Development of Short-Range Repulsive Potentials by Short-Chain Surfactants in Aqueous Slurries

The effect of both cationic ( n -trimethylammonium bromide, C _n TAB) and zwitterionic (phosphocholine, C _n C _n PC) surfactants on the properties of aqueous slurries of Si₃N₄ was studied. These surfactants were expected to adsorb to the surface of Si₃N₄ particles to produce short-range repulsive interparticle potentials that might be useful for the colloidal processing of advanced ceramic powders. Electrophoretic, adsorption, and viscosity measurements showed that longer-chain-length surfactants (C _n TAB, n ≥ 12, and C _n C _n PC, n ≥ 9) strongly adsorb. Surfactants with shorter chain lengths were highly soluble and did not adsorb. Although the C _n TAB, n ≥ 12, surfactants produced very weak particle networks with a low viscosity, the packing density during consolidation was very low, and the bodies were brittle (cracked before plastic deformation). The less-soluble, longer-chained C _n C _n PC, n ≥ 9, surfactants did produce high particle-packing densities but also produced brittle bodies. In all cases, it appeared that the surfactants could be pushed away from the surface during particle packing.     

Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si₃N₄ and SiC slurries was studied by monitoring the pH versus time. Various Si₃N₄ and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO₂/(m² AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si₃N₄ and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).     

Interparticle Potentials in Nonaqueous Silicon Nitride Suspensions

The rheological properties of nonaqueous silicon nitride suspensions are studied. Suspensions were prepared to volume fractions of solids of 0.21, 0.25, 0.29, and 0.33, and dispersed with phosphate ester in a mixture of solvents (methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone). Expanded viscosity curves were obtained by measuring under controlled rate and stress conditions, and the experimental data were fitted to the Cross model that provides the high shear limiting viscosity (η_∞). The evolution of viscosity with volume fraction of solids was fitted to the Krieger-Dougherty equation, to predict the maximum packing fraction (φ_m). The electrostatic pair potential was calculated based on the DLVO theory by evaluating the dielectric constant of the three-component solvent and the Hamaker constant of the Si₃N₄–solvent system. The surface potential was calculated by measuring the elastic modulus through dynamic rheological measurements. The steric potential was also evaluated from the available models. It has been observed that phosphate ester provides a purely steric stabilization at short separation distances (up to 9 nm), while electrostatic forces dominate at larger separation distances.     

Dispersant-Binder Interactions in Aqueous Silicon Nitride Suspensions

The interaction of dispersant and binder on the surface of particles was studied to identify the effect of these additives on aqueous ceramic powder processing. Poly(methacrylic acid) (PMAA) and poly(vinyl alcohol) (PVA) were used as the dispersant and binder, respectively. The adsorption isotherms of the organic additives on silicon nitride were determined. The adsorption of PMAA was differentiated from PVA in the mixed additive system via ultraviolet spectroscopy. The electrokinetic behavior of silicon nitride was measured by using an electrokinetic sonic amplitude analyzer. As the PMAA concentration increased, the isoelectric point (pH_iep) of silicon nitride shifted from pH 6.7 ± 0.1 to acidic pH values. The magnitude of the shift depended on the surface coverage of PMAA. PVA did not affect the pH_iep of suspensions but did cause a moderate decrease in the near-surface potential. Finally, the rheological behavior of silicon nitride suspensions was measured to assess the stability of particles against flocculation in aqueous media; this behavior was subsequently correlated with the electrokinetic and adsorption isotherm data.     

Corrosion of Silicon Nitride Materials in Aqueous Solutions

Silicon nitride ceramics are used under conditions where high strength, hardness, and wear resistance are necessary. The increasing use of Si₃N₄ ceramics in different environments demands an understanding of the relationships between microstructure and corrosion behavior. This study gives an overview of the behavior of silicon nitride in acids, bases, and hydrothermal conditions. It not only summarizes the literature data but also attempts to explain the mechanisms and to give some guidelines for the use of the materials in different environments. The stability of the ceramics against corrosion in acids and bases up to the boiling point is mostly controlled by the stability of the grain boundary. The stability can be predicted using the glass network theory. Materials with grain boundaries exhibiting a strong network, i.e., a high amount of SiO₂ in the grain boundary, are stable in acids, but less stable under hydrothermal conditions and in basic solutions. Therefore, tailoring the grain boundaries can change the corrosion stability by several orders of magnitude. At temperatures above 200°C–250°C, the dissolution of the Si₃N₄ grains becomes a decisive factor determining the stability.     

Aqueous Gel-Forming of Silicon Nitride Using Carrageenans

Much effort has been devoted recently to the development of near-net-shaping processes in water. Agarose has been demonstrated to be a suitable gelling agent for aqueous forming. However, its high cost and the difficulties in controlling the rheological properties have restricted large-scale applications. In this work a novel gelling binder, namely carrageenan, is proposed as a low-cost and high-gel-strength additive for gel-forming ceramic powders. The capability to obtain silicon nitride parts by carrageenan gelation is described. Aqueous silicon nitride slips are prepared at pH >11 to a solids content of 70 wt% using tetramethylammonium hydroxide as a dispersant. Dissolution and gelation of carrageenan are studied by continuous measurements of viscosity with temperature for solutions prepared at different pH values. The final injection slips prepared by mixing at <60°C the previously heated suspension and the carrageenan solution are rheologically characterized also. After the blend is injected into cooled nonporous molds, gelling occurs in a few seconds and samples can be dried in air for 24–48 h. Green densities of 52% of theoretical are obtained.     

Rheological Behavior of Slurries and Consolidated Bodies Containing Mixed Silicon Nitride Networks

Two particle networks, each formulated with a different interparticle potential, were mixed to control the rheological properties of ceramic slurries and to develop claylike plasticity in consolidated bodies. A weakly attractive network, containing silicon nitride powder, alkylated with hexadecanol, was mixed with a second slurry containing flocculated (nonalkylated) silicon nitride powder. The elastic modulus and apparent yield stress of concentrated suspensions containing each constituent and their mixtures were found to increase with volume fraction according to a previously reported power law function (exponents of 4.8 and 3.75, respectively). Because of the large difference in the relative strengths of the two networks, the flocculated network overwhelmingly controlled the behavior of the mixed slurries when its volume fraction (relative to total solids) exceeded 0.30. Slurries were consolidated by pressure filtration, and the saturated bodies were tested in uniaxial compression. Bodies containing only alkylated powder packed to a high volume fraction and deformed at a low flow stress. The addition of small amounts of the flocculated network increased the flow stress to produce a body with rheological properties similar to clay.     

Dissolution and Deagglomeration of Silicon Nitride in Aqueous Medium

Silicon nitride undergoes hydrolysis and dissolution when subjected to an aqueous environment. Molecular dynamics simulations suggest that hydrolysis proceeds through nucleophilic attack of water with the formation of an intermediate molecular complex involving a pentacoordinated silicon. We found that the dissolution of an oxidized silicon nitride powder resembles that of silica; the dissolution rate could be described using a simple kinetic equation with a dissolution activation energy of 52 kJ·mol⁻¹. The deagglomeration of a fine silicon nitride powder under mild agitation was evaluated; we show that the peptization kinetics at room temperature is dominated by the breakup of particle–particle bonds due to hydrodynamic friction and cluster attrition. For breakup of hard agglomerates of small particles the dissolution of interparticle necks will play an important role.     

Improved Aqueous Dispersion of Silicon Nitride with Aminosilanes

The addition of a standard fiberglass surfactant, 3-aminopropyltriethoxysilane (APS), improves whisker and powder dispersion of silicon nitride aqueous suspensions. Aqueous suspensions of APS-coated silicon nitride have lower viscosities, increased consolidation, and higher dried green-body densities compared to uncoated silicon nitride in suspensions with pH values ≤8. The APS coating shifts the isoelectric point (IEP) of silicon nitride to a more basic value, dependent on the concentration of APS coating. Suspension pH measurements indicate that APS extracts one hydrogen ion for each APS molecule either chemisorbed on the particle surface or dissolved in the solution. Optical microscopy reveals that dilute suspensions coated with APS at pH 10 are qualitatively more dispersed than uncoated silicon nitride at pH 7. Our results show increased dispersion of APS-coated silicon nitride in acidic environments, with a 12% increase in green density under identical wetpressing conditions.     

Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries

Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C₆C₆PC to C₉C₉PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long-range attractive van der Waals force and produced a repulsive short-range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C₆C₆PC to C₉C₉PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes.     

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.     

Corrosion of Silicon Nitride Ceramics in Aqueous Hydrogen Chloride Solutions

The leaching behavior of pressureless sintered and hotpressed Si₃N₄ containing Y₂O₃, Al₂O₃, and AlN and hot isostatically pressed Si₃N₄ without additives was studied in 0.1 M to 10 M HCl aqueous solutions at 50° to 100°C. Y and Al ions contained in the grain-boundary phase dissolved in HCl solutions, but dissolution of the Si ion from the matrix was negligible. The dissolution of Y and Al ions in HCl solutions was adequately described by a surface-chemical-reaction-controlled, shrinking-core model in <1 M HCl solutions and by a diffusion-controlled, shrinking-core model in >5 M HCl solutions. The rates of dissolution of both Y and Al ions decreased as the degree of crystallinity of the grain-boundary phase increased. The fracture strength of the corroded samples linearly decreased with increasing degree of dissolution of soluble Y and Al ions.     

氮化硅粉体水基悬浮液电动特性的研究

通过测量氮化硅粉体在水溶液中的Zeta电位和颗粒大小，研究了不同条件下氮化硅水基悬浮液的电动特性。结果表明，溶液的pH值以及引入的聚电解质不同均会使氮化硅颗粒表面的荷电状态发生变化，进而导致粉体在水溶液中的分散状况发生改变。在碱性条件下添加适量的阴离子聚电解质有利于氮化硅水基悬浮液的稳定分散。     

氮化硅粉体水基悬浮液电动特性的研究

通过测量氮化硅粉体在水溶液中的Zeta电位和颗粒大小j研究了不同条件下氮化硅水基悬浮液的电动特性。结果表明,溶液的pH值以及引入的聚合电解质不同均会使氮化硅颗粒表面的荷电状态发生变化,进而导致粉体在水溶液中的分散状况发生改变．在碱性条件下添加适量的阴离子聚电解质有利于氮化硅水基悬浮液的稳定分散。     

Processing of Concentrated Aqueous Silicon Nitride Slips by Slip Casting

The optimization of concentrated Si₃N₄ powder aqueous slurry properties to achieve high packing density slipcast compacts and subsequent high sintered densities was investigated. The influence of pH, sintering aid powder (6% Y₂O₃, 4% Al₂O₃), NH₄PA dispersant, and Si₃N₄ oxidative thermal treatment was determined for 32 vol% Si₃N₄ slurries. The results were then utilized to optimize the dispersion properties of 43 vol% solids Si₃N₄-sintering aid slurries. Calcination of the Si₃N₄ powder was observed to result in significantly greater adsorption of NH₄PA dispersant and effectively reduced the viscosity of the 32 vol% slurries. Lower viscosities of the optimized dispersion 43 vol% Si₃N₄-sintering aid slurries resulted in higher slipcast packing density compacts with smaller pore sizes and pore volumes, and corresponding higher sintered densities.     

氮化硅粉体的表面化学性质和水中的胶体特性

陶瓷粉体的表面性质和胶体行为对成型、烧结等工艺过程有很大影响。本文回顾了近年来对氮化硅粉体的表面化学性质和水中的胶体特性的研究成果。系统介绍了氮化硅粉体表面元素的键合状态、表面官能团、氧在颗粒表面和内部的分布和表面富氧层的厚试行表面化学性质,以及氮化硅颗粒在水中的荷电机理、等电点和Ｚｅｔａ电位与表面氧含量的关系、含烧结助剂氮化硅多相体系的胶体特性和分散剂的作用机理等胶体特性。     

Chemical-Vapor-Infiltrated Silicon Nitride, Boron Nitride, and Silicon Carbide Matrix Composites

Composites of carbon/chemical-vapor-deposited (CVD) Si₃N₄, carbon/CVD BN, mullite/CVD SiC, and SiC yarn/CVD SiC were prepared to determine if there were inherent toughness in these systems. The matrices were deposited at high enough temperatures to ensure that they were crystalline, which should make them more stable at high temperatures. The fiber-matrix bonding in the C/Si₃N₄ composite appeared to be too strong; the layers of BN in the matrix of the C/BN were too weakly bonded; and the mullite/SiC composite was not as tough as the SiC/SiC composites. Only the SiC yarn/CVD SiC composite exhibited both strength and toughness.     

Chemisorption of Organofunctional Silanes on Silicon Nitride for Improved Aqueous Processing

Different chem-adsorbed silane molecules have been used to produce weakly attractive silicon nitride particle networks for aqueous colloidal processing. Silanes with diamino and poly(ethylene glycol) hydrophilic heads yielded slurries with the lowest viscosity, longest sedimentation stability, and highest packing density. Chem-adsorbed silane molecules protected silicon nitride and yttrium oxide, a common processing aid, from hydrolysis at pHs between 5.5 and 11. A novel approach was used to produce short-range repulsive potentials necessary to yield the weakly attractive networks. Addition of salt to dispersed silicon nitride slurries with particles coated with poly(ethylene glycol)-silane caused the collapse of the 22-atom-long chains and residual electrical double layer. This produced a weakly attractive network which persisted during consolidation to yield a plastic body with a flow stress that was dependent on the counterion size. When 0.5 M tetramethylammonium chloride was used at pH 10, plastic bodies had a flow stress similar to clay, whereas lithium counterions produced bodies with a much higher flow stress.     

低成本制备氮化硅材料的研究

采用工业用纯净水作分散介质,制备了氮化硅陶瓷试样。并对试样进行了力学性能的测试,结果表明其常温力学性能指标略低于用酒精作分散介质材料的试样,断裂韧性为6.42MPa·m~(1/2),硬度为1522kgf·mm~(-2),证明了这种材料在常温下是可以使用的。     

氮化硅陶瓷粉料造粒的研究进展

氮化硅陶瓷具有高强度、高硬度、良好的断裂韧性等优异力学性能,并具有独特的自润滑性能,而日益受到重视.氮化硅陶瓷粉料是制备氮化硅陶瓷的关键原料,粉料的处理方式是影响陶瓷性能的关键步骤.本文介绍了目前较为常见的氮化硅陶瓷粉料的处理方法,并对各处理工艺的优缺点进行了分析,重点阐述了喷雾造粒工艺及其目前研究进展.最后分析了目前氮化硅粉料处理方法存在的问题,并对氮化硅粉料处理方法的发展提出了展望.