Si2N2O Grains Research Articles

Robust super-hydrophobic inorganic nanoparticles modified layered structure Si2N2O membrane for membrane distillation

Robust super-hydrophobic ceramic membranes consisting of layered structure Si2N2O grains and organosilane-derived inorganic nanoparticles were successfully fabricated and employed for membrane distillation. First, phase inversion and sintering method were used to prepare porous Si2N2O membranes. The slurry composition and sintering temperature were optimized to obtain a pure phase Si2N2O membrane with high bending strength, tailored average pore size, and high permeability. Then, the Si2N2O membranes were modified with organosilane-derived inorganic nanoparticles through ammonolysis and pyrolysis reactions. Due to the micro and nano-hierarchical rough structures and the presence of -Si-CH3 groups, the membranes showed super-hydrophobicity with a water contact angle of 152 ± 1°. Finally, the membranes were applied to desalinate seawater by sweeping gas membrane distillation. A stable water flux of 76 ± 0.9 L/(m2 day) with a salt rejection of > 99% was recorded during 30 h distillation test at 75 °C, demonstrating the stability and durability of the membranes.

Toughening mechanism of SIALON/Si2N2O multiphase ceramics and finite element simulation of stress field at crack tip

A finite element model of crack propagation in the fracture process of SIALON/Si2N2O ceramic was established. The influence of columnar crystals on the stress field at crack tip was analyzed. The toughening mechanism of SIALON/Si2N2O ceramics is mainly grain toughening of long rod-shaped Si2N2O. The pull-out, deflection, fracture and bridging of Si2N2O grains can increase the fracture resistance and improve the mechanical properties of the composites. Simulation results show that the fracture, pull-out, deflection, and mixed effect of fracture and pull-out of crystals will all play a hindrance role to the stress concentration at the crack tip. However, with certain strength of the reinforcing phase and the number and bonding strength with the substrate, the stresses at the crack tip are extremely close, but the paths of the crack propagation are different. More columnar crystals mean higher equivalent stress at the crack tip and higher strength of the material.

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si3N4/Si2N2O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si2N2O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si2N2O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si3N4/Si2N2O and fine grained β- Si3N4 ceramics.

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si2N2O ceramics were prepared by plasma activated sintering using nanosized amorphous Si3N4 powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N4−n–On (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si2N2O crystals were nucleated where the local structure was similar with Si2N2O. After sintering at 1600°C, the Si2N2O ceramic was composed of elongated plate‐like Si2N2O grains and amorphous phase. The Si2N2O grains showed a width of less than 100 nm and a very high aspect ratio.

Fabrication of Nanocrystalline Dense Silicon Nitride Ceramics

Since the properties of the bulk ceramics are dependent on the grain size the ceramic materials with nanoscale grain size is of interest. Sintering of nanosized powder compacts to full density with minimum grain growth is a difficult task to achieve due to its high surface energy. Two step sintering methode was developed to enhance densification with minimum grain growth for silicon nitride ceramics with liquid phase. Starting with nano-sized silicon nitride powder two step sintering methode gives rise to a very fine-grained b-Si3N4 matrix with large agglomerated Si2N2O grains due to its high surface oxygen content. Addition of Y2O3 shifts the composition point to a primary phase field with no Si2N2O, gives rise to b-Si3N4 with nano scale grain size and near full density. Carbothermal reduction method was employed to reduce the oxygen content in nano-sized silicon nitride powder to give nanocrystalline dense silicon nitride ceramics without Si2N2O formation. Use of SPS was effective to suppress the grain growth and to give near full density. Microstructural development and mechanical properties will be reported.

Effects of talc and clay addition on pressureless sintering of porous Si3N4 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.

<i>In Situ</i> Synthesis and Mechanical Properties of TiN-Si<sub>2</sub>N<sub>2</sub>O-Si<sub>3</sub>N<sub>4</sub> Composites

Si3N4-Si2N2O-TiN composite ceramics were in-situ fabricated by using following reactions of (1) 3TiO2 + Si3N4 → 3TiN + 3SiO2 + N2 and (2) Si3N4+ SiO2 → 2Si2N2O. The mixed powder of α-Si3N4, Al2O3, Y2O3 and TiO2 was hot-pressed at 24 MPa and 1800°-1900°C for 1-4 h in N2. Sintered composite ceramics were characterized by XRD, SEM, TEM, four-point bending test and Vickers indentation method. XRD results and TEM observation showed that TiN and amorphous SiO2 were formed at 1250°C by the reaction (1), and the Si2N2O phase formed by reaction (2) above 1800°C. Si3N4-Si2N2O-TiN composites consisted of ≥2 m sized Si2N2O grains with TiN and Si3N4 grains. Hardness and fracture strength of the composites were comparable to those of Si2N2O ceramics, with fracture toughness being improved at 5vol% TiN containing composites.

Liquid Phase Sintering Behavior of Amorphous Nano-Sized Silicon Nitride Powders

The amorphous nano-sized silicon nitride powders were sintered by liquid phase sintering method. Si3N4-Si2N2O composites were in-situ fabricated. The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% was obtained at a sintering temperature of 1650°C and reduced when the sintering temperature increased or decreased, which indicates that the reaction is reversible. The mass loss, relative density and average grain size increase with increasing of sintering temperature. The average grain size is less than 500nm when the sintering temperature is below 1700°C. During the sintering procedure, there is a complex crystallization and phase transition: amorphous Si3N4 → equiaxial α-Si3N4→ equiaxial β-Si3N4 → rod-likeSi2N2O → needle-like β-Si3N4. Small round-shaped β-Si3N4 particles are entrapped in the Si2N2O grains and a high density of staking faults are situated in the middle of Si2N2O grains at a sintering temperature of 1650°C.

Fabrication of fine-grained Si3N4-Si2N2O composites by sintering amorphous nano-sized silicon nitride powders

Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering (LPS). The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5 O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% in the volume was obtained at a sintering temperature of 1650°C and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1700°C. The sintering procedure contains a complex crystallization and a phase transition: amorphous silicon nitride→equiaxial α-Si3N4→equiaxial β-Si3N4→rod-like Si2N2O→needle-like β-Si3N4. Small round-shaped β-Si3N4 particles were entrapped in the Si2N2O grains and a high density of staking faults was situated in the middle of Si2N2O grains at a sintering temperature of 1650°C. The toughness increased from 3.5 MPa·m1/2 at 1600°C to 7.2 MPa·m1/2 at 1800°C. The hardness was as high as 21.5 GPa (Vickers) at 1600°C.

In-situ Synthesis and Evaluation of Mo5Si3 Particle Reinforced Si3N4 and Si2N2O Composites

Mo5Si3 particle reinforced silicon nitride and silicon oxynitride composites were fabricated by sintering green compacts of silicon nitride, added with the molybdenum powder or a precursor for molybdenum oxide powder, at 1850°C in nitrogen atmosphere. During the sintering process, molybdenum powders reacted with Si3N4 to form the Mo5Si3 particles at grain boundary junctions in Si3N4, and MoO3 reacted with Si3N4 to generate Mo5Si3 particles and Si2N2O grains simultaneously. However, because a small amount of oxygen was included in the molybdenum powder, a little amount of Si2N2O grain generated in the Mo5Si3-Si3N4 composite. The Si2N2O grains were needlelike crystals in Mo5Si3-Si3N4 composite, but the shape changed into the tabular in the Mo5Si3-Si2N2O composite. The flexural strengths of Mo5Si3-Si3N4 and Mo5Si3-Si2N2O composites improved slightly by incorporating the Mo5Si3 particles, and were 946 and 712MPa, respectively. Because of pulling-out of elongated Si2N2O grains during fracture and the effect of fracture toughening by internal stresses due to the Mo5Si3 and matrix thermal expansion coefficient mismatch, the fracture toughness of Mo5Si3-Si3N4 composite was 6.9MPa·m1/2, -6% higher than that of normal Si3N4. However, the fracture toughness of the Mo5Si3-Si2N2O composite was affected by the tabular crystal Si2N2O which has a low value of fracture toughness, and reached the same valu as Si2N2O.

Effect of Si2N2O content on the microstructure, properties, and erosion of silicon nitride–Si2N2O in situ composites

Silicon nitride–Si2N2O in situ composites were prepared by hot pressing powder mixtures of α–Si3N4, 6 wt% Y2O3, 1 wt% Al2O3, and 0–12 wt% SiO2. X-ray diffraction (XRD) analysis indicated that the volume percents of Si2N2O were 0, 13, 31, and 54 for the composites prepared with 0, 4, 8, and 12 wt% SiO2, respectively. XRD results also indicated that both silicon nitride grains and Si2N2O grains were laid down perpendicular to hot pressing direction. As the volume percent of Si2N2O increased, the width and the amount of elongated silicon nitride grains decreased, but the fracture toughness increased. Young's modulus of the in situ composites decreased as the Si2N2O content was increased. The erosion rate decreased as the Si2N2O content was increased, in part, due to both the increased fracture toughness and the reduced grain size. Erosion of the composites occurred primarily due to the grain dislodgment. The sample without Si2N2O experienced micro-chipping due to transgranular fracture.

Microstructures and Properties of Mo5Si3‐Particle‐Reinforced Si3N4‐Matrix Composites

Mo5Si3‐particle‐reinforced Si3N4‐matrix composites were fabricated by sintering molybdenum‐solution‐infiltrated porous Si3N4. Fine Mo5Si3 particles, with an average diameter of ∼0.13–0.17 μm, grew in situ from the reaction between MoO3 and Si3N4 in the grain boundary of the Si3N4. The Mo5Si3 particles resided in the grain‐boundary glassy phase and reinforced the grain boundaries. The four‐point flexural strength and fracture toughness of a 2.8 wt% Mo5Si3‐Si3N4 composite were 1060 MPa and 7.7 MPa·m1/2, respectively. This was higher than those for normally sintered Si3N4 by ∼17% and ∼18%, respectively. The fracture toughness of the Mo5Si3‐Si3N4 composite increased as the content of Mo5Si3 particles increased, but the flexural strength decreased. Improvement in fracture toughness was attributed to a thermal expansion mismatch among the Mo5Si3, the Si3N4, and the grain‐boundary amorphous phases in the Mo5Si3‐Si3N4 composites. Another reason for the improved fracture toughness was the pullout of elongated Si2N2O grains that formed as a result of oxygen gas released from the reaction between the molybdenum‐solution‐obtained MoO3 and the Si3N4. The infiltration method for incorporating desired elements or compounds into a ceramic matrix holds promise as a process for fabricating submicrometer‐ or nanometer‐sized composites with high strength and high fracture toughness.

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Abstract Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si3N4 and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si2N2O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si3N4 matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si2N2O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si2N2O grains, the texture formation, and the coarsening of β-Si3N4. Fracture toughness, however, was still low (∼5.2 MPa m1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si2N2O-matrix interfacial bond and nearly equiaxed β-Si3N4 with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.

Texture Development in Silicon Nitride–Silicon Oxynitride In Situ Composites via Superplastic Deformation

Silicon nitride–silicon oxynitride (Si3N4–Si2N2O) in situ composites have been fabricated via either the annealing or the superplastic deformation of sintered Si3N4 that has been doped with a silica‐containing additive. In this study, quantitative texture measurements, including pole figures and X‐ray diffraction patterns, are used in conjunction with scanning electron microscopy and transmission electron microscopy techniques to examine the degree of preferred orientation and texture‐development mechanisms in these materials. The results indicate that (i) only superplastic deformation can produce strong textures in the β‐Si3N4 matrix, as well as Si2N2O grains that are formed in situ; (ii) texture development in the β‐Si3N4 matrix mainly results from grain rotation via grain‐boundary sliding; and (iii) for Si2N2O, a very strong strain‐dependent texture occurs in two stages, namely, preferred nucleation and anisotropic grain growth.

Superplastic Deformation in Silicon Nitride–Silicon Oxynitride In Situ Composites

Starting with a mixture of ultrafine β‐Si3N4 and a SiO2‐containing additive, a superplastic Si3N4‐based composite was developed, using the concept of a transient liquid phase. Significant deformation‐induced phase and microstructure evolutions occurred in the nonequilibrium, fine‐grained Si3N4 material, which led to the in situ development of a Si3N4–22‐vol%‐Si2N2O composite and strong texture formation. The unusual ductility of the composites with elongated Si2N2O grains was attributed to the fine‐grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si2N2O grains. The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa·m1/2).

Superplasticity in a fine-grained beta-silicon nitride ceramic containing a transient liquid

Compressive deformation in a dense fine-grained β-Si 3N 4 sintered with 7 wt% cordierite was investigated over a wide range of temperatures (1450–1650°C) and strain rates (1×10 −5–1×10 −3/s). The superplastic characteristics and the results of microstructural examination are presented. These indicate that: (a) fine-grained β-Si 3N 4 ceramics are capable of high rates of deformation (about 10 −5–10 −4/s) at 1550°C without strain hardening occurring; (b) remarkable phase and microstructural evolutions, such as formation of elongated and strongly textured Si 2N 2O grains occurred during deformation; (c) a transient liquid phase present in this material system which facilitated the formation of Si 2N 2O grains enhanced the superplastic flow, and well-aligned Si 2N 2O grains formed in situ did not have a detrimental impact on the superplasticity; and (d) the mechanism of deformation is grain boundary sliding accommodated by diffusion-controlled solution–precipitation creep.

Nucleation control for hot-working of silicon oxynitride based ceramics

An attempt was made to develop an engineering ceramic plastically deformable at high temperatures with low flow stress at high strain rates, and without strain hardening. Dense ceramic preforms were fabricated by pressureless sintering Si3N4 + SiO2 mixed powders with an addition of MgAl2O4 at 1500 °C. A transient liquid, which occurs during the reaction sintering of Si2N2O, was utilized for subsequent net-shape forming. The ceramic (6Φ × 6 mm column) was deformed without any cracks and cavities in compression tests at high strain rates (10−2−10−3:s−1) at 1500 °C, but this was not achieved in a test at lower strain rates for a long time, because of the growth of elongated Si2N2O grains during the test. Potassium fluoride (KF) was used as a dopant for retardation of nucleation of Si2N2O during sintering and hot-working. The KF-doped preforms were successfully plastically deformed even in the test for a long time.

Nucleation and Growth of Silicon Oxynitride Grains in a Fine‐Grained Silicon Nitride Matrix

Annealing induces significant microstructural changes in β‐Si3N4 ceramics densified with a silica containing additive, as signified by the precipitation of elongated Si2N2O grains in the fine‐grained β‐Si3N4 matrix, leading to a reinforced structure. The phases and microstructure evolution and the interface structural features in this composite system have been systematically studied by using X‐ray diffraction, and conventional and high‐resolution transmission electron microscopy. Traced from the indispensable features of the Si2N2O grains, typically the spatial distribution of the stacking faults and the size, morphology, phase, and the spatial distribution of the particles trapped within the Si2N2O, it has been generally established that Si2N2O grains nucleate heterogeneously on the β‐Si3N4 grain surface and grow rather quickly. The low nucleation rate and high growth speed of Si2N2O indicate a high nucleation barrier of Si2N2O and fast diffusion of (N,O) in the liquid. Experimentally, no epitaxial orientation relationship between Si2N2O grains and intragranularly trapped β‐Si3N4 grains which either serve as a nucleation site for Si2N2O or are simply encapsulated during Si2N2O grain growth can be observed; instead, the interface between the Si3N4 and the Si2N2O features an amorphous phase of uneven thickness. Sialon polytypoid phase particles similarly trapped in Si2N2O show an epitaxial relationship with Si2N2O grains. From the observed interface structure between the Si2N2O and β‐Si3N4, a possible partial ordering of a glass film in the form of Si2N2O is also discussed.

Fabrication of silicon nitride–Silicon oxynitride in-situ composites

Fine-grained β-Si 3N 4 ceramics were fabricated from fine and uniform β-powder by hot-pressing at 1700–1750°C using different sintering aids (cordierite, Y 2O 3–MgO, and Y 2O 3–Al 2O 3). The microstructural development of hot-pressed bodies was studied in annealing at 1750°C for 0·5 to 8 h. When Y 2O 3–MgO and Y 2O 3–Al 2O 3 were used as sintering aids, fine and uniform microstructures were maintained because of the low driving force for grain growth of β-Si 3N 4. When cordierite was used as an aid, β-Si 3N 4 reacted with the SiO 2 in the additive and developed a bimodal microstructure with large Si 2N 2O grains in fine β-Si 3N 4 matrix grains after annealing. Local segregation of β-Si 3N 4 grains was observed after hot-pressing. Nucleation of Si 2N 2O occurred at a part of the segregation surface in the early stage of annealing. Si 2N 2O grains grew selectively with annealing time, resulting in improved fracture toughness.

Factors affecting mechanical properties of silicon oxynitride ceramics

The effect of additives and impurities on the mechanical properties of silicon oxynitride ceramics was investigated. The toughening of the ceramics was affected by three factors: (1) the thermal tensile stress in the intergranular glassy phase and the compressive stress in Si 2N 2O grains, developed by a thermal expansion mismatch between Si 2N 2O grains and the intergranular glassy phase; (2) the large grain size of Si 2N 2O; and (3) the concentration of impurities in the intergranular glassy phase. The degradation of high-temperature strength was primarily dependent on the basic chemical composition (the kind of additives (MeO x ) and Me Si ratio) of the intergranular phase and impurities.