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1.
Single crystals of CaO-stabilized ZrO2 containing between 15.3 and 18.9 mol% CaO were heat-treated for 5000 h at 1200°C to study the stability of the ordered defect-fluorite phase, CaZr4O91). Subsequent TEM analysis of the equiaxed φ1 domains in samples richer than approximately 18 mol% in CaO showed a random distribution of φ1 variants, with no preferred interfacial habit planes. A critical review of the literature, combined with the new data, supports the Stubican-Hellmann-Hannon version of the phase diagram in the region 15 to 26 mol% CaO and 1000° to 1400°C and strongly suggests that φ1 is a stable phase in the ZrO2-CaO systems.  相似文献   

2.
The effect of ZrO2 on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 (BZT) ceramics was investigated. A small amount of ZrO2 disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO2, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO2, but it decreased when the ZrO2 content was increased. Variation of the dielectric constant with ZrO2 addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO2 amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO2, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO2 sintered at 1550°C for 10 h.  相似文献   

3.
Phase equilibria in the system ZrO2─InO1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO1.5 is soluble in t -ZrO2 at 1500°C. The martensitic transformation temperature m → t of ZrO2 containing InO1.5 is compared with that of ZrO2 solid solutions with various other trivalent ions with different ionic radii. The diffusionless c → t ' A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO1.5 is found at 1700°C in the cubic ZrO2 phase. The eutectoid composition and temperature for the decomposition of c -ZrO2 solid solution into t -ZrO2+InO1.5 solid solutions were determined. A maximum of about 1 mol% ZrO2 is soluble in bcc InO1.5 phase. Metastable supersaturation of ZrO2 in bcc InO 1.5 and conditions for phase separation are discussed.  相似文献   

4.
MgO addition to 3 mol% Y2O3–ZrO2 resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO2 in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO2 grains with tetragonal ZrO2 precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.  相似文献   

5.
Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures   总被引:1,自引:0,他引:1  
High-temperature plastic flow in Al2O3-10 wt% ZrO2 (2.5 mol% Y2O3) has been examined at temperatures between 1400° and 1500°C. Al2O3-10 wt% ZrO2 (2.5 mol% Y2O3) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al2O3. The suppression of grain growth with ZrO2 dispersion into Al2O3 is not effective for improving the tensile ductility. The limited ductility in Al2O3-10 wt% ZrO2 (2.5 mol% Y2O3) is associated with the increment of flow stress caused by ZrO2. The ZrO2 dispersion or segregation in Al2O3/Al2O3 boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.  相似文献   

6.
Pure Ba2Ti9O20 (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO2 powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO2. The addition of ZrO2 possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled.  相似文献   

7.
Compression creep measurements at constant load on ZrO2-6 mol% Sc2O3 (grain size ∼1 μm), ZrO2-6 mol% Y2O3 (grain size ∼17 μm), and heat-treated ZrO2-6 mol% Sc2O3 (grain size ∼2 μm) yield activation energies of 89, 86, and 74 kcal/mol, respectively. The creep rates are linearly proportional to the inverse square of the grain size of the material. A stress exponent, n , of 1.5 was found for the scandia-doped zirconia and two regimes, with n =1 and 6, were found for the yttria-doped zirconia. These data, supported by metallographic evidence, are interpreted as showing that n =1 is associated with cation diffusion control of creep, n =6 with local propagation of inter-crystalline cracks, and n =1.5 with a transition region.  相似文献   

8.
Aqueous processing of Al2O3─ZrO2 (123 mol% CeO2) composites, combined with sintering conditions, was used to control the microstructure and its influence on the martensitic transformation temperature of t -ZrO2 and the transformation-toughening contribution at room temperature. The resultant ZrO2 grain sizes in the dense composites were related to the transformation-toughening behavior of t -ZrO2. The data show that (1) the best processing conditions exist when the electrophoretic mobilities of the two solids are positive, adequately high to ensure colloidal stability, efficient packing,and uniform ZrO2 distribution but differ greatly in magnitude, (2) the colloidal stability of ZrO2 controls the overall stability and the rheological and processing behavior of this mixture, (3) the grain size distribution in dense pieces sintered for 1 h at 1500°C is comparable to the particle size distribution of the powders, (4) the martensite start temperature for the tetragonal to-monoclinic transformation in Al2O3 containing 20 and 40 vol% ZrO2 increases and can approach 0°C with increasing average ZrO2 grain size, and as a result, (5) the fracture toughness values at room temperature are raised from 4–5 MPa.m1/2 to 9–12 MPa.m1/2 for these two compositions.  相似文献   

9.
The effect of Y2O3 content on the flexure strength of melt-grown Al2O3–ZrO2 eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y2O3 content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO2 platelets oriented along the growth axis and embedded in the continuous Al2O3 matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y2O3 in relation to ZrO2(Y2O3). Higher Y2O3 content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al2O3–ZrO2 interface during the martensitic transformation of ZrO2 on cooling and propagated into the Al2O3 matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y2O3 retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y2O3 dropped rapidly with temperature as a result of extensive microcracking.  相似文献   

10.
The phase diagram of the system ZrO2-CeO2 was rein-vestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO2, respectively, at 1400°C, and about 16 and 80 mol% CeO2, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO2 at 1000°C, and 1.5 and 98 mol% CeO2 at 800°C, respectively. The compound Ce2Zr3O10 is not found in this system.  相似文献   

11.
ZrO2–Y2O3–CuO nanocrystalline powders have been synthesized using a chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies show that the presence of CuO accelerates the densification process of ZrO2(3Y). A fully dense (>96%) pellet of ZrO2(3Y)/5 mol% CuO was obtained after sintering at 900°C, with a very small grain size of 44 nm calculated by X-ray line broadening.  相似文献   

12.
Euctectoid decomposition of cubic ( c ) ZrO2 in MgO-partially-stabilized ZrO2 (Mg-PSZ) has been studied using optical, scanning electron, and transmission electron microscopy. Alloys containing from 8.1 to 18.6 mol% MgO were decomposed by annealing between 1100° and 1300°C for times up to 16 h. The eutectoid products nucleated heterogeneously at the grain boundaries and advanced into the adjoining two-phase grains. Decomposition proceeded as a "cellular" reaction involving the cooperative growth of MgO and a low-solute ZrO2 phase with either monoclinic ( m ) or tetragonal ( t ) symmetry. The MgO morphology is rodlike and exhibits a well-defined orientation relationship to m -ZrO2. The rate of eutectoid decomposition was a maximum at 1200°C and was greater in the solute-rich materials at all temperatures. At 1100°C, SrO doping decreased the nucleation rate of the eutectoid product in a 9.7 mol% alloy, thus strongly suppressing the decomposition rates; at the higher decomposition temperatures, the SrO was less effective.  相似文献   

13.
In the system ZrO2-Al2O3, cubic ZrO2 solid solutions containing up to 40 mol% Al2O3 crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al2O3 content. At higher temperatures, the solid solutions transform into tetragonal ZrO2 and α-Al2O3. Pure ZrO2 crystallizes in the tetragonal form at 415° to 440°C.  相似文献   

14.
The pseudoternary system ZrO2-Y2O3-Cr2O3 was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO3, was observed on the Y2O3−Cr2O3 join. ZrO2 and Y2O3 formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO2, Cr2O3, and YCrO3 three-phase region was located at =17 mol% Y2O3 (83 mol% ZrO2). Below 17 mol% Y2O3, ZrO2 solid solution coexisted with Cr2O3. Cr2O3 appears to be slightly soluble in ZrO2(ss).  相似文献   

15.
This paper clarifies the formation reaction of ZrO2 crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF2 purified by sublimation. BaO reacts with ZrF4 to form ZrO2 at 600°C during a glass-melting process. The ZrO2 formation reaction is influenced by H2O. Ba(OH)2, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF4 to form ZrO2 at 450° to 520°C. When low-oxide-content BaF2 is used for fiber preparation, scatterers significantly decrease.  相似文献   

16.
Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO2(3Y) and Al2O3 have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The microstructures are such that tetragonal ZrO2 (0.3 μm) and Al2O3 (0.5 μm) particles are located at the grain boundaries of the CoAl (8.5 μm) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO2(3Y)/Al2O3= 16/4 mol% are 3.87 MPa·m1/2 and 1080 MPa, respectively, and high strength (>600 MPa) can be retained up to 1000°C.  相似文献   

17.
ZrO2-2 mol% Y2O3 crystals with average grain sizes from 0.51 to 0.96 µm were prepared by sintering in air at 1400°C for 2 to 100 h. The tetragonal-to-monoclinic phase transformation associated with the low-temperature degradation was investigated to clarify how the presence of water directly affects the influence of grain size on transformation. The specimens were exposed to water at 80–120°C, a temperature range in which transformation by thermal activation is difficult in the absence of water. Contrary to expectations, this type of low-temperature transformation did not accelerate monotonously with increasing grain size. Instead, the amount of phase transformation first decreased, reaching a constant value, and then increased with increasing grain size. Such interesting results can be explained satisfactorily by the combined influences of grain size on the nucleation process, because of preferential dissolution of yttrium at the grain boundaries, and the intrinsic transformability of Y2O3-doped tetragonal ZrO2 grains.  相似文献   

18.
Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO2–3 mol% Y2O3 powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO2–3 mol% Y2O3 produced mixed results. In the case of submicrometer ZrO2–3 mol% Y2O3, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO2–3 mol% Y2O3, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO2–3 mol% Y2O3 specimens.  相似文献   

19.
Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc2O3-doped cubic-ZrO2 by the presence of 1 mol% Bi2O3. The powders of 1-mol%-Bi2O3–10-mol%-Sc2O3-doped ZrO2 were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO2 phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO2 sintered at 1200°C was >2 μm because of grain growth in the presence of Bi3+. Cubic, stabilized Bi-Sc-doped ZrO2 sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.  相似文献   

20.
The phase diagram for the system ZrO2-Y2O3 was redetermined. The extent of the fluorite-type ZrO2-YzO3 solid solution field was determined with a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. Long range ordering occurred at 40 mol% Y2O3 and the corresponding ordered phase was Zr3Y4OL12. The compound has rhombohedra1 symmetry (space group R 3), is isostructural with UY6Ol2 and decomposes above 1250±50°C. The results indicate that the eutectoid may occur at a temperature <400°C at a composition between 20 and 30 mol% Y2O3 Determination of the liquidus line indicated a eutectic at 83± 1 mol% Y2O3 and a peritectic at 76 ± 1 mol% Y2O3.  相似文献   

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