Preparation and characterization of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics prepared by compression molding and slip casting methods

Porous silicon nitride (Si₃N₄) ceramics were fabricated by compression molding and slip casting methods using petroleum coke as pore forming agent, and Y₂O₃-Al₂O₃ as sintering additives. Microstructure, mechanical properties and gas permeability of porous Si₃N₄ ceramics were investigated. The mechanical properties and microstructure of porous Si₃N₄ ceramics prepared by compression molding were better than those which were prepared by slip casting method, whereas slip casting method is suitable for the preparation of porous Si₃N₄ ceramics with higher porosity and excellent gas permeability.     

Effects of organic additives on microstructure and mechanical properties of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Green bodies of porous Si₃N₄ ceramics were shaped by extrusion technique using different organic additives as binder during extrusion molding. Different porosity, microstructures and mechanical properties after the extrusion, drying, debinding and sintering stages were investigated. The solid slurry content of 70–75% and extrusion pressure of 0.5–1.0 MPa had played a decisive role in the smooth realization of extrusion molding. The porous Si₃N₄ ceramics were obtained with excellent properties using 4% hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) of molecular weight, 1000, as plasticizer with a density of 1.91 g cm⁻³, porosity of 41.70%, three-point bending strength of 166.53 ± 20 MPa, fracture toughness of 2.45 ± 0.2 MPa m^1/2 and Weibull modulus (m) of 20.75.     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Design of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si₃N₄/Si₃N₄-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si₃N₄/Si₃N₄-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si₃N₄/Si₃N₄-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Mechanical Properties and Thermal Shock Resistance Analysis of BNNT/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Composites

BNNT/Si₃N₄ ceramic composites with different weight amount of BNNT fabricated by hot isostatic pressing were introduced. The mechanical properties and thermal shock resistance of the composites were investigated. The results showed that BNNT-added ceramic composites have a finer and more uniform microstructure than that of BNNT-free Si₃N₄ ceramic because of the retarding effect of BNNT on Si₃N₄ grain growth. The addition of 1.5 wt.% BNNT results in simultaneous increase in flexural strength, fracture toughness, and thermal shock resistance. The analysis of the results indicates that BNNT brings many thermal transport channels in the microstructure, increasing the efficiency of thermal transport, therefore results in increase of thermal shock resistance. In addition, BNNT improves the residual flexural strength of composites by crack deflection, bridging, branching and pinning, which increase the crack propagation resistance.     

Microstructure of brazed joints between mechanically metallized Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and stainless steel

Brazing has been increasingly used to join metals to advanced ceramics. Brazing covalent materials requires either the use of active filler alloys or the previous metallization of the surface. To that end, a new and simple mechanical technique has been applied to metallize advanced ceramics, thus avoiding the use of costly Ti-based active filler alloys. The mechanical metallization of Si₃N₄ with Ti was employed as an alternative route to deposit active metallic films prior to brazing with stainless steel using 72% Ag--28% Cu or 82% Au—18% Ni eutectic alloys. The brazing temperatures were set to 40°C or 75°C above the eutectic temperature of each filler alloy. Ti-films of average thickness 4 μm produced adequate spreading of both filler alloys onto Si₃N₄ substrates, which were subsequently brazed to stainless steel. The interface of Si₃N₄/310 stainless steel basically consisted of a reaction layer, a precipitation zone and an eutectic microconstituent. Mechanically sound and vacuum-tight joints were obtained, especially upon brazing at relatively lower temperatures. Increasing the brazing temperature resulted in thermal cracking of the Si₃N₄, possibly due to increased thermal stress.     

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.     

Microstructure and properties of SiC-whisker-reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics with calcium aluminate additions

Silicon-nitride-based ceramics containing Al₂O₃-CaO sintering aids and reinforced with silicon carbide whiskers have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and their microstructure, phase composition, and mechanical properties have been studied. The results indicate that the Si₃N₄ ceramic containing 15 wt % calcium aluminate additions and 10 wt % SiC fibers has a dense microstructure with a uniform distribution of skeletal and dendritic silicon carbide crystals. The observed variations in the morphology of the crystals are tentatively attributed to the secondary crystallization of silicon carbide from the eutectic calcium aluminate melt during cooling.     

Facile and scalable synthesis of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> nanoparticles via solid-state route in an autoclave

A novel method of the synthesis of titanium silicide nanoparticles via solid-state route in an autoclave at 700°C is reported. The reaction of titanium silicide could be described briefly as: 5TiO₂ + 3Si + 20Li = Ti₅Si₃ + 10Li₂O. XRD pattern indicated that the product was hexagonal Ti₅Si₃. The Ti₅Si₃particle size (about 20–40 nm) is confirmed by the TEM images. Furthermore, the thermal stability and oxidation resistance of the titanium silicide nanoparticles were also investigated.     

Joining of Al-plasma-sprayed Si3N4ceramics

Aluminium was coated on silicon nitride ceramics by a low-pressure plasma spraying method, in order to form a tight bond between aluminium and the ceramics. Aluminium nitride formed as a interfacial reaction product between the aluminium coating layer and the ceramics. Two pieces of the aluminium-coated Si₃N₄ ceramics were then joined using the aluminium coating layers as filler metal in a vacuum of 1.3×10^–3 Pa at 973 K. The average bending strength and Weibull modulus of the joint are 340 MPa and 6.3 respectively, considerably higher than the 230 MPa and 0.9 of a Si₃N₄ ceramics joint brazed with an aluminum plate under the same condition.     

Efficient visible-light driven photocatalysts: coupling TiO<Subscript>2</Subscript>(AB) nanotubes with g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanoflakes

The wide application of the titanium dioxide (TiO₂) as the photocatalysts is greatly hindered by its intrinsic large band gap and usually fast electron–hole recombination. Here, we reported the exploration of coupling g-C₃N₄ nanoflakes to TiO₂ nanotubes with the anatase and TiO₂(B) mixed phases (TiO₂(AB)) toward the efficient visible-light-driven hybrid photocatalyst. It is found that coupling TiO₂(AB) nanotubes with g-C₃N₄ nanoflakes could bring a profoundly extension the visible light adsorption capacity and enhanced photogenerated carrier separation. Accordingly, they exhibit much higher efficient photocatalytic activities toward the degradation of sulforhodamine B under the visible light irradiation, which is enhanced for nearly 15 times to those of the TiO₂(AB) and g-C₃N₄, suggesting their promising practical applications as novel and efficient semiconductor photocatalysts for the water purification.     

N-TiO<Subscript>2</Subscript>/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript>/Up-conversion phosphor composites for the full-spectrum light-responsive deNO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> photocatalysis

The ternary composites consisted of nitrogen-doped titanium dioxide, carbon nitride and up-conversion phosphors (UP) were successfully prepared by a solvothermal method. The heterojunction could be formed when N-TiO₂ and g-C₃N₄ were combined together. The composite of N-TiO₂/g-C₃N₄@UP had excellent ultraviolet, visible and infrared light absorption, indicating the possibility for the utilization of full spectrum of solar light. When N-TiO₂ was coupled with g-C₃N₄ and up-conversion phosphors to form a composite, the visible light and NIR light absorption of the samples increased. The ternary composite N-TiO₂/g-C₃N₄@G-UP presented reasonable deNO _x performance of about 8.0% under the irradiation of IR light of 980 nm. The intensification of the photocatalysis might be realized by utilizing up-conversion phosphors, which could convert low-energy NIR light into high-energy photons (visible light) and increase the efficient irradiation on the surface of photocatalyst.     

Microstructure,chemical reaction and mechanical properties of TiC/Si3N4 and TiN-coated TiC/Si3N4 composites

Silicon nitride containing various compositions of as-received TiC and TiN-coated TiC, were hot pressed at 1800 °C for 1 h in a nitrogen atmosphere. In TiN-coated TiC/Si₃N₄ composites, TiC reacted first with the TiN coating to form a titanium carbonitride interlayer at 1450 °C, which essentially reduced further reactions between TiC and Si₃N₄ and enhanced densification. TiN-coated TiC/Si₃N₄ composites exhibited better densification, hardness, flexural strength and fracture toughness than those of as-received TiC/Si₃N₄. The toughening mechanisms for as-received TiC/Si₃N₄ and TiN-coated TiC/Si₃N₄ composite were attributed to crack deflection, load transfer and crack impedence by the compressive thermal residual stress.     

Novel Bi<Subscript>12</Subscript>TiO<Subscript>20</Subscript>/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> composite with enhanced photocatalytic performance through Z-scheme mechanism

Novel Bi₁₂TiO₂₀/g-C₃N₄ composite was successfully prepared with Bi₁₂TiO₂₀ nanoparticles embedded within the fluffy crumpled g-C₃N₄ nanosheets. Bi₁₂TiO₂₀/g-C₃N₄ composites exhibit superior photoactivity and stability. As compared with g-C₃N₄ and Bi₁₂TiO₂₀, the photocatalytic efficiency of Bi₁₂TiO₂₀/g-C₃N₄ is effectively enhanced about 1.8- and 4.9-fold, respectively. Based on the trapping experiment, ^·OH and ^·O₂^? radicals are the dominant reactive oxygen species involved in the photocatalytic process. The proposed Z-scheme mechanism of charge transfer markedly promotes the carriers’ migration and separation, leading to the enhanced photocatalytic performance.     

Enhanced visible light photocatalytic activity for g-C<Subscript>3</Subscript>N<Subscript>4</Subscript>/SnO<Subscript>2</Subscript>:Sb composites induced by Sb doping

In this paper, g-C₃N₄/SnO₂:Sb composite photocatalysts were fabricated by in situ loading Sb-doped SnO₂ (SnO₂:Sb) nanoparticles on graphitic carbon nitride (g-C₃N₄) nanosheets via a facile hydrothermal method. The synthesized g-C₃N₄/SnO₂:Sb composites delivered enhanced visible light photocatalytic performance for degradation of rhodamine B in comparison with g-C₃N₄/SnO₂ composites without doping Sb. Various techniques including XRD, SEM, TEM, FTIR, XPS, PL and electrochemical method were employed to demonstrate the successful fabrication of g-C₃N₄/SnO₂:Sb composite and to investigate the enhanced mechanism of photocatalytic activity. The improvement of visible light absorption and the promotion of separation efficiency and interfacial transfer of photogenerated carriers induced by Sb doping were responsible for the enhancement of photocatalytic activity. This study provides a simple and convenient method to synthesize a visible light responsive catalyst with promising performance for the potential application in environmental protection.     

R-Curve Behavior of Si3N4/BNNT Composites

The crack propagation rsistance behavior of Si₃N₄ ceramics reinforced by boron nitride nanotubes (BNNT) has been discussed in the work. And, bending strength and fracture toughness of Si₃N₄/BNNT composites were tested by three point bending method. It is shown that crack propagation resistance of BNNT/Si₃N₄ composites is increased distinctly owing to addition of BNNT. It is attributed to the pinning and bridging roles of BNNT. One kind of mathematical model was constructed for calculating crack propagation resistance of Si₃N₄ ceramics and BNNT/Si₃N₄ composites. Crack resistance curve (R-Curve) of Si₃N₄ ceramics and BNNT/Si₃N₄ composites was also calculated. Crack propagation of them was simulated using finite element methods. The results show that strong shielding area is formed in crack tip owing to existence of BNNT and crack propagation is prevented by strong stress shielding roles.     

Mass spectrometry of carbon nitride C<Subscript>3</Subscript>N<Subscript>4</Subscript>

Carbon nitride has been synthesized in macroscopic amounts by means of the original method based on an ecologically safe technology using inorganic initial compounds. The product has been characterized by mass spectrometry (MS), X-ray diffraction, and quantitative chemical analysis. The MS and thermochemical data show that stoichiometry of the samples of carbon nitride obtained using the proposed method corresponds to the empirical formula C₃N₄.     

Studies on machinability of Al/Si<Subscript>p</Subscript> + SiC<Subscript>p</Subscript> composite materials

This paper presents the experimental results on the machinability of silicon and silicon carbide particles (SiC_p) reinforced aluminium matrix composites (Al/Si_p + SiC_p) during milling process using a carbide tool. The total volume fraction of the reinforcements is 65 vol%. The milling forces, flank wear of the tool and the machined surface quality of composites with different volume fraction of SiC_p were measured during experiments. The machined surfaces of composites were examined through SEM. The results showed that the flexural strength and Vickers hardness are improved when certain volume fraction of silicon particles are replaced by silicon carbide particles with the same volume fraction and particle size and the effect of SiC_p on machinability is optimal when 9 vol% silicon particles in Al/Si_p was replaced by silicon carbide particles with the same volume fraction and the same particle size. Cracks and pits were found on the machined surfaces of composites due to the intrinsic brittleness of silicon particles.     

Evaluation of Si3N4 joints: bond strength and microstructure

Joining of pressurelessly sintered silicon nitride ceramics was carried out using adhesive slurries in the system Y-Si-Al-O-N in a nitriding atmosphere. The effects of bonding parameters, such as joining temperature (1450–1650°C), applied pressure (0– MPa) and holding time (10–60 min), on the bond strength of joint were evaluated. A typical microstructure of the joint bonded with the optimum adhesive was investigated. The three point bend testing of joined samples with 3 × 4 × 36 mm³ in dimension was employed to study the bond strength of joints. The results show that an optimum joining process was achieved by holding at 1600°C for 30 min under an external pressure of 5 MPa and the maximum bond strength was 550 MPa, compared to 700 MPa of unbonded Si₃N₄ ceramic, using the adhesive having the Si₃N₄/(Y₂O₃ + SiO₂ + Al₂O₃) ratio of 0.39. The good bond strength is attributed to the similarity in microstructure and chemical composition between joint zone and ceramic substrate. The fracture modes were classified into two types according to the values of bond strength. This revised version was published online in September 2006 with corrections to the Cover Date.