Influence of Granule Character and Compaction on the Mechanical Properties of Sintered Silicon Nitride

The influence of granule character and compaction on the mechanical properties of sintered silicon nitride was studied as a function of the pH of the spray-dry slurry. The character and the compaction behavior of the spray-dried silicon nitride granules considerably affect the mechanical properties of the sintered body. Dense and hard granules resulting from a well-dispersed slurry retained their shape in green compacts and caused numerous pore defects in sintered body. Decreasing the slurry pH to a certain value (e.g., 7.9) caused slurry flocculation and reduced the granule density as well as the diametral compression strength of the granules. Sintered bodies fabricated with these weak granules contained fewer defects and showed remarkable strength increase.     

Preparation and Sintering of Homogeneous Silicon Nitride Green Compacts

Interactions between silicon nitride particles and hydroxide precipitates were investigated using electrophoresis measurements. Conditions under which stable suspensions of silicon nitride particles and flocculation and heteroflocculation of silicon nitride/hydroxide mixtures occur were Identified. On the basis of the observations, a method for producing uniform mixtures of silicon nitride powders and additive precipitates was formulated and used to produce green compacts of improved compositional homogeneity. The effect of the mixing process on the sintering of green silicon nitride compacts was investigated and compared to the sintering behavior of conventionally prepared green compacts. The results show that the improved homogeneity obtained using the precipitation mixing process leads to enhanced sintering of the green compacts.     

Development of Short-Range Repulsive Potentials in Aqueous, Silicon Nitride Slurries

The properties of aqueous, dispersed, silicon nitride slurries, with an isoelectric point of pH 5.5, can be changed with additions of NH₄CI. At pH 10 the effect of adding NH₄Cl is similar to that suggested by DLVO theory; namely, for concentrations .0.5 M , the viscosity vs shear rate behavior, the elastic modulus, and the relative packing density are identical to those for slurries prepared at the isoelectric point. On the other hand, the effect of salt on dispersed slurries prepared at pH 2 differs from the behavior implied by classic DLVO theory; i.e., measurement of the same properties showed that the attractive interparticle potential was much weaker relative to that produced at the isoelectric point. As previously reported for alumina slurries, the results suggest that a short-range, repulsive interparticle potential is developed in salt-added slurries prepared at pH 2 which prevents attractive particles in the slurry from touching and aids particle packing. The same short-range potential apparently is not developed with salt additions at pH 10. The difference between silicon nitride and alumina slurries is apparent when the slurries are consolidated. Bodies consolidated from any silicon nitride slurry are elastic (i.e., they fracture before they flow) unlike salt-added alumina slur-ries, which are plastic.     

低介电常数多孔氮化硅陶瓷材料的制备

采用有机泡沫前躯体浸渍工艺制备了低介电、低密度的氮化硅陶瓷。以氮化硅粉体为主要原料,制备粘度和流动性合适的水基料浆,并以软质聚氨酯泡沫塑料为载体,在真空状态下浸渍,然后在氧化气氛下排塑,在氮气气氛下烧结,得到了低介电常数的多孔氮化硅陶瓷材料。所制备的材料性能可达到：容积密度为0.12g/cm3、介电常数为1.15、介电...     

Fracture properties of spray-dried powder compacts: Effect of granule size

Inappropriate mechanical properties of spray-dried powder compacts lead to significant green product losses, entailing considerable costs in ceramic tile manufacture as well as serious environmental problems. In addition, green strength can be indicative of how well a ceramic processing system is working.In this study, granules were prepared by spray drying a red clay slurry used in floor tile manufacture. The resulting granules were characterised and their porosity, morphology, and mechanical behaviour were determined.The study analyses the fracture properties of green ceramic materials using Linear Elastic Fracture Mechanics (LEFM), which has been widely used for fired materials, but whose application to green compacts has drawn much less attention. Two types of tests for determining fracture parameters (fracture toughness, fracture energy, and crack size) in green materials are also critically examined. Finally, the fracture parameters have been correlated to the microstructural characteristics of the compacts, in particular to granule size and the topography of the fracture.     

Influence of particles packing in granules on the particles orientation in compacts

Particles orientation during compaction was studied in alumina granules of different packing structures and deformation properties. These granules were classified mainly in two types: loose granules prepared with flocculated slurries and dense granules prepared with dispersed slurries. Particles orientations in the granules and in the compacts were examined quantitatively with the cross-polarized light microscopy. A large difference was noted in the packing structures of granules and compacts. Orientation of particles was detected only in the surface vicinity of the dense granules. These dense granules show only a slight change in particle orientation locally and its initial structures were mostly preserved even after compaction. As a result, the green compacts containing these granules also show very low net particles orientation. In contrast, loose granules show no orientated particles. However, a major rearrangement of the particles was noted during compaction, resulting in a high net particle orientation in the compacts. The particles orientation in the green compacts affected the anisotropy in the sintering shrinkage significantly; high anisotropy was observed in compacts of high particle orientation fabricated from the loose granules.     

Characterisation of the green machinability of AlN powder compacts

The aim of this work is to study the machinability of aluminium nitride green bodies obtained from dry pressing of spray-dried granules. The characterisation of the green machinability of ceramics is not easy. Indeed, to date, no conventional mechanical test able to represent the machining behaviour of green powder ceramic compacts. Therefore, the first target of this work is to determine possible correlations between mechanical properties, microstructure and machinability and to propose suitable tests. Three types of aluminium nitride granules are investigated. Two of theses types of granules are commercial grades using a thermoplastic binder: a conventional grade for pressing and a specific grade especially developed for green machining. The third grade is under development and uses a thermoset binder. From this work it appears that the compacts containing the thermoset binder exhibit simultaneously the highest mechanical properties and the best behaviour at machining. The good machinability has been correlated to a high work of fracture together with a transgranular mode of fracture of the material.     

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.     

Formation and Control of Agglomerates in Alumina Powder

High-purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as-received powder. After the slurries were dried, fine alumina particles cohered to form agglomerates, which were "hard" when formed in a water slurry, and "soft" in an ethanol slurry. Firing of the powder compacts with "hard" agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with "soft" agglomerates. The reasons for their formation are discussed and experimentally confirmed. In the case of water slurry, a small amount of transition alumina reacted with water at low pH. After drying, alumina gels were formed and acted as a strong binder between α-alumina particles to form "hard" agglomerates.     

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.     

Dispersion,slip casting and reaction nitridation of silicon–silicon carbide mixtures

The dispersing behaviour of silicon, silicon carbide and their mixtures in aqueous media were monitored by particle size, sedimentation, viscosity and zeta potential analyses as a function of pH of the slurry. The pH values for optimum dispersion were found to be 4 and 8 for silicon, 10 for SiC and 9 for Si+SiC mixtures. Optimum slips of Si+SiC mixtures were slip cast to obtain green compacts which were nitrided once at 1450°C for 2 or 4 h or successively and cumulatively for 8 (2+6) and 10 (4+6) h in a resistively heated graphite furnace. The binding phases in the nitrided products were found to be fibrous/needle like α-Si₃N₄, flaky grains of β-Si₃N₄ and Si₂ON₂. The products containing 19–47% of silicon nitride as bond/matrix possessed flexural strength (three-point bending) values of 50–85 MPa. ©     

Synthesis of Porous Silicon Nitride with Unidirectionally Aligned Channels Using Freeze-Drying Process

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze-drying process. Freezing of a water-based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze-drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si₃N₄ had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si₃N₄ matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.     

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.     

Interaction of encapsulation glass and silicon nitride ceramic during HIPing

Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and borosilicate glass. Attention has been focused on what happens when the pressure is first applied in the HIP-cycle, i.e. between about 1200 and 1500°C. At this stage the pore system of the ceramic green body is still rather unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass compositions, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Darcy's law it was predicted that the encapsulation glass with the lowest viscosity should penetrate about 1200 microns into the still open pore structure at 1450°C, but this was not observed experimentally. In the calculations no chemical reactions were assumed to take place. However, increases in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen increases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibited a phase composition gradient of silicon oxynitride, down to approximately 100–200 microns into the bulk. ©     

Refined grain size of ITO ceramic targets prepared by pressure slip casting and two-step sintering

Using high-efficiency bead milling dispersion technology, weakly agglomerated mixed nanoparticles were dispersed into a high solid content and low viscosity ITO slurry consisting of uniform single particles. The rheological properties of the slurry were studied as a function of the change in solid content and dispersant amount. The green ITO compacts with more uniform microstructure were prepared by mixing the slurry with high solid loading. Densification activation energies of the ITO compacts in different sintering atmospheres and different heating rate conditions were calculated by the MSC method based on shrinkage data. Finally, ITO ceramic targets with a grain size range of 3–6 μm were obtained by the two-step sintering method, and their phase structure was uniformly distributed.     

一种三元复合絮凝剂的合成及性能研究

以聚硅酸、聚合氯化铝(PAC)、阳离子型聚丙烯酰胺(C-PAM)为原料,制备了复合型无机高分子絮凝剂PCSM。考察了加药量、铝与硅的物质的量比、pH值等因素对絮凝效果的影响。结果表明,少量添加C-PAM有助于改善聚合铝硅(PSAC)的稳定性和絮凝效果。与PSAC相比,PCSM有较宽的pH值适用范围,絮体颗粒粗大、密实,沉降速度快,余浊低。     

Fabrication of transparent MgAl2O4 spinel via spray freeze drying of microfluidized slurry

Spherical granules were prepared from a monodispersed slurry by combining the microfluidization (MF) method and the spray freeze-drying (SFD) process. Starting with the prepared granules, transparent MgAl₂O₄ ceramics were fabricated through pressureless sintering followed by hot isostatic pressing. A comparison with a polydispersed slurry prepared by the ball-milling method showed successful fabrication of a mono-dispersed state by microfluidization method, and 80% visible in-line transmittance was obtained at a 600-nm wavelength from a process starting with a monodispersed slurry of low solid content. Microstructural analysis of the green bodies, the pre-sintered bodies, and the hot isostatic pressed bodies of the prepared MgAl₂O₄ ceramics revealed that the slurry dispersion should be controlled to a high level in order to suppress scattering sources such as pores and microcracks, which affect the in-line transmittance of visible light.     

Processing of Al2O3/SiC nanocomposites—part 2: green body formation and sintering

Pressure filtration was used to form green compacts from aqueous slurries of alumina with 5 vol.% silicon carbide. Green densities of 64%TD were achieved for slurries containing a 50 vol.% solids loading. Lower green densities were obtained for a very fine alumina due to the practical limits on maximum slurry solids loading when using finer powders. The samples were dried in a purposely built humidity cabinet to limit sample cracking. It was found that a higher consolidated layer permeability gave a higher initial drying rate. Near fully dense (99% TD) nanocomposites were produced, via pressureless sintering at 1900 °C. Poor sintered densities were obtained in the case of the fine alumina because of localised sintering of these low green density compacts. The required intra/inter-granular nanocomposite microstructures have been obtained for several different systems, with an average grain size of approximately 5 μm. Abnormal grain growth was noted for samples containing the larger particle size silicon carbide. This shows that a maximum particle size limit exists when selecting the powders for a 5 vol.% nanocomposite.     

Development and analysis of a high refractive index liquid phase Si3N4 slurry for mask stereolithography

Ceramic mask stereolithography additive manufacturing technology has the abilities of fast and accurate printing complex and personalized ceramic parts. However, since the ultraviolet light is strongly blocked by the ceramic particles in the slurry during exposure, the slurry with high refractive index ceramic particles has low cure depth, and serious light scattering. This investigation developed a curable silicon nitride slurry with a high refractive index liquid phase, which significantly reduced the refractive index difference between the liquid phase and the silicon nitride ceramic particles. The high refractive index acrylate monomer and solvent were chosen to increase the cure depth while ensuring the curing accuracy. Compared with the slurry whose liquid phase was pure HDDA, the addition of OPPEA and ACMO in the liquid phase showed a better refractive index increasing effect and the strongest cure depth and dimensional accuracy improvement. POE was verified as a suitable non-reactive high refractive solvent. The composition of 20 wt% OPPEA, 10 wt% ACMO, 40 wt% HDDA, and 30 wt% POE were optimized as the high refractive index liquid phase for 40 vol% solid phase silicon nitride slurry. A finite element analysis model of silicon nitride slurry was established, and the distribution of ultraviolet light intensity in silicon nitride slurry with high refractive index liquid phase was numerically simulated.     

Packaging of Sol–Gel-Made Porous Nanostructured Titania Particles by Spray Drying

Porous titania granules (1–20 μm) were made by spray drying titania solutions with and without peptization. An unpeptized titania slurry was prepared by hydrolyzing titanium tetraisopropoxide, while a peptized sol was prepared by adding nitric acid to the slurry. The characteristics of the granules depended on the feed solution properties. In particular, granules from the unpeptized slurry showed a bimodal pore size distribution (PSD), with a rough surface attributed to hard aggregates; granules from the peptized sol showed a monomodal PSD, with a smooth surface of well-packed nanoparticles. The effect of dopants (alumina and zirconia) also was investigated. Doping slowed the crystallite growth of the titania granules, as well as the anatase-to-rutile phase transformation. All of the peptized granules exhibited a monomodal PSD of intraparticle pores, from 2 to 15 nm. However, the PSD of the pure titania granules collapsed at 600°C, mainly by phase transformation.