Metal-organic Surfactants as Sintering Aids for Silicon Nitride in an Aqueous Medium

Interaction between two commercial silicon nitride powders and various metal-organic surfactants was investigated in an aqueous medium to obtain a homogeneous distribution of sintering aid. Conditions under which the silicon nitride particles could be coated with metal oxide sintering aids were identified. Acoustophoresis, XPS, TEM, and FTIR were used to characterize the interaction between the metalorganic and the silicon nitride powder. From these data, the mechanism of attachment of the coatings was proposed. The surface area and surface chemistry of the powder, the pH of the solution, the structure of the surfactant, and the surface charge behavior of the powder and the coating were found to be the crucial parameters. The control of these parameters offered a simple and reproducible method to homogeneously distribute various metal oxides. Multicomponent coatings were also possible. The coated powders containing 0.5–4 wt% sintering aid could be compacted to 72% green density.     

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.     

Fundamentals of Preparing Suspensions of Silicon and Related Ceramic Powders

A dispensability model previously developed for silicon powder is extended to theoretically predict the behavior of silicon nitride and silicon carbide powders, and their nonstoichiometric analogs, in a variety of potential dispersing media.     

氮化硅结合碳化硅复合材料性能优化的研究进展

氮化硅结合碳化硅具有优异的力学性能、抗蠕变性能、抗热冲击性能、热学性能和抗化学侵蚀性能,被广泛应用于冶金、化工、机械和国防军工等领域。氮化硅结合碳化硅的服役环境恶劣,材料的力学性能、抗热震性能、抗侵蚀性能和抗氧化性能是影响其服役寿命的关键。本文针对如何提升氮化硅结合碳化硅材料的服役性能,对氮化硅结合碳化硅力学性能、抗热震性能、抗侵蚀性能和抗氧化性能的优化工艺和技术原理进行了总结与归纳。目前,氮化硅结合碳化硅性能优化的主要措施有:(1)合理选择原料的组分配比、颗粒级配、结合系统和烧结制度;(2)添加含Fe、Co、Ni或Cr等催化剂,促进硅粉的氮化和氮化硅晶须的生长;(3)添加烧结助剂或增强相,促进材料的烧结致密化或材料中高熔点第二相的生成。     

Effect of Powder Surface Modifications on the Properties of Silicon Nitride Ceramics

The effect of surface oxygen concentration of silicon nitride powders on the properties of resulting ceramics was studied. A high-purity silicon nitride powder was treated physically and chemically to modify its surface oxygen content. The resulting powders were hot-pressed into dense ceramics using 6 wt% yttria as a sintering aid. Strength and oxidation resistance of these ceramics were measured and correlated with the powder and ceramic compositions as well as the resulting intergranular phases. Results show that the phases developed in yttria-containing silicon nitride ceramics vary with slight changes in the initial powder oxygen content, as predicted, and that strength can be correlated to initial oxygen concentration. The mechanical strength vs oxygen content curve has a definite maximum; i.e., there is a small oxygen concentration range at which optimum ceramic strength is realized. Best results are obtained when the oxygen content is increased by thermal oxidation; other techniques such as chemical oxidation or addition of silica are not as effective, particularly in attaining high strength at elevated temperatures.     

Formation of Boron Nitride Inclusions in Hot Isostatically PressedSilicon Nitride-Silicon Carbide Composites

Small graphite-like boron nitride inclusions have been detected, using transmission electron microscopy, in silicon nitride particulate-reinforced silicon carbide composites hot isostatically pressed in tantalum cans. The inclusions are thought to arise indirectly from fine particles of boron nitride sprayed onto the internal surface of the tantalum can to prevent a chemical reaction between the tantalum and the silicon carbide. Boron oxide present on the surface of these particles is able to evaporate and penetrate the powder compacts during hot isostatic pressing and then react with silicon nitride and silicon carbide to form the observed inclusions.     

Evaluation of oxygen content on silicon nitride powder surface from the measurement of the isoelectric point

The oxygen content associated to the surface of silicon nitride powder particles was characterized using a method which is based on the measurement of the specific surface area and the isoelectric point in aqueous solution. From the dissociation model of hydrolyzed surface groups a linear dependence between the fraction of silanol groups (SiOH) and the pH of the isoelectric point is predicted, which was confirmed by electrophoretic measurements of various commercial silicon nitride powders. The proposed method provides rapid information on the particle surface composition, which is of particular relevance for colloidal processing of silicon nitride powders.     

Interfacial Characterization of Silicon Nitride Powders

The composition of the surface and the behavior in aqueous suspensions of three silicon nitride powders were investigated using electron spectroscopy for chemical analysis (ESCA), potentiometric titrations, leaching experiments, and electrophoretic mobility. ESCA shows that the as-received powders have a surface-layer composition similar to that identified as an intermediate state between silica and silicon oxynitride. The original differences in pH_iep between the three powders disappears by aging the powders. The common pH_iep of 6.8 ± 0.3 for the three powders is interpreted as the equilibrium pH_iep for silicon nitride in aqueous suspensions.     

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric-Field-Assisted Sintering

Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm³) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si₃N₄ increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide.     

A thermodesorption method for the investigation of hydrogen adsorption on electrodes; its application to tungsten carbide,along with electrochemical methods

A method for direct determination of hydrogen adsorption on large surface area electrodes or on electro-conductive powders in electrolyte solutions, at any given potential, is proposed. It consists of hydrogen desorption from the electrode by heating (thermodesorption) after complete adsorption and a quantitative chromatographic analysis of the gas collected during thermodesorption. The proposed method was used in this work to investigate hydrogen adsorption on tungsten carbide powder in a H₂SO₄ solution. The potential-dependence of adsorption was found, confirming the authors previous suggestion of an extensive coverage of tungsten carbide by chemisorbed hydrogen and of the potential region of adsorption (up to 0·35–0·4 V).The tungsten carbide surface was also investigated by the method of charging and potentiodynamic curves. Reversibility of these curves up to 0·35–0·4 V is shown. Hydrogen adsorption is correlated with the quantity of electricity calculated from the charging curve and it is shown that a considerable portion of the charge is associated with hydrogen chemisorption but some portion of the total charge is consumed even in the hydrogen region for other non-defined redox processes occurring on the tungsten carbide surface.     

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.     

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.     

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.     

氮化硅粉体的行星式球磨工艺研究

主要研究了各种球磨工艺参数在行星式球磨过程中对粉料的粒度及形貌的影响。由于超细粉碎过程中特有的团聚现象，当颗粒尺寸达到极限值时，进一步延长球磨时间，反而使球磨的效果变差，降低球磨效率。通过对氮化硅粉体行星球磨过程的分析，研究了不同球磨工艺参数(如料球比、球磨转数、球磨时间等)对氮化硅粉料球磨效果的影响，从而优化行星球磨工艺参数。     

Highly Creep-Resistant Silicon Nitride/Silicon Carbide Nano–Nano Composites

A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/silicon carbide composite with a nano–nano structure (nanosized SiC and Si₃N₄ in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si–C–N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nano-sized SiC included in sub-micron Si₃N₄) to nano–nano type. Nanocrystalline silicon nitride/silicon carbide ceramic composite with 30–50 nm grain size was synthesized without using sintering additive.     

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。     

Combustion Synthesis of Silicon Nitride-Silicon Carbide Composites

The feasibility of synthesizing silicon nitride-silicon carbide composites by self-propagating high-temperature reactions is demonstrated. Various mixtures of silicon, silicon nitride, and carbon powders were ignited under a nitrogen pressure of 30 atm (∼ 3 MPa), to produce a wide composition range of Si₃N₄-SiC powder products. Products containing up to 17 vol% of SiC, after being attrition milled, could be hot-pressed to full density under 1700°C, 3000 psi (∼ 21 MPa) with 4 wt% of Y₂O₃. The microhardness and fracture toughness of these composites were superior to those of the pure β-Si₃N₄ matrix material and compared very well with the properties of "traditionally" prepared composites.     

Effect of Continuous-Wave High-Intensity Laser Radiation on the Strength of Silicon Nitride and Silicon Carbide

Laser interaction with silicon nitride and silicon carbide ceramics shows that hot-pressed silicon nitride was less affected than reaction-bonded silicon nitride at the same energy levels. Both of these showed greater thermal-shock resistance than hot-pressed silicon carbide.     

Silicon Nitride Colloidal Probe Measurements: Interparticle Forces and the Role of Surface-Segment Interactions in Poly(acrylic acid) Adsorption from Aqueous Solution

Direct measurements of forces between silicon nitride surfaces in the presence of poly(acrylic acid) (PAA) are presented. The force-distance curves were obtained at pH > pH_iep with an atomic force microscopy (AFM) colloidal-probe technique using a novel spherical silicon nitride probe attached to the AFM cantilever. We found that PAA adsorbs onto the negatively charged silicon nitride surface, which results in an increased repulsive surface potential. The steric contribution to the interparticle repulsion is small and the layer conformation remains flat even at high surface potentials or high ionic strength. The general features of the stabilization of ceramic powders with PAA are discussed; we suggest that PAA adsorbs onto silicon nitride by sequential adsorption of neighboring segments ("zipping"), which results in a flat conformation. In contrast, the long-range steric force found in the ZrO₂/PAA system at pH > pH_iep arises because the stretched equilibrium bulk conformation of the highly charged polymer is preserved via the formation of strong, irreversible surface-segment bonds on adsorption.     

Dispersion of Boron Nitride Powders in Aqueous Suspensions with Cellulose

The dispersion of nitride and carbide ceramic particles in water is difficult, due to the absence of hydroxyl groups on their surface. Boron nitride (BN) is not an exception and despite its numerous applications, no effective dispersant has been identified so far. We demonstrate here the dispersion properties of two cellulose derivatives for hexagonal BN powders, hydroxyl ethyl cellulose and methyl cellulose. The effect of particle size and cellulose concentration was investigated. The adsorption of cellulose onto the surface of the BN particles was confirmed by isotherm adsorption. Zeta potential measurements show a charge screening effect of the cellulose The suspensions obtained were highly loaded and stable versus pH, and thus could lead to homogeneous codispersion of BN particles and sintering additives.