Synthesis of silicon and silicon nitride powders by vapour-phase reactions

The Bureau of Mines is investigating vapour-phase reactions for syntheses of high-purity, nanometre-sized silicon and Si₃N₄ powders suitable for industrial applications. In this study, submicrometre-sized silicon powder was formed by a vapour-phase reaction of SiCl₄, NH₃, and magnesium vapor at temperatures ranging from 950–1050°C. The silicon was subsequently nitrided to form Si₃N₄ at 1150°C.     

Oxygen distribution in silicon nitride powders

The oxygen content of ten different silicon nitride powders was determined by bulk chemical analysis and surface-sensitive X-ray photoemission spectroscopy (XPS). In silicon nitride powders prepared from silicon and silica by nitridation and carbothermal reduction in a nitrogen atmosphere, respectively, only a minor part of the total oxygen content of 0.9 to 2.5 wt% was found in a surface layer of less than 1 nm thick, whereas an appreciable amount can be attributed to oxygen dissolved in the bulk. Powders made by silicon diimide decomposition, however, are characterized by a higher oxygen concentration at the particle surface relative to the bulk, which may be further reduced by chemical treatment. The surface layer composition corresponds to an intermediate state between silica and silicon oxynitride.     

Characterization of wurtzite indium nitride synthesized from indium oxide by In-115 MAS NMR spectroscopy

Wurtzite indium nitride (w-InN) powders synthesized from the reaction of indium oxide (In₂O₃) with ammonia were characterized by ¹¹⁵In magic-angle spinning (MAS) NMR spectroscopy and nitrogen analyzer. The powders were not a single phase of w-InN but a mixture of w-InN and In-incorporated w-InN. The incorporation of In metal in InN lattice due to thermal decomposition caused the ¹¹⁵In MAS NMR peak of w-InN to be downfield shifted and might be responsible for the increase in the band gap of w-InN.     

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 Si₃N₄ 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 Si₃N₄ 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.     

Interference coatings based on synthesized silicon nitride

Silicon nitrides are synthesized by ion-assisted deposition with only one coating material and a nitrogen-ion-beam source. All the SiN(x) films are amorphous and mechanically strong. A wide range of refractive indices from 3.43 to 1.72 at a wavelength of 1550 nm is obtained. Near-IR antireflection coating and a bandpass filter based on the multilayers of SiN(x) and Si are demonstrated.     

Hot-press sintering studies of amorphous silicon nitride powders

The hot-press sintering behaviour of amorphous Si₃N₄ powders prepared from the ammonia pyrolysis of polycarbosilane or hydridopolysilazane polymers was studied. In the presence of yttria and alumina, the amorphous powders sintered to > 98% of theoretical density at 2023 K. Both the microstructure and average four-point MOR bend strengths, of test bars machined from the sintered compacts were comparable to those obtained from UBE SN-E-10 Si₃N₄ powder processed under the same conditions. However, in contrast to commercial crystalline powders, between approximately 1690 and 1700 K the amorphous Si₃N₄ powders underwent a rapid shrinkage corresponding to 50–60% of the total densification. In this narrow temperature regime, a radical change in morphology and phase composition of the amorphous powder occurred. Prior to 1690 K, the Si₃N₄ powders were totally amorphous as determined by X-ray diffraction analysis and consisted of angular shards with an average particle size of 2–3 m. Samples quickly cooled after heating to 1700 K, consisted of a 53/47 mixture of equiaxed and Si₃N₄ crystallites with an average particle size of 0.1–0.3 m. Thus, the rapid densification at R~ 1700 K is identified with the amorphous to ( + ) transition. Beyond 1700 K, these samples gradually densified to the maximum density as mentioned. The fully densified samples consisted of 100% -Si₃N₄ phase.     

Characterization of silicon nitride single crystals and polycrystalline reaction sintered silicon nitride by microhardness measurements

Identification of - and -phases of Si₃N₄ single crystals grown from Si melt could be made with the help of Vickers microhardness measurements. The effect of chemical additives, e.g. metallic Fe and BaF₂, on the microhardness of Si₃N₄ was also determined. Different constants involved in the empirical Meyer relationship between load and indentation diameters could be correlated with the porosity and microhardness of Si₃N₄ single crystals and polycrystalline, reaction sintered Si₃N₄.     

Phase and morphology evolution of magnesium niobate powders synthesized by solid-state reaction

Magnesium niobate (MgNb₂O₆; MN) powders have been prepared and characterized by TG-DTA, XRD, SEM and EDX techniques. The effect of calcination temperature, dwell time and heating/cooling rates on phase formation, morphology and chemical composition of the powders are examined. The calcination temperature and dwell time have been found to have a pronounced effect on the phase formation of the calcined magnesium niobate powders. It has been found that the minor phases of unreacted MgO and Nb₂O₅ phases tend to form together with the columbite-type MgNb₂O₆ phase, depending on calcination conditions. It is seen that optimisation of calcination conditions can lead to a single-phase MgNb₂O₆ in an orthorhombic phase. Higher calcination times and heating/cooling rates clearly favoured particle growth and the formation of large and hard agglomerates.     

Characterization of wurtzite type boron nitride synthesized by shock compression

Graphite-like boron nitride was shock-compressed up to 550 kbar. The X-ray diffraction pattern of the recovered specimen identified the wurtzite structure whose hexagonal cell dimensions were $\text{a}{}_0\text{= 2.553±0.003}\text{A}\text{?}$, $\text{c}{}_0\text{= 4.228±0.004}\text{A}\text{?}$. No trace of zinc blende form was found. The conversion rate was related to the crystallization of the starting material. The density of the wurtzite type boron nitride was proved to be smaller by about 1 % than that of zinc blende type boron nitride synthesized under static compression.     

Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si₃N₄) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si₃N₄ powders was studied, and the results were compared with a commercial powder. The α-Si₃N₄ powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm^?3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si₃N₄ powders produced by CRN from silica initially containing 5 wt.% Y₂O₃. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.     

Characterization of amorphous niobium silicates powders synthesized by polymeric precursor method

In this paper 4.5SiO₂–3Al₂O₃–xNb₂O₅–2CaO powders have been synthesized using a chemical process the Polymeric Precursor Method. The process of glass formation has been investigated by XRD and DTA, the results confirm that the prepared powders are glasses. Experimental data show that amount of Nb₂O₅ had a considerable effect on the T_g values. The structures of glasses prepared have been determined by ²⁹Si and ²⁷Al MAS NMR and the results indicated that the network is formed by SiO₄ and AlO₄ tetrahedral linked and probably Si–O–Nb bonds are present in the vitreous network.     

Cavitation asymmetry in silicon nitride by scanning laser acoustic microscopy

Scanning laser acoustic microscopy (SLAM) was used to visualize creep damage distribution in the gas-pressure-sintered silicon nitride after creep at 1300°C in 4-point bending. SLAM revealed asymmetrical distribution of creep damage beneath the tensile surface and in a narrow zone that spread continuously across the neutral axis toward the compression surface. Scanning and transmission electron microscopy confirmed the presence of cavities in tensile zone of bending bar. The combination of the homogeneous cavity development in the zone of tensile stresses and formation of the damage zone ahead of main crack was proposed to explain such cavity distribution. Current SLAM observation is a direct evidence of cavitation asymmetry in vitreous bonded ceramics. Such distribution supports the model of simultaneous presence of cavitation and non-cavitation creep mechanisms in silicon nitride and similar ceramics.     

Structures and purification of boron nitride nanotubes synthesized from boron-based powders with iron particles

Boron nitride (BN) nanotubes, nanohorns and nanocoils were synthesized by annealing Fe₄N/B, FeB and Fe/B powders at 1000 °C for 1–24 h in nitrogen gas atmosphere, and large amounts of BN nanotubes were obtained by annealing Fe₄N/B. The growth mechanism and atomic structures were investigated on cup-stacked BN nanotubes synthesized from Fe₄N/B by X-ray diffraction, high-resolution electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. As-produced BN soot was purified by removing non-BN nanomaterials such as metal catalyst particles and unreacted boron, and high purity BN nanotubes were obtained.     

Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si₃N₄ green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si₂N₂O was formed through reaction between Si₃N₄ and silica sol at a temperature above 1550 °C. Si₃N₄/Si₂N₂O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180 MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39 GHz) were obtained by varying infiltration cycle and sintering temperature.