Silicon Nitride Powder Research Articles

Effect of Aluminium on High-Energy Ball Milling and Spark Plasma Sintering of Silicon Nitride

A commercial silicon nitride powder with oxide sintering additives was ground with high-energy ball mill to obtain nano-sized powder. Metallic aluminium powder was added as a grinding additive. Effect of high-energy ball milling was evaluated by X-ray diffraction analysis. After milling, height of background increased and peak height of silicon nitride decreased in XRD chart, which suggested that vitrification and/or decrease in grain size of silicon nitride occurred. The milled powders were sintered by spark plasma sintering system. Aluminium nitride was formed during sintering by reaction of aluminium and atmospheric nitrogen. Dense nano-ceramics, which were composed of silicon nitride and sialon, were obtained by sintering at 1550 oC.

Kinetics of SiO vapor ammonolysis for nano-sized silicon nitride powder synthesis

The kinetics of SiO vapor ammonolysis to produce nano-sized silicon nitride powder was studied with an 80-mm-diameter vertical tubular flow reactor operated at temperatures ranging from 1300 °C to 1400 °C. The molar feed rate of NH 3 was maintained 2–3 orders of magnitude in excess of stoichiometric ratio over SiO. The rate of reaction to form nano-sized powder was represented by a pseudo-first-order rate expression with respect to SiO concentration, independently of NH 3 concentration. The temperature dependency of the pseudo-first-order rate constant was determined. The effects of mean residence time as well as the reaction temperature on the growth of nano-sized particles were investigated, and the minimum residence time to produce particles of critical size was estimated.

A novel route for preparing MgSiN 2 powder by combustion synthesis

A new route for the preparation of magnesium silicon nitride (MgSiN 2) powder by combustion synthesis (CS) in nitrogen gas from several different starting mixtures (Mg + Si, Mg 2Si, Mg + Si 3N 4) was investigated. The results showed that the compositions of the combustion products depended on the starting mixtures. Single-phase MgSiN 2 powder can be fabricated from Mg 2Si or the mixtures of Mg and Si 3N 4 powders. The oxygen content of the resulting powder was as low as 0.356 wt.%. However, the product from Mg and Si mixtures consists mainly of MgSiN 2 together with free Si, MgO and not identified phases.

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si 3N 4 powders coated with thin layers of MgO. The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si 3N 4 powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.

Total oxidation of methane over Pd catalysts supported on silicon nitrideInfluence of support nature

Abstract Non-oxide refractory materials such as silicon nitride showing high thermal stability and thermal conductivity can be used as catalytic supports. Silicon nitride with different specific area and crystallinity: amorphous silicon nitride (SiN-am), amorphous silicon nitride annealed at 1450 °C (120 min) under nitrogen flow (SiN-annl) and α-Si 3 N 4 were chosen as supports for Pd catalysts. Commercial amorphous silicon nitride contains a small amount of crystalline phase with α/β Si 3 N 4 ratio of 2.3. Annealing of the amorphous silicon nitride leads to changes in the phase composition, as well as in the morphology of the powder. An increase of the amount of crystalline phases up to 80% with α/β Si 3 N 4 ratio of 6.6 was obtained after annealing. Palladium catalysts were prepared by impregnation of Pd(II) acetylacetonate in toluene solution of silicon nitride powder and were tested in the methane total oxidation reaction. A strong influence of the phase composition and the crystalline state of supports on the catalytic properties of Pd catalysts was found. The activity of Pd catalysts deposited on the amorphous Si 3 N 4 is very poor. It increases with the α-Si 3 N 4 content of the support.

Suppression of Viscous Grain-Boundary Sliding in a Dilute SiAION Ceramic

Aluminum nitride (AIN) and alumina (Al 2 O 3 ) additives were blended into a high-purity silicon nitride (Si 3 N 4 ) powder, and optimized processing conditions were applied for promoting a sintering process, which was shown to partly remove the amorphous silica-rich (SiO 2 -rich) remains from grain boundaries. Such an approach led to the formation of a dilute SiAION structure. The microstructure of the obtained SiAION material consisted of two distinct types of matrix phases, namely, a main network of equiaxed (dilute) β-SiAlON grains, with a minor fraction of (oxygen-rich) acicular O'-SiAION grains. As a consequence of the resulting solid solution of aluminum plus oxygen in the Si 3 N 4 lattice, many grain boundaries were free from any amorphous interlayer, as revealed by high-resolution electron microscopy (HREM) observation. Other than some boundaries that were still wetted, small amounts of residual glass were noted at some triple-grain pockets. However, the average size of such glass pockets was only a few nanometers. The observed microstructural characteristics were critical for the anelastic response of the present SiAION material, as compared with undoped Si 3 N 4 , whose grains were continuously encompassed by an amorphous SiO 2 film. Internal-friction data that were collected up to very high temperatures showed that, in the present SiAION material, no anelastic relaxation peak was monitored. Thus, viscous sliding along grain boundaries was inhibited. In addition, the partial elimination of the amorphous SiO 2 film also implied that no continuous path for oxygen diffusion was available along grain boundaries. Because of this important circumstance and despite the possible softening effect due to solid solution, the viscoelastic (background) component of the internal-friction data shifted toward higher temperatures and the creep resistance of the material was improved.

Superplasticity and Sinter-Forging of Fine-Grained Si<sub>3</sub>N<sub>4</sub>-Si<sub>2</sub>N<sub>2</sub>O Composite

The Si3N4- Si2N2O composites are fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method in this article. XRD analysis shows that the sintered body consists of β-Si3N4 and Si2N2O. SEM experiment conforms that the average grain size of sintered body is less than 300nm. The superplastic deep-drawing forming can be proceed at a low temperature of 1550°C with a forming velocity of 0.2mm/min. There are only a few small sintered defects before forming, but there are a lot of cavity groups after forming. Cavitation failure occurs by nucleation, growth and interlinkage of cavities. The complex-shape gears can be formed by a sinter-forging technology when the sintering temperature is 1600°C and the superplastic forging temperature is 1550°C.

The influence of lubricants on uniaxial dry pressing of silanised silicon nitride powder

The influence of three surfactants (stearic acid, octadecylamine and octadecyl-trimethoxysilane) on the dry-pressing behaviour of silicon nitride powders was investigated with an instrumented compacting tool. The study reveals that friction processes have a large impact on the compaction behaviour as well as on the properties of green bodies (density, strength, gradients, macroscopic defects). All surfactants reduce friction by boundary lubrication. However, the functional groups cause different interactions of the surfactant molecule with the powder surface, and therefore, different lubrication properties. Octadecyl-trimethoxysilane can form a stable, covalent bond to the powder surface. Hence, it effectively reduces powder–powder friction, but is of little influence on powder–wall friction. This surfactant causes a significant increase of the green density but has only a limited effect on axial density gradients and strength of the green bodies. Stearic acid and octadecylamine have functional groups, which cause only adsorption processes. Therefore, such compounds exhibit an opposite lubrication behaviour with a high efficacy on the die wall. For silicon nitride, stearic acid is more effective than the octadecylamine due to a stronger adsorption of the amine to the silica-like powder surface.

Improvement in the colloidal processing of concentrated silicon nitride slips with ammonium polyacrylate by an yttria–alumina surface coating

Silicon nitride (Si 3N 4) powders coated with 6 wt.% Y 2O 3 and 4 wt.% Al 2O 3 were prepared by coprecipitation. For comparison, the Si 3N 4 powder containing the same amount of additives was also prepared by mechanical mixing. The resulting powders were dispersed in water with ammonium polyacrylate (NH 4PA) to produce 32 vol.% slips. The adsorption behavior and interfacial charge properties of coated Si 3N 4 slips with NH 4PA were investigated and compared with those of the mixed Si 3N 4 slips. Then, the influence of pH on the rheological properties of 32 vol.% coated and mixed Si 3N 4 slips was determined. The density of green cast samples was related to the degree of slip dispersion. In addition, the influence of the surface coating of Si 3N 4 on the sintering behavior was investigated. The surface coating improved the dispersion of 32 vol.% Si 3N 4 slips with NH 4PA; lower viscosity values of the coated Si 3N 4 slips with respect to the mixed ones were found in the pH range studied. The better slip dispersion of the coated Si 3N 4 slips resulted in higher slip cast packing density compacts. The second stage of densification (solution precipitation) and the α- to β-phase transformation were promoted by the surface coating.

Effect of mechanical activation on synthesis of ultrafine Si3N4–MoSi2 in situ composites

Abstract Si 3 N 4 –MoSi 2 in situ composite has been synthesized by reacting powders of molybdenum (Mo) and silicon nitride (Si 3 N 4 ). Mo and Si 3 N 4 powders mixture in a molar ratio of 1:3 were ball milled for 0–100 h. The milled and unmilled powder mixtures were reacted at different temperatures between 1000 and 1600 °C in an argon atmosphere. The effect of mechanical activation (MA) induced by milling has been studied through X-ray diffraction (XRD), differential thermal analysis (DTA), and thermo-gravimetric analysis (TGA). No peaks of Mo in the XRD pattern have been observed after 70 h of milling. The crystallite size of the Mo has been found to be the lowest (41 nm) after milling for 30 h. Similarly, a 100 nm lowest size of crystallite of Si 3 N 4 was observed after milling for 50 h. DTA and TGA results show that the reaction between Mo and Si 3 N 4 enhances with increase in milling time. Milling for 10 h lowers the pyrolysis temperature by 150 °C. Additional milling upto 100 h does not lead to further reduction in the pyrolysis temperature. The intensities of peaks of MoSi 2 in the pyrolysed samples increased with increase in milling time. MoSi 2 particles of size less than 1 μm were observed to be uniformly distributed through out the Si 3 N 4 matrix.

Studies on Atmospheric Ageing of Nanosized Silicon Nitride Powders

Ageing of nanosized silicon nitride powder produced by the vapor phase reaction of sili- con tetrachloride and ammonia has been studied in argon gas, in dry air and in air of 80% humidity, respectively, for 3 to 540 days. No remarkable changes were observed either in the bulk or the sur- face properties on storage in argon gas for 90 days. However, samples stored in dry air and humid air, respectively, exhibited remarkable oxidation and formation of N-H species. It was proved that nanosized silicon nitride powders should be stored and processed in oxygen-free conditions.

Fabrication of Mixed-Oxide Sequential Deposited Silicon Nitride Powders

Fabrication of Mixed-Oxide Sequential Deposited Silicon Nitride Powders

Modelling of High-Power Grinding for Sintering Activation of Silicon Nitride Powders

Modelling of High-Power Grinding for Sintering Activation of Silicon Nitride Powders

Simultaneous direct determination of aluminum, calcium and iron in silicon carbide and silicon nitride powders by slurry-sampling graphite furnace AAS.

A fast and accurate analytical method was established for the simultaneous direct determination of aluminum, calcium and iron in silicon carbide and silicon nitride powders by graphite furnace atomic absorption spectrometry using a slurry sampling technique and a Hitachi Model Z-9000 atomic absorption spectrometer. The slurry samples were prepared by the ultrasonication of silicon carbide or silicon nitride powders with 0.1 M nitric acid. Calibration curves were prepared by using a mixed standard solution containing aluminum, calcium, iron and 0.1 M nitric acid. The analytical results of the proposed method for aluminum, calcium and iron in silicon carbide and silicon nitride reference materials were in good agreement with the reference values. The detection limits for aluminum, calcium and iron were 0.6 microg/g, 0.15 microg/g and 2.5 microg/g, respectively, in solid samples, when 200 mg of powdered samples were suspended in 20 ml of 0.1 M nitric acid and a 10 microl portion of the slurry sample was then measured. The relative standard deviation of the determination of aluminum, calcium and iron was 5 - 33%.

Effect of silicon nitride addition on the thermal and mechanical properties of magnesium silicon nitride ceramics

The effect of silicon nitride (Si 3N 4) addition on the sintering of magnesium silicon nitride (MgSiN 2) powder has been examined using a hot-pressing technique. The MgSiN 2 powder compacts with 1, 4, and 9 mol% of Si 3N 4 addition [sintering aid: 1 mol% ytterbium oxide (Yb 2O 3)] were hot-pressed at a temperature between 1550 and 1800 °C for 90 min in a nitrogen (N 2) atmosphere under the pressure of 75 MPa. The relative density of a MgSiN 2 compact with 4 mol% of Si 3N 4 addition hot-pressed at 1600 °C for 90 min showed a maximum (98.6%), although the relative density was reduced by the addition of Si 3N 4 at other hot-pressing temperatures. The maximum fracture toughness (6.6 MPa·m 1/2) was achieved for the MgSiN 2 specimen with 4 and 9 mol% of Si 3N 4 addition hot-pressed at 1600 °C for 90 min. This value was 2.5 times higher than that of MgSiN 2 specimen without Si 3N 4 addition (2.6 MPa·m 1/2). The improvement of the fracture toughness appeared to be due to the elongation of the Si 3N 4 grains, which had occurred in the presence of a liquid phase during the hot pressing. The thermal conductivity of the MgSiN 2 specimen with 9 mol% of Si 3N 4 addition hot-pressed at 1750 °C for 90 min increased to 34.1 W m −1 K −1, which is to our knowledge, the highest among the values so far reported for MgSiN 2 ceramics.

Band gap engineering of SiCN film grown by pulsed laser deposition

The band gap tuning of amorphous silicon carbon nitride thin (a-SiCN) film was demonstrated in the range of 2.3–3.0 eV by pulsed laser deposition using mixed targets. a-SiCN films were grown on silicon and quartz glass substrates at room temperature in a vacuum. Targets were fabricated by compacting a mixture of silicon carbide and silicon nitride powders. The stoichiometry of the film could be varied by the mixing ratio of the target. Ternary phase SiCN films were deposited at 30– 70 wt. % SiC in a target and their band gaps were controlled by appropriate adjustment of the carbon content. These findings indicate that the growth of a-SiCN films with a mixed target and their subsequent use as optoelectronic materials is a possibility.

Effect of surface silanisation on the dry pressing behaviour of silicon nitride powder

The influence of surface modification of silicon nitride powders with alkylsilanes on their compaction behaviour was studied with an instrumented compacting tool. The powder–powder interactions were investigated by viscosity measurements additionally. All silanes , even those with short alkyl chains, reduce interparticular interactions (e.g. hydrogen bridging) and therefore improve the compaction behaviour of ceramic granules in general.

Gas Pressure Sintering of Silicon Nitride Powder Coated with Al2O3 and TiO2

Gas pressure sintering kinetics of silicon nitride powder coated with 10 wt% (9:1) Al2O3 and TiO2 have been studied at 1850°C with a pressure schedule of 0.3 MPa in the first stage and 1 MPa in the second stage. The rates have been analyzed with a liquid‐phase sintering model. Diffusion‐controlled intermediate‐stage kinetics have been observed. The role of second‐step pressurization with nitrogen and argon has been determined by monitoring the kinetics. Pressurization at an earlier stage (∼90% relative density) reduces the densification rate but produces a denser material at the final stage. Although final density is greater, a porous surface layer forms on samples sintered with argon pressurization at the second stage. No such porous layer is formed in the case of pressurization with nitrogen. The mechanism of the intermediate‐stage kinetics has been discussed with respect to the nature of the product analyzed by XRD after sintering.

Qualitative Analysis of Silicon Nitride Synthesized by Ammonolysis of Silicon Tetrachloride

Silicon nitride powder synthesis, performed by silicon tetrachloride ammonolysis, is described in this work. Gaseous ammonia reacts with liquid SiCl 4 , at low temperature and inert atmosphere. At a second step, a powder pyrolysis of silicon diimide is performed between 1300 and 1500°C, for ammonium chloride removal and Si 3 N 4 formation. Phase formation is evaluated by X-ray diffraction. Particle medium size and form is determined by Scanning Electron Microscopy.

Erosion characteristics of silicon nitride ceramics

Abstract Silicon nitride ceramics with three different microstructures were prepared using three different silicon nitride powders and β-silicon nitride whiskers in order to study their erosion characteristics. Samples with a coarse microstructure were prepared by using a commercially available silicon nitride powder seeded with the whiskers. Samples with fine microstructures were obtained using fine silicon nitride powders that had been milled in toluene or in distilled water. The powder milled in toluene gained both carbon and oxygen during milling. Heating the powder at 1673 K for 10 h in a flowing nitrogen atmosphere reduced carbon and oxygen contents to the level of as-received powder. The powder milled in distilled water gained a significant amount of oxygen during milling, and silicon oxynitride (Si2N2O) was detected by X-ray diffraction analysis (XRD) from the sample sintered with it. The sample sintered with the powder milled in distilled water showed the finest microstructure and the lowest erosion rate among the samples. Erosion occurred mainly by dislodgment of grains.