Kinetics of direct nitridation of pelletized silicon grains in a fluidized bed: experiment, mechanism and modelling

A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of -Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp340 kJ mol-1. © 1998 Chapman & Hall     

Thermal behaviour of a polytitanocarbosilane-derived fibre with a low oxygen content: the Tyranno Lox-E fibre

The chemical composition, microstructure and mechanical properties of Tyranno Lox-E fibre were studied in the as-received state and after annealing in inert atmosphere. The fibre consists of SiC nanocrystals of 2–3 nm, free carbon aggregates of 4–5 distorted aromatic layers and 1–3 nm in length and an amorphous silicon (titanium) oxycarbide phase. Except for evolution of residual hydrogen and a slight densification, the fibre is chemically and structurally stable and retains a high strength up to 1300°C. Beyond 1300°C, superficial degradation resulting from decomposition of the oxycarbide into SiO(g) and CO(g) induces a decrease of strength. Compared with bulk polycrystalline SiC, the fibre has a low creep resistance at high temperature, mainly because of the nanometric size of the SiC crystals but also because of the presence at the grain boundary of the oxycarbide phase (viscous and chemically unstable) and of the poorly organized free carbon phase (chemically and structurally unstable). © 1998 Chapman & Hall     

A batch process to deposit amorphous metallic Mo–Si–N films

A process for depositing amorphous electrically conducting Mo–Si–N films in a batch-type reactive sputtering system has been developed. Each elemental constituent in the film is individually adjustable: molybdenum and silicon through the electrical power applied to the separate targets, and nitrogen through the gas flow rate. Argon is used for the tuning of the intrinsic stress. The amorphous structure of a Mo₃₁Si₁₈N₄₅ film is confirmed by cross-sectional transmission electron microscopy and electron diffraction. The structure remains unchanged up to at least 700 °C for 1 min of annealing in an argon ambient. In the process, the room-temperature resistivity decreases from an initial value of about 1.1 to about 1.0 m cm with no change in the film thickness. After 1100 °C for one minute, grains nucleate and the film resistivity falls by two-thirds. The intrinsic stress in Mo–Si–N films is significantly more uniform throughout the film area than in polycrystalline molybdenum films. These results hold promise for applications of Mo–Si–N films in micromechanical devices. Self-supported beams and membranes have been successfully delaminated from their silicon substrates; molybdenum-rich films are more ductile than silicon-rich films.     

Fabrication and properties of novel composites in the system Al–Zr–C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 m long and 133 m thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 m diameter and a 1–2 m thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.     

Some properties of mullite powders prepared by chemical vapour deposition

The sinterability of mullite (3Al₂O₃·2SiO₂) powder prepared by chemical vapour deposition was examined to improve the conditions for fabricating dense mullite ceramics. The starting powder contained not only mullite, but also a small amount of -Al₂O₃ (Al-Si spinel) and amorphous material. Although the compressed powder was fired at a temperature between 1550 and 1700 °C for 1, 3 and 5 h, the relative densities of the sintered compacts were limited to 90%: (i) due to the creation of pores/microcracks during the solid state reaction (1100–1350 °C), and (ii) due to restriction on the rearrangement of grains because the amount of liquid phase (1550–1700 °C) was insufficient. Calcination of the starting powder was effective for preparation of easily sinterable powder with homogeneous composition. When the compact formed by compressing the calcined powder at 1400 °C for 1 h was fired at 1650 °C for 3 h, the relative density was raised up to 97.2%; moreover, mullite was the only phase detected from the sintered compact. The sintered compact was composed of polyhedral grains with sizes of 1–2 m and elongated grains with long axes of 6 m.     

High pressure sintering of diamond-SiC composite

Sintered polycrystalline compacts in the system diamond-10–50 wt% SiC having average grain size of less than 1 m were prepared at pressure of 6 GPa and temperature between 1400 and 1600 °C. Knoop indentation hardness of the compacts increased with diamond content and sintering temperature, and specimens with a Knoop indentation hardness greater 40 GPa were obtained. It was found that small amount of Al addition into the starting diamond-SiC powder was effective to improve relative density and Knoop indentation hardness of the compacts. The formation of graphite was also suppressed by the addition of Al. Microstructure observation by SEM and TEM suggested that Al segregated at the grain boundary and promoted the bonding between grains. Thin microtwins were observed in diamond grains, whereas fine wavy structures with slightly different orientations were observed in SiC grains, with or without Al addition.     

Preparation of TiB2 sintered compacts by hot pressing

A sintered compact of titanium diboride (TiB₂) was prepared by hot pressing of the synthesized TiB₂ powder, which was obtained by a solid-state reaction between TiN and amorphous boron. Densification of the sintered compact occurred at 20 MPa and 1800° C for 5 to 60 min with the aid of a reaction sintering, including the TiB₂ formation reaction between excess 20 at % amorphous boron in the as-synthesized powder (TiB₂ + 0.2B) and intentionally added 10 at % titanium metal. A homogeneous sintered compact of a single phase of TiB₂, which was prepared by hot pressing for 30 min from the starting powder composition [(TiB₂ + 0.2B) + 0.1 Ti], had a fine-grained microstructure composed of TiB₂ grains with diameters of 2 to 3 m. The bulk density was 4.47 g cm^–3, i.e. 98% of the theoretical density. The microhardness, transverse rupture strength and fracture toughness of the TiB₂ sintered compact were 2850 kg mm^–2, 48 kg mm^–2 and 2.4 MN m^–3/2, respectively. The thermal expansion coefficient increased with increasing temperature up to 400° C and had a constant value of 8.8 x 10^–6 deg^–1 above 500° C.     

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Structure of chemically vapour deposited silicon carbide for coated fuel particles

The silicon carbide coating layers prepared under various conditions were examined by density measurement, X-ray diffractometry, and optical and scanning electron microscopies in order to clarify the relation between deposition conditions and structure of the coating layers. It was found that the deposition temperature was the main parameter affecting the content of free silicon, density, crystallite size and lattice distortion, and microstructure. The dependence of these properties on the coating rate and the composition of fluidizing gas was not observed clearly. Free silicon was co-deposited with-SiC at temperatures lower than 1400 to 1500° C, and the content of free silicon increased with decreasing deposition temperature. The density of the layers without free silicon was more than 3.210 Mg m^–3 and the density decreased with increasing content of free silicon. Crystallite size increased with deposition temperature and lattice distortion decreased with increasing deposition temperature. The outer surfaces of the layers without free silicon consisted of large interlocked grains, whereas those of the layers with free silicon showed a cauliflower-like structure of which the apparent grain size was small.     

Sintering Behavior of Hot-Pressed Ca–αSialon Ceramics

On the basis of phase relationships in the Ca–Si–Al–O–N system, a Ca––sialon ceramic was synthesized using the hot-pressing technique. The reaction sequences and densifications of the Ca––sialon vs. firing temperatures have been characterized in detail. The present experiments reveal a reaction sequence as follows: at 1250°C the reactant mixture started to soften, at 1300°C a gehlenite phase was produced, at 1500°C the gehlenite phase was resolved into a liquid phase and a Ca––sialon started to form, and at 1600°C the formation of Ca––sialon was complete. The product was stable and almost entirely single phase Ca––sialon. Accompanying to the above sequences, densification also proceeded via a liquid-phase sintering, particle rearrangement, solution–reprecipitation, and grain growth process. In the final microstructure elongated grains of Ca––sialon were obtained, improving the fracture toughness of this Ca––sialon ceramic.     

Influence of annealing on properties and structure of alumina

Alumina bodies were prepared from pure alumina powder (98.9% Al₂O₃ consisting of 82% > 53m). The powder was compacted by hot-pressing at 1200° C, Compacted bodies were annealed at 1300, 1400 and 1500° C. Annealing continued at each maximum temperature for 25, 50 and 100 h. Strong bodies were obtained with maximum bulk density of 2.32 g cm^–3 and minimum apparent porosity of 30.21%. The change in sintering parameters with annealing was correlated with developed structure.     

Sintering of pre-mullite powder obtained by chemical processing

The crystallization and sintering behaviour of a premullite powder which had been synthesized from aluminium sulphate [Al₂(SO₄)₃· 16H₂O] and colloidal silicon dioxide have been studied. Calcination of the mixture at 860 °C for 12 h gives a very active powder (surface area – 188 M²g^–1) in the form of spine] and mullite forms via this spinel phase. The non-mullitized powder can be reactively sintered at 1500–1550°C to 97%–99% density in 3–5 h with a very fine microstructure.     

Structural factors determining the change in young's modulus of polycrystalline carbon materials in heat treatment

The structural factors determining the change in Young's modulus of polycrystalline carbon materials based on fired coke (type ARV) and unfired coke (type MPG) in relation to treatment temperature t in the 1000–3200°C range were investigated. The microstructure of the specimens was characterized by the density and coefficient of coherence, which was determined from the ratio of the electrical conductivity of the material to the electrical conductivity of its porosity-free volumes. The degree of perfection of the layers of specimens with a graphitic structure was determined from the magnetic resistance. It was established that the change in Young's modulus in this range of treatment temperatures is determined by the change in Young's modulus of the porosity-free volumes E₀ c₄₄ and in the coefficient of coherence. The value of E₀ decreases sharply with an increase in t from 1000 to 1800°C and after 2400°C it is practically independent of the degree of perfection of the graphite-like layers. The coefficient of coherence increases in density with an increase in t from 1000 to 2000°C and decreases as the result of appearance of disk-shaped cracks in a change in t from 2400 to 3200°C. The physical reasons for the rules found were analyzed.Translated from Problemy Prochnosti, No. 1, pp. 69–73, January, 1990.     

The sintering and morphology of interconnected porosity in UO2 powder compacts

Uranium dioxide powder compacts of 46% green density were sintered in flowing hydrogen at temperatures between 1500 and 1700° C. On annealing, the compacts readily formed an interconnected system of pores stabilized by grain boundaries. The volume of open porosity decreased with an activation energy of 4.6 J mol^–1 at a rate controlled by grain growth. The grain-boundary migration removed the restraint on the porosity allowing shrinkage to commence. The compact surface area decreased with a higher activation energy of 6.0 J mol^–1. The mechanism proposed for the diminishing area was the smoothing of the faceted powder grains. Nucleation of atomic layers on the facets was shown to account for the high activation energy. The equilibrium shapes that may be adopted by interconnected porosity were calculated using a model in which simpler geometry was substituted for the real anticlastic surface curvature. The model demonstrated the stabilizing effect of increasing grain-boundary energy and the formation of closed pores.     

Infrared transmission of sintered 3 mol% Y2O3-ZrO2 gel

Translucent ZrO₂ film was successfully prepared by gelling hydrothermally produced nano-ZrO₂ powders. The film (300 m thick) was found to transmit light to 6.5 m (40% transmission) when sintered at 1200 °C, but transmission was totally lost after sintering at 1300 °C for 1 h. Residual organic material such as urea, which was used for preparing the powder, dominated the transmission of the film in the region between 1.3 and 4.5 m when sintered below 1000 °C. When sintered above 1000 °C, the microstructure controlled the transmission. Both organic residuals and the microstructure of the zirconia were found to determine the transmission in 4.5–6.5 m region.     

Influence of carbide formation on the strength of carbon fibres on which silicon and titanium have been deposited

Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10^–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation _m d ^–1/2, where d is the thickness of the TiC layer.     

The microstructure of experimentally deformed limestones

The paper presents preliminary results of TEM studies of the microstructure of fine-grained Solnhofen limestone which was deformed in compression experiments at various strain-rates, temperatures and confining pressures. The aims of the tests were to study changes in the patterns of preferred orientation and possible changes in boundaries of the stability fields of the phases, induced by non-hydrostatic stress or large shearing strains. The TEM work on sputter-etched specimens followed extensive investigations by X-ray methods.Low-temperature deformation (200 °C, 26% strain, =10^–4% sec^–1) produces heavily cold-worked structures, with many sub-cells containing unresolvable defects. At 300 °C the microstructure is slightly less confused: some grains contain only tangles of dislocations while others exhibit ragged deformation bands. At higher temperatures but low strain-rates (600 °C, 36% strain, =10^–6% sec^–1) and within the aragonite stability field, the microstructure is more inhomogeneous, with slip and extensive faulting in both the aragonite and the residual calcite grains. Preferred orientation is indicated by significant alignment between fault structures in neighbouring grains. At yet higher temperatures (900 °C), twinning still occurs in the calcite but it is less profuse; dislocation densities are generally lower and preferred orientation is weaker, as shown by X-ray results.     

Electrocal,thermal, thermoelectric and related properties of magnesium silicide semiconductor prepared from rice husk

Polycrystalline, 10m size magnesium silicide was prepared by alloying 99.9% purity polycrystalline silicon obtained from rice husk ash and high-purity magnesium powder. The material in sintered pellet form was characterized for its structural, electrical, thermal, thermoelectric and other properties. A typical sintered pellet exhibited a room-temperature (30°C) thermoelectric power of 565 V K^–1 and an electrical resistivity of 35 cm. On the other hand, the material was found to be thermally quite stable up to 650°C with a room-temperature thermal conductivity of 6.3×10^–3cals^–1cm^–1K^–1 (2.6 J s^–1 m^–1 K^–1). These properties of the material indicate that the material can find potential applications as a thermoelectric generator and in other semiconductor devices. Furthermore, an indigenous technology for large-scale production of silanes (SiH₄) can be developed using this Mg₂Si which could be prepared in large quantities by a simple and low-cost process.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of -SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at 1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol^–1 in argon and 340–420 kJ mol^–1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.