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1.
Synthesis of Glass-like Cordierite from Metal Alkoxides and Characterization by 27 Al and 29 Si MASNMR
Ulagaraj Selvaraj Sridhar Komarneni Rustum Roy 《Journal of the American Ceramic Society》1990,73(12):3663-3669
A sol-gel process for the preparation of glass-like cordierite and ceramic oxide powder is described. Metal alkoxide precursors were dissolved in 2-methoxyethanol and the metal cations were complexed using 2,4-pentanedione to overcome different hydrolysis rates of metal alkoxides which lead to microscopic inhomogeneities during gelation. Heating the gel in an aerated furnace resulted in ultrafine cordierite powder of stoichiometric composition at a relatively low temperature. Cordierite gel and glass were also prepared by other methods and compared with the above gel. The environments of aluminum and silicon in the glass and gels heated at various temperatures were studied using 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Most of the heated gels and the glass showed resonances due to pentacoordinated aluminum along with 4- and 6-coordinated aluminum. The homogeneities of the resulting gels and glass are compared using the MASNMR data. 相似文献
2.
M. Backhaus-Ricoult V. Guerin A.-M. Huntz V. S. Urbanovich 《Journal of the American Ceramic Society》2002,85(2):385-392
High-temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive-free α-, β-, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in α-phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si3 N4 phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO2 crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the α-polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen-rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. β-Si3 N4 is observed to form oxygen-rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen-rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse-grained oxidation scales with thick intergranular glass film. 相似文献
3.
Stephen Kwan Judith LaRosa Michael W. Grutzeck 《Journal of the American Ceramic Society》1995,78(7):1921-1926
Strätlingite (2CaO·Al2 O3 ·SiO2 ·8H2 O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag-containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al2 O3 ·6H2 O)-katoite (3CaO·Al2 O3 ·SiO2 ·4H2 O)] and calcium silicate hydrate (C-S-H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using 29 Si, with and without cross-polarization (CP), and 27 Al solid-state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q2 , Q2 (1Al), and Q2 (2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the AlIV /A1VI ratio. 相似文献
4.
Daniel J. O'Donnell T. T. Peter Cheung George F. Schuette Maziar Sardashti 《Journal of the American Ceramic Society》1991,74(8):2025-2028
Solid-sample magic angle spinning (MAS) nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS), in conjunction with scanning electron microscopy (SEM), were used to investigate the fate of boron used as a sintering aid for silicon carbide. The results of the NMR studies indicated that the boron penetrated the silicon carbide grain boundaries during sintering, and was incorporated in a tetrahedral form in the bulk, regardless of the gas used during the process. The NMR spectrum of a sample sintered under nitrogen indicated the formation of a trigonal form of boron as well. XPS identified this trigonal boron as boron nitride; however, no boron was detected by XPS in any form on the fracture surface of the silicon carbide sintered under argon, even though the NMR results confirmed the presence of tetrahedral boron in the bulk sample. The SEM results indicated that the fracture process for these materials was predominantly intergranular. This suggested that the boron in the silicon carbide sintered under argon penetrated the grains and left the grain boundaries depleted of boron. 相似文献
5.
Hideki Ishida Yoshihiko Okada Takeshi Mitsuda 《Journal of the American Ceramic Society》1992,75(2):359-363
The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca2 (SiO3 )(OH)2 ) was studied over a period of 7 to 224 d using 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q2 and Q1 silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q1 and Q2 formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q2 /Q1 increases as the reaction progresses and as the curing time becomes longer. From the values of Q2 /Q1 , it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q2 /Q1 ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD. 相似文献
6.
Steve W. Martin Debra Bain Karim Budhwani Steven Feller 《Journal of the American Ceramic Society》1992,75(5):1117-1122
7.
Guo-Dong Zhan Mamoru Mitomo Toshiyuki Nishimura Rong-Jun Xie Taketo Sakuma Yuichi Ikuhara 《Journal of the American Ceramic Society》2000,83(4):841-847
The deformation behavior of a hot-pressed, fine-grained β-Si3 N4 ceramic was investigated in the temperature range 1450°—1650°C, under compression, and the results for strain rate and temperature dependence of the flow stress are presented here. The present results show that the material is capable of high rates of deformation (∼10−4 —10−3 s−1 ) within a wide range of deformation temperatures and under a pressure of 5—100 MPa; no strain hardening occurs in the material, even at slow deformation rates, because of its stable microstructure; Newtonian flow occurs, with a stress exponent of approximately unity; and the material has activation energy values for flow in the range 344—410 kJ·mol−1 . Grain-boundary sliding and grain rotation, accommodated by viscous flow, might be the mechanisms of superplasticity for the present material. 相似文献
8.
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
9.
Whiskers of α- and β-Si3 N4 were grown on or near a carbon black substrate, respectively, 10 mm downstream from a mixed starting powder of low-grade silica and carbon, in flowing nitrogen gas at 1400°C. The parameters (flowing nitrogen gas rate, growth time, grade of silica, and type of carbon) that promoted growth of the whiskers were examined in view of increasing the whisker yield. The shapes and sizes of both types of whiskers were observed by scanning electron microscopy (SEM). The separate growth of the whiskers is discussed here, based on X-ray diffraction analysis and SEM observation with energy-dispersive X-ray analysis. 相似文献
10.
Dong-Duk Lee Suk-Joong L. Kang Gunter Petzow Duk N. Yoon 《Journal of the American Ceramic Society》1990,73(3):767-769
By using α-Si3 N4 and β-Si3 N4 starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si3 N4 ·9Al2 O3 ·9Y2 O3 (wt%) and 80Si3 N4 ·13Al2 O3 ·5AIN·5AIN·2Y2 O3 . When α-Si3 N4 powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification. 相似文献
11.
Young-Hag Koh Hae-Won Kim Hyoun-Ee Kim 《Journal of the American Ceramic Society》2002,85(11):2840-2842
A three-layered composite, composed of a strong outer layer (monolithic S3 N4 ) and a tough inner layer (fibrous Si3 N4 /BN monolith), was fabricated by hot-pressing. For the inner layer, a Si3 N4 –polymer fiber made by extrusion was coated by dipping it into a 20 wt% BN-containing slurry. The three-layered composite exhibited excellent mechanical properties, including high strength, work of fracture, and crack resistance, because of the combination of a strong outer layer and a tough inner layer. In other words, the strong outer layer withheld the applied stress, while the tough inner layer promoted crack interactions through the weak BN cell boundaries. Also, the residual thermal stress on the surface due to the anisotropy in the coefficient of thermal expansion of BN affected a median/radial crack generation after indentation. 相似文献
12.
Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics 总被引:1,自引:0,他引:1
A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary. 相似文献
13.
Shinichi Sakida Satoshi Hayakawa Toshinobu Yoko 《Journal of the American Ceramic Society》2001,84(4):836-842
The structures of M2 O3 –TeO2 (M = Al and Ga) glasses have been investigated by means of 125 Te, 27 Al, and 71 Ga NMR spectroscopies. The structural units of respective cations in M2 O3 –TeO2 glasses were quantitatively analyzed. The fractions of TeO4 trigonal bipyramid, AlO6 and GaO6 octahedra decreased and those of TeO3 trigonal pyramid, AlO4 , AlO5 , and GaO4 polyhedra increased with increasing M2 O3 content. Based on the local structures around Te, Al, and Ga atoms, the structure models of M2 O3 –TeO2 glasses were proposed. 相似文献
14.
Joseph Homeny Lynn J. Neergaard Keith R. Karasek Jeffry T. Donner Steven A. Bradley 《Journal of the American Ceramic Society》1990,73(1):102-105
The physical, chemical, and structural properties of a commercially available β-Si3 N4 whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y-rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM analysis. 相似文献
15.
The microstructure of silicon nitride containing different percentages of β-seeds was investigated. The average grain size and volume fraction of large grains increased with the incorporation of β-seeds. The length and aspect ratio of large grains in sintered Si3 N4 initially increased by incorporating β-seeds, and then decreased. Similar trends were observed in the fracture toughness. The toughening mechanisms and fracture behaviors were correlated with the grain morphology measured from image analysis. Plots of measured toughness versus volume fraction of large grains fit very well with the equation derived by Becher and Budiansky, which strongly suggested the essential role of debonding length in the toughness of silicon nitride. Further improvement of fracture toughness in silicon nitride may be possible by tailoring the amount of incorporated β-phase seeds and leading to optimize the volume fraction, grain size, and aspect ratio of the elongated grains. 相似文献
16.
Salan U. Al-Dulaijan Gwilym Parry-Jones Abdul-Hamid J. Al-Tayyib Abdulaziz I. Al-Mana 《Journal of the American Ceramic Society》1990,73(3):736-739
This paper presents 29 Si magic-angle-spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist-cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration ( Q 1 + Q 2 ) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases. 相似文献
17.
Masamichi Tsuji Sridhar Komarneni Prakash Malla 《Journal of the American Ceramic Society》1991,74(2):274-279
18.
Seeding effects on the microstructure and mechanical properties of single-phase Ca-α-SiAlON ceramics have been investigated. Whereas a small amount of seeds can transform the microstructure from one of fine equiaxed grains to one consisting of many needle-like grains, the highest fracture toughness of 8 MPa·m1/2 is not reached until 8% seeding. This contrasts with the much higher seed efficiency in Y-SiAlON, where the peak toughness is reached at 1% seeding. The difference and the general trend of mechanical properties of seeded α-SiAlONs are discussed in terms of α-SiAlON formation and toughening mechanisms. 相似文献
19.
Rong-Jun Xie Mamoru Mitomo Gou-Dong Zhan Li-Ping Huang Xi-Ren Fu 《Journal of the American Ceramic Society》2001,84(2):471-73
A superplastic β-SiAlON was used as an interlayer to diffusionally bond a hot-pressed silicon nitride to itself. The bonding was conducted in a graphite furnace under a constant uniaxial load of 5 MPa at temperatures varying from 1500° to 1650°C for 2 h, followed by annealing at temperatures in the range of 1600° to 1750o C for 2 h. The bonds were evaluated using the four-point-bend method at both room temperature and high temperatures. The results indicate that strong, void-free joints can be produced with the superplastic β-SiAlON interlayer, with bond strengths ranging from 438 to 682 MPa, and that the Si3 N4 joints are heat resistant, being able to retain their strength up to 1000°C (635 MPa), and therefore have potential for high-temperature applications. 相似文献
20.
Silicon Nitride and Related Materials 总被引:22,自引:0,他引:22
Frank L. Riley 《Journal of the American Ceramic Society》2000,83(2):245-265
Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness. 相似文献