Bonding mechanism between silicon carbide and thin foils of reactive metals

Pressureless-sintered SiC pieces and SiC single crystals were joined with foils of reactive metals at 1500° C (1773 K) for titanium and zirconium foils or at 1000° C (1273 K) for Al/Ti/Al foils. Bend testing at various temperatures up to 1400° C (1673 K), optical and electron microscopy, and electron-probe X-ray microanalysis studies were carried out on the specimens. From the results, it was concluded that the fairly high bond strength of titanium-foil joined SiC specimens might be attributed to the formation of a Ti₃SiC₂ compound, since good lattice matching between SiC and Ti₃SiC₂ was obtained in the SiC single crystals. Also in the Al/Ti/Al-foil joined SiC, high bond strength was obtained, but it decreased steeply at 600° C (873 K) because of a retained aluminium phase. The bond strength in the zirconium-foil joined SiC was low.     

Interfacial reaction and adhesion between SiC and thin sputtered cobalt films

Thin sputtered cobalt films on SiC were annealed in an Ar/4 vol% H₂ atmosphere at temperatures between 500 and 1450 °C for various times. The reaction process and the reaction-product morphology were characterized using optical microscopy, surface profilometry, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The relative adhesive strength between the film and substrate was determined by the scratch test method. Below 850 °C sputtered cobalt with a thickness of 2 m on SiC showed no detectable reaction products. Cobalt initially reacted with SiC at 850 °C producing Co₂Si and unreacted cobalt in the reaction zone. At 1050 °C the first-formed Co₂Si layer reacted to CoSi, and carbon precipitates were formed in the reaction zones. Sputtered thin cobalt layers reacted completely with SiC after annealing at 1050 °C for 2 h. Above 1250°C only CoSi was observed with carbon precipitates having an oriented structure in the reaction zone. Above 1450°C, a significant amount of graphitic carbon in the reaction zone was detected.     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Si-rich a-SiC:H thin films: Structural and optical transformations during thermal annealing

Amorphous hydrogenated silicon-rich silicon carbide (a-Si_0.8C_0.2:H) thin films were prepared by plasma enhanced chemical vapour deposition and were thermally annealed in a conventional resistance heated furnace at annealing temperatures up to 1100 °C. The annealing temperatures were varied and the samples were characterised with Auger electron spectroscopy, glancing incidence X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy. As-deposited a-Si_0.8C_0.2:H thin films contain a large amount of hydrogen and are amorphous. When annealing the films, the onset of Si crystallisation appears at 700 °C. For higher annealing temperatures, we observed SiC crystallites in addition to the Si nanocrystals (NCs). The crystallisation of SiC correlates with the occurrence of a strong PL band, which is strongly reduced after hydrogen passivation. Thus PL signal originates from the SiC matrix. Si NCs exhibit no PL yield due to their inhomogeneous size distribution.     

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T _p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si0₂ and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T _p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T _p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.     

Microstructural evolution of a cold-deformed 6061Al reinforced with SiC particles during subsequent annealing

The microstructural development during annealing of a cold-deformed 6061Al metal matrix composite (MMC) reinforced with either 3 or 20 m diameter SiC particles has been investigated. The composites were compressed to low (< 10%) levels of strain and then annealed at either 350 or 450°C for different times. Microstructure examination was carried out by transmission electron microscopy and optical microscopy. The results reveal that prior grain boundaries and constituent particles are the dominant sites for recrystallization in both composites, although some nucleation was observed adjacent to the larger SiC particles. The concurrent presence of Mg₂Si precipitates affected the progress of recrystallization.     

Phase separation and toughening of SiC-AIN solid-solution ceramics

SiC-AIN solid-solution ceramics were prepared by pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. The resulting ceramics were subjected to annealing treatments over a range of temperatures from 1400 °C to 1800 °C in the spinodal region. The fracture toughness of the annealed ceramics was examined, by the indentation method, in relation to the annealing temperature and annealing time. X-ray diffraction profiles revealed that phase separation occurred during annealing. In ceramics containing 50 mol % SiC annealed at 1800 °C, the morphology of the phase separation is the characteristic modulated stratiform structure. Energy-dispersive X-ray spectroscopy (EDS) showed that the structure consisted of alternations of silicon-rich and aluminium-rich composition. The fracture toughness of the annealed ceramics increased compared to the as-sintered solid-solution ceramics. The phase separation is expected to contribute to the toughening of ceramics with nanometre-scale texture.     

Chemical reactivities of hafnium and its derived boride, carbide and nitride compounds at relatively mild temperature

MB₂/SiC composites are materials of choice for ultra-high-temperature structural applications, primarily in the aerospace arena. These composites are processed in a hot-press operation at a temperature range of 1900 to 2200°C. This article assesses potential mild-temperature (below 1500°C) chemical reactions that may lead to structures and coatings made of HfB₂/SiC under pressureless or mild-pressure conditions. The reactions are anticipated to be involved in reactive and shape-forming processes, where ceramic precursors and/or reactive powders are incorporated. This article pays special attention to exothermic reactions as well as to formers of a liquid phase; both can aid the desired phase formation, microstructure development, and sintering of the composite under milder conditions than currently practiced. Reactions between loosely mixed powders with melting points significantly above 1500°C were detected by X-ray diffraction (XRD) analyses. Significant solid-phase reactions of the loose powder mixtures were observed at this mild temperature in powder form. Preliminary microstructural studies using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray Spectroscopy (EDX) techniques have confirmed the presence of unique reaction mechanisms between the loosely connected particles.Good examples are the reactions between Hf powder and powders of BN or B₄C, all having melting points above 2200°C, which form at 1500°C, or below HfB₂/HfN and HfB₂/HfC crystalline domains, respectively. These reactions are less intuitive than the reaction with B₂O₃, which forms HfB₂/HfO₂, potentially via molten or gaseous phases of boron oxide.     

Synthesis and phase evolution of Si-C-Ti powder derived from poly(methylsilaacetylene) and Ti

Si-C-Ti powder was synthesized by reactive pyrolysis of poly(methylsilaacetylene)(PSCC) precursor mixed with metal Ti powder. Pyrolysis of PSCC/Ti mixture with certain atomic ratio was carried out in argon atmosphere between 1300 °C and 1500 °C. The metal-precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy equipped with EDX. Ti₃SiC₂ phase was obtained from reaction of PSCC and Ti for the first time. Ti₃SiC₂ formation started at 1300 °C and its amount increased significantly at 1400 °C. In addition, liquid formed by additive CaF₂ could promote the formation of Ti₃SiC₂ phase.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

Characterization of the pyrolytic conversion of polysilsesquioxanes to silicon oxycarbides

A series of silsesquioxane copolymers synthesized by hydrolysis and condensation of phenyl- and methyltrimethoxysilanes have been studied as preceramic polymers. The pyrolytic conversion to ceramics was characterized by thermogravimetric analysis, ²⁹Si and ¹³C nuclear magnetic resonance and Raman spectroscopy. The pyrolysed materials were further characterized by differential thermal analysis. X-ray diffractometry and transmission electron microscopy. The ratio of phenyl to methyl groups in the copolymer was found to control polymer structure and rheology, as well as ceramic composition and char yield. On pyrolysis to 1000 °C under inert conditions, silicon oxycarbides were formed, along with glassy carbon. On heating from 1200 °C to 1400 °C, the oxycarbide structure diminished, and the materials were comprised primarily of amorphous silica, amorphous Si-C, some small crystallite SiC and graphitic carbon. The carbon content increased, and char yield decreased, with increasing concentration of phenyl groups in the copolymer. The presence of free carbon appears to inhibit the crystallization of silica. Significant carbothermal reduction was observed only above 1500 °C. Oxidation studies of the pyrolysed materials indicated the presence of at least two forms of carbon.     

Study of SiO2 encapsulation for aluminum and phosphorus implant activation in 4H-SiC

SiO₂ encapsulation layer was studied for aluminum (Al) and phosphorus (P) implant activation anneal in 4H-SiC. Both Al^- and P⁺ implantation were carried out at 650 °C followed by activation anneal at 1400 °C to 1500 °C. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and four-point-probe (FPP) measurements were performed to examine surface stoichiometry, roughness, and sheet resistance of the implanted SiC regions. The effect of using SiO₂ encapsulation layer for Al implant activation on the performance of 4H-SiC p-i-n diodes with both p-type active region and JTE region formed by Al implantation was also investigated. Forward and reverse characteristics including saturation current density J₀, ideality factor η, reverser leakage current density J_L and threshold breakdown voltage V_BR have been extracted. The results show that SiO₂ encapsulation effectively protects the SiC surface during high temperature implant activation for both Al^- and P⁺.     

Sensing behaviour of nanosized zinc-tin composite oxide towards liquefied petroleum gas and ethanol

A chemical route has been used to synthesize composite oxides of zinc and tin. An ammonia solution was added to equal amounts of zinc and tin chloride solutions of same molarities to obtain precipitates. Three portions of these precipitates were annealed at 400, 600 and 800 °C, respectively. Results of X-ray diffraction and transmission electron microscopy clearly depicted coexistence of phases of nano-sized SnO₂, ZnO, Zn₂SnO₄ and ZnSnO₃. The effect of annealing on structure, morphology and sensing has been observed as well. It has been observed that annealing promoted growth of Zn₂SnO₄ and ZnSnO₃ at the expense of zinc. The sensing response of fabricated sensors from these materials to 250 ppm LPG and ethanol has been investigated. The sensor fabricated from powder annealed at 400 °C responded better to LPG than ethanol.     

Effect of the sintering temperature on the properties of Ce0.85La0.10Ca0.05O2−δ electrolyte material

Rare earth and alkaline earth co-doped Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte material with the powder obtained by solid-state reaction method was sintered at 1300, 1400, 1500 and 1600 °C respectively. The results showed that the ionic conductivity of the sample sintered at 1400 °C was slightly lower compared to that sintered at 1500 °C in the temperature range of 300-550 °C, while the sample sintered at 1400 °C showed the highest ionic conductivity in all the samples above 550 °C. The ionic conductivity of ∼0.021 S/cm at 600 °C and the relative density of 98.2% were observed for the sample sintered at 1400 °C. In addition, the highest flexural strength with 145 MPa was also obtained for the sample sintered at 1400 °C. It suggested that the sintering temperature for Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte may be reduced to as low as 1400 °C with desired properties.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Preparation of SiC nanopowder using low-temperature combustion synthesized precursor

SiC nanopowder was synthesized by carbothermal reduction of a low-temperature combustion synthesized (LCS) precursor derived from silicic acid, polyacrylamide (PAM), nitric acid, urea, and glucose mixed solution. The results showed that the LCS precursor is a kind of porous blocky particles. The precursors were subsequently calcined under argon at 1100–1500 °C for 2 h. The transformation of SiO₂ to SiC occurred at 1200 °C, and complete transformation of SiO₂ to SiC was achieved at 1500 °C. The SiC powder synthesized at 1500 °C is mostly composed of near-spherical particles with the diameter of 50–100 nm. Moreover, the SiC powder also contains very rare amount of whiskers with a diameter of 80 nm and a length of up to several micrometers. It is proposed that the present holes in the precursor particles during calcination are responsible for the formation of whiskers. Furthermore, the formation of mainly SiC near-spherical nanoparticles is ascribed to coarse surface of precursor particles during calcination, intimate contact among SiO₂ and C particles, uniformly formed free space during reduction reaction, and separation effect of unreacted carbon.     

Microstructure evolution and ordering in commercial Mg-PSZ

Sintered commercial ZrO₂-9 mol % MgO (PSZ) alloy was heat-treated at different temperatures in the range 900 to 1400° C. The microstructure was studied using transmission electron microscopy (TEM). The as-sintered material was characterized by either fine tetragonal precipitation in cubic matrix grains, or coarser precipitates which had transformed martensitically to the monoclinic symmetry. The diffuse scattering intensity (DSI) was observed to originate from the cubic lattice, and was correlated with the short-range ordering of the oxygen vacancies present in the cubic matrix. However, by annealing at temperatures below 1100° C for relatively short times, long-range ordering occurred in the cubic matrix. The ordered phase was-Mg₂Zr₅O₁₂ with a rhombohedral symmetry, which belongs to the homologous series of M _n O _2n-2 (M₇O₁₂) defect structures derived from the CaF₂-type structure. The ordering process is characteristic only for the cubic regions between the fine-tetragonal precipitates. This microstructure is considered to be a pseudo-equilibrium state and is related to the limited extent of diffusion.     

Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Surface nano-structured silicon carbide thin film produced using hot filament decomposition of ethylene at low temperature on silicon wafer

The structure and spectroscopic properties of nano-structured silicon carbide (SiC) thin films were studied for films obtained through deposition of decomposed ethylene (C₂H₄) on silicon wafers via hot filament chemical vapor deposition method at low temperature followed by annealing at various temperatures in the range 300-700 °C. The prepared films were analyzed with focus on the early deposition stage and the initial growth layers. The analysis of the film's physics and structural characteristics was performed with Fourier transform infrared spectroscopy and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. The conditions for forming thin layer of cubic SiC phase (3C-SiC) are found. X-ray diffraction and Raman spectroscopy confirmed the presence of 3C-SiC phase in the sample. The formation conditions and structure of intermediate SiC layer, which reduces the crystal lattice mismatch between Si and diamond, are essential for the alignment of diamond growth. This finding provides an easy way of forming SiC intermediate layer using the Si from the substrate.