共查询到20条相似文献,搜索用时 10 毫秒
1.
The kinetics of partial melting in a β-spodumene glass-ceramic was measured by holding fully crystallized material above the solidus but below the liquidus temperature. The kinetics was apparently limited by the rate of diffusion of silica in the crystalline phase. The measured lattice diffusivity of silica was 8×10−10 exp(-251 kJ mol−1 /RT ) m2 s−1 with a possible error of ±60 kJ mol−1 in the activation energy. The partially molten samples were heat-treated below the solidus to obtain the kinetics of crystallization, which was found to be interface-reaction-controlled. 相似文献
2.
Timothy Jochum Ivar E. Reimanis Michael J. Lance Edwin R. Fuller Jr. 《Journal of the American Ceramic Society》2009,92(4):857-863
A pressure-induced phase transformation in the lithium aluminum silicate β-eucryptite was studied with in situ Raman spectroscopy. Dense β-eucryptite composites were made via powder synthesis followed by sintering. The specimens were then subjected to diamond indentation up to applied contact stresses of about 8 GPa, while collecting Raman spectra. The appearance of a Raman peak (∼520 cm−1 ) at a contact stress of about 3 GPa likely corresponds to the reversible phase transformation of β-eucryptite to the orthorhombic phase ɛ-eucryptite. Loading and unloading in situ Raman indentation experiments are discussed with regards to this transformation. 相似文献
3.
The rate of dissolution of β-Si3 N4 into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si3 N4 -SiO2 -MgO such that Si2 N2 O rather than β-Si3 N4 was the equilibrium phase. Dissolution was driven by the chemical reaction Si3 N4 (c)+SiO2 ( l )→Si2 N2 O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si3 N4 ) and the precipitating phase (Si2 N2 O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si3 N4 , crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol−1 , at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si3 N4 hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si3 N4 . 相似文献
4.
Martin Krämer 《Journal of the American Ceramic Society》1993,76(6):1627-1629
α-Si3 N4 core structures within β-Si3 N4 grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si3 N4 crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism. 相似文献
5.
《Journal of the American Ceramic Society》1977,60(9-10):471-472
6.
Simple ionic defect theory has not been successful in describing the observed defect and transport behavior in β-Ta2 O5 . The available data were reviewed, and a revised model is proposed which more adequately describes the defect behavior of nominally pure or lightly doped β-Ta2 O5 . 相似文献
7.
Håkan Björklund Jonas Wasén Lena K. L. Falk 《Journal of the American Ceramic Society》1997,80(12):3061-3069
The three-dimensional grain size distribution in an experimental β-Si3 N4 material has been determined using the hexagonal prism as a model of β-Si3 N4 grain shape. Results from quantitative microscopy of polished and etched sections were compared with computer-generated two-dimensional stereological parameters of hexagonal prisms with different aspect ratios in order to determine an average grain shape (i.e., aspect ratio) in the microstructure. Section parameter distributions for the average grain shape were obtained from the computer simulations and used in a three-dimensional reconstruction of the microstructure. The results showed that this Si3 N4 ceramic had the postulated fibrous microstructure and a broad grain size distribution. 相似文献
8.
9.
Hideki Ishida Katsumi Mabuchi Kaori Sasaki Takeshi Mitsuda 《Journal of the American Ceramic Society》1992,75(9):2427-2432
Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca2 (SiO3 )(OH)2 ) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low-crystalline β-Ca2 SiO4 , which is stable at room temperature and has a remarkably large specific surface area of about 7 m2 /g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topotactically. 相似文献
10.
The kinetics of anisotropic β-Si3 N4 grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si3 N4 grains follows the empirical growth law Dn – D0 n = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si3 N4 growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time. 相似文献
11.
Ellen Y. Sun Kathleen B. Alexander Paul F. Becher Shyh-Lung Hwang 《Journal of the American Ceramic Society》1996,79(10):2626-2632
Interfacial microstructures in βP-Si3 N4 ( w )-Si-Al-Y-O-N-glass systems were investigated by systematically varying the nitrogen content and the Al:Y ratio of the glass matrix. High-resolution and analytical transmission electron microscopy (HREM and AEM) studies revealed that the interfacial microstructure is a function of the glass composition. No interfacial phases were formed in glasses with low Al:Y ratios and in glasses with high Al:Y ratios and low nitrogen content, whereas epitaxial growth of an interfacial layer (100–200 μm thick) on the βP-Si3 N4 ( w ) occurred in a glass matrix with high Al:Y ratio and high nitrogen content. The interfacial layer was identified to be a β'-SiAION phase. Interfaces containing the SiAION layer exhibited high debonding energy compared to Si3 N4 ( w )–glass interfaces. HREM studies indicated that the lattice-mismatch strain in the SiAION layer was relieved by dislocation formation at the SiAION–Si3 N4 ( w ) interface. The difference in interfacial debonding energy was, hence, attributed to the local atomic structure and bonding between the glass-β-Si3 N4 and the glass–β'-SiAION phases. This observation was clear evidence of the strong influence of glass chemistry on the interfacial debonding behavior by altering the interfacial microstructure. 相似文献
12.
JAMES D. HODGE 《Journal of the American Ceramic Society》1983,66(3):166-169
Mixtures of α-A12 O3 and NaAlO2 corresponding to the eutectic composition 65 mot % A12 O 3 were heated in sealed platinum crucibles above the eutectic temperature (measured to be 1572°C in this study). Once melting was complete, the liquid was quenched and the resultant solid examined by X-ray diffraction. Sodium aluminate, beta alumina, and beta" alumina were detected in all cases. However, the relative amounts of beta and beta" present were a function of the cooling rate, with the amount of beta" increasing with a decreasing cooling rate. Subsequent annealing (in platinum envelopes) of the solidified eutectic at 1400° and 1530°C showed that the amount of beta" present decreased with increasing annealing time. Based on this observation, it is apparent that, in the temperature range studied, beta"-is metastable. Analysis of the time-transformation data shows that the reaction kinetics are consistent with the diffusion-controlled growth of the beta phase at the expense of the beta"-phase. 相似文献
13.
Mikito Kitayama Kiyoshi Hirao Akira Tsuge Koji Watari Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》2000,83(8):1985-1992
Dense β-Si3 N4 with various Y2 O3 /SiO2 additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y2 O3 /SiO2 ratio increased. The oxygen contents in the β-Si3 N4 crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si3 N4 that was sintered at various Y2 O3 /SiO2 additive ratios has been clarified. 相似文献
14.
Tetsuo Nakayasu Tetsuo Yamada Isao Tanaka Hirohiko Adachi Seishi Goto 《Journal of the American Ceramic Society》1998,81(3):565-570
First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si3 N4 and the interface between prismatic planes of β-Si3 N4 and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln3+ ions at the grain boundary and no solubility of Ln3+ into bulk β-Si3 N4 . Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si3 N4 during liquid-phase sintering. 相似文献
15.
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
16.
Beta-type CVD-Si3 N4 plates (up to 1.1 mm thick) have been prepared by adding TiCl4 vapor to the system SiCl4 -NH3 -H2 at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si3 N4 was obtained without TiCl4 vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si3 N4 matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si3 N4 were examined. 相似文献
17.
Koichiro Fukuda Iwao Maki Suketoshi Ito 《Journal of the American Ceramic Society》1997,80(6):1595-1598
Crystals of β-Ca2 SiO4 (space group P 121 / n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (αi ) found from the matrix algebra analysis was as follows: α1 = 20.492 × 10−6 + 16.490 × 10−9 ( T - 25)°C−1 , α2 = 7.494 × 10−6 + 5.168 × 10−9 ( T - 25)°C−1 , α3 =−0.842 × 10−6 − 1.497 × 10−9 ( T - 25)°C−1 . The expansion coefficient α1 , nearly along [302] was approximately 3 times α2 along the b -axis. Very small contraction (α3 ) occurred nearly along [ 01]. The volume changes upon martensitic transformations of β↔αL ' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity. 相似文献
18.
The model proposed to explain the defect behavior of nominally pure β-Ta2 O5 was extended to the case of defect equilibria and charge transport in β-Ta2 O5 doped with a multivalent cation, i.e., Nb2 O5 -doped P-Ta2 O5 . This case is of practical interest in the electronics industry, where Nb2 O5 , is a dominant impurity in capacitor-grade β-Ta2 O5 . 相似文献
19.
K. HIRAGA M. HIRABAYASHI S. HAYASHI T. HIRAI 《Journal of the American Ceramic Society》1983,66(8):539-542
Microstructures of Si3 N4 -TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si3 N4 . Crystallographic orientation relations between β-Si3 N4 and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si3 N4 crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si3 N4 matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix. 相似文献
20.
BetaSi3 N4 coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl4 -NH3 -N2 at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied. 相似文献