Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si₃N₄) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si₃N₄ powders was studied, and the results were compared with a commercial powder. The α-Si₃N₄ powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm^?3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si₃N₄ powders produced by CRN from silica initially containing 5 wt.% Y₂O₃. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.     

Carbothermal nitridation synthesis of α-Si3N4 powder from pyrolysed rice hulls

The carbothermal nitridation synthesis of α-Si₃N₄ was studied using a high-temperature tube furnace to react a precursor, comprised of pyrolysed rice hulls (C/SiO₂) and additive “seed” Si₃N₄, with N₂. The experimental design for synthesis was a three-level factorial surface response design for determining the effect of temperature (1300–1380°C) and reaction time (1–5 h) on kinetics. In addition, all precursors were reacted at 1460, 1480 and 1500°G for 5 h in order to ensure high conversion suitable for product powder evaluation (composition and morphology). Following excess carbon removal, the product Si₃N₄ was >95% α-phase and had a surface area of 7.7 m²g^?1 with an oxygen content of 3.6 wt% O. The powder was comprised of a bimodal size distribution of submicrometre solid α-Si₃N₄ crystallites centred at 0.03 and 0.22 μm. No whiskers or high aspect ratio elongated crystallites were found in the powder. The addition of carbon black to the seeded pyrolysed rice hull C/SiO₂ mixture had no significant impact on the reaction rate or product powder properties. The reaction was modelled using a nuclei-growth rate expression as $$\begin{gathered} (kt)^{0.58} = - ln(1 --- X) \hfill \\ k = 1.09 \times 10^{10} exp (--- 50502/T) \hfill \\ \end{gathered} $$ k=1.09×10¹⁰ exp (?50502/T) where (1573 K<T<1653 K), (3600<t<18000 s), (0<X<1), andk=rate in s^?1.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Calculated influences of starting materials composition on carbothermal nitridation synthesis of silicon nitride/silicon carbide composite powders

Equilibrium phase composition at 1 atm total pressure is quantitatively calculated as the function of starting materials composition for carbothermal nitridation synthesis of Si₃N₄/SiC composite powders. N₂ flow is simulated by the N₂ amount in the starting materials. At low starting N₂ content (molar ratio N₂/SiO₂ 100), Si₃N₄ and SiC can not exist in equilibrium at the same temperature. At higher starting N₂ content, the Si₃N₄-SiC coexisting temperature range appears above 1760 K. The upper limit of the temperature range becomes higher with increasing starting N₂ content. Starting carbon content affects the composition in the equilibrium powder products. The calculation results are compared with experiments.     

Synthesis of silicon nitride by conventional and microwave carbothermal reduction and nitridation of rice hulls

Rice hulls were used as raw material to produce Si₃N₄ by conventional and microwave carbothermal reduction and nitridation. The precursor was made by the digestion of rice hulls with 12 M HNO₃, followed by pyrolisis of digested hulls to reduce the organic carbon. The precursor was then pelletized and reacted in conventional and microwave furnaces. The intimate of SiO₂ and carbon mixture favored the complete formation of Si₃N₄ in conventional reaction. However, SiC residual phase was present in the product of microwave reaction probably owing to the rapid temperature increasing of the reaction, caused by the thermal runaway effect of alumina pipe reactor.     

Optimization of time-temperature schedule for nitridation of silicon compact on the basis of silicon and nitrogen reaction kinetics

A time-temperature schedule for formation of silicon-nitride by direct nitridation of silicon compact was optimized by kinetic study of the reaction, 3Si + 2N₂ = Si₃N₄ at four different temperatures (1250°C, 1300°C, 1350°C and 1400°C). From kinetic study, three different temperature schedules were selected each of duration 20 h in the temperature range 1250°-1450°C, for complete nitridation. Theoretically full nitridation (100% i.e. 66.7% weight gain) was not achieved in the product having no unreacted silicon in the matrix, because impurities in Si powder and loss of material during nitridation would result in 5–10% reduction of weight gain. Green compact of density < 66% was fully nitrided by any one of the three schedules. For compact of density > 66%, the nitridation schedule was maneuvered for complete nitridation. Iron promotes nitridation reaction. Higher weight loss during nitridation of iron doped compact is the main cause of lower nitridation gain compared to undoped compact in the same firing schedule. Iron also enhances the amount of Β-Si₃N₄ phase by formation of low melting FeSi_x phase.     

High temperature phase equilibrium of SiC-based ceramic systems: SiC–Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–R<Subscript>2</Subscript>O<Subscript>3</Subscript> (R = Gd,Y) systems

The phase relations of the ternary systems SiC–Si₃N₄–R₂O₃ (R = Gd and Y) have been investigated by XRD phase analyses of the hot-pressed samples from SiC, Si₃N₄, and R₂O₃ powders. Their subsolidus phase diagrams were presented. In situ SiO₂ impurity in the powders always led to form some oxygen-richer rare-earth siliconoxynitrides and extend the ternary systems to the quaternary systems of SiC–Si₃N₄–SiO₂–R₂O₃ (R = Gd, Y). Within these systems all rare-earth siliconoxynitrides coexist with SiC. The phase diagrams of the quaternary systems of SiC–Si₃N₄–SiO₂–R₂O₃ (R = Gd, Y) were established, in which the subsolidus diagram of Si₃N₄–SiO₂–Gd₂O₃ system was first reported.     

Effect of gas phase on SiC and Si3N4 formations from SiO2

During the synthesis of SiC, Si₃N₄ and sialon whiskers by carbothermal reduction of SiO₂, a localized formation of amorphous phases or Si₂N₂O powders was observed beneath these whiskers. Because these whiskers were formed by the vapour/solid mechanism, the controlling gas phase was of primary importance to obtain whiskers of tailored morphology and chemistry. To elucidate the effect of the gas phase composition on the reaction mechanisms of SiC and Si₃N₄, the oxygen partial pressure was measured during the synthesis with a ZrO₂ solid electrolyte. The carbothermal reduction of SiO₂, as well as evolution of gases, were accelerated by a formation of a molten fluorosilicate with an auxiliary halide bath. The oxygen partial pressure steadily increased with increasing temperature and reached a maximum level of 10^–1110^–12 atm in the early stage of reaction at 1623 K, then decreased again towards the end of reaction in both cases. Effects of the gas phase on the SiC and Si₃N₄ formations were not the same: p _CO and and their ratio were important factors in the SiC formation, while the higher formed an oxynitride phase in the Si₃N₄ formation.     

Synthesis and photoluminescence of belt-shaped Si3N4 whiskers

Belt-shaped Si₃N₄ whiskers have been synthesized by a carbothermal reduction and nitridation method. The whiskers had an average width of 800 nm, width-to-thickness ratios of 4-6, and a length in the range of several tens of microns to hundreds of microns. Photoluminescence (PL) spectrum of the whiskers showed a strong blue emission peak at 410 nm, and PL lifetime measurement exhibited a rapid decay within a few nanoseconds. The growth of the whiskers was supposed to be dominated by a vapor-solid (VS) mechanism.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.     

Novel synthesis of α-Si3N4 fibres

α-Si₃N₄ fibres have been synthesized by carbothermal reduction and nitridation of pre-oxidized SiO_1·7. The fibres were characterized using X-ray diffraction, infrared spectroscopy and electron microscopic techniques. The likely mechanism of reaction has been outlined. Communication No. 224 from MRC     

Evaluation of processing parameters effects on the formation of Si3N4 wires synthesized by means of ball milling and nitridation route

Crystalline silicon nitride (Si₃N₄) wires have been synthesized by means of ball milling and nitridation route. The influence of temperature of reaction and starting condition of the powder (milled or unmilled) on the synthesis of Si₃N₄ wires were studied. The reduced size of silicon particle during the milling process led to an increased degree of nitridation.Silicon powders with higher surface energy can react incessantly with nitrogen to form silicon nitride wires. The results show that the Si₃N₄ was fully formed with two kinds morphologies including globular and wire with a width of 100–300 nm and a length of several microns at temperature of 1300 °C for 1 h by employing the milled silicon powder. The infrared adsorption of wires exhibit absorption bands related to the absorption peaks of Si–N band of Si₃N₄.     

Preparation of nano-TiN powders by Ni-catalysed carbothermal reduction nitridation

TiN nanoparticles were prepared at 900–1100 °C by Ni-catalysed carbothermal reduction nitridation from sol-gel using tetrabutyl titanate, citric acid monohydrate and nickel chloride hexahydrate as starting materials. The catalytic effects of nickel on the carbothermal reduction nitridation of xerogels were investigated. Ni has a crucial promoting effect on the carbothermal reduction reaction of the xerogels and visibly enhanced carbothermal reduction and nitridation reactions. The dry gel with 5% NiCl₂ was added to obtain nano-TiN at 900 °C. The transmission electron microscopy analysis and Materials Studio simulation results showed that the facilitating effect of Ni on carbothermal reduction nitridation reaction was ascribed to the provision of heterogeneous nucleation sites for amorphous titania, promoting crystallisation and adsorption of N₂, resulting in its smooth dissociation into highly active N atoms and consequently enhancing the carbothermal reduction nitridation.     

Direct bonding of Cu to oxidized silicon nitride by wetting of molten Cu and Cu(O)

When pressureless sintered silicon nitride with the main additives Y₂O₃ and Al₂O₃, having a thermal conductivity K = 20 W/m K, was oxidized at 1240–1360 °C in still air, the resulting surface oxide layer easily bonded to a copper plate in the temperature region between 1065 and 1083 °C, and in the oxygen concentration range of 0.008–0.39 wt%, as shown in a Cu–O phase diagram. The oxide on the silicon nitride was characterized as Y₂O₃·2SiO₂ and mixed silicate glass with additives and impurities that diffused through the grain boundary. The bonding strength of Cu/Si₃N₄ depends on the amount or layer thickness of silicate glass and reaches as high as 100 MPa by shear at room temperature. Detailed analysis of the oxidation layer and the peeled-off surfaces of directly bonded Si₃N₄/Cu reveal that the main mechanism of bonding is wetting to glassy silicate phase by mixtures of molten Cu and α-solid solution Cu(O), which solidify to α + Cu₂O below 1065 °C by a eutectic reaction. The direct reactive wetting of molten Cu, supplied from the grain boundary of a Cu plate, on the glassy phase enables very tight chemical bonding via oxygen atoms.     

Effect of grain-boundary crystallization on the high-temperature strength of silicon nitride

Si₃N₄ specimens having the composition 88.7 wt% Si₃N₄-4.9wt% SiO₂-6.4wt% Y₂O₃ (85.1 mol% Si₃N₄-11.1 mol% SiO₂-3.8mol% Y₂O₃) were sintered at 2140° C under 25 atm N₂ for 1 h and then subjected to a 5 h anneal at 1500° C. Crystallization of an amorphous grainboundary phase resulted in the formation of Y₂Si₂O₇. The short-time 1370° C strength of this material was compared with that of material of the same composition having no annealing treatment. No change in strength was noted. This is attributed to the refractory nature of the yttrium-rich grain-boundary phase (apparently identical in both glassy and crystalline phases) and the subsequent domination of the failure process by common processing flaws.Chemical analysis of the medium indicated 5.25 wt% O₂, 0.46 wt% C, 0.8 wt% Al, and expressed in p.p.m. 670 Ca, 30 Cu, 2000 Fe, <2 Ti, 370 Cr, 130 Mg, 90 Mn, <10 V, <20 Zr, 2000 Mo, 240 Ni, 130 Zn, <30 Pb, <60 Sn.     

Control of particle size in Si3N4 powders prepared by high-pressure carbothermal nitridation

The grain size variation in “unseeded” Si₃N₄ powders, prepared by high-pressure carbothermal nitridation of SiO₂ (in stoichiometric 1∶2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and “Sedigraph” measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO₂ areaA(SiO₂) ≈ 50 m² g^?1 in the reaction mixture, the resulting mean Si₃N₄ particle diameter,d(Si₃N₄), is very sensitive to the carbon surface area,A(C), such that the minimumd(Si₃N₄) ≈ 1 μm was obtained withA(C)=30m²g^?1 and the maximumd(Si₃N₄) ≈ 7μm withA(C)=115m²g^?1. Using mixtures withA(SiO₂)=50m²g^?1 andA(C)=115m²g^?1, a slight dependence ofd(Si₃N₄) on the furnace heating rate was also observed; larger grains (≈ 7 μm) were obtained with 20deg min^?1 than with 2deg min^?1 (≈5μm). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3–6.5 MPa), annealing temperature (1470–1830°C) and gas flow rate (2–20 l_(stp) min^?1).     

Synthesis of β-SiC nanostructures via the carbothermal reduction of resorcinol–formaldehyde/SiO2 hybrid aerogels

The novel resorcinol–formaldehyde/SiO₂ (RF/SiO₂) hybrid aerogels were chosen to synthesize the cubic silicon carbide (β-SiC) nanostructures via a carbothermal reduction route. In this process, the in situ polymerized RF/SiO₂ aerogels were used as both the silicon and carbon sources. The morphologies and structures of SiC nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and high-resolution transmission electron microscope (HRTEM) equipped with EDS. The effects of C/Si atomic ratios in RF/SiO₂ aerogels and heat treatment temperatures on the formation of SiC nanomaterials were investigated in detail. It was shown that β-SiC nanowhiskers with diameters of 50–150 nm and high crystallinity were obtained at the temperatures from 1400 to 1500 °C. The role of the interpenetrating network of RF/SiO₂ hybrid aerogels in the carbothermal reduction was discussed and a possible mechanism was proposed.     

Microstructure characterization of in situ synthesized porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–Si<Subscript>2</Subscript>N<Subscript>2</Subscript>O composites using feldspar additive

Porous Si₃N₄–Si₂N₂O bodies fabricated by multi-pass extrusion process were investigated depending on the feldspar addition content (4–8 wt% Si) in the raw silicon powder. The diameter of the continuous pores was about 250 μm. The polycrystalline Si₂N₂O fibers observed in the continuous pores as well as in the matrix regions of the nitrided bodies can increase the filtration efficiency. In the 4 wt% feldspar addition, the diameter of the Si₂N₂O fibers in the continuous pores of the nitrided bodies was about 90–150 nm. A few number of rope typed Si₂N₂O fibers (∼4 μm) was found in the case of 8 wt% feldspar addition. However, in the 8 wt% feldspar addition, the matrix showed highly porous structure composed of large number of the Si₂N₂O fibers (∼60 nm). The relative densities of the Si₃N₄–Si₂N₂O bodies with 4 wt% and 8 wt% feldspar additions were about 65% and 61%, respectively.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

Synthesis of Si3N4 using sepiolite and various sources of carbon

Sepiolite of Turkish origin was used as Si precursor in the syntheses of silicon nitride (Si₃N₄) powders by carbothermal reduction-nitridation (CRN) by mixing with several reducing agents i.e. charcoal, carbon black and petroleum coke as discrete particles and acrylonitrile as an intercalation medium. Purified sepiolite samples with a pre-determined C/SiO₂ ratio of 4 yielded Si₃N₄ powders after firing at temperatures 1300–1475°C under continuous nitrogen flow. The various sepiolite-reducing agent combinations were evaluated. The / ratio and secondary phase content of the powders after CRN were found to depend on temperature, time, heating rate and on the physicochemical properties of the precursor used such as, surface area and mixing of the reactants.