Sintering of Si3N4 in the presence of additives from Y2O3-SiO2-Al2O3 system

The sintering process of Si₃N₄ in the presence of a liquid phase from the Y₂O₃-SiO₂-Al₂O₃ system was investigated. The starting composition of liquid phase was varied according to data in the phase diagram of the Y₂O₃-SiO₂-Al₂O₃ system, in order to lower the temperature of liquid formation because it might exhibit an influence on the sintering behaviour of Si₃N₄. Densification as well as phase analysis were followed as a function of composition and the amount of liquid phase, both in the sintered and in hot pressed samples.     

Subsolidus phase relations of the Y2O3-SrO-CuO system

The 960 °C subsolidus phase relations of Y₂O₃-SrO-CuO system have been determined by X-ray diffraction. The section at room temperature in the Y₂O₃-SrO-CuO ternary system can be divided into six three-phase and two-phase regions. In this system, there are five binary compounds: Y₂SrO₄, Y₂Cu₂O₅, Sr₂CuO₃, SrCuO₂ and Sr₁₄Cu₂₄O₄₁ as well as a solid solution Y_xSr_14–x Cu₂₄O₄₁ (0 <x 4). The structure of the solid solution is discussed. The variation of the lattice parameters of the solid solution with increasing yttrium content has been measured. Electrical resistivity measurements of the solid solution show that there is no superconductive phase in the Y-Sr-Cu-O system.     

Microstructure and phase composition of ZrO2-CeO2-Al2O3 materials modified with MgO and Y2O3

This paper presents our findings on phase formation processes during heat treatment of sol-gel synthesis products with the composition (mol %) 65(88ZrO₂ + 12CeO₂) + 35Al₂O₃ modified with 1 mol % MgO or Y₂O₃. The composites modified with 1 mol % MgO have been shown to differ significantly in phase composition from the parent nanopowders. Sintering is accompanied by partial decomposition of the tetragonal zirconia (T-ZrO₂) based solid solution and the formation of a monoclinic zirconia (M-ZrO₂) based solid solution and two aluminum-containing phases: corundum and the mixed oxide MgAl₁₁CeO₁₉. The addition of 1 mol % Y₂O₃ leads to successive formation of T-ZrO₂ and corundum and improvement of their structural perfection. The observed differences in phase formation during heat treatment result in different grain size compositions in the microstructure of the composites.     

An anomalous behaviour in the phase stability of the system Fe2O3 and NiO

Iron-nickel mixed oxides containing up to 50 mol% of NiO were prepared by firing the corresponding co-precipitated hydrous oxides; characterization was performed by X-ray diffraction, infrared spectroscopy, magnetic susceptibility, electrical conductivity and thermoelectric power measurements. A non-stoichiometric ferrite phase was formed when a sample containing 20 mol% NiO was sintered at 1050°C. This phase had two- to three-fold higher conductivity than either Fe₂O₃ or the stoichiometric ferrite (NiFe₂O₄). The thermoelectric power of this phase indicated a sharp change of charge carriers from n- to p-type near 350°C. This non-stoichiometric ferrite phase was stable only in a small temperature range and dissociated into -Fe₂O₃ and stoichiometric ferrite above 1200°C. Samples containing 5 and 10 mol% NiO also had small fractions of this non-stoichiometric ferrite phase when sintered at 1050°C.     

Phase development and activation energy of the Y2O3-Al2O3 system by a modified sol-gel process

The phase development of Y₃Al₅O₁₂ (YAG), YAlO₃ (YAP), and Y₄Al₂O₉ (YAM) synthesized by a modified sol-gel method has been studied. A chelate agent (ethyl acetoacetate) is added to control the hydrolysis and to modify the sol-gel process. The sol-gel-derived powder is then heat-treated to induce crystallization. The X-ray diffraction method is employed to determine the crystalline phases of the resulting powders. The formation of YAG, YAP, and YAM is described. For both YAP and YAM compositions, there is a phase transformation from YAP polymorphism to perovskite YAP. The activation energies of YAG are estimated at about 69 kcal/mol by the isothermal process as fitted with the Johnson-Mehl-Avrami equation and about 222 ± 2 kcal/mol by the continuous heating method as fitted with Kissinger and Sotgiu plots. The difference of activation energies calculated from the two methods is mainly due to the complicated crystallization of YAG.     

Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites

P/M 316L austenitic stainless steel has been reinforced with yttria and alumina particles. In order to improve the sintering behaviour of these composite materials, chromium diboride and boron nitride were added. The sinterability of the different materials has been characterised through dilatometry and sintering curves (sintered density vs. sintering temperature). A metallographic study by SEM coupled with microprobe has also been performed. Composites materials present a good densification. Chromium diboride and boron nitride react with the matrix in different manners, but they both greatly improve the sinterability of reinforced materials. The optimum sintering temperature for these composites materials is 1250 °C.     

Studies of the system CaO-SiO2-Al2O3-MgO in relation to the stability of blastfurnace slag

Phase equilibria in the 5% magnesia plane of the quaternary system CaO-SiO₂-Al₂O₃-MgO and in the high lime region of one of the bounding ternary systems, namely CaO-Al₂O₃-SiO₂, have been investigated by high temperature microscopy and X-ray analysis and substantial new data is presented. This study is one of a series designed to determine the extent of dicalcium silicate formation in the system CaO-SiO₂-Al₂O₃-MgO which accounts for over 95% of the composition of blastfurnace slag. Phase diagrams are presented based on an examination of approximately 150 synthetic compositions. The results are relevant to the stability of blastfurnace slag. The mineralogical information assembled will be of use also however in the wider application of industrial by-products some of which contain compounds occurring in the system lime-silica-alumina-magnesia. The results are also relevant to cement chemistry and to the study of igneous rocks.     

Phase equilibria in the CaF2-Al2O3-CaO system

Computations of phase equilibria in the CaF₂-Al₂O₃-CaO system have been carried out on the basis of experimentally found thermodynamic properties of all intermediate phases and melts. Coordinates of the phase equilibrium boundaries were determined by solving a system of equations expressing equality of chemical potentials of the components in coexisting phases. The nature and quantity of the coexisting phases were established by a search for the Gibbs energy minimum of the system. All the phases of the CaF₂-Al₂O₃-CaO system were taken into consideration. Calculated phase diagrams of the CaO-CaF₂, CaO-Al₂O₃ and CaF₂-Al₂O₃ binary subsystems are in good agreement with the data available in the literature. Isotherms of the CaF₂-Al₂O₃-CaO system were calculated at 1600, 1650, 1723 and 1773 K. A wide region of liquid separation into two phases is observed in the system. One phase is composed of practically pure CaF₂ with additions of several mol% of CaO and Al₂O₃, and the other consists of 50 to 65 mol% of CaF₂ only. Eleven invariant points of the CaF₂-Al₂O₃-CaO system include seven ternary eutectics, two ternary peritectics and two points of four-phase monotectic transition. The primary fields of crystallization of all the phases are alongated toward the CaF₂ apex, the CaO field being the widest and the 3CaO·Al₂O₃ field the narrowest. Seven junctions of the CaF₂-Al₂O₃-CaO phase diagram were represented. Computed saturation lines of CaF₂-Al₂O₃-CaO melt with CaO, Al₂O₃, CaO·6Al₂O₃ and CaO·2Al₂O₃, and also the positions of a number of characteristic points, agree well with the experimental data available. The present calculations reveal a number of details and peculiarities of the constitution of the CaF₂-Al₂O₃-CaO phase diagram.     

Kinetics of non-isothermal precipitation process of the perovskite phase in CaO-TiO2-SiO2-Al2O3-MgO system

Kinetics of non-isothermal precipitate process and crystal growth of perovskite (CaO·TiO₂) phase in CaO-TiO₂-SiO₂-Al₂O₃-MgO system were studied. The experimental results show that the relative volume fraction can be described by the equation given by Matusita et al. and the experiential expression of average crystal radius was obtained. The particle coarsening in non-isothermal process has important effects on the crystal growth of perovskite phase.     

Mechanical properties and microstructures of co-precipitation derived tetragonal Y2O3-ZrO2-Al2O3 composites

Y-TZP Al₂O₃ specimens (2.5 mol% Y₂O₃-ZrO₂ and 5 to 30 wt% Al₂ 03) were prepared from coprecipitated powders and their mechanical properties were studied. The addition of alumina to Y-TZP improves the attainable density of the materials after sintering at 1500° C and reduces the degradation of their densities due to porosity formation when the materials are sintered above 1500° C. Near theoretical density could be achieved for most of the samples after HIPing at 1500° C for 1/2 h at 200 M Pa pressure. The fracture strength of the HIPed specimens was in the range 2.0 to 2.4 GPa and the stress intensity factor was in the range 3.5 to 6.0 MPa m^1/2. The mechanical strength of the materials was not degraded seriously after autoclaving in water at 175° C for 24 h. The surface layer of transformed monoclinic zirconia was less than 70 m thick even after autoclaving at 175° C for 5 days.     

Crystallization of gels in the SiO2-Al2O3-ZrO2 system

The preparation of glasses and ceramics from gels in the ternary system of SiO₂-Al₂O₃-ZrO₂ has been investigated. The phase relations in this system suggest the existence of a stable joint between mullite and ZrO₂ in addition to the joint between mullite and zircon. The gel technique allows metastable ZrO₂ particles to be readily dispersed in alumino-silicate matrices.     

SiO2-Al2O3 metastable phase equilibrium diagram without mullite

A metastable binary phase diagram between SiO₂ (cristobalite) and-Al₂O₃ (corundum) in the absence of any mullite phase is presented. A eutectic is indicated at a temperature of 1260° C and a composition of 18 wt% ( 12 mol%) Al₂O₃. The liquidi of the proposed metastable system were positioned on the basis of the thermodynamic data calculated from the stable equilibrium diagram of Aksay and Pask [2]. Experimental evidence is also presented. A SiO₂-Al₂O₃ melt containing 80 wt% Al₂O₃ cooled at a slow rate in sealed molybdenum crucibles shows crystalline Al₂O₃ plus a glass phase whose composition followed the calculated extension of the stable Al₂O₃ liquidus to lower temperatures. Compacts of cristobalite—corundum mixtures were fired at subsolidus temperatures to estimate the eutectic temperature experimentally. The proposed metastable phase diagram effectively explains the formation of non-crystalline phases in subsolidus reactions, and microstructure obtained on solidification of high alumina melts.     

超重力辅助燃烧合成Y2O3-Al2O3-SiO2玻璃/Y3Al5O12陶瓷梯度复合材料

以超重力场辅助燃烧合成技术制备了呈梯度分布的Y2O3-Al2O3-SiO2透明玻璃/Y3Al5O12陶瓷梯度复合材料.结果表明,梯度布置两种组分的原料既可防止下层熔体的喷溅,又能提高上层玻璃的透过率.该梯度材料沿超重力方向依次为YAS透明玻璃层、YAS-YAG玻璃陶瓷层和YAG陶瓷层.     

Fine spherical particles of narrow size distribution in the Cr2O3-Al2O3 system

The preparation of spherical particles of composite alumina-chromia nature and amorphous character by a procedure of homogeneous precipitation at 100°C for 1 h, from aqueous Al(III) and Cr(III) nitrate solutions containing sodium sulphate, urea and polyvinylpyrrolidone, is described. This method allows controlling the Cr/Al ratio and the mean size of the particles (in the nanometer and the micrometer range) by adjusting the reagents concentrations. The structural and compositional changes originated in the particles on calcination were evaluated. It was found that for alumina-rich compositions (Cr/Al mole ratio 0.13), the particles transformed into fully developed corundum solid solutions at much lower temperature (1000°C) than those needed for samples prepared by previously reported procedures (1300°C), which was attributed to the high degree of chemical homogeneity at the atomic level attained in our case. It was also observed that an increase in the Cr/Al mole ratio of the composite systems resulted in an increase of the heating temperature required for the full development of the corundum solid solution.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.