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Si3N4
Terms modified by Si3N4 Selected AbstractsCrystallization behaviors of carbon fiber reinforced BN-Si3N4 matrix compositeCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2007Bin Li Abstract The crystallization behaviors of a new carbon fiber reinforced composite with a hybrid matrix comprising BN and Si3N4 prepared by precursor infiltration and pyrolysis were investigated by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The results show that the as-received composite is almost amorphous, and its main composition is BN and Si3N4. When heat treated at 1600°C, the composite is crystallized and shows a much better crystal form. When heat treated at 2100°C, Si3N4 in the matrix is decomposed, and BN exhibits a relatively complete crystallization. The existence of B4C and SiC is detected, which indicates the interfacial chemical reactions between nitride matrices and carbon fibers. The surface morphology of carbon fibers in the composite changed significantly when heated from 1600 to 2100°C, which also proved the occurrence of interfacial chemical reactions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Microstructure and Microfabrication Considerations for Self-Supported On-Chip Ultra-Thin Micro-Solid Oxide Fuel Cell MembranesFUEL CELLS, Issue 5 2009B.-K. Lai Abstract La0.6Sr0.4Co0.8Fe0.2O3,,,, (LSCF) has been sputtered on bare Si and Si3N4 and yttria-stabilised zirconia (YSZ) thin films to investigate annealing temperature- and thickness-dependent microstructure and functional properties, as well as their implications for designing failure-resistant micro-solid oxide fuel cell (,SOFC) membranes. The LSCF thin films crystallise in the 400,450,°C range; however, after annealing in the 600,700,°C range, cracks are observed. The formation of cracks is also thickness-dependent. High electrical conductivity, ,520,Scm,1 at 600,°C, and low activation energy, ,0.13,eV, in the 400,600,°C range, are still maintained for LSCF films as thin as 27,nm. Based on these studies, a strong correlation between microstructure and electrical conductivity has been observed and an annealing temperature-thickness design space that is complementary to temperature-stress design space has been proposed for designing reliable membranes using sputtered LSCF thin films. Microfabrication approaches that maintain the highest possible surface and interface quality of heterostructured membranes have been carefully examined. By taking advantage of the microstructure, microfabrication and geometrical structural considerations, we were able to successfully fabricate large-area, self-supported membranes. These results are also relevant to conventional or grid-supported SOFC membranes using ultrathin nanocrystalline cathodes and ,SOFCs using cathode thin films other than LSCF. [source] Properties of Porous Si3N4/BN Composites Fabricated by RBSN TechniqueINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 4 2010Ji-Xuan Liu Reaction bonding of silicon nitride (RBSN) technique combined with slip-casting shaping process was used to fabricate porous Si3N4/BN ceramic composites. Si/BN slurry with chemical stability, good dispersibility, and viscosity was prepared using glycerol trioleate (GTO) covering on Si surface and poly(acrylic acid) (PAA) as dispersant. The hydrolysis of Si was strongly prevented by GTO coating. The dispersibility of covered Si and BN suspensions were improved by PAA dispersant. Twenty volume percent covered Si/BN slurries with low viscosity were successfully casted. The cast bodies were dried at room temperature, debindered at 750°C and nitrided below 1450°C. The nitrided samples mainly consist of ,-Si3N4, ,-Si3N4, and h-BN. The composites exhibit homogeneous microstructure consisting of faceted particles, ,-Si3N4 nanowires and a large amount of pores. The porosity is 52.64% and the pore size is in the range of 60,300 nm. The composites show compressive strength of 16.6±1.5 MPa. The dielectric constant of the composite is about 3.1 and the dielectric loss is below 0.5% under different frequencies. [source] Microstructure and Mechanical Properties of Lu2O3 -Doped Porous Silicon Nitride Ceramics Using Phenolic Resin as Pore-Forming AgentINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 3 2010Xiaowei Yin The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si3N4 ceramics with controlled microstructure. Lu2O3 and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu2O3 -doped ceramics using 12,57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 ,m. The porous ceramic infiltrated with CVI Si3N4 had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness. [source] Synthesis and Luminescence Properties of Orange,Red-Emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) Light-Emitting Diode Conversion Phosphors by a Simple Nitridation of MSi2INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 4 2009Hui-Li Li Eu2+ -doped M2Si5N8 (M=Ca, Sr, Ba) orange,red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi2, Eu2O3, and ,-Si3N4 as the starting materials under N2,H2 (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M2Si5N8 for M=Ca, Sr, and a little amount of BaSi7N10 impurity phase for M=Ba. Under the blue light excitation, M2Si5N8:Eu2+ shows a typical broad band emission of Eu2+ ranging from orange to red (585,620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca Near-Net Shape ,-Si4Al2O2N6 Parts by Hydrolysis Induced Aqueous Gelcasting ProcessINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 1 2009Ibram Ganesh In this paper, a new net-shaping process, an hydrolysis-induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of ,-Si4Al2O2N6 ceramics from aqueous slurries containing 48,50 vol%,-Si3N4, ,-Al2O3, AlN, and Y2O3 powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect-free thin wall ,-Si4Al2O2N6 crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered ,-Si4Al2O2N6 ceramics exhibited comparable properties with those reported for similar materials in the literature. [source] Phase Transformation and Densification Behavior of Microwave-Sintered Si3N4,Y2O3,MgO,ZrO2 SystemINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 1 2009Sreekumar Chockalingam A 2.45 GHz microwave-sintered Si3N4,Y2O3,MgO system containing various amounts of ZrO2 secondary additives have been studied with respect to phase transformation and densification behavior. The temperature dependent dielectric properties were measured from 25°C to 1400°C using a conventional cavity perturbation technique. Phase transformation behavior was studied using X-ray diffractometry. Microwave sintered results were compared with those of conventional sintered results. It has been found that , to , phase transformation was completed at a lower temperature in microwave-sintered samples than those of the conventionally sintered samples. Density of the microwave-sintered samples increased up to 2.5 wt% of ZrO2 addition and thereafter it showed a tendency to decrease or remain constant. The decrease in density is attributed to the pore generation caused by decomposition due to the localized over heating. [source] Microstructural Characterization and Mechanical Properties of Si3N4 Formed by Fused Deposition of CeramicsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2008S. Iyer We present processing (green and sintered), part shrinkage and warping, microstructural characterization, and mechanical properties of Si3N4 made by fused deposition of ceramics (FDC), using optical microscopy, scanning electron microscopy, and X-ray diffraction. The mechanical properties (fracture strength, fracture toughness, and Weibull modulus) are also reported. Proper FDC build parameters resulted in dense, homogeneous, near-net-shape Si3N4, with microstructures and mechanical properties similar to conventionally processed material. Mechanical properties are shown to be isotropic, while there is some degree of microstructural texturing (preferred ,-Si3N4 grain orientation) in sintered components. [source] Simultaneous IR Material Recognition and Conductivity Mapping by Nanoscale Near-Field Microscopy,ADVANCED MATERIALS, Issue 17 2007J. Huber IR scattering-type near-field microscopy is applied to simultaneously map material composition and conduction properties in cross-sectional samples of industrial bipolar and metal-oxide- semiconductor devices with nanoscale spatial resolution. Within a single mid-IR image, all relevant materials such as metals, Si, Si3N4, and oxides can be identified by material-specific amplitude and phase contrasts. [source] Preparation and tribological properties of polyetheretherketone compositesJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2010Dangsheng Xiong Abstract Polyetheretherketone (PEEK) is a thermoplastic engineering plastic with excellent mechanical properties. In this article, PEEK and its composites filled with ultrahigh-molecular-weight polyethylene (UHMWPE) were prepared by vacuum hot-pressing method. Tribological properties of these materials were investigated by block-on-ring friction and wear rig. An alloy (CoCrMo) ring and a ceramic (Si3N4) ring were used as friction pairs. The experiments were conducted under deionized water lubrication (DWL), saline lubrication (SL), and calf serum solution lubrication (CSSL). Worn surfaces morphology was observed and analyzed by metallographic microscope. The results indicated that friction coefficients of PEEK/UHMWPE composites were effectively reduced when compared with pure PEEK. When the materials slid against the alloy (CoCrMo) ring, wear rates of PEEK/UHMWPE were also effectively reduced when compared with pure PEEK. Wear mechanisms of PEEK were mainly ploughing and slight scratches under CSSL condition, whereas the quantity of the ploughing and scratches for PEEK/20%UHMWPE were significantly reduced. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2010 [source] Micromechanical analysis of silicon nitride: a comparative study by fracture mechanics and Raman microprobe spectroscopyJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2002Shigemi Tochino Raman microprobe spectroscopy was used to characterize in situ microstress fields which develop during fracture of a toughened silicon nitride (Si3N4) polycrystal. Maps of microscopic stress were collected in the neighborhood of a propagating crack both at zero and at critical loading conditions. Micromechanics results by Raman spectroscopy were analyzed and compared with conventional fracture mechanics assessments, such as the evaluation of rising R -curve behavior and crack opening displacement. Outcomes of these assessments illustrate that, despite the approximations involved in the piezo-spectroscopic equations used for calculating the microstress field from a local Raman shift, Raman microprobe spectroscopy is a viable method for semi-quantitative investigations of microfracture mechanisms in advanced ceramic materials. Copyright © 2002 John Wiley & Sons, Ltd. [source] Boson Peak, Elastic Properties, and Rigidification Induced by the Substitution of Nitrogen for Oxygen in Oxynitride GlassesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Nicolas F. Richet The effects of the substitution of nitrogen for oxygen on the heat capacity and vibrational entropy of three yttrium aluminosilicate glasses with 0, 3.4, and 7.4 mol% Si3N4 have been investigated from 10 to 300 K. The partial molar heat capacity and entropy of Si3N4 calculated from these and previous measurements indicate stronger average bonding than for SiO2 units, whereas the values derived for Y2O3 are consistent with the dual network modifying and Al3+ charge-compensating role of yttrium. The low-frequency part of the vibrational densities of states g(,) and the boson peaks g(,)/,2 derived from the inversion of the heat capacities indicate that nitrogen rigidifies the TO4 (T=Si, Al) tetrahedral network and that yttrium hampers the librational motion of the AlO4 tetrahedra, which contribute to the excitations associated with the boson peak. Along with data reported previously for borate and silicate glasses, these results for oxynitrides show a general monotonically increasing relation between transverse acoustic velocities and the temperature of the calorimetric boson peak. Illustrating the universal phenomenology of the boson peak, all these data collapse on the same master curve when plotted in a reduced form. [source] Growth and Mechanism of Network-Like Branched Si3N4 NanostructuresJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Zhijian Peng The high-yield synthesis of network-like branched silicon nitride (Si3N4) nanostructures by a simple template catalyst-assisted pyrolysis of a polymer precursor, perhydropolysilazane, was reported. The templates were silicon wafers deposited with Fe films of 5,20 nm in thickness. The processes simply involved thermal cross-linking of the preceramic polymer, crushing of the solidified polymer chunks into fine powder, and thermal pyrolysis of the powder under flowing high-purity nitrogen. The collected white network-like branched nanostructures are ,-Si3N4 of hexagonal phase, and their microstructures, in which the diameters of each linear part of the network-like nanostructure varied in a very wide range from tens of nanometers to hundreds of nanometers, strongly depend on the applied growth parameters, where the key factors are the heating rate and catalyst thickness for change in the diameters. It was proposed that the Si3N4 nanonetworks were formed through "metal-absorption on the surface of nanostructures" model by vapor,liquid,solid mechanism. The reaction mechanism of Si3N4 nanonetworks was also discussed. [source] Blue-emitting AlN:Eu2+ Powder Phosphor Prepared by Spark Plasma SinteringJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2010Hyoung-SeoK Do Blue-emitting AlN:Eu2+ powder phosphor was synthesized by spark plasma sintering (SPS) using AlN, Si3N4, and Eu2O3 as the starting materials, and its luminescence properties were investigated. A single-phase Eu- and Si-co-doped AlN powder was successfully fabricated by SPS in the range of 1650°,1800°C for 5 min. The AlN:Eu2+ obtained exhibited a strong blue emission at 480 nm under the excitation of ,exc=340 nm and an electron beam. The highest photoluminescence intensity was observed in the phosphor sintered at 1700°C, which was comparable to that of the phosphor prepared by gas pressure sintering at 1750°C for 4 h. [source] Fast and Almost Complete Nitridation of Mesoporous Silica MCM-41 with Ammonia in a Plug-Flow ReactorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010Fumitaka Hayashi The title reaction proceeded well to yield silicon (oxy)nitride at 973,1323 K using a plug-flow reactor. The degree of nitridation was studied as a function of temperature and time of nitridation, the sample weight, and the flow rate of ammonia. It was dependent on the reaction temperature and the amount of ammonia supplied per sample weight. The nitridation at 1273 K for 10,25 h yielded the oxynitride with 36,39 wt% nitrogen, which was very close to 40 wt% of Si3N4. Characterization with X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy measurements, and nitrogen adsorption revealed the conversion of MCM-41 to the corresponding oxynitride without essential loss of the mesoporous structure, the decrements of the lattice constant and the pore diameter by 20,35%, and the increments of the wall thickness by ca. 45%. Solid-state 29Si nuclear magnetic resonance spectra during the nitridation clearly showed fast decrease in SiO4 species and slow in SiO3(OH). Various intermediate species, SiOxNy(NH2 or NH)z, were observed to be formed and finally, ca. 70% SiN4 species, ca. 20% SiN3(NH2 or NH), and ca. 10% SiON2(NH2 or NH) were produced, being consistent with the results of the above mentioned elemental analysis. [source] Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2009Gui-hua Peng Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°,1600°C for 3,12 min, using ,-Si3N4 powders as raw materials and MgSiN2 as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%,-Si3N4 phase left in the sintered material. ,-Si3N4 was completely transformed to ,-Si3N4 after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K),1 was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate ,-Si3N4 ceramics with high thermal conductivity when appropriate additives are used. [source] Microstructure and Creep Behavior of a Si3N4,SiC MicronanocompositeJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2009Monika Ka, iarová The microstructure and its influence on the creep behaviour of carbon derived Si3N4 -SiC micro/nanocomposite tested in bending at temperatures from 1200° to 1400°C in air has been studied. No phase and microstructure change after creep test implied that material is stable at tested temperature range. After creep test only partial crystallization of glassy intergranular phase has been observed. Creep parameters n close to 1, apparent activation energy around 350 kJ/mol together with TEM observation indicated that the main creep mechanisms is solution precipitation controlled by interface reaction in combination with grain boundary sliding caused by the amorphous intergranular phases present in microstructure. However, the grain boundary sliding is hindered by local SiC particles interlocking neighboring Si3N4 grains. [source] Thermodynamically Stable SiwCxNyOz Polymer-Like, Amorphous Ceramics Made from Organic PrecursorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2008Riham M. Morcos This communication reports new results on the enthalpy of formation of pseudo-amorphous ceramic compounds constituted from silicon, carbon, oxygen, and nitrogen (SiCNO), made from the polymer route. Again, like the SiCO materials, although with one exception, the enthalpy of formation from crystalline components (SiO2 cristobalite, ,-Si3N4, SiC, and excess C) is negative. Some of the alloyed oxygen,nitrogen compositions yield enthalpies that are much more negative (,100 kJ/g·atom) in comparison with compositions that contain mainly oxygen or nitrogen (,20 kJ/g·atom). The exception, having a N/O ratio near 2, has a positive value for the enthalpy. This may reflect the presence of nanoclusters of stoichiometric Si2N2O instead of the pseudo-amorphous nanodomain structure seen for the other samples. [source] Phase Evolution in Heat-Treated Si3N4 with Additions of Yb2O3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008Gang Feng Guo The heat treatment of silicon nitride (Si3N4) ceramics with additions of 8, 12, and 16 wt% Yb2O3 was carried out at different temperatures and the evolution of grain boundary (GB) phase was investigated systematically by X-ray diffraction (XRD) as well as scanning electron and transmission electron microscopic analyses. XRD results reveal that the extent and the ease of GB crystallization increase with increasing the Yb2O3 content, and that high heat-treatment temperatures in general favor crystallization of the quaternary compounds such as the Yb4Si2O7N2 phase. These results provide an insight into the GB phase evolution in the Yb-system Si3N4 ceramics subjected to a postsintering heat treatment. [source] Biomorphic Silicon Nitride Ceramics with Fibrous Morphology Prepared by Sol Infiltration and Reduction,NitridationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2007Min Luo Biomorphic silicon nitride (Si3N4) ceramics with fibrous morphology were fabricated by combining sol,gel infiltration with carbothermal reduction nitridation from wood precursor. Y2O3 -incorporated silica sol was used as the infiltrated solution to promote the formation of fibrous Si3N4 grain at 1600°C under high nitrogen pressure (0.6 MPa). The influence of sintering conditions (additive and temperature) on the phase composition and microstructure of sintering bodies was analyzed, and the reaction mechanism is discussed. [source] Pressureless Sintering of ,-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming AgentJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2007Fei Chen A new method for preparing high bending strength porous silicon nitride (Si3N4) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H3PO4) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si3N4 ceramics are investigated, as a function of the content of H3PO4. The resultant high porous Si3N4 ceramics sintered at 1000°,1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H3PO4 and by the continous reaction of SiP2O7 binder, which could bond on to the Si3N4 grains. Porous Si3N4 ceramics with a porosity of 42%,63%, the bending strength of 50,120 MPa are obtained. [source] Laser-Assisted Machining of Damage-Free Silicon Nitride Parts with Complex Geometric Features via In-Process Control of Laser PowerJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2006Yinggang Tian A methodology to achieve optimal operating conditions for laser-assisted machining (LAM) is developed for silicon nitride parts with complex geometric features by applying a three-dimensional, transient thermal model and in-process laser power control. Complex silicon nitride parts are successfully produced by the LAM operation, where the maximum and material removal temperatures are carefully designed and controlled to achieve good machining results and avoid thermal damage on the final part. On-line temperature and laser power measurements are conducted and compared with prescribed values to show the effectiveness of the power control scheme. Scanning electron microscopy examination reveals virtually no subsurface microcrack or thermal damage on the silicon nitride parts. The X-ray diffraction (XRD) study shows the preservation of the silicon nitride microstructure and no phase transformation of ,-Si3N4 during the LAM experiments. XRD residual stress measurements show moderate compressive residual stresses on the silicon nitride workpieces produced by the LAM operation. [source] Fractographic Montage for a Si3N4,SiC NanocompositeJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2006Monika Ka, iarová A silicon nitride,silicon carbide nanocomposite has been prepared by an in situ method that utilizes C+SiO2 carbo-thermal reduction during the sintering process. The materials consist of a silicon nitride matrix, with an average grain size of 140 nm, and inter- and intragranular SiC particles with sizes of approximately 250 and 45 nm, respectively. The four-point bending strength and its distribution were investigated. The fracture origins were identified and characterized using fractographic methods, and a fractographic montage of the Weibull plot and fracture origins was constructed. The fracture origins were subsurface and volume located processing defects with sizes from 5 to 460 ,m, mainly in the form of clusters of pores, together with clusters of large SiC grains. [source] Modeling of Thermal Stresses in Joining Two Layers with Multi- and Graded InterlayersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2006C. H. Hsueh The technique of introducing interlayers has been used extensively to mitigate residual thermal stresses in joining dissimilar materials. Finite-element analyses have often been used to quantify thermal stresses in these layered structures in case-by-case studies. Recently, simple analytical models containing only three unknowns have been developed to derive closed-form solutions for elastic thermal stresses in both multilayer systems and two layers joined by a graded junction. The analytical solutions are exact for locations away from the free edges of the system. Application of these solutions is shown here to provide a systematic study of thermal stresses in Si3N4 and Al2O3 layers joined by various sialon polytypoid-based multi- and graded interlayers. The effects of the thickness, stiffness, and coefficient of thermal expansion of the interlayer on thermal stresses in the system are examined. The differences in thermal stresses resulting from multi- and graded interlayers are shown. [source] Electronic Structure and Bonding of All Crystalline Phases in the Silica,Yttria,Silicon Nitride Phase Equilibrium DiagramJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2004Wai-Yim Ching This paper reviews the structures and properties of 10 binary, ternary, and quaternary crystals within the equilibrium phase diagram of the SiO2,Y2O3,Si3N4 system. They are binary compounds SiO2, Y2O3, Si3N4; ternary compounds Si2N2O, Y2Si2O7, and YSi2O5; and quaternary crystals Y2Si3N4O3 (M-melilite), Y4Si2O7N2, (N-YAM), YSiO2N (wallastonite), and Y10(SiO4)6N2 (N-apatite, N-APT). Although the binary compounds are well-known and extensively studied, the ternary and the quaternary crystals are not. Most of the ternary and the quaternary crystals simply have been referenced as secondary phases in the processing of nitrogen ceramics. Their crystal structures are complex and not precisely determined. In the quaternary crystals, there exists O/N disorder in that the exact atomic positions of the anions cannot be uniquely determined. It is envisioned that a variety of cation,anion bonding configurations exist in these complex crystals. The electronic structure and bonding in these crystals are, therefore, of great interest and are indispensable for a fundamental understanding of structural ceramics. We have used ab initio methods to study the structure and bonding properties of these 10 crystals. For crystals with unknown or incomplete structural information, we use an accurate total energy relaxation scheme to obtain the most likely atomic positions. Based on the theoretically modeled structures, the electronic structure and bonding in these crystals are investigated and related to various local cation,anion bonding configurations. These results are presented in the form of atom-resolved partial density of states, Mulliken effective charges, and bond order values. It is shown that Y,O and Y,N bonding are not negligible and should be a part of the discussion of the overall bonding schemes in these crystals. Spectroscopic properties in the form of complex, frequency-dependent dielectric functions, X-ray absorption near-edge structure (XANES), and the electron energy-loss near-edge structure (ELNES) spectra in these crystals also are calculated and compared. These results are discussed in the context of specific bonding configurations between cations (silicon and yttrium) and anions (oxygen and nitrogen) and their implications on intergranular thin films in polycrystalline Si3N4 containing rare-earth elements. [source] Preparation and Properties of Porous Aluminum Nitride,Silicon Carbide Composite CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004Eirik Hagen Microporous two-phase AlN,SiC composites were prepared using Al4C3 and either Si (N2 atmosphere) or Si3N4 (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al4C3 under N2 atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN,SiC materials. The homogeneous two-phase AlN,SiC composites had a grain size in the range of 1,5 ,m, and the porosity varied in the range of 36%,45%. The bending strength was 50,60 MPa, in accordance with the high porosity. [source] Photoluminescence of Cerium-Doped ,-SiAlON MaterialsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004Rong-Jun Xie Cerium-doped ,-SiAlON (MxSi12,(m+n)Alm+nOnN16,n) materials have been prepared by gas-pressure sintering and post-hot-isostatic-press (HIP) annealing, using four powder mixtures of ,-Si3N4, AlN, and either (i) CeO2, (ii) CeO2+ Y-,-SiAlON seed, (iii) CeO2+ Y2O3, or (iv) CeO2+ CaO. Cerium-containing CeAl(Si6,zAlz)(N10,zOz) (JEM) phase, rather than Ce-,-SiAlON phase, forms in the sample with only CeO2, whereas a single-phase ,-SiAlON generates in samples with dual doping (CeO2+ Y2O3 and CeO2+ CaO). On ultraviolet-light excitation, JEM gives one broad emission band with maximum at 465 nm and a shoulder at 498 nm; ,-SiAlON shows an intense and broad emission band that peaks at 500 nm. The unusual long-wavelength emissions in JEM and ,-SiAlON are due to increases in the nephelauxetic effect and the ligand-field splitting of the 5d band, because the coordination of Ce3+ in JEM and ,-SiAlON is nitrogen enriched. [source] Densification of Si3N4 with LiYO2 AdditiveJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2004Branko Matovic This paper deals with the densification and phase transformation during pressureless sintering of Si3N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from ,-Si3N4 to ,-Si3N4 taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of ,-Si3N4 to ,-Si3N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li2O,Y2O3 system can provide very effective low-temperature sintering additives for silicon nitride. [source] In Situ Synthesis and Microstructures of Tungsten Carbide-Nanoparticle-Reinforced Silicon Nitride-Matrix CompositesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004Tateoki Iizuka A W2C-nanoparticle-reinforced Si3N4 -matrix composite was fabricated by sintering porous Si3N4 that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W2C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W2C particles resided in the grain-boundary junctions of the Si3N4, had an average diameter of ,60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W2C particles in the composite, the sinterability of the Si3N4 was improved, and a W2C,Si3N4 composite with almost full density was obtained. The flexural strength of the W2C,Si3N4 composite was 1212 MPa, ,34% higher than that of standard sintered Si3N4. [source] Oxidation and Volatilization of Silica Formers in Water VaporJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2003Elizabeth J. Opila At high temperatures, SiC and Si3N4 react with water vapor to form a SiO2 scale. SiO2 scales also react with water vapor to form a volatile Si(OH)4 species. These simultaneous reactions, one forming SiO2 and the other removing SiO2, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady-state oxide thickness, the time to achieve steady state, and the steady-state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water-vapor-containing environment. Accordingly, maps have been developed to show these steady-state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO2 formers in water-vapor-containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si3N4. [source]
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