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51.
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53.
Hydraulic permeability is studied in porous media consisting of randomly distributed monodisperse spheres by means of computational fluid dynamics (CFD) simulations. The packing of spheres is generated by inserting a certain number of nonoverlapping spherical particles inside a cubic box at both low and high packing fractions using proper algorithms. Fluid flow simulations are performed within the interparticulate porous space by solving Navier-Stokes equations in a low-Reynolds laminar flow regime. The hydraulic permeability is calculated from the Darcy equation once the mean values of velocity and pressure gradient are calculated across the particle packing. The simulation results for the pressure drop across the packing are verified by the Ergun equation for the lower range of porosities (<0.75), and the Stokes equation for higher porosities (∼1). Using the results of simulations, the effects of porosity and particle diameters on the hydraulic permeability are investigated. Simulations precisely specified the range of applicability of empirical or semi-empirical correlations for hydraulic permeability, namely the Carman-Kozeny, Rumpf-Gupte, and Howells-Hinch formulas. The number of spheres in the model is gradually decreased from 2000 to 20 to discover the finite-size effect of pores on the hydraulic permeability of spherical packing, which has not been clearly addressed in the literature. In addition, the scale dependence of hydraulic permeability is studied via simulations of the packing of spheres shrunk to lower scales. The results of this work not only reveal the validity range of the aforementioned correlations, but also show the finite-size effect of pores and the scale-independence of direct CFD simulations for hydraulic permeability. 相似文献
54.
We consider an approach for ex post evaluation of approximate solutions obtained by a well known simple greedy method for
set packing. A performance bound is derived that is a function of the highest average reward per item over subsets as well
as the number of allocated subsets and ground items. This a posterior bound can enable much revelation of optimality when
the solution is near optimal. One of the advantages of the ex post analysis is that it does not require computing the optimal
solution to the LP relaxation. The ex post bound will not be guaranteed to reveal substantial levels of optimality for all
problem instances but can be a useful tool that is complementary to other traditional methods for ex post evaluation for the
set packing problem. 相似文献
55.
Carolyn Pratt Brock 《Helvetica chimica acta》2023,106(2):e202200170
A survey has been made of the well determined (R≤0.050) organic crystal structures that have only a single glide plane (e. g., are in space group Pc) and that have more than one crystallographically independent formula unit (Z′>1); the goal was to discover what fraction have additional approximate symmetry. Of the 377 unique structures examined 8 % should almost certainly been refined in a higher-symmetry unit cell; 86 % of the remaining 347 have approximate symmetry that is periodic in at least one dimension. While these percentages are similar to those found for P1 structures (C. P. Brock, Acta Crystallogr., Sect. B 2022 , 78, 576–588), the types of approximate symmetry differ because 89 % of the P1, Z′>1 structures were composed of enantiopure material. The 347 reliable Pc structures include 118 that are slightly distorted or mimicked Cc and P21/c structures, 15 of which were reported as having been determined at room temperature. The distortions in another 72 are so large that the approximate symmetry must be seen as periodic in two dimensions only. These results suggest that symmetry lowering may accompany transformation of a crystal nucleus to a macroscopic crystal. 相似文献
56.
H. Aoki S. Kosakabe M. Inumaru A. Kuboki S. Ohira M. Kodama 《Journal of Thermal Analysis and Calorimetry》2008,92(2):443-449
D-erythro sphingomyelines (SM) having a defined acyl chain were synthesized with sphingosylphosphorylcholine as a starting material,
and both a structural property and its relating phase transition phenomenon were compared between a symmetric chain length
SM (palmitoyl-SM: C16-SM) and asymmetric chain length SMs (behenoyl-SM: C22-SM, lignoceryl-SM: C24-SM). Furthermore, effect
of increasing a content of asymmetric chain SMs in the mixture systems of C22-SM/C16-SM, and C24-SM/C16-SM was investigated.
The present calorimetric and electron microscopic studies revealed that (1) The main transition enthalpy is smaller for the
asymmetric chain SMs than for the symmetric chain SM by about 3 kJ mol−1, although the acyl chain length is longer for the former than for latter; (2) Relatively small size vesicles (100∼200 nm
diameters) surrounded by one or more lamellae are observed for the asymmetric chain SMs, in contrast to large multilamellar
vesicles (1500∼2500 nm diameters) having at least fifteen stained lamellae for the symmetric chain SM and (3) The coexisting
asymmetric chain SMs cause the decrease in size and multiplicity for the MLV of the symmetric chain SM, simultaneously with
a decrease in the main transition enthalpy. 相似文献
57.
We consider a generalized one-dimensional bin packing model in which the cost of a bin is a nondecreasing concave function of the utilization of the bin. We show that for any given positive constant ?, there exists a polynomial-time approximation algorithm with an asymptotic worst-case performance ratio of no more than 1 + ?. 相似文献
58.
Edward G. Coffman Jr. Jnos Csirik Lajos Rnyai Ambrus Zsbn 《Discrete Applied Mathematics》2008,156(14):2810-2816
The average-case analysis of algorithms usually assumes independent, identical distributions for the inputs. In [C. Kenyon, Best-fit bin-packing with random order, in: Proc. of the Seventh Annual ACM-SIAM Symposium on Discrete Algorithms, SIAM, 1996, pp. 359–364] Kenyon introduced the random-order ratio, a new average-case performance metric for bin packing heuristics, and gave upper and lower bounds for it for the Best Fit heuristics. We introduce an alternative definition of the random-order ratio and show that the two definitions give the same result for Next Fit. We also show that the random-order ratio of Next Fit equals to its asymptotic worst-case, i.e., it is 2. 相似文献
59.
Jean-Paul Penot 《Set-Valued Analysis》2001,9(1-2):131-157
We show that three important topics in nonlinear analysis and optimization are intimately related: the theory of perturbations, the notion of well-posedness and variational principles in the sense of Ekeland, Borwein–Preiss and Deville–Godefroy–Zizler. The concept of genericity and the new notion of flexible perturbation play a key role in these connections. This notion enables one to consider topologies on spaces of functions which have been introduced recently. A link between the Asplund and Ekeland–Lebourg methods and the Palais–Smale condition, another important topic in nonlinear analysis, is pointed out. 相似文献
60.
Einar L. Hinrichsen Jens Feder Torstein Jøssang 《Journal of statistical physics》1986,44(5-6):793-827
By sequentially adding line segments to a line or disks to a surface at random positions without overlaps, we obtain configurations of the one- and two-dimensional random sequential adsorption (RSA) problem. We have simulated the one- and two-dimensional problem with periodic boundary condition. The one-dimensional simulations are compared with the exact analytical solutions to give an estimate of the accuracy of the simulation. In two dimensions the geometrical properties of the RSA configuration are discussed and in addition known results of the RSA process are reproduced. Various statistical distributions of the Voronoi-Dirichlet (VD) network corresponding to the RSA disk configuration are analyzed. In order to characterize pores in the RSA configuration, we introduce circular holes. There is a direct correspondence between vertices of the VD network and these holes, and also between direct/indirect geometrical neighbors and these holes. The hole size distribution is found to be a parabola. We also find general relations that connect the asymptotic behavior of the surface coverage, the correlation function, and the hole size distribution. 相似文献