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1.
Gas-pressure sintering of α-Si3 N4 was carried out at 1850 ° to 2000°C in 980-kPa N2 . The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3 N4 powder was similar to that in the materials from β-Si3 N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3 N4 ceramics. 相似文献
2.
Solid-state reactions of equimolar mixtures of Bi2 O3 and Fe2 O3 from 625° to 830°C and their kinetics were investigated. The reaction rates were determined from the integrated X-ray diffraction intensities of the strongest peaks of the reactants and products. The activation energy for the formation of BiFeO3 was 96.6±9.0 kcal/mol; that for a second-phase compound, Bi2 Fe4 O9 , which formed above 675°C, was 99.4±9.0 kcal/mol. Specific rate constants for these simultaneous reactions were obtained. The preparation of single-phase BiFeO3 from the stoichiometric mixture of Bi2 O3 and Fe2 O3 is discussed. 相似文献
3.
Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing 总被引:1,自引:0,他引:1
Uwe Schulz 《Journal of the American Ceramic Society》2000,83(4):904-910
Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO2 stabilized by 7 wt% Y2 O3 were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y2 O3 content in the phases were calculated. At the start of transformation, small grains of a low-Y2 O3 t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y2 O3 , an m phase, and a c phase formed after cooling, with the Y2 O3 contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings. 相似文献
4.
Masahiko Tajika Hideaki Matsubara William Rafaniello 《Journal of the American Ceramic Society》1999,82(6):1573-1575
Thermal diffusivity of AlN-based ceramics was studied as a function of second-phase amount and heat-treatment time. The Y2 O3 ·Al2 O3 contents varied over the range of 13-31 vol%. The thermal diffusivity decreased as the amount of second phase increased. After sintering at 1850°C, the AlN ceramics consisted of rounded, largely isolated grains. Heat treatment of these samples for 5-50 h at 1800°C resulted in microstructures that consisted of largely contiguous AlN grains. There was a substantial increase in the thermal diffusivity after the heat-treatment step, and the incremental improvement was essentially constant for the three compositions that have been studied. The amount of second phase was unchanged during heat treatment; therefore, the increase in thermal diffusivity is assumed to be a direct result of the enhanced contiguity of AlN grains. 相似文献
5.
Porous Cr3 C2 grains (∼300 to 500 μm) with ∼10 wt% of Cr2 O3 were prepared by heating a mixture of MgCr2 O4 grains and graphite powder at 1450° to 1650°C for 2 h in an Al2 O3 crucible covered by an Al2 O3 lid with a hole in the center. The porous Cr3 C2 grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m2 /g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr3 C2 grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics. 相似文献
6.
Nobuyasu Mizutani Chung-Hsin Lu Kuzuo Shinozaki Masanori Kato 《Journal of the American Ceramic Society》1990,73(5):1214-1220
A high-temperature liquid phase (rather than a low-temperature liquid phase at 690°C as reported recently) has been demonstrated to form at 860°C on heating and to solidify at 840°C on cooling in PbFe2/3 O3 . This liquid phase not only promotes densification, but also induces the formation of rounded PbFe2/3 W1/3 O3 grains during sintering at 870°C. Through slow cooling at a rate of 25°C/h after sintering, platelike grains, designated G phase, are found to form in a thin surface layer of specimens. This formation of platelike G phase is considered to be related to the solidification and recrystallization of the liquid phase exuded from the interior. The amount of the G phase on the surfaces decreases with the increase of cooling rates, indicating that fast cooling will lead the liquid phase to be solidified in the bulk of specimens. These results reveal that the microstructure of PbFe2/3 W1/3 O3 is greatly affected by the high-temperature liquid phase; additionally, the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PbFe2/3 W1/3 O3 . 相似文献
7.
Hong Zheng Ian M. Reaney William E. Lee Nik Jones Howard Thomas 《Journal of the American Ceramic Society》2002,85(1):207-212
Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb0.88 Sr0.12 Zr0.54 Ti0.44 Sb0.02 O3 (PSZT) soft piezoelectric ceramics ( d 33 ≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO2 , accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO2 and tetragonal PZT (PbZrO3 –PbTiO3 ) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C. 相似文献
8.
Inger-Lise Tangen Yingda Yu Tor Grande Ragnvald Høier Mari-Ann Einarsrud 《Journal of the American Ceramic Society》2004,87(9):1734-1740
AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al2 O3 mixtures (3.7–16.6 mol% Al2 O3 ) using Y2 O3 (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al2 O3 content in the polytypoids varied systematically with the overall Al2 O3 content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y2 O3 in the polytypoids was demonstrated, and Y2 O3 was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al2 O3 –Y2 O3 parallel to the AlN–Al2 O3 join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids. 相似文献
9.
Mineo Mizuno 《Journal of the American Ceramic Society》1991,74(12):3017-3022
The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575°C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600°C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al2 O3 contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al2 O3 –SiO2 phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase. 相似文献
10.
Phase Transformation of Diphasic Aluminosilicate Gels 总被引:1,自引:0,他引:1
Aluminosilicate gels with compositions Al2 O2 /SiO2 and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC2 H5 )4 . Upon heating the pseudo-boehmite transforms to γ-Al2 O3 around 400°C, then to δ-Al2 O3 at 1050°C, and at 1200°C reacts with amorphous SiO2 to form mullite. Some twinned θ-Al2 O3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al2 O3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape. 相似文献
11.
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
12.
Tzer-Shin Sheu 《Journal of the American Ceramic Society》1994,77(9):2345-2353
The in situ β-Si3 N4 /α'-SiAlON composite was studied along the Si3 N4 –Y2 O3 : 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si3 N4 /α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si3 N4 during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si3 N4 fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m1/2 . α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions. 相似文献
13.
Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects 总被引:1,自引:0,他引:1
The Mode I fracture toughness ( K I C ) of a small-grained Si3 N4 was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si3 N4 grains was <0.4 µm, with aspect ratios of 2–8. K I C at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K IC was observed through 1200°C, to ∼4.1 MPa·m1/2 , before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in K IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K IC . Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C. 相似文献
14.
Mamoru Mitomo Yoh-ichiro Sato Nobuo Ayuzawa Isamu Yashima 《Journal of the American Ceramic Society》1991,74(4):856-858
Plasma etching of β-Si3 N4 , α-sialon/β-Si3 N4 and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si3 N4 grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si3 N4 grains. The size, shape, and distribution of β-Si3 N4 grains in the α-sialon/β-Si3 N4 composite ceramics were revealed by the present method. 相似文献
15.
Alumina Dissolution into Silicate Slag 总被引:1,自引:0,他引:1
Shaowei Zhang Hamid Reza Rezaie Hossain Sarpoolaky William Edward Lee 《Journal of the American Ceramic Society》2000,83(4):897-903
Dissolution of commercial white fused and tabular Al2 O3 grains into a model silicate slag was investigated after 1 h at 1450° and 1600°C. Formation of CA6 and hercynitic spinel layers was observed at all Al2 O3 /slag interfaces. The spinel layer was not always continuous, and so, compared with the CA6 layer, it had a less-significant effect on the dissolution process. The CA6 layer that formed adjacent to the tabular Al2 O3 was incomplete at both temperatures, so that its dissolution was not a totally indirect process. These incomplete CA6 and spinel layers meant that slag penetrated into the tabular Al2 O3 grains, which, thus, were corroded and disintegrated by the penetrating slag. There was evidence of liquid in the CA6 layer adjacent to the fused Al2 O3 after 1 h at 1450°C, which also enabled direct dissolution. After 1 h at 1600°C, fused Al2 O3 revealed a thick (∼60 μm), continuous and unpene-trated CA6 layer, indicating fully indirect dissolution at this temperature. 相似文献
16.
Feng Ye Michael J. Hoffmann Stefan Holzer Yu Zhou Mikio Iwasa 《Journal of the American Ceramic Society》2003,86(12):2136-2142
The yttrium–sialon ceramics with the composition of Y0.333 Si10 Al2 ON15 and an excess addition of Y2 O3 (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y2 O3 could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K IC = 6.3 MPa·m1/2 ) via grain debonding and pullout effects. Although the addition of 5 wt% Y2 O3 could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness. 相似文献
17.
Zong-Yang Shen Yuhua Zhen Ke Wang Jing-Feng Li 《Journal of the American Ceramic Society》2009,92(8):1748-1752
Microstructure characteristics, phase transition, and electrical properties of (Na0.535 K0.485 )0.926 Li0.074 (Nb0.942 Ta0.058 )O3 (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d 33 is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO3 ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k p =46.2%, thickness electromechanical coupling factor k t =36%, mechanical quality factor Q m =18, remnant polarization P r =21.1 μC/cm2 , and coercive field E c =1.85 kV/mm. 相似文献
18.
Compression creep tests were performed on fully dense specimens of UC1.01 , UC1.05 , UC1.01 .+ 4 wt% W, and U0.9 Zr0.1 C1.01 + 4 wt% W. Steady-state creep rates were measured from 1400° to 1800°C in a vacuum of 1.33 × 10-3 N/m2 (1 × 10-5 torr) at stresses of 4.55 to 69.0 MN/m2 (660 to 10,000 psi). The data for UC1.01 could best be fit by an expression of the form ɛ= 1773σ6.024 exp (106.5/RT) , where σ is the steady-state creep rate (h-l ), σ is the applied stress (MN/m2 ), and the creep activation energy is given in kcal/mol. The stress dependence for creep of UC1.05 decreased with decreasing temperature because of second-phase precipitation; therefore, a unique creep activation energy could not be established for this U/C ratio. At all temperatures, the creep strength of UC1.05 exceeded that of UC1.01 . For example, at 1700 ° C steady-state creep rates for UC1.05 are ∼1/4 those for UC1.01 , but at 1400°C the creep rates are ∼ 3 orders of magnitude less. At 1700°C, creep rates for UC alloys are ∼4 orders of magnitude lower than those for unalloyed UC1.01 . 相似文献
19.
The thermodynamic data for the YO1.5 –BaO, BaO-CuOx , and YO1.5 –CuOx quasi-binary systems were optimized from experimental phase diagrams. They were used to calculate tentative phase diagrams for the YO1.5 –BaO—CuOx quasi-ternary system. The equilibrim liquidus surface and the isothermal sections of the ternary system at 900°, 925°, 950°, 975°, and 1000°C were calculated. The isopleths containing YBa2 Cu3 O7-δ were also calculated. 相似文献
20.
T. D. CHIKALLA 《Journal of the American Ceramic Society》1963,46(7):323-328
The melting points of UO2 and PuO2 in a helium atmosphere were found to be 2730°× 30° C and 2280°× 30° C, respectively. With the exception of a melting maximum at the composition 90 UO2 –10 wt% PuO2 , the liquidus exhibits good continuity and agrees well with that calculated from thermodynamic data. X-ray diffraction data on melted PuO2 and UO2 -PuO2 solid solutions indicate that oxygen is evolved during melting but that no reduction to a second-phase plutonium suboxide occurs. The oxygen-plutonium atomic ratio of melted PuO2 is 1.62, so that the 2280° C temperature reported here is the result of a dissociation reaction and is considered to be a pseudo melting point. Lattice parameters of melted UO2 –PuO2 samples vary linearly with composition but are approximately 0.2% greater than anticipated because of an oxygen deficiency. 相似文献