Kinetics of Silicon Monoxide Ammonolysis for Nanophase Silicon Nitride Synthesis

Silicon monoxide vapor generated from Si/SiO₂ mixed-powder compacts was used with NH₃ to synthesize silicon nitride in a tubular flow reactor operated at temperatures in the range of 1300°-1400°C. The ammonolysis of SiO with excess NH₃ was very rapid, yielding three different types of silicon nitride at different longitudinal locations in the reactor: amorphous nanophase powder of an average size of about 20 nm, amorphous whiskers of a few micrometers in diameter, and α-polycrystals. The amorphous products were heat-treated for crystallization at temperatures between 1300° and 1560°C in a stream of dissociated NH₃, N₂, or N₂/H₂ mixture gas. When dissociated NH₃ was used, nanophase powder was crystallized at 1300°C. The yield of nanophase silicon nitride from SiO varied from 13% to 43%, depending on operating conditions.     

"Alumina" Surface Modification of Silicon Nitride for Colloidal Processing

Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si₃N₄ behave like Al₂O₃ in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO₃)₃ during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri- sec -butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and rheological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si₃N₄ powder, all rheological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al₂O₃ powder. Although electrophoretic and rheological data showed that the coated powder behaved like Al₂O₃, consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al₂O₃ slurries.     

Highly Creep-Resistant Silicon Nitride/Silicon Carbide Nano–Nano Composites

A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/silicon carbide composite with a nano–nano structure (nanosized SiC and Si₃N₄ in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si–C–N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nano-sized SiC included in sub-micron Si₃N₄) to nano–nano type. Nanocrystalline silicon nitride/silicon carbide ceramic composite with 30–50 nm grain size was synthesized without using sintering additive.     

Sintered Reaction-Bonded Silicon Nitride with High Thermal Conductivity and High Strength

Sintered reaction-bonded silicon nitride (SRBSN) materials were prepared from a high-purity Si powder doped with Y₂O₃ and MgO as sintering additives by nitriding at 1400°C for 8 h and subsequently postsintering at 1900°C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four-point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high-thermal conductivity silicon nitride ceramics that were prepared by sintering of Si₃N₄ powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Aqueous Processing of Sintered Reaction-Bonded Silicon Nitride: I, Dispersion Properties of Silicon Powder

An aqueous-based system (Si-Al₂O₃-Y₂O₃-Fe₂O₃) for processing sintered reaction-bonded silicon nitride (SRBSN) was investigated with an emphasis on chemical control of suspension component interactions. Chemical stability and dispersion properties of a commercial silicon powder were characterized using electroacoustic, adsorption isotherm, and rheological measurements. The interactions of silicon with nitriding agent, sintering aids, dispersants, and binder were considered. The effects of pH, electrolyte, aging, particle size, and solids loading were examined. The suspension properties of the silicon powder were influenced by the native oxide film and powder treatment history. The silicon-oxide composite particles exhibit dispersion behavior similar to silica, characterized by a negative surface potential above pH 2. A method to improve the dispersion and homogeneity of suspension components based on the use of quaternary amine dispersants is proposed.     

Combustion Synthesis of Si3N4 by Selective Reaction of Silicon with Nitrogen in Air

Silicon nitride (Si₃N₄) was synthesized by a selective combustion reaction of silicon powder with nitrogen in air. The α/β-Si₃N₄ ratio of the interior product could be tailored by adjusting the Si₃N₄-diluent content in the reactant mixtures. The synthetic β-Si₃N₄ showed a well-crystallized rod-like morphology. Mechanical activation greatly enhanced the reactivity of silicon powder, and the slow oxidation of silicon at the sample surface promoted the combustion reaction in air. The formation mechanism of Si₃N₄ was analyzed based on a proposed N₂/O₂ diffusion kinetic model, and the calculated result is in good agreement with the experimental phenomenon.     

Reaction Synthesis of Magnesium Silicon Nitride Powder

The synthesis of magnesium silicon nitride (MgSiN₂) by direct nitridation of a Si/Mg₂Si/Mg/Si₃N₄ powder mixture is described. A nucleation period at 550°C and stepwise heat-treatment schedule up to 1350°C was adopted for the synthesis of MgSiN₂ powder, based on TG-DTA measurements. The influence of the ratio of constituents on the final phase composition also has been studied. The content of magnesium and silicon in the starting powder should fulfill the conditions Mg₂Si/Mg ≥ 3 and Si₃N₄/Si_tot≥ 0.5 to obtain single-phase MgSiN₂. The silicon particle size of <0.5 μm is preferable to decrease the time of nitridation. The oxygen content of as-synthesized powders is in the range 0.9–1.2 wt%. However, the oxygen content of MgSiN₂ powder decreases further by the addition of 2 wt% CaF₂ or 0.75 wt% carbon and reaching the lowest value of 0.45 wt% oxygen after carbothermal reduction in an alumina-tube furnace.     

Solid-State 29Si NMR Analysis of Amorphous Silicon Nitride Powder

Solid-state ²⁹Si NMR techniques were used to characterize laser-synthesized silicon nitride powder prepared from the reaction of silane with ammonia. When the powder is exposed to water vapor, a hydrated layer rapidly forms at the surface. A comparison of ²⁹Si cross polarization (CP) and Bloch decay (BD)-MAS-NMR spectra revealed differences between surface and bulk compositions. CP-NMR identified Si-NH_x (x = 1, 2) species with a chemical shift of −45 ppm in the as-synthesized ( unexposed ) powder. In BD-NMR spectra, the nitride resonance is observed at −48 ppm. For the hydrated powder, CP-NMR identified additional =Si-OH ( Q³ ), =Si-(OH)₂ ( Q² ), and SiO₂ ( Q⁴ ) species present at the surface. The CP-NMR spectra were corrected for T_lpH relaxation effects and deconvoluted into individual components in order to extract quantitative measurements of the various species present.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

Combustion Synthesis of Silicon Nitride-Silicon Carbide Composites

The feasibility of synthesizing silicon nitride-silicon carbide composites by self-propagating high-temperature reactions is demonstrated. Various mixtures of silicon, silicon nitride, and carbon powders were ignited under a nitrogen pressure of 30 atm (∼ 3 MPa), to produce a wide composition range of Si₃N₄-SiC powder products. Products containing up to 17 vol% of SiC, after being attrition milled, could be hot-pressed to full density under 1700°C, 3000 psi (∼ 21 MPa) with 4 wt% of Y₂O₃. The microhardness and fracture toughness of these composites were superior to those of the pure β-Si₃N₄ matrix material and compared very well with the properties of "traditionally" prepared composites.     

New Strategies for Preparing NanoSized Silicon Nitride Ceramics

We report the preparation of nanosized silicon nitride (Si₃N₄) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si₃N₄ and sintering additives of 5-mol% Y₂O₃ and 2-mol% Al₂O₃ was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline β-Si₃N₄ particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si₃N₄ grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness.     

Thermal Stability, Phase Evolution, and Crystallization in Si-B-C-N Ceramics Derived from a Polyborosilazane Precursor

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazane, {B[C₂H₄Si(H)NH]₃} _n , were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X-ray diffraction (XRD). The phase evolution was additionally followed by ¹¹B and ²⁹Si MAS NMR as well as by FT-IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.     

Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

Titanium diboride (TiB₂) was hot-pressed at a temperature of 1800°C, and silicon nitride (Si₃N₄) was added as a sintering aid. The amount of Si₃N₄ that was added had a significant influence on the sinterability and mechanical properties of the TiB₂. When a small amount (2.5 wt%) of Si₃N₄ was added, the Si₃N₄ reacted with titania (TiO₂) that was present on the surface of the TiB₂ powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO₂). The elimination of TiO₂ suppressed the grain growth effectively, which led to an improvement in the densification of TiB₂. The formation of SiO₂ also was deemed beneficial for densification. The mechanical properties-especially, the flexural strength-were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si₃N₄ was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si-Ti-O-N phase at the grain boundaries.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric-Field-Assisted Sintering

Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm³) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si₃N₄ increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide.     

Simulation of Cold Compaction Densification Behavior of Silicon Nitride Ceramic Powder

The cold-compaction densification behavior of silicon nitride (Si₃N₄) ceramic powder was analyzed using two models: the Shima model and the Cam-Clay model. Triaxial-compression experimental data were used to evaluate these two models. Shima models that used Si₃N₄ matrix material with different yield stresses were discussed. It is clear that the Cam-Clay model can effectively simulate the cold-compaction densification behavior of Si₃N₄ ceramic powder. The Shima model that used a very high yield stress for the Si₃N₄ matrix material slightly overestimated the experimental data, and the Shima model that used the actual yield stress for Si₃N₄ matrix material largely overestimated that data.     

Dissolution Rate of Silicon Nitride Ceramics into Molten CaO–Al2O3–SiO2 Slags

The dissolution rates of silicon nitride (Si₃N₄) ceramics into CaOAl₂O₃SiO₂ slags were investigated by using a rotating specimen method in the temperature range of 1773–1873 K. Dissolution rates in the present study increased as the revolution speed and temperature increased and decreased as the SiO₂ content of the slags increased. The nitrogen content of the slags increased after the Si₃N₄ ceramics had been immersed into them. The slags contained two types of nitrogen ions—N³⁻ and CN^-—because a graphite crucible was used for the experiment. N³⁻ ions were confirmed in all the slags that were used in the present work; the CN^- content was much lower than that of the N³⁻ ions, except in the slag without SiO₂. Based on those results, Ficks law of diffusion was used to analyze the dissolution rates. The dissolution mechanism of the Si₃N₄ ceramics into CaO–Al₂O₃SiO₂ slags has been discussed in this paper.     

Formation of Reaction-Bonded Silicon Nitride Using Microwave Heating

Rod-shaped silicon compacts were nitrided in a microwave applicator using a minimum of insulation in order to maximize the temperature gradients to effect an inside-out reaction. These specimens exhibited very nonuniform conversion to Si₃N₄, and fully nitrided areas had poor microstructures consisting of alternating regions of high and low density. The key factor was found to be sintering of the silicon powder during initial heating which caused the specimens to be electrically conducting during the early stages of the reaction and led to large changes in the microwave heating behavior of the specimens as they nitrided. The temperature and composition profiles in the specimens were simulated using a numerical model. The results of the model indicated that the observed microstructures were caused by high temperatures and temperature gradients in the areas of maximum nitridation rate which caused silicon vapor to diffuse within the specimens. Some compacts were made from a mixture of silicon powder and silicon nitride powder to avoid sintering of the silicon particles. These specimens nitrided uniformly with inside-out composition profiles, indicating that microwave heating would be beneficial for the nitridation of pure silicon powder compacts if sintering could be avoided.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.