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1.
An electrochemical study of structurally-integrated xLi2MnO3•(1 x)LiMn0.5Ni0.5O2 ‘composite’ materials has been undertaken to investigate the stability of electrochemically-activated electrodes at the Li2MnO3-rich end of the Li2MnO3–LiMn0.5Ni0.5O2 tie-line, i.e., for 0.7  x  0.95. Excellent performance was observed for x = 0.7 in lithium half-cells; comparable to activated electrodes that have significantly lower values of x and are traditionally the preferred materials of choice. Electrodes with higher manganese content (x  0.8) showed significantly reduced performance. Implications for stabilizing low-cost, manganese-rich, layered lithium-metal-oxide electrode materials are discussed.  相似文献   
2.
We consider a nonlinear mathematical model of hematopoietic stem cell dynamics, in which proliferation and apoptosis are controlled by growth factor concentrations. Cell proliferation is positively regulated, while apoptosis is negatively regulated. The resulting age-structured model is reduced to a system of three differential equations, with three independent delays, and existence of steady states is investigated. The stability of the trivial steady state, describing cells dying out with a saturation of growth factor concentrations is proven to be asymptotically stable when it is the only equilibrium. The stability analysis of the unique positive steady state allows the determination of a stability area, and shows that instability may occur through a Hopf bifurcation, mainly as a destabilization of the proliferative capacity control, when cell cycle durations are very short. Numerical simulations are carried out and result in a stability diagram that stresses the lead role of the introduction rate compared to the apoptosis rate in the system stability.  相似文献   
3.
Long term stability of organic selenium compounds (selenocystine, selenomethionine, trimethylselenonium ion) has been studied over a one year period for 2 analyte concentrations: 25 and 150 μg/L Se, at pH 4.5 in the dark, under different storage conditions: temperature of –20°C, 4°C, 20°C, 40°C; in Pyrex, Teflon, or polyethylene containers; in an aqueous matrix or in the presence of a chromatographic counter ion (pentyl sulfonate at 10–4 mol/L concentration). Light effects have also been tested. The stability of the selenium species was monitored by HPLC-ICP/MS. Storage conditions can drastically alter the stability of organic selenium species. Organoselenium compounds were shown to be stable in the dark over a one year period in an aqueous matrix at pH 4.5 in Pyrex containers at both 4°C and 20°C. Pyrex vials exposed to natural sunlight at room temperature resulted in a steady decrease of the selenoamino acid concentration. Teflon containers caused losses of less than 25% at both 4° C and 20° C in the dark. However, polyethylene vials presented, at all temperatures tested, a rapid decrease of the TMSe+ concentration. The stability of the Se species studied did not show significant differences between 4° C and 20° C in any container material used. Storage of solutions at 40° C led to slight differences between the Pyrex and Teflon containers. However, polyethylene presented a drastic decrease of the three species over time at this higher temperature. Solutions frozen at –20° C in polyethylene vials did not stabilize the TMSe+ signal. Finally, concentrations and matrices of the samples did not significantly affect the stability of the species. Received: 15 July 1996 / Revised: 14 July 1997 / Accepted: 18 July 1997  相似文献   
4.
This paper is devoted to factorial designs generated by Patterson's (1976) DSIGN method. As stated by Bailey (1977), the identification of confounded effects may be done here by constructing annihilators of given subgroups of the treatment group.  相似文献   
5.
Résumé L'évolution des propriétés viscoélastiques linéaires de polymères en solution est caractérisée par l'étude des variations des trois paramètres caractéristiques de la viscoélasticité linéaire (viscosité limite 0, complaisance limiteJ e 0 et module de plateauG N 0 ) en fonction de la concentration dans différents solvants. Les polymères étudiés sont des polybutadiènes linéaires et branchés en étoile, à distribution étroite de masses moléculaires. Les solvants étudiés ont des températures de transition vitreuse inférieure, égale ou supérieure à celle du polybutadiène (respectivement le tétradécane, un polybutadiène de faible masse moléculaire et une huile commerciale). Les résultats obtenus ( 0 c 4,0,G N 0 c 2,25 etJ e 0 c –2,25) sont en désaccord avec le comportement conforme aux modèles de champ moyen généralement admis, et semble partiellement en accord avec les modèles récents de la relaxation et de la thermodynamique des longues chaînes flexibles.
Summary The linear viscoelastic properties of polymer solutions are studied through the concentration dependence of the three characteristic parameters of linear viscoelasticity (zero-shear viscosity 0, limiting complianceJ e 0 and plateau modulusG N 0 ) in various solvents. The polymers are narrow-distribution linear and starbranched polybutadienes. The glass transition temperatures of the solvents are below, equal to, and above, theT g of polybutadiene (respectively tetradecane, low molecular weight polybutadiene, and a commercial oil). The results ( 0 c 4.0,G N 0 c 2.25 andJ e 0 c –2.25) do not agree with the usual mean field behavior assumed for the viscoelastic properties of polymer solutions, and seem to agree partly with recent models for the relaxation and thermodynamics of long chains.

Zusammenfassung Das linear-viskoelastische Verhalten von Polymerlösungen wird anhand der Abhängigkeit der drei charakteristischen Parameter (Nullviskosität 0, NullkomplianzJ e 0 und PlateaumodulG N 0 ) von der Konzentration in verschiedenen Lösungsmitteln untersucht. Bei den Polymeren handelt es sich um lineare und sternförmig verzweigte Polybutadiene mit enger Molmassenverteilung. Die Glasübergangstemperaturen der Lösungsmittel (Tetradecan, niedermolekulares Polybutadien bzw. ein kommerzielles Ol) sind niedriger, gleich oder höher als diejenigen des Polybutadiens. Die Ergebnisse ( 0 c 4,0,G N 0 c 2,25 undJ e 0 c –2,25) stimmen nicht mit den Voraussagen der üblicherweise zugrundegelegten, durch Mittelwertbildung gekennzeichneten Theorien überein, sondern scheinen zumindest zum Teil denen neuer thermodynamischer bzw. Relaxationstheorien langkettiger Polymerer zu entsprechen.


Avec 13 figures et 6 tableaux  相似文献   
6.
The finite field approach has been implemented in the periodic ab initio CRYSTAL program and been used for calculating the dielectric constants of crystalline LiF and MgO (FCC structure) and BeO (wurtzite structure). To maintain the periodicity along the applied field direction, a "sawtooth" potential is used in conjunction with a supercell scheme. Supercells four to five times longer than the primitive cell in the direction of the applied field provide well-converged results. The influence of the computational parameters is discussed. An alternative scheme has also been implemented, for inner check, that consists of applying a static electric field to a slab of increasing thickness in the direction orthogonal to the surface; the dielectric response at the center of the slab is shown to converge rapidly to the bulk value evaluated with the sawtooth field. The method is accurate and permits the determination of nonlinear corrections to the dielectric constant. When used in conjunction with the local density approximation (LDA) scheme, it provides for the dielectric constant of the three above-mentioned compounds values close to those recently obtained with a time-dependent density functional theory approach.  相似文献   
7.
Ventcel boundary conditions are second order differential conditions that appear in asymptotic models. Like Robin boundary conditions, they lead to well-posed variational problems under a sign condition of the coefficient. This is achieved when physical situations are considered. Nevertheless, situations where this condition is violated appeared in several recent works where absorbing boundary conditions or equivalent boundary conditions on rough surfaces are sought for numerical purposes. The well-posedness of such problems was recently investigated: up to a countable set of parameters, existence and uniqueness of the solution for the Ventcel boundary value problem holds without the sign condition. However, the values to be avoided depend on the domain where the boundary value problem is set. In this work, we address the question of the persistency of the solvability of the boundary value problem under domain deformation.  相似文献   
8.
The need for speciation-related information in environmental chemistry is critically evaluated. Recent advances in the analysis for volatile or volatizable organometallic compounds and redox states are presented. They include microwave-assisted extraction for the leaching of analytes and the solubilization of tissues, multicapillary gas chromatography, and integration of the sample preparation and separation steps in a single speciation-dedicated instrument. Particular attention is given to the needs for speciation analysis in environmental biota: plant and animal tissues. The areas discussed include speciation of selenium and arsenic, analytical chemistry of metal complexes with phytochelatins and metallothioneins, the need for the identification of metal enzymes and co-enzymes and their role in the biosynthesis of metal complexes. The status of the rapidly emerging field of bioinorganic analytical chemistry is discussed.  相似文献   
9.
Traceability is now considered to be a key concept in chemical and biological measurement sciences. While this concept is increasingly used in many areas, it is still prone to misunderstandings with respect to more ‘classical' terms such as e.g. accuracy. Traceability implies that measurement data are linked to stated references through an unbroken chain of comparisons, all with stated uncertainties. What does the word ‘stated references' mean when it is applied to complex environmental analysis? This paper discusses how traceability can be conceived in the context of environmental monitoring, in particular the various stated references (documented standards, reference materials, environmental specimens) to which chemical environmental data may be linked to.  相似文献   
10.
The aim of this study was to characterize colloids associated with uranium by using an on-line fractionation/multi-detection technique based on asymmetrical flow field-flow fractionation (As-Fl-FFF) hyphenated with UV detector, multi angle laser light scattering (MALLS) and inductively coupling plasma-mass spectrometry (ICP-MS). Moreover, thanks to the As-Fl-FFF, the different colloidal fractions were collected and characterized by a total organic carbon analyzer (TOC). Thus it is possible to determine the nature (organic or inorganic colloids), molar mass, size (gyration and hydrodynamic radii) and quantitative uranium distribution over the whole colloidal phase. In the case of the site studied, two populations are highlighted. The first population corresponds to humic-like substances with a molar mass of (1500 ± 300) g mol−1 and a hydrodynamic diameter of (2.0 ± 0.2) nm. The second one has been identified as a mix of carbonated nanoparticles or clays with organic particles (aggregates and/or coating of the inorganic particles) with a size range hydrodynamic diameter between 30 and 450 nm. Each population is implied in the colloidal transport of uranium: maximum 1% of the uranium content in soil leachate is transported by the colloids in the site studied, according to the depth in the soil. Indeed, humic substances are the main responsible of this transport in sub-surface conditions whereas nanoparticles drive the phenomenon in depth conditions.  相似文献   
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