Effect of Fe‐Contained Species on the Preparation of α‐Si3N4 Fibers in Combustion Synthesis

Herein, Si₃N₄ powders of comparatively high α‐phase but with distinct morphologies, especially α‐Si₃N₄ fibers, were successfully prepared by a developed combustion synthesis (CS) strategy. Different proportions of Fe and Fe₂O₃ were innovatively doped in reactants as additives to control the phase constitution and their relative percentage, as well as morphologies of final microstructures. One step further, the effects of Fe‐contained impurities on the CS process were rationally proposed and verified based on a series of meticulous designed experiments. It turns out that two contradictory effects of metal Fe on the formation of α‐Si₃N₄ synergistically play vital roles in the CS reaction. The existence of metal Fe can accelerate the crystallization of the amorphous SiO₂, which act as protection layer outside the Si powders and subsequently promote the generation of gaseous SiO. These gaseous SiO easily reacts with N₂ and eventually form α‐Si₃N₄. On the other hand, the formation of β‐Si₃N₄ will be promoted by the assistance of some liquid phases, and in this case, they mainly come from the reaction between Fe and Si. For this study, when the content of doped Fe is below 2 mol%, the prior effect on promoting α‐phase content is pronounced. Otherwise, the latter dominates the CS process as the content of Fe additive is further increased above 2 mol%. In a different way, Fe₂O₃ mainly encourages the formation of β phase through the large amount of newly generated liquid phases, although the reduced SiO₂ and Fe may still promote the α/β ratio on some extent.     

Preparation of Porous Si3N4 Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si3N4 Addition

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐Si₃N₄ powder with a various content ratios. The effects of the contents of the fibrous α‐Si₃N₄ powder (0%–100%) on the microstructure and mechanical properties of porous Si₃N₄ ceramics were studied. As the increase of the fibrous α‐Si₃N₄ powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si₃N₄ particle. XRD analysis proved the complete formation of single β‐Si₃N₄ phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐Si₃N₄ porous ceramics was almost composed of fine elongated β‐Si₃N₄ grains with high aspect ratio while numerous coarse elongated β‐Si₃N₄ grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐Si₃N₄ powder increased. With the increase in content of the fibrous α‐Si₃N₄ powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.     

Silicon nitridation mechanism in reaction‐bonded Si3N4–SiC and Si3N4‐bonded ferrosilicon nitride

Reaction‐bonded Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe₃Si–Si₃N₄ particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N₂ at 1723K. There is columnar β‐Si₃N₄ in both Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride. However, fibrous α‐Si₃N₄ is only observed in Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride contains much more Si₂N₂O than Si₃N₄–SiC. By analyzing the oxidation thermodynamics of Si and Si₃N₄, it is known that in the process of producing Si₃N₄–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si₃N₄ is generated with SiO and N₂. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si₃N₄‐bonded ferrosilicon nitride, Si₃N₄ is easier to be oxidized than Si and Si₂N₂O is generated on the surface of Si₃N₄ hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe₃Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors β‐Si₃N₄ generation from Si direct nitridation and fibrous α‐Si₃N₄ transformation, which used to exist in ferrosilicon nitride raw materials.     

Synthesis of well‐dispersed β‐Si3N4 with equiaxed structures using carbothermal reduction–nitridation method

Well‐dispersed β‐Si₃N₄ powders with a novel equiaxed structure and eminent crystal integrity were prepared by carbothermal reduction–nitridation (CRN) strategy with the assistance of CaF₂ additive. The growth mechanism of Si₃N₄ particles in the CRN process was elucidated. It is proposed that the liquid phase formed by SiO₂ and CaF₂ additive is crucial to the formation of equiaxed β‐Si₃N₄, and with an appropriate content of CaF₂, Si₃N₄ powders with pure β phase, superior dispersity and crystal integrity can be obtained.     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

Synthesis and growth mechanism of single crystal β‐Si3N4 particles with a quasi‐spherical morphology

Single‐crystal β‐Si₃N₄ particles with a quasi‐spherical morphology were synthesized via an efficient carbothermal reduction‐nitridation (CRN) strategy. The β‐Si₃N₄ particles synthesized under an N₂ pressure of 0.3 MPa, at 1450°C and with 10 mol% unique CaF₂ additives showed good dispersity and an average size of about 650 nm. X‐ray photoelectron spectroscopy analysis revealed that there was no SiC or Si–C–N compounds in the β‐Si₃N₄ products. Selected‐area electron‐diffraction pattern and high‐resolution image indicated single crystalline structure of the typical β‐Si₃N₄ particles without an obvious amorphous oxidation layer on the surface. The growth mechanism of the quasi‐spherical β‐Si₃N₄ particles was proposed based on the transmission electron microscopy and energy dispersive X‐ray spectroscopy characterization, which was helpful for controllable synthesis of β‐Si₃N₄ particles by CRN method. Owing to the quasi‐spherical morphology, good dispersity, high purity, and single‐crystal structure, the submicro‐sized β‐Si₃N₄ particles were promising fillers for preparing resin‐based composites with high thermal conductivity.     

Effects of diazenedicarboxamide additive on the content of α-Si3N4 synthesized by combustion method

Combustion synthesis (CS) of high content of α-Si₃N₄ powders was carried out using Si and Si₃N₄ powders as reactants with the addition of diazenedicarboxamide (AC) at a relatively low N₂ pressure of 3 MPa. Effects of diazenedicarboxamide contents on the phase compositions and Si₃N₄ particle morphologies were studied. In addition, the reaction mechanisms were discussed. The results indicated that the additive diazenedicarboxamide promoted the nitridation of Si. The α-Si₃N₄ content in the combustion-synthesized products showed great dependence on the additive contents, which reached 85.21 wt% with 24 wt% diazenedicarboxamide added. N₂, CO and NH₃ produced by the decomposition of diazenedicarboxamide leaded to a change of compact porosity and the formation of micro-pores in the reactive area, which was responsible for the increasing contents of α-Si₃N₄ and the discrepancy morphology of the products.     

Formation mechanisms of Si3N4 microstructures during silicon powder nitridation

Silicon nitride (Si₃N₄) was synthesized under a nitrogen gas flow (100 mL/min) using a molten salt nitriding method to investigate the effects of the temperature and NaCl content on the α-Si₃N₄ content in products and their micro-morphologies. Adding NaCl and β-Si₃N₄ in silicon powders resulted in Si nitridation products divided into two layers. Analysis of the lower product using X-ray diffraction revealed a change in the α-Si₃N₄ content with changes in the temperature and NaCl content. Analysis of the lower and upper layers using scanning electron microscopy revealed that the upper layer contained Si₃N₄ nanowires, Si₃N₄ nanobelts, and clastic oxide impurities; the lower one contained short needle-like and blocky Si₃N₄. From the microstructures of the products, the product morphology related to that the dry mixing procedure did not correspond to homogenization of the starting Si-Si₃N₄-NaCl mixtures and the different concentrations of raw materials resulted in different morphologies.     

Fabrication and Characterization of In Situ Porous Si3N4‐Si2N2O‐BN Ceramic

Porous Si₃N₄‐Si₂N₂O‐BN ceramic was fabricated at 1750°C using Si₃N₄, BN, and (NH₄)₂HPO₄ as starting materials. During the sintering process, oxygen from the decomposed products of (NH₄)₂HPO₄ would bond Si and N in the liquid phase to form Si₂N₂O. The microstructure and properties of the porous ceramics were investigated. With the (NH₄)₂HPO₄ content varied from 10 to 50 vol.%, porosity of the porous Si₃N₄‐Si₂N₂O‐BN ceramic increased from 43.5% to 51%. The microstructure, mechanical, and dielectric properties was well controlled by adjusting (NH₄)₂HPO₄ contents. The present technique offers a more simple way of synthesizing porous Si₃N₄‐Si₂N₂O‐BN ceramics.     

Synthesis and growth mechanism of approximate spherical β‐Si3N4 particles via carbothermal reduction‐nitridation method

In this work, an efficient carbothermal reduction‐nitridation (CRN) strategy was rationally designed to directly synthesize β‐Si₃N₄ powders with eminent dispersity and granularity uniformity. With the aid of CaO additive, the obtained β‐Si₃N₄ particles were endowed with approximate spherical morphology and smooth surface. The size of β‐Si₃N₄ particles could be regulated in submicro and microscale by altering N₂ pressures. More significantly, the underlying growth mechanism of the β‐Si₃N₄ under elevated N₂ pressure was comprehensively analyzed and tentatively put forward. Benefiting from the remarkable merits, the as‐synthesized β‐Si₃N₄ powders showed great potential for alternative fillers in the application of high thermal conductivity plastic packages.     

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.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

Thermodynamic Analysis on the Codeposition of SiC–Si3N4 Composite Ceramics by Chemical Vapor Deposition using SiCl4–NH3–CH4–H2–Ar Mixture Gases

A thermodynamic calculation on the chemical vapor deposition of the SiCl₄–NH₃–CH₄–H₂–Ar system was performed using the FactSage thermochemical software databases. Predominant condensed phases at equilibrium were SiC, Si₃N₄, graphite, and Si. The equilibrium conditions for the deposition of condensed phases in this system were determined as a function of the deposition temperature, dilution ratio (δ), and reactant ratios of CH₄/SiCl₄ and NH₃/SiCl₄. The CVD phase diagrams were used to understand the reactions occurring during the formation of Si–C–N from the gas species and determine the area of SiC–Si₃N₄. The concentration of condensed‐phase products was used to determine the deposition conditions of CVD SiC–Si₃N₄. The present work was helpful for further experimental investigation on CVD Si–C–N.     

3D printing Si3N4-bonded SiC refractories fabricated using colloidal films-containing slurries based on non-spherical SiC and Si powders

Stable slurries for Si₃N₄-bonded SiC refractories for direct ink writing (DIW) were successfully prepared from a mixture of non-spherical silicon carbide (SiC) and silicon (Si) powders with an average particle size of D₅₀ = 41.98 μm. The rheological properties and printability of slurries prepared using polyvinyl alcohol (PVA; 4–16 wt %) or hydroxypropyl methylcellulose (HPMC, 0.5–2 wt.%) were investigated with the effect of sintering temperature on the mechanical performance, phase, and microstructure of Si₃N₄-bonded SiC refractory products. The results indicated that slurries prepared with the HPMC solution showed better printability than those prepared with the PVA solution because colloidal films formed by HPMC in slurries play a role in encasing particles, preventing solid−liquid separation and contributing to plasticity and lubrication, which guarantees the smooth extrusion and homogeneity of slurries. The successful printing of SiC–Si slurries is not only related to proper viscosity, yield value, and shear thinning characteristics but it is also crucial for maintaining the homogeneity of slurries under extrusion pressure. Optimal SiC–Si slurries containing 52 vol % SiC–Si and 1.5 wt% HPMC exhibited proper viscosity, shear thinning, and homogeneity characteristics during printing. The obtained specimens achieved the best printing performance with height and section retention rates of 98.7% and 97.6%, respectively. When sintered at 1450 °C, Si₃N₄ fibres grow further and reach a diameter of 342.5 nm, the nitriding rate is 92.43%, the fibres tend to form a full network structure, and the mechanical properties of Si₃N₄-bonded SiC products are the best.     

Architectural design and cryogenic synthesis of Si3N4@(TiN–Si3N4) for high conductivity

Si₃N₄@(TiN–Si₃N₄) composites with heteroshelled structure were designed for enhanced conductivity and successfully synthesized through the simultaneous reduction and in‐situ cocoating process in liquid ammonia at around ?40°C. The heteroshells were composed of nanosized TiN and Si₃N₄ particles, which were amorphous with the size ranging from 10 to 40 nm. Using spark plasma sintering, dense bulk composite with >98.1% relative density of theoretical value were obtained and their electrical conductivity were increased to an adequate value (6.62 × 10² S·cm^?1) for electrical discharge machining by compositing 15 vol% TiN to Si₃N₄, which is superior to the previous reports. The excellent electric performance could be attributed to the heteroshelled structure which guarantees the conductive network can be formed and kept with minimal TiN content. The nanosized Si₃N₄ powders in the shells reduce the content of conductive powders and limit the growth of TiN particles.     

Combustion synthesis of SiC/Si3N4-NW composite powders: The influence of catalysts and gases

Composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized by combustion synthesis, using elemental Si, carbon black, PTFE and small amount of metal powders as raw materials. The catalyst types and environmental gases and pressures have been altered to study their influence upon the crystal growth and the nature of the products. The products were characterized by X-ray diffraction, scanning and transmission electron microscopy. Results reveal that the metal/silicon liquid (e.g. Ni₂Si and Fe₃Si) formed during the combustion process is a key factor for the growth of Si₃N₄-NWs in nitrogen. For the process carried out in non-nitrogen gas (Ar, CO₂ or mixed CO₂/O₂), pure SiC particles were obtained. The rise in nitrogen pressure can promote the growth of Si₃N₄-NWs as well as large SiC particles. The growth of Si₃N₄-NWs could be explained by the SLGS mechanism, and the growth of SiC particles was involved in the gas-phase and liquid-phase mechanisms.     

Combustion synthesis of MgSiN2 powders and Si3N4-MgSiN2 composite powders: Effects of processing parameters

In the present work, high-quality MgSiN₂ powders were prepared by a combustion synthesis method, based on Mg-Si₃N₄-N₂ system. The effects of additive content (NH₄Cl or NH₄F) and N₂ pressure on phase composition and crystal morphology of products were investigated. The results suggested that both NH₄Cl and NH₄F additives alleviated agglomeration and decreased the grain size of MgSiN₂. In addition, NH₄Cl did not result in the formation of an extra impurity, while MgF₂ residue was found when NH₄F was introduced. Therefore, NH₄Cl additive was considered as a better choice for the preparation of MgSiN₂ powders in comparison with NH₄F additive. According to the observation of some MgSiN₂ whiskers and notably increased Si content under low N₂ pressure, the pressure of N₂ over 0.5 MPa was essential to have a complete reaction in the present work. Finally, MgSiN₂-Si₃N₄ composite powders with quasi-spherical morphology were prepared by adding excessive Si₃N₄ and NH₄Cl additive in reactant. The formation and growth mechanism of MgSiN₂ grains was reasonably speculated based on the experimental results. Because of the remarkable merits, the as-prepared Si₃N₄-MgSiN₂ composite powders showed great potential for alternative sintering aid in the sintering of Si₃N₄-based ceramics.     

A Novel Si3N4–SiC Ceramic Used for Volumetric Receivers

Si₃N₄–SiC composite ceramics used for volumetric receivers were fabricated by pressureless sintering of micrometer SiC, Si₃N₄, andalusite, and other minor additions powders. Mechanical, thermal expansion, thermal conductivity, and thermal shock resistance properties were tested at different sintering temperatures. The best sintering temperature of optimum formula A2 is 1360°C, and the bending strength reaches 79.60 Mpa. And moreover, its thermal expansion coefficient is 6.401 × 10^?6/°C, thermal conductivity is 7.83 W/(m K), and no crack occurs even subjected to 30 cycles thermal shock with a bending strength increase rate of 4.72%. X‐ray diffraction results show that the phase constituents of the sintered products mainly consist of SiC, Si₃N₄, mullite, and quartz. Microstructure that is most appropriate and exhibits maximal thermal shock resistance was detected using SEM. The porosity of Si₃N₄–SiC ceramic foam prepared from formula A2 is 95%, which provides a rapid and steady action for the receiver. The evaluation of the present foam shows that Si₃N₄–SiC ceramic composite is a good candidate for volumetric receivers.     

Reaction Pathway of Combustion Synthesis of Ti5Si3 in Cu–Ti–Si System

The reaction pathway of combustion synthesis (CS) of Ti₅Si₃ in Cu–Ti–Si system was explored through a delicate microstructure and phase analysis on the resultant products during differential thermal analysis (DTA). The formation of Cu–Si eutectic liquids plays a key role in the reaction pathway, which provides easy route for reactant transfer and accelerates the occurrence of complete reaction. Cu initially reacted with Si to form Cu₃Si by a solid‐state diffusion reaction, which further reacted with Cu to form Cu–Si liquids at the eutectic point of ~802°C; then Ti was dissolved into the surrounding Cu–Si liquids and led to the formation of Cu–Ti–Si ternary liquids; finally, Ti₅Si₃ was precipitated out of the saturated liquids by a solution–reaction–precipitation mechanism. The reaction pathway in CS of titanium silicide (Ti₅Si₃) could be described briefly as: Cu_(s) + Ti_(s) + Si_(s)→Cu₃Si_(s) + Ti_(s) + Si_(s)→(Cu–Si)_(l) + Ti_(s)→(Cu–Ti–Si)_(l)→Cu_(l) + Ti₅Si_3(s).     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.