Ammoxidation of toluene over the perovskites YBa2Cu3O6.1, YBa2Cu2CoO6.7 and YBaCuCoO4.9 was investigated at 400 °C. At low partial pressures of O2 benzonitrile was selectively formed, while CO2 was the main product at high pressures of O2. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O2 pressures, Cu-sites are active for nitrile formation, while Co-sites give CO2. At high O2 pressures, the activity for CO2 of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies. 相似文献
The relationship between chemical degradation and thickness loss of an unpigmented, non UV-stabilized, crosslinked amine-cured
epoxy coating exposed to three UV conditions was investigated. Spin-coated samples having a thickness of approximately 7 μm
on an Si substrate were prepared from a stochiometric mixture of a bisphenol A epoxy resin and a tetra-functional amine curing
agent. Samples were exposed outdoors and to two accelerated laboratory UV environments. Chemical degradation and thickness
loss were measured by transmission Fourier transform infrared spectroscopy (FTIRS) and laser scanning confocal microscopy
(LSCM), respectively. In addition, surface roughness and morphological changes were measured by atomic forcemicrosocopy (AFM)
and LSCM. Substantial chemical degradation, thickness loss, and morpholocal changes occurred in the exposed films, and the
rate of chemical degradation was greater than that due to the thickness loss. This additional chemical loss was attributed
to an inhomogeneous degradation process in which nanoscale localized depressions initiate at certain sites on the surface,
which then enlarge and deepen with exposure time. The results of this study provide a better understanding of the degradation
mechanism and should lead to the development of scientific-based models for predicting the service life of crosslinked amine-cured
epoxy coatings.
Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago,
IL 相似文献
The most commonly used bearing couple in prosthetic hip or knee joint replacements consists of a cobalt–chrome (CoCr) metal alloy articulating against ultrahigh-molecular-weight polyethylene. Ceramics have been used as an alternative to metal-on-polyethylene in joint replacement surgery of arthritic hips and knees since the 1970s. In prosthetic hip and knee bearings, ceramic surfaces offer a major benefit of drastically reduced wear rates and excellent long-term biocompatibility, which can increase the longevity of prosthetic hip and knee joints. This benefit is important clinically because hip and knee replacement has become a very common surgical procedure, particularly in the United States, and because these procedures are being increasingly performed in younger patients who place greater demands on the prosthetic bearings. However, ceramics are brittle and the risk of catastrophic bearing failure in vivo , while rare, is a major concern. Improvements in material quality, manufacturing methods, and implant design have resulted in a drastic reduction of the incidence of such failures, so that modern ceramic bearings are safe and reliable if used with components of proven design and durability. Future material improvements are actively being investigated to reduce the risk of ceramic-bearing failures even further. The purpose of this article is to review the structure, properties, applications, and limitations of the ceramics that have been used in orthopedic bearings, and to describe the new ceramic composite materials and surface treatments that will be available for joint replacement surgery in the near future. 相似文献
The use of a periodic flow reactor is described for the oxidation of butane to maleic anhydride to compare the catalytic performance
of vanadium phosphate catalysts operating under aerobic and anaerobic conditions. It is found that for the catalyst prepared
via a standard VPO method, operation in the absence of oxygen leads to a very small enhancement in selectivity when butane
concentrations in the range 0.9–2.9% are used. Operation in the absence of oxygen leads to very small differences in conversion
such that the overall yield is enhanced and this effect is maximised for reactor feeds containing 1.5% butane. However, the
enhancement is negligible when the catalyst is operated at high conversion required for commercial operation, indicating that
reactors operating with continuous flow with aerobic conditions are preferred. Similar experiments are conducted for a catalyst
prepared by the VPD method and, in contrast, this catalyst gives lower butane conversion and maleic anhydride selectivity
when operated in the absence of oxygen. 相似文献
We present preliminary results of the first NMR T1-T2 two-dimensional relaxation correlation experiments performed using a one-sided NMR system in cement based materials. Two-dimensional correlation relaxometry has itself only recently been demonstrated in cement paste where it proved to be a particularly sensitive probe of pore-water dynamics providing direct information on exchange of water between the gel and capillary pore networks. Further to this we have observed differences in the structural development of a selection of cement pastes throughout the early stages of hydration and verified the theoretical frequency dependence of the ratio T1 / T2. When coupled with instrumentation developments in one-sided NMR magnets the way is opened to detailed, spatially resolved studies of the development of hydration and porosity in the surface layers (top 50 mm) of cementitious materials. A new magnet, suitable for such applications, is discussed. 相似文献
Vinyl stearate was studied as a major internal plasticizer in terpolymers containing vinyl acetate and vinyl chloride. The terpolymers were prepared by systematically replacing vinyl acetate by close increments of vinyl stearate starting with combinations of vinyl acetate and vinyl chloride, in increments, over all compositions. For comparison of properties, a complete range of copolymers of vinyl stearate and vinyl chloride, as well as mixtures of poly(vinyl chloride) and di-2-ethylhexyl phthalate (DOP) were also made. The external plasticizer was more efficient in reducing the glass temperature than was vinyl stearate. The decline in Tg with weight fraction of plasticizer was linear for the copolymers and terpolymers but concave downward with the liquid diluent. The linear decline was shown to involve mere additivity of the free volume contributed by each side-chain methylene (or methyl) group in both vinyl esters to reducing Tg. The mechanism of the diluent system was more complex. However, the magnitude of the reduction of tensile modulus at a given weight fraction of DOP could be equaled or exceeded by the same amount of vinyl stearate, by increasing the vinyl acetate content of the base copolymer to 40 mole-% or more. Unfortunately, the ultimate strengths and elongations of internally plasticized systems were reduced more than those of the mixtures at comparable compositions. Vinyl stearate was found to markedly retard photolytic degradation compared to both vinyl acetate and the external plasticizer in unstabilized samples having nearly the same thermal treatment. The effect was greater than could be ascribed to dilution by the long alkyl group. The production of a stearoyl radical more stable than the radicals initiating dehydrochlorination is suggested as a possible mechanism. 相似文献
Laboratory robotics has been firmly established in many non-QC
laboratories as a valuable tool for automating pharmaceutical
dosage form analysis. Often a single project or product line is used
to justify an initial robot purchase thus introducing robotics to the
laboratory for the first time. However, to gain widespread acceptance
within the laboratory and to justify further investment in robotics,
existing robots must be used to develop analyses for existing manual
methods as well as new projects beyond the scope off the original
purchase justification. The Automation Development Group in
Analytical Research and Development is a team of analysts
primarily devoted to developing new methods and adapting existing
methods for the robot. This team approach developed the expertise
and synergy necessary to significantly expand the contribution of
robotics to automation in the authors'' laboratory. 相似文献
Large scale online kernel learning aims to build an efficient and scalable kernel-based predictive model incrementally from a sequence of potentially infinite data points. Current state-of-the-art large scale online kernel learning focuses on improving efficiency. Two key approaches to gain efficiency through approximation are (1) limiting the number of support vectors, and (2) using an approximate feature map. They often employ a kernel with a feature map with intractable dimensionality. While these approaches can deal with large scale datasets efficiently, this outcome is achieved by compromising predictive accuracy because of the approximation. We offer an alternative approach that puts the kernel used at the heart of the approach. It focuses on creating a sparse and finite-dimensional feature map of a kernel called Isolation Kernel. Using this new approach, to achieve the above aim of large scale online kernel learning becomes extremely simple—simply use Isolation Kernel instead of a kernel having a feature map with intractable dimensionality. We show that, using Isolation Kernel, large scale online kernel learning can be achieved efficiently without sacrificing accuracy.