共查询到20条相似文献,搜索用时 750 毫秒
1.
Monika Agarwal Mark R. de Guire Arthur H. Heuer 《Journal of the American Ceramic Society》1997,80(12):2967-2981
Undoped or Y2 O3 -doped ZrO2 thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO4 )·4H4 O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO2 and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO2 . Amorphous Y2 O3 -containing ZrO2 films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y2 (SO4 )3 8·H2 O), and urea (NH2 CONH2 ) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C. 相似文献
2.
JENQ-GONG DUH HSING-TAO DAI BI-SHIOU CHIOU 《Journal of the American Ceramic Society》1988,71(10):813-819
A wet-chemical approach is applied to derive fine powders with compositions 11 mol% CeO2 -ZrO2 , 1 mol% YO1.5 -10 mol% CeO2 -ZrO2 , 12 mol% CeO2 -ZrO2 , and 2 mol% YO1.5 -10 mol% CeO2 -ZrO2 by the coprecipitation method. The characteristics of the as-derived powders are evaluated through thermal analysis and electron microscopy. The sintering behavior of the calcined powders is carried out at 1400° and 1500°C for 1 to 10 h. Sintered density higher than 98% of theoretical is achieved for sintering at 1400°C for several hours. The as-sintered density dependence on the sintering condition is related to the extent of tetragonal-to-monoclinic phase transformation as well as the associated microcracks. Partial substitution by Y2 O3 in CeO2 -ZrO2 results in reduced grain size and tends to stabilize the tetragonal structure. Y2 O3 is more effective than CeO2 with respect to the grain size refinement and tetragonal stability. In addition, Y2 O3 substitution in CeO2 -ZrO2 increases the hardness, while it decreases the fracture toughness. 相似文献
3.
The ionic conductivity of cubic solid solutions in the system CaO -Y2 O3 -ZrO2 was examined. Particular Y2 O3 -ZrO2 binary compositions were more conductive at elevated temperatures (>600°C) than either CaO-ZrO2 binary or CaO-Y2 O3 -ZrO2 ternary compositions. The higher ionic conductivity appears to be related to a lower activation energy rather than to the number of oxygen vacancies dictated by composition. Those compositions of highest conductivity lie close to the cubic-monoclinic solid-solution phase boundary. Conductivity-temperature data are presented that indicate a reversible order-disorder transition for Y2 O3 -ZrO2 cubic solid solutions containing 20 and 25 mole % Y2 O3 . The transference number for the oxygen ion at 1000°C for Y2 O2 -ZrO2 cubic solid solutions is greater than 0.99. 相似文献
4.
ZrO2 -2 mol% Y2 O3 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 Y2 O3 -doped tetragonal ZrO2 grains. 相似文献
5.
The fracture toughness of 3 mol% Y2 O3 -ZrO2 (3Y-PSZ) composites containing 10–30 vol% Al2 O3 with different particle sizes was investigated. It was found that Al2 O3 dispersion of up to 30 vol% increased the fracture toughness by 17% to 30%, and the toughness increase was more remarkable in the composite dispersed with Al2 O3 particles of larger sizes. By combining the effects of the dispersion toughening and phase transformation toughening, the toughness change in the present materials was theoretically predicted, which was in good agreement with the experimental data. 相似文献
6.
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
7.
In the presence of a fluorine mineralizer, highly aggregated, <5 μm α-Al2 O3 platelet particles form by vapor transport during the thermal transformation of γ-alumina. Platelet aggregation was determined to occur by platelet inter-growth and by edge nucleation on primary α-Al2 03 platelets. The addition of 1010 α-alumina seed particles/cm3 γ–Al2 O3 resulted in the development of discrete particles during the initial stage of transformation. Impingement of the growing platelets during the latter stage of transformation, however, resulted in intergrowth, a process which was not changed by seeding. Particle size distribution broadening was observed to increase with increasing HF and H2 O concentrations because vapor reactant supersaturation increases the degree of edge nucleation. When initially low HF and H2 O concentrations were used in seeded systems, however, essentially aggregate-free α-Al2 O3 platelets of 10–15 μm were obtained. 相似文献
8.
F. G. BUTTLER L. S. DENT GLASSER H. F. W. TAYLOR 《Journal of the American Ceramic Society》1959,42(3):121-126
Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al2 O3 . 13H2 O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca2 (OH)7 - 3H2 O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A12 O3 .13H2 O, from which it is derived by substitution of CO3 2- for 20H- + 3H2 O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al2 O3 .Ca Y 2 - xH2 O and 3CaO Al2 O3 Ca Y xH2 O. On dehydration, 4CaO.Al2 O3 .13H2 O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)2 and 4CaO.3Al2 O3 .3H2 O are formed and, by 1000°C., CaO and 12CaO.7Al2 O8 . The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al2 O3 .3H2 O is formed. 相似文献
9.
Kentaro Fukuda Teiichi Hanada Setsuhisa Tanabe Takeshi Yao 《Journal of the American Ceramic Society》2002,85(4):915-920
Amorphous films in the system Al2 O3 –Y2 O3 were prepared by the rf sputtering method in the range of 0–76 mol% Y2 O3 , and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al2 O3 –Y2 O3 films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y2 O3 content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al2 O3 –Y2 O3 films, the Al K α X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y2 O3 content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y2 O3 , the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y2 O3 content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al2 O3 –Y2 O3 , the change of the coordination state of aluminum ions has an important effect on physical properties. 相似文献
10.
Paste samples of tricalcium aluminate alone, with CaCl2 , with gypsum, and with gypsum and CaCl2 were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl2 to tricalcium aluminate resulted in the formation of 3CaO · Al2 O3 · CaCl2 ·10H2 O and 4CaO · Al2 O3 · 13H2 O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al2 O3 · 6H2 O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al2 O3 · CaSO4 · 12H2 O and 3CaO · Al2 O3 · CaCl2 · 10H2 O were the main hydration products. 相似文献
11.
The influence of citric acid on paste hydration of 3CaO· Al2 O3 in the presence of CaSO4 ·2H2 O and Ca(OH)2 was studied using X-ray diffraction, scanning electron microscopy, and conduction calorimetry. The time at which the citric acid is added (either prior to or with the mixing water) determines how it affects the reactivity of the aluminate. Immediately after the paste is gaged citric acid promotes a more rapid reaction, but later reactions are retarded. Hexagonal calcium aluminate hydrates, ettringite, and monosulfate were all detected as early hydration products. The influence of citric acid on the hydration of 3CaO·Al2 O3 slabs immersed in saturated CaSO4 ·2H2 O solutions was also studied and a reaction scheme proposed. 相似文献
12.
Oriented samples of Al2 O3 -ZrO2 (Y2 O3 ) eutectics consisting of an alumina matrix with zirconia dispersoids were grown by directional solidification. Preferred growth directions and epitaxial relations were determined from X-ray and electron diffraction analyses. Imaging of interfaces was performed by high-resolution transmission electron microscopy on oriented platelets. Semicoherent interfaces were observed with faceting along crystallographic planes of both phases. 相似文献
13.
Masayuki Nogami Naoya Hayakawa Naoya Sugioka Yoshihiro Abe 《Journal of the American Ceramic Society》1996,79(5):1257-1261
Samarium ions (Sm2+ ) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na2 O-Al2 O3 SiO2 synthesized by the hydrolysis of Si(OC2 H5 )4 , Al(OC4 H9 )3 , CH3 COONa, and SmCl3 ·6H2 O were heated in air at 500°C, then reacted with H2 gas to form Sm2+ ions. Whereas Al3+ ions effectively dispersed the Sm3+ ions in the glass structure, Na+ ions were not effective. The Al2 O3 -SiO2 glasses proved appropriate for reacting the Sm3+ ions with H2 gas and exhibited the intense photoluminescence of Sm2+ ions. The reaction of Sm3+ ions with H2 in the Al2 O2 -SiO2 glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm2+ ions was achieved within 20 min. 相似文献
14.
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
15.
Boron oxide glasses with compositions corresponding to B2 O3 , B2 O3 .0.42H2 O, B2 O3 .0.50H2 O, and B2 O3 .0.63H2 ) have been prepared and their structures have been studied. Diffractometric X-ray scattering measurements have been made, and from these, radial distribution functions have been computed. Comparison of the radial distribution functions indicates that the fundamental triangular coordination characteristic of vitreous B2 O3 is maintained in the water-containing glasses. Consideration of the results of complementary infrared and nuclear magnetic resonance studies along with preliminary results of physical property measurements indicates that the glasses of high water content are heterogeneous. It is probable that disordered regions of high hydrogen bond density, approaching HBO2 in composition, are embedded in a matrix approximating vitreous B2 O3 . 相似文献
16.
Basak Yazgan Kokuoz Karn Serivalsatit Baris Kokuoz Olt Geiculescu Exley McCormick John Ballato 《Journal of the American Ceramic Society》2009,92(10):2247-2253
Nanoparticles of erbium-doped yttria (Er:Y2 O3 ) are important precursors to transparent ceramics for high-power solid-state lasers systems. As structure influences properties and, subsequently, performance the purpose of this work is to compare the morphological and chemical nature of the nanoparticles synthesized using two common methods: solution precipitation and combustion synthesis. A thorough characterization of as-prepared and calcined powders was performed using Fourier transform infrared spectroscopy, X-ray diffraction, conventional and high-resolution transmission electron microscopy, and Brunauer–Emmet–Teller methods. Solution precipitation was found to lead to two different precursor compositions (yttrium carbonate or yttrium hydroxide) depending on the precipitating reagent whereas combustion synthesis yielded only phase-pure, cubic Er:Y2 O3 . The hydroxide precipitation and combustion synthesis methods exhibited agglomerated particles with low surface area after calcining the precursors at 900°C. The addition of a small amount of ammonium sulfate during combustion synthesis was found to reduce the level of agglomeration, resultant particle size, and degree of crystallinity of the calcined Er:Y2 O3 nanoparticles. The amount of carbon dioxide (CO2 ) and water (H2 O) on the surface of the Er:Y2 O3 powders is dependent on the powder surface area, however, increasing levels of gas absorption on the particle surfaces do not have a detrimental effect on the sinterability. The sintered density increases with increasing surface area and decreasing agglomeration. 相似文献
17.
Phase equilibria have been determined in the system CaO-Al2 O3 -H2 O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)2 , 3CaO·Al2 O3 ·6H2 O, and 4CaO·3Al2 O3 -3H2 O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al2 O3 and 3CaO·Al2 O3 have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed. 相似文献
18.
The effects of heat treatment in Ar-O2 and H2 -H2 O atmospheres on the flexural strength of hot isostatically pressed Si3 N4 were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si3 N4 was exposed at 1400°C to (1) H2 with water vapor pressure ( P H2O ) greater than 1 × 10−4 MPa or (2) Ar with oxygen partial pressure ( P O2 ) of between 7 × 10−6 and 1.5 × 10−5 MPa. However, the strength of the material was degraded when the P H2O in H2 was lower than 1 × 10−4 MPa, and essentially unaffected when the P O2 in Ar was higher than 1.5 × 10−5 MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y2 Si2 O7 layer formed during exposure. 相似文献
19.
Tai-Il Mah Kristin A. Keller Sankar Sambasivan Ronald J. Kerans 《Journal of the American Ceramic Society》1997,80(4):874-878
Heat treatments in several environments were performed on a series of compounds in the Al2 O3 and Y2 O3 system: Al2 O3 Y3 Al5 O12 eutectic, Y3 Al5 O12 , YAlO3 , Y4 Al2 O9 , and Y2 O3 . The yttrium aluminates were found to be stable at high temperatures under vacuum and in air. However, when they were heat-treated under vacuum in proximity to SiC, degradation was observed. This was found to be primarily a result of carbothermal reduction. In a similarly reducing environment without Si, the yttrium aluminates, and Al2 O3 and Y2 O3 , all exhibited degradation by carbothermal reduction. Based upon the experimental results, a degradation mechanism for yttrium aluminates was proposed. 相似文献
20.
The phase equilibrium relations in the systems Y2 O3 -Al2 O3 and Gd2 O3 -Fe2 O3 were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd2 O3 -Fe2 O3 but only metastable in the system Y2 O3 -AI2 O3 . This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O3 . A new 2:1 molar ratio cubic phase has been found in the system Y2 O3 -A12 O3 . Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination. 相似文献