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991.
Dawson K Kopff LA Zhu L McMahon RJ Yu L Richert R Ediger MD 《The Journal of chemical physics》2012,136(9):094505
Physical vapor deposition of organic molecules can produce glasses with high kinetic stability and low enthalpy. Previous experiments utilizing wide-angle x-ray scattering (WAXS) have shown that, relative to the ordinary glasses prepared by cooling the supercooled liquid, such glasses exhibit excess scattering characteristic of anisotropic packing. We have used vapor deposition to prepare glasses of four isomers of tris-naphthylbenzene (TNB), and measured both the WAXS patterns and the kinetic stability. While vapor-deposited glasses of all four TNB isomers exhibit high and nearly uniform kinetic stability, the level of excess scattering varies significantly. In addition, for α,α,β-TNB, glasses of essentially identical kinetic stability can have excess scattering levels that vary by a factor of two. These results indicate that anisotropic packing is not the source of kinetic stability in vapor-deposited glasses but rather a secondary feature that depends upon the chemical structure of the glass-forming molecules. We also show that the time required for these stable vapor-deposited glasses to transform into the supercooled liquid greatly exceeds the structural relaxation time τ(α) of the liquid and scales approximately as τ(α) (0.6). The kinetic stability of the vapor-deposited TNB glasses matches that expected for ordinary glasses that have been aged for 10(2) to 10(7) years. 相似文献
992.
M Ahmed A Ganesan F Wang V Feyer O Plekan KC Prince 《The journal of physical chemistry. A》2012,116(33):8653-8660
X-ray photoelectron spectra of the core and valence levels of the fundamental building blocks of β-lactam antibiotics have been investigated and compared with theoretical calculations. The spectra of the compounds 2-azetidinone and the 2- and 4-isomers of thiazolidine-carboxylic acid are interpreted in the light of theoretical calculations. The spectra of the two isomers of thiazolidine-carboxylic acid are rather similar, as expected, but show clear effects due to isomerization. Both isomers are analogues of proline, which is well-known to populate several low energy conformers in the gas phase. We have investigated the low energy conformers of thiazolidine-4-carboxylic acid theoretically in more detail and find some spectroscopic evidence that multiple conformers may be present. The measured valence levels are assigned for all three compounds, and the character of the frontier orbitals is identified and analyzed. 相似文献
993.
Heaney KD Campbell RL Murray JJ 《The Journal of the Acoustical Society of America》2012,131(2):1680-1688
During the CALOPS 2007 experiment, off the coast of Fort Lauderdale, Florida, three-dimensional (3D) multipath was observed using a bottom mounted horizontal line array during source tows along the 200 m isobath [Kevin D. Heaney and James J. Murray, J. Acoust. Soc. Am. 125(4), 1394-1402 (2008)]. In this paper a hybrid modeling approach is presented to model the 3D sound on the Florida shelf, nearly shaped like the canonical wedge. The hybrid approach combines vertical acoustic normal modes with the parabolic equation solution (in range/cross-range). The approach is shown to satisfy the 3D Cartesian-coordinate wave equation in the limit of adiabatic mode propagation. In the adiabatic mode parabolic equation (AMPE) approach modal phase speeds vs position are used as the input to the parabolic equation computation with dimensions of easting (km) and northing (km). Vertical adiabatic modes and horizontal rays are also computed to illustrate the 3D multipath arrival. The AMPE field is computed for all the modes for each element of the horizontal array. Beamforming vs source range is then conducted and excellent agreement with data is achieved. 相似文献
994.
Jeffrey Humpherys Gregory Lyng Kevin Zumbrun 《Archive for Rational Mechanics and Analysis》2009,194(3):1029-1079
Extending recent results in the isentropic case, we use a combination of asymptotic ODE estimates and numerical Evans-function
computations to examine the spectral stability of shock-wave solutions of the compressible Navier–Stokes equations with ideal
gas equation of state. Our main results are that, in appropriately rescaled coordinates, the Evans function associated with
the linearized operator about the wave (i) converges in the large-amplitude limit to the Evans function for a limiting shock
profile of the same equations, for which internal energy vanishes at one end state; and (ii) has no unstable (positive real
part) zeros outside a uniform ball |λ| ≦ Λ. Thus, the rescaled eigenvalue ODE for the set of all shock waves, augmented with
the (nonphysical) limiting case, form a compact family of boundary-value problems that can be conveniently investigated numerically.
An extensive numerical Evans-function study yields one-dimensional spectral stability, independent of amplitude, for gas constant
γ in [1.2, 3] and ratio ν/μ of heat conduction to viscosity coefficient within [0.2, 5] (γ ≈ 1.4, ν/μ ≈ 1.47 for air). Other values may be treated similarly but were not considered. The method of analysis extends also to the
multi-dimensional case, a direction that we shall pursue in a future work. 相似文献
995.
Photomechanics of light-activated polymers 总被引:1,自引:0,他引:1
Kevin N. Long H. Jerry Qi Martin L. Dunn 《Journal of the mechanics and physics of solids》2009,57(7):1103-1508
Light-activated polymers are an exciting class of modern materials that respond mechanically when irradiated by light at particular wavelengths. While details of the mechanisms that connect the optical excitation to mechanical behavior are complex and differ from material to material, there is sufficient commonality among them to permit the development of a generalized modeling framework to describe the photomechanics. The features shared by light-activated polymers involve light interacting with the material, which triggers photochemical reactions that alter the structure of the crosslinked polymer network. Many such structural alterations result in an evolution of the polymer network, and subsequent macroscopic deformation. When this process is appropriately executed it can enable a photomechanical shape-memory effect. In this paper, we develop a three-dimensional finite-deformation modeling framework to describe the photomechanical response of light-activated polymer systems. This framework integrates four coupled phenomena that contribute to macroscopic photomechanical behavior: photophysics, photochemistry, chemomechanical coupling, and mechanical deformation. The chemomechanical coupling consists of chemically induced structural alterations of the crosslinked network that result in subsequent deformation. We describe this behavior through a decomposition of the crosslinked network into two components consisting of an original network and a photochemically altered network; both evolve during photomechanical deformation. The modeling framework presented in this paper is sufficiently general that it is applicable to light-activated polymer systems that operate with various mechanisms in each of the four areas. Using this modeling approach, we develop constitutive models for two recently developed light-activated polymer systems [Lendlein, A., Hongyan, J., Junger, O., Langer, R., 2005. Light-induced shape-memory polymers. Nature 434 (7035) 879; Scott, T.F., Schneider, A.D., Cook, W.D., Bowman, C.N., 2005. Photoinduced plasticity in crosslinked polymers. Science 308 (5728) 1615]. For the material developed by Scott and his co-workers we validate our model by measuring and numerically simulating photo-induced stress relaxation and bending deformation and obtain good agreement between measurements and predictions. Finally, we use the model to study the effects of photomechanical parameters (applied strain magnitude, irradiation time and intensity, and photoabsorber concentration) and the behavior of the network evolution rule on the material response. 相似文献
996.
Determination of contact parameters for discrete element method simulations of granular systems 总被引:2,自引:0,他引:2
Both linear-spring-dashpot (LSD) and non-linear Hertzian-spring-dashpot (HSD) contact models are commonly used for the calculation of contact forces in Discrete Element Method (DEM) simulations of granular systems. Despite the popularity of these models, determination of suitable values for the contact parameters of the simulated particles such as stiffness, damping coefficient, coefficient of restitution, and simulation time step, is not altogether obvious. In this work the relationships between these contact parameters for a model system where a particle impacts on a flat base are examined. Recommendations are made concerning the determination of these contact parameters for use in DEM simulations. 相似文献
997.
Particles have been shown to adsorb at the interface between immiscible homopolymer melts and to affect the morphology of
blends of those homopolymers. We examined the effect of such interfacially active particles on the morphology of droplet/matrix
blends of model immiscible homopolymers. Experiments were conducted on blends of polydimethylsiloxane and 1,4-polyisoprene
blended in either a 20:80 or 80:20 weight ratio. The effects of three different particle types, fluoropolymer particles, iron
particles, and iron oxyhydroxide particles, all at a loading of 0.5 vol.%, were examined by rheology and by direct flow visualization.
Particles were found to significantly affect the strain recovery behavior of polymer blends, increasing or decreasing the
ultimate recovery, slowing down or accelerating the recovery kinetics, and changing the dependence of these parameters on
the applied stress prior to cessation of shear. These rheological observations were found to correlate reasonably well with
particle-induced changes in drop size. The particles can both increase as well as decrease the drop size, depending on the
particle type, as well as on which phase is continuous. The cases in which particles cause a decrease in drop size are analogous
to the particle stabilization of “Pickering emulsions” well-known from the literature on oil/water systems. We hypothesize
that cases in which particles increase drop size are analogous to the “bridging–dewetting” mechanism known in the aqueous
foam literature. 相似文献
998.
Margaret Beck Björn Sandstede Kevin Zumbrun 《Archive for Rational Mechanics and Analysis》2010,196(3):1011-1076
In order to understand the nonlinear stability of many types of time-periodic travelling waves on unbounded domains, one must
overcome two main difficulties: the presence of embedded neutral eigenvalues and the time-dependence of the associated linear
operator. This problem is studied in the context of time-periodic Lax shocks in systems of viscous conservation laws. Using
spatial dynamics and a decomposition into separate Floquet eigenmodes, it is shown that the linear evolution for the time-dependent
operator can be represented using a contour integral similar to that of the standard time-independent case. By decomposing
the resulting Green’s distribution, the leading order behavior associated with the embedded eigenvalues is extracted. Sharp
pointwise bounds are then obtained, which are used to prove that time-periodic Lax shocks are linearly and nonlinearly stable
under the necessary conditions of spectral stability and minimal multiplicity of the translational eigenvalues. The latter
conditions hold, for example, for small-oscillation time-periodic waves that emerge through a supercritical Hopf bifurcation
from a family of time-independent Lax shocks of possibly large amplitude. 相似文献
999.
Boundary Layer Stability¶in Real Vanishing Viscosity Limit 总被引:5,自引:0,他引:5
In the previous paper [20], an Evans function machinery for the study of boundary layer stability was developed. There,
the analysis was restricted to strongly parabolic perturbations, that is to an approximation of the form u
t
+(F(u))
x
=ν(B(u)u
x
)
x
$ (ν≪1) with an “elliptic” matrix B. However, real models, like the Navier–Stokes approximation of the Euler equations for a gas flow, involve incompletely
parabolic perturbations: B is not invertible in general.
We first adapt the Evans function to this realistic framework, assuming that the boundary is not characteristic, neither for
the hyperbolic first order system u
t
+(F(u))
x
= 0, nor for the perturbed system. We then apply it to the various kinds of boundary layers for a gas flow. We exhibit some
examples of unstable boundary layers for a perfect gas, when the viscosity dominates heat conductivity.
Received: 27 November 2000/ Accepted: 16 March 2001 相似文献
1000.
Finite obstruction set characterizations for lower ideals in the minor order are guaranteed to exist by the graph minor theorem. In this paper we characterize several families of graphs with small feedback sets, namely k1-F
V
S
, k2-F
E
S
and (k1,k2)-F
V
/E
S
, for small integer parameters k1 and k2. Our constructive methods can compute obstruction sets for any minor-closed family of graphs, provided the pathwidth (or treewidth) of the largest obstruction is known. 相似文献