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1.
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
2.
The system Al 2O 3–ZrO 2 was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO 2. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al 2O 3. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO 2 transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO 2, with 0.7±0.2% mass Al 2O 3 at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO 2 from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO 2 saturated solid solution. 相似文献
3.
The formation of cubic solid solutions in the system La 2O 3–ZrO 2 by mechanochemical activation of a mixture of the oxides (molar ratio ZrO 2 82%–La 2O 3 18%) is studied. After 6 h of activation at room temperature, a poorly crystalline cubic solid solution is formed, with ultimate crystallite sizes in the nanometer range. The mixtures activated during 1–3 h form the solid solution on subsequent heating at 1000 °C, while the non-activated mixture does not react, even after thermal treatment at 1200 °C. The solid solution obtained at room temperature undergoes partial structural ordering at temperatures between 800 and 1000 °C. Long time heating at temperatures of 1000 °C and above results in the formation of La 2Zr 2O 7 and rejection of the excess ZrO 2. Mechanochemical activation offers interesting possibilities for the synthesis of these materials at temperatures lower than those used in conventional processing, and for the control of their physicochemical and microstructural properties. 相似文献
4.
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
5.
Zirconia (ZrO 2) addition effects on densification and microstructure of tricalcium phosphate–26.52 wt% fluorapatite composites were investigated, using X-ray diffraction, scanning electron microscopy and by analysis using 31P nuclear magnetic resonance. The tricalcium phosphate–26.52 wt% fluorapatite–zirconia composites densification increases versus temperature. At 1300 °C, the composites apparent porosity reaches 9% with 5 wt% zirconia. XRD analysis of the composites reveals the presence of tricalcium phosphate, fluorapatite and zirconia without any other structures. Above 1300 °C, the densification was hindered by grain growth and the formation of both intragranular porosity and new compounds. The 31P MAS-NMR analysis of composites sintered at various temperatures or with different percentages of zirconia reveals the presence of tetrahedral P sites. At 1400 °C, XRD analysis of the tricalcium phosphate–26.52 wt% fluorapatite–20 wt% zirconia composites shows the presence of calcium zirconate and tetracalcium phosphate. This result indicated that partial decomposition of tricalcium phosphate during sintering process of composites when 20 wt% or less ZrO 2 was added. Thus, zirconia reacts with tricalcium phosphate forming calcium zirconate and tetracalcium phosphate. 相似文献
6.
Zirconium doped SiC with a surface area from 88 to 200 m 2 g −1 was synthesized using the shape memory concept method followed by calcination in air at a temperature of ≤480°C. The material obtained was composed of β-SiC and small ZrO 2 particles dispersed throughout the material matrix and a significant amount of an amorphous phase containing Si, Zr and O. Molybdenum oxycarbide, the active isomerization phase, supported on such a material displayed a similar behavior to that obtained on pure SiC for the n-heptane isomerization reaction. A comparison made with the molybdenum oxycarbide catalyst supported on pure ZrO 2 showed that the Zr doped SiC was not simply made of silicon carbide coated with a layer of ZrO 2 on the surface but probably an amorphous phase containing Si, Zr and O which displays a similar behavior as pure SiC. 相似文献
7.
Mullite–zirconia porous bodies were prepared by reaction sintering of zircon and alumina derived from oxidation reaction of Al at sintering temperatures between 1200 and 1600 °C. The results show that the incorporation of TiO 2 improves the oxidation reaction of Al, dissociation of zircon subsequently formation of mullite and zirconia. Composites containing TiO 2 obtain a high tetragonal concentration at 1500 °C, which reduces by increasing sintering temperature to 1600 °C. No tetragonal zirconia phase was detected at 1500 °C in TiO 2-free composites while tetragonal concentration was increased over this temperature. The major oxidation reaction of Al proceeds with a liquid–gas mechanism that is suitable for producing low dense ceramics. In spite of the higher porosity of the composites containing TiO 2, they possess almost the same flexural strength values as obtained from the TiO 2-free composites. 相似文献
8.
A hydrothermal technique was used to prepare micropowders of ZrO 2 solid solutions stabilized by the mixture of rare earth oxides and yttria (Ln 2O 3) recovered from phosphogypsum. Phase compositions and particle sizes of the powders were measured. Zirconia solid solutions of cubic and tetragonal symmetry are the major phases. A larger Ln 2O 3 concentration promotes a greater cubic phase content. Powders having low Ln 2O 3 concentrations have small amounts of monoclinic phase present. The phase composition, fracture toughness (KIc) and Vickers hardness of samples sintered at temperatures ranging from 1250 to 1350°C were measured. Tetragonal and, in some cases, monoclinic zirconia solid solutions are found in the microstructure of the sintered bodies. Two ordered phases with the formulae Ln2Zr2O7 and Ce2Zr3O10 were also detected in the system. Tetragonal zirconia polycrystals (TZP) with high (10 MPaM0·5) fracture toughness were found. 相似文献
9.
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
10.
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
11.
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
12.
A 3 mol% Y 2O 3 zirconia stabilized powder has been synthesized by destabilization of an aqueous zirconia sol prepared by the alkoxide hydrolysis method. The powder calcined at 500°C is ultrafine with tetragonal crystallites of about 8 nm, slightly agglomerated and with a narrow pore size distribution having an average pore size of 5.2 nm. Zirconia ceramics with density higher than 92%TD and grain size on the order of 100 nm have been obtained by uniaxial pressing at 500 MPa and vacuum sintering at 1000°C. Electrical conductivity of sintered samples, evaluated by complex impedance spectroscopy measurements, indicated that the zirconia stabilized with 3 mol% Y 2O 3 can potentially be used as an oxygen semipermeable dense membrane, but only at a relatively high temperature. 相似文献
13.
Microstructural development associated with diffusionless phase transformation was investigated in sintered ZrO 2-10–60 mol% CeO 2 ceramics cooled rapidly from a high temperature, using TEM and XRD techniques. The results show that (112) reflections appeared and a domain structure was found in ZrO 2-20–40 mol% CeO 2 samples, which is a result of c-t′ diffusionless transition, while the structure of the ZrO 2-60 mol% CeO 2 sample was fully stabilized zirconia, in which no forbidden reflections of c-ZrO 2 appeared. Finally, plate martensite and lath martensite structure were found in the ZrO 2-10 mol% CeO 2 sample; the former is the tetragonal phase with internal twins and the latter is the twinned monoclinic phase. 相似文献
14.
The NiSO 4 supported on Fe 2O 3-promoted ZrO 2 catalysts were prepared by the impregnation method. Fe 2O 3-promoted ZrO 2 was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe 2O 3–ZrO 2. The addition of nickel sulfate (or Fe 2O 3) to ZrO 2 shifted the phase transition of ZrO 2 (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe 2O 3) and ZrO 2. 15-NiSO 4/5-Fe 2O 3–ZrO 2 containing 15 wt.% NiSO 4 and 5 mol% Fe 2O 3, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO 4/Fe 2O 3–ZrO 2 catalysts was very effective for ethylene dimerization even at room temperature, but Fe 2O 3–ZrO 2 without NiSO 4 did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe 2O 3 up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe 2O 3 and ZrO 2 and due to consequent formation of Fe–O–Zr bond. 相似文献
15.
An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO 42−-ZrO 2 with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO 42−-ZrO 2 catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H 2 was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work. A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO42−-ZrO2 is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates. 相似文献
16.
The synthesis, superplastic deep drawing and cavitation behavior of a fine-grained alumina–zirconia ceramic composite have been investigated. The results show that dense Al 2O 3/ZrO 2 samples with average grain diameter of 230 nm can be elongated to a dome height of at least 12 mm at the punch rate of 0.6 mm·min −1 at 1400 °C. Deformed microstructure of the material indicates that not only the nucleation and growth of internal cavities are effectively suppressed but also the intragranular dislocation structures and sub-boundaries are observed around the nano-particles in alumina grains, which proves that intragranular dislocation creep is playing an important role in deformation. While the research on the deep drawing of the materials with the initial average grain size of 450 nm conforms that a plasticity controlled cavity growth process takes place during deformation. 相似文献
17.
Changes in microstructure and phase composition of ceria stabilized tetragonal zirconia polycrystals (Ce-TZP), magnesia and yttria partially stabilized zirconia [(Mg,Y)-PSZ] and magnesia partially stabilized zirconia (Mg-PSZ) were studied in diluted aqueous HCl, H 2SO 4 or H 3PO 4 solutions at a temperature of 390° C and a pressure of 27 MPa. Ce-TZP is corrosion resistant under these conditions in HCl, while Mg-PSZ is attacked severely and (Mg,Y)-PSZ undergoes a surface tetragonal to monoclinic phase transformation. All investigated zirconia ceramics suffer severe weight losses and transformation to the monoclinic phase on the surface in H 2SO 4. Only a small weight gain and a slight increase of m-phase on the surface of the ceramics is found in H 3PO 4. 相似文献
18.
A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO 2 (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO 2 on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO 2 slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO 2. ZrO 2 containing samples always show better resistance to thermal shock than the ZrO 2 free samples. Scanning electron micrography shows that ZrO 2 occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering. 相似文献
19.
The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse-grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high-temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO 2 was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m 2. The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m 2, for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/surface area gave surface enthalpies of 4.2 J/m 2 for the monoclinic form and 0.9 J/m 2 for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO 2 was estimated to be 10±1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol. 相似文献
20.
The change of mechanical properties of zirconia-toughened mullite composites, aged at low temperature in air, was investigated in this paper. The results indicated that the existence of microcracks, which were formed by the transformation of the zirconia tetragonal phase to the monoclinic during the cooling stage of processing was an important factor for the degradation of mechanical properties during subsequent ageing at 200–300°C. The increasing of flexural strength of the composites aged at 500–600°C was attributed to the relaxation of stress by reverse transformation of ZrO 2. 相似文献
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