TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water

TiO₂ photocatalyst loaded on Si₃N₄ (TiO₂/Si₃N₄) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO₂ photocatalysts (TiO₂/SiO₂, TiO₂/Al₂O₃, and TiO₂/SiC) loaded on various types of supports (SiO₂, Al₂O₃, and SiC). The formation of the well-crystallized anatase phase of TiO₂ was observed on the calcined TiO₂/Si₃N₄ photocatalyst, while a small anatase phase of TiO₂ was observed on the TiO₂/SiC photocatalyst and amorphous TiO₂ species was the main component on the TiO₂/SiO₂ and TiO₂/Al₂O₃ photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO₂/Si₃N₄ photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO₂/Si₃N₄ photocatalyst decomposed 2-propanol diluted in water into acetone, CO₂, and H₂O, and finally, acetone was also decomposed into CO₂ and H₂O. The TiO₂/Si₃N₄ photocatalyst showed higher photocatalytic activity than TiO₂ photocatalyst loaded on other supports. The well-crystallized TiO₂ phase deposited on Si₃N₄ and the hydrophobic surface of Si₃N₄ support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions.     

Synthesis of carbon-free Si3N4/SiC nanopowders using silica fume

We have investigated a possible method of synthesizing carbon-free, nano-silicon nitride-silicon carbide (Si₃N₄/SiC) powders from the waste silica fume for the first time, using the integrated mechanical and thermal activation (IMTA) process. This novel process results in the formation of nano-Si₃N₄/SiC powders at 1465 °C with crystallite sizes as small as 45 nm. In order to synthesize carbon-free nano-Si₃N₄/SiC powders, two different approaches, one using the H₂ gas and the other using air, have been studied for their effectiveness in removing the free carbon present. It is found that the H₂ treatment is not very effective although both Si₃N₄ and SiC are stable during the H₂ treatment. In contrast, removing the free carbon using air is effective, and the limited oxidation of nano-Si₃N₄ and SiC can be achieved if the air treatment is terminated soon after the free carbon is eliminated. This study has provided a clear pathway and understanding for effectively synthesizing carbon-free, nano-Si₃N₄/SiC powders from the silica fume.     

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.     

Preparation of Ni-coated Si3N4 powders via electroless plating method

The Ni/Si₃N₄ coated powders were successfully prepared via electroless plating method by using hydrazine hydrate (N₂H₄·H₂O) as a reducing agent. The coated powders were characterized with several techniques such as scanning electron microscope, energy dispersive spectrometer, Transmission electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction to determine particle size, composition, phase and morphology. It indicated that the core–shell structure of Ni/Si₃N₄ has been constructed in the present method, the Ni layer on the surface of Si₃N₄ particles was relatively continuous and uniform, but it is inevitable that only in very small area occurred the aggregation of Ni particles. In principle, the coated process was successful and expectable.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Characterization of SiO2-encapsulated WSi2/W5Si3 nanoparticles synthesized by a mechanochemical route

This study reports on the mechanochemical synthesis (MCS) of SiO₂-encapsulated WSi₂/W₅Si₃ nanoparticles starting from tungsten oxide (WO₃), silicon dioxide (SiO₂) and magnesium (Mg) powder blends. MCS process, carried out in a high-energy ball mill, was evaluated in terms of various milling time and initial composition of WO₃-SiO₂-Mg powders. A subsequent purification step using aqueous HCl solution was conducted on the as-synthesized powders. Compositional, microstructural and thermal properties of the powders were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimeter (DSC). Based on the adiabatic temperature values, SiO₂-encapsulated WSi₂/W₅Si₃ nanoparticles exhibit a high potential as a result of a mechanically induced self-sustaining reaction. The exothermic reaction took place after 20?min of milling, and WSi₂, W₅Si₃, W, MgO, Mg₂SiO₄ and residual Mg phases were detected. The utilization of an optimized amount of excess Mg (3.5?mol) resulted in the elimination of unwanted Mg₂SiO₄ and W phases, leaving behind WSi₂, W₅Si₃, Mg₂Si, MgO and Mg. After removal of Mg-based by-products by leaching process, WSi₂, W₅Si₃ and a very small amount of SiO₂ phases were obtained. TEM analysis revealed that W silicide nanoparticles with an average size of 97?nm were encapsulated by SiO₂ layers with an average thickness of 15?nm.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

Partial nitridation of Li4SiO4 and ionic conductivity of Li4.1SiO3.9N0.1

The Li content and anion lattice of Li₄SiO₄ were modified to improve ionic conductivity. Li₂CO₃ and Si₃N₄ were mixed in a ratio of Li/Si?=?4.5 and heated in NH₃ at 820?°C, which resulted in the formation of the oxynitride, Li_4.1SiO_3.9N_0.1. Powder X-ray diffraction analyses revealed Li_4.1SiO_3.9N_0.1 and Li₄SiO₄ to be isostructural with a subtle variation in the lattice constants. Diffuse-reflectance absorption spectroscopy, however, showed a significant decrease in the band gap, from 5.6?eV in Li₄SiO₄ to 4.8?eV in Li_4.1SiO_3.9N_0.1. X-ray photoelectron spectra of the Li 1s and Si 2p levels revealed enhanced lattice covalency in Li_4.1SiO_3.9N_0.1 compared to the oxide phase. The ionic conductivity of Li₄SiO₄ and Li_4.1SiO_3.9N_0.1 were measured by ac impedance spectroscopy over the temperature range 100–400?°C. Non-linear fitting analysis of the equivalent circuit revealed that the ionic conductivity of Li_4.1SiO_3.9N_0.1 was approximately one order of magnitude higher than that of Li₄SiO₄.     

In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

Influence of H2/NH3 mixtures on the composition of SiCNYO and SiCNAl(O) nanopowders

We performed pyrolysis of SiCNAlH and SiCNYOH nanopowder precursors under a reactive atmosphere (Ar/NH₃/H₂) with various compositions of ammonia (NH₃) and dihydrogen (H₂) to diminish C content, which is deleterious for thermal stability and sintering of the powders. This paper continues a previous work on the fabrication of an Si₃N₄/SiC composite without free C by studying the effect of H₂ on the C/N atomic ratio of the powder. We studied the influence of the nature of the gaseous mixture (Ar/NH₃/H₂) on the powder composition. Elemental analysis showed that the introduction of H₂ in the pyrolysis atmosphere limited the decomposition of NH₃ and allowed for control of the C/N ratio. This behaviour can be explained by the structural evolution observed by ²⁹Si NMR spectrometry but also by Fourier transform infrared and Raman spectroscopy. An Si₃N₄/SiC composite, with traces of free C, was obtained after post-pyrolysis heat treatment of the powders synthesized with 10 wt.% of H₂ and 25 wt.% NH₃.     

Effects of grinding and firing conditions on CaAl2Si2O8 phase formation by solid-state reaction of kaolinite with CaCO3

The effects of grinding and firing conditions on CaAl₂Si₂O₈ phase formation by solid-state reaction of kaolinite with CaCO₃ were investigated by differential thermal analysis (DTA)–thermogravimetry (TG), X-ray powder diffraction (XRD) and ²⁹Si and ²⁷Al MAS NMR. Unground and ground samples showed similar crystallization behavior at about 850 °C, and the crystallizing temperature was relatively unaffected by grinding. On the other hand, the crystalline products were strongly influenced by the grinding. Gehlenite (Ca₂Al₂SiO₇) was the dominant phase in the unground samples but layer-structured CaAl₂Si₂O₈ was dominant in the ground samples, together with a small amount of anorthite, which is the stable phase. The amount of anorthite gradually increased with higher firing temperature, the sample fired at 1000 °C being almost completely anorthite. Grinding treatment before firing was effective in accelerating the decomposition of CaCO₃ and extending the temperature range for the formation of CaAl₂Si₂O₈, a phase with local structure similar to that of layered CaAl₂Si₂O₈.     

Combined effect of Fe-Si alloys and carbon on Si3N4 stability at elevated temperatures

The combined effect of carbon and Fe-Si alloys on Si₃N₄ was explored by heat treating Si₃N₄ materials at 1500?°C and 1600?°C in flowing nitrogen. The phase compositions and microstructures were characterized by XRD and SEM, respectively. The reaction degree was analysed based on the mass variation in the system. Combined with a thermodynamic assessment, the reaction mechanism was studied and proposed. The results show that the coexistence of Fe-Si alloys and carbon accelerates the phase transformation from Si₃N₄ to SiC and worsens the strength of Si₃N₄ materials. Fe-Si alloys accelerate the deposition of CO gas to free carbon and accelerate the decomposition of Si₃N₄ to Si. The in situ-formed Si can react with carbon, thus accelerating the thermodynamic and kinetic formation of SiC. Along with the growth of pores and the deterioration of the wettability of Fe-Si alloys during this process, the microstructure changes from a network constituted by Si₃N₄ columns/whiskers to porous SiC particles with weak linkages, which leads to the failure of Si₃N₄ materials. Therefore, the combined effect of Fe-Si alloys and carbon is harmful for Si₃N₄ materials at 1500–1600?°C.     

High temperature mechanical properties of porous Si3N4 prepared via SRBSN

The high temperature strength and fracture behavior of porous Si₃N₄ ceramics prepared via reaction bonded Si₃N₄ (RBSN) and sintered reaction bonded Si₃N₄ (SRBSN) were investigated at 800–1400?°C. The weight gain after oxidation for 15?min and the microstructure of the edge and center of the fracture surface clearly show that the internal oxidation of porous SRBSN is unavoidable with porosity of ~ 50% and mean pore size of 700?nm. The oxidation of Si₃N₄ and intergranular Y₂Si₃O₃N₄ phase may responsible for the high temperature strength degradation of SRBSN. Porous Si₃N₄ ceramics prepared with addition of 1?wt% C showed low strength degradation at temperature >?1200?°C.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

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.     

High temperature reactions between Si3N4 bonded SiC materials and Cu,Cu2O and matte

Si₃N₄ bonded SiC (Si₃N₄-SiC) is a conventional refractory material and has broad applications. In the present study, Si₃N₄-SiC refractory materials were systematically investigated in the copper-making environment. Si₃N₄-SiC was reacted with Cu, Cu₂O, industrial matte, Cu₂S and FeS melts at 1200 °C in argon gas atmosphere, and all samples were directly quenched in water after the experiments. Phase changes and compositions of the phases were measured by electron probe X-ray microanalysis. The present investigations demonstrate that Cu and Cu₂S do not react with Si₃N₄-SiC at high temperatures and the wettability between this material and the melts is low. However, significant reactions occur between Si₃N₄-SiC and Cu₂O, industrial matte and FeS. The results imply that Si₃N₄-SiC material has limited oxidation-resistance and can only be used under reducing conditions.     

Transformation of polysilicon cutting waste into SiC/α-Si3N4 composite powders via electromagnetic induction heating

In order to use polysilicon cutting waste more efficiently, SiC/α-Si₃N₄ composite powders were prepared via electromagnetic induction heating at ambient pressure. Factors affecting synthesis and reaction mechanisms of the SiC/α-Si₃N₄ composite powders were investigated. Results show that the morphology of obtained α-Si₃N₄ consists of particles equiaxed with diameter in the micron range and one-dimensional Si₃N₄ nanowires with diameter of tens to hundreds of nanometers and length of up to tens of microns. It was found that the addition of NH₄Cl can promote nitridation by controlling reaction temperature and participating in the reaction. In addition, the existence of Fe₂O₃ impurities was found to be beneficial to the formation of α-Si₃N₄ in reaction process by removing the SiO₂ film from silicon and providing FeSi₂ liquid phase as effective pass for vapor phase. Mechanisms used to grow Si₃N₄-NWs were vapor-solid (VS) and vapor-liquid-solid (VLS) depositions.     

Structural and spectroscopic properties of Nd:Y2Si2O7 phosphors

Phosphors of α-Y₂Si₂O₇ doped with Nd³⁺ ions were prepared using the sol–gel technique. Nano-sized crystalline phosphor powders were obtained by annealing the dried gels at 960 °C. The crystallization properties of the phosphor powders were determined from their XRD patterns. The α-Y₂Si₂O₇ phase was the only phase observed in all compositions. As the amount of amorphous SiO₂ in the composition was increased, the crystalline sizes and the widths of the size distribution curves were found to decrease from 17.8 nm to 10.6 nm and from 15.6 nm to 12.2 nm, respectively. The spectroscopic properties of the powders were studied by measuring the luminescence and the decay patterns of the ⁴F_3/2→ ⁴I_9/2 and ⁴F_3/2→ ⁴I_11/2 transitions between 50 K and 310 K. No appreciable effect of the crystallite sizes on the average lifetime of the ⁴F_3/2 level was observed at temperatures below 100 K. The effect of temperature, however, becomes relevant above 100 K as the size of α-Y₂Si₂O₇ nano-crystal becomes smaller.