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.     

Nitridation of silicon powder studied by XRD, 29Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

Sintering of Si3N4 with liquid in the system Ce2O3---AIN---SiO2

Liquid phase sintering of Si₃N₄ with melts from the system Ce₂O₃---AIN---SiO₂ has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si₃N₄ with two melt compositions selected from outside the glass forming region yields fully dense Si₃N₄. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si₃N₄ sintered with liquids in the system Ce₂O₃---AIN---SiO₂ were found to be inferior to those of liquids selected from Y₂O₃---AIN---SiO₂, but superior to those selected from the system MgO---AIN---SiO₂.     

La–Si–O–N glasses: Part I. Extension of the glass forming region

The nitrogen-rich part of the glass forming region in the La–Si–O–N system has been the subject of a comprehensive study. Glasses were prepared by heating powder mixtures of La metal, Si₃N₄ and SiO₂ in a nitrogen atmosphere at 1650–1800 °C. By this new synthesis route, glasses containing up to 68 e/o of N and 62 e/o of La were prepared, showing that the glass forming region is significantly larger than previously reported. The glasses were characterized by elemental analysis, differential thermal analysis, X-ray powder diffraction, and scanning electron microscopy. They were found to be X-ray amorphous and homogenous, with the majority of them containing small amounts of crystalline La silicides and elemental Si. Glass transition temperatures (T_g) were found to vary between 900 and 1100 °C and crystallization to occur typically 120 °C above T_g. The forming of the glasses was investigated by characterizing samples taken out at various steps of the heating cycle. The results indicate that the glass formation is strongly dependent on reaction kinetics. A strong exothermal reaction occurs at temperatures 900–1100 °C, leading to the formation of assemblies of amorphous and crystalline (oxy)nitride phases that melt upon further heating at 1650–1800 °C.     

Crystalline galliosilicate molecular sieves with the beta structure

Crystalline galliosilicates with the beta structure have been synthesized from alkali-free hydrogels with the composition Ga₂O₃:xSiO₂:12(TEA)₂O:1285H₂O, where x=40, 60, 80 and 100. The addition of an alkali (Ga₂O₃/Na₂O=1.0) to the hydrogel in the form of sodium gallate decreased crystallization times at 135°C to 10 days from 14 days. Crystallization reactions are not stoichiometric and the crystals' silica-to-gallia ratio (SGR) is always less than that in the parent hydrogel. Thermal analysis has shown that TEA ions first decompose and are then thermally desorbed, probably in the form of Hofmann decomposition products. Residual hydrocarbons are burned at T>400°C. The calcined crystals have surface area in the 500–625 m² g⁻¹ range. FTIR experiments with chemisorbed pyridine have shown that the isomorphous substitution of Al(IV) with Ga(IV) atoms decreases the acid-site strength and changes the relative B/L acid-site ratio of the crystals. IR spectra at a desorption temperature of 500°C revealed that all the pyridine desorbed from B sites but not from L sites in Ga-beta, whereas the Al-beta analog retained pyridine on both L and B sites. Microcalorimetry experiments with ammonia at 150°C have revealed the existence of different acid-site (B+L) strengths and site populations. The total number of sites available to NH₃ chemisorption and the number of strong acid sites show the same dependence on the SGR value of the crystals. ²⁹Si MAS-NMR spectra contain a resonance at −111 ppm attributed to T(4Si,0Ga) groups and a second resonance at −102 ppm attributed to T(3Si,1Ga) groups. ⁷¹Ga NMR spectra confirm that Ga(IV) is the dominant species and that Ga(VI) formation depends, in part, on the thermal pretreatment applied to the crystals. ¹H NMR results have revealed that after calcination in air at 500°C, there are residual hydrocarbon compounds in the beta microporous structure. This can be avoided if the organic template is decomposed in nitrogen at 500°C for 1 h and the decomposition products are removed oxidatively by a second calcination step in air at 550°C for 2 h.     

The effect of thermal cycling on oxide scale structure and morphology on a hot-pressed silicon nitride

Dense Si₃N₄ hot-pressed with the aid of 9 wt% Y₂O₃ was oxidised in ‘dry’ synthetic air at both 1000 and 1200°C for up to 500 h under thermal cycling conditions. The experiments revealed that thermal cycling has little effect on oxidation kinetics although the morphology of surface oxidation products is affected by the cycling frequency. Cracks formed in the oxide layers on cooling healed immediately on re-exposure to high temperature, and there was no apparent change in the oxidation rate controlling mechanism over the time period investigated. The exposure of the material at 1000°C did not result in catastrophic oxidation as observed for some other Y₂O₃-doped hot-pressed Si₃N₄ compositions. Additionally, it was observed that crystallisation of the oxide layer with time (assisted in part by the outward diffusion of intergranular phase cations from the bulk ceramic to the surface scale) leads to non-parabolic kinetics owing to reduced rates of diffusion through crystalline phases in the surface scale. ©     

Development, characterization and oxidation behaviour of Si3N4---Al2O3 ceramics

Five Si₃N₄---Al₂O₃ ceramic grades were prepared by hot pressing at 1650°C. Backscattered electron micrographs revealed four different phases. Quantitative electron probe microanalysis allowed the identification of these phases as X-sialon, -Al₂O₃, O'-sialon and β-sialon. The maximum solubility of Al₂O₃ in Si₃N₄ and in Si₂N₂O at 1650°C was determined as well as the chemical composition of X-sialon. Based on these results, a slightly revised phase diagram is proposed. The general features describing the microstructure of the various phases have been investigated by transmission electron microscopy. The influence of the various phases on the mechanical properties was investigated; hardness, fracture toughness and elastic modulus were measured. The oxidation behaviour has been studied in air at 1300°C and 1450°C. The metastable phase diagram of Al₂O₃---SiO₂ in the absence of mullite can be used to predict the oxidation products and relative amounts formed in the oxide layers.     

Synthesis, densification and electrical properties of strontium cerate ceramics

Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO₃/SrCe_0.95Yb_0.05O_3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO₃) and 1200 °C (SrCe_0.95Yb_0.05O_3−δ). Dense SrCeO₃ and SrCe_0.95Yb_0.05O_3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO₃ and 1–2 μm for SrCe_0.95Yb_0.05O_3−δ. The electrical conductivity of SrCe_0.95Yb_0.05O_3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere.     

Synthesis and characterization of non-oxide ceramic coatings on graphite substrates by solid–vapor reaction process

Silicon carbide (SiC) and silicon carbide/silicon nitride (SiC/Si₃N₄) were successfully synthesized on graphite substrates by the use of solid–vapor reaction (SVR) process. Layers of SiC and SiC/Si₃N₄ were synthesized on graphite substrate through the reaction between SiO and substrate the (SiO_(vapor) + 2C_{(from graphite)}) and N₂ and substrate the (3SiO_(vapor) + 2N_{2 (vapor)} + 3C_{(from graphite)}), respectively. With the increase of dwell time and synthesis temperature the thickness of SiC layers increases up to 1500 °C temperature. Also, with the increase of synthesis temperature hardness value of SiC coatings is increased, which is 10–15 times higher than the substrate. The critical load of SiC coatings for wear resistance is about 22 N, which was observed by scratch tests. The synthesized SiC coatings appear to consist of a β-SiC phase mixed with a minor amount of an -SiC phase, and its thickness is mainly affected by porosity of the substrate. The thickness of SiC/Si₃N₄ coatings is much thinner than that of SiC coatings, but gives higher value of surface hardness.     

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.     

High-quality, oriented and mesostructured organosilica monolith as a potential UV sensor

We synthesized high-quality and oriented periodic mesoporous organosilica (PMO) monoliths through a solvent evaporation process using a wide range of mole ratios of the components: 0.17–0.56 1,2-bis(triethoxysilyl)ethane (BTSE): 0.2 cetyltrimethylammonium chloride (CTACl): 0–1.8 × 10⁻³ HCl: 0–80 EtOH: 5–400 H₂O. X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images indicated that the mesoporous channels within the monolith samples were oriented parallel to the flat external surface of the PMO monolith and possessed a hexagonal symmetry lattice (p6mm). The PMO monolith synthesized from a reactant composition of 0.35 BTSE: 0.2 CTACl: 1.8 × 10⁻⁶ HCl: 10 EtOH: 10 H₂O had a pore diameter, pore volume, and surface area – obtained from an N₂ sorption isotherm – of 25.0 Å, 0.96 cm³ g⁻¹ and 1231 m² g⁻¹, respectively. After calcination at 280 °C for 2 h in N₂ flow, the PMO monolith retained monolith-shape and mesostructure. Pore diameter and surface area of the calcined PMO monolith sample were 19.8 Å, 0.53 cm³ g⁻¹ and 1368 m² g⁻¹, respectively. We performed ²⁹Si and ¹³C CP MAS NMR spectroscopy experiments to confirm the presence of Si–C bonding within the framework of the PMO monoliths. We investigated the thermal stability of the PMO monoliths through thermogravimetric analysis (TGA). In addition, rare-earth ions (Eu³⁺, Tb³⁺ and Tm³⁺) were doped into the monoliths. Optical properties of those Eu³⁺, Tb³⁺ and Tm³⁺-doped PMO monoliths were investigated by photoluminescence (PL) spectra to evaluate their potential applicability as UV sensors.     

Synthesis of nano-crystalline Gd0.1Ce0.9O2−x for IT-SOFC by aerosol flame deposition

Nano-sized gadolinia-doped ceria (GDC) can be used as an IT-SOFC electrolyte, oxygen gas sensor or abrasives. In this study, nano-sized GDC powders with bimodal particle distribution of about 10 nm and 200 nm particle size were successfully synthesized by aerosol flame deposition (AFD). The resulting effects of sintering temperature on microstructure and electrical properties were investigated in the sintering temperature range 1100–1400 °C. The pellet had a completely dense microstructure after sintering at 1400 °C for 10 h. Raman measurement showed an increase of oxygen vacancy due to shift between reduced and oxidized states (Ce³⁺ ↔ Ce⁴⁺) with increasing sintering temperature. The formation of oxygen vacancies noticeably increased the ionic conductivity above 1300 °C.     

The effect of TiN and TiC dispersoids on the high-temperature deformation mechanisms of Si3N4

The compression creep properties of pressureless sintered Si₃N₄ matrix, Si₄N₄−(TiN + TiC) and Si₃N₄−N composites, and of a hot pressed Si₃N₄−TiC composite were studied in air between 1260 and 1340°C. Creep characteristics have been compared in relation to microstructure. The addition of soft TiC particles resulted in better mechanical strength, particularly in the creep ductility. The deformation of Si₃N₄---TiC, dominated by a more refractory vitreous phase, was slower than for the other composites. Further, based on a comparison of creep parameters obtained experimentally in this work and on the nature of dispersoids, deformation mechanisms are proposed.     

Effect of atmosphere on the reaction sintering of Si2N2O

The reaction sintering of Si₂N₂O from an equimolar mixture of Si₃N₄ and SiO₂ with 5 wt% Al₂O₃ addition was investigated in 98 or 980 kPa N₂ at 1600–1850°C. At the initial stage, Si₃N₄ densification occurred through a liquid phase of SiO₂---Al₂O₃ system. Further densification was observed together with the formation and exaggerated grain growth of Si₂N₂O. High N₂ pressure was useful for the prevention of thermal decomposition of Si₂N₂O and bloating of the compact. Among various packing powders, which have various SiO partial pressures, an equimolar mixture of Si₃N₂O and SiO₂ was the most effective for the densification. The effect of N₂ and packing powder on reaction sintering of Si₂N₂O was discussed in relation to observed kinetics and thermodynamic calculations. Bending strength of sintered materials was 310–320 MPa.     

Low-temperature synthesis/densification and properties of Si2N2O prepared with Li2O additive

Dense Si₂N₂O was successfully synthesized using 2 mol% Li₂O as an additive by a hot-pressing method at 1500 °C. Compared to other metal oxide additives, Li₂O can significantly decrease the sintering temperature of Si₂N₂O, which is ascribed to the lower melting point of Li₂O–SiO₂ and the formation of less viscous liquid phase. Increasing Li₂O content has no apparent influence on the mechanical and dielectric properties of dense Si₂N₂O, which is due to the easy evaporation of Li₂O at sintering temperature. The mechanical properties of Si₂N₂O with Li₂O additive are comparable to those of Si₂N₂O synthesized with other additives. The as-prepared bulk Si₂N₂O with 2 mol% Li₂O additive exhibits both low dielectric constant (6.17 at 1 MHz) and loss tangent (0.0008 at 1 MHz) and combines good mechanical performance, indicating it is a potential high-temperature structural/functional material.     

Preparation of hollow alumina microspheres by microwave-induced plasma pyrolysis of atomized precursor solution

Hollow alumina microspheres have been prepared by microwave-induced (MI) plasma pyrolysis of atomized aerosols of precursor solutions and subsequent calcination at 1300 °C for 2 h. When an aqueous solution of 0.5 mol dm⁻³ Al(NO₃)₃ without any additives was used as a precursor, hollow -Al₂O₃ microspheres with a thick shell wall were prepared after post-calcination at 1300 °C. The addition of a polypropylene (PO)–polyethylene(EO) blockcopolymer (molecular weight: 2900–6500) to the precursor solution was effective for increasing the yield of hollow microspheres, but resulted in the formation of many cracks and holes in the thinned shell wall. Hollow alumina microspheres with a thin, but strong, shell layer could be prepared by the simultaneous addition of tetraethylorthosilicate.     

Dense silicon carbonitride ceramics by pyrolysis of cross-linked and warm pressed polysilazane powders

This study reports on the pyrolysis and densifaction behavior of cross-linked poly(hydridomethylsilazane) powders. The influence of the cross-linking procedure such as temperature and annealing time of the polymer powders on the compaction behavior under cold and warm pressing conditions is discussed. The degree of cross-linking is determined by thermal mechanical analysis (TMA). In addition to particle sliding which is assumed to be the compaction mechanism obtained by cold-pressing, the polymer powder consolidates by plastic deformation applying warm-pressing. A continuous 3-dimensional polysilazane network is formed after a dwelling time under these conditions. Pyrolysis of the cross-linked and compacted polysilazane powder in argon at 1100°C gives crack-free amorphous silicon carbonitride Si_3+xC_x+yN₄ with compositions ranging from x=1·47 and y=0·88 for cold pressed samples to x=1·47 and y=1·86 for warm pressed materials. The residual open porosity is significantly reduced from 10–15 vol% in the cold pressed specimens to 1·3–5 vol% by the warm pressing procedure. The weight loss during pyrolysis between room temperature and 1300°C is about 5 wt% lower than that for cold pressed specimens. This result is explained by a reduced methane evolution during the polymer-to-ceramic conversion and is in accordance with the enhanced carbon content of the warm pressed material.     

Oxidation of pressureless sintered Si2N2O materials

The oxidation behaviour of pressureless sintered Si₂N₂O materials prepared from both amorphous and crystalline starting powders has been examined. The materials exhibited excellent oxidation resistance at temperatures of up to 1350°C. Thin protective oxide scales formed which had a duplex morphology after long exposures to air at high temperatures. Substantial crystallisation of the intergranular glass phase with formation of Y₂Si₂O₇ occurred during oxidation at 1200°C and 1350°C. Catastrophic oxidation occurred at temperatures ≥ 1400°C. This behaviour is enhanced by an oxidation-induced shift in the composition of the material to a liquid-forming region in the Y-Si-Al-O-N system.     

Microstructure development and properties of novel Ba-doped phase Sialon ceramics

Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba₂Si_12−xAl_xO_2+xN_16−x (x = 20.2). Appropriate amount of the precursor powders (BaCO₃, -Si₃N₄, AlN, Al₂O₃) with a targeted composition of BaAlSi₅O₂N₇ was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m^1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms.     

Low temperature sintering and microwave dielectric properties of 0.5LaAlO3–0.5SrTiO3 ceramics using copper oxide additions

The microwave dielectric properties and the microstructures of 0.5LaAlO₃–0.5SrTiO₃ ceramics with CuO addition prepared with conventional solid-state route have been investigated. Doping with CuO (up to 1 wt.%) can effectively promote the densification and remain comparable dielectric properties of 0.5LaAlO₃–0.5SrTiO₃ ceramics. It is found that 0.5LaAlO₃–0.5SrTiO₃ ceramics can be sintered at 1400 °C due to the sintering aid effect resulted from CuO as addition observed by scanning electron microscopy. The dielectric constant as well as the Q×f value decreases with increasing CuO content. At 1460 °C, 0.5LaAlO₃–0.5SrTiO₃ ceramics with 0.25 wt.% CuO addition possess a dielectric constant (_r) of 35.2, a Q×f value of 24 000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −13.5 ppm/°C.