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.     

Catalytic Hydrocracking of a Bitumen‐Derived Asphaltene over NiMo/γ‐Al2O3 at Various Temperatures

Hydrocracking of a bitumen‐derived asphaltene over NiMo/γ‐Al₂O₃ was investigated in a microbatch reactor at varying temperatures. The molar kinetics of asphaltene cracking reaction was examined by fitting the experimental data. Below a defined temperature, the molar reaction showed the first‐order kinetic feature while at higher temperatures secondary reactions such as coke formation became significant, causing deviation of the reaction behavior from the proposed first‐order kinetic model. Selectivity analysis proved that dominant products varied from gases to liquids to gases with increasing temperature, shifting the dominant reaction from C–S bonds cleavage to C–C bonds cleavage.     

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.     

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.     

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.     

Synthesis of tercopolymer BA–MMA–VTES and surface modification of nano‐size Si3N4 with this macromolecular coupling agent

A new macromolecular coupling agent butyl acrylate (BA)‐methyl methacrylate (MMA)‐vinyl triethoxy silane (VTES) tercopolymer was synthesized using solution polymerization initiated by free radical initiator benzoyl peroxide (BPO) and dicumyl peroxide (DCP). Dodecylthiol is choosed as the chain transfer to control the molecule weight of this tercopolymer. The terpolymer's molecular structure was confirmed by FTIR and NMR, and its average molecular weight was determined by GPC. In this work, the tercopolymer BA–MMA–VTES is used for surface modification of silicon nitride (Si₃N₄) nanopowder. The structure surface properties and thermal stability of modified nano‐Si₃N₄ were systematically investigated by FTIR, TGA, TEM, and size distribution analyzer. The results show that the macromolecular coupling agent bonds covalently on the surface of nano‐sized Si₃N₄ particles and an organic coating layer is formed. The optimum loading of this macromolecular coupling agent BA–MMA–VTES tercopolymer is 5% (wt %) of nano‐sized Si₃N₄. TEM also reveals that modified nano‐Si₃N₄ possesses good dispersibility. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

Fabrication and properties of surface‐modified β‐Si3N4 whiskers reinforced dental resin composites

In this article, three kinds of surface‐modified methods were used to treat β‐Si₃N₄ whiskers before being incorporated into Bis‐GMA/TEGDMA dental resin matrix in order to improve the whiskers' reinforcing effect. The experimental results showed that composites with directly heat‐treated and then silanized β‐Si₃N₄ whiskers had the best reinforcing effect. They had flexural strength of 160 ± 7.0 MPa (mean ± SD; n = 6), compressive strength of 371 ± 1.4 MPa (mean ± SD; n = 5) and HRA of 48.4 ± 0.5(mean ± SD; n = 5), respectively. In addition, water sorption and solubility test demonstrated that the composites were reliable to use as the dental restoration materials. Therefore, the directly heat‐treated and then silanized β‐Si₃N₄ whiskers (better than β‐Si₃N₄ whiskers mixed with SiO₂ nanoparticles or SiO₂ sols) were most suitable fillers to reinforce dental resin composites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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.     

Surface modification of nano‐sized silicon nitride with BA‐MAA‐AN tercopolymer

In this work, a macromolecular coupling agent (BA‐MAA‐AN tercopolymer) was used for surface modification of native nano‐sized silicon nitride (Si₃N₄) powder. This modification strategy was designed for preparing nano‐Si₃N₄/NBR composites. The structure and surface properties of modified nano‐Si₃N₄ were systematically investigated by FTIR, XPS, TGA, TEM, Size Distributions Analyzer, and Contact Angle Measurement. It was found that, the optimum loading of BA‐MAA‐AN tercopolymer coated on the surface of nano‐sized Si₃N₄ is 10% of nano‐Si₃N₄. According to the spectra of FTIR, XPS and TGA, it can be inferred that this macromolecular coupling agent covalently bonds on the surface of nano‐sized Si₃N₄ particles and an organic coating layer is formed. The contact angle experiments show that the hydrophobic property of nano‐sized Si₃N₄ modified with macromolecular coupling agent is improved obviously. TEM reveals that modified nano‐Si₃N₄ possesses good dispersibility and the average diameter in NBR is less than 100 nm. It has also been found that the oil resistance of NBR based nanocomposites is improved greatly due to the modified nano‐Si₃N₄. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The Reaction Mechanism and Kinetics of α‐BN Powder in Wet Air at 1273 K

The reaction behavior of α‐Boron nitride (BN) powder in wet air was investigated at 1273 K using thermogravimetric (TG) analysis, X‐ray diffraction (XRD), and mass spectrometry. Scanning electron microscopy and electron probe X‐ray microanalysis (EPMA) were used to analyze the morphological and elemental development of the sample. The results show that the reaction process consists of two stages: the oxidation of BN powder to form B₂O₃ and the further reaction of B₂O₃ with H₂O. In the first stage, the oxidation reaction occurs very quickly and can be described by Chou's model. In the second stage, the reaction rate followed the linear rate law.     

Crystal Growth in the Combustion Synthesis of α‐Si3N4 Using Si with Different Particle Sizes

In this present work, Si₃N₄ powders with high α‐phase contents and distinct crystal morphologies were prepared via a promising approach of combustion synthesis (CS), using Si powders with different particle sizes as reactants. The influence of Si particle size on phase composition and crystal morphologies in the products was systematically investigated. Two unique crystal morphologies, radial‐spheroidal‐cluster and flowerlike, were observed in the Si₃N₄ products. The crystal growth mechanisms of Si₃N₄ granules in the CS system with disparate Si have been proposed based on experiments and thermokinetic calculations. As conclusion, the α‐phase content in the final product was synergetically dominated by the vaporization process of Si particles and the α–β phase transformation of Si₃N₄ during the after‐burn period. Si₃N₄ powders with high α‐phase content can be obtained from Si powders with an appropriate particle size.     

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.     

Microstructure and mechanical properties of the Si3N4/42CrMo steel joints brazed with Ag–Cu–Ti + Mo composite filler

The Si₃N₄ ceramic was joined to 42CrMo steel using Ag–Cu–Ti + Mo composite filler. Effect of Mo particles content on the microstructure and mechanical properties of the joints were investigated. Defect-free joints were received when the Si₃N₄/42CrMo steel joints were brazed with Ag–Cu–Ti + Mo composite filler. The results show that a continuous reaction layer which is composed of TiN and Ti₅Si₃ was formed near the Si₃N₄ ceramic. A double reaction layer which consists of Fe₂Ti and FeTi was also formed adjacent to 42CrMo steel, with Fe₂Ti being located near the steel. The central part of the joint is composed of Ag based solid solution, Cu based solid solution, Mo particles and some Cu–Ti intermetallic compounds. The maximal bending strength reached 587.3 MPa with 10 vol.% Mo particles in the joint, at which the joint strength was 414.3% higher than the average strength for the case without Mo particles addition.     

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.     

Brazing of porous Si3N4 ceramic to Invar alloy with a novel Cu–Ti filler alloy: Microstructure and mechanical properties

Porous Si₃N₄ (P–Si₃N₄) ceramic was successfully joined to Invar alloy using a Cu–Ti active brazing alloy for the first time. The interfacial reactions between the Cu–Ti filler and two dissimilar substrates were studied. The influence of brazing process on the microstructure evolution of the joint was revealed, along with the formation of an infiltration layer that permitted the bonding of P–Si₃N₄ substrate. Ti reacted with Si₃N₄ to form TiN and Ti₅Si₃ compounds, resulting in the decomposition of Si₃N₄. In addition, the reaction and diffusion dual-layer formed at the Cu–Ti/Invar interface, which was attributed to the interaction of alloy elements between the braze filler and the Invar alloy. Fe–Ti and Ni–Ti intermetallics together with Cu solid solution (s,s) constituted the microstructure of the brazing seam. In addition, the optimal shear strength of the brazed joint was 20 MPa and the fracture propagation occurred in the P–Si₃N₄ ceramic substrate adjacent to the infiltration layer during the shearing tests.     

Corrosion behavior of porous silicon nitride ceramics in different atmospheres

The corrosion behavior of silicon nitride (Si₃N₄) ceramics with a porosity of 46% at 1200–1500 °C under different conditions including dry O₂, O₂ containing 20 vol% H₂O and Ar containing 20 vol% H₂O is compared. The results show that porous Si₃N₄ ceramics exhibit good oxidation resistance up to 1200 °C. Their corrosion behavior varies depending on the temperature and atmosphere. Water vapor can obviously affect the morphology of the reaction product and thus accelerate the corrosion rate due to its specific inward diffusion mechanism and devitrified effect at high temperature. In view of the reaction kinetics, it proceeds in a diffusion-controlled manner in dry O₂ while follows the parabolic-linear law at water-containing atmosphere. Furthermore, a new model considering both oxidation and volatilization reactions is established. These provide a baseline for expanding the application fields of non-oxide porous ceramics such as Si₃N₄ and silicon carbide (SiC) etc.     

Densification and Thermal Stability of Hot‐Pressed Si3N4–ZrB2 Ceramics

Densification and thermal stability of hot‐pressed Si₃N₄–ZrB₂ ceramics with and without additives were investigated in N₂ atmosphere. The addition of MgO–Yb₂O₃, MgO–Y₂O₃, and Al₂O₃–Yb₂O₃ resulted in significant increase in relative density of the ceramics hot‐pressed at 1500°C from 48.5% to 98.0%, 97.3%, and 95.6%, respectively. There was weak reaction of ZrB₂ with N₂ to form ZrN in hot‐pressed ceramics. Then heat treatment at 1550°C resulted in the further reactions to produce ZrN, ZrSi₂, and BN. The Si₃N₄–ZrB₂ ceramics with MgO–Yb₂O₃ showed much better thermal stability as compared to the ceramics with Al₂O₃–Yb₂O₃. The small difference in density led to the obvious difference in thermal stability. Therefore, Si₃N₄–ZrB₂ ceramics should be densified to full density, to obtain high thermal stability.     

Hydrogenation of α‐Keto Ethers: Dynamic Kinetic Resolution with a Heterogeneous Modified Catalyst and a Heterogeneous Base

The first successful example of the asymmetric hydrogenation of substituted α‐keto ethers with Cinchona‐modified Pt/Al₂O₃ is reported. In the absence of an additional base, kinetic resolution of the racemic starting material was observed with high diastereoselectivity and ee's up to 98% at conversions of <50%. Addition of KOH gave a strong reaction acceleration but racemic product. Immobilization of OH ^– on solid ion exchangers resulted in the desired dynamic kinetic resolution, and ee's of >80% were obtained at >95% conversion. These effects are rationalized on the basis of a simple kinetic and structural model.     

3D printing of honeycomb Si3N4 ceramic

In this paper, a honeycomb Si₃N₄ ceramic was fabricated by 3D printing with a well-preserved structure. The effects of Si₃N₄ content on the rheological properties of Si₃N₄/sol–silica ink and the printing resolution of products were investigated. The microstructure, phase composition, liner shrinkage rate, and fracture behavior of printed samples before and after sintering were systematically characterized in detail. The results showed that the modified inks had the optimized rheological properties, and the stress–shear rate curves corresponding to each slurry could be well described by Bingham and Herschel–Bulkley fluid models. The corresponding slump rates of the printed samples with different Si₃N₄ to sol–silica mass ratios were all lower than 4%, and the linear shrinkage rate of all of the samples after sintering was below 20%. The fracture behavior under compressive loading of the honeycomb Si₃N₄ ceramics tended to be non-catastrophic fractures both before and after sintering. The compressive strengths of all of the printed samples decreased with the increase of the Si₃N₄ content, and the highest compressive strength of the honeycomb ceramics could reach 131.2 MPa after sintering at 1600°C, which was about 366.9% higher than that of the samples in green state prior to the sintering.