were characterized. The high ionic exchange capability of 45S5 was demonstrated by the pH rise, the significant weight loss and the amorphous calcium phosphate nucleation, upon its immersion in aqueous solution. The addition of PVA did not affect the dispersion properties of the 45S5 powder, which were basically controlled by its negative surface charge. 30 wt% 45S5 slips with 4 wt% PVA exhibited a yield stress and an adequate viscosity in the low shear rate range, to produce a bioglass coating into the ZrO₂ scaffold. The glass-ceramic coating was distributed along the strut surfaces, forming a thin film without altering the porosity and the strut thickness of the original ZrO₂ scaffold structure.     

Synthesis and pyrolysis of polysilazane precursors containing linear–cyclic structures for Si/N/C-based ceramics

A new kind of polysilazane precursor containing linear–cyclic structure was synthesized from the de-lithium chloride condensation reaction of hexamethylcyclotrisilazane lithium salts (D) and 1,3-dichlorotetraorganodisilazanes [(ClR₁R₂Si)₂NH]. Pyrolyses of these precursors and some comparable linear polysilazanes were carried out at 900 °C in nitrogen. The results indicated that the existence of linear–cyclic structure in precursors provided relatively crosslinked or branched structures, which were helpful for improving ceramic yields. The precursor containing a linear–cyclic structure as well as reactive vinyl groups gave the highest pyrolytic yields. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2827–2831, 2001     

Pyrolysis synthesis of Si–B–C–N ceramics and their thermal stability

Si–B–C–N ceramics were synthesized by co-pyrolyzing hybrid polymeric precursors of polycarbosilane (PCS) and polyborazine (PBN). The pyrolysis behavior and structural evolution of the hybrid precursor, the microstructure and composition of the prepared Si–B–C–N ceramics were fully investigated. It was found that the copyrolysis of hybrid polymeric precursors in Ar led to the release of CH₄, CH₃NH₂ and CH₃CN gases at temperatures ranging from 200 to 1100 °C, and finally resulted in the formation of amorphous Si–B–C–N ceramics. In particular, the Si–B–C–N ceramics formed from the hybrid precursor with PBN/PCS mass ratio of 1 could keep amorphous state up to the annealing temperature of 1800 °C with weight change of only 2.08%. But this amorphous ceramics would decompose to form crystalline SiC, BN and Si₃N₄ at 2000 °C. Additionally, compared with PCS-derived SiC ceramics, the Si–B–C–N ceramics showed improved anti-oxidation performance up to 1300 °C due to the formation of borosilicate layers covering the ceramics.     

Feasibility of SiAlON–Si3N4 composite ceramic as a potential bone repairing material

In this study, SiAlON–Si₃N₄ composite ceramic are prepared by direct nitridation and investigated to overcome the limitations associated with ceramic Si₃N₄, which includes the difficulty in fabricating ceramic Si₃N₄ into shaped parts for use in the human body. Phase composition and microstructure of the SiAlON–Si₃N₄ composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the porosity, bulk density, compressive strength, and ion release were also measured. The biological properties were evaluated by bone cell cultures on the ceramic surfaces. Results show that Si₄Al₂O₂N₆ is formed by the reaction of Al, Si, and Al₂O₃ with nitrogen at high temperature that forms Si₃N₄, thereby fabricating SiAlON–Si₃N₄ composite ceramics. Some α-Si₃N₄ grains underwent a phase transition from α-to β-Si₃N₄ fiber at high temperature. Porosity of the samples increases with increasing Si₃N₄ content, while the bulk density of the samples decreases. The compressive strength increases and then slightly decreases with increasing Si₃N₄ content. Water leaching experiments of the SiAlON–Si₃N₄ composite ceramics reveal that the composites exhibit outstanding chemical stability. Studies using bone cell culture indicate that the cells present a fusiform and extend two or three thin pseudopodia. The phenomena demonstrate that MC3T3-E1 cells have excellent growth activity and have the potential ability to proliferate to osteocytes on the surfaces of the samples, thus suggesting that SiAlON–Si₃N₄ based ceramics are biocompatible and could be implemented as a potential bone-repairing material.     

Preparation and characterization of nanocrystalline Fe–Ni–Cr alloy electrodeposits on Fe substrate

Fe–Ni–Cr alloy layers were prepared by electrodeposition from trivalent chromium plating bath in chloride-sulfate based solution. The influences of bath composition and plating parameters on the alloy electrodeposition process and the properties of deposited alloy were studied. The effects of plating parameters and bath composition such as current density, bath pH, bath temperature, the concentrations of FeSO₄ · 7H₂O and CrCl₃ · 6H₂O on the contents of Fe and Cr in Fe–Ni–Cr alloy layer were investigated. Electrodeposited Fe–Ni–Cr alloy layers on Fe substrate were characterized by X-ray diffraction (XRD), Electronic Differential System (EDS) and a CHI600B electrochemistry workstation. The composition of the Fe–Ni–Cr coatings depends on bath composition and plating conditions including pH, current density, and temperature. The internal structure of the alloy is nanocrystalline, the average grain size is 87 nm, and the corrosion resistance of the alloy layers is better than that of pure nickel layers.     

含Si–O–C界面层的C/Si–C–N复合材料的抗氧化性能(英文)

为了研究利用Si–O–C界面层来提高碳纤维增强陶瓷基复合材料的抗氧化性能,利用化学气相浸渗和聚合物浸渗裂解工艺制备了以Si–O–C为界面的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–O–C/Si–C–N）和无界面层的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–C–N）。研究了C/Si–O–C/Si–C–N和C/Si–C–N在600、900℃和1 200℃空气环境中的氧化行为。结果表明：采用Si–O–C界面层后可提高复合材料的抗氧化性能;Si–O–C界面层较高的氧化抗力是碳纤维增强Si–C–N复合材料抗氧化性能提高的主要原因。     

Synthesis of Ti(C,N) ultrafine powders by carbothermal reduction of TiO2 derived from sol–gel process

Experimental results demonstrate that TiC_1−XN_X ultrafine powders can be synthesized by the sol–gel process. The factors influencing the powder synthesizing process, such as temperature, C/Ti ratio in raw materials, holding time and flow rate of nitrogen gas, are discussed. TiC_0.5N_0.5 powders with particle sizes less than 100 nm were produced at 1550°C. The microhardness of hot-pressed TiC_0.5N_0.5 samples at 1750°C was 19.6 GPa and the relative density was 98.9%.     

Correlation of boron content and high temperature stability in Si–B–C–N ceramics

The preparation and characterization of precursor derived Si–B–C–N ceramics with similar Si/C/N ratios but variable boron content are reported. The polymeric precursors were prepared via hydroboration of poly(methylvinylsilazane) using different BH₃·SMe₂/polymer stoichiometries. High temperature thermogravimetric analysis of as-pyrolysed ceramics as well as XRD studies of post-annealed samples display a retarding effect of boron on both crystallization of SiC and Si₃N₄ and stabilization of crystalline β-Si₃N₄.     

Preparation of translucent YAG glass/ceramic at temperatures below 900 °C

The work presented here deals with the preparation of bulk yttrium aluminium garnet (YAG) glass-ceramics and YAG ceramics from glass microspheres with a YAG composition. Sol-gel prepared YAG powder was fed into a high temperature methane-oxygen flame where the particles melted and glass microspheres, with a YAG composition, were formed. Viscous flow sintering of the microspheres was then performed to prepare bulk YAG glass-ceramics or ceramics in a hot press.Rapid crystallization of YAG glass was traced during hot pressing through a change in the heating rate slope due to release of latent (crystallization) heat. This allowed control of crystallization and enabled preparation of YAG-based materials with different amounts of residual glass. YAG ceramic with relative density of 94.2 % was prepared at 891 °C without isothermal heating; additionally, YAG glass-ceramic reached relative density > 99 % at temperature 815 °C without isothermal heating.     

Preparation and properties of ferrimagnetic glass–ceramic foams based on pyrite slag

By employing a melt-sintering method, we prepared a new type of ferrimagnetic glass–ceramic foam (FGCF) using ferrimagnetic glass–ceramic and foaming agent SrCO₃. The ferrimagnetic glass–ceramics were fabricated based on pyrite slag by a melt-quenching method. The effects of foaming agent content, sintering temperature and time on microstructure, magnetic properties, microwave absorption performance, compressive strength, and thermal conductivity of the as-obtained FGCF were analyzed. This foaming process at 1100°C for 40 min with 3-wt% SrCO₃ provided an FGCF with a bulk density of .693 g/cm³, a porosity of 63.60%, a specific saturation magnetic moment of 5.2 A m²/kg, a compressive strength of 2.61 MPa, a thermal conductivity of .241 W/(m K), and the calculated reflection loss of −12.1 dB for a layer thickness of 9 mm.     

Thermodynamic calculations on the chemical vapor deposition of Si–C–N from the SiCl4–NH3–C3H6–H2–Ar system

Based on the Gibbs free energy minimum principle and Factsage software, the thermodynamic phase diagram for the SiCl₄–NH₃–C₃H₆–H₂–Ar system was calculated. The effects of temperature, dilution ratio of H₂, total pressure on product types and distribution regions of reacted solid products were discussed. The results show that: (1) The area of SiC–Si₃N₄ increases at first, then decreases with the rising of temperature and reaches the maximum value at 1273.15 K. (2) The ratio of C/Si is the main factor for the deposition of SiC in the double phase of SiC–Si₃N₄. (3) The preferred deposition conditions of Si₃N₄ are: T=1173.15 K, H₂:SiCl₄=10:1, and P_Total=0.01 atm. Taking the deposition of SiC into consideration, the deposition of Si₃N₄ influences the formation of Si–C–N directly. (4) According to the influencing factors of depositing SiC and Si₃N₄, the suitable parameter for Si–C–N deposition can be determined. (5) Through the experimental verification, it can be demonstrated that Si–C–N can be obtained by low-pressure chemical vapor deposition (CVD), its product being amorphous and mainly constituted by Si–N and Si–C bonds. The obtained Si–C–N ceramics can transform to α-Si₃N₄ and SiC nano-crystal when heat-treated at 1773.15 K in N₂ for 2 h.     

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB₂ and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO₂ and ZrO₂ mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B₂O₃ melt, SiO₂ melt, borosilicate glass, ZrSiO₄ melt and ZrO₂ oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.