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21.
Microstructural Evolution in Liquid-Phase-Sintered SiC: Part III, Effect of Nitrogen-Gas Sintering Atmosphere 总被引:1,自引:0,他引:1
Angel L. Ortiz Tania Bhatia Nitin P. Padture Giuseppe Pezzotti 《Journal of the American Ceramic Society》2002,85(7):1835-1840
Effects of N2 sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid-phase-sintered (LPS) SiC were studied. It was found that, for the β-SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar-atmosphere case. Chemical analyses of the specimens sintered in N2 atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β-SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N2 atmosphere. For specimens sintered in N2 atmosphere, a remarkable shift of the grain-boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α-SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N2 atmosphere open the possibility of tailoring their microstructures for room-temperature mechanical properties and for making high-temperature materials that are highly resistant to coarsening and creep. 相似文献
22.
Microstructural Evolution in Liquid-Phase-Sintered SiC: Part I, Effect of Starting Powder 总被引:1,自引:0,他引:1
Huiwen Xu Tania Bhatia Swarnima A. Deshpande Nitin P. Padture Angel L. Ortiz Francisco L. Cumbrera 《Journal of the American Ceramic Society》2001,84(7):1578-1584
The effect of starting SiC powder (β-SiC or α-SiC), with simultaneous additions of Al2 O3 and Y2 O3 , on the microstructural evolution of liquid-phase-sintered (LPS) SiC has been studied. When using α-SiC starting powder, the resulting microstructures contain hexagonal platelike α-SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α-SiC. When using β-SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high-aspect-ratio SiC grains is proposed. 相似文献
23.
Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ca XVII. The configurations used are 2s2, 2s2p, 2p2, 2l3l′, 2l4l′ and 2s5l′, with l=s,p, and l′=s,p,d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at seven incident energies (15, 30, 75, 112.5, 150, 187.5 and 225 Ry) for the transitions within the three lowest configurations corresponding to the 10 lowest energy levels, and at five incident energies (75, 112.5, 150, 187.5 and 225 Ry) for transitions between the lowest five levels and the n=3,4,5 configurations. Calculations have been carried out using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-Matrix results for the 2s2, 2s2p, 2p2 configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of at an electron temperature of logTe(K)=6.7, corresponding to the maximum abundance of Ca XVII. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This dataset will be made available in the next version of the CHIANTI database. 相似文献
24.
Strategies to encode or label small particles or beads for use in high-throughput screening and bioassay applications focus on either spatially differentiated, on-chip arrays or random distributions of encoded beads. Attempts to encode large numbers of polymeric, metallic or glass beads in random arrays or in fluid suspension have used a variety of entities to provide coded elements (bits)--fluorescent molecules, molecules with specific vibrational signatures, quantum dots, or discrete metallic layers. Here we report a method for optically encoding micrometre-sized nanostructured particles of porous silicon. We generate multilayered porous films in crystalline silicon using a periodic electrochemical etch. This results in photonic crystals with well-resolved and narrow optical reflectivity features, whose wavelengths are determined by the etching parameters. Millions of possible codes can be prepared this way. Micrometre-sized particles are then produced by ultrasonic fracture, mechanical grinding or by lithographic means. A simple antibody-based bioassay using fluorescently tagged proteins demonstrates the encoding strategy in biologically relevant media. 相似文献
25.
Montecito: a dual-core, dual-thread Itanium processor 总被引:2,自引:0,他引:2
Intel's Montecito is the first Itanium processor to feature duplicate, dual-thread cores and cache hierarchies on a single die. It features a landmark 1.72 billion transistors and server-focused technologies, and it requires only 100 watts of power. Intel's Itanium 2 processor series has regularly delivered additional performance through the increased frequency and cache as evidenced by the 6-Mbyte and 9-Mbyte versions. 相似文献
26.
The two glycosylated N- and C-terminal lobes of buffalo lactoferrin have been produced by limited proteolysis using proteinase K. Lactoferrin is a single chain glycoprotein of molecular mass 80 kDa with two iron-binding sites and two structural lobes connected by a short peptide. Purified samples of lactoferrin, isolated from buffalo colostrum, were subjected to hydrolysis using trypsin, chymotrypsin, pepsin, subtilisin and proteinase K. The first three proteinases produced two major fragments of approximately 35 and 23 kDa together with small molecular mass peptides. Trypsin and chymotrypsin partly digested lactoferrin, while pepsin converted all the intact lactoferrin into fragments. Subtilisin hydrolysis produced fragments of 40 and 26 kDa together with low molecular mass peptides. However, SDS-PAGE of the proteinase K hydrolysis product gave a clear band at 40 kDa together with a band indicating a substantial quantity of low molecular mass peptides (< 14.4 kDa). Upon ion-exchange chromatography this product gave two major fractions, which were further purified by gel filtration and identified as the C and N lobes from their N-terminal sequences. Thus, the 40 kDa band in SDS-PAGE of the proteinase K hydrolysis product contained two fragments of equal molecular mass. On further hydrolysis with proteinase K, the N lobe was completely hydrolysed into low molecular mass peptides, while only a small fraction of the C lobe was converted into small products. This suggested that an inhibitory fragment was present in the C lobe that was released on hydrolysis to small fragments and prevented complete digestion of the C lobe by high-affinity binding to the active site of proteinase K. This fragment was isolated from the lactoferrin-proteinase K complex and its sequence determined to be Val-Ala-Gln-Gly-Gly-Ala-Ala-Gly-Leu-Ala. Circular dichroism studies indicated a high alpha-helical content in the native lactoferrin while comparatively lower helical structures were present in the N and C lobes. In addition, the iron saturations of the N and C lobes appeared to be lower than that of the native protein. 相似文献
27.
28.
This paper describes the investigations carried out on the kinetics of the solid-gas reaction between terephthalic acid and methyl alcohol in presence of phosphoric acid as the catalyst. As phosphoric acid itself reacts with the alcohol forming methyl phosphates, the kinetic data obtained have been analysed taking this reaction into account. The rate of formation of monomethyl phosphate has been found to be proportional to the square of the mole fraction of the acid present at any instant. The activation energy involved in this reaction is 11.3 kcal/g mole. The data obtained on terephthalic acid esterification show that; (i) the conversion sharply increases with an increase in the catalyst concentration between 30–35 per cent (w/w), (ii) the ‘shrinking particle’ model, proposed by Levenspiel for solid-gas reactions, can be conveniently adapted to represent the reaction, and (iii) the temperature dependency of the reaction follows the Arrhenius law, with activation energy being 11.5 kcal/g mole. 相似文献
29.
Mesocarbon microbeads are now-a-days used as a prominent self-sintering precursor for the production of high density monolithic graphite. The quality of this graphite is highly dependent on the characteristics of these microbeads, such as the quinoline and toluene insoluble contents, -resins content and volatile matter content, which in turn, can be controlled to desired values by suitable treatments of their extraction and calcination. In the present paper, the authors give an account of the study conducted to see the effect of calcination conditions of mesocarbon microbeads on the characteristics of the resulting graphite. A calcination at a temperature in the range of 280–320 °C for 30 min. under an ambient pressure of nitrogen, or at a temperature of 245–310 °C for 10 min. under a reduced pressure (5 cm Hg) of nitrogen, results in mesocarbon microbeads having a quinoline insoluble content of 83.6–89.8%, toluene insoluble content of 94.4–99.7%, -resins content of 6.8–11.9% and a volatile matter content of 10.2–13.5%. Such microbeads have been found to lead to a monolithic graphite possessing a bulk density 1.91–2.02 g cm–3, bending strength of 62–70 MPa, Shore hardness of 58–69, electrical resistivity of 2.1–2.6 m cm and a degree of anisotropy of 1.02–1.05. 相似文献
30.
Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Mg VI. The configurations used are 2s22p3, 2s2p4, 2p5, 2s22p23s, 2s22p23p, and 2s22p23d, giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 12, 24, 36, 48, and 60 Ry. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved at electron densities covering the range of 108-1014 cm−3 at an electron temperature of log Te (K) = 5.6, corresponding to maximum abundance of Mg VI. Relative and absolute spectral line intensities are calculated and compared with observations of a solar active region. 相似文献