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1.
The eutectic composition between Y4 Al2 O9 and Y2 O3 was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y2 O3 and 29.5 mol% Al2 O3 ). Based on the current results and other published data, the pseudobinary system between Al2 O3 and Y2 O3 is revised. 相似文献
2.
W. J. BUYKX 《Journal of the American Ceramic Society》1979,62(7-8):326-332
Several microstructures were produced in dense polycrystalline UO2 -U4 O9 specimens by cooling UO2+x , solid solutions at different rates. Void formation (intragranular pores, grain-boundary microcracks) was observed in specimens which had a nonuniform distribution of the U4 O9 precipitate; such formation was probably caused by the relief of stresses generated by differential thermal contraction. The microstructures were assessed quantitatively and used to calculate thermal conductivities from theoretical models. The effect of microcracking was calculated. Thermal conductivities were also determined from thermal diffusivity values measured by the laser-flash method. The thermal conductivity was primarily influenced by the amount of U4 O9 , present and the occurrence of voids, whereas the effect of U4 O9 , distribution was minor. Good agreement was obtained between theoretical predictions of thermal conductivity based on microstructure and experimental values. 相似文献
3.
JOHN M. KERR 《Journal of the American Ceramic Society》1967,50(9):449-453
A device was built to provide the capability for out-reactor simulation of the microstructures seen in irradiated fuel. Out-reactor simulation of nuclear heating makes possible the precise measurement of temperature gradients in the fuel and provides data for correlating microstructural alterations with temperature. The simulation apparatus was designed to produce controlled thermal gradients sufficient to produce both equiaxed and columnar grain growth in the specimens. Provision was made for calorimetric measurements from which thermal conductance could be determined. Micro-structural alterations in UO2 and ThO2 -UO2 materials were achieved, and thermal conductivity data were recorded. Microstructural alterations in the ThO2 -10 wt% UO2 specimens were similar to those that other investigators have observed in UO2 at lower temperatures. 相似文献
4.
Crack growth resistance studies of a Sic-whisker-reinforced Al2 O3 -matrix composite have been correlated with the composite microstructure to determine the active fracture toughening mechanism, at each of three test temperatures through 1400°C. Evidence of cumulative toughening at all temperatures, as reported in the literature, was validated by R -curves; however, isolation of the following wake zone effects from that of the frontal process zone elicits a departure from published assumptions. A frontal zone mechanism, presumably microcrack toughening, dominates at room temperature, while a following wake zone mechanism of crack face whisker-bridging controls at temperatures near 1200°C. 相似文献
5.
Hyoungjoon Park Hae-Won Kim Hyoun-Ee Kim 《Journal of the American Ceramic Society》1998,81(8):2130-2134
The oxidation behaviors of monolithic Si3 N4 and nanocomposite Si3 N4 -SiC with Yb2 O3 as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb2 Si2 O7 and SiO2 (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si3 N4 -SiC compared to the monolithic Si3 N4 . The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb3+ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented. 相似文献
6.
Jian-Xi Tong Qi-Long Zhang Hui Yang Jia-Li Zou 《Journal of the American Ceramic Society》2007,90(3):845-849
The effects of LiF and ZnO–B2 O3 –SiO2 (ZBS) glass combined additives on phase composition, microstructures, and microwave dielectric properties of Ca[(Li1/3 Nb2/3 )0.84 Ti0.16 ]O3−δ (CLNT) ceramics were investigated. The LiF and ZBS glass combined additives lowered the sintering temperature of CLNT ceramics effectively from 1150° to 880°C. The main diffraction peaks of all the specimens split due to the coexistence of the non-stoichiometric phase (A) and stoichiometric phase (B), which all possess CaTiO3 -type perovskite structures. The transformation from A into B became accelerated with the increase of LiF or ZBS content. ZBS glass restrained the volatilization of lithium salt, which greatly affected the microstructures and microwave dielectric properties. CLNT ceramics with 2 wt% LiF and 3 wt% ZBS sintered at 900°C for 2 h show excellent dielectric properties: ɛr =34.3, Q × f =17 400 GHz, and τf =−4.6 ppm/°C. It is compatible with Ag electrodes, which makes it a promising ceramic for low-temperature cofired ceramics technology application. 相似文献
7.
Transformation-toughened MgO-partially-stabilized ZrO2 exhibits a crack-wake-induced load-point displacement in fracture samples. The equations of linear elastic fracture mechanics were self-consistently reformulated to include the residual displacement from the transformation wake. Application of these equations to double-cantilever-beam specimens of transformation-toughened ZrO2 , with the martensitic start transformation temperature close to the testing temperature, gives a decreasing KR curve and initial toughness values near 30Mpa·ml/2 . 相似文献
8.
John J. Petrovic Maria I. Pena H. Harriett Kung 《Journal of the American Ceramic Society》1997,80(5):1111-1116
Details of the fabrication and microstructures of hot-pressed MoSi2 reinforced–Si3 N4 matrix composites were investigated as a function of MoSi2 phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi2 and Si3 N4 at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi2 phase content. The MgO densification aid was present only in the Si3 N4 phase. An amorphous glassy phase was observed at the MoSi2 –Si3 N4 phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi2 –Si3 N4 composites, despite the presence of a substantial thermal expansion mismatch between the MoSi2 and Si3 N4 phases. The critical MoSi2 particle diameter for microcracking was calculated to be 3 μm. MoSi2 particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi2 phase. 相似文献
9.
The effects of various liquid-phase sintering aids on (Pb0.6 SrO0.4 )TiO3 ceramics have been investigated. The relationships between electrical properties and microstructures have been scrutinized. It has been found that, among the sintering aids studied, only SiO2 exhibits a significant effect on the grain growth of (Pb0.6 SrO4 )TiO3 . The optimum firing profiles for sound microstructure and good electrical properties of (Pb0.6 SrO0.4 )TiO3 + 5.0 mol% SiO2 have been established. The V-shaped electrical behavior is prominent, and a PTCR jump of about 102.9 is observed. The formation of cation vacancies may increase the resistivity of the over-fired specimens. Various milling methods to pulverize the calcined powder and the optimum amount of packing protection powder during sintering are also discussed. 相似文献
10.
Sintering studies of Si3 N4-Y2 O3/Al2 O3 compositions indicated that increased densification could be produced using a dual N2 pressure process. RT modulus of rupture values and microstructures were found to be similar to hot-pressed material. The technique should be adaptable to complex shaped Si3 N4 bodies where high density and strength are required . 相似文献
11.
The microwave dielectric properties and the microstructures of (Mg1− x Co x )2 TiO4 ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg1− x Co x )2 TiO4 ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛr ∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τf ∼−52.5 ppm/°C) was achieved for (Mg0.95 Co0.05 )2 TiO4 ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg0.95 Co0.05 )TiO3 was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications. 相似文献
12.
Xingsen Gao Zhaohui Zhou Junmin Xue John Wang 《Journal of the American Ceramic Society》2005,88(4):1037-1040
Effects of excess Bi2 O3 content on formation of (Bi3.15 Nd0.85 )Ti3 O12 (BNT) films deposited by RF sputtering were investigated. The microstructures and electrical properties of BNT thin films are strongly dependent on the excess Bi2 O3 content and post-sputtering annealing temperature, as examined by XRD, SEM, and P – E hysteresis loops. A small amount of excess bismuth improves the crystallinity and therefore polarization of BNT films, while too much excess bismuth leads to a reduction in polarization and an increase in coercive field. P – E loops of well-established squareness were observed for the BNT films derived from a moderate amount of Bi2 O3 excess (5 mol%), where a remanent polarization 2P r of 25.2 μC/cm2 and 2E c of 161.5 kV/cm were shown. A similar change in dielectric constant with increasing excess Bi2 O3 content was also observed, with the highest dielectric constant of 304.1 being measured for the BNT film derived from 5 mol% excess Bi2 O3 . 相似文献
13.
Won-Ho Lee Hyoun-Ee Kim Seong-Jai Cho 《Journal of the American Ceramic Society》1997,80(10):2737-2740
Microstructural evolution of gas-pressure-sintered Si3 N4 with Yb2 O3 as a sintering aid was observed. Microstructures typical for in situ toughened Si3 N4 , i.e., large elongated grains randomly distributed in a fine matrix, were observed. However, the size of the elongated grains near the surface was much larger than that at the center, resulting in two distinct regions: an inner region and an outer region. The smaller the amount of Yb2 O3 added, the larger the difference in the size of the elongated grains between the outer and inner regions. The difference between microstructures was diminished when 16 wt% Yb2 O3 was added. The microstructural change with Yb2 O3 content was attributed to the evaporation of Yb-containing liquid phase from the surface. 相似文献
14.
Composites containing 30 vol%β-Si3 N4 whiskers in a Si3 N4 matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m1/2 , compared to 4.0 MPa · m1/2 for unreinforced polycrystalline Si3 N4 . The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms. 相似文献
15.
Won-Hyuk Lee Yoshihiro Abe Eikichi Inukai 《Journal of the American Ceramic Society》1993,76(4):849-856
Ag2 O-doped superconducting Bi2 Sr2 Ca1 Cu2 O x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag2 O-doped, as-cast specimens exhibit superconductivity ( T c = around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag2 O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag2 O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi2 Sr2 Ca1 Cu2 O x ceramics sealed in metals or glasses. The addition of Ag2 O to Bi2 Sr2 Ca1 Cu2 O x melt, however, had deleterious influences on the superconducting properties ( T c and J c ) of the resultant ceramics when obtained by heat treatment in air. 相似文献
16.
Liang Fang Hui Zhang Qian Yu Heping Su Bolin Wu Xueming Cui 《Journal of the American Ceramic Society》2009,92(2):556-558
The microwave dielectric properties of dense ceramics of a new A4 B3 O12 type cation-deficient hexagonal perovskite Sr3 LaNb3 O12 are reported. Single-phase powders can be obtained from the mixed-oxide route at 1320°C and dense ceramics (>96% of the theoretical X-ray density) with uniform microstructures (5–12 um) can be obtained by sintering in air at 1430°C. The ceramic exhibits a moderate dielectric constant ɛr ∼36, a high quality factor Q × f ∼45 327 GHz, and a low temperature coefficient of resonant frequency τ f of −9 ppm/°C. 相似文献
17.
Mark I. Jones Hideki Hyuga Kiyoshi Hirao Yukihiko Yamauchi 《Journal of the American Ceramic Society》2004,87(4):710-713
α-SiAlONs with equiaxed and elongated microstructures stabilized with Y2 O3 and Lu2 O3 were produced by hot pressing, and the phase structure and room- and high-temperature mechanical properties were assessed. Additional liquid added to the starting composition in the form of 5 wt% rare-earth monosilicate resulted in the formation of elongated microstructures and improvements in room-temperature strength and fracture toughness. The elongated grain growth was promoted by the additional liquid phase, which crystallized to form a secondary grain-boundary phase thought to be J ' (Re4 Si2– x Al x O7+ x N2– x ). For the equiaxed and the elongated samples, those sintered with Lu2 O3 showed higher hardness than the comparable Y2 O3 -sintered materials, and, at elevated temperature, the strength retention of the elongated Lu2 O3 SiAlON was much higher than that of the Y2 O3 sample, which was attributed to properties of the residual grain-boundary phase associated with the difference in the cationic radius of the stabilizing cation. 相似文献
18.
Jonathan D. French Helcn M. Chan Martin P. Harmer Gary A. Miller 《Journal of the American Ceramic Society》1992,75(2):418-423
Composites of Al2 O3 (A) and cubic ZrO2 (Z) (8 mol% Y2 O3 ) (with c -ZrO2 volume fractions ranging from O to 1) were fabricated by pressureless sintering of mechanically mixed powders. The microstructures of the AZ composites were duplex, with the grains of both phases exhibiting similar size. Room-temperature mechanical properties including Young's modulus (determined from elastic wave velocity measurements), strength and toughness (by indentation-strength-in-bending), and Vickers microhardness were evaluated for the full range of compositions. All properties exhibited a linear decrease with increasing c -ZrO2 content, and no R -curve behavior was seen in any of the composite compositions. Fracture morphology, observed from cracks emanating from microhardness indentations, changed from essentially intergranular for the tougher Al2 O3 to transgranular for c -ZrO2 , with AZ composites exhibiting mixed morphology. 相似文献
19.
The 1:1 compound, CeA1O3 , in the system Ce2 O3 –Al2 O3 has been synthesized from the oxides and shown to have a perovskite-like tetragonal unit cell with the lattice parameters a = 3.763 and c = 3.792 Å. A new XRD pattern is suggested for CeA1O3 . This compound is shown to be stable up to 1950°C. The 1:11 compound, CeAl11 O18 , has also been synthesized and shown to possess a magnetoplumbite-like hexagonal unit cell with the lattice parameters α= 5.558 and c = 22.012 å. An XRD pattern is suggested for CeAl11 O18 for the first time. The evolution of eutectic-like microstructures was observed and reported in the Ce2 O3 -rich side of this binary system. 相似文献
20.
Powders of composition Ba0.65 Sr0.35 TiO3 were prepared from catecholate precursor phases, BaTi(C6 H4 O2 )3 and SrTi (C6 H4 O2 )3 . The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000. 相似文献