共查询到20条相似文献,搜索用时 0 毫秒
1.
Naoto Hirosaki Yoshio Akimune Mamoru Mitomo 《Journal of the American Ceramic Society》1994,77(4):1093-1097
Fine β-Si3 N4 powders with or without the addition of 5 wt% of large β-Si3 N4 particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y2 O3 and Al2 O3 as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si3 N4 and the rim was β-SiAION. These results show that the large β-Si3 N4 particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum. 相似文献
2.
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
3.
Peelamedu D. Ramesh Rainer Oberacker Michael J. Hoffmann 《Journal of the American Ceramic Society》1999,82(6):1608-1610
This paper describes a method for the preparation of silicon nitride (Si3 N4 ) seeds that have an average aspect ratio of ∼4. The seeds are prepared via heat treatment of a powder mixture that contains alpha-phase-rich Si3 N4 and 0.5 wt% Y2 O3 at a temperature of 1800°C and a nitrogen pressure of 35 kPa. A Y-Si-O-N liquid forms during heat treatment; this liquid acts as a flux for seed precipitation. During cooling, the Y-Si-O-N liquid transforms to a thin intergranular grain-boundary phase and causes strong agglomeration of the seeds. The seeds can be isolated by dissolving the grain-boundary phase in hot phosphoric acid, followed by an ultrasonic treatment (for 30 min). The method can be used to produce large quantities of seeds. 相似文献
4.
Daniel Muscat Martin D. Pugh Robin A. L. Drew Helen Pickup Don Steele 《Journal of the American Ceramic Society》1992,75(10):2713-2718
β-Si3 N4 whisker-reinforced β'SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original β-Si3 N4 whiskers in a matrix of fine SiAlON grains. SEM of plasma-etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X-ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original β-Si3 N4 whiskers. The composition of the sheath was qualitatively identical to that of the fine β' SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the β'SiAlON during liquid-phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the β'SiAlON grew epitaxially on the β-Si3 N4 whiskers, resulting in a heavily faulted SiAlON layer. 相似文献
5.
M. Mitomo M. Tsutsumi H. Tanaka S. Uenosono F. Saito 《Journal of the American Ceramic Society》1990,73(8):2441-2445
The microstructures of gas-pressure-sintered materials from β-Si3 N4 powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si3 N4 powder despite the smaller aspect ratio of the grains 相似文献
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7.
Valter Sergo Giuseppe Pezzotti Gen Katagiri Naoki Muraki Toshihiko Nishida 《Journal of the American Ceramic Society》1996,79(3):781-784
The stress dependence of the Raman bands of β-silicon nitride,β-Si3 N4 , has been investigated. In the stress range examined (from-200 to +200 MPa), low-frequency shift bands (namely the 183, 205, and 226 cm−1 lines) do not show any frequency change with the stress, whereas the high-frequency shift bands (862, 925, and 936 cm−1 ) have been found to have a linear stress dependence. The pertinent piezo-spectroscopic coefficients have been determined and are found to depend strongly on the additives used to promote densification presumably being taken into solid solution into the β-Si3 N4 phase. 相似文献
8.
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
9.
DORIS H. STUTZ SVANTE PROCHAZKA JOSEF LORENZ 《Journal of the American Ceramic Society》1985,68(9):479-482
The effect of additions of B, Al, and B + Al on the pressureless sintering of β-Sic was examined. The influence of the sintering atmosphere and heating schedule on densification behavior, polytype transformation, and microstructure development was also studied. High densities were obtained at 1940°C by the simultaneous addition of B and Al. The decrease in the sintering temperature is attributed to the presence of a liquid phase which results in the formation of platelets (up to 200 # in size) of an α-polytype, predominantly 4H and 6H. Polytype transformation and exaggerated grain growth could be prevented by annealing the compact at 1650° to 18500°C for 0.5 to 1 h. This procedure results in a better redistribution of the sintering aids, giving a fine-grained microstructure, constituted primarily of the cubic 3C polytype. 相似文献
10.
Kiyoshi Hirao Takaaki Nagaoka Manuel E. Brito Shuzo Kanzaki 《Journal of the American Ceramic Society》1994,77(7):1857-1862
The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3 N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation D n –D0 n = kt , with growth exponents of 3 and 5 for the c -axis direction and the a -axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2 , retaining high strength levels of about 1 GPa. 相似文献
11.
Mikito Kitayama Kiyoshi Hirao Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》1999,82(10):2931-2933
A powder mixture of α-Si3 N4 , Y2 O3 , and SiO2 was heat-treated in a loose powder state in the temperature range of 1750°–1900°C for 2 h; then, the mixture was acid-rinsed to remove the glassy phase. The widths and lengths of the resulting β-Si3 N4 crystals were analyzed quantitatively. The width–aspect-ratio distribution of the β-Si3 N4 crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width-aspect-ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the β-Si3 N4 crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model. 相似文献
12.
Z. G. Cambaz G. N. Yushin Y. Gogotsi K. L. Vyshnyakova L. N. Pereselentseva 《Journal of the American Ceramic Society》2006,89(2):509-514
Carbon was synthesized on β-SiC whiskers by extraction of Si atoms from SiC. In this study, three different elevated temperature extraction methods were used to remove Si atoms from SiC: treatments in either Cl2 or HCl and vacuum decomposition. In all chlorination experiments and vacuum treatment at 1700°C, carbon preserved the original shape of SiC whiskers. At higher temperatures (2000°C), vacuum decomposition led to a distortion in the shape of the whiskers. High-resolution transmission electron microscopy and Raman spectroscopy showed that the structure of carbide-derived carbon depends on the Si extraction method and the process parameters. Chlorination of SiC resulted in the formation of mostly amorphous nanoporous carbon. High-temperature treatment of SiC in HCl environment produced fullerene-like structures, while high-temperature vacuum decomposition resulted in the formation of graphite. Transmission electron microscopy studies of the carbon coating thickness produced in Cl2 at various chlorination times revealed linear reaction kinetics at 700°C. Raman studies showed that the carbon structure became more ordered with increasing chlorination temperature. The results obtained demonstrate that by using the silicon extraction technique, one can precisely control the thickness and morphology of the carbon coating. 相似文献
13.
Crystallizing the grain-boundary glass of a liquid-phase-sintered Si3 N4 ceramic for 2 h or less at 1500° led to formation of δ-Y2 Si2 O7 . After 5 h at 1500°, the δ-Y2 Si2 O7 had transformed to β-Y2 Si2 O7 with a concurrent dramatic increase in dislocation density within β-Si3 N4 grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material. 相似文献
14.
Xin Xu Toshiyuki Nishimura Naoto Hirosaki Rong-Jun Xie Yoshinobu Yamamoto Hidehiko Tanaka 《Journal of the American Ceramic Society》2006,89(5):1745-1747
The effect of sintering additives on superplastic deformation of nano-sized β-Si3 N4 ceramics has been studied by compression tests at 1500°C. The sintering additives were (i) Y2 O3 +Al2 O3 ; (ii) Y2 O3 +MgO; and (iii) Y2 O3 . Nano-sized Si3 N4 ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be ∼2 at a lower stress region and ∼1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y2 O3 additives had only one region, which had a stress exponent of ∼1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress. 相似文献
15.
Martin K. M. Hruschka Wenjie Si Samuele Tosatti Thomas J. Graule Ludwig J. Gauckler 《Journal of the American Ceramic Society》1999,82(8):2039-2043
A water-based route for processing ß-Si3 N4 from alpha-Si3 N4 , Al2 O3 , and Y2 O3 powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al2 O3 and to pH 6 for Y2 O3 . Aluminum hydroxide (Al(OH)3 ) showed strong interaction with the Si3 N4 powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries. 相似文献
16.
An atomic-scale approach has been applied to the examination of both the physical and electronic structures of stable surfaces of β-Si3 N4 . Sterical constraints prevent the (001) surface from effective chemical reaction with the interface. The theoretical surface-to-surface bonding is investigated by using a periodical tight-binding approach. Based on the interpretation of the density of states, the balance of the number of states and electrons is performed for stoichiometric Si3 N4 , ideal N-terminated (110) surfaces, oxygen-overlayered (110) slabs, and the metal monolayer with which the slabs are brought into contact. The stable electronic configuration, which is attained when the cation binds to the interface, represents the electronic driving force behind the diffusion of the additive and/or impurity atoms toward grain boundaries. The different bonding propensities of the (001) and (110) surfaces imply that effective bonding of the planes parallel to the c -direction to the interphase restrains the crystal from growth in the lateral direction. Conversely, geometry-constrained bonding of the (001) planes allows the crystal growth that produces the rod-shaped β-grains. 相似文献
17.
Rachid Belkada Toshiya Shibayanagi Masaaki Naka Masanori Kohyama 《Journal of the American Ceramic Society》2000,83(10):2449-2454
The atomic and electronic structure of the β-silicon nitride (β-Si3 N4 ) crystal have been determined using the ab initio pseudopotential method based on the density functional theory. We have obtained the stable lattice parameters and the stable positions of 14 atoms in the unit cell for the structure P 63 / m for the first time. The electronic structure and the charge distribution indicate that the Si–N bond has both ionic and covalent characters. The band structure is in good agreement with the other first-principles results and consistent with the experiments. 相似文献
18.
Guo-Dong Zhan Mamoru Mitomo Yuichi Ikuhara Taketo Sakuma 《Journal of the American Ceramic Society》2000,83(12):3179-3184
The influence of grain shape and size on superplastic behavior and deformation mechanisms was investigated in annealed β-silicon nitride materials and compared with the results for hot-pressed material. The microstructure of the annealed materials consisted of fine equiaxed β-grains together with some elongated ones. Similar to the deformation behavior in the hot-pressed material, strain hardening did not occur in these annealed materials. Moreover, in contrast to the deformation behavior under tension, grain alignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elongated grains had a flow-stress dependency of n = 1, whereas other annealed materials with large elongated grains exhibited a flow-stress dependency of n = 1.6. In terms of texture development and the effect of grain shape on the creep rate when diffusion was the rate-controlling mechanism, a single curve with a stress exponent of ∼1 and a grain-size exponent of 3 were obtained for all materials. This suggests that the deformation mechanism in these annealed materials was the same as that of fine equiaxed β-silicon nitride. 相似文献
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20.
Guo-Dong Zhan Yuichi Ikuhara Mamoru Mitomo Rong-Jun Xie Taketo Sakuma Amiya K. Mukherjee 《Journal of the American Ceramic Society》2002,85(2):430-436
The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed. 相似文献