铁精粉还原炉用Si3N4（Sialon）/SiC材料在H2O—H2—N2气氛中热力学计算分析

根据热力学原理对Si—C—N—H—O五元系统进行了平衡状态下的相稳定性计算,绘制了在1 073 K和1 223 K下的SiC、Si₃N₄、Si₂N₂O和SiO₂4个稳定相的稳定性与N₂分压和H₂O分压的关系图,即优势区域图,分析了其凝聚相的稳定区域。同时结合SEM显微结构分析氢气还原炉中Si₃N₄/SiC和Sialon/SiC制品抗H₂O—H₂—N₂气氛的侵蚀性能。     

Development of a tape casting process for making thin layers of Si3N4 and Si3N4 + TiN

A process for the tape casting of silicon nitride ceramics has been developed and is described in detail. A solvent (ethanol) based recipe was developed using polyvinyl butyral and polyethylene glycol as the binder and plasticizer, respectively. The effect, of milling times, dispersant, solvent, plasticizer and binder contents were all investigated as well as that of the binder to plasticizer ratio. In addition the beneficial effect of multi-stage milling of the slip was evaluated. The removal of the ceramic tape from the carrier film is described. In addition the recipe and process used for producing silicon nitride tape was successfully adapted for the production of two different silicon nitride + titanium nitride composite based tapes. From the tapes produced it was possible to hot press dense multi-layer laminate structures with the thinnest layers being 45 μm thick.     

Densification and phase transformation of Si3N4 in the presence of mechanically activated BaCO3–Al2O3–SiO2 mixture

Densification as well as the →β phase transformation of Si₃N₄ were monitored as a function of activation time of the BaCO₃–Al₂O₃–SiO₂ additive mixture. The composition of the ternary mixture corresponded to celsian (BaAl₂Si₂O₈—BAS). Previously, mechanically activated powder mixtures for various lengths of time were added to Si₃N₄ in the amount of 10–30%. Sintering was performed at 1650–1700°C in nitrogen atmosphere up to 8 h. The changes in densification degree, as well as phase composition, were followed as a function of heating time and the time of mechanical activation of the additive mixture. The results obtained showed that mechanical activation retarded densification in samples heated up to 1700°C. On the other hand, for the constant sintering time, →β transformation of Si₃N₄ was enhanced with increasing activation time, and the amount of additives.     

Toughening of MgAl2O4 spinel/ Si3N4 nanocomposite fabricated by spark plasma sintering

The main objective of this work is to compare the hardness, fracture toughness, and optical transparency of MgAl₂O₄ spinel (magnesium aluminate), MgAl₂O₄ spinel/ Si₃N₄ nanocomposite, and the heat-treated spinel/Si₃N₄ nanocomposite. For this purpose, the commercial spinel nanopowder and the laboratory-made spinel/ Si₃N₄ nanocomposite powder were sintered using spark plasma sintering (SPS). A heat treatment at 1000?°C for 4?h was carried out on the as-sintered nanocomposite. The field emission scanning electron microscopy (FESEM), Energy dispersive X-ray (EDX) mapping, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Nanoindentation, and Vickers microhardness analyses were used to determine microstructure, elemental analysis, functional group, hardness, and indentation toughness of the samples. The results showed that the hardness and toughness of the heat-treated sample are more than those of the as-SPSed nanocomposite as much as 15.7% and 25.7%, respectively. Also, the values of optical transmission of the nanocomposite sample in the visible range (400–800?nm) and infrared region (800–2000?nm) were lower than those of pure spinel.     

Combustion synthesis of Si3N4–TiN composite powders

Si₃N₄–TiN composite powders have been prepared by self-propagating high-temperature synthesis (SHS). Y₂O₃ plays a dominant role on the formation of rod-like Si₃N₄ whiskers. Full nitridation occurred of the Si–Ti mixtures. In the SHS process, TiN formation preceeds that of Si₃N₄. The formation of TiN inhibits the grain growth of rod-like Si₃N₄ crystals. Meanwhile some solid solutions between Si₃N₄ and TiN have formed.     

Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders

Silicon nitride ceramics have been produced by microwave sintering at 28 GHz with Y₂O₃, Al₂O₃ and MgO as sintering additives. The effect of initial β content of the Si₃N₄ starting powder on the microstructural development has been assessed by scanning electron microscopy (SEM) and quantitative image analysis. Phase transformation behaviour was assessed by X-ray diffraction. Mechanical properties of the sintered bodies were determined through assessment of hardness and fracture toughness. It was found that the samples sintered from powders with lower initial β content developed larger grains than those from higher β content powders, due to fewer nucleation sites during the →β transformation. However, attempts to develop a more bimodal microstructure by using a mixture of the two grades of powder, in an effort to increase both fracture toughness and fracture strength, were unsuccessful. In this case the microstructure was similar to that developed in the materials produced from higher β content powders. The mechanical properties of the sintered bodies were very similar, despite differences in microstructure. This was attributed to the strong bonding between the grains and grain boundary phase resulting in crack paths in all the materials that were predominantly transgranular, with little debonding or crack deflection. Under these circumstances the effect of larger grains is eliminated.     

Si3N4 with comparable permeability to SiC

Porous sintered reaction-bonded silicon nitrides (SRBSNs) with comparable permeability to SiC were fabricated using presintered Si-additive mixture granules. By increasing the granule strength through the adjustment of the presintering conditions, the strengthened sturdiness of the granules led to an increase in the pore channel size. Porous SRBSNs with ≥60% porosity were achieved without employing a pore former due to the formation of intragranular narrow pore channels as well as intergranular wide pore channels. As the specific pore area of the developed SRBSN is nearly 26 times that of SiC with similar permeability, superior filtering efficiency for nano-sized particulate matter is expected.     

愈合温度对SiC/Si_3N_4复合材料强度的影响

用压痕法,在SiC/Si3N4复合陶瓷试样表面引入裂纹,对含有裂纹的试样进行愈合处理,来研究温度对材料抗弯强度的影响规律.结果表明,经1300℃保温4h愈合处理后,抗弯强度恢复至647.5MPa,与完好试样的抗弯强度值(685.5MPa)非常接近.     

Corrosion of Si3N4-ceramics in aqueous solutions Part I: Characterisation of starting materials and corrosion in 1 N H2SO4

The corrosion behaviour of silicon nitride materials in acids strongly depends on the composition and amount of the grain boundary. But there exist no systematic investigations of the relation between the corrosion behaviour and the composition and amount of the grain-boundary phase. The aim of the series of these papers is the systematic investigation of these relations. In the first part the methods of determination of the amorphous grain-boundary phases are described in detail. Additionally, the correlation between the corrosion behaviour and the composition of the grain-boundary phases are given. The structural reasons and the mechanisms behind the observed changes in the corrosion behaviour will be given in part II of this paper.     

In situ reaction synthesis and characterization of Ti3Si(Al)C2/SiC composites

The reaction route, microstructure, and properties of Ti₃Si(Al)C₂/SiC composites with 5–30 vol.% SiC content prepared by in situ hot pressing/solid–liquid reaction synthesis process are investigated. In contrast to monolithic Ti₃Si(Al)C₂, the SiC particle-reinforced composites exhibit higher elastic modulus, Vickers hardness, fracture toughness, improved wear, and oxidation resistance, but have a slight loss in flexural strength. The improvement in the properties is mainly ascribed to the contribution of SiC particles, and the strength degradation is due to the residual tensile stresses in the matrix.     

Fabrication and mechanical behavior of ZrB2 strengthened SiCf/SiC composites prepared by hybrid processing route

A SiC fiber-reinforced composite containing a SiC-ZrB₂ mixed matrix (SiC_f/(SiC-ZrB₂)) with high density and enhanced mechanical properties was fabricated. ZrB₂ at 5 or 40?vol% was added to a (SiC + C) slurry to be infiltrated into the voids of 2D woven Tyranno?-SA grade-3 fabrics by electrophoretic deposition. Subsequent hot pressing at 1300?°C and 10?MPa for 1?h, followed by liquid silicon infiltration (LSI) at 1600?°C for 5?h in an Ar atmosphere resulted in the formation of the reaction-bonded SiC matrix, which revealed a composite density close to 97%. SiC_f/(SiC-ZrB₂) having open porosities of 0.2–0.6% showed peak strengths of 398 and 320?MPa for 5 and 40?vol% ZrB₂ addition, respectively. The large mismatch in the coefficient of thermal expansion and Young's modulus between the SiC and ZrB₂ phases was attributed to a reverse trend in the strength of composites. Brittle behavior of the composites in flexure can be explained by the strong bonding between the matrix and fibers formed by the reaction of interphase with molten Si during LSI. Strength retention after oxidation at 1000 and 1400?°C for 2?h was also compared in terms of ZrB₂ amount contained in the composites.     

Oxidation behavior of hot-pressed Si3N4 with Re2O3 (Re=Y, Yb, Er, La)

Four different β-Si₃N₄ ceramics with silicon oxynitrides [Y₁₀(SiO₄)₆N₂, Yb₄Si₂N₂O₇, Er₂Si₃N₄O₃, and La₁₀(SiO₄)₆N₂, respectively] as secondary phases have been fabricated by hot-pressing the Si₃N₄–Re₄Si₂N₂O₇ (Re=Y, Yb, Er, and La) compositions at 1820°C for 2 h under a pressure of 25 MPa. The oxidation behavior of the hot-pressed ceramics was characterized and compared with that of the ceramics fabricated from Si₃N₄–Re₂Si₂O₇ compositions. All Si₃N₄ ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C and the oxidation products of the ceramics were SiO₂ and Re₂Si₂O₇. The Si₃N₄–Re₄Si₂N₂O₇ compositions showed inferior oxidation resistance to those from Si₃N₄–Re₂Si₂O₇ compositions, owing to the incompatibility of the secondary phases of those ceramics with SiO₂, the oxidation product of Si₃N₄. Si₃N₄ ceramics from a Si₃N₄–Er₄Si ₂N₂O₇ composition showed the best oxidation resistance of 0·198 mg cm⁻² after oxidation at 1400°C for 192 h in air among the compositions investigated herein.     

Processing of ultrafine ZrO2 toughened WC composites

The interrelationships between the dispersion of the secondary ZrO₂ phase and the material properties of WC-based composites with up to 10 vol% of ZrO₂ are investigated. The homogeneity of the ultrafine WC–nanometric ZrO₂ powder mixtures was optimized by means of multidirectional milling and bead milling. In an alternative route, zirconium butoxide was used as a ZrO₂ source. The composites were fully densified by means of pulsed electric current sintering (PECS), also known as spark plasma sintering, within a few minutes at 1700 °C allowing to maintain an ultrafine grained microstructure combining a hardness of 2600 kg/mm² with an indentation toughness of 6 MPa m^1/2. The ZrO₂ content and Y₂O₃ stabilization were found to strongly influence the mechanical properties and especially strength of the WC–ZrO₂ composites.     

反应烧结形成氮化硅/碳化硅材料

介绍了反应烧结形成氮化硅／碳化硅材料的制备工艺，对该材料的微观结构、性能及其影响因素进行了阐述，并介绍了该材料的应用背景。     

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.     

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.     

Influence of intergranular phases on edge integrity of Si3N4/SiC wood cutting tools

Si₃N₄/SiC composites used for industrial wood cutting were processed by a near net shape route involving gas pressure sintering with sintering additives such as Al₂O₃, La₂O₃, Y₂O₃ and MgO. The cutting edge integrity of these knives was tested in a cutting trial and compared to knives made by a hot pressing route. It was found that the intergranular phase has a crucial influence on the cutting edge integrity. The boundary phase was analysed by EFTEM and EDX mapping on TEM samples: in gas pressure sintered composites the crystallisation of the apatite Y₅Si₃O₁₂N phase was identified. In the hot pressed composite the boundary phase consisted only of silicates. These composites showed better edge stability than cutting tools with a Y-N-apatite phase. The formation of the type of intergranular phase was found to be determined by the amount of MgO sintering aid and the temperature of the post sintering heat treatment.     

超分子前体制备g-C3N4/g-C3N4同质结及光催化性能研究

通过混合煅烧二氰二胺水热法和三聚氰胺-三聚氰酸共沉淀法制备的超分子前驱体,制备了兼具多孔纳米片和中空纳米管形貌的g-C₃N₄/g-C₃N₄同质结（CN-HP）。通过X射线衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、紫外-可见漫反射光谱（UV-Vis DRS）、光致发光光谱（PL）等对光催化剂进行了表征。结果表明,CN-HP-1/1在75 min内对亚甲基蓝（MB）的降解率可达99.8%,伪一级动力学常数为4.02 h^-1,是块体g-C₃N₄的5倍,同时单位比表面积的反应常数提升至单体的1.8倍。PL测试表明,合适比例的三聚氰胺-三聚氰酸（MCA-H、MCA-P）超分子制备的同质结,其光生电子-空穴复合率较单体g-C₃N₄进一步降低。自由基捕获实验表明,光生空穴是光催化降解过程中的主要活性物种,并结合X射线光电子能谱价带谱测试提出了Z型电子传输机制。     

Laser surface nanostructuring for reliable Si3N4/Si3N4 and Si3N4/Invar joined components

IR pulsed laser radiation in air was applied to Si₃N₄ and Invar to obtain reliable Si₃N₄/Si₃N₄ and Si₃N₄/Invar adhesive bonded components. The laser pre-treatment produced a homogeneous nanostructured oxide layer on the surfaces, which effectively increased the adhesion at the adhesive/adherends interface and led to cohesive failure in the joining material. The mechanical strength of Si₃N₄/ Si₃N₄ and Si₃N₄/Invar joined components was measured, with and without laser nanostructuring, before and after thermal cycling from room temperature to 50?K, and it resulted that the exposure to extremely low temperatures did not affect the mechanical integrity of the joints. It was also demonstrated that this laser pre-treatment did not alter the mechanical properties of the ceramic substrate.     

氮化硅结合碳化硅管状制品的研制

简介了氮化硅结合碳化硅管状制品研制的必要性 ,确定了原材料及粒度配比 ,选择了合适的成型方法及添加剂 ,介绍了氮化烧成机理。研制出的制品其质量和使用性能接近同类进口产品 ,具有良好的市场前景和广泛的推广应用价值。