Catalytic Effects of Metals on Direct Nitridation of Silicon

Catalytic effects were investigated on the direct nitridation of silicon granules, impregnated with 0.125–2.0% by mass of calcium, yttrium, iron, copper, silver, chromium, or tungsten, in a stream of nitrogen with 10% hydrogen, using a tubular flow reactor operated at temperatures ranging from 1200° to 1390°C. Calcium and yttrium suppressed the formation of β-silicon nitride while iron enhanced the formation of β-silicon nitride over the temperature range investigated. An addition of 0.125% calcium resulted in about 99% overall conversion with 100%α-phase and a 2.0% yttrium addition yielded an overall conversion over 98% with an α-phase content above 97%. Copper promoted not only the nitridation but the formation of α-silicon nitride at 1200°C, but enhanced the β-phase formation at higher temperatures. The role of liquid phases on the formation of α-/β-silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium, and tungsten.     

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.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

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.     

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.     

Nitridation Effect on Properties of Stannous-Lead Phosphate Glasses

Stannous-lead phosphorus oxynitride (Sn-Pb-P-O-N) glasses were prepared by remelting under an anhydrous ammonia atmosphere. Glasses that contained up to ∼4.2 wt% (9 at.%) of nitrogen were obtained. The rate of nitrogen dissolution was studied as a function of remelting time (3–66 h) and temperature (400°–600°C). The onset nitridation temperature was extrapolated to be 315°C; higher nitridation temperatures accelerated nitrogen dissolution. Nitridation of the stannous-lead oxyphosphate (Sn-Pb-P-O) glasses decreased the dissolution rate in water and the thermal expansion coefficient; however, it increased the dilatometric softening temperature, the glass-transition temperature, the microhardness, and the density. The chemical durability of the nitrided glasses increased more than four orders of magnitude with 3.0 wt% of nitrogen content. An increase in the lead oxide content in the stannous phosphate glasses also improved the chemical durability. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy suggested that nitrogen replaces the terminating hydroxyl ion and the bridging and nonbridging oxygen atoms in the PO4 tetrahedra to form the functional groups –NH<, –N<, and –N=, which leads to enhanced crosslinking of the glass network. Quantitative results regarding these bondings have been given.     

Mechanism of Nitridation of Silicon Powder in a Fluidized-Bed Reactor

Direct nitridation of 400 μm average-sized silicon granules, composed of 2 μm average-sized particles, was carried out in a fluidized-bed reactor. Nitridation progress was studied by transmission electron microscopy (TEM). TEM photomicrographs suggested that the formation of surface nitride layers on individual silicon particles and the subsequent spallation of these layers were the dominant process of the direct nitridation of silicon. The spallation was modeled based on a simple crack theory, and the critical thickness of nitride layers leading to spallation was estimated to be 57 nm. This agreed reasonably well with the experimentally observed values of 20–100 nm.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Synthesis of Oxynitride Powders via Fluidized-Bed Ammonolysis, Part I: Large, Porous, Silica Particles

Reaction of silica (SiO₂) with triethanolamine (TEA, N(CH₂CH₂OH)₃) and ethylene glycol (EG) under conditions (∼200°C) where byproduct water is removed resulted in the formation of the neutral silatrane glycolate complex, N(CH₂CH₂O)₃SiOCH₂CH₂OH (or TEASiOCH₂CH₂OH) in essentially quantitative yield. Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500°C, formed porous ceramic powders with high specific surface areas (>500 m²/g). These powders were nitrided via ammonolysis in a fluidized-bed reactor at temperatures of 700°-1000°C. The resulting nitrided powders were characterized by thermal and chemical analyses, diffuse reflectance infrared spectroscopy, gas sorption, and X-ray photoelectron spectroscopy. The apparent activation energy for the nitridation process was determined to be 54 kJ/mol. Following nitridation, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas >300 m²/g at 1000°C. Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density. This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow. The linear density increase also suggested that pore trapping did not occur under these processing conditions. This work serves as a model for ongoing studies on the nitridation of high-surface-area ceramic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxides.     

Kinetics and Mechanisms for Nitridation of Floating Aluminum Powder

Aluminum powder entrained by ammonia-containing nitrogen gas was reacted at various temperatures and time to form aluminum nitride powder. The kinetics of nitride formation were determined by a quantitative X-ray analysis and compared with those determined by a nitrogen analysis of the product. The conversion to aluminum nitride increased with the reaction time following a sigmoidal rate law. The reaction time for full conversion decreased as the temperature increased in the range 1050°–1300°C. The reaction rate constant at a given temperature was evaluated using the Avrami equation. The activation energy for the reaction was 1054 ± 91 kJ/mol in the temperature range of 1050°–1200°C, and decreased to 322 ± 70 kJ/mol above 1200°C. Comparative analysis of powders formed below and above 1200°C suggested strongly that the rate-controlling step changed from chemical reaction to mass transport above 1200°C.     

α-β Sialon Ceramics Made from Different Silicon Nitride Powders

The present work is concerned with the sintering of an α-β sialon ceramic using five different silicon nitride powders from a single source. The parameters varied in the silicon nitride were the amount of "free' silicon, iron content, α:β ratio, and grain size as measured by BET surface. The sintering atmosphere was varied by use of protective powder beds with passive (boron nitride) and active (SiO-generating) properties. Five sintering temperatures between 1600° and 1800°C were used. Microstructural characterization as well as density, hardness, and fracture toughness measurements were carried out. The sintering conditions were found to be critical for obtaining fully dense materials and low weight change. The optimum sintering temperature was 1750°C. The silicon nitride powder with a high content of free silicon resulted in a material which was more susceptible to the sintering atmosphere conditions. An α-β sialon made from a silicon nitride powder with a high β-α phase ratio resulted in a higher β-α ratio in the sintered material.     

High-Temperature Fracture Energy of Superplastically Forged Silicon Nitride

The fracture energy of superplastically forged silicon nitride, where rodlike silicon nitride grains are aligned in one direction, was investigated at high temperatures from 1100° to 1300°C and at room temperature. Bending tests using chevron-notched beams were conducted at two displacement rates, 0.05 and 0.005 mm/min. The superplastically forged silicon nitride showed remarkably high fracture energies, 200–630 J/m². The fracture energy was largely dependent on the temperature and the displacement rate. The high fracture energy was attributed to grain pullout enhanced by the softened grain-boundary glassy phase and the aligned rodlike grains.     

Effects of Sodium Silicate Exposure at High Temperature on Sintered α-Silicon Carbide and Siliconized Silicon Carbide

Sintered α-silicon carbide and siliconized silicon carbide were exposed to combustion off-gas containing sodium silicate vapors and particulates in a combustion test facility for 24 to 373 h at 900° to 1050°C. Degradation was evaluated by measuring dimensional changes, by measuring loss in strength due to changes in flaw population, and by evaluating surface corrosion morphology. It is suggested that passive oxidation and dissolution of the silica oxidation scale play an important role in the corrosion process. These mechanisms were enhanced by the continuous removal and replenishment of corrosive material by the high-velocity gas. These degradation phenomena caused surface pitting and an approximately 50% reduction in strength for both materials after long-term exposure (>100 h). Morphological evaluation suggested that the grain boundaries in the α-silicon carbide were oxidized more rapidly than the grains, while for the case of the siliconized silicon carbide the silicon phase was oxidized rapidly along with preferential oxidation of the silicon carbide grains parallel to the {0001} plains.     

Nitridation of Silicon Powders Catalyzed by Cobalt Nanoparticles

The effect of Co nanoparticles (NPs) on the nitridation of silicon (Si) was studied. Co NPs were deposited homogeneously on the surfaces of Si powders using an in situ reduction method using NaBH₄ as a reducing reagent. Si powders impregnated with 0.5–2.0 wt% Co NPs were nitrided in 1200°C–1400°C for 2 h. The resultant silicon nitride powders were characterized by XRD, FE‐SEM, TEM, and EDS. The results showed that: (1) Co NPs significantly decreased the Si nitridation temperature, and the nitridation could be completed at 1300°C upon using 2 wt% Co NPs as catalysts. For comparison, the Si conversion could not be completed even at a temperature as high as 1400°C in the case without using a catalyst; (2) many Si₃N₄ whiskers with 80–320 nm in diameter and tens micrometers in length were generated and uniformly distributed in the final products. They were single‐crystalline α‐Si₃N₄ grown along the [101] direction. The enhanced nitridation in the case of using Co NPs as a catalyst was attributed two following factors, the increased bond length and weakened bond strength in N₂ caused by the electron donation from the Co atoms to the N atoms.     

Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite

The substitution of between 0 and 1.6 wt% silicon (Si-HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si-HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si-HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.     

Densification and Cell Performance of Gadolinium-Doped Ceria (GDC) Electrolyte/NiO–GDC Anode Laminates

A thin film (60 μm thick) of a gadolinium-doped ceria (GDC) electrolyte was prepared by the doctor blade method. This film was laminated with freeze-dried 42 vol% NiO–58 vol% GDC mixed powder and pressed uniaxially or isostatically under a pressure of 294 MPa. This laminate was cosintered at 1100 °–1500 °C in air for 4–12 h. The laminate warped because of the difference in the shrinkage of the electrolyte and electrode during the sintering. A higher shrinkage was measured for the electrode at 1100 °–1200 °C and for the electrolyte at 1300 °–1500 °C. The increase of the thickness of anode was effective in decreasing the warp and in increasing the density of the laminated composite. The maximum electric power density with a SrRuO₃ cathode using 3 vol% H₂O-containing H₂ fuel was 100 mW/cm² at 600 °C and 380 mW/cm² at 800 °C, respectively, for the anode-supported GDC electrolyte with 30 μm thickness.     

Flexure Strength of Melt-Infiltration-Processed Titanium Carbide/Nickel Aluminide Composites

TiC/Ni₃Al composites were prepared using a simple melt-infiltration process, performed at either 1300° or 1400°C, with the Ni₃Al content varied over the range of 8–25 vol%. Densities >96% of theoretical were obtained for all composites. Four-point flexure strengths at 22°C increased as the Ni₃Al content increased (i.e., ∼1100 MPa at 20 vol% Ni₃Al), with the highest strengths being observed for composites processed at 1300°C, because of reduced TiC grain size. Strengths at elevated temperatures increased with test temperature, up to ∼1000°C. As with the yielding behavior of the Ni₃Al alloy used, a maximum in composite strength (∼1350 MPa) versus temperature was observed; this occurred at 950°C, which is ∼300°C above the yield maximum for the alloy. Extensive plastic strain was achieved in the composites even at high loading rates at 1135°C, and the yield stress was dependent on the applied loading rate.     

Synthesis of Silicon Carbide Thin Films with Polycarbosilane (PCS)

Polycarbosilane (PCS) thin films were deposited on silicon (and other) substrates and heat treated under vacuum (∼10_--6>torr)at temperatures in the range of 200°–1200°C. At temperatures in the range of 1000°–1200°C, the initially amorphous PCS films transformed to polycrystalline ß-silicon carbide (ß-SiC). Although PCS films could be deposited at thickness up to 2 μm, the films with thicknesses >1 μm could not be transformed to SiC without extensive cracking. The resulting SiC coatings were characterized using Fourier transform infrared spectroscopy, glancing-angle X-ray diffractometry, secondary-ion mass spectroscopy, Raman spectoscopy, transmission electron microscopy, and scanning electron microscopy. The temperature and time dependence of the amorphous-to-crystalline transition could be associated with the evolution of free carbon, oxygen, and hydrogen in the films.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

Processing of biomorphic Si3N4 ceramics by CVI-R technique with SiCl4/H2/N2 system

Biomorphic porous silicon nitride Si₃N₄ ceramics have been produced by chemical vapor infiltration (CVI) of carbonized paper preforms with silicon, followed by gas–solid chemical reaction (R) of nitrogen with the infiltrated silicon. The paper was first carbonized in inert atmosphere to obtain a biocarbon (C_b) template. In a second step, silicon tetrachloride in excess of hydrogen was used to infiltrate silicon into the pores of the C_b template and to deposit silicon onto the C_b fibers. Finally, a gas–solid chemical reaction between nitrogen and infiltrated silicon in a temperature range of 1300–1450 °C took place in N₂ or N₂/H₂ atmosphere to form reaction bonded silicon nitride (RBSN) ceramics. After nitridation, the samples consist mainly of α-Si₃N₄ phase for thermal treatment below the melting point of silicon (1410 °C) or of β-Si₃N₄ phase and β-Si₃N₄/SiC-mixed ceramics for treatment at temperatures above.The crystalline phases α- and β-Si₃N₄ were identified by X-ray diffraction (XRD) analysis and the microstructure of these samples was investigated by scanning electron microscopy (SEM). Energy-dispersive X-ray analysis (EDX) was used to detect the presence of silicon, nitrogen, carbon and oxygen, whereas Raman spectroscopy was applied to identify the presence of Si and SiC. Using thermal gravimetric analysis (TGA), residual carbon was determined. It was found, that addition of 10% H₂ to the nitridation gas at temperatures near the melting point of silicon allows to increase the conversion of Si as well as to control the exothermic nitridation reaction obtaining the preferable needle-like microstructure.