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排序方式: 共有644条查询结果,搜索用时 234 毫秒
121.
122.
This paper deals with evaluation of the dependability (considered as a generic term, whose main measures are reliability, availability, and maintainability) of software systems during their operational life, in contrast to most of the work performed up to now, devoted mainly to development and validation phases. The failure process due to design faults, and the behavior of a software system up to the first failure and during its life cycle are successively examined. An approximate model is derived which enables one to account for the failures due to the design faults in a simple way when evaluating a system's dependability. This model is then used for evaluating the dependability of 1) a software system tolerating design faults, and 2) a computing system with respect to physical and design faults. 相似文献
123.
The synthesis of poly(dimethylsiloxane) (PDMS) by anionic polymerization of octamethylcyclotetrasiloxane (D4) in aqueous emulsion using an emulsifying agent acting also as initiator (benzyldimethyldodecylammonium hydroxide) is described. Stable emulsions of α,ω-dihydroxy PDMS with an easily controlled molar mass (up to 15 000), a low polydispersity index and high yields were obtained. The amount of cyclics formed (essentially D4 to D7) is lower than that observed in bulk. Kinetics analysis and computerized simulations are in good agreement with a reaction scheme involving simultaneous polyaddition/polycondensation processes. At high monomer consumption, polycondensation predominates leading to a broadening of the molecular weight distribution (MWD). The apparent rate constants of initiation, propagation and condensation at various temperatures and the corresponding activation energies were determined. 相似文献
124.
This paper proposes here a new method for seeking biquadratic cells. The language Prolog is used to create a programe which searches the large number of possible cases. This article describes sixteen optimal cells which have been selected from a set of more than two million cells. These sixteen cells comply with a set of realistic but dissimilar requirements, which would make an algebraic approach very difficult. 相似文献
125.
Abdelaziz El Madani Jean-Claude Favier Patrick Hmery Pierre Sigwalt 《Polymer International》1992,27(4):353-357
Cyclization of α,ω;-dilithiopolyisoprenes has been achieved in hexane solution in the presence of tetrahydrofuran (15% in volume) at ?40 to - 50°C. Using as linking agent the non-conjugated diene 1,2-bis(isopropenyl-4-phenyl)-ethane, macrocyclic polyisoprenes could be obtained with yields of at least 90%, even for molecular weights as high as 30 000 mol g?1. With SiCl4 as coupling agent no gelation occurred and yields of about 70% in cyclic dimers, probably bicyclic, have been observed. These results are compared to those obtained by various authors with α,ω;-dilithiopolystyrenes or α,ω;-disodiopolystyrenes and are attributed mainly to the presence, in the polyisoprene case, of predominant cyclic aggregates even at low organolithium concentration. 相似文献
126.
Cecile Mazière Jean-Claude Mazière Liliana Mora Martine Auclair Jacques Polonovski 《Lipids》1988,23(5):419-423
A 24-hr pretreatment of cultured human fibroblasts with trifluoperazine induced a marked increase in incorporation of saturated
(stearic, palmitic) and unsaturated (oleic, arachidonic) fatty acids into phospholipids (1.5- to 2-fold for 5.10−5 M trifluoperazine). Concomitantly, incorporation into cholesteryl esters was strongly inhibited (20% of control for 5.10−5 M trifluoperazine). The drug did not change the phospholipid composition of treated cells. The effect of trifluoperazine
on oleic acid incorporation into phospholipids was time-dependent and reached a maximum after a six-hr preincubation with
the drug. Trifluoperazine also induced an increase in the rate of chase of oleic acid from the different phospholipid classes.
In vitro preincubation of cell-free extracts with trifluoperazine resulted in activation of phospholipid acyltransferases,
whereas cholesterol acyltransferase activity was decreased. The rapid effect of trifluoperazine together with its effect on
a cell-free system suggests a direct action of this amphiphilic drug on the acyltransferase activities, probably by modification
of the structural organization of cellular membranes. 相似文献
127.
Francius G Hemmerlé J Ohayon J Schaaf P Voegel JC Picart C Senger B 《Microscopy research and technique》2006,69(2):84-92
A homemade colloidal probe atomic force microscope was used to perform nanoindentation with a spherical probe of 5 microm in diameter, at different approach velocities in order to extract the Young's modulus, E0, of poly(L-lysine)/hyaluronan (PLL/HA) films. This parameter is of prime importance to control cellular adhesion. The films were either kept in their native form or cross-linked with a mixture of 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide (EDC) and N-hydrosulfosuccinimide (sulfo-NHS), where the EDC concentration was varied from 1 up to 100 mg mL(-1) (approximately from 5 to 500 mM). A model based on Hertz mechanics was used to account for the interactions between film and probe. It is shown that the Young's modulus varies with the approach velocity for the native (PLL/HA) films, whereas for cross-linked ones, E0 is independent from the velocity over the whole range investigated. It is found that for native films, E0 takes a value of 3 kPa at low approach velocities, a velocity domain that should be relevant in cellular adhesion processes. The Young's modulus increases with the EDC concentration used to cross-link the films and levels off at a value of about 400 kPa for EDC concentrations exceeding 40 mg mL(-1). Thus, it is possible by crosslinking PLL/HA films to control their elastic properties with the aim to alter their behavior as to the cellular adhesion. 相似文献
128.
The oligomer reaction products between urea and formaldehyde were preliminarily studied by 60-MHz NMR spectroscopy. This analytical method was extended to urea–formaldehyde copolymers and provided means to attempt to determine their molecular stucture. Prior to NMR analysis, dilution water was removed by freeze drying in order to avoid degradation and advancement of the resin. Lithium chloride was found to enhance polycondensate solubility in deuterated dimethylsulfoxide and to improve resolution of the spectra by reducing overlapping of the signals. It was then possible to proceed to a qualitative and quantitative analysis of urea–formaldehyde copolymer structure. Structural differences appeared between one-step synthesized resins and those involving a second urea addition; in the latter case, the percentage of unreacted urea was calculated. The shape of the unresolved broad signal assigned to the various methylene groups yielded further information on the resins. A particular shoulder on the methylene signal gives evidence for polyoxymethylene sequences occurring in polycondensates synthesized from concentrated aqueous formaldehyde solutions. 相似文献
129.
Jean-Claude Loulergue Sen-Lu Xu 《Journal of Infrared, Millimeter and Terahertz Waves》1986,7(1):171-182
A thermal imaging system using a liquid-liquid interface has been studied, which utilizes the extreme temperature sensitivity of surface tension (Marangoni Effect). Theoretical considerations for the design and the operating conditions of the device are given. The experimental results using a detection by polarization interferometer indicate that the limitations on such a device are primarily due to the finite film thickness of the upper solution (on the order of 50–80 μm). This system was able to image 1°C with a limiting resolution of ten lines pair per millimeter. 相似文献
130.
Sara Bals Jo Verbeeck Gustaaf Van Tendeloo Yi-Lin Liu Jean-Claude Grivel 《Journal of the American Ceramic Society》2005,88(2):431-436
The microstructural and compositional evolution during initial annealing of a superconducting (Bi,Pb)2 Sr2 Ca2 Cu3 O10+δ /Ag tape is studied using quantitative transmission electron microscopy. Special attention is devoted to the occurrence of Pb-rich liquids, which are crucial for the Bi2 Sr2 CaCu2 O8+δ to (Bi,Pb)2 Sr2 Ca2 Cu3 O10+δ transformation. Ca and/or Pb-rich (Bi,Pb)2 Sr2 CaCu2 O8+δ grains dissolve into a liquid, which reacts with Ca-rich phases to increase the liquid's Ca-content. This leads to (Bi,Pb)2 Sr2 Ca2 Cu3 O10+δ formation. Apparently, a Ca/Sr ratio of around 1 is sufficient to keep (Bi,Pb)2 Sr2 Ca2 Cu3 O10+δ nucleation going. It is confirmed that Ag particles are transported from the Ag-sheath into the oxide core by the liquid and not by mechanical treatment of the tape. 相似文献