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1.
《Scripta Metallurgica》1989,23(8):1243-1248
2.
Mixtures of pure nanometer-sized amorphous silica and -alumina with the atomic ratio SiAl=1 were reduced by a stoichiometric amount of carbon between 1100 and 1450 °C in flowing nitrogen in order to produce -sialon powder. Using aqueous suspensions of starting materials, compacts with different microstructures were prepared for reaction. Silica reduction to SiO occurred at a temperature as low as 1300 °C and part of it was removed with flowing nitrogen. Carbothermal reaction involving nitrogen stated at 1350 °C and Si2N2O was found as an intermediate together with SiC, resulting in -sialon formation. Loss of silica from the system led to AlN formation. Decomposition of -sialon into sialon polytypoids (15R, 12H) was observed as a result of sialon and AlN reaction at 1450 °C. The reaction rate of sialon formation was slowed down compared to the carbothermal reduction of kaolin because of the lack of impurities. The microstructure of the reacted pellets influenced the reaction products, and the narrow pore size distribution as well as good homogeneity enhanced -sialon formation.On leave, from Silesian Technical University, Krasiskiego 8, 40-019 Katowice, Poland. 相似文献
3.
Zuxiong Chen 《Journal of Materials Science》1993,28(22):6021-6025
-sialon whiskers were synthesized from silica fume and -alumina via the carbothermal reduction reaction under nitrogen flow. The growth of whiskers was found to be conducted by the vapour-solid liquid crystal growth mechanism involving SiO, FeCl3 and other vapour species. SEM observation and EDAX reveal that the changing morphology of whiskers: branching, varying size, shape and growing direction, is a result of variations in vapour composition. 相似文献
4.
β-sialon/nano-size SiC composite ceramic with DyAG(Dy3Al5O12) as grain boundary phase was fabricated through hot-pressing. The effect of nano-size SiC on densification, phase composition, microstructure and mechanical properties of the materials was studied 相似文献
5.
K. J. D. Mackenzie R. H. Meinhold G. V. White C. M. Sheppard B. L. Sherriff 《Journal of Materials Science》1994,29(10):2611-2619
27Al and 29Si magic-angle spinning(MAS) nuclear magnetic resonance(NMR)and complementary X-ray diffraction (XRD) studies of carbothermal formation of sialons from kaolinite and halloysite confirm that the reaction involves the initial formation of mullite (3Al2O3·2SiO2) and amorphous silica. In the presence of carbon, Si-N bonds are formed at 1200 °C, giving a continuum of silicon oxynitride compositions which become progressively more N-rich. These do not become sufficiently ordered to be detected by XRD until later in the reaction, when crystalline silicon oxynitride, possibly containing a little Al (O-sialon) and x-phase sialon are formed, followed by -sialon. The O- and x-phase sialons are transitory, but the -sialon persists throughout the reaction. Si-O bonds survive the destruction of the mullite and persist throughout the reaction, especially with kaolinite starting material. The 29Si MAS NMR results indicate that Si-C bonds are formed later in the reaction than previously suggested, the SiC phase behaving more like a secondary product than a transitory intermediate. Al-N bonds are not detectable by 27Al MAS NMR until very late in the reaction (after 8 h firing at 1400 °C), and coincide with the appearance of the secondary product AlN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are discussed. 相似文献
6.
The reaction hot-pressing at 1700 and 1750° C of a number of compositions in the system Al-Si-N-O corresponding to points spanning the-sialon phase line in the region of z=0.8 has been studied. Measurements have been made of densification rate and of the rate of conversion of-Si3N4 to-sialon. The densification process for these compositions may be described in terms of a liquid-phase assisted fast particle rearrangement process succeeded by Coble creep. The rates of these processes are sensitive to the volume of grain-boundary liquid phase present, which in turn is determined by the position of the compositional point in relation to the-sialon line. For systems containing very little grain-boundary phase, the vapour-phase transport of material may become important. 相似文献
7.
Z. Jouini O. O. Kurakevych H. Moutaabbid Y. Le Godec M. Mezouar N. Guignot 《Journal of Superhard Materials》2016,38(1):66-70
Understanding of the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. It has been found that silicon clathrates have the archetype structures, which can serve an existent model compounds for superhard clathrate frameworks Si–B, Si–C, B–C and C with intercalated atoms (e.g., alkali metals or even halogens) that can assure the metallic properties. Here we report our in situ and ex situ studies of high-pressure formation and stability of clathrates Na8Si46 (structure I) and Na24+x Si136 (structure II). Experiments have been performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in the Na–Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration has been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~6 GPa, Na24+x Si136 (structure II) is stable at lower temperatures as compared to Na8Si46 (structure I). 相似文献
8.
对原位TiN/O′-sialon纳米复相陶瓷(NTS)和采用“二步法”制备的TiN/O′-sialon复相陶瓷(TS)的常温导电性能进行了对比研究,并对材料TS进行了放电加工。研究结果表明,初始原料中20%(质量分数)和25%(质量分数)左右的TiO2加入量是决定材料NTS和TS中TiN能否形成导电网络的最低TiO2加入量,该导电临界值同基相O′-sialon与导电相TiN的颗粒尺寸比有关,此时相应材料的电阻率为1.6×10^-2和1.8×10^-2Ω·cm,满足放电加工的需要。烧结温度升高,两种材料的电阻率略有降低。随放电加工速度的增加,材料TS加工表面粗糙度明显增加。 相似文献
9.
A large number of δ particles precipitated atlow-angle grain boundaries have been observed byTEM in a dilute A1-Li-Zn-Zr alloy during ageingat the suitable temperatures.An explanation forthis phenomenon was proposed,which involves therole of excess vacancies and capillarity effects. 相似文献
10.
W. I. Archer R. D. Armstrong D. P. Sellick W. G. Bugden J. H. Duncan 《Journal of Materials Science》1980,15(8):2066-2072
A comparison is made between the microstructures, resistivities and strengths of sodium /-alumina samples prepared under different firing conditions. Double peak firing schedules are shown to result in samples of higher strength with a narrower grain-size distribution and lower resistivity than those produced by single peak firing. Complex plane a.c. impedance and admittance data show the sodium ion conduction over the temperature range 173 to 300 K to be governed primarily by intergranular regions of the sample (with the resistance varying with grain size) whilst resistivity measurements at 623 K show the conduction to be primarily intragranular controlled at elevated temperatures. 相似文献
11.
Nam-Kyun Kim Doh-Yeon Kim A. Kranzmann E. Bischoff Suk-Joong L. Kang 《Journal of Materials Science》1993,28(16):4355-4358
The aluminium concentration in -sialon grains of the 74Si3N4 ()-6Al2O3-20Nd2O3 (wt%) specimen has been determined. As the - transformation proceeded, the average aluminium concentration in -sialon grains decreased. For individual grains, the aluminium concentration at the centre was higher than that at the edge. The observed variation of aluminium concentration has been explained in terms of the compositional change of the liquid matrix during sintering. 相似文献
12.
The thermal conductivity of the solid phases I and III of urea was measured at temperatures in the range 50–370 K for pressures up to 1 GPa. Phase III, previously detected only at pressures above 0.5 GPa, was observed here at low pressures ( <0.07 GPa) below about 230 K. Extrapolation of the I–III phase line indicates that phase III might be obtained at 218 K at atmospheric pressure and, consequently, that urea might exhibit two solid phases at atmospheric pressure. The temperature dependence of the thermal conductivity of both phase I and phase III could be described by the Debye model for thermal conductivity assuming phonon scattering by three phonon umklapp processes only. Despite a volume decrease at the I III transition, the thermal conductivity decreased by about 20%. Normally, thermal conductivity increases at a phase transition at which volume decreases. This rather unusual behavior of urea might be due to an increase in the nearest-neighbor distance at the I III transition. 相似文献
13.
The microstructure variation of-Al2O3 with different compositions during hot-pressing and its effect on ionic conductivity were studied. It is found that, besides the phase composition, the conductivity of hot-pressed-Al2O3 materials of the same composition is also related to the measurement direction, microstructure and density. The resistivity in the direction parallel to hot-pressing is about three times higher than that perpendicular to the hot pressing. The pores in the material are particularly detrimental to the ionic conduction. It was observed that the shape of complex impedance plane spectrum changed with measurement temperature and such change was affected by chemical composition and sample preparation condition. This phenomenon is explained successfully with the help of the ideal equivalent circuit of polycrystalline fast ionic conductor and the results of the conductivity measurement of hot-pressed-Al2O3. 相似文献
14.
Previous investigations of the replacement of silicon by aluminium and nitrogen by oxygen in -silicon nitride have been based primarily on X-ray powder diffraction studies. In the present work this technique is coupled with parallel infra-red studies. X-ray analyses of sialons over a wide composition range confirm previous observations that increasing substitution of aluminium for silicon and nitrogen for oxygen in -silicon nitride is accompanied by an increase in cell size, with no evidence of any other structural modification. Parallel infra-red analyses show shifts in certain of the infra-red absorption bands to lower wavenumbers as the degree of substitution increases. Changes in the infrared spectra at the composition Si2Al4N4O4 indicate structural modifications which are not apparent from the X-ray investigations. It is suggested that these changes are a result of the ordering of the different atom types at this composition. 相似文献
15.
A newly designed experimental apparatus has been used to measure the speed of sound u in high-purity water on nine isotherms between 274 and 394 K and at pressures up to 90 MPa. The measurement technique is
based on a traditional double-reflector pulse-echo method with a single piezoceramic transducer placed at unequal distances
from two stainless steel reflectors. The transit times of an acoustic pulse are measured at a high sampling rate by a digital
oscilloscope. The distances between the transducer and the reflectors were obtained at ambient temperature and pressure by
direct measurements with a coordinate measuring machine. The speeds of sound are subject to an overall estimated uncertainty
of 0.05 %. The acoustic data were combined with available values of density ρ and isobaric heat capacity cp along one isobar at atmospheric pressure to calculate the same quantities over the whole temperature and pressure range by
means of a numerical integration technique. These results were compared with those calculated from the IAPWS-95 formulation
with corresponding relative deviations which are within 0.1%.
Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A. 相似文献
16.
Data are presented on K3Nb3O6(B2O6)1–x
(Si2O7)
x
solid solutions in the K3Nb3B2O12–K3Nb3Si2O13 system. The system is shown to contain two partial solid-solution series in the composition ranges 0 < x< 0.2 (KNB:Si) and 0.8 < x< 1 (KNS:B). KNB:Si crystals are grown from off-stoichiometric melts, and their polymorphism and physical properties are studied. Similar to K3Nb3B2O12, KNB:Si crystals exhibit a complex polymorphism and combine ferroelectric, ferroelastic, and superionic properties. Si substitution for B is shown to have a significant effect on the phase-transition temperatures and electrical properties of K3Nb3B2O12 crystals. The results obtained for K3Nb3Si2O13 crystals demonstrate that, in spite of their hexagonal habit, their room-temperature structure possesses orthorhombic or lower symmetry. The room-temperature dielectric permittivity 33 of K3Nb3Si2O13 crystals is as high as 4000. 相似文献
17.
《低温学》1986,26(7):392-395
Cryogenic mechanical property data compiled at the National Bureau of Standards, USA, have been used to analyse the relationship between yield strength and fracture toughness for austenitic stainless steel welds at 4 K. This study demonstrates that there is an inverse linear correlation between yield strength and fracture toughness for the stainless steel welds at 4 K, and that the welds have significantly lower toughness than base materials of comparable strength. 相似文献
18.
Young's, shear and bulk moduli, Poisson's ratio and Lamé parameters, longitudinal and transverse internal friction values and acoustic wave velocity anisotropy factors for three kinds of polycrystalline compounds, -alumina, yttria-stabilized tetragonal zirconia polycrystal, (Y)TZP, and -sialon, (Si,Al)3(N, O)4, were simultaneously measured over a temperature range 295–1773 K, by an ultrasonic pulse sing-around method. These elastic moduli and Lamé parameters decreases and Poisson's ratio increases with increasing temperature, suggesting activation of a shear mode in the high-temperature region. The high-temperature shear internal friction for (Y)TZP and sialon were more sensitive to relief of strain and softening of glassy phase at grain boundaries, respectively, compared with the dilational friction. 相似文献
19.
The products of carbothermal reduction in N2 of a nanocomposite between dodecylammonium-exchanged montmorillonite and polyacrylonitrile (PAN) have been studied by solid-state 27Al and 29SiNMR spectroscopy, X-ray diffraction, transmission electron microscopy and thermogravimetry. Comparison with analogous reactions involving sodium-exchanged montmorillonite and dodecylammonium-exchanged montmorillonite (without PAN) shows that in the presence of PAN, the formation of silica, cordierite or mullite is almost completely suppressed. The only crystalline phase detected between 1000 and 1300 °C was a -sialon, having a much higher SiAl ratio (7.051) than that of the precursor clay (2.441). Reduction of the octahedral AlO6 begins near 1200 °C, forming increasing amounts of Al(N,O)4 tetrahedra with temperature, so that by 1600 °C, complete reduction to AlN4 (i.e. bulk AlN) has occurred. In contrast, reduction of the tetrahedral SiO4 is appreciable at 1100 °C, and is almost complete (SiN4 tetrahedra only) by 1200 °C. No intermediate Si(N,0)4 environments are found. By 1600 °C, only the SiC4 environment (i.e. bulk SiC) remains. A mechanism is suggested, involving the formation of alternating slabs of an amorphous aluminosilicate and carbon at 1000 °C, followed by diffusion of silicon from the outer regions of the aluminosilicate band towards the centre, and sequential reduction of Si(OSi)4 and Si(OSi)3 (OAl) groups. 相似文献