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1.
The subsolidus phase relations in the entire system ZrO2 -Y2 O3 were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y2 O3 and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y4 Zr3 O12 +hexagonalphase Y6 ZrO11 at 45 mol% Y2 O3 and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y6 ZrO11 at ∼72 mol% Y2 O3 and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y2 O3 with ideal formula Y4 Zr3 O12 , and another, a new hexagonal phase, at 75 mol% Y2 O3 with formula Y6 ZrO11 . They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO2 -Y2 O3 . 相似文献
2.
Kuan Zong Fung Anil V. Virkar David L. Drobeck 《Journal of the American Ceramic Society》1994,77(6):1638-1648
Cubic solid solutions in the Y2 O3 -Bi2 O3 system with ∼25% Y2 O3 undergo a transformation to a rhombohedral phase when annealed at temperatures ≤ 700°C. This transformation is composition-invariant and is thermally activated, and the product phase can propagate across matrix grain boundaries, indicating that there is no special crystallo-graphic orientation relationship between the product and the parent phases. Based on these observations, it is proposed that cubic → rhombohedral phase transformation in the Y2 O3 -Bi2 O3 system is a massive transformation. Samples of composition 25% Y2 O3 -75% Bi2 O3 with and without aliovalent dopants were annealed at temperatures ≤ 700°C for up to 10000 h. ZrO2 as a dopant suppressed while CaO and SrO as dopants enhanced the kinetics of phase transformation. The rate of cubic/rhombohedra1 interface migration (growth rate or interface velocity) was also similarly affected by the additions of dopants; ZrO2 suppressed while CaO enhanced the growth rate. Diffusion studies further showed that ZrO2 suppressed while CaO enhanced cation interdiffusion coefficient. These observations are rationalized on the premise that cation interstitials are more mobile compared to cation vacancies in cubic bismuth oxide. The maximum growth rate measured was ∼10−10 m/s, which is orders of magnitude smaller than typical growth rates measured in metallic alloys. This difference is explained in terms of substantially lower diffusion coefficients in these oxide systems compared to metallic alloys. 相似文献
3.
The tetragonal ( t ) and cubic ( c ) ZrO2 solid solutions in two-phase ZrO2 -8 wt% Y2 O3 ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y2 O3 content. The enrichment in Y2 O3 content of the c -ZrO2 grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO2 increases by the nucleation and growth of cap-shaped t -ZrO2 lenses. The interfaces between the c -ZrO2 matrix and the growing t -ZrO2 lenses are semicoherent. 相似文献
4.
Single-phase, cubic solid solutions of baseline composition 25% Y2 O3 —75% Bi2 O3 with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO2 , or ThO2 was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO2 or ThO2 remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO2 - and ThO2 -doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice. 相似文献
5.
GEORGE C. WEI T. EMMA WILLIAM H. RHODES STEVEN HORVATH MARTIN HARMER 《Journal of the American Ceramic Society》1988,71(10):820-825
Transmission electron microscopic analyses defined the structures and compositions in single-phase and two-phase La2 O3 -doped Y2 O3 materials fabricated by the transient solid second-phase sintering. The composition in single-phase, 10-mol%-La2 O3 -doped, sintered and annealed samples was found to be uniform, indicating that diffusivity was sufficiently high for homogenization in the single-phase field. Two-phase, 16-mol%-La2 O3 -doped, sintered and annealed samples showed two morphologies: (1) intragranular, lath-like, monoclinic second-phase particles (twinned and untwinned) and (2) equiaxed cubic matrix. The second-phase particles were identified as the monoclinic phase derived from the high-temperature hexagonal phase through a rapid phase transition. A short, high-temperature anneal (2200°C for 1 min) of 9 mol% La2 O3 -Y2 O3 composition was found to retain the hexagonal phase. Microchemical analyses of the phases suggested adjustments to the Y2 O3 -La2 O3 phase diagram. Observation of the interactions of the intragranular second-phase particles with crack propagation indicated crack deflection as one of the mechanisms responsible for toughening (1.5 vs 0.9 MPa · m1/2 ). 相似文献
6.
Masahiro Yoshimura Manikpurage Jayaratna Shigeyuki Smiya 《Journal of the American Ceramic Society》1982,65(10):c166-c168
The pseudoternary system ZrO2 -Y2 O3 -Cr2 O3 was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO3 , was observed on the Y2 O3 −Cr2 O3 join. ZrO2 and Y2 O3 formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO2 , Cr2 O3 , and YCrO3 three-phase region was located at =17 mol% Y2 O3 (83 mol% ZrO2 ). Below 17 mol% Y2 O3 , ZrO2 solid solution coexisted with Cr2 O3 . Cr2 O3 appears to be slightly soluble in ZrO2 (ss). 相似文献
7.
α-Al2 O3 -doped (8 mol % Sc2 O3 )ZrO2 composite solid electrolyte has been investigated in the fabrication of solid-state ceramic gas sensors. The microstructure and electrical conductivity of the composite solid electrolyte have been measured over a range of temperature from 240°C to 596°C. The composite solid electrolyte has been found to exhibit a higher conductivity compared with the commonly used (8 mol% Y2 O3 )ZrO2 at temperatures above ∼448°C. The sensing characteristics for NO2 detection have been studied in the temperature range of 500–650°C at the low concentration from 10 to 30 ppm and at high concentration from 100 to 500 ppm of NO2 . The NO2 sensor was found to respond reproducibly and rapidly to the variations of NO2 , concentration, indicating that the composite solid electrolyte has promising application as a solid electrolyte for on-board exhaust gas monitoring. 相似文献
8.
Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH4 OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO2 crystallites at elevated temperatures was strongly inhibited by Al2 O3 derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al2 O3 , and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO2 is lower than that of the monoclinic form. With increasing A12 O3 content, however, the transformation temperature gradually increased, although the growth of ZrO2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO2 . The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy. 相似文献
9.
F.F. LANGE 《Journal of the American Ceramic Society》1986,69(3):240-242
The average grain size of ZrO2 (+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y2 O3 content. The average grain size decreased by a factor of 4 to 5 for Y2 O3 contents between 0.8 and 1.4 mol% and increased at Y2 O3 contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y2 O3 -rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y2 O3 . Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent. 相似文献
10.
The electrical conductivity of M2 O3 -ZrO2 compositions containing 6 to 24 mole % M2 O3 , where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm2 O3 , Y2 O3 , and Yb2 O3 formed cubic solid solutions with ZrO2 over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO2 at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc2 O3 ). Lanthana additions to ZrO2 produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M2 O3 . 相似文献
11.
The phase diagram for the system ZrO2 -Y2 O3 was redetermined. The extent of the fluorite-type ZrO2 -Yz O3 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% Y2 O3 and the corresponding ordered phase was Zr3 Y4 OL12 . The compound has rhombohedra1 symmetry (space group R 3), is isostructural with UY6 Ol2 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% Y2 O3 Determination of the liquidus line indicated a eutectic at 83± 1 mol% Y2 O3 and a peritectic at 76 ± 1 mol% Y2 O3 . 相似文献
12.
A. DOMINGUEZ-RODRIGUEZ V. LANTERI A. H. HEUER 《Journal of the American Ceramic Society》1986,69(3):285-287
Precipitation hardening was observed in two-phase (cubic plus tetragonal) Y2 O3 -partia1ly-stabilized ZrO2 single crystals deformed at 1400°C. Slip was activated on (001) 〈110〉, primarily in Luders bands. 相似文献
13.
MgO addition to 3 mol% Y2 O3 –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
14.
J. LLorca José Y. Pastor Pedro Poza José I. Peña Isabel de Francisco Angel Larrea Victor M. Orera 《Journal of the American Ceramic Society》2004,87(4):633-639
The effect of Y2 O3 content on the flexure strength of melt-grown Al2 O3 –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 Y2 O3 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 Al2 O3 matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y2 O3 in relation to ZrO2 (Y2 O3 ). Higher Y2 O3 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 Al2 O3 –ZrO2 interface during the martensitic transformation of ZrO2 on cooling and propagated into the Al2 O3 matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y2 O3 retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y2 O3 dropped rapidly with temperature as a result of extensive microcracking. 相似文献
15.
Several unusual microstructural features, i.e., 90° tetragonal ZrO2 twins containing antiphase domain boundaries, tetragonal ZrO2 precipitates in a colony morphology, and precipitate-free zones at the perimeter of cubic ZrO2 grains containing fine tetragonal ZrO2 precipitates, were observed in a single ZrO2 -12 wt% Y2 O2 ceramic annealed at 1550°, 1400°, and 1250°C, respectively. The type of phase transformation responsible for each microstructural feature is described. 相似文献
16.
The phase equilibria in the zirconia-rich part of the system ZrO2 −Yb2 O3 −Y2 O3 were determined at 1200°, 1400°, and 1650°C. The stabilizing effects of Yb2 O3 and Y2 O3 were found to be quite similar with <10 mol% of either being necessary to fully stabilize the cubic fluorite-structure phase at 1200°C. The two binary ordered phases, Zr3 Yb4 O12 and Zr3 Y4 O12 , are completely miscible at 1200°C. These were the only binary or ternary phases detected. The ionic conductivities of ternary specimens in this system were measured using the complex impedance analysis technique. For a given level of total dopant, the substitution of Yb2 O3 for Y2 O3 gives only minor increases in specimen conductivity. 相似文献
17.
Above 755°C, compounds along the spinel join LiFe5 O8 -Li4 Ti5 O12 form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe5 O8 and the disordered spinel phase. At 800° and 900°C, cubic LiFeO2 ( ss ) and monoclinic Liz Ti03 ( ss ) exist on the monoxide join LiFeO2 -Li2 TiO3 . The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li2 O-Fe2 O3 -TiO2 at 800° and 900°C were determined for compositions containing ∼50 mol% Li2 O. 相似文献
18.
EIJI TANI MASAHIRO YOSHIMURA SHIGEYUKI SOMIYA 《Journal of the American Ceramic Society》1983,66(7):506-510
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 Ce2 Zr3 O10 is not found in this system. 相似文献
19.
Kui Du Chang-Hoon Kim Arthur H. Heuer Richard Goettler Zhien Liu 《Journal of the American Ceramic Society》2008,91(5):1626-1633
X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc2 O3 -stabilized ZrO2 as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc2 O3 samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc2 O3 samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc2 O3 samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)t XRD peak of Sc2 O3 -stabilzed tetragonal ZrO2 accompanies the change(s) in the electrical conductivity. 相似文献
20.
The subsolidus phase diagram of the quasiternary system Si3 N4 -AlN-Y2 O3 was established. In this system α-Si3 N4 forms a solid solution with 0.1Y2 O3 : 0.9 AIN. The solubility limits are represented by Y0.33 Si10.5 Al1.5 O0.5 N15.5 and Y0.67 Si9 A13 ON15 . At 1700°C an equilibrium exists between β-Si3 N4 and this solid solution. 相似文献