共查询到20条相似文献,搜索用时 31 毫秒
1.
Hae-Won Kim Young-Hag Koh Hyoun-Ee Kim 《Journal of the American Ceramic Society》2000,83(11):2863-2865
The densification behavior and mechanical properties of B4 C hot-pressed at 2000°C for 1 h with additions of Al2 O3 up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al2 O3 . The improvement was attributed to the enhanced mobility of elements through the Al2 O3 near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al2 O3 exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B4 C and Al2 O3 . 相似文献
2.
Fabrication and Microstructure of Al2 O3 –SiC–YAG Hybrid Composites Prepared by Particulate Dispersion
Stable Al2 O3 –SiC–YAG hybrid composites were successfully fabricated by reaction of Al2 O3 and Y2 O3 and incorporation of SiC. The hot-pressed bodies consisted of uniformly dispersed grains of microsized YAG particulates and nanosized SiC particulates in an Al2 O3 matrix. Although the grain size of monolithic A12 O3 increases markedly with increased temperature, the grain size of the Al2 O3 –SiC–YAG hybrid composites was effectively restrained due to grain-boundary pinning by the particulates. 相似文献
3.
Koji Tsukuma Kuniyoshi Ueda Masahiko Shimada 《Journal of the American Ceramic Society》1985,68(1):4-C-5
Composites of Al2 O3 and Y2 O3 partially-stabilized ZrO2 were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al2 O3 resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al2 O3 was 2400 MPa, and its fracture toughness was 17 MN·w−3/2 相似文献
4.
The effect of Al8 B4 C7 used as an antioxidant in MgO–C refractories and the behavior of Al8 B4 C7 in CO gas were investigated in the present study. Al8 B4 C7 was found to react with CO gas, to form Al2 O3 ( s ), B2 O3 ( l ), and C( s ), at temperatures >1100°C. The Al2 O3 reacts with MgO to form MgAl2 O4 near the surface of the material. At the same time, B2 O3 ( l ) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B2 O3 and MgO. The coexistence of the liquid and MgAl2 O4 makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C. 相似文献
5.
Yury G. Gogotsi Franz Porz Viktor P. Yaroshenko 《Journal of the American Ceramic Society》1992,75(8):2251-2259
The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al2 O3 –42 wt% TiN, differing in the AIN/Al2 O3 ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al2 O3 and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al2 O3 -rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters. 相似文献
6.
Oxidation studies were conducted on Al2 O3 -SiC and mullite-SiC composites at 1375° to 1575°C in O2 and in Ar-1% O2 . The composites were prepared by hot-pressing mixtures of Al2 O3 or mullite and SiC powders. The reaction products contained alumina, mullite, an aluminosilicate liquid, and gas bubbles. The parabolic rate constants were about 3 orders of magnitude higher than those expected for the oxidation of SiC. Higher rates are caused by higher oxygen permeabilities through the reaction products than through pure silica. Our results suggest that oxygen permeabilities are comparable in the three condensed phases observed in the reaction products. 相似文献
7.
Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2 总被引:1,自引:0,他引:1
Takayuki Ide Katsutoshi Komeya Takeshi Meguro Junichi Tatami 《Journal of the American Ceramic Society》1999,82(11):2993-2998
We investigated the effect of characteristics of raw Al2 O3 powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF2 was added as a catalytic material. Four types of Al2 O3 powders were selected. An Al2 O3 /C molar ratio of 0.29 was fixed, and the amount of CaF2 was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N2 flow. The nitridation rate tended to increase with decreasing particle size of raw Al2 O3 and was found to depend on the Al2 O3 synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al2 O3 powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al2 O3 and CaO·2Al2 O3 were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al2 O3 with CaF2 addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF2 -CaO·6Al2 O3 -CaO·2Al2 O3 . 相似文献
8.
Phase Transformation and Thermal Conductivity of Hot-Pressed Silicon Carbide Containing Alumina and Carbon 总被引:1,自引:0,他引:1
Phase transformation and thermal conductivity of hot-pressed β-SiC with Al2 O3 and carbon additions were studied. Densification rate was a complex function of both Al2 O3 and carbon. Simultaneous additions of Al2 O3 and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high-temperature hot-pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other. 相似文献
9.
Koji Matsui Takanori Yamakawa Masato Uehara Naoya Enomoto Junichi Hojo 《Journal of the American Ceramic Society》2008,91(6):1888-1897
The isothermal shrinkage behavior of 2.9 mol% Y2 O3 -doped ZrO2 powders with 0–1 mass% Al2 O3 was investigated to clarify the effect of Al2 O3 concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al2 O3 addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β0 n ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al2 O3 addition and both nQ and β0 n increased with increasing Al2 O3 concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al2 O3 addition largely depends on the increase of β0 n , that is, the increases of n with GBD→VD change and β0 with an increase in Al2 O3 content, although the nQ also increases with Al2 O3 addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al2 O3 at ZrO2 grain boundaries. 相似文献
10.
Asoke C. D. Chaklader Sankar Das Gupta Edmond C. Y. Lin Boris Gutowski 《Journal of the American Ceramic Society》1992,75(8):2283-2285
Al2 O3 and SiC composite materials have been produced from mixtures of aluminosilicates (both natural minerals and synthetic) and carbon as precursor materials. These composites are produced by heating a mixture of kaolinite (or synthetic aluminosilicates) and carbon in stoichiometric proportion above 1550°C, so that only Al2 O3 and SiC remain as the major phases. A similar process has also been used for synthesizing other composite powders having mixtures of Al2 O3 , SiC, TiC, and ZrO2 in different proportions (all compounds together or selective mixtures of some of them), as desired. The microstructure of hot-pressed dense compacts, produced from these powders, revealed that the SiC phase is distributed very homogeneously, even occasionally within Al2 O3 grains on a nanosize scale. The homogeneous distribution of SiC particles within the system produced high fracture toughness of the hot-pressed material (KIC ∼ 7.0 MPa · m1/2 ) and having Vicker's hardness values greater than 2000 kgf/mm2 . 相似文献
11.
Thermochemical calculations are performed to evaluate the chemical compatibility of oxides with carbon at temperatures of the order of 1650°C and to evaluate the implications of carbon oxidation. These calculations indicate that Al2 O3 , BeO, Y2 O3 , and Ce2 O3 are stable with carbon provided there are no cracks or pores for CO gas to escape. In the presence of cracks or interconnected pores, both the direct reduction of oxides with carbon as well as the oxidation of carbon can proceed rapidly. The calculations also indicate that even in the absence of such cracks the oxidation of carbon can generate high enough CO partial pressures to degrade the composite properties by formation of CO gas bubbles or of cracks in the oxide. The results of these calculations are confirmed by conducting experiments on samples of pyrolytic and vitreous carbon disks hot-pressed within Al2 O3 and Y2 O3 disks and exposing them in argon and oxygen environments at 1650–1800°C. An oxidation model is then proposed that includes thermodynamic and kinetic considerations as well as creep behavior of oxides. The model quantitatively predicts the times required to initiate the formation of gas bubbles and qualitatively considers the factors influencing the growth rate of bubbles and formation of cracks. 相似文献
12.
F. F. LANGE 《Journal of the American Ceramic Society》1971,54(12):614-620
Fracture energy and strength were determined for three series within a sodium borosilicate glass-Al2 O3 dispersed composite system. The average particle sizes of the Al2 O3 dispersions were
, and
μm. Within each series, composites containing 0.10, 0.25, and 0.40 vol fractions of the Al2 O3 dispersed phase were vacuum hot-pressed. The fracture energy was determined at 77°K with the double cantilever specimen configuration. Strength was measured by a 4-point flexural test. A significant increase in fracture energy was observed (up to 5 times the fracture energy of the glass without second-phase dispersion). The fracture energy depended on the interparticle spacing and average particle size of the Al2 O3 dispersion. These results could best be explained by a previously proposed model for the interaction of a crack front with a second-phase dispersion. Surface roughness also contributed to the increased fracture energy. Some composites were strengthened significantly relative to the glass without a dispersion. Calculation of the crack size showed that the Al2 O3 dispersion increased the crack size of the glass by ∼1 to 3 times the average particle size of the Al2 O3 dispersion. Thus, the dispersion increased both the fracture energy and the crack size. These opposing parameters ultimately determined the strength behavior of these composites. 相似文献
, and
μm. Within each series, composites containing 0.10, 0.25, and 0.40 vol fractions of the Al
13.
Hyun M. Jang Jong H. Moon Cheol W. Jang 《Journal of the American Ceramic Society》1992,75(12):3369-3376
Al2 O3 –ZrO2 –SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO2 phase on the kinetically stable colloid particles of Al2 O3 and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO2 and SiC whisker phases throughout the Al2 O3 matrix. The fracture toughness values of the Al2 O3 –15 vol% ZrO2 –20 vol% SiC whisker composites (∼12 MPa.m1/2 ) are substantially greater than those of comparable Al2 O3 –SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites. 相似文献
14.
Jalajakumari Nair Padmakumar Nair Fujio Mizukami Jan G. Van Ommen Giel B. M. Doesburg Julian R. H. Ross Anthonie J. Burggraaf 《Journal of the American Ceramic Society》2000,83(8):1942-1946
Pure Al2 O3 and different compositions of La2 O3 –Al2 O3 samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La2 O3 ·11Al2 O3 and La2 O3 ·13Al2 O3 had higher surface area compared to the pure Al2 O3 and the La2 O3 ·Al2 O3 composition. Ethanol washing is an effective way for improving the textural stability of pure Al2 O3 and La2 O3 –Al2 O3 samples. The effect of steam on the thermal stability of La2 O3 ·11Al2 O3 has also been studied. La2 O3 ·11Al2 O3 sample is found to be stable in steam. 相似文献
15.
Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina
The microstructure of two hot-pressed silicon nitrides containing Y2 O3 and Al2 O3 was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y2 O3 +2 wt% Al2 O3 , as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y2 O3 +5 wt% Al2 O3 was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si3 N4 grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y2 O3 /Al2 O3 fluxing. 相似文献
16.
The electrical conductivity and ion/electron transference numbers in Al3 O3 were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O2 partial pressure over single-crystal Al2 O3 at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O2 (1 atm) the conductivity above 1400°C was σ≃3×103 exp (–80 kcal/ RT ) Ω−1 cm−1 , which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al2 O3 and MgO-doped Al2 O3 showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O2 partial pressure (below 10−10 atm). 相似文献
17.
Satoshi lio Masakazu Watanabe Masaru Matsubara Yasushi Matsuo 《Journal of the American Ceramic Society》1989,72(10):1880-1884
The improvement of mechanical properties of Al2 O3 /SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al2 O3 matrix. 相似文献
18.
Koji Matsui Nobukatsu Ohmichi Michiharu Ohgai Naoya Enomoto Junichi Hojo 《Journal of the American Ceramic Society》2005,88(12):3346-3352
The sinterabilities of fine zirconia powders including 5 mass% Y2 O3 were investigated, with emphasis on the effect of Al2 O3 at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al2 O3 increased the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact including Al2 O3 was lower than that of a powder compact without Al2 O3 . The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al2 O3 included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al2 O3 enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage. 相似文献
19.
Corrosion Behavior of Alumina Ceramics in Caustic Alkaline Solutions at High Temperatures 总被引:2,自引:0,他引:2
Tsugio Sato Shigeki Sato Akitsugu Okuwaki Syun-ichiro Tanaka 《Journal of the American Ceramic Society》1991,74(12):3081-3084
The corrosion rate and changes in the microstructure and fracture strength of alumina ceramics (93.0% Al2 O3 and 99.5% Al2 O3 ) were studied in 0.1 m to 25 m NaOH solutions at 150°C to 200°C, where m = mol/(kg of H2 O). The attack of the caustic alkaline solution started at the grain boundaries. Consequently, the corrosion resistance increased with decreasing SiO2 content in Al2 O3 ceramics, and the corrosion resistance of 99.5% pure Al2 O3 was similar to that of Si3 N4 ceramics. Since large pits are formed by corrosion, the surface area increased first and the apparent corrosion rate increased with time in the initial stage of the corrosion. The corrosion rate of Al2 O3 increased linearly with increasing NaOH concentration, and the activation energy was 102 kJ/mol. The fracture strength of corroded Al2 O3 decreased monotonically as the degree of dissolution of alumina increased. 相似文献
20.
Reaction-bonded Al2 O3 (RBAO) ceramics were fabricated starting from mechanically alloyed Al2 O3 /Al, Al2 O3 / Al/ZrO2 , and Al2 O3 /Al/ZrO2 /Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO2 and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al2 O3 , it is 112 and 152 kJ/mol at ∼64% and ∼74% TD, respectively, while above the melting temperature, it lies in the range ∼ 26–33 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO2 additions exhibit nearly the same activation energies as ZrO2 -free samples, though ZrO2 has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported. 相似文献