Effect of Y2O3 additive on nitridation of diamond wire silicon cutting waste

The effect of Y₂O₃ additive on the nitridation of diamond wire silicon cutting waste (DWSCW) was studied by using X-ray diffraction, thermo gravimetry, differential thermal analysis, scanning electron microscope equipped with energy-dispersive spectrometry, and an equivalent alternative method, and the individual particles of DWSCW were simulated using cubic polycrystalline silicon blocks. The results showed that the native SiO₂ film on the surface of DWSCW can be disrupted at low temperature (1300°C) by adding Y₂O₃ additive, which provide good channels for the diffusion of SiO and N₂ and improve the overall conversion of DWSCW. Y₂O₃ additive can also reduce the initial nitriding temperature of cutting waste, change the nitriding kinetic behavior, and promote the formation of β-Si₃N₄ through accelerating the nitridation of cutting waste at high temperature (≥1500°C). In addition, when 8 wt% Y₂O₃ additive is added to the cutting waste, the complete nitridation is achieved, at 1350°C, and ω_α + ω_β reaches a maximum of 83.6 wt%.     

Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers

Porous Si₃N₄-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si₃N₄ was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si₃N₄-bonded SiC ceramics were SiC, α-Si₃N₄, and β-Si₃N₄, respectively. The contents of β-Si₃N₄ were increased following the sintering temperature. The morphology of Si₃N₄ whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sintering-temperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si₃N₄ bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6?MPa. The α-Si₃N₄ whiskers were derived from the reaction between N₂ and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si₃N₄ was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si₃N₄ was dissolved into the liquid then the β-Si₃N₄ was precipitated successfully.     

Mechanical properties of Si3N4 ceramics from an in-situ synthesized a-Si3N4/β-Si3N4 composite powder

Sintered Si₃N₄ ceramics were prepared from an ɑ-Si₃N₄/β-Si₃N₄ whiskers composite powder in-situ synthesized via carbothermal reduction at 1400–1550 °C in a nitrogen atmosphere from SiO₂, C, Ni, and NaCl mixture. Reaction temperatures and holding time for the composite powder, and mechanical properties of sintered Si₃N₄ were investigated. In the synthesized composite powder, the in-situ β-Si₃N₄ whiskers displayed an aspect ratio of 20–40 and a diameter of 60–150 nm, which was mainly dependent on the synthesis temperature and holding time. The flexural strength, fracture toughness and hardness of the sintered Si₃N₄ material reached 794±136 MPa, 8.60±1.33 MPa m^1/2 and 19.00±0.87 GPa, respectively. The in-situ synthesized β-Si₃N₄ whiskers played a role in toughening and strengthening by whiskers pulling out and crack deflection.     

Relation between crystal structure and lattice oxygen content of sintered reaction-bonded silicon nitride

When reaction-bonded silicon nitride containing MgO/Y₂O₃ additives is sintered at three different temperatures to form sintered reaction-bonded silicon nitride (SRBSN), the thermal conductivity increases with sintering temperature. The β-Si₃N₄ (silicon nitride) crystals of SRBSN ceramics were synthesized and characterized to investigate the relation between the crystal structure and the lattice oxygen content. The hot-gas extraction measurement result and the crystal structure obtained using Rietveld analysis suggested that the unit cell size of the β-Si₃N₄ crystal increases with the decrease in the lattice oxygen content. This result is reasonable considering that the lattice oxygen with the smaller covalent radius substitutes nitrogen with the larger one in the β-Si₃N₄ crystals. The lattice oxygen content decreased with increasing sintering temperature which also correlated with increase in thermal conductivity. Moreover, it is noteworthy from the viewpoint that it may be possible to apply the lattice constant analysis for the nondestructive and simple measurement of the lattice oxygen content that deteriorates the thermal conductivity of the β-Si₃N₄ ceramics.     

Fabrication of β-Si3N4 whiskers from a GPSed-RBSN sponge using 6Y2O3–2MgO additives

In this study, β-Si₃N₄ whiskers with a diameter of 0.05–1.5 μm and length of about 70 μm were successfully fabricated using a gas pressure sintered (GPSed)-reaction boned silicon nitride (RBSN) process at 1550 °C for 9 h. The β-Si₃N₄ whiskers grew on the frame and filled fully in the pores of the GPSed-RBSN sponge. 6Y₂O₃–2MgO additives played a significant role in the growth of β-Si₃N₄ whiskers and α-Si₃N₄ whiskers, while α-Si₃N₄ whiskers, which were grown inside the RBSN sponge through the vapor–solid mechanism, had a diameter ranging from 50 to 100 nm and length of about 80 μm.     

Open-cell reaction bonded silicon nitride foams: Fabrication and characterization

In this study, a newly designed fabrication procedure was utilized to produce silicon nitride foams. The main goal of the present study was to obtain Si₃N₄ foams with high levels of porosity and pore interconnectivity via an economical fabrication procedure including sacrificial template technique, gel-casting and reaction bonding processes. The fabrication procedure was studied and optimized in terms of suspension preparation and rheology, gel-casting parameters, and reaction bonding conditions. The produced foams have a precisely controlled level of porosity which can be varied up to 87 vol%. BET analysis showed that the surface area of the foam is of the order of 2.01 m²/g. The pore interconnectivity of the foam was investigated via polyester resin infiltration. Based on XRD and SEM analysis, the dominant nitriding reactions are the gas-phase reactions which lead to α-Si₃N₄ in the form of whiskers.     

Function mechanism of Y2O3 additive in silicon powder nitridation

Effect of sole Y₂O₃ additive on the nitridation behavior of silicon powder was systematically studied using thermo gravimetry, differential thermal analysis, particle size analysis, X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscope and thermodynamic analysis in this paper. The thermo gravimetry results showed that Y₂O₃ additive can significantly decrease the initial nitriding temperature and increase the nitriding rate. This phenomenon can be attributed to the much lower reaction temperature of the silica film and Y₂O₃ additive than that of the silica film and silicon. In addition, Y₂O₃ additive has little effect on the nitridation of silicon powder at 1300°C. However, it can obviously enhance the nitridation of silicon powder and the formation of β-Si₃N₄ at 1400°C, which is evidenced by the fact that the overall conversion increases from 58.1% to 100% and the fraction of β-Si₃N₄ in generated Si₃N₄ increases from 7.9% to 68.2% with increasing the content of Y₂O₃ additive from 0 to 10 wt%.     

Effects of alumina sources on the microstructure and properties of nitrided Al2O3-C refractories

Fused white alumina, tabular alumina with a plate-like morphology, reactive alumina powder with a high specific surface area and industrial alumina composed of 40 to 76% γ-Al₂O₃ and 60 to 24% α-Al₂O₃ were selected as alumina sources and grouped in this work. The effects of alumina sources on the microstructure of nitrided Al₂O₃-C refractories were investigated. A large number of β-Sialon phases and a small number of SiC phases were formed when the alumina source varied, and α-Si₃N₄ phase was only formed when using tabular alumina. The β-Sialon phase was deemed to be the major ceramic bonding phase, which generated the morphologies of column and tabular column. Columned β-Sialon crystals with conical tips were formed by the direct nitriding of liquid silicon following a VLS (vapor-liquid-solid) growth mechanism, the transformation of the VLS growth mechanism into a VS (vapor-solid) growth mechanism was observed when using industrial alumina with smooth tips on the β-Sialon crystals. β-Sialon crystals with a morphology of tabular column were formed through nitriding of SiO _(g) and Si _(g) following the VS growth mechanism. SiC whiskers were formed by the reacting of CO _(g) and SiO _(g) following a CVD (chemical-vapor-deposition) growth mechanism. The physical, mechanical and thermal properties of these groups after the nitriding process were also investigated and compared. When using reactive alumina powder and fused white alumina as the alumina sources, the optimal cold crushing strength (CCS) and cold modulus of rupture (CMOR) were generated due to the dense reticular structure, and also the optimal hot modulus of rupture (HMOR) was achieved due to the formation of large size of O’-Sialon tabular whiskers in the test atmosphere. Improved thermal shock resistance and oxidation resistance were also observed.     

Study on rapid nitridation process of molten silicon by thermogravimetry and in situ Raman spectroscopy

The melt of silicon, hindering nitridation for its agglomeration, should be avoided in the direct nitridation of silicon to synthesize silicon nitride powders, although liquid phase facilitates nitridation. Therefore, we proposed a method to nitride molten silicon without agglomeration. Thermogravimetric and in situ Raman studies on the nitridation process of molten silicon were performed. The as-prepared silicon nitride samples were found to be micron clusters composed of submicron grains with high α-Si₃N₄ content. The nitridation of molten silicon at 1500°C was completed after 500 s and 109 times faster than the nitridation of solid silicon at 1350°C. β-Si₃N₄ is produced dominantly by α–β-phase transition. Less nitridation time and low temperature can decrease the β-Si₃N₄ content. The rapid nitridation was owning to core–shell structure Si@Si₃N₄, which was formed after the initial nitridation of silicon particles and hindered the agglomeration of molten silicon.     

Defect structures and dopant solution states of Hf-doped Si3N4 ceramics

Transition-metal-doped silicon nitride ceramics have attracted much attention as gate materials for semiconductors because of their electrical properties as well as chemical and thermal stability. The present study aims to clarify the defect structures of Hf-doped β-Si₃N₄ by theoretical calculations and scanning transmission electron microscopy (STEM). First-principles calculations based on a hybrid functional method were performed. It was found that Hf dopants are mainly substituted for the Si sites and can be occasionally located at interstitial sites in the lattice of β-Si₃N₄. The substitution sites of Hf dopants predicted by the first-principles calculations were also confirmed by the high-resolution STEM images.     

A facile method to produce rodlike β-Si3N4 seed crystallites for bimodal structure controlling

β-Si₃N₄ rodlike seed crystallites were successfully produced by single-step heat treatment of commercial α-Si₃N₄ powder at 1900°C for 20 h under an N₂ gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α- ⇀ β-Si₃N₄ phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α-Si₃N₄ dissociation temperature (1933°C) under N₂ gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α-Si₃N₄ crystal lattice was suggested as the driving force for the present oxide additive-free α- ⇀ β-Si₃N₄ phase transformation. β-Si₃N₄ ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd₂O₃. Starting α-Si₃N₄ powder with 10 vol% rodlike β-Si₃N₄ seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β-Si₃N₄ ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m^−1/2 and 109.3 ± 0.4 W m⁻¹ K⁻¹, respectively, retaining a high fracture strength of about 1 GPa.     

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.     

The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si₃N₄ and diameter of post-sintered β-Si₃N₄ are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y₂O₃ as sintering additive, nano Y₂O₃ can promote the formation of small β-Si₃N₄ nuclei, but the large amount of β-Si₃N₄ (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si₃N₄ grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si₃N₄ grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si₃N₄ grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.     

Effect of Reinforcement of a Silicon Nitride Matrix by Silicon Carbide Whiskers

A fine-grain high-density tough matrix is a general prerequisite for synthesis of high-strength and crack-resistant composite materials, as well as its self-reinforcement with extended grains of β-Si₃N₄ and reinforcement with β-SiC crystals. Both approaches are used simultaneously in the present work. It is shown that the reinforcement of a silicon nitride matrix based on ultradisperse powder compositions by SiC whiskers of grade TWS provides hot-pressed ceramics with a high ultimate bending strength (950 MPa) and crack resistance (10 MPa · m^1/2). Reinforcement by fine or coarse whiskers increases the crack resistance by 30% with respect to monolithic Si₃N₄. Composites reinforced by TWS silicon carbide whiskers preserve their high strength up to 1500°C.     

Synthesis of α-Si3N4 powder by high energy ball milling assisting molten salt nitridation method at low temperature

A new method for synthesizing high purity α-Si₃N₄ powder is introduced in this paper. α-Si₃N₄ powder was synthesized by high energy ball milling assisting molten salt nitridation method at low temperature, using Si powder as starting material and mixed powder of NaCl–NaF as molten salt. The effects of synthesis temperature and holding time on phase composition and micro-morphology of the samples were discussed. The results showed that Si powder will be completely nitrided at 1200 °C for 4 h, and the content of α-Si₃N₄ in the sample is 96%. It indicates that this method can effectively reduce the synthesis temperature of silicon nitride and improve the purity of product. The growth of α-Si₃N₄ whiskers occurs via a vapor-solid (VS) mechanism.     

Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.     

Ab initio calculation of the evolution of [SiN4-nOn] tetrahedron during β-Si3N4(0001) surface oxidation

The easy-going oxidation of silicon nitride (Si₃N₄) at high temperature greatly hampers its potential applications. Here, we explored the reaction mechanism between β-Si₃N₄ and O₂ via density functional theory (DFT) calculation, which discloses that O atoms are preferentially adsorbed on the top of Si atoms and N₂ starts to be generated as the dominant gas product at 2/3 monolayer (ML) O coverage. The vacancies formed by N₂ removal attract the O adatoms to transfer to the site of the N vacancy, which accelerates the adsorption of O and the formation of Si–O bonds toward the growth of SiO₂ product. The surface oxidation of β-Si₃N₄ (0001) has been clarified by the unambiguous evolution of [SiN_4-nO_n] (n = 0-4) tetrahedrons going through from [SiN₄] tetrahedron to [SiO₄] tetrahedron, providing a deep insight into intrinsic oxidation process of Si₃N₄ ceramic.     

Facile synthesis of α-Si3N4 nanoneedles and their photoluminescence properties

Needle-like α-Si₃N₄ with unique structures has potential application in both field emission devices and tips for atomic force microscopes. Single-crystalline, α-Si₃N₄ nanoneedles have been prepared by using an improved chemical vapor deposition (CVD) method at 1200°C for 3 hours. The phases, chemical composition, and microstructure of the as-prepared products were determined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) equipped with EDS. The as-synthesized products show a needle-like morphology with hundreds of micrometers in length and nanometer-featured tips. The nanoneedles show a α-Si₃N₄@SiOx core-shell heterostructure as characterized by TEM, with single-crystalline α-Si₃N₄ core and an insulating amorphous silicon oxide shell. The growth of α-Si₃N₄ nanoneedles corresponds to vapor-liquid-solid cap-growth and base-growth mechanism. Photoluminescence (PL) properties of the products were also characterized. An obvious emission peak at 400 nm under 254 nm UV excitation was observed. The nanoneedle morphology and photoluminescence properties of the products have potentials to be used in future optoelectronic nanodevices.     

Electromagnetic wave-transparent porous silicon nitride ceramic prepared by gel-casting combined with in-situ nitridation reaction

To achieve the balance between mechanical properties and electromagnetic wave-transparent properties of porous silicon nitride (Si₃N₄), the key is to form an interlocking microstructure constituted by columnar β-Si₃N₄ crystals. This structure can be realized by liquid-phase sintering. However, grain boundaries which affect high temperature properties and volume shrinkage during sintering are inevitable. We proposed a strategy to realize this structure by gel-casting of β-Si₃N₄ whisker (Si₃N_4w) and Si powder followed by in-situ nitridation of Si. To achieve chemically-stable slurry containing micro-sized Si with low viscosity, a novel formulation was developed. Two key structural parameters of the interlocking Si₃N_4w network, i.e., density of the Si₃N_4w skeleton and inter-whisker bonding mode, were adjusted by composition of raw materials and nitridation temperature. The flexural strength, dielectric constant and loss of the porous ceramics are 44.9 MPa, 2.7 and 2 × 10⁻³, when the volume fraction of Si₃N_4w/Si is 5 and the nitriding temperature is 1400 °C.     

Influence of ceramic phase content and its morphology on mechanical properties of MgO–C refractories under high temperature nitriding

To evaluate the influences of ceramic phase content and its morphology on the mechanical properties of MgO–C refractories, pre-prepared Si powder-phenolic resin loaded with Fe₂O₃ was introduced to prepare MgO–C refractories via catalytic nitridation. The effects of nitriding temperature and Fe₂O₃ content on the phase composition, microstructure evolution and properties of MgO–C refractories were studied and compared. The results show that the increase of the nitriding temperature was conducive to the in-situ formation of the ceramic phases, and a new phase of Mg₂SiO₄ was formed at temperatures ≥1450 °C. Both the increase in nitriding temperature and the addition of catalyst could inhibit the growth of α-Si₃N₄ to promote the formation of β-Si₃N₄ and MgSiN₂. In addition, the formation of excessive ceramic phases caused samples after nitriding to expand violently and form more porous, thereby reducing the physical properties of MgO–C refractories.