影响氮化硅浆料流变特性的因素

通过对影响氮化硅浆料流变性因素的研究发现：不同的氮化硅粉体具有不同的等电点（ｉｓｏｅｌｅｃｔｒｉｃ ｐｏｉｎｇ,简称ＩＥＰ）。分散剂的引入明显改善浆料的流动性。对于粉体Ａ,当分散剂用量达１．２ｗｔ％时浆料具有最佳的流动性。颗粒尺寸及其形貌对浆料的流动性均有较大的影响：颗粒尺寸越大,颗粒形貌越不规则,浆料的流动性就越差。     

Comparison of the Surface Charge Behavior of Commercial Silicon Nitride and Silicon Carbide Powders

The adsorption and desorption of protons from aqueous solution onto the surfaces of a variety of commercial silicon carbide and silicon nitride powders has been examined using a surface titration methodology. This method provides information on some colloidal characteristics, such as the point of zero charge (pzc) and the variation of proton adsorption with dispersion pH, useful for the prediction of optimal ceramic-processing conditions. Qualitatively, the magnitude of the proton adsorption from solution reveals small differences among all of the materials studied. However, the results show that the pzc for the various silicon nitride powders is affected by the powder synthesis route. Complementary investigations have shown that milling can also act to shift the pzc exhibited by silicon nitride powder. Also, studies of the role of the electrolyte in the development of surface charge have indicated no evidence of specific adsorption of ammonium ion on either silicon nitride or silicon carbide powders.     

高固含量氮化硅浆料的制备工艺

本文通过沉降,Ｚｅｔａ电位的测定以及流变测量的方法研究了制备高固体含量氮化硅浆产的工艺条件,结果发现：Ｓｉ３Ｎ４悬浮粒子的等电点在ｐＨ＝４．２其最大的Ｚｅｔａ电位在ｐＨ＝１１附近,分散剂的引入有效地提高悬浮粒子的Ｚｅｔａ电位,改善浆料的分散性,最佳分散剂用量在１．０－１．２ｗｔ％之间,并且,最佳分散性的用量不随浆料固含量的变化而改变,球磨８ｈ已能获得较好的流动性,进一步延长球磨时间并不能有效的改善     

Colloidal Processing of Silicon Nitride with Poly(acrylic acid): I, Adsorption and Electrostatic Interactions

The role of poly(acrylic acid) (PAA) in the dispersion of silicon nitride suspensions was investigated experimentally. The effects of concentration, relative molecular mass, and suspension pH were evaluated. The ionization of PAA was characterized by potentiometric titration and indicated a pH-dependent conformational transition. The isoelectric point for silicon nitride decreased from pH 6.3 to pH 3 as the PAA concentration increased, roughly independent of relative molecular mass. A broadening of the stability region for silicon nitride was attributed to this effect. Redispersion in alkaline media, subsequent to destabilization by surface-charge neutralization, improved following preadsorption of PAA in acidic media. This effect was attributed to formation of a protective polymer adlayer on the surface, which prevented primary minimum aggregation; an electrosteric contribution also may be present. The influence of free polymer on the suspension properties also is discussed.     

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 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.     

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.     

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.     

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.     

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 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.     

表面处理对碳化硅浆料流变性的影响

本文通过对SiC粉体洗涤、表面氧化和聚乙二醇(PEG)包覆等处理后制得的SiC浆料的zeta电位和粘度的测试,研究了表面处理对SiC粉体流变性能的影响.研究表明:通过用酸碱洗涤过后再经过在780℃氧化处理或经PEG包覆所得的SiC粉体,都可以使其浆料的zeta电位的等电点向酸性方向偏移,从而使在pH值为11.8时的电位差大幅度增加,改善了浆料的流变性能.而且用PEG包覆过的SiC粉体浆料的电位差增幅达20左右,使其流变性改善的效果更佳.     

Criteria of the Applicability of Various Powders of Silicon Nitride in the Hot-Pressing Technology for Fabricating High-Density and High-Strength Materials

The possibility of providing a high level of mechanical properties in ceramic materials is proved experimentally by using hot pressing of highly active powders of silicon nitride obtained by plasmachemical synthesis (PCS) and self-propagating high-temperature synthesis (SHS). The requirements placed on the powders of silicon nitride used for the production of high-density and high-strength materials widen substantially.     

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.     

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.     

Excellent corrosion protection performance of epoxy composite coatings filled with silane functionalized silicon nitride

Silicon nitride was firstly used as anticorrosive pigment in organic coatings. An effective strategy by combining inorganic fillers and organosilanes was used to enhance the dispersibility of silicon nitride in epoxy resin. The formed nanocomposites were applied to protect Q235 carbon steel from corrosion. The anticorrosive performance of modified silicon nitride with silane (KH-570) was investigated by electrochemical impedance spectroscopy (EIS), water absorption and pull-off adhesion methods. With the increase of immersion time, the corrosion resistance as well as adhesion strength of epoxy resin coating and unmodified silicon nitride coating decreased significantly. However, for the modified silicon nitride coating, the corrosion resistance and adhesion strength still maintained 5.7×10¹⁰ Ω cm² and 7.6 MPa after 2400-h and 1200-h immersion, respectively. The excellent corrosion resistance performance could be attributed to the chemical interactions between KH-570 functional groups and silicon nitride powders, which mainly came from the easy formation of Si-O-Si bonds. Furthermore, the modified silicon nitride coating formed a strong barrier to corrosive electrolyte due to the hydrophobic of modified silicon nitride powder and increased bonds.     

Surface modification of ultrafine silicon nitride powders by calcination

Calcination was used for surface modification of ultrafine silicon nitride powders, which improved the flowability and dispersion stability of their slurries. Surface characteristics of the powder were investigated by X-ray photoelectron spectroscopy, which showed the generation of an oxide layer and partial elimination of amine groups on the particle surfaces. Ion analysis results suggested a drop in both ion conductivity and supernatant concentration after calcination. For the as-received powder, poor flowability and dispersion stability was caused by high-valence cations, which were detrimental to colloidal processing according to the Schulze–Hardy rule, and surface amine groups were also present; both of these were partly removed by calcination. The oxide layer formed on the particle surfaces hydrolyzes into silanol groups when the powder is dispersed in aqueous solutions, which was favorable for surface charging and formation of higher diffuse layer potential on the particles. These factors contributed to the improved flowability and dispersion stability of the slurries.     

Pyrolysis of Preceramic Polymers in Ammonia: Preparation of Silicon Nitride Powders

The pyrolysis of cross-linked polycarbosilanes, polysilanes and polysilazanes in an ammonia atmosphere to 1473 K yields amorphous, low-carbon silicon nitride powders. The efficacy of carbon removal is independent of the polymer's structure or functionality but is partially dependent on the initial cross-linking temperature. The amorphous silicon nitride powders partially crystallize to α-Si₃N₄ by 1773 K.     

Effect of surface silanisation on the dry pressing behaviour of silicon nitride powder

The influence of surface modification of silicon nitride powders with alkylsilanes on their compaction behaviour was studied with an instrumented compacting tool. The powder–powder interactions were investigated by viscosity measurements additionally. All silanes, even those with short alkyl chains, reduce interparticular interactions (e.g. hydrogen bridging) and therefore improve the compaction behaviour of ceramic granules in general.     

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.