Silicon Nitride Powder Research Articles

Study of AlN and Si 3N 4 powders synthesized by SHS reactions

The particular characteristics of aluminum nitride and silicon nitride powders obtained by SHS technology are reported. The synthesized powders showed high particle sizes that are not suitable for sintering. Consequently, the powders were subjected to an energetic milling process using an attritor with different milling media and times. Silicon nitride powders were milled with Si 3N 4 balls while for aluminum nitride powders different media (Si 3N 4, Al 2O 3, ZrO 2) were used. Particle size and specific surface area were determined in both powders as a function of the milling variables. The increase in the level of impurities associated with the milling procedure was measured. The morphology of each powder was analyzed before and after milling by scanning electron microscopy. The results were evaluated by comparing with characteristics of typical commercial powders of AlN and Si 3N 4, to establish the differences with the SHS powders. ©

Carbothermal reduction and nitridation of silica: nuclei planar growth controlled by silicon monoxide diffusion on the reducer surface

The carbothermal reduction and nitridation of silica as a source of low-cost silicon nitride powders prepared from cheap raw materials, lamp black and fumed silica, was investigated in the temperature range of 1200–1450°C. Carbon and silica with a specific surface area of 27 and 176 m 2 g −1, respectively, were wet mixed to form gels. The degree of mixing of carbon and silica, as given by microscopic observation, and the Si 3N 4 reaction yield were optimised in the study of a set of mixed gels that were dispersed using water and ethanol as dispersing liquids. The Si 3N 4 reaction yield at 1450°C was further increased to a value of about 96% of the conversion ratio of silica into Si 3N 4 and silica loss as SiO(g) was controlled to a value of below 1% by using lamp black with a specific surface area of 390 m 2 g −1. Quantitative analysis of the reaction kinetics of the carbothermal reduction and nitridation of silica reveals that it is of exponential type, as given by the Johnson–Mehl–Avrami law for processes controlled by fast nucleation followed by diffusion-limited crystal growth. By selecting carbon sources of high specific surface area, the Si 3N 4 nuclei density number is increased, yielding nearly spherical particles of Si 3N 4 powders with sub-micron sizes.

Direct determination of trace copper and chromium in silicon nitride by fluorinating electrothermal vaporization inductively coupled plasma atomic emission spectrometry with the slurry sampling technique

A method has been developed for the determination of trace impurities in silicon nitride (Si3N4) powders by fluorination assisted electrothermal vaporization (ETV) /ICP-AES using the slurry sampling technique. Polytetrafluoroethylene (PTFE) emulsion as a fluorinating reagent not only effectively destroys the skeleton of Si3N4, but also carries out selective volatilization between the impurity elements (Cu, Cr) and the matrix (Si). The experimental parameters influencing fluorination reactions were optimized. The detection limits for Cu and Cr are 1.05 ng/mL ( Cu) and 1.58 ng/mL (Cr), the RSDs are in the range of 1.9–4.2%. The proposed method has been applied to the determination of Cu and Cr in Si3N4 ceramic powders. The analytical results were compared with those obtained by independent methods.

Morphology and Sinterability of Thermally Treated Carbothermally Synthesized Silicon Nitride Powders

Carbothermally produced alpha‐Si3N4 powder is thermally treated (1400°‐1500°C for 4‐12 h) in nitrogen‐gas to determine the impact of thermal treatment on powder composition, particle size, morphology, and sinterability. Thermal treatment reduces residual carbon and oxygen content and reduces internal porosity (lower surface area), which results in a rounding of the particles. The lower oxygen content of the heat‐treated, carbothermally produced alpha‐Si3N4 powder allows Y2O3‐Al2O3‐SiO2 sintering formulation to be adjusted so that an improvement in the early stage of densification can be achieved.

α-SiC layers grown on silicon via nano-sized silicon nitride precursor route

α-SiC thin layers were successfully grown on silicon (111) by pyrolysis of the organic liquid dip-coating films and polyimide Langmuir–Blodgett (LB) films containing nanometer sized amorphous silicon nitride powders (NASNP) at 900–1000°C in vacuum. The products were characterized by X-ray diffraction (XRD), infrared absorption (IR) and Raman spectra, the results of which strongly support the formation of preferred orientated 4H-SiC in the layers by pyrolysis of LB films containing NASNP. Based on the IR spectra of samples pyrolyzed at different temperatures and the computed values of the Gibbs free energy changes of the reactions, the mechanism of formation of α-SiC is discussed.

The study of shock-activation and sintering of silicon nitride powder materials

Abstract In this work, Si 3 N 4 ceramic has been fabricated by means of shock-activating and hot-press sintering. Two kinds of shock pressure, i.e. 15.7 and 25.6 Gpa, obtained by explosive loading, were used in the experiments. TEM, SEM, and X-ray diffraction have been used to analyse the properties of the Si 3 N 4 powders and ceramics. The results show that the shock-activation has an obvious positive action in the densification in the sintering process, and that the bending strength of the sintered ceramics activated by shock waves is additionally related to sintering temperature. This is because, for Si 3 N 4 , the phase transformation of α → β is an essential factor in the strength, and the sintering temperature shows an important effect on phase transformation.

Comparison of oxidation behaviour of sintered ceramics produced from various silicon nitride powders

Sintered ceramics produced from two kinds of silicon nitride powder were oxidized at 1000 and 1400°C. The density, the mechanical properties and the phase composition of the oxide layer and the internal region (without the oxide scale) of one material were compared with those of the other. It was found that the difference in the starting powder properties caused differences in the oxide layer thickness, and the phase composition and the mechanical properties of the internal part.

Effects of dispersant on rheological behavior of silicon nitride aqueous suspension

Colloidal processing techniques such as slip casting, centrifugal slip casting or pressure slip casting are considered as a powerful route to fabricate silicon nitride ceramic components with complex shapes [1, 2]. For this purpose, Si3N4 powder should be dispersed efficiently in liquid medium thus forming a desirable suspension with the lowest viscosity and highest solid content. Colloidal behavior of Si3N4 suspension with either aqueous or organic medium has been studied widely [3–7]. The same Si3N4 powder behaves differently depending on the various processing parameters applied. In order to achieve promising powder suspensions, dispersants are frequently used [7–10]. After that the particle surface has been modified with dispersant by either changing the surface charge, i.e. by electrolytes, or by coating with a large amount of organic barrier, i.e. by polymers, or by a combined action of both factors, i.e. by polyelectrolytes, Si3N4 particles can be stabilized to a promising level in liquid media. The introduction of dispersant to suspension generally has important influence on the electrokinetics of suspended particles and the rheological behavior of suspension. In this letter, some experimental results about the effects of dispersant on the rheological behavior of Si3N4 aqueous suspension will be discussed. Silicon nitride powder (H.C. Starck, Germany) with an average particle size of 0.60 μm and a BET specific surface area of 21.0 m2 g−1 was used for the present study. The dispersant used was an inorganic compound which stabilized the suspension by changing the surface charge of Si3N4 particles with its negatively charged anion group in electrostatic mechanism. The zeta potential of Si3N4 particles was measured with diluted Si3N4 dispersion (volume fraction, φ < 0.005) at 25 ◦C on Zetasizer 4 (Malvern, UK). The rheological behavior of Si3N4 suspension was characterized in steady shear mode with a couette (cup radius: 34.0 mm, bob radius: 32.0 mm, bob length: 33.3 mm) on Rheometric Fluid Spectrometer II (RFS II). The zeta potential of Si3N4 particles as a function of pH of suspension is shown in Fig. 1. As can be seen the isoelectric point (IEP) of Si3N4 particles is at the pH value of 4.2 and the particles have a maximum zeta potential at about pH = 11. The introduction of dispersant increases efficiently the zeta potential of Si3N4 particles in magnitude and shifts the IEP to acid region. Meanwhile, the pH values corresponding to the maximum of zeta potential for Si3N4 particles with different dispersant concentration remain constant. Therefore, in this work, all suspensions investigated are kept at pH = 11 expecting a good fluidity obtained. It seems that the changes of zeta potential and IEP of Si3N4 particles result from the increasing of the charging density of the particles caused by the adsorption of negatively charged anion groups of dispersant on the surface of Si3N4 particles. Fig. 2 illustrates the flow curves of 23.9 vol % Si3N4 aqueous slurry containing various amounts of dispersant. As is shown, the behavior of slurry transfers from an anti-thixotropy via near Bingham fluid to thixotropy with increasing amount of dispersant. It is clear that the Si3N4 suspension studied is a non-Newtonian time-dependent fluid which exhibits a pronounced thixotropy. When the amount of dispersant is below 0.8 wt % (weight percent, on a powder basis) the suspension exhibits an anti-thixotropic behavior and the anti-thixotropy decreases with increasing the dispersant amount. The anti-thixotropic behavior of the suspension is considered as due to the irregular morphology of Si3N4 particles. At appropriate dispersant range from 0.8–1.2 wt %, the particles are well dispersed in media so that the anti-thixotropic behavior disappeared and a near Bingham behavior is exhibited. With further increasing of the dispersant, the excessive dispersant moleculars are bridged together to produce some particle networks in suspension which could be broken down in the high applied shear field, so the suspension exhibits a thixotropic behavior at higher dispersant amounts.

Preparation of magnesium silicon nitride powder by the carbothermal reduction technique

Magnesium silicon nitride (MgSiN 2) powder was prepared by carbothermally reducing a magnesium metasilicate whose chemical composition corresponded to MgO SiO 2 or MgSiO 3. About 0.2 g of the powder mixture of magnesium metasilicate and carbon (C) with the molar ratio of C to MgO SiO 2 equal to 6.0 was heated at 1250°C for 7 h in nitrogen atmosphere. The crystalline phase of the carbothermally reduced powder was only MgSiN2. The residual carbon could be removed by heating the powder at 600°C for 1 h in air. The yield of MgSiN 2 powder was ∼ 70%. The resulting powder contained 3.22 mol% oxygen. The primary particle sizes were ranging from 0.1 to 0.5 μm.

Investigation of Fine-Dispersion Silicon Nitride and Oxinitride Powders Manufactured by Titanium–Magnesium Production Waste Nitriding

The nitriding of titanium–magnesium production waste containing chlorine was studied using X-ray, IR, EPR, and EPMA techniques as well as chemical and mass spectrometry analyses. It was established that the high-dispersion system, consisting mainly of silicon nitride and oxinitride, formed in the temperature range of 1273–1673 K. The complex defective structure of initial silicon particles and their size determine the mechanism of nitriding and phase composition of the product formed. The base of this mechanism is the formation of free Si— and Si— bonds.

Slurry sampling of silicon nitride powder combined with fluorination assisted electrothermal vaporization for direct determination of titanium, yttrium and aluminum by ICP-AES

P. Tianyou, J. Zucheng and Q. Yongchao, J. Anal. At. Spectrom., 1999, 14, 1049 DOI: 10.1039/A809221D

Weight loss and gas evolution during sintering of silicon nitride powders synthesized by silica reduction and imide decomposition

The relationships among weight loss, gas evolution, and other changes during sintering were investigated for silicon nitride powders synthesized by silica reduction and imide decomposition. Small weight losses were detected for all powders at low temperatures (below 1400 °C), and large weight losses were found to occur for the silica-reduction powders at high temperatures (1400–1800 °C). Only CO was detected in the sampled gases during sintering, and CO evolution peaks were seen at around 900–1200 (broad), 1400, and 1600 °C. The main deposited material around the samples was SiO. The weight losses at low temperatures were thought to be caused by evaporation of residual binder or adsorbed substances. The main causes of the large weight losses at high temperatures were thought to be CO and SiO evolutions, and CO evolution at around 1600 °C probably originated from the liquid phase accompanied by α–Si3N4 to β–Si3N4 transformation.

Shock-enhanced alpha to beta phase transformation in Si 3 N 4 powders

Abstract Shock-compression was used to make Si 3 N 4 powder compacts (80–85% theoretical density) in order to investigate its effect on the α – β phase transformation occurring during post-shock thermal treatments. Crystallite size reduction to half of the starting powder size was attained, and residual strains in crystallites in excess of 10 −3 –10 −2 , corresponding to dislocation densities of the order of 10 15 –10 17 cm −2 , were generated due to shock compression. Transmission electron microscopy showed evidence of defects in the form of irregular striations (fringes), hexagonal dislocation arrays, sub-grains, and vacancy clusters in the interiors of the individual crystallites, with contrast-free amorphous regions along interparticle boundaries. Upon subsequent annealing (sintering) of the shock-densified compacts, the α – β phase transformation was observed to occur at temperatures of ≈200°C below that in unshocked powders, with no obvious change in density. The amount of α – β phase transformation increased with increasing temperature and time, with up to 87% conversion to β -phase in the compact heated for 5 h at 1650°C. The apparent activation energy for the α – β phase transformation in the shock-compressed silicon nitride powder compacts was determined based on the measured fraction transformed as a function of time and temperature, and was found to be in the range of 154–286 kJ mol −1 . The results provide evidence that shock compression activates and enhances the reactivity of Si 3 N 4 powders, by creating defect sites for heterogeneous nucleation of the β -phase, and thereby causing the α – β phase transformation to occur in the solid state and at reduced temperatures.

Dispersion Properties of Silicon Nitride Powder Coated with Yttrium and Aluminium Precursors

A coated silicon nitride (Si3N4) powder with yttria and alumina precursors as sintering additives was prepared by a heterogeneous precipitation method. The rheological and electrophoretic properties of the suspensions obtained from the coated (CO) powder were investigated and compared with those of pure Si3N4powder and of the mechanically mixed (MM) powders of Al2O3, Si3N4, and Y2O3. The results showed that the CO powder calcined at 500°C exhibited improved dispersion properties compared with the pure Si3N4powders. The CO powder possessed the surface character of Al2O3and Y2O3particles, that made it easier to process in aqueous media, yielding a higher solid loading than the pure Si3N4powder. These improvements were attributed to a change in the resultant interaction forces between particles from attractive (pure Si3N4, and MM powders) to repulsive in the case of the CO powder. A homogeneous distribution of sintering additives in the Si3N4matrix was obtained.

Microstructure and Properties of Nanosemicrystalline Si3N4 Ceramics with Doped Sintering Additives: Part I. Microstructural Characterization of Nanosemicrystalline Si3N4 Powders

The characteristics of nanosized silicon nitride powders with doped Y2O3 and Al2O3 fabricated by a plasma-reacted chemical process were investigated. The chemical compositions of the powders were analyzed by wet chemical analysis. The morphology and the size distribution were determined by transmission electron microscopy (TEM). TEM with energy dispersive spectroscopy (EDS) was used to verify the existence of sintering additives in each individual particle. The crystal structure of the powders was identified by the selected area diffraction pattern (SADP). X-ray diffraction (XRD) technique was used for phase analysis and the measurement of degree of crystallinity. The characteristics of chemical bonding was analyzed by using Fourier transform infrared spectroscopy (FTIR).

Particle formation paths in the synthesis of silicon nitride powder in a laser-heated aerosol reactor

The Laser-chemical vapour precipitation of Si 3N 4 powder by reacting SiH 4 and NH 3 was studied using a newly developed laminar gas mixing system. Thermophoretic sampling enabled monitoring of the formation process on a milli-second basis and shows the existence of two synthesis paths: the nitridation of Si particles and the sintering of directly formed Si 3N 4. The latter route leads to almost stoichiometric α- and β-Si 3N 4, with particle sizes up to 150 nm. The Si 3N 4 particles do not decompose above the thermodynamic decomposition temperature, probably due to kinetic reasons since the residence time is of the order of milliseconds.

Desorption of gases from the surface of silicon nitride powders

The influence of fabrication and treatment techniques and of carbon coating onto silicon nitride powders on the process of gaseous component desorption has been studied. The N2, H2O, CO2, and CO molecules, as well as OH and NH radicals, were revealed to be absorbed on the particle surface. Different hydrocarbons are the products of reaction in the gas phase and on the surface. Carbon coating on the powder particle surface alloys one to protect the surface from absorbed gases and from chemosorbed compounds, in particular hydroxylic groups. The results obtained permit one to determine the conditions of intended modification of surface and thermal treatment of powders in order to activate their sintering.

CEramics based on α-sialon

Ceramics containing pure α-sialon and α-sialon in combination with β-Si3N4, produced from two types of silicon nitride powder, are described. It is shown that the high-temperature strength of the ceramics increases with increase in the content of α-sialon in the composition. Data on the microstructure, phase composition, and mechanical characteristics of the synthesized materials are presented.

Silicon nitride: From sintering to joining

In earlier work, silicon nitride (Si3N4) powders were sintered with a mixture of rare-earth oxide (RE2O3) and silica (SiO2). When the chemistry and sintering schedule is carefully controlled, the intergranular phase is a crystalline RE2Si2O7 phase, which imparts improved mechanical and oxidation properties to the Si3N4. Based on the premise that a material that is successful in joining individual grains will also be suitable for joining bulk pieces, the process has been extended to join bulk Si3N4. This method of joining causes the elimination of the joint, as the mechanism of joining is the same as that by which Si3N4 powder sinters. By analogy with the results for sintered compacts, it is predicted that the rare-earth oxide joined parts may also exhibit improved high temperature performance.

Silicon nitride: From sintering to joining

In earlier work, silicon nitride (Si3N4) powders were sintered with a mixture of rare-earth oxide (RE2O3) and silica (SiO2). When the chemistry and sintering schedule is carefully controlled, the intergranular phase is a crystalline RE2Si2O7 phase, which imparts improved mechanical and oxidation properties to the Si3N4. Based on the premise that a material that is successful in joining individual grains will also be suitable for joining bulk pieces, the process has been extended to join bulk Si3N4. This method of joining causes the elimination of the joint, as the mechanism of joining is the same as that by which Si3N4 powder sinters. By analogy with the results for sintered compacts, it is predicted that the rare-earth oxide joined parts may also exhibit improved high temperature performance.