Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si₃N₄ compacts, containing ≅7 wt% of both BeSiN₂ and SiO₂ as densification aids, can be reproducibly sintered to relative densities >99% by a gas-pressure sintering process. Nearly all densification takes place via liquid-phase sintering of transformed β-Si₃N₄ grains at T =1800° to 2000°C. Compacts with high density are produced by first sintering to the closed-pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N₂ pressure at 2000°C and then increasing the N₂ pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T = 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas-filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.     

Effect of Sintering Atmosphere on Densification of MgO-Doped Al2O3

The densification behavior of Al₂O₃-MgO (0.1 wt%) has been studied in O₂ and N₂ atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O₂ becomes higher than that in N₂. When the sintering atmosphere is changed from O₂ to N₂, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.     

Effect of Nitrogen Atmosphere on the Densification of a 3-mol%-Yttria-Doped Zirconia

The densification behavior of a 3-mol%-Y₂O₃-doped ZrO₂ (3Y-ZrO₂) has been investigated under N₂ and O₂ atmospheres. Powder compacts have been sintered at 1550° and 1400°C for various times. The density of the specimen sintered at 1550°C is higher in N₂ than in O₂, while the contrary result is obtained in the case of the specimen sintered at 1400°C. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO₂ matrix. Because nitrogen solubility into the ZrO₂ increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO₂ and its role on the densification behavior of 3Y-ZrO₂.     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.     

Nitridation Kinetics and Thermodynamics of Silicon Powder Compacts

The nitridation kinetics of Si powder compacts were studied by measuring the flow rate dependence of N₂ and N₂–H₂ gas mixtures during slow heating of Si compacts while simultaneously monitoring the oxygen partial pressure of the egress gas. The reaction was found to occur in two distinct stages. In pure nitrogen the initial stage was interpreted in terms of devitrification of the native silica layer, catalyzed by Fe impurities, and the exposure of the underlying Si. The reaction sequence at that point has been unequivocally shown to be followed by the formation of oxynitride according to as evidenced by a large increase in the oxygen partial pressure simultaneously with the onset of the initial stage. In the absence of hydrogen, both reactions are rapidly suppressed as the oxygen liberated competes with the nitrogen for the exposed silicon surface and reoxidizes it. However, in the presence of hydrogen, the oxygen liberated reacts with the hydrogen and prevents the reoxidation of the Si. The net reaction in this case is and/or with the second reaction being thermodynamically favored. The main nitridation reaction occurring between 1300° and 1400°C appears to be diffusion controlled and depends on the nature of the passivating layer that forms during the initial stage.     

Preparation of Magnesium Nitride Powder by Low-Pressure Chemical Vapor Deposition

Submcrometer-scale magnesium nitride (Mg₃N₂) powder was prepared by low-pressure chemical vapor deposition, i.e., the reaction of Mg vapor with mixed NH₃–N₂ gases at 800°C under a pressure of ∼1 kPa. The mixing ratios of NH₃–N₂ gases were 0% NH₃–100% N₂ (pure N₂), 20% NH₃–80% N₂, 40% NH₃–60% N₂, 60% NH₃–40% N₂, 80% NH₃–20% N₂, and 100% NH₃–0% N₂ (pure NH₃). The reactions between Mg vapor and NH₃–N₂ gases produced platy Mg₃N₂ particles <0.2 μm and acicular particles with long-axis length of ∼0.2 μm, whereas the reaction of Mg vapor with pure NH₃ gas produced spherical Mg₃N₂ particles with diameters of ∼0.1 μm.     

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.     

Densification of Si3N4-Y2O3/Al2O3 by a Dual N2 Pressure Process

Sintering studies of Si₃N4-Y₂O3/Al₂O₃ compositions indicated that increased densification could be produced using a dual N₂ pressure process. RT modulus of rupture values and microstructures were found to be similar to hot-pressed material. The technique should be adaptable to complex shaped Si₃N₄ bodies where high density and strength are required .     

Low-Temperature Pressureless Sintering of Sr- and Mg-Doped Lanthanum Gallate Ceramics by Sintering Atmosphere Control

The present investigation reports a new processing technique that can reduce the sintering temperature of Sr- and Mg-doped lanthanum gallate (LSGM), a good candidate material for the electrolyte of the solid oxide fuel cell (SOFC). When LSGM was sintered at 1623 K for 5 h in N₂ or O₂, the samples were densified over 98% relative density. In contrast, only 93% relative density was achieved after sintering in air, the conventional sintering atmosphere. As a result of better densification in N₂ or O₂, the electrical conductivity of the N₂-sintered and air-annealed or the O₂-sintered sample was higher than that of the air-sintered sample by 30%. This result shows the beneficial effect of N₂ or O₂ sintering of LSGM and provides a high possibility of a low-temperature preparation of an LSGM-based SOFC.     

Effect of Agglomeration on Mechanical Properties of Porous Zirconia Fabricated by Partial Sintering

Porous ZrO₂ ceramics were fabricated by compacting a fine ZrO₂ powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO₂ specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO₂ grains. It was revealed that the ZrO₂ agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO₂ ceramics.     

Effects of Trace O2 Levels on the Nitriding Kinetics of High-Purity Silicon Powders

The effects of trace O₂ levels on the nitridation of compacts made from silane-derived Si powders were studied in N₂ atmospheres, with oxygen levels of either 5 ppm or 10 ppb (approximately). The nitriding kinetics were studied by thermogravimetric analysis as a function of temperature (1100–1200°C) and heating rate (5°C/min and 100°C/min). Reducing the O₂ level in the nitriding gas enhanced conversion to Si₃N₄ at lower temperatures, reduced the composition variations within the samples, and decreased the α/β ratios. The results suggest that nucleation and rapid growth of Si₃N₄ at relatively low temperatures are possible only when the oxygen partial pressure in the system is below the threshold value for passive oxidation.     

Through Pore Size Distribution and Kinetics of the Carbonation Reaction of Portland Cement Mortars

Through pores of hardened cement mortars were studied by gas transfer experiments. The permeabilities were the same for N₂ and H₂. The pore size distribution of through pores was examined by back diffusion; the method was simplified by combining H₂ and N₂ gases. The permeability calculated from these results agreed with the experimental values of N₂ and H₂. The neutralization rate of hardened cement was expressed as L²=kθ(t-t_i). The rate of neutralization was determined by the gas diffusion of both CO₂ and H₂O.     

Densification of Alumina-Silicon Carbide Powder Composites: II, Microstructural Evolution and Densification

Microstructural evolution adn densification kinetics of Al₂O₃-SiC powder composites were studied using two different SiC powders. Examination of the microstructural evolution of Al₂O₃-fine SiC powder composites showed three well-defined stages of densification: the first was characterized by constant pore size and no grain growth; the second involved rapid pore coarsening and grain growth; the third was characterized by pore shrinkage and slow grain growth. Studies of the densification kinetics of Al₂O₃-coarse SiC powder composites exhibited two stages of densification: in the first stage there were no significant differences in densification rate between pure Al₂O₃ compacts and composites; in the second stage, however, differences in densification behavior between pure Al₂O₃ compacts and composites became pronounced.     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

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.     

Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization

Partially crystalline Si₃N₄, with nanosized crystals and a specific surface area greater than 200 m²/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH₃ to 1000°C. This polymer does not contain N initially. Crystallization to high-purity α-Si₃N₄ proceeds with additional heating above 1400°C under N₂. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N₂, are consistent with an amorphous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes.     

Colloidal Processing and Mechanical Properties of Whisker-Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD-processed ultrafine mullite powder (<0.1 μm), -Si₃N₄ whisker and -mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze-dried porous green compacts were hot-pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N₂ atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si₃N₄ whisker increased the density, flexural strength, and fracture toughness of the hot-pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Reaction of UC with Nitrogen from 1475° to 1700°C

The reaction of nitrogen with arc-melted UC was studied from 1475° to 1700°C at an N₂ pressure of about 400 torr. The reaction appeared to proceed toward equilibrium in distinct stages: (1) UC and N₂ reacted to form UC₂ and UC_0.8N_0.2; (2) UC₂ reacted with N₂ to produce more UC_0.3N_0.2 and free C; and (3) this U(C,N) reacted with N₂ to produce more free C and the equilibrium product, a high-N U(C,N). These results are consistent with known thermodynamic and phase behavior in the U-C-N system.