Colloidal Filtration of Silicon Nitride Aqueous Slips,Part II: Slip Casting and Pressure Casting Performance

In a previous part, the rheological behaviour of silicon nitride aqueous slips was optimized by dispersing with TMAH up to pH 11·5 using different mixing procedures and including different concentrations of sintering aids (Al₂O₃ and Y₂O₃). In this part, the obtention of pressureless sintered silicon nitride bodies by colloidal filtration techniques is studied. The kinetics of the different compositions is studied for both slip casting and pressure casting. The pressure casting kinetics is up to 20 times faster than that of slip casting, which allows the scale-up of the process for a low cost production, The obtained green density is slightly >58%th for slip casting and 57–55%th for pressure casting, depending on the applied pressure. This small difference does not influence sintered density. At 1750°C/2h, a final density around 96%th is obtained. The sintering conditions are studies considering the time, temperature, atmosphere and sintering bed. The best results are obtained when the sintering bed has the same composition to that of the sample to be sintered. The room temperature properties of the sintered materials show a K_IC value higher than 6 MPam^1/2, comparable to those found in the literature for pressure sintered materials.     

氮化硅流延膜的制备

流延成型是一种制备高质量陶瓷基片的成型方法.氮化硅是一种高热导率的材料,有望在电子基片领域获得应用.本文利用流延成型制备了具有较好柔韧性和一定强度的氮化硅流延素坯膜.研究了无水乙醇、无水乙醇/丁酮作为溶剂时对浆料粘度的影响.通过优化流延浆料添加剂的各种配比,得出了适合氮化硅粉体(SN-E10)流延的最佳配方.     

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.     

Molding Silicon Carbide Articles by Freezing Aqueous Slip

Molding silicon carbide articles by freezing aqueous slip is considered. The freezing conditions are determined. The slip composition is selected. Firing in a nitrogen-hydrogen medium at 1850°C made it possible to obtain samples whose properties are typical of this material.     

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.     

Aqueous Slip Casting of Transparent Aluminum Oxynitride

Highly transparent Aluminum oxynitride (AlON) body has been produced using aqueous slip casting technique. High‐purity alumina and AlN were used as raw materials for the synthesis of single‐phase AlON powder. As‐synthesized AlON powder was surface modified to enable the AlON powders resistant to hydrolysis in water during aqueous slip casting. High solid loaded aqueous AlON slip was prepared for casting followed by drying and sintering to produce transparent AlON. Phase formation and stability was characterized by XRD, pH, and viscosity measurements. AlON powders before and after surface treatments were characterized. Sintered transparent AlON samples were characterized for their mechanical, microstructural, and optical properties. Sintered and polished AlON produced in this study has shown inline transparency up to 80% between 0.22 and 6 μm wavelength region.     

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).     

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.     

Boehmite Coating as a Consolidation and Forming Aid in Aqueous Silicon Nitride Processing

Silicon nitride powders were coated by boehmite via a solgel process to improve the consolidation and forming of aqueous silicon nitride suspensions. The coated silicon nitride suspensions had a significantly higher solids loading than the pure silicon nitride in water. Viscosity measurements and centrifugation showed that the coating changed the long-range interaction between the silicon nitride particles, whereas the pressure filtration study indicated that the oxide coating modified the short-range interaction at particle contacts. Preliminary rheological studies indicated that at pH 3, the boehmite-coated silicon nitride suspensions can gel, as indicated by a wide linear viscoelastic region in dynamic rheological tests. The gelation of coated silicon nitride at pH 3 offers the potential for net-shape forming. Our study establishes a water-based green body consolidation and forming technique of silicon nitride without polymeric additives or organic solvents.     

Nonaqueous Processing of Silicon for Reaction-Bonded Silicon Nitride

Ethanolic silicon suspensions, with and without a polyethoxylated amine of low molecular weight, were studied by rheological, adsorption, electrophoretic, and sedimentation methods. Pellets were pressure-cast and nitrided to form reaction-bonded silicon nitride. Density and binding strength in the green state relate well to rheological behavior and other collodial aspects of the suspensions used, particularly the additive's role and distribution. Density and degree of nitridation in the final state are not importantly affected by the additive's use. Its greatest benefit is to modify the binding strength in the green state. The mode by which this small molecule affects the processing of silicon consists of adsorption, combined with an increased electrostatic interparticle repulsion which increases the suspension viscosity and that of undried pellets. Although the improved binding strength is accompanied by decreased green and nitrided densities, high degrees of conversion to silicon nitride are still achieved.     

Effect of Acid Cleaning and Calcination on Rheological Properties of Concentrated Aqueous Suspensions of Silicon Nitride Powder

The powder characteristics of two types of Si₃N₄ (referred to as FD1 and FD2), as well as the rheological properties of their aqueous suspensions, were studied in this paper. There are distinctive differences in size distribution, soluble counterions, and surface groups. Highly concentrated aqueous slurries could not be prepared from these two as-received powders. Acid cleaning and calcination improved the solids loading of their aqueous slurries, but the improvement varied with the powder. For the as-received FD1 powder, poor dispersibility was caused by high-valence counterions, which can be eliminated through acid-cleaning. However, for the as-received FD2 powder, it was the surface group of amine structures and carbon-hydrogen bonding that limited the dispersibility. The calcination of FD2 can remove the amine structure and carbon-hydrogen bonding and improve the slurry's rheological properties almost perfectly. For acid-cleaned and calcined FD1, and calcined FD2, the solids loading of their aqueous suspensions reached 50 vol% with a viscosity below 300 mPa·s.     

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.     

Rheological Properties of Concentrated, Nonaqueous Silicon Nitride Suspensions

The rheological properties of nonaqueous silicon nitride powder suspensions have been investigated using steady shear and viscoelastic measurements. The polymeric dispersant, Hypermer KD-3, adsorbed strongly on the powder surfaces, and colloidally stable, fluid suspensions up to a volume fraction of φ= 0.50 could be prepared. The concentrated suspensions all displayed a shear thinning behavior which could be modeled using the high shear form of the Cross equation. The viscoelastic response at high concentrations was dominated by particle interactions, probably due to interpenetration of the adsorbed polymer layers, and a thickness of the adsorbed Hypermer KD-3 layer, Δ∼10 nm, was estimated. The volume fraction dependences of the high shear viscosity of three different silicon nitride powders were compared and the differences, analyzed by using a modified Krieger-Dougherty model, were related to effective volume effects and the physical characteristics of the powders. The significantly lower maximum volume fraction, φ_m= 0.47, of the SN E-10 powder was referred to the narrow particle size distribution and the possibility of an unfavorable particle morphology.     

Viscous Slip along Grain Boundaries in Chlorine-Doped Silicon Nitride

The effect of chlorine doping on the anelastic-relaxation and torsional-creep behavior of a silicon nitride (Si₃N₄) polycrystalline body was studied. Two model polycrystals—one undoped and the other doped with a small fraction of chlorine—were investigated. Their microstructures consisted of equiaxed and well-faceted Si₃N₄ grains whose boundaries were separated by a continuous, nanometer-sized film of silica (SiO₂) glass. The actual presence of chlorine in the doped polycrystal was ascertained by ion chromatography and is thought to be enriched at the grain boundaries. The effect of chlorine on the intergranular film structure was characterized by high-resolution electron microscopy. The micromechanical response of the SiO₂ grain boundary under shear stress was monitored up to very high temperatures (i.e., ∼2000°C) by internal-friction and torsional-creep experiments. The presence of the chlorine dopant, which is a network modifier of SiO₂ glass that also causes a widening of the grain-boundary film, significantly lowered the bulk viscosity of the residual glass. As a consequence of the change in grain-boundary chemistry, the internal-friction curve of the chlorine-doped material shifted toward lower temperatures and the torsional-creep rate markedly increased, as compared to the undoped material. According to a viscoelastic model of the Si3N4 polycrystal, the internal-friction data resulted as a superposition of two individual components: (i) a relaxation peak that is related to the anelastic slip mechanism along grain boundaries and (ii) a background component that results from an irreversible diffusional-creep mechanism.     

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.     

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.     

冷冻注凝制备氮化硅陶瓷基耐高温复合材料

采用硅溶胶冷冻胶凝陶瓷成型技术制备Si3N4/BAS陶瓷复合材料,分析了硅溶胶冷冻胶凝技术原理和特点,并对Si3N4/BAS陶瓷复合材料性能及微观形貌进行了研究。结果表明:该成型方法所获得的坯体干燥无变形无开裂,收缩率小于1%;陶瓷烧结体密度为2.9 g/cm3时,烧结体抗弯强度、弯曲弹性模量、断裂韧性以及洛氏硬度分别为350 MPa、193GPa、6.2 MPa·m1/2和58。该成型技术实现了陶瓷界多年来对先进陶瓷高效、低成本、原位近净尺寸成型的追求。     

High Thermal Conductivity Aluminum Nitride Substrates Prepared by Aqueous Tape Casting

High thermal conductivity aluminum nitride (AlN) substrates were prepared by aqueous tape casting. The characteristics of surface-treated AlN powder were studied in aqueous ball-milling media. The oxygen content of AlN powder with the dispersant was lower than that of AlN powder without the dispersant at the same ball-milling time. The isoelectric points of the surface-treated AlN with and without a dispersant were, respectively, at pH∼3.35 and pH∼3.90. The atmosphere had considerable effects on organic additive burnout of aqueous AlN green sheets. The composition of grain boundaries changed with increasing holding time at 1850°C. A translucent AlN substrate with a uniform microstructure and a thermal conductivity of 263 W·(m·K)⁻¹ was obtained by pressureless sintering at 1850°C for 6 h in a nitrogen atmosphere.     

Nonisothermal Microwave Processing of Reaction-Bonded Silicon Nitride

The size and density of reaction-bonded silicon nitride (RBSN) specimens are limited by the reduction in pore size and pore volume associated with the nitridation reaction. In particular, under conventional heating, pores at the surface of dense compacts close before the center has reacted fully. Microwave heating offers a unique advantage over conventional heating for the processing of RBSN. A temperature gradient can be maintained within the compact, which causes the reaction to occur preferentially in the interior. This increases the amount of silicon converted to Si₃N₄ because the center of compacts with a high green density finishes reacting before the porosity near the surface closes. This study follows the reaction process and shows that partially nitrided silicon compacts have composition gradients in the radial direction. Microwave processing also facilitates control of the reaction rate.