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1.
Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al 2O 3 and quartz, with an Y 2O 3 addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y 2O 3 content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y 2O 3-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density. 相似文献
2.
A suspension stabilizer-coating technique was employed to prepare x mol% Yb 2O 3 ( x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y 2O 3 co-doped ZrO 2 powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb 2O 3 and Y 2O 3 co-doped zirconia ceramics. Fully dense ZrO 2 ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb 2O 3 content and the sintering parameters. Polycrystalline tetragonal ZrO 2 (TZP) and fully stabilised cubic ZrO 2 (FSZ) were achieved in the 1.0 mol% Y 2O 3 stabilised ceramic, co-doped with 1.0 mol% Yb 2O 3 and 4.0 mol% Yb 2O 3, respectively. The amount of stabilizer needed to form cubic ZrO 2 phase in the Yb 2O 3 and Y 2O 3 co-doped ZrO 2 ceramics was lower than that of single phase Y 2O 3-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m 1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb 2O 3 and 1.0 mol% Y 2O 3 ceramic at 1450 °C for 1 h. 相似文献
3.
Yttria- and ceria-doped tetragonal zirconia polycrystals ((Y, Ce)-TZP) with compositions 2·5 mol% YO 1·5-4 mol% CeO 2---ZrO 2, 4 mol% YO 1·5-4 mol% CeO 2---ZrO 2, and 2·5 mol% YO 1·5-5·5 mol% CeO 2---ZrO 2 were prepared from zirconia sols obtained hy hydrolysis of ZrOCl 2 solution, and their sintering, microstructure and thermal stability were studied. Sintered bodies with 99% TD were obtained by firing at 1400°C for 2 h in air. The grain size of (Y, Ce)-TZP increased with decreasing Y 2O 3 content in Y 2O 3---CeO 2---ZrO 2. (Y, Ce)-TZP was resistant to tetragonal-to-monoclinic (t → m) phase transformation during low temperature ageing as compared with 3Y-TZP. 相似文献
4.
The microstructure and mechanical properties of 8 mol% Y 2O 3 fully stabilized zirconia (8Y-FSZ) with BaTiO 3 additive were investigated. The introduction of BaTiO 3 additive would significantly increase the density and the grain size of 8Y-FSZ ceramics. XRD, Raman spectroscopy, and dielectric measurement were performed. A rhombohedral Ba(Ti 1−xZr x)O 3 ferroelectric phase resulted in the composite with 5 mol% additive, while for those with higher additive content, the secondary phase changes to cubic Ba(Ti 1−xZr x)O 3. The fracture toughness of the xBaTiO 3/(1− x)8Y-FSZ composites reached a maximum and then decreased with increasing the amount of additive. The highest value reached 6.1 MPa m 1/2 for 0.05BaTiO 3/0.95(8Y-FSZ) sintered at 1475 °C for 3 h, where the piezoelectric/ferroelectric secondary phase toughening played an important role. Moreover, the fracture toughness of the composites increased firstly and then decreased with increasing sintering temperature. 相似文献
5.
Polydimethylsiloxane–zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in different molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO 2 content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400°C and the structural evolution of samples treated at different temperatures has been followed by X-ray diffraction, transmission electron microscopy, infrared and 29Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N 2 sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600°C of high surface area materials with pore sizes below 3 nm. Samples are amorphous up to 800°C. At 1000°C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO 2 content. At 1400°C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO 2 phases are present with ZrO 2 average crystallite dimensions never exceeding 20 nm, for ZrO 2 content ≤50 mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. © 相似文献
6.
An organic precursor synthesis of 8 mol% yttria stabilized zirconia (YSZ) powder from Zr–Y composite nitrate solution and sucrose has been studied. Oxidation of sucrose in Zr–Y composite nitrate solution containing excess nitric acid in situ generates hydroxy carboxylic acids that forms a white sol which showed peaks at 1640 cm −1 and 1363 cm −1 in IR spectrum corresponding to hydroxy carboxylic acid complexes of Zr and Y. Precursor mass obtained by drying the sol on calcinations at 600 °C produced loosely agglomerated particles of cubic YSZ. Deagglomerated YSZ contain submicron particles with D50 value of 0.5 μm and the particles are aggregates of nanocrystallites of nearly 10 nm size. Compacts prepared by pressing the YSZ powder sintered to 96.7% TD at 1450 °C. The sintered YSZ ceramic showed an average grain size of 2.2 μm. 相似文献
7.
In this research, solid oxide fuel cell electrolytes were fabricated by aqueous tape-casting technique. The basic compositions for SOFC electrolyte systems were focused on yttria-stabilized zirconia (YSZ) system. The powders used in this study were from different sources. ZrO 2-based system doped with 3, 8, and 10 mol% of Y 2O 3, and 8YSZ electrolyte tape illustrated the desirable properties. The grain size of the sintered electrolyte tapes was in the range of 0.5–1 μm with 98–99% of theoretical density. Phase and crystal structure showed the pure cubic fluorite structure for 8–10 mol% YSZ and tetragonal phase for 3 mol% doped. The electrolyte tapes sintered at 1450 °C for 4 h had the highest ionic conductivity of 30.11 × 10 −3 S/cm which was measured at 600 °C. The flexural strengths were in the range of 100–180 MPa for 8–10 mol% YSZ, and 400–680 MPa for 3 mol% YSZ. 相似文献
8.
Homogeneous-eutectic microstructure of Y 3Al 5O 12–Al 2O 3 system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y 2O 3 and Al 2O 3 having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y 3Al 5O 12 and Al 2O 3 phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al 2O 3), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed. 相似文献
9.
SiC/Si 3N4 composites with rare earth oxide additions have been prepared by glass encapsulated hot isostatic pressing at 1850 °C and 200 MPa pressure. Mechanical properties and microstructures of the sintered samples have been studied. It is shown that different molar ratios of La 2O 3 to Y 2O 3 and the total amount of La 2O 3 and Y 2O 3 additions can affect the mechanical properties significantly. With 3 wt% La 2O 3 + Y 2O 3 additions, lower La 2O 3/Y 2O 3 molar ratio exhibits higher bending strength and median fracture toughness, but relatively lower Vickers hardness. For addition of 6 wt% La 2O 3 + Y 2O 3, the higher bending strength, Vickers hardness and fracture toughness correspond to a certain La 2O 3/Y 2O 3 molar ratio of 1.5, 1.0 and 0.5, respectively. SEM observation shows that the SiC matrix composite with fine grain size and homogeneous microstructure can be obtained. 相似文献
10.
Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi 2O 3) 0.75(RE 2O 3) 0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi 2O 3 and RE 2O 3 at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE 3+, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE 3+, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE 3+. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively. 相似文献
11.
Reforming of methane with carbon dioxide to synthesis gas (CO/H 2) has been investigated over rhodium supported on SiO 2, TiO 2, γ-Al 2O 3, MgO, CeO 2, and YSZ (ZrO 2 (8 mol% Y 2O 3)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using 13CH 4) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al 2O 3 catalyst during reforming reaction at 750°C are primarily derived from the CO 2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH 4 molecular route is found to be very small. 相似文献
12.
Densification of nanocrystalline yttria stabilized zirconia (YSZ) powder with 8 mol% Y 2O 3, prepared by a glycine/nitrate smoldering combustion method, was investigated by spark plasma sintering, hot pressing and conventional sintering. The spark plasma sintering technique was shown to be superior to the other methods giving dense materials (≥96%) with uniform morphology at lower temperatures and shorter sintering time. The grain size of the materials was 0.21, 0.37 and 12 μm after spark plasma sintering, hot pressing and conventional sintering, respectively. Total electrical conductivity of the materials showed no clear correlation with the grain size, but the activation energy for spark plasma sintered materials was slightly higher than for materials prepared by the two other densification methods. The hardness, measured by the Vickers indentation method, was found to be independent on grain size while fracture toughness, derived by the indentation method, was slightly decreasing with increasing grain size. 相似文献
13.
Rhodium catalysts, supported on six γ-Al 2O 3 supports with different crystallinities, were exposed to sequential treatments in hydrogen at 500°C, in oxygen at 760°C, in hydrogen at 500°C and at 760°C, respectively. Samples were characterized by X-ray diffraction and hydrogen chemisorption at various stages in the sequential treatment. Based on the characterization results, it is concluded that the formation of crystalline Rh 2O 3 is a function of γ-Al 2O 3 crystallinity; formation of crystalline Rh 2O 3 increased with increasing crystallinity of γ-Al 2O 3 during treatment in oxygen at 760°C. The crystalline Rh 2O 3 formed during treatment in oxygen at 760°C was reduced to Rh metal by hydrogen at 500°C, but most of the Rh did not adsorb hydrogen at room temperature. Subsequent treatment in hydrogen at 760°C increased the hydrogen adsorption capacity by as much as a factor of three. X-ray line broadening measurements showed that oxygen treatment of reduced Rh/γ-Al 2O 3 at 760°C followed by hydrogen reduction at 500°C resulted in significant increases in Rh crystal size; further treatment in hydrogen at 760°C resulted in additional sintering of Rh. 相似文献
14.
Lanthanum-doped Pd/γ-Al 2O 3 and Pd/γ-Al 2O 3 membranes were prepared by sol-gel methods. The thermal stability of the unsupported Pd/γ-Al 2O 3 and La/Pd/γ-Al 2O 3 membranes was investigated with BET (including average pore size, pore volume and BET surface area), XRD, and DTA techniques. The average pore size of the Pd/γ-Al 2O 3 membranes increased sharply after sintering at temperatures higher than 1000°C. Addition of 3 mol% lanthanum can raise the temperature of the γ-Al 2O 3 to-Al 2O 3 phase transformation significantly. This improves the thermal stability of the Pd/γ-Al 2O 3 catalytic membranes. 相似文献
15.
Dense Si 3N 4 hot-pressed with the aid of 9 wt% Y 2O 3 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 Y 2O 3-doped hot-pressed Si 3N 4 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. © 相似文献
16.
In this paper, a strategy is described to develop high toughness yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) composites reinforced with hard TiB 2 particles. The experimental results revealed that fully dense Y-TZP composites with 30 vol.% TiB 2 can be obtained with a moderate hardness of 13 GPa, a high strength up to 1280 MPa and an excellent indentation toughness up to 10 MPa m 1/2 by hot pressing in vacuum at 1450 °C. The toughness of the composites can be tailored between 4 and 10 MPa m 1/2 by varying the yttria stabiliser content of the ZrO 2 matrix between 3 and 2 mol%. An optimum composite toughness was achieved for a ZrO 2 matrix with an overall yttria content of 2.5 mol%, obtained by mixing pure monoclinic and 3 mol% Y 2O 3 co-precipitated ZrO 2 starting powders. An important observation is that the thermal residual tensile stress in the ZrO 2 matrix due to the TiB 2 addition, needs to be taken into account when optimising the transformability of the ZrO 2 matrix in order to develop high toughness Y-TZP composites. 相似文献
17.
Pb 0.99Nb 0.02[(Zr 0.60Sn 0.40) 0.94Ti 0.06] 0.98O 3 powders were synthesized from oxide mixtures of Pb 3O 4, Nb 2O 5, ZrO 2, SnO 2, and TiO 2 using a Fritsch P4™ vario-planetary ball milling system. The perovskite structure of PNZST powder can be obtained well after 14 h milling and crystallite size of the powders greatly reduced to 20–30 nm. The resultant powders have better sinterability characteristics to obtain dense ceramic bodies at low temperature. The dielectric constant of samples shows a maximum at 1150 °C and the PE MC to PE SC transformation temperature shifts to higher temperature with an increase in grain size. The field-induced strain reaches maximum longitudinal strain of 0.32% and the P– E hysteresis loop shows much higher squareness at 1150 and 1200 °C. The measured dielectric and antiferroelectric properties of the PNZST ceramics were found to be comparable to those from other processing techniques. 相似文献
18.
An attempt was made to prepare various F-doped β-, O-, X-, and -SiAlONs from a mixture of Si 3N 4, SiO 2, Al 2O 3, AlN, or Y 2O 3 using AlF 3 or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and -SiAlONs by X-ray diffraction. When AlF 3 was used, a single phase ceramic (O-SiAlON) was produced from a mixture of -Si 3N 4 and SiO 2 at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of -Si 3N 4, AlN, and Y 2O 3 with AlF 3 produced β-/Y--SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of -Si 3N 4 and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from -Si 3N 4 and SiO 2 at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y--SiAlON mixture showed the growth of O- and β-SiAlON and Y--SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses. 相似文献
19.
The purpose of this work was to select materials for applications in oxidizing atmospheres at temperatures of a minimum of 1500°C. Several requirements had to be fulfilled, e.g. long-term oxidation resistance, low vapour pressure, high creep resistance and low gas permeability. Another important point was to avoid reactions and sintering in material combinations with silica forming materials. The investigation was based on a detailed literature screening, followed by compatibility tests at 1600°C in air with potential materials (Al 2O 3, ZrO 2, CeO 2, La 2O 3, Y 2O 3, HfO 2, MgO·Al 2O 3) and Mosi 2 as an example of a silica-forming material. The result of these experiments was, that only Y 2O 3 and HfO 2 did not show severe reactions in contact with MoSi 2. Since most of the materials available did not fulfill all the requirements, some new materials were investigated: CIPed MoSi 2 with different additives (SiC, ZrB 2) and recrystallized SiC (RSiC) with a polymer (polysiloxane) derived MoSi 2-filled coating. The oxidation behaviour of these materials was evaluated by continuous thermogravimetric measurements at 1500°C over a period of 100 h in air and by detailed postexperimental investigations. 相似文献
20.
In zirconia-graphite refractories, calcia-stabilized zirconia is sensitive to silica coming from oxidized additives (Si, SiC). Accordingly, ceria is considered here as an alternative cheap stabilizer. Ceria-doped zirconia has been prepared and the monoclinic/tetragonal distribution has been analysed. At 1500°C in oxygen, the solution of ceria in zirconia is sluggish; to obtain a pure tetragonal phase, a 16 mol% ceria concentration (equal to the solubility limit) is necessary. Such samples are stable below 1050°C, in spite of previous claims to the contrary. At 1300°C, silica does not destabilize these samples, even in reducing conditions. However, at low oxygen pressure, the pyrochlore Ce2Zr2O7 is formed, and the remaining zirconia transforms into the monoclinic modification. Nevertheless, below 1175°C, a metastable, purely tetragonal modification is obtained, because the activation energy needed for the formation of Ce2Zr2O7 is too high. In any case, the reduction does not destroy the polycrystalline texture. This fact, combined with the resistance to silica, favours the use of these materials in steelmaking. 相似文献
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