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.     

Dispersion,slip casting and reaction nitridation of silicon–silicon carbide mixtures

The dispersing behaviour of silicon, silicon carbide and their mixtures in aqueous media were monitored by particle size, sedimentation, viscosity and zeta potential analyses as a function of pH of the slurry. The pH values for optimum dispersion were found to be 4 and 8 for silicon, 10 for SiC and 9 for Si+SiC mixtures. Optimum slips of Si+SiC mixtures were slip cast to obtain green compacts which were nitrided once at 1450°C for 2 or 4 h or successively and cumulatively for 8 (2+6) and 10 (4+6) h in a resistively heated graphite furnace. The binding phases in the nitrided products were found to be fibrous/needle like α-Si₃N₄, flaky grains of β-Si₃N₄ and Si₂ON₂. The products containing 19–47% of silicon nitride as bond/matrix possessed flexural strength (three-point bending) values of 50–85 MPa. ©     

Solid Solutions of Silicon Oxide in Mullite

An explanation is proposed to account for the possibility of formation of solid solutions of silicon oxide in mullite. The difference in the diffusion coefficients of cations in complex oxides leads to the formation of a solid solution of the oxide containing the cation with the lower diffusion coefficient in the complex oxide and a phase containing the cation with the higher diffusion coefficient or rich in this oxide. Using yttrium or scandium additives, which are chemisorbed on the surface of aerosil (SiO₂ source), it is possible to make the rate of diffusion mass transfer of aluminum cations higher than that of silicon cations and thus obtain a solid solution of silicon oxide in mullite.     

Porous Si3N4 ceramics prepared via slip casting of Si and reaction bonded silicon nitride

Slip casting process combined with reaction bonded silicon nitride (RBSN) was used to prepare porous Si₃N₄ ceramic with near-net and complex shape. A butyl stearate (BS) coated process was introduced to restrain the hydrolysis of Si, and ammonium polyacrylate (NH₄PAA) was used to enhance the dispersion of coated Si. The measured oxygen content showed that the hydrolysis of Si was strongly prohibited by BS coating, and relatively low viscosity was obtained with the addition of 0.25-1.5 wt% NH₄PAA to the 60 wt% solid load slurry. 40-60 wt% solid load slurries were used for slip casting in the experiment. After vacuum degassing, slip casting, debindering and nitridation, a density of 1.57-1.92 g/cm³ (porosity 50.9-40%) and a flexural strength of 47-108 MPa were obtained. The samples without vacuum degassing showed a large number of nanowires grown in the large pores.     

Synthesis of Ti3SiC2 by mechanically induced self-sustaining reaction: Some mechanistic aspects

Titanium silicon carbide Ti₃SiC₂ was prepared by mechanically induced self-sustaining reaction (MSR) in 3Ti/Si/2C powder blends. After mechanical alloying (MA) for 7 h, the MRS yielded the powdered and granulated products containing Ti₃SiC₂, TiC, TiSi₂, and Ti₅Si₃. Discussed are some important accompanying phenomena.     

Thermal degradation behaviors and flame retardancy of epoxy resins with novel silicon‐containing flame retardant

A novel macromolecular silicon‐containing intumescent flame retardants (Si‐IFR) was synthesized, and its structure was a caged bicyclic macromolecule containing phosphorus‐silicon characterized by IR. Epoxy resins (EP) were modified with Si‐IFR to get the flame retardant EP, whose flammability and burning behavior were characterized by UL 94 and limiting oxygen index (LOI). Twenty percentage of weight of Si‐IFR was doped into EP to get 27.5% of LOI and UL 94 V‐0. The degradation behavior of the flame retardant EP was studied by thermogravimetry, differential thermogravimetry, scanning electron microscopy, and X‐ray photoelectron spectroscopy analysis. The experimental results exhibited that when EP/Si‐IFR was heated, the phosphorus‐containing groups firstly decompose to hydrate the char source‐containing groups to form a continuous and protective carbonaceous char, which changed into heat‐resistant swollen char by gaseous products from the nitrogen‐containing groups. Meanwhile, SiO₂ reacts with phosphate to yield silicophosphate, which stabilizes the swollen char. The barrier properties and thermal stability of the swollen char are most effective in resisting the transport of heat and mass to improve the flame retardancy and thermal stability of EP. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of the nature of silicon oxide structures on the activity of an alumina-chromium catalyst in the reaction of iso-butane dehydrogenation

The formation of silicon oxide structures in the composition of an alumina-chromium catalyst for the dehydrogenation of iso-butane is studied by means of nitrogen adsorption, XRD analysis, solid-state ²⁹Si NMR spectroscopy, temperature-programmed desorption of ammonia, and UV-Vis and Raman spectroscopy. It is established that 0.5–1.2 wt % silicon was distributed on the catalyst surface in the form of Si(OSi)₄ structures. As the silicon content was increased to 2.2–3.6 wt %, Si(OSi)₃(O-) structural elements were present on the surface in addition to Si(OSi)₄. The formation of silicon oxide structures on the catalyst surface was responsible for an increase in the concentration of Cr(III) ions and a decrease in the surface acidity; the activity and selectivity of the catalysts in the reaction of iso-butane dehydrogenation increased.     

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.     

Development of multi-phase B–Si–C ceramic composite by reaction sintering

A dense ceramic composite in the system B–C–Si has been synthesized by the reaction sintering technique based on infiltration of silicon melt at 1550 °C under vacuum into a porous compact made of boron carbide and petroleum coke powder. The final material is around 99% dense and microstructurally contains B₄C, SiC and Si as the major phases. The B₄C-phase reacted at its interface with Si-phase, which is explained in terms of dissolution of Si in the carbide phase.     

The influence of reaction parameters on the free Si and C contents in the synthesis of nano-sized SiC

Uniform nano-sized beta-silicon carbide (β-SiC) powder was synthesized from the reaction of silicon (Si) and carbon black (C). Mixed Si and C-black powder were pressed into pellets and the influence of four parameters, temperature (1250, 1300 and 1350 °C), heating rate (20 and 50 °C/min), soaking time (1 and 3 h) and atmosphere (vacuum and argon), were tested. It was found that higher temperatures, higher heating rates and longer soaking times in a vacuum system lead to lower free Si content in the SiC powder created. Temperature was the parameter with the greatest influence on the Si content of the SiC powder. This study also found that the Si–C reaction occurs through gas–solid (SiO–C) and solid–solid (Si–C) reactions that occur simultaneously.     

Chemistry of the direct synthesis of methylchlorosilanes from methyl + chlorine monolayers on a Cu3Si surface

The direct process for synthesizing methylchlorosilanes from methyl chloride + silicon in the presence of catalytic amounts of copper has been studied in vacuum using a sample of Cu₃Si alloy, the bulk phase that is present in active regions of the catalytic direct process. From the melt containing 23% of excess silicon, a two-phase Cu₃Si + Si sample was prepared. Free silicon phase served both to replenish silicon reacted from Cu₃Si and to provide the grain boundaries found in an industrial process. Atomically clean surfaces of this material with varying Cu/Si atomic ratios were prepared by ion bombardment over a range of temperatures. While the dissociative adsorption of CH₃Cl was observed to be immeasurably slow on these surfaces under ultrahigh vacuum conditions, methyl + chlorine monolayers generated by the coadsorption of methyl radicals and Cl₂ led to selective formation of dimethyldichlorosilane. Adsorption of methyl groups alone produced trimethylsilane from two different active sites with very different kinetics. Adsorption of chlorine alone produced SiCl₄. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of heating rate on the hydrothermal formation of kilchoanite from lime-quartz mixtures

If mixtures of Ca(OH)₂ and quartz with Ca : Si = 1.3–2.0 are processed hydrothermally at 350 °C and 400–500 bars, the products depend on the heating rate. If this is sufficiently rapid, the main product at Ca : Si = 1.5 is kilchoanite (Ca₆(SiO₄)(Si₃O₁₀)); this is accompanied at Ca : Si = 1.3 by xonotlite and at Ca : Si = 2.0 by calciochondrodite. If the heating rate is slower, the main products are combinations of xonotlite, foshagite and dellaite. If the heating rate is rapid, kilchoanite is formed at 300 °C, and, if it is rapid and sufficiently finely divided quartz is used, at 250 °C; it was not obtained at 200 °C. Broadly similar results were obtained at saturated steam pressures. At Ca : Si = 1.5, 250–350 °C and 400–500 bars, kilchoanite is possibly stable relative to the other solid phases, formation of which at slow heating rates is attributed to nucleation while the temperature is rising. To obtain the fastest heating rates, a new method was used, in which water is not admitted into the reaction vessel until the working temperature has been reached.     

Synthesis of magnesium silicon nitride nanopowder by employing two-step method

In this work, a new method for the preparation of magnesium silicon nitride (MgSiN₂) nanopowder was studied using a two-step process in nitrogen gas from Mg/Si starting mixtures. This method is known as mechanical alloying followed by heat treatment. The results showed that the compositions of the combustion products depended on the starting mixtures. In addition, the content of magnesium and silicon in the starting powder should fulfil the condition Mg/Si?=?2 to obtain single phase MgSiN₂.Single phase MgSiN₂ nanopowder can be fabricated from Mg and Si powders with mole ratio of 2:1 at 1350°C in N₂ atmosphere. Composition and structure of reactants and products were examined by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy.     

Preparation of hollow SiC spheres by combustion synthesis

Hollow spherical β-SiC was successfully prepared in argon by combustion synthesis using Si powder and polytetrafluoroethylene (PTFE) powder. The phase composition and morphology of spherical products can be controlled by adjusting the Si/C₂F₄ molar ratio (M_Si/(C2F4)). When M_Si/(C2F4) = 3, the phase content of β-SiC is the highest (up to 85.54%), and hollow spherical products obtained; When M_Si/(C2F4) ≥ 5, the Si/SiC microspheres are solid. The synthesis mechanism of hollow β-SiC microspheres is as follows: Si particles react with PTFE releasing heat. Then unreacted Si absorbs heat to form liquid phase microspheres, which is equivalent to the core template to form β-SiC microspheres by reaction with cracked C. Meanwhile, the silicon diffuses from the core to the shell to form the cavity. This method can synthesize the hollow spherical β-SiC in a simple way without prearranged spherical template and long synthesis cycle.     

Effect of annealing treatments on photoluminescence and charge storage mechanism in silicon-rich SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H films

In this study, a wide range of a-SiN _x :H films with an excess of silicon (20 to 50%) were prepared with an electron-cyclotron resonance plasma-enhanced chemical vapor deposition system under the flows of NH₃ and SiH₄. The silicon-rich a-SiN _x :H films (SRSN) were sandwiched between a bottom thermal SiO₂ and a top Si₃N₄ layer, and subsequently annealed within the temperature range of 500-1100°C in N₂ to study the effect of annealing temperature on light-emitting and charge storage properties. A strong visible photoluminescence (PL) at room temperature has been observed for the as-deposited SRSN films as well as for films annealed up to 1100°C. The possible origins of the PL are briefly discussed. The authors have succeeded in the formation of amorphous Si quantum dots with an average size of about 3 to 3.6 nm by varying excess amount of Si and annealing temperature. Electrical properties have been investigated on Al/Si₃N₄/SRSN/SiO₂/Si structures by capacitance-voltage and conductance-voltage analysis techniques. A significant memory window of 4.45 V was obtained at a low operating voltage of ± 8 V for the sample containing 25% excess silicon and annealed at 1000°C, indicating its utility in low-power memory devices.     

Production of silicon containing particles by laser induced reaction of silane with methane,ethane and acetylene

Silicon containing ultrafine particles (Si and SiC) have been obtained by pulsed IR laser irradiation of gaseous SiH₄/hydrocarbon (CH₄, C₂H₆ and C₂H₂) mixtures. The chemical composition and structure of the powders formed were determined by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD) and I.R. and UV-v spectroscopy. The particles are small, uniform, loosely agglomerated spheres with a mean size of approximately 19 nm. To complete the characterisation of these ultrafine silicon-containing particles, the X-ray absorption spectra (XAS) were obtained using synchrotron radiation at the silicon absorption K-edge. We show that the use of pulsed laser radiation results in the generation of amorphous Si and crystalline SiC particles depending on the gas mixtue used, among other parameters such as composition of the mixture, laser fluence, pressure and number of laser pulses.     

Transesterification reaction between acetylated wood and trialkoxysilane coupling agents

A novel dibutyltin oxide‐catalyzed transesterification reaction between acetylated maritime pine sapwood (Pinus pinaster Soland) and methyltrimethoxysilane (MTMS) was studied. The transesterification exchange between the acetyl moities and MTMS was confirmed by weight percent gain calculations, Fourier‐transform infrared spectroscopy, as well as solid state ¹³C and ²⁹Si crosspolarization with magic‐angle spinning nuclear magnetic resonance spectroscopy. The silicon atoms in the MTMS‐transesterified wood were found to exist in the form of more or less condensed structures, concurrently formed by condensation between neighboring Si(OMe)₃ groups. An increase in the acetyl/MTMS exchange rate was also noted when the catalyst amount, temperature, or reaction time were increased. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of technological parameters on densification of reaction bonded Si/SiC ceramics

Si/SiC composite ceramics was produced by reaction sintering method in process of molten silicon infiltration into porous C/SiC preform fabricated by powder injection molding followed by impregnation with phenolic resin and carbonization. To optimize the ceramics densification process, effect of slurry composition, debinding conditions and the key parameters of all technological stages on the Si/SiC composite characteristics was studied. At the stage of molding the value of solid loading 87.5% was achieved using bimodal SiC powder and paraffin-based binder. It was found that the optimal conditions of fast thermal debinding correspond to the heating rate of 10?°C/min in air. The porous C/SiC ceramic preform carbonized at 1200?°C contained 4% of pyrolytic carbon and ～25% of open pores. The bulk density of Si/SiC ceramics reached 3.04?g/cm³, silicon carbide content was 83–85?wt.% and residual porosity did not exceed 2%.     

Kinetics of Silicon Monoxide Ammonolysis for Nanophase Silicon Nitride Synthesis

Silicon monoxide vapor generated from Si/SiO₂ mixed-powder compacts was used with NH₃ to synthesize silicon nitride in a tubular flow reactor operated at temperatures in the range of 1300°-1400°C. The ammonolysis of SiO with excess NH₃ was very rapid, yielding three different types of silicon nitride at different longitudinal locations in the reactor: amorphous nanophase powder of an average size of about 20 nm, amorphous whiskers of a few micrometers in diameter, and α-polycrystals. The amorphous products were heat-treated for crystallization at temperatures between 1300° and 1560°C in a stream of dissociated NH₃, N₂, or N₂/H₂ mixture gas. When dissociated NH₃ was used, nanophase powder was crystallized at 1300°C. The yield of nanophase silicon nitride from SiO varied from 13% to 43%, depending on operating conditions.     

New method of free silicon determination in pressureless sintered silicon nitride by Raman spectroscopy and XRD

Formation of silicon affects different physical properties of silicon nitride ceramics. Decomposition of Si₃N₄ and formation of free Si are highly important processes and depend on many factors. The proposed method of combined nano-Raman spectroscopy and X-ray diffraction (XRD) allows quantitative analysis of Si in silicon nitride. Raman spectroscopy enables the determination of atomic bonds and rapid and easy identification of free silicon. Further analysis of the crystalline phases by XRD enables the calculation of the amount of free silicon. The proposed complex method allows the characterization of such complicated processes as silicon nitride decomposition, microstructure formation and, in particular, the formation of the nanoscale grain boundary phase because Si nanosized precipitates are the nucleants of secondary phases during crystallization. Strong 522.8 cm⁻¹ mode and 943.1–984.3 cm⁻¹ transverse optical modes of free Si were clearly observed in the investigated silicon nitride that was subjected to pressureless sintering at 1800 °C. Reported ceramics demonstrated typical microstructures with elongated grains and relatively high microhardness and Young's modulus. It was shown that the aspect ratio depended linearly on the microhardness and Young's modulus. High values of the Young's modulus (more than 290 GPa) and microhardness (more than 1800 HV) were shown for reported silicon nitride produced by hot pressing and pressureless sintering via cold isostatic pressing with a higher quantity of sintering agent. The features of molecular structure of the reported Si₃N₄ ceramics were clearly described and discussed in detail and were found to be in good agreement with the microstructure and phase composition of these ceramics.