Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Preparation of Silicon Nitride Whiskers from Diatomaceous Earth: I, Reaction Conditions

Whiskers and powder of silicon nitride were prepared by the carbothermal reduction and nitridation of diatomaceous earth in the presence of flowing N₂ and NH₃. The optimum temperature for the formation of Si₃N₄ whiskers was 1350°C and the yield reached almost 20% after 24 h. The α-Si₃N₄ content decreased with increasing nitridation temperature. Yields of the whiskers were dependent on NH₃ concentration and the total gas feed rate. The maximum yield of inside whiskers was obtained for a 25 vol% NH₃/N₂ mixture, while the maximum quantity of outside whiskers was produced for 75 vol% NH₃/N₂. The sum of the yield of the inside and outside whiskers increased with decreasing total gas feed rate. However, no nitridation of SiO₂ was observed at a feed gas rate below 0.18 mmol·min⁻¹. The yield of the inside whiskers increased gradually with increasing reaction time up to 36 h, whereupon a constant value was attained. Although the amount of outside whiskers produced was relatively small, the quantity seemed to increase until 60 h.     

Synthesis and Characterization of Amorphous Si2N2O

Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si₂ p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si₂ p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The ²⁹Si NMR analysis showed the formation of (amorphous) Si₃N₄ (Si–N₄) and possibly two oxynitride phases (Si–N₃O, Si–N₂O₂). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si₂N₂O) phase, the formation of phase–pure, amorphous Si₂N₂O is extremely difficult via this route.     

Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions

The influence of ammonium chloride (NH₄Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO₂) suspensions of varying solids volume fraction (φ_s) was studied. SiO₂ suspensions with low NH₄Cl concentration (≤0.05 M , pH 5.2) exhibited Newtonian behavior and a constant settling velocity ( U ). The volume fraction dependence was well described by the Richardson–Zaki form, U = U ₀(1 −φ_s) ⁿ , where n = 4.63 and U ₀= 1.0419 × 10⁻⁵ cm/s. At higher NH₄Cl concentrations (0.07–2.0 M , pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5 M NH₄Cl and exhibited a similar dependence on salt concentration. Above 0.5 M NH₄Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.     

Co-Additions of TiO2 and SiO2 Crystalline Fillers to Tailor the Properties of BaO–ZnO–B2O3–SiO2 Glass for Application to Barrier Ribs of Plasma Display Panels

The influence of co-additions of crystalline TiO₂ and SiO₂ fillers (10 wt% addition in total) to BaO–ZnO–B₂O₃–SiO₂ glass on resultant properties was investigated from the viewpoint of applying the material to the barrier ribs of plasma display panels. The substitution of SiO₂ for TiO₂ reduced the dielectric constant significantly, while it maintained high optical reflectance and appropriate coefficient of thermal expansion (CTE) in the case when TiO₂ alone was used. A 5–7.5 wt% SiO₂ addition with 2.5–5 wt% TiO₂ under the constraint of 10 wt% total fillers demonstrated an optical reflectance of about 55%, a CTE of about 8.3 × 10⁻⁶ K⁻¹ (compatible with glass panels), and a dielectric constant of about 7.5, which are promising properties for the barrier rib application.     

Oxidation Behavior of Silicon Yttrium Oxynitride

The oxidation behavior of the silicon yttrium oxynitride Y₁₀Si₇O₂₃N₄, so-called H-phase, in the temperature range 700–1400°C has been investigated. A nitrogen retention phenomenon in the oxidation product Y_4.67(SiO₄)₃O (O-apatite) is discussed. The H-phase is one of the four quaternary compounds identified in hot-pressed Si₃N₄ materials fabricated within the Si₃N₄–SiO₂–Y₂O₃ pseudoternary system.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Preparation of Fiberlike Silicon Nitride from Diatomaceous Earth

Fiberlike Si₃N₄ was prepared by the carbothermal reduction of diatomaceous earth in a flow of nitrogen and ammonia. Diatomaceous earth, which is an inexpensive raw material, is composed of 82.5 wt% SiO₂, 5.69 wt% Al₂O₃, and a very small amount of metal oxides (K₂O, CaO, and Fe₂O₃). Two types of fiberlike Si₃N₄ were obtained, short needlelike fiber and woollike fiber with Fe droplets at 1350°C.     

Grain-Boundary Microstructure and Chemistry of a Hot Isostatically Pressed High-Purity Silicon Nitride

Two high-purity Si₃N₄ materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser-derived Si₃N₄ powder with different oxygen contents. High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) analysis of the Si₃N₄ materials showed the presence of an amorphous SiO₂ grain-boundary phase in the three-grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The chemical composition of the grain-boundary film was SiN_xO_y, (x ∼ 0.53 and y ∼ 1.23), and the triple pocket corresponded to the amorphous SiO₂ containing ∼2 wt% nitrogen. The equilibrium grain-boundary-film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO₂ grain-boundary phase, because the concentration of foreign ions has been shown to affect the grain-boundary thickness.     

Alkali-Resistant Magnesium Aluminosilicate Oxynitride Glasses

Glasses containing up to 3.3 wt% nitrogen were prepared in the system MgO─Al₂O₃─SiO₂─AIN─Si₃N₄. GlasGlass transition temperature, density, and elastic properties increased with increasing nitrogen content. Resistance against aqueous alkaline solutions was determined by weight loss measurements, solution and surface analysis. The good durability of both oxide and oxynitride glasses is caused by a protective surface layer. The introduction of nitrogen decreases the alkaline attack. The data suggest that nitrogen is incorporated as Si─N boxhs.     

Nitridation of Amorphous Silica with Ammonia

The reaction between amorphous silica and ammonia in the temperature range 200° to 1230°C has been investigated. The reaction process was monitored with respect to the nitrogen content of the reaction product, the specific surface area of the amorphous nitrided silica, and the decomposition of ammonia. A surface reaction was observed at temperatures between 300° and 500°C, but in agreement with other studies bulk reaction only occurred above 800°C, reaching its maximum rate at about 1000°C. It is suggested that the decomposition of ammonia, which also becomes important above 800°C, is essential for the bulk nitridation reaction. At temperatures above 1050°C the nitridation yield decreases, until gas-phase reaction between SiO( g ) and N₂ or NH₃ becomes dominant at 1230°C, leading to the formation of α-Si₃N₄.     

Phase Relations in the System Iron Oxide–NiO–SiO2 under Strongly Reducing Conditions

Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO₂ under strongly reducing conditions obtained by using CO₂–CO gas mixtures in controlled proportions. The phase relations were determined with the well-known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted- and reflected-light microscopy and X-ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe₂SiO₄–Ni₂SiO₄ solid solutions), oxide ("FeO"–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The "ternary peritectic" point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%"FeO", 47 wt% NiO, 34 wt% SiO₂.     

Synthesis and Mechanism of Nanosized AlN from an Aluminum Oleic Emulsion Using Carbothermal Reduction at Low Temperatures

Nanosized Al₂O₃ particles homogeneously dispersed in a matrix of amorphous carbon (a-C) were prepared by decomposition of an aluminum oleic emulsion at 600°C in Ar. Nanosized aluminum nitride (AlN) grains were prepared by carbothermal reduction and nitridation (CRN) of this Al₂O₃–a-C mixture in NH₃ using graphite, BN, and alumina crucibles or boats. The phases formed by CRN were identified by X-ray diffraction analysis. The morphology and grain size of the AlN were determined by transmission electron microscopy. The formation of single-phase AlN was achieved at temperatures as low as 1150°–1200°C in NH₃ using a cylindrical graphite crucible with holes in its two flat faces. Mass spectroscopy (MS) showed that a significant amount of HCN and a minor amount of C₂H₂ are formed at 500°C by reaction of NH₃ with carbon at the decomposition temperature of NH₃. A most probable formation mechanism of the AlN from nanosized Al₂O₃ and a-C in NH₃ is discussed on the basis of MS results and thermodynamic considerations.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Impurity-Dependent Morphology and Grain Growth in Liquid-Phase-Sintered Alumina

The alumina grains in liquid-phase-sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO + BaO + Na₂O + K₂O) ratio in the chemical composition controls the morphology in LPS alumina that is 91–94 wt% pure. Within a given range of SiO₂ content (i.e., 4.3–5.2 wt% in the chemical composition), a low MgO:(CaO + BaO + Na₂O + K₂O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO₂ contents outside the 4.3–5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91–94 wt% when both the MgO:(CaO + BaO + Na₂O + K₂O) ratio and the SiO₂ content are relatively low. The sintered density generally increases as the SiO₂ content in the chemical composition decreases.     

Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition

Thermodynamic calculations in the systems Al-Cl, Al-Cl-N, and H-Al-Cl-N were used to assess the capabilities of AlCl₃ or mixtures of AlCl₃ with Al to produce AlN by chemical vapor deposition (CVD) techniques. Direct nitridation (N₂ as reaction agent) is possible only at high temperatures (≥1500 K), using AlCl₃–Al mixtures. Reaction with NH₃ at equilibrium gives low yields but the suppression of NH₃ dissociation yields near 100%, which makes the method suitable for powder production, coating, and single-crystal growth. AlN with less than 1 wt% oxygen was obtained from technical grade AlCl₃ by this process. The formation of both amorphous AlN powder and crystalline AlN coatings was observed. It is assumed that the formation of AlCl₃· x NH₃ adducts by mixing of Al-Cl vapor and NH₃ at temperatures ≤1273 K prevents NH₃ dissociation and favors the production of amorphous AlN.     

Acid-Base Equilibria in the System PbO–SiO2

The acid-base equilibria in the liquid silicates in the system PbO–SiO₂ are discussed, Data reported by Richardson and Webb, wherein the PbO activity is determined over a composition range of 0 to 60 mole % SiO₂, are used for comparison with activities computed from structural models with consideration of the acid-base equilibria. The results suggest that the liquid silicates in the system PbO–SiO₂, for the composition and temperature ranges studied, are constituted of a relatively low number of anionic species and that these anions are of a relatively small size (i.e., O_2–, SiO_4–, (SiO₃)₃⁶⁻. and (SiO_2.5)₆⁶⁻).     

Mechanically Enhanced Reactivity of Silicon for the Formation of Silicon Nitride Composites

The chemical reactivity of silicon can be enhanced by mechanical activation. Ball milling elemental silicon powders with a small amount of carbon addition at ambient temperature has resulted in an enhanced nitridation of silicon at high temperatures. In comparison with powder mixtures without milling, the nitridation process at 1250°C has been accelerated by a factor of 9 and 23 by milling powder mixtures in N₂ and NH₃, respectively, before nitridation. The enhanced nitridation process for powders milled in N₂ is primarily attributed to the mechanical activation, whereas for powders milled in NH₃, the trapped nitrogen within the powder mixtures during milling also contributes to the enhanced nitridation process, in addition to the effect of the mechanical activation.     

Microstructural Characterization of Superplastic SiO2-doped TZP with a Small Amount of Oxide Addition

The microstructures of 5 wt% SiO₂-doped TZP, 5 wt% (SiO₂+ 2 wt% MgO)-doped TZP, and 5 wt% (SiO₂+ 2 wt% Al₂O₃)-doped TZP are characterized by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. An amorphous phase is formed at multiple grain junctions but not along the grain-boundary faces in these three materials. A small addition of MgO and Al₂O₃ into the SiO₂ phase results in a marked reduction in tensile ductility of SiO₂-doped TZP. This reduction seems to correlate with segregation of magnesium or aluminum ions at grain boundaries and a resultant change in the chemical bonding state.     

Reaction Sintering of Cubic Boron Nitride Using Volatile Catalysts

Reaction sintering behavior of cBN, which is accompanied by the transformation from hBN to cBN in the presence of 30 wt% diamond seed grains, was investigated under high pressure conditions (6.0–7.5 GPa, 1400–1700°C, 0–30 min) using volatile catalysts such as NH₄NO₃, NH₄Cl, and NH₂NH₂. Fully dense sintered compacts having a Vickers microhardness of more than 5000 kg/mm² were prepared with no residual catalytic solid components. The activation energy for the conversion from hBN to cBN was 200–230 kJ/mol. Adsorbed N₂, H₂, and/or NH _x components which were formed by decomposition of these catalysts during high pressure and temperature treatments, would have a favorable kinetic effect on cBN formation from hBN and its simultaneous sintering.