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排序方式: 共有2160条查询结果,搜索用时 46 毫秒
981.
Anil Kaza M. John Matthewson Dale Niesz Richard L. Haber Mark A. Rossi 《Journal of the American Ceramic Society》2009,92(11):2517-2527
Carbon, which is often used as an additive to silicon carbide powder, is thought to facilitate densification during sintering by aiding the removal of the native SiO2 layer, which is present on the starting SiC powder. The mechanism is the reduction of SiO2 to SiC with the formation of primarily CO gas, which diffuses out from the porous compact at a temperature below the normal sintering temperature. It has been found beneficial to hold the compact at an intermediate temperature to allow time for the CO and other gases to diffuse out before the pores close. We investigate this process using a computational model based on codiffusion of multiple gas species, which enables prediction of the gas and condensed phase compositions as a function of time and position in the specimen. The results are used to determine the optimum holding time for complete SiO2 removal as a function of key parameters, such as specimen thickness, particle size, temperature, etc., as well as the necessary amount of C additive. The results of the modeling are consistent with the experimentally observed spatial variation of density and composition in SiC compacts. 相似文献
982.
The magnetic behaviour of two ferromagnetic oxides and two ferrimagnetic oxides (ferrites) are compared to study the effect
of magnetocrystalline anisotropy on thermal-history-dependence of magnetization of these ordered magnetic systems. All four
compounds show thermomagnetic irreversibility (M
FC >M
ZFC) below a certain temperature,T
irr. The highly anisotropic ferromagnetic oxide, SrRuO3 and the hard ferrite, SrFe12O19 show sharp peaks belowT
c in theirM
ZFC(T) curves, whereas for the soft ferrite Ni0.8Zn0.2Fe2O4 and the low anisotropic ferromagnetic oxide La0.7Ca0.3MnO3 only a broad maximum is observed inM
ZFC, when measured in small magnetic fields. The shapes of theM
ZFC(T) curves are inversely-related to the magnitude of the coercivities (H
c) of the compounds in relation to the applied field, and the temperature dependence of Hc.M
FC andM
ZFC are related through the coercivity for all four magnetic systems. 相似文献
983.
Suresh Kumar Anil Kumar Anupam Shukla G. Rohini Devi A.K. Gupta 《Journal of the European Ceramic Society》2009,29(13):2849-2855
Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10?6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10?6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min. 相似文献
984.
Abstract PZT thin films have been prepared via a sol-gel route using standard butoxide and acetate precursors. The solution compositions were modified by the addition of acetylacetone in 2 methoxyethanol, which has the effect of changing the solution complex and therefore its deposition and drying characteristics. Varying amounts of excess lead were included in some cases. The films were prepared by spin coating onto Pt/Ti electroded silicon with an intermediate barrier layer of either boron phosphate silica glass (BPSG) or thermal silicon oxide and the resulting samples subjected to a range of thermal annealing conditions in an oxidising atmosphere. Several methods have been investigated including two regimes using rapid furnace annealing (RFA) up to 700°C and slow annealing at 450°C in air. The latter method yielded highly (111) oriented and crack-free perovskite films with no evidence of other orientations, whereas the former route showed the presence of (100) and (110) orientations. The correlation between the thermal anneal method and orientation will be discussed. Surface electrode dots have been evaporated onto the film to allow electrical measurements. The ferroelectric and fatigue properties of these films have been assessed with respect to non-volatile memory applications. The implications of film quality and thermal processing on the overall ferroelectric performance will be discussed. 相似文献
985.
986.
Asim Tewari S. Vijayalakshmi Shashank Tiwari Pinaki Biswas Sooho Kim Raja K. Mishra Robert Kubic Anil K. Sachdev 《Metallurgical and Materials Transactions A》2013,44(5):2382-2398
The application of lightweight aluminum sheets to fabricate automotive components for vehicle weight reduction continues to be limited due to their low formability and high cost. This report summarizes a metallurgical investigation of the influence of various microstructural attributes on the forming and failure characteristics of aluminum sheets produced by lower cost continuous casting processes. The study has identified the combination of microstructural attributes, such as grain size, texture, and second phase particle distribution, in the sheets which make some sheets more formable than others and has traced the origin of these features to the processing history. The results show that the microstructural features present in the sheets have their origin in the casting, rolling, and recrystallization processes involved in their fabrication. 相似文献
987.
The sinterability of a class F fly ash was investigated as a function of processing conditions including sintering temperature (1050-1200 °C) and sintering time (0-90 min). Density, shrinkage, splitting tensile strength, water absorption and residual loss on ignition (RLOI) were evaluated as measures of sintering efficiency. Scanning electron microscopy (SEM), X-ray microanalysis and X-ray diffraction was used to examine microstructure and phase development due to processing. The results show that premature densification can inhibit complete carbon removal and that carbon combustion is influenced by both internal and external mass transfer conditions. 相似文献
988.
989.
ZnS nanocrystallites were synthesised by pyrolysis method using Zn (cinnamtscz)2 and ZnCl2 (cinnamtsczH)2 (cinnamtsczH = cinnamaldehyde thiosemicarbazone) as single source precursors. The prepared ZnS nanocrystallites were characterized
by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction, UV-Vis and
fluorescence spectroscopy. The peak broadening in XRD and emission at shorter wavelength in fluorescence spectra showed the
presence of nanocrystallites. The blue shift in UV-Vis absorption spectroscopy also proved the formation of nanocrystallites.
TEM images show presence of plate-like and spherical ZnS nanoparticles obtained from Zn(cinnamtscz)2 and ZnCl2 (cinnamtsczH)2 respectively. 相似文献
990.
Bruce A. Mover Yanpei Deng Yunfu Sun Richard A. Sachleben Anil K. Batra R. Bruce Robinson 《溶剂提取与离子交换》2013,31(5):791-810
ABSTRACT The extraction of cesium nitrate from a high concentration of sodium nitrate by a family of 21-crown-7 ethers in 1,2-dichloroethane has been investigated. Dibenzo-21-crown-7 (DB21C7) and bis[4(5),4′(5′)-rm-butylbenzo]-21-crown-7 (BtBB21C7) ethers have higher selectivity for cesium but lower extraction efficiency than dicyclohexano-21-crown-7 (DC21C7) ether. As measured by the distribution coefficient ratio Dcs/DNa, the cesium selectivity averaged 78 and 93 for DB21C7 and BtBB21C7, respectively. However, in the case of DC21C7, the cesium selectivity was lower and decreased approximately three-fold from 10 to 3 as cesiüm loading increased. Alkyl substitution on the benzo group has only a small effect on the extraction behavior. It was shown by use of the equilibrium modeling program SXLSQI that the extraction of sodium and cesium could be adequately modeled by augmenting the previously determined cesium nitrate equilibrium model with a 1:1 Na+:crown organic-phase complex. No evidence for a mixed-metal species was found. In accord with our previous study, cesium nitrate extraction entails formation of a 1:1 Cs+:crown complex, but a minor 1:2 complex also forms in the case of DC21C7. Although the sodium and cesium made possible by the U. S. Department of Energy Postgraduate Research Program administered by the Oak Ridge Associated Universities. 相似文献