Silicon Nitride Powder Research Articles

Ammonium polyacrylate adsorption on “aluminium hydroxides and oxyhydroxide” coated silicon nitride powders

Si 3N 4 powders coated with pseudoboehmite, Al hydroxide with a OH/Al mol ratio ( R) lower than 3 and bayerite, were prepared from different coating solutions and dispersed in water with an ammonium polyacrylate (NH 4PA) to produce 11.8 vol% slips. The coated powders showed isoelectric points at pH 7, 7.5 and 7.5 close to that of the precipitate coatings of pseudoboehmite, Al hydroxide ( R<3) and bayerite, respectively, indicating that the surface coverages were almost complete. The bayerite coated powders had the lowest specific surface area values. The coatings on Si 3N 4 particles were shown to improve the adsorption of NH 4PA dispersant on Si 3N 4. The maximum adsorption of the polyelectrolyte at pH 9.5 increased from 0.01 for the uncoated powder to 0.03, 0.055 and 0.08 mg/m 2 for the coated powders with pseudoboehmite, Al hydroxide ( R<3) and bayerite, respectively.

Systematic Approach for Dispersion of Silicon Nitride Powder in Organic Media: II, Dispersion of the Powder

A novel dispersant—O‐(2‐aminopropyl)‐O′‐(2‐methoxyethyl)‐polypropylene glycol (AMPG)—was developed to disperse submicrometer‐sized Si3N4 powder in nonaqueous media, based on the surface chemistry of the powder. The dispersing phenomena and mechanisms have been studied systematically, both in model systems (using atomic force microscopy and ellipsometry) and in powder systems (using rheological behavior and adsorption isotherms). The results from the model systems correlated well with those from wet powder systems. It is demonstrated that highly concentrated (with a solids volume fraction of &gt;0.50) and colloidally stable nonaqueous Si3N4 suspensions can be realized using AMPG.

Systematic Approach for Dispersion of Silicon Nitride Powder in Organic Media: I, Surface Chemistry of the Powder

To develop novel dispersants for submicrometer‐sized Si3N4 powder, the surface chemistry of a powder has been investigated using thermodesorption, carrier‐gas heat extraction, X‐ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, and zeta potential measurements. This study indicates that the powder surface is composed mainly of silanol groups and exhibits acidic behavior. Furthermore, the interaction affinity of various surface probe molecules with the powder surface has been studied using adsorption isotherms. The detailed description of the surface chemistry can be used as a guide for designing efficient dispersants, as will be presented in part II.

Particle size distribution and dislocation density determined by high resolution X-ray diffraction in nanocrystalline silicon nitride powders

Two silicon nitride powders were investigated by high resolution X-ray diffraction. The first sample was crystallized from the powder prepared by the vapour phase reaction of silicon tetrachloride and ammonia while the second was a commercial powder produced by the direct nitridation of silicon. Their particle size and dislocation density were obtained by the recently developed modified Williamson–Hall and Warren–Averbach procedures from X-ray diffraction profiles. Assuming that the particle size distribution is log-normal the size distribution function was calculated from the size parameters derived from X-ray diffraction profiles. The size distributions determined from TEM micrographs were in good correlation with the X-ray results. The area-weighted average particle size calculated from nitrogen adsorption isotherms was in good agreement with that obtained from X-rays. The powder produced by silicon nitridation has a wider size distribution with a smaller average size than the powder prepared by vapour phase reaction. The dislocation densities were found to be between about 10 14 and 10 15 m −2.

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.

Effect of Granule Properties on the Fracture Origin of Silicon Nitride Sintered Bodies.

The effect of granule properties on the microstructure of sintered silicon nitride bodies and their strength variation was examined. Commercially available silicon nitride, alumina and yttria powders were used as the raw materials. The granules were fabricated under three different spray drying conditions. Employed were several characterization techniques which are capable of directly observing the internal structures of granules, green bodies and of sintered bodies. The strength variation of the sintered bodies was correlated with the dimension of large pores detected by the direct observation of the internal structure of sintered bodies. These pores were originated from the inherent flaws in green compacts. These inherent flaws were introduced by non-uniform packing of powder particles via the incomplete collapse of dimples in granules and also from the cavities between granules. Furthermore, it was emphasized that these large pores in the sintered body was controlled by the granule size, dimple size, as well as the yield stress of granules.

Kinetics of the reaction between silicon nitride and carbon

Through partial reduction of silicon nitride (Si3N4) powders, both silicon carbide (SiC) whiskers [1] and nanometer-sized SiC particles [2] have been produced in situ in the Si3N4 matrix powders. The resultant Si3N4/SiC composite powders were found to have more uniform distribution of SiC than those obtained via mechanically mixing Si3N4 and SiC powders. Wang et al. have reported the reduction kinetics at 1280–1350 ◦C in atmospheric helium [3]. In this paper the reduction kinetics at above 1530 ◦C in atmospheric nitrogen is reported. The preparation procedure of the mixture of Si3N4 and carbon powders has been reported in a previous paper [2], using novolac-type phenolic resin as the carbon source. The raw Si3N4 powder was 95% α-phase, with mean particle size of ∼1 μm measured via sedimentation method, containing 3.18 wt% O, 0.080 wt% Fe, 0.12 wt% free Si, and 0.0047 wt% Cl. The carbon produced in situ via pyrolysis of the resin was amorphous, flakelike, and around the Si3N4 particles. The reduction was conducted at 1530–1570 ◦C in a graphite-resistant furnace filled with 1 atm nitrogen. The heating rate was 10 ◦C /min and the cooling rate was 20 ◦C/min. Reduction at higher temperature was regarded not suitable for the present kinetic study because of its high reaction rate, for example, for the sample containing 10.3 wt% carbon before reduction, at 1600 ◦C the reaction was close to complete within only 10 min. Since nitrogen was released during the reduction, weight loss in the heating procedure was measured to calculate the fraction reacted. According to thermogravimetric analysis conducted at a heating rate of 10 ◦C/min [4], at blow 1300 ◦C SiO2 (the existing form of oxygen impurity in Si3N4 powder) would convert to Si3N4 in the presence of nitrogen, or to SiC in the absence of nitrogen, and the reaction between Si3N4 and carbon starts at 1510 ◦C in atmospheric nitrogen. Therefore, weight loss at 1350 ◦C for 2 min was set as the reference zero point. The fraction reacted, α, was calculated according to

Synthesis of ultrafine silicon nitride powders

Silicon nitride powders were synthesised during vapour phase reaction of silicon tetrachloride (SiCl4) and ammonia (NH3) in a hot wall flow reactor. The effect of process variables on powder composition was investigated by means of thermogravimetric analysis (TGA), specific surface area measurements (BET) and elementary analysis. The powders obtained consist of amorphous silicon nitride (Si3N4) and contain various amounts of volatile ammonium chloride (NH4Cl). The amount of by-products is strongly dependent on reactor temperature. At high temperatures (T = 1200°C), the amount of volatile compounds is less than 35%, chloride is less than 19% and the specific surface area is around 40m²g-1. The initial NH3/SiCl4 ratio used does not significantly affect the ammonium chloride formation, indicating that temperature and flow rate rather than concentration of ammonia determine the formation of NH4Cl. At temperatures 900°C and lower all chloride originating from SiCl4 was converted to ammonium chloride.

Surface Modified Silicon Nitride Powder with Highly Dispersed Sintering Aid via Aqueous Processing.

Highly dispersive loading of Y3+ ions, as a sintering aid, on Si3N4 powder via aqueous processing has been presented. The chemical stability of Si3N4 powder in pressured and saturated water vapor was improved by a chemical surface modification with dicarboxylic sebacic acid, while maintaining hydrophilic surface. The treatment of the sebacic acid-modified powder with yttrium acetate tetrahydrate resulted in strong immobilization of Y3+ ions at a highly dispersive level on the Si3N4 powder surface through ion-exchange with the free carboxyl groups of the sebacic acid molecules attached to the Si3N4 surface. The highly dispersive sintering aid promoted hot-press densification of Si3N4 even with a small loading amount, in comparison with materials added with Y2O3 powder as a sintering aid.

The effect of heat-treatment on the grain-size of nanodisperse plasmathermal silicon nitride powder

Nanodisperse silicon nitride has been synthesized by vapor phase reaction of silicon tetrachloride and ammonia in a thermal plasma reactor and crystallized at temperatures 1250, 1350, 1450 and 1500°C. The average grain-size and the dislocation density of the samples were determined by the recently developed modified Williamson-Hall and Warren-Averbach procedures from X-ray diffraction profiles. A new numerical method provided log-normal grain-size distributions from the size parameters derived from X-ray diffraction profiles. It has been shown that the average grain-size in the amorphous phase is lower than that observed in the crystalline fraction. On the other hand, the average grain-size in the crystalline fraction decreases up to 1450°C while it increases during heat-treatment at 1500°C. The size distribution and the area-weighted average grain-size obtained by X-rays were in good agreement with those determined by TEM and from the specific surface area, respectively. The dislocation density was found to be of the order of 1014 and 1015 m−2.

Various Types of Bubble-Containing Silica Glasses Fabricated by Sintering Powder Mixtures of Silica with Silicon Nitride.

Bubble-containing silica glasses were fabricated by sintering amorphous silica powders with small amount of silicon nitride powders. Sintering was performed at 1800°C in N2 gas atmosphere. It was found that the addition of Si3N4 is effective for forming bubbles. By controlling the Si3N4 content, various types of bubble-containing silica glasses, such as the so-called the opaque silica glass and the foamed silica glass, could be obtained. Addition of 0.004mol% Si3N4 produced highly opaque silica glasses containing a large number of small bubbles, while 0.431mol% Si3N4 resulted in low-density foamed silica glasses containing large 1mm-sized bubbles. Wide variations of bubble size and density were observed on these glasses. Thermal conductivities were estimated and compared with several literature values. The relation between thermal conductivity and bubble density was discussed based on a model simulation.

Plastic Forming of Silicon Nitride Powders via Colloidal Processing

Colloidal powder processing improved the reliability and strength of structural ceramics by reducing the size of strength degrading heterogeneities which is accomplished by filtering the powder prior to consolidation as demonstrated with silicon nitride ceramics. Ceramic colloidal powder processing attempts to develop the science of how advanced ceramic powders can be formed similar to traditional clay based ceramics. A shortrange repulsive potential is developed to produce plastic bodies. Knowing how to develop these potentials and maintain them during particle packing has lead to the production of saturated powder compacts that enable the forming of complex, engineering shapes by plastic deformation.

Nitridation of silicon powder studied by XRD, 29Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and 29Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si 2N 2O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si 3N 4, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the 29Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si 3N 4 resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©

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. ©

Crystallization of an amorphous silicon nitride powder produced in a radiofrequency thermal plasma

Crystallization behavior of an amorphous silicon nitride powder produced in an RF thermal plasma by the vapor-phase reaction of silicon tetrachloride and ammonia has been investigated. Effects of annealing conditions such as temperature and duration of heat treatment on the properties of powders were studied. Changes in the chemical and phase compositions, as well as in the morphology of powders were measured and interpreted. Annealing of the amorphous silicon nitride powder at 1450°C for 120 min resulted in a powder of about 80% crystalline phase content with an α/β ratio of about 6.5. ©

Densification of nanosized amorphous and crystalline silicon nitride powders

The hot-pressing behavior of two amorphous and three crystalline silicon nitride powders, including both experimental and commercial samples has been investigated in the presence of MgO-Y 2O 3 sintering aid. The powders were characterized in terms of bulk and surface chemistry, phase composition and morphology. The sintering behavior was assessed on the basis of green and final densities, weight loss on densification and chemical and phase compositions of the dense material. ©

Resistance heating of the gasket in a gem-anvil high pressure cell

Resistance heating of the gasket strip in a gem-anvil high pressure cell was successful in obtaining sample temperatures up to 1100 °C, under pressures up to 4.0 GPa. The heating capabilities, as well as the mechanical and chemical stability, of several different gasket strips (two Ni-based alloys, Ta, Pt/Rh, and a Re/Mo alloy) with different design shapes, and two different single-crystal anvil materials (diamond and cubic zirconia) were investigated. Two gasket-strip designs were found to provide optimum uniform heating conditions while decreasing the required current needed to achieve 1100 °C. Two anvil systems were investigated to reduce the temperature increase of the pressure cell body. Cubic zirconia anvils reduced the cell-body temperature to 100 °C at sample temperatures up to 1100 °C. However, zirconia anvils often failed during heating and almost always failed during cooling. Diamond anvils with cubic zirconia mounting plates also permitted temperatures up to 1100 °C to be reached without anvil failure. However, the cell-body temperature increased to 300 °C. A sealed vacuum-type chamber was employed to eliminate the problem with gasket and anvil oxidation. The optimized operating parameters reported here provide a routine method for high temperature-high pressure studies. The method was used to densify and sinter nanosize amorphous silicon nitride and γ-alumina powders at high temperatures and high pressures.

Ceramic innovations in the 20th century

Ceramic innovations in the 20th century

Influence of chemical pretreatment on the surface properties of silicon nitride powder

Adsorption energy distributions for nitrogen at the surface of silicon nitride samples with different pretreatment have been obtained and related to the heats of adsorption determined by Brunauer–Emmett–Teller analysis of adsorption isotherms. This has confirmed the conjecture that, in normal conditions, the surface of silicon nitride is covered by a layer of silica, as suggested by earlier studies. An attempt to correlate adsorption data with ab initio potentials of the adsorbate–adsorbent interaction has been made. The charge-regulated behaviour of the surface of silicon nitride in aqueous electrolytic solutions has been investigated in relation to charge titration and electrokinetic experiments. An important role of surface hydrolysis in the interpretation of experimental data has been pointed out. Based on the surface ionization parameters extracted from charge titration data, interparticle forces governing the stability of colloidal dispersions have been calculated in the framework of Derjaguin’s approximation.

Synthesis of Silicon Nitride/Silicon Carbide Nanocomposite Powders through Partial Reduction of Silicon Nitride by Pyrolyzed Carbon

An alternative method to incorporate nanometer‐sized silicon carbide (SiC) particles into silicon nitride (Si3N4) powder was proposed and investigated experimentally. Novolac‐type phenolic resin was dissolved in ethanol and mixed with Si3N4 powder. After drying and curing, the resin was converted to reactive carbon via pyrolysis. Si3N4 powder was partially reduced carbothermally using the pyrolyzed carbon, and nanometer‐sized SiC particles were produced in situ at 1530°‐1610°C in atmospheric nitrogen. At temperatures &lt;1550°C, the reduction rate was low and the SiC particles were very small; no SiC whiskers or barlike SiC was observed. At 1600°C, the reduction rate was high and the reaction was close to completion after only 10 min, with the appearance of SiC whiskers as well as curved, barlike, and equiaxial SiC, all of which were dozens of nanometers in diameter; this size is greater than that at observed temperatures &lt;1550°C. A longer soaking time at 1600°C led to agglomerates. SiC particles were close to the surface of the Si3N4 particles. The SiC content could be adjusted by changing the carbon content before reduction and the reduction temperature. A reaction mechanism that involved the decomposition of Si3N4 has been proposed.