Si3N4 Powders Research Articles

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength, oxidation resistance, and dielectric properties

The high-temperature mechanical and dielectric properties of Si2N2O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si2N2O was prepared at 1700 °C by hot-pressing oxidized amorphous Si3N4 powder without sintering additives. A homogeneous network with short-range order and a SiN3O structure was formed in the oxidized amorphous Si3N4 powder during the hot-pressing process. Si2N2O crystals preferentially nucleated at positions within the SiN3O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si2N2O and some residual amorphous phases were obtained. The as-prepared Si2N2O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10−3 at 1000 °C.

Glass Foam Made with Silicon Nitride and Manganese Oxide by Microwave Irradiation

A high mechanical strength (6.1 MPa) glass foam was produced by sintering/foaming at 830 ºC in an experimental 0.8 kW-microwave oven. The basic raw material was a colorless flat glass waste and the foaming agent was Si3N4 powder (2 wt.%). As an oxygen supplying agent, a MnO2 powder (3.1 wt.%) was used. The main physical, mechanical, thermal and morphological characteristics of the optimal sample were: apparent density of 0.47 g/cm3, porosity of 77.6%, thermal conductivity of 0.105 W/m·K, compressive strength of 6.1 MPa and pore size between 0.15-0.40 mm. The optimal glass foam sample has the required characteristics of a thermal insulation material usable under mechanical stress conditions in civil engineering. The originality of the paper is the application of the unconventional microwave heating technique, faster and more economical, unlike the other papers in the same area published in the literature, followers of the traditional conventional heating technique.

Effect of geometric parameters at open turbine combined structure on flow field distribution of dry granulation for Si3N4 powder

To improve the uniformity of the flow field and the poor axial velocity in the chamber of Si3N4 dry granulation, the influence of geometric parameters at open turbine combined structure on the flow field distribution is studied. The Euler–Euler gas-solid two-phase flow model is established and the physical model of dry granulation chamber under the combined structure is simplified. Under the same radial structure, the volume distribution and velocity field of Si3N4 particles in the granulation chamber with a different number and angle of the axial structure at the open turbine are analyzed by the CFD method. The influence of the axial structure at the open turbine on the flow field distribution of Si3N4 particles under different geometric parameters is compared. The results show that the axial structure of the open turbine in the granulation chamber is the most uniform when the number of blades is 6 and the inclination angle is [Formula: see text], and the circulating flow of the upper and lower parts of Si3N4 powder is strong.

Tribology of Si3N4 containing in-situ grown Si2N2O processed from oxidized α - Si3N4 powders

Silicon nitride ceramics containing in-situ grown Si2N2O were prepared from oxidized silicon nitride powders by hot isostatic pressing (HIP) at 1500 °C and 1700 °C in a nitrogen gas environment under 20 MPa for 3 h. In case of systems sintered at 1700 °C, the volume fraction of the silicon oxynitride (Si2N2O) increased, and the β - Si3N4 phase decreased with the increase of oxidation time. The tribological behavior of the materials was studied by the ball-on-disc method under dry sliding conditions at room temperature under an applied load of 5 N with a sliding speed of 0.05 m/s using Si3N4 ball as a tribo-partner. For the systems sintered at 1700 °C, the coefficient of friction (COF) values were between 0.8 and 0.76 at the start, and started decreasing with the increasing sliding distance, at 1000 m of sliding distance the values were between 0.65 and 0.6. The lowest COF value was measured for the system with unoxidized α-Si3N4 powder sintered at 1700 °C with the value of 0.65. The lowest wear rate, 1.224 x 10-4 mm3/N•m, was measured for the system with 10 h oxidized α-Si3N4 powder sintered at 1500 °C. Fractographical analyses of the wear track revealed that the main wear mechanisms were abrasive wear with grain pull-out, micro-cracking, and debris formation together with tribo-film formation.

A novel method to fabricate porous single phase O’-sialon ceramic and improve its mechanical property

Nowadays, the O?-sialon ceramics are synthesized by the reaction of Si3N4, SiO2 and Al2O3. However, it is difficult to achieve the single phase materials. Here, we have successfully developed porous single phase O?-sialon ceramics by pre-oxidation combined with gas-pressure sintering method. The effects of ?-Si3N4 powder on the microstructure, phase evolution, mechanical property were investigated. The result illustrated that the main crystal phase of the porous ceramics was composed of the single O?-sialon phase. The pores were well distributed and generated from the decomposition of Si2N2O. The elongated O?-sialon grains were found and formed around pore walls. Additionally, the addition of ?-Si3N4 powder was beneficial for improving the bending strength because of the reduction of porosity and pore size. The porous O?-sialon ceramics with uniform pores obtained the excellent bending strength when the ?-Si3N4 powder was 6 wt%.

Re-examination of phase relations between solid solutions in R4Al2O9-R4Si2N2O7-R4Si2O10 (R=Y, Gd, Yb) systems

Samples in Si–Al-R-O-N (R = Y, Gd, Yb) systems were prepared by solid-state reactions using R2O3, Al2O3, SiO2 and Si3N4 powders as starting materials. X-ray diffraction was done to investigate RAM-J(R) solid solutions [RAM = R4Al2O9, J(R) = R4Si2N2O7] formation and their equilibrium with RSO (R4Si2O10). Phase relations between RAM, J(R) and RSO at 1700 °C were summarized in a phase diagram. It was determined that a limited solid solution of RAM and RSO could be formed along RAM-RSO tie-line, while RAM and J(R) form a continuous solid solution along RAM-J(R) tie-line. In RAM-J(R)-RSO ternary systems, the RAM-J(R) tie-lines were extended towards the RSO corner to form a continuous solid solution area of JRAMss (R = Y, Gd, Yb). The established phase relations in the Si–Al-R-O-N (R = Y, Gd, Yb) systems may facilitate compositional selections for developing JRAMss as monolithic ceramics or for SiC/Si3N4 based composites using the solid-solutions as a second refractory phase.

Lu-atom-ordered oxonitridoaluminosilicate Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1.

A single crystal of Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1 (barium cerium lutetium aluminosilicate nitride oxide) was obtained by heating a mixed powder of Ba3N2, Si3N4, Al, Lu2O3, and CeO2 at 2173 K for 1 h under N2 gas at 0.85 MPa. X-ray single-crystal structure analysis revealed that the title oxynitride is hexa-gonal (lattice constants: a = 6.0378 (5) Å, c = 9.8133 (9) Å; space group: P63 mc) and isostructural with BaYbSi4N7. (Ba,Ce) and Lu atoms occupy twelvefold and sixfold coordination sites, respectively.

Laser powder bed fusion of in-situ composites using dry-mixed Ti6Al4V and Si3N4 powder

Abstract Herein, we report laser powder bed fusion (L-PBF) of dry-mixed Ti6Al4V + Si3N4 powder to create in-situ titanium matrix composites. The dry-mixed Ti6Al4V powder with 5 wt.% Si3N4 was processed using l -PBF at varying laser energy densities, between 44 and 133 J/mm3, by changing the laser scan speed (400–1200 mm/s) at constant laser power of 96 W, layer thickness of 20 μm and scan spacing of 90 μm. The selected samples were examined for microstructural evolution, in-situ reaction products and hardness. The results showed that the in-situ reaction between liquid titanium and Si3N4 forms fine TiN and Ti5Si3 reinforcements in these l -PBF processed samples. However, the irregular shape and fine size of Si3N4 reduced the feedstock flowability, and the composites could not be processed with laser energy density (E) l -PBF but further improvement is required through adjusting Si3N4 powder attributes (size, shape) and concentration.

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si3N4) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH2-BNNSs) and Si3N4 powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si3N4 composites were obviously enhanced by adding NH2-BNNSs. The fracture toughness and bending strength of Si3N4 composites added 0.75 wt% NH2-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si3N4. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si3N4 ceramics are also improved after the addition of NH2-BNNSs.

Synthesis of α-Si3N4 whiskers or equiaxed particles from amorphous Si3N4 powders

Silicon nitride (Si3N4) particles with different morphologies have been used in many fields. In this work, α-Si3N4 whiskers and granular particles with high-phase purity were successfully tailored by the controllable crystallisation process of amorphous Si3N4 powders under different N2 pressure. Impressively, α-Si3N4 whiskers were prepared by simply heat treating amorphous Si3N4 powders at 1550 °C for 2 h under the low N2 pressure of 0.2 MPa, whereas equiaxed α-Si3N4 particles with uniform size of ~280 nm were obtained under an elevated N2 pressure of 2.0 MPa. With the evaluated N2 pressures and temperatures, large scale α-Si3N4 whiskers or equiaxed α-Si3N4 particles could be produced. The growth mechanisms of the α-Si3N4 particles with distinct morphologies were rationally proposed, and these consist of two main growth processes. First, amorphous Si3N4 powders decomposed into Si(g) and N2(g) under high-temperature treatment. Subsequently, N2(g) dominated the recombination of the evaporated chemical with the Si3N4 molecule. The initial N2 concentration, which plays a key role in tailoring the shape and size of products, was controlled by the N2 pressure.

Directional solidification of photovoltaic silicon in re-useable graphite crucibles

Graphite crucibles are potentially interesting for the directional solidification processing of photovoltaic silicon, because, contrarily to standard silica crucibles, they can be used many times. In the present work, two types of graphite crucibles are studied: i) graphite directly coated with the Si3N4 powder classically used as a releasing layer for standard silica crucibles, and ii) graphite protected by a dense SiC layer before deposition of the releasing coating. In both cases, spontaneous detachment of the silicon ingot is observed to occur during cooling. Results are given and discussed concerning the impurities introduced in the silicon during the melting-solidification cycle, as well as their consequences on the photo-electrical characteristics of silicon. These results are compared with those obtained using the standard silica crucible. It is shown that re-useable SiC protected graphite crucibles lead to the same performances as standard silica. Reproducible results are found after four runs with the same crucible, and no degradation of the crucible is observed.

Rheological properties of Si3N4 and Si3N4 with sintering additive CaO-Al2O3 powders

In this paper were considered the rheological characteristics of Si3N4 powders and the mixture of Si3N4 powders with sintering additive based on CaO-Al2O3. The dependence of optimal density on the applied stress during pressing was established. The powders compressibility coefficients were studied. It was shown that upon addition of a sintering additive CaO-Al2O3 the compressibility coefficient decreases and the compressibility modulus increases.

Formation mechanism of stereolithography of Si3N4 slurry using silane coupling agent as modifier and dispersant

Silicon nitride (Si3N4) ceramic slurry with high-solid loading, low-viscosity and high curing depth was prepared by using a surface modifier (silane coupling agent) directly as the dispersant. The effect of the interaction between modified Si3N4 powders and prepolymer on rheological property, kinetic stability, wettability and curing property of the slurry was investigated. The stereolithography forming mechanism of the modified Si3N4 ceramic slurry was elucidated. It is revealed that the epoxy group of KH560 can easily form an ether covalent bond with the hydroxyl group of bisphenol A epoxy acrylate (EA). This makes submicron sized Si3N4 powder well-dispersed in the premixed liquid and leads to improved rheological properties and kinetic stability of the ceramic slurry. In particular, the ether covalent bond formed between surface modified Si3N4 powder and EA reduces the surface tension of the slurry and enhances the wettability of the cured slurry layer. KH560 acts as a molecular bridge linking EA to Si3N4 powder surface and forms an EA shell structure. Therefore, the refractive index difference between the Si3N4 powder and prepolymer reduces significantly by two orders of magnitude, leading to an increase in the curing depth. These findings could be crucial for the stereolithographic fabrication of high refractive index ceramics parts with high precision and complex geometries.

Effect of ball-milled MgO and Si3N4 addition on the physical, mechanical and thermal behaviour of aluminium based composite developed by hybrid casting technique

ABSTRACTComposite material via hybrid casting techniques. Ball-milling of Si3N4 and MgO powder was carried out for 100 hours to obtain in single entity particles. MgO powder is used to enhance wettability property of composite with Si3N4 reinforcement in AA2024 aluminium alloy. However, MgO was also responsible to improve tensile strength and hardness of composite material. Microstructure results showed the fair distribution of reinforcement particles developed by the hybrid casting technique. Maximum tensile strength and hardness were found to be 219.55 MPa and 80.86 BHN for ball-milled AA2024/7.5 % Si3N4/5% MgO composite material as heat treated. Minimum thermal expansion was also found for composition AA2024/7.5% Si3N4/5% MgO composite. However, minimum corrosion loss was observed for composition AA2024/10% Si3N4/2.5% MgO composite material. Though, toughness and ductility were reduced by adding the Si3N4 and MgO particles in AA2024 aluminium alloy. XRD result showed the presence of SiO2, Mg, Al, Si3N4, Cu and MgO in ball-milled AA2024/7.5% Si3N4/5% MgO composite material.

Properties of MWCNTs added Si3N4 composites processed from oxidized silicon nitride powders

Si3N4/3wt.% multi-walled carbon nanotubes (MWCNTs) composites were prepared from oxidized ?-Si3N4 powders and sintering additives by hot isostatic pressing (HIP). The Si3N4 powders were oxidized at 1000?C in ambient air environment for 10 and 20 h and the powder mixture was sintered at 1700?C for 3 h under the pressure of 20MPa in nitrogen. The relationship between oxidation, microstructure, tribology and mechanical properties was studied. The dispersion of MWCNTs was not optimal, as they were found mainly in the form of bundles between the ?-Si3N4 grains. Neither Si2N2O nor other oxide phase were observed after the sintering. This was probably caused by the presence of MWCNTs at the grain boundaries of the Si3N4 grains causing reduction of oxide phases. The density, hardness, bending strengths and friction coefficient of the composites increased with the oxidation time of the Si3N4 powder.

Mechanical and wear properties of Si3N4 reinforced titanium composites

In the present study, Si3N4 reinforced titanium composites were produced by powder metallurgy method. The effect of various percentages of Si3N4 (0-9 wt.%) on the microstructure, density, hardness and compressive strength of titanium (Ti) composites have been investigated. After sintering at 1100oC for 120 min., the mechanical properties were significantly developed up to 3 wt.% Si3N4. The highest hardness and the greatest compressive strength were obtained for 3 wt.% Si3N4 reinforced composites (698.5 HV and 1093 MPa) when compared to pure titanium (414.2 HV and 826 MPa). Si3N4 addition developed the wear properties of composites when compared pure titanium. The lowest wear rate (1.36x10-5, 2.75x10-5 and 5.15x10-5 mm3/Nm for 10 N, 20 N and 30 N) were obtained for 3 wt.% Si3N4 reinforced titanium composite. The scanning electron image, elemental mapping and line analyses confirm the uniform distribution of Si3N4 powder in Ti matrix. Above this ratio, the properties of composites were deteriorated due to agglomeration tendency of Si3N4 powder. There were no in-situ formed second phases of composite structure from the X-ray diffraction analyses.

Surface characterization of silicon nitride powder and electrokinetic behavior of its aqueous suspension

The dispersion stability of aqueous suspensions containing silicon nitride nanopowder prepared by direct nitridation had been studied. The Si3N4 powder was calcined at 200 °C, 400 °C and 600 °C, respectively. It was aimed to improve dispersion stability of Si3N4 suspension, and the effects were evaluated by measuring the median diameters of the samples taken from the slurries after settling experiments. The relationship between the median diameter and the viscosity of the slurry was discussed on the basis of Stoke's law. High resolution transmission electron microscopy (HRTEM) clearly revealed that the calcined Si3N4 particles were covered by an amorphous surface layer, which was mainly composed of silica. It can also be easily verified according to the results of X-ray photoelectron spectroscopy (XPS) analyses and diffuse reflectance Fourier transform infrared spectroscopy (DRTIFT) measurements. The formation of oxide layer also led to an increase in the total oxygen content of Si3N4 powder. Inductively coupled plasma atomic emission spectrometry (ICP-OES) analyses showed that the soluble impurities decreased after thermal oxide process, which also contributed to the lower of viscosity according to Jones-Dole expression. Electrokinetic behavior of suspensions also indicated that the absolute value of ζ-potential increased after powder calcination.

Recycling of waste flat glass in the fabrication of Si3N4 powder by the carbo‐thermal reduction and nitridation

AbstractThe recycle rate of waste flat glass is quite low in spite of higher SiO2 content and its pre‐melted characteristic. This study searched a novel approach for the fabrication of Si3N4 powder by carbo‐thermal reduction and nitridation from waste glass. For comparison, starting powder mixtures were prepared from silica and waste SLS (soda‐lime‐silica) glass. Carbo‐thermal reduction and nitridation process was conducted using two temperatures and different atmospheres. The results showed that equiaxed, micron size β‐Si3N4 particle could be synthesized from waste glass. The use of waste glass reduced the amount of fiber‐like Si3N4 particles and SiC phase by increasing the amount of the liquid phase formed. This study proved the utilization of waste flat glass as a low cost and environment friendly SiO2 source.

Recycling of silicon kerf loss derived from diamond-wire saw cutting process to prepare silicon nitride

Abstract In this paper, silicon kerf loss derived from diamond-wire saw cutting process was recycled to synthesize Si3N4 powders and ceramics via the incorporation of carbothermal nitridation and direct nitridation. The effects of sintering temperatures and CaO additives on the nitridation behavior were studied in detail. The results showed that the main impurities were SiC and Si2N2O in Si3N4 powders products, and the content of impurities could be reduced with the increasing of sintering temperatures and CaO additives. In addition, the acceleration mechanisms of sintering temperatures and CaO additives on the nitridation were investigated and proposed by the thermodynamics calculation and density functional theory, respectively. Owing to the acceleration effect of CaO additives on the nitridation, the properties of the obtained Si3N4 ceramics were obviously improved. When the content of CaO additives was 5 wt%, the as-prepared Si3N4 ceramic had a bulk density of 2.31 g cm−3, apparent porosity of 25.42% and a compressive strength of 109 Mpa. In a word, this study presented a feasible approach for recycling silicon kerf loss to fabricate high-value Si3N4 materials, which could be applied to other harmful waste resources.

YB2C2: A new additive for fabricating Si3N4 ceramics with superior mechanical properties and medium thermal conductivity

Si3N4 ceramics were hot-pressed with MgO and self-synthesized YB2C2 as sintering additives at 1800 °C for 2 h under the pressure of 60 MPa. The addition of YB2C2 played a significant role in the simultaneous achievement of superior mechanical properties and medium thermal conductivity of Si3N4 ceramics. Due to the existence of multilayer YB2C2 nanoplatelets, the flexural strength and fracture toughness of Si3N4 ceramics reached up to 1189.6 ± 43.7 MPa and 9.46 ± 0.14 MPa m1/2, respectively. Owing to the reaction between YB2C2 and native SiO2 on the surface of Si3N4 powders, the lattice oxygen was removed and the thermal conductivity of Si3N4 ceramics was about 77.0 W/(m·K).