β-Si3N4 Powder Research Articles

Synthesis of Si3N4 powder with a controllable alpha/beta ratio by sol-gel assisted carbothermal reduction and nitridation

The controllable α/β ratio of Si3N4 powder was synthesized using tetraethyl orthosilicate (TEOS), glucose, MgCl2 and YCl3·6H2O by sol-gel assisted carbothermal reduction and nitridation (CRN). This study aimed to investigate the impact of various parameters, including a H2O/TEOS mol ratio of the sol, a C/Si mol ratio, sintering aids, and the temperature of CRN on the phase transformation of α-Si3N4. The results demonstrated that the H2O/TEOS ratio significantly influenced the relative content of α and β phase in Si3N4 by changing the morphological characteristics of SiO2. When the H2O/TEOS ratio was 100, β-Si3N4 powder with an approximate β content over 88 wt% was synthesized at 1500 °C. This remarkable phase transformation was likely facilitated by the eutectic liquid phase consisting of MgSiO3/Mg2SiO4. The SEM image revealed rod-like whiskers with a length around 1.4 μm for the β-Si3N4 grains. Due to the presence of liquid phase, the synthesis of spherical-like α-Si3N4 nano-powder with an α-phase content of 91 wt% was achieved at a lower temperature of 1450 °C, with an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The size of the particles was about 70 nm due to the reduced reaction temperature. Furthermore, without the addition of any sintering aids or Si3N4 seeds, high purity α-Si3N4 with an α phase content approaching 96 wt% could be synthesized at 1500 °C, using an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The resulting α-Si3N4 powder exhibited an extremely regular hexagonal prism shape with a length of approximately 2 μm.

Synthesis of spherical-like β-Si3N4 powder by direct nitridation of silicon saw dust

ABSTRACT Spherical-like β-phase silicon nitride (β-Si3N4) powder has been synthesized by direct nitridation of silicon saw dust recovered from the photovoltaic industry in ammonia gas (NH3) at 1300°C with calcium fluoride (CaF2) as additive. The X-ray diffraction (XRD) analysis indicates that the formation of β-Si3N4 is enhanced and accelerated in NH3 compared to nitrogen (N2). The scanning electron microscope (SEM) examination shows that, compared with N2, NH3 contributes to the formation of spherical-like particles, and all irregular silicon particles are transformed into spherical-like β-Si3N4 particles in the NH3 atmosphere. These results indicate that the NH3 atmosphere is conducive to the formation of submicron spherical-like β-Si3N4 particles. A complete submicron spherical-like Si3N4 powder with 93.12 wt% β phase has been synthesized by direct nitridation of silicon saw dust assisted with 5 wt% CaF2 additives in NH3. The mechanism for the formation of the spherical-like β-Si3N4 powder has also been discussed.

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

Effect of Additives on Thermal Conductivity of Si<sub>3</sub>N<sub>4</sub> Ceramics

Compared with traditional ceramics, Si3N4 ceramics have the characteristics of high theoretical thermal conductivity, high thermal shock resistance, high oxidation resistance, high strength, and strong current carrying capacity. It is a potential high-speed circuit and high-power device for heat dissipation and heat dissipation. Sealing material. For applications in 5these fields, β-Si3N4 with a relatively stable structure and high thermal conductivity is an ideal material. However, β-Si3N4 powder is difficult to sinter as a raw material. Therefore, the prepared Si3N4 generally has a low density, and there are various defects in the crystal. The existence of these defects will cause interference and scattering of heat in the transfer process. Limits the application of β-Si3N4 ceramics. Studies have shown that the introduction of different additives can form a liquid phase at high temperatures, which can effectively reduce the firing temperature of the sample and increase the density. At the same time, it can also remove lattice oxygen, weaken the intercrystalline phase, and promote the α→β phase transition. Thereby improving the thermal conductivity and sintering performance of Si3N4 ceramics. Therefore, this article reviews the types of additives and their effects on the properties of Si3N4 ceramics and their mechanism. Trying to find an additive system for the preparation of high thermal conductivity Si3N4 ceramics with excellent comprehensive performance, hoping to provide help for the work and researchers engaged in the research on the thermal conductivity of Si3N4 ceramics.

The effect of sintering regime on superhydrophobicity of silicon nitride modified ceramic surfaces

ABSTRACT The most common method of obtaining a superhydrophobic surface is to create a specific surface morphology and then coat it with a hydrophobic polymer. Numerous such morphological surfaces have been developed but are often fragile. Ceramic-based coatings show longer life with high wear resistance. In this study, surface micro-nano surface morphology was developed with β-Si3N4 powder and the influence of sintering regime on contact angle of ceramic surfaces was investigated. The contact angle, surface energy and surface roughness were determined from the developed surfaces and surface morphology analyzed by scanning electron microscopy, phase evolution was determined by X-Ray diffraction. Changes in sintering regimes lead to different phase evolutions, roughness, surface topography, surface free energies and contact angles. The superhydrophobicity resulted mainly due to the surface structure/topography in the micro-nano hybrid structures of β-Si3N4 crystals. The highest water contact angle achieved was 166º of the samples sintered at 980°C for 5 min.

Preparation, Properties and Growth Mechanism of Low-Cost Porous Si3N4 Ceramics with High Levels of β-Si3N4 Powders

Nowadays, porous Si3N4 ceramics are fabricated by high purity α-Si3N4 powders, resulting in a higher cost of production. To reduce cost and save energy, in this research, high levels of β-Si3N4 powders were effectively utilized to produce excellent and low-cost porous Si3N4 ceramics. The effects of high levels of β-Si3N4 powders on the microstructural evolution and properties were investigated in detail. The results suggested that, with the increase in β-Si3N4 powders from 0 wt.% to 80 wt.%, the aspect ratio of β-Si3N4 grains gradually decreased because of the anisotropic growth of grains significantly impinged by adjacent other β-Si3N4 grains. Additionally, the bending strength and dielectric constant of porous silicon nitride ceramics declined and their values were 685 MPa - 220 MPa, and 7.58–5.57, respectively, while the porosity increased from 5.9% to 28.5%. Similarly, the residual bending strength of Si3N4 ceramics degraded from 615 MPa to 172 MPa at 1000 °C for 20 h owing to the formation of SiO2 on the surface of silicon nitride ceramics after oxidizing.

Polymer Materials Reinforced with Silicon Nitride Particles for 3D Printing

Comprehensive research into the production of a ceramic-reinforced polymer material from highdensity polyethylene or polypropylene and β-Si3N4 powder was conducted. The incorporation of silicon nitride ceramic particles (5 and 10 vol.%) into the polymers to make polymer–ceramic filaments was studied step by step. High-quality polypropylene–ceramic filaments could be obtained at an extrusion temperature of 150°C with an extrusion speed of 20 cm/min and polyethylene–ceramic filaments at 160°C and 30 cm/min. Data on the shape and size distribution of Si3N4 particles were used to simulate the elementary volume of the filaments to determine the mechanical properties of the composites applying a 2D finite-element model. The reinforcement of the polypropylene/polyethylene-matrix material by 10 vol.% Si3N4 particles was not sufficient because the composite elastic modulus increased insignificantly and the critical strain decreased substantially to incorporate a greater volume of hard particles to improve the elastic modulus. To assess the quality of the polymer–ceramic filaments, parts of different shape (washer and auger) from reinforced and unreinforced filaments were designed and printed. The printed polymer–ceramic parts demonstrated a smooth surface and had no ledges or discontinuous areas. The mechanical (Vickers and Brinell hardness) and tribological (volume wear) properties of the materials were examined. Wear tests of the polyethylene–Si3N4 composite showed that its wear resistance tended to improve with increasing ceramic content of the filament. The low abrasive wear of the Si3N4-reinforced polypropylene/polyethylene material and the behavior of ceramic particles in contact with the indenter indicate that the composite has high fracture resistance in 3D printing.

Precisely controlled carbothermal synthesis of spherical β-Si3N4 granules

High purity spherical β-Si3N4 particles were synthesized via an efficient carbothermal reduction–nitridation strategy by precise regulation. The results showed that nano-sized reactants with an appropriate ratio was significant for acquiring Si3N4 granules with pure β phase. c/a,b-axis length ratio of the β-Si3N4 particles could be adjusted by using various additives. Fine spherical β-Si3N4 particles can only be obtained with the help of CaO additive by L-S mechanism. Moreover, variations of underlying reaction mechanisms when changing the raw materials were comprehensively revealed. Owing to the high purity and fine spherical morphology, as-obtained β-Si3N4 powders are promising fillers for preparing resin-based composites with high thermal conductivities.

Direct Nitridation Synthesis of Quasi-Spherical β-Si3N4 Powders with CaF2 Additive.

In this work, the quasi-spherical β-Si3N4 powders were synthesized via an efficient direct nitridation strategy with CaF2 as the catalytic material under NH3 atmosphere. The effect of CaF2 on phase composition and crystalline morphology was studied. CaF2 additive can accelerate the nitridation of silicon powders, and the particles of nitridation products tend to have an equiaxed structure with the CaF2 additive increasing. When 4 wt% CaF2 additive or more was added, submicron β-Si3N4 particles with quasi-spherical morphology and eminent crystal integrity were obtained. In contrast, irregular α-Si3N4 particles appear as the main phase with less than 4 wt% CaF2 additive. The growth mechanism of Si3N4 particles was also discussed. CaxSiyOz liquid phase is crucial in the nitridation of silicon powders with CaF2 additive.

Carbothermal synthesis of micron-sized β-Si3N4 with tailored morphology

Micron-sized β-Si3N4 powders with tailored morphology were successfully synthesized by carbothermal reduction–nitridation (CRN) strategy. β-Si3N4 granules with low aspect ratios were prepared with 10mol% of CaO additive and the size of particles could be regulated by altering N2 pressures. β-Si3N4 particles with high aspect ratios were synthesized when the content of CaO was less than 5%. Increasing content of additive could reduce the diameter of β-Si3N4 granules. More significantly, the underlying growth mechanism of the β-Si3N4 was tentatively put forward.

The effect of silicon nitride powder characteristics on SiAlON microstructures, densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si3N4 powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si3N4 powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si3N4 powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si3N4 powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si3N4 powders were coarser, but the fine β-Si3N4 powders yielded a bimodal microstructure. The critical particle diameter of the β-Si3N4 powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.

Effect of β-Si3N4 Initial Powder Size on Texture Development of Porous Si3N4 Ceramics Prepared by Gel-Casting in a Magnetic Field

ABSTRACTTextured porous Si3N4 ceramics with fine and coarse β-Si3N4 powders as the raw materials were prepared by gel casting in a strong magnetic field of 6T and subsequent pressureless sintering. These results showed that highly textured porous Si3N4 ceramics could be obtained by adjusting the ratio between fine and coarse β-Si3N4 powders in 6T. The Lotgering orientation factor in green bodies showed the value of 0.72∼0.74 with the increase of coarse powder content. The sintering process promoted texture development of the samples by Ostwald ripening owing to the big difference in the β-Si3N4 initial powder size. After sintering at 1800oC for 2 h, the degree of texture (Lotgering orientation factor) in the sample, prepared with fine and coarse powders at the ratio of 1:1, reached the maximum value (0.91), and it had relatively high density (66.91%) and low apparent porosity (34.51%). The crystallographic texture in porous Si3N4 ceramics led to the anisotropic bending strength. The method combined with gel casting in a strong magnetic field was beneficial for preparing highly textured porous ceramics with big and complex shape.

Effect of β-Si<sub>3</sub>N<sub>4</sub> Powder on Thermal Conductivity of Silicon Nitride Ceramics

To investigate the influence of β-Si3N4 powder on thermal conductivity of silicon nitride, coarse, fine β-Si3N4 powder and various β-Si3N4/α-Si3N4 ratios of starting powders were adopted to fabricate ceramics by spark plasma sintering at 1600°Cand subsequent high-temperature heat treatment at 1900°C with the sintering additives of Y2O3 and MgO. It is found that with more fine β-Si3N4 powder in the starting powder, β-Si3N4 grains exhibit high thermal conductivity, which is partly resulted from the compaction of β-Si3N4 grains.

Sr-anorthite glass ceramic with enhanced crack resistance, reinforced with silicon nitride particles

It is demonstrated that composite materials based on Sr-anorthite glass ceramic can be synthesized with finely dispersed α-Si3N4 and β-Si3N4 powders introduced as fillers. A study of the influence exerted by the polymorphic forms of fillers, their morphology, dispersity, and concentration and also by the nature of the matrix on the synthesis, structure, and properties of the composites demonstrated that the mechanism and temperature range of sintering in syntheses of materials are determined by the nature of the matrix and the degree of densification and the structure of the composite depend on the concentration, dispersity, and shape of filler particles, their phase composition, and wettability of the glass-phase matrix. In syntheses of glass-crystalline composites by the hot compaction method, raising the filler content from 10 to 70 vol % leads to a substantial decrease in the density of the materials (by 20%); the maximum content of the filler should not exceed 30 vol %. The use of both α-Si3N4 and β-Si3N4 in an amount of 30% made it possible to raise the critical stress intensity coefficient of the strontium-aluminosilicate matrix by more than a factor of 2. A study of the thermal properties of the composites demonstrated that the glass-crystalline matrix can protect nonoxide fillers from oxidation by atmospheric oxygen at elevated temperatures, with the oxidation onset temperature increased by 300°. The developed class of composites is promising for application in airspace technology, chemical industry, and motor-vehicle construction.

Protective Engobes for Si3N4 Ceramics Sintered in Air-Atmosphere Furnace

Silicon nitride ceramics were sintered at 1640°C in an air-atmosphere furnace using an alumina crucible, alumina packing powder, and protective Al2O3 engobes as well as glass ceramic engobes obtained from the system SiO2-ZnO-Al2O3 The Si3N4 composite was prepared from β-Si3N4 powder with addition of MgO and Al2O3 as sintering aids. It was found that the densest silicon nitride ceramic with only partial oxidation was the sample covered with the Al2O3 engobe. Three phases were identified by XRD analysis: β-Si3N4, α-Si3N4, and Si2N2O. The obtained results are promising and encourage continuous research in this field to ensure protection from and prevention of intensive oxidation.

Fabrication of textured Si3N4 ceramics with β-Si3N4 powders as raw material by gel-casting under strong magnetic field

The textured Si3N4 ceramics with β-Si3N4 powders as raw material were fabricated by gel-casting under a strong magnetic field of 6T and subsequent pressureless sintering. The results show that the a or b-axis of β-Si3N4 crystal is oriented parallel to the direction of magnetic field. The degree of texture in the green body reaches 0.72. With the increase of sintering temperature, the degree of texture in the sample increases from 0.74 at 1700°C to 0.81 at 1800°C. The bending strength of the Si3N4 ceramic depends on the orientation of grains.

Effects of nitrogen pressure and diluent content on the morphology of gel-cast-foam-assisted combustion synthesis of elongated β-Si3N4 particles

Elongated β-Si3N4 particles were prepared by gel-cast-foam-assisted combustion synthesis, which is a combination method of gel-casting-foam method and combustion synthesis. Gel-casting-foam method was used to prepare starting powder green body of high porosity and sufficient strength. Combustion synthesis reaction was conducted on the as-made green body. The effects of N2 pressure and diluent content on the combustion velocity, conversion rate and morphology of the resulting particles are studied. With high porosity (~80%), the conversion rate more than >90% was achieved at low N2 pressure (1.5MPa) and low diluent content (even nil). In the epoxy composites, elongated β-Si3N4 powder was confirmed a better filler than its equiaxed counterpart in improving the thermal conductivities of the resultant composites.

Composites based on aluminum-silicate glass ceramic with discrete fillers

It is shown that composite materials (CM) can be synthesized on the basis of Sr-anortite glass ceramic with BNhex, α-Si3N4, β-Si3N4 and TiC powders introduced as disperse fillers. It is shown that the mechanism and temperature range of sintering in obtaining CM are determined by the chemical composition and dispersity of the matrix glass powder, while the degree of compaction and the structure of the composite depend on the dispersity of the fillers, particle shape, particle proneness toward aggregation and wettability by the glass phase of the matrix. It is established that the introduction of BNhex particles decreases the dielectric permittivity of the Sr-anortite glass ceramic. Silicon nitride powder increased the critical coefficient of intensity of the stresses in the matrix by more than a factor of 2.5. Reinforcement with TiC particles increases the elastic modulus and microhardness and decreases the coefficient of friction of CM compared with the initial glass ceramic.

Synthesis and reaction process of β-Si3N4 by means of carbothermal nitridation of serpentine

Abstract The reaction process of carbothermal nitridation synthesis of β-Si3N4 from serpentine is investigated in this paper. Samples with different molar ratios of carbon black to serpentine were calcined under nitrogen atmosphere at 1300 °C, 1400 °C and 1500 °C for 3 hrs. The phase composition and microstructure of the synthesized powders were determined using X-ray diffraction and scanning electron microscopy. The reaction process was analyzed by means of differential thermal analysis. The results show that the main reaction was that forsterite from serpentine reacted with carbon black to form SiO, Mg vapor and CO, then SiO and N2 generated β-Si3N4. Fine β-Si3N4 powders were produced under nitrogen atmosphere at 1500 °C when the carbon black/serpentine molar ratio was 6.0. The synthesized β-Si3N4 powders had uniform grain size and high β-Si3N4 content.

Processing, Characterization and Mechanical Properties of SiAlONs Produced from Low Cost <i>β</i>-Si<sub>3</sub>N<sub>4</sub> Powder

SiAlON ceramics have been known for many years as prime candidate materials in structural applications at ambient and high temperatures involving superior mechanical and/or chemical processes. In spite of their excellent properties, the utilization of SiAlONs has remained limited till today due to the high cost of raw materials and processing. In order to circumvent this problem, low cost refractory grade, coarse, impure, less sinter active β-Si3N4 powder was used to produce SiAlON ceramics with satisfactory mechanical properties.In this article, the processing challenges in the production of SiAlON ceramics with β-Si3N4 powder were discussed. The process parameters obviously affect the phase assemblage, densification, microstructural and mechanical properties of final SiAlON ceramics. Processed β-Si3N4 powder characteristics are majorly investigated by SEM-EDX, XRD, XRF, laser particle sizer and elemental analyser. The existence of undesirable impurities in the β-SiAlON crystal structure because of the use of impure Si3N4 powders has been shown to be tolerable by TEM microstructural analysis. Mechanical properties are in general evaluated by Vickers indentation method. Wear behaviour of the cost effective SiAlONs were compared with commercially available ceramic materials which are commonly being used in wear applications. Initially the use of such powders to produce materials for engineering applications proved challenging, however, satisfactory results have been obtained by the optimization of the initial chemical composition and process parameters.