Ultrafine Si3N4 Research Articles

SHS of ultrafine and nanosized Si3N4 powders: The effect of inorganic and organic additives on the microstructures, morphology, and phase compositions of products

The relationship between the microstructure and phase composition of a product obtained by selfpropagating high-temperature synthesis (SHS) and different reaction mechanisms is studied using the example of silicon nitride. The formation of nanosized silicon nitride particles upon the combustion of silicon in nitrogen and other phenomena typical of substances of this class, such as self-organization and self-assembly, are observed. The chemical condensation SHS of silicon nitride is studied in the presence of inorganic salts as modifying additives. The resulting Si3N4 powders have a structure of secondary spherical particles being composed of primary ultrafine and nanosized ones. The Si3N4 powders under study were fractionated depending on the particles size and microstructure by chemical dispersion. Ultrafine particles of the light fraction form as homogeneous rods by the conventional vapor–liquid–crystal mechanism. The fine-crystalline fractions are formed by hollow crystals assembled into globules. Microstructural analysis shows that the crystal walls consist of nanosized Si3N4 particles. The effects of organic additives introduced into the charge stock on the phase formation, microstructure formation, and particle size of SHS silicon nitride are studied. The combustion products are found to be composite powders whose phase compositions depend on the additive and the synthesis conditions. According to the data from X-ray powder diffraction and chemical analysis, the final products can contain up to 100% of β-Si3N4, up to 60% of α-Si3N4, up to 80% of SiC, and up to 70% of Si2N2O. It is shown that silicon nitride-based composite powders containing silicon carbide and/or silicon oxynitride can be obtained in one step. The role of ferric chloride hexahydrate as the SHS catalyst is studied. An ultrafine structurally homogeneous powder of β-Si3N4 is obtained in the presence of FeCl3 · 6H2O.

Injection molding of ultra-fine Si3N4 powder for gas-pressure sintering

The ceramic injection molding technique was used in the gas-pressure sintering of ultra-fine Si3N4 powder. The feedstock’s flowability, debinding rate, defect evolution, and microstructural evolution during production were explored. The results show that the solid volume loading of less than 50vol% and the surfactant mass fraction of 6wt% result in a perfect flowability of feedstock; this feedstock is suitable for injection molding. When the debinding time is 8 h at 40°C, approximately 50% of the wax can be solvent debinded. Defects detected during the preparation are traced to improper injection parameters, mold design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green body. The bulk density, Vickers hardness, and fracture toughness of the gas-pressure-sintered Si3N4 ceramic reach 3.2 g/cm3, 16.5 GPa, and 7.2 MPa·m1/2, respectively.

Nanopowder processing of ultrafine Si3N4 with improved wear resistance

The processing, microstructure, and material properties of monolithic Si3N4 were investigated by using two amorphous Si3N4 nanopowders doped with (1) 6 wt% Y2O3 and (2) 6 wt% Y2O3 + 8 wt% Al2O3. The orthorhombic Si2N2O-based phase was found in the two as-sintered bulks. Carbothermal reduction treatment (CRT) was thus applied at 1400 °C for 10 h with 3 wt% and 6 wt% carbon black added to the precursor powders of the Si–Y–O–N and Si–Y–Al–O–N systems, respectively, resulting in an increased nitrogen-to-oxygen (N:O) ratio and elimination of Si2N2O within the sintered bulks. The possible mechanisms of nucleation and grain growth of Si3N4 are discussed during CRT and the sintering process. The best wear resistance was achieved in ultrafine Si3N4 doped with Y2O3, which has good hardness, indentation toughness and a protective tribo-film.

Development of SiAlON - From Mechanical to Optical Applications

Various rare-earth-doped α-SiAlON powders with high purity were prepared to study mechanical and optical properties of SiAlON-based functional materials in connection with ionic radius and electronic structure of rare-earth elements. Single phase rare-earth-doped α-SiAlON powders were obtained at a temperature as low as 1873 K by heating powder mixtures of rare-earth oxide, AlN and highly active ultrafine amorphous Si3N4. Bending strength of highly dense rare-earth-doped α/β-SiAlON-based ceramics was increased with decreasing radii of rare-earth ions, i.e., Yb-SiAlON-based ceramics exhibited excellent high-temperature strength and oxidation resistance caused by the small ionic radius of ytterbium. As for optical application, α-SiAlON is an excellent host lattice with good thermal and chemical stability for doping rare-earth element which activates photoluminescence. Europium-doped Ca-α-SiAlON phosphor formulated as CaxEuy(Si,Al)12(O,N)16 (where 0<x+y<2) was prepared to obtain high quality phosphor with high brightness and desired emission characteristics. Photoluminescence spectra of the resultant Europium-doped Ca-α-SiAlON exhibited high emission intensity at peak wavelength of 580-600 nm giving the better yellow color tone than Cerium-doped yttrium aluminum garnet for applying white LED. It was demonstrated that nitrides or oxynitrides were the innovative materials for the diverse range of high performance specialty applications.

Effect of mechanical activation on synthesis of ultrafine Si3N4–MoSi2 in situ composites

Abstract Si 3 N 4 –MoSi 2 in situ composite has been synthesized by reacting powders of molybdenum (Mo) and silicon nitride (Si 3 N 4 ). Mo and Si 3 N 4 powders mixture in a molar ratio of 1:3 were ball milled for 0–100 h. The milled and unmilled powder mixtures were reacted at different temperatures between 1000 and 1600 °C in an argon atmosphere. The effect of mechanical activation (MA) induced by milling has been studied through X-ray diffraction (XRD), differential thermal analysis (DTA), and thermo-gravimetric analysis (TGA). No peaks of Mo in the XRD pattern have been observed after 70 h of milling. The crystallite size of the Mo has been found to be the lowest (41 nm) after milling for 30 h. Similarly, a 100 nm lowest size of crystallite of Si 3 N 4 was observed after milling for 50 h. DTA and TGA results show that the reaction between Mo and Si 3 N 4 enhances with increase in milling time. Milling for 10 h lowers the pyrolysis temperature by 150 °C. Additional milling upto 100 h does not lead to further reduction in the pyrolysis temperature. The intensities of peaks of MoSi 2 in the pyrolysed samples increased with increase in milling time. MoSi 2 particles of size less than 1 μm were observed to be uniformly distributed through out the Si 3 N 4 matrix.

Magnetic Flux Penetration into Polycrystalline Superconducting (Bi,Pb)2Sr2Ca2Cu3O10 + xCeramics Containing Additions of Inorganic Compounds

The magnetic flux distribution in superconducting (Bi,Pb)2Sr2Ca2Cu3O10 + x ceramics containing small (0.05–0.3 wt %) additions of ultrafine HfN, Ta3N5, Si3N4, NbC, TaC, and SiC was studied by magneto-optic imaging and scanning Hall magnetometry. The ceramics were prepared by mixing (Bi,Pb)2Sr2Ca2Cu3O10 + x powder with ultrafine additions, followed by pressing and sintering at 840°C for 10 h. The samples were found to differ in magnetic flux penetration and trapping. The critical current density in the samples was determined as a function of temperature and magnetic field.

Preparation, structure, and properties of (Bi, Pb)2Sr2Ca2Cu3O10+δ ceramics with Si3N4 additions

The microstructure, phase composition, texture, and superconducting properties (Tc, ΔTc, jc(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)2Sr2Ca2Cu3O10 + δ ceramics (sintering at 840°C for 36 h) with ultrafine Si3N4 additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si3N4 is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.

Synthesis and Characterization of Ultra-Fine Si3N4 by Electric Arc Plasma

Under conditions of electric-arc low-temperature plasma (LTP), ultra-finely dispersed Si3N4 particles have been synthesized by using silicon powder and nitrogen as raw materials. The prepared samples are characterized by x-ray diffraction (XRD), scanning electron spectroscopy (SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The result indicates that the basic phase in Si3N4 produced is α- and β-Si3N4. The particle size of Si3N4 sample is in the range of 30-500 nm.

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si3N4−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m2/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si3N4, β-Si3N4, β-SiC, and X-ray amorphous Si3N4. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders.

Microstructures of ultrafine Si3N4 powder compacts induced by rapid heating under controlled thermograms

The rapid heating of ultrafine Si3N4 powder compacts with relatively high oxygen content was investigated with particular attention to their microstructures. The specimens were heated without resorting to additives and pressure under controlled thermograms attained by an Xe image heating apparatus. The effects of particle size and oxygen contents, as well as heating conditions, were investigated. When fired to 1700°C within 15 s and then immediately held at 1350°C for 10 min in N2 atmosphere, significant densification took place in the limited region, in addition to decreasing the oxygen content to less than 0.3 wt%. This decrease of oxygen content was drastic and found to be a prominent feature of this heating process; especially, weakly oxidized ultrafine powder less than 30 nm was found to be advantageous to obtain uniform and homogeneous Si3N4 microstructures.

Fourier transformation infrared investigation of surface oxidation of ultrafine Si3N4 powders

ne univ,dept chem,shenyang 100006,peoples r china.;li, yl (reprint author), acad sinica,inst met res,state key lab rsa,72 wenhua rd,shenyang 110015,peoples r china

Sintering and microstructure of silicon nitride prepared by plasma CVD

Ultrafine Si3N4 powder with average particle size of 30 nm prepared by a thermal plasma CVD was sintered at 1750 °C in nitrogen for 1 h. The sintering behaviour of the powder was characterized by the crystallization of the powder and the resultant sintered bodies were observed with microscopes. It was found that the sinterability depended strongly on the green density and the degree of crystallization. If the powder was homogeneously mixed with sintering additives, it sintered to 98% density at 1750 °C in a nitrogen atmosphere. The microstructure of the sintered bodies observed by SEM indicated that they consist of needle-like grains with an aspect ratio of about 4. The microstructure of a thin film of the sintered body observed by TEM indicated that the grains with crystal habits were wet with liquid phase. TEM also clarified that two kinds of grain boundaries were present; one was wet with liquid phase along a grain boundary and the other was a coincident one without liquid phase. The lattice fringes of liquid phase suggested the presence of Y-apatite which would be generated during cooling.

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si3N4 and Si3N4+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio‐frequency plasma and an arc jet. The reactant, SiCl4, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH4 and/or NH3, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si3N4 powder synthesis, reaction effieciency increased significantly by multistage injection compared to single‐stage injection. The most striking result is that amorphous Si3N4 with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH3/SiCl4 molar ratio of ∼ 1.33, although the crystallinity depended on the NH3/SiCl4 molar ratio and the injection method. For the preparation of Si3N4+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH3 and CH4 reactant gases and the H2 quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.

Hydrothermal Synthesis and Sintering of Hydroxyapatite Powders with Si<sub>3</sub>N<sub>4</sub> Whiskers Dispersion

Hydroxyapatite (HAp) powders with Si3N4 whiskers dispersion were synthesized hydrothermally at 200°C under 2MPa for 10h. The transmission electron microscopy (TEM) demonstrated that the homogeneous mixture of ultra-fine HAp single crystals and Si3N4 whiskers was obtained by this technique. The mixture was hot-pressed at 1000°C under 30MPa for 1h in Ar, and then post-sintered by HIP at the same temperature under 200MPa of Ar for 1h. The product had almost pore free microstructures consisting of HAp and TCP matrix and Si3N4 whiskers. The value of KIC was 3.1MPa⋅m1/2, and was 3 times as high as that of the transparent HAp ceramics.

超微粒子の粒径評価に関する一考察

The size distributions of ultrafine Si3N4 and TiO2 particles prepared by CVD reactions were measured by the sedimentation photo extinction (SPE) method and were compared with the results obtained by a TEM photograph of the particles. Relative changes of both particle sizes were well indicated by the SPE method, although the obtained values might be biased by the dependence of the extinction coefficients on the particle diameter and other factors. The two different methods showed good agreement if the sample particles were well dispersed, and it was shown that the SPE method could be applied to examine the size distribution of ultrafine particles.For Si3N4 samples, the temperature and concentration dependence of the particle diameter was examined. Agglomeration of TiO2 particles was discussed by comparing the distributions obtained by the two methods.

Pulsed CO2 laser-driven production of ultrafine silicon, silicon carbide, silicon nitride and silicon oxynitride powders

Ultrafine Si, Si3N4, SiC and silicon oxynitride powders have been produced by irradiating gas-phase reactants by means of a CO2 laser. The mechanism of SiH4 CO2 laser-induced absorption and dissociation is discussed on the basis of the results of the spectral and time-resolved measurement of fragment chemiluminescence. The role played by the SiH2 radical in the powder formation is investigated. The quality of Si, Si3N4, SiC and silicon oxynitride powders is checked by means of several off-line diagnostics (IR spectroscopy, X-ray diffraction at wide and small angle, BET analysis). The possibility of controlling powder stoichiometry and doping from the gas-phase reactant concentration is discussed.

Silicon Nitride Synthesis by Laser Pyrolysis of an Aerosol-Dispersed Precursor

AbstractSurface layers of ultrafine Si3N4 particles (30–100 nm. diameter) have been synthesized by laser-pyrolysis of a liquid precursor, dispersed as an aerosol. This aerosol was formed in a nitrogen environment (at near ambient pressure) using an ultrasonically activated nozzle. The organosilazane precursor, which was specially designed for CO2 laser pyrolysis, was of the type (CH3SiHNH)x, with x = 3 or 4. In the current experiments deposition was performed on Ni substrates using a (i) 7 cm/s. substrate translational velocity, and (ii) ˜2.8 × 102 w/cm2 power density.

XPS characterization of ultrafine Si3N4 powders

AbstractThe mechanical properties of Si3N4 based ceramic bodies are largely controlled by the phases formed on the grain boundaries during sintering. For this reason determination of the state and the chemical composition of the surface of the starting Si3N4 powder is of prime theoretial and practical importance, especially in the cases when advanced ultrafine powders are used. In this work an ultrafine Si3N4 powder, obtained by high temperature plasma nitridation of silicon, has been characterized by XPS. Oxygen, carbon and also a small amount of potassium were detected as surface impurities. The Si 2p and Si (KLL) spectral lines could be decomposed into two components corresponding to silicon in Si3N4 and SiO2 phases. From angle‐resolved and Ar‐ion depth‐profiling experiments a layer model, consisting of a Si3N4 core covered by a relatively thick SiO2 layer and a carbon contaminant overlayer, could be elucidated.

Synthesis of ultrafine Si 3N 4 powder by plasma process and powder characterization

An ultrafine Si3N4 powder was synthesized by the vapor phase reaction of SiCl4 and NH3 in a thermal plasma arc jet. The as-prepared powder was white and amorphous, and had an average particle size of 30 to 40nm. The production rate was 200g/h. The heat treatment above 1380°C decreased the specific surface area and increased the crystallinity, implying that the grain growth and the crystallization occurred simultaneously. The oxygen content was reduced to 1wt% at 1450°C. The powder thus obtained was the crystallized hexagonal grain with the average size 0.2μm. X-ray diffraction indicated the presence of α-Si3N4 phase more than 95% and β phase as remainder. The sintering at 1700°C with the aid of 5wt% Al2O3 and 5wt% Y2O3 resulted in the relative density 98%, Vicker's hardness 16 GN/m2 and the fracture toughness 7 MN/m3/2.

Synthesis of Ultrafine Si<sub>3</sub>N<sub>4</sub> Powder by Plasma Process and Powder Characterization

An ultrafine Si3N4 powder was synthesized by the vapor phase reaction of SiCl4 and NH3 in a thermal plasma arc jet. The as-prepared powder was white and amorphous, and had an average particle size of 30 to 40nm. The production rate was 200g/h. The heat treatment above 1380°C decreased the specific surface area and increased the crystallinity, implying that the grain growth and the crystallization occurred simultaneously. The oxygen content was reduced to 1wt% at 1450°C. The powder thus obtained was the crystallized hexagonal grain with the average size 0.2μm. X-ray diffraction indicated the presence of α-Si3N4 phase more than 95% and β phase as remainder. The sintering at 1700°C with the aid of 5wt% Al2O3 and 5wt% Y2O3 resulted in the relative density 98%, Vicker's hardness 16 GN/m2 and the fracture toughness 7 MN/m3/2.