全文获取类型
收费全文 | 23117篇 |
免费 | 3889篇 |
国内免费 | 2096篇 |
专业分类
化学 | 7929篇 |
晶体学 | 285篇 |
力学 | 5207篇 |
综合类 | 212篇 |
数学 | 2748篇 |
物理学 | 12721篇 |
出版年
2024年 | 61篇 |
2023年 | 273篇 |
2022年 | 830篇 |
2021年 | 829篇 |
2020年 | 847篇 |
2019年 | 771篇 |
2018年 | 671篇 |
2017年 | 804篇 |
2016年 | 1096篇 |
2015年 | 939篇 |
2014年 | 1258篇 |
2013年 | 2078篇 |
2012年 | 1355篇 |
2011年 | 1533篇 |
2010年 | 1327篇 |
2009年 | 1445篇 |
2008年 | 1537篇 |
2007年 | 1604篇 |
2006年 | 1394篇 |
2005年 | 1141篇 |
2004年 | 927篇 |
2003年 | 898篇 |
2002年 | 791篇 |
2001年 | 671篇 |
2000年 | 633篇 |
1999年 | 541篇 |
1998年 | 561篇 |
1997年 | 367篇 |
1996年 | 309篇 |
1995年 | 236篇 |
1994年 | 256篇 |
1993年 | 199篇 |
1992年 | 147篇 |
1991年 | 127篇 |
1990年 | 107篇 |
1989年 | 81篇 |
1988年 | 68篇 |
1987年 | 72篇 |
1986年 | 52篇 |
1985年 | 61篇 |
1984年 | 38篇 |
1983年 | 24篇 |
1982年 | 38篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1976年 | 12篇 |
1971年 | 7篇 |
1957年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Relative permeability functions for immiscible displacements in porous media show a wide range of profiles. Although, this
behavior is well known, its impact on the stability of the displacement process is unexplored. Our analysis clearly demonstrates
for the first time that the viscous instability characteristics of two-phase flows are governed not only by their end point
values, but are strongly dependent on the actual profile of relative permeability functions. Linear stability analysis predicts
the capacity of the flow to develop large scale fingers which can result in substantial bypassing of the resident fluid. It
is observed that relative permeability functions attributed to drainage processes yield a more unstable displacement as compared
to functions related to imbibition processes. Moreover, instability is observed to increase for those relative permeability
functions which result from increased wettability of the wetting fluid. High accuracy numerical simulations show agreement
with these predictions and demonstrate how large amplitude viscous fingers result in significant bypassing for certain relative
permeability functions. In the nonlinear regime, the finger amplitude grows at a rate ∝ t1/2 initially, drops to t1/4 at a later time and finally grows ∝ t. The basic mechanisms of finger interaction, however, are not substantially influenced by relative permeability functions. 相似文献
82.
A particle imaging technique has been used to collect droplet displacement statistics in a round turbulent jet of air. Droplets are injected on the jet axis, and a laser sheet and position-sensitive photomultiplier tube are used to track their radial displacement and time-of-flight. Dispersion statistics can be computed which are Lagrangian or Eulerian in nature. The experiments have been simulated numerically using a second-order closure scheme for the jet and a stochastic simulation for the particle trajectories. Results are presented for non-vaporizing droplets of sizes from 35 to 160 μm. The simulations have underscored the importance of initial conditions and early droplet displacement history on the droplet trajectory for droplets with large inertia relative to the turbulence. Estimates of initial conditions have been made and their effect on dispersion is quantified. 相似文献
83.
W. Schindler 《Computational Geometry》1994,4(6):327-343
Stochastic simulations on manifolds usually are traced back to
n via charts. If a group G is acting on a manifold M and if the respective distribution v is invariant under this group action then in many cases of practical interest there exists a more convenient approach which uses equivariant mappings. The concept of equivariant mappings will be discussed intensively at the instance of the Grassman manifold in which case G equals the orthogonal group. Further advantages of this concept will be demonstrated by applying it to a probabilistic problem from the field of combinatorial geometry. 相似文献
84.
We develop a model of distributed damage in brittle materials deforming in triaxial compression based on the explicit construction of special microstructures obtained by recursive faulting. The model aims to predict the effective or macroscopic behavior of the material from its elastic and fracture properties; and to predict the microstructures underlying the microscopic behavior. The model accounts for the elasticity of the matrix, fault nucleation and the cohesive and frictional behavior of the faults. We analyze the resulting quasistatic boundary value problem and determine the relaxation of the potential energy, which describes the macroscopic material behavior averaged over all possible fine-scale structures. Finally, we present numerical calculations of the dynamic multi-axial compression experiments on sintered aluminum nitride of Chen and Ravichandran [1994. Dynamic compressive behavior of ceramics under lateral confinement. J. Phys. IV 4, 177-182; 1996a. Static and dynamic compressive behavior of aluminum nitride under moderate confinement. J. Am. Soc. Ceramics 79(3), 579-584; 1996b. An experimental technique for imposing dynamic multiaxial compression with mechanical confinement. Exp. Mech. 36(2), 155-158; 2000. Failure mode transition in ceramics under dynamic multiaxial compression. Int. J. Fracture 101, 141-159]. The model correctly predicts the general trends regarding the observed damage patterns; and the brittle-to-ductile transition resulting under increasing confinement. 相似文献
85.
李林 《Annals of Differential Equations》2004,20(2):118-124
This paper considers the scalar differential delay equation x(t) = -μx(t)-f(x(t-Τ(t)), t). By using the mapping method we obtain that the solutionswill be ultimately in some interval. 相似文献
86.
介绍了虚拟技术在实验教学改革中的作用与意义,探讨了电子信息系统实验教学改革的新思路与实现措施,并总结了实验教学改革的初步成果. 相似文献
87.
An analogy between social and hydrodynamic processes is developed. The relation of the state system to the passionarity theory suggested by L. N. Gumilev is discussed. 相似文献
88.
Theoretical models for Monte Carlo simulation of radiative processes, i.e. bremsstrahlung and characteristic x-ray emission, are presented. Possible strategies for simulating electron transport are briefly described. For mechanisms involving energy loss and angular deflections, difficulties for strict implementation of accurate numerical differential cross sections still remain due to the strong correlations between these variables. Practical solutions for the case of inelastic collisions and bremsstrahlung emission are described. Comparisons of simulation results with experimental data for several problems of interest in electron probe microanalysis are presented. 相似文献
89.
S. F. Chekmarev R. Mitri V. Bonaci-Koutecký 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):45-48
The ab initio molecular
dynamics (AIMD) [1] is combined with the heuristic, successive
confinement method of surveying a potential energy surface (PES)
[2], thereby offering a framework for the simulation study of
kinetics and equilibrium properties of metallic clusters. This
approach is applied to the study of Au4,
a cluster possessing a simple but specific PES, which consists
of very shallow and deep basins and due to this presents a
challenge to the conventional AIMD methods. Among other things,
the probabilities of the transitions between isomers have been
found, and on this basis, both the time-dependent and
equilibrium populations of the isomers have been calculated for
the conditions typical of the NeNePo experiments [3] in the
femtosecond pump-probe spectroscopy. 相似文献
90.
LI Na ZHAO Degang & YANG Hui State Key Laboratory on Integrated Optoelectronics Institute of Semiconductors Chinese Academy of Sciences Beijing China 《中国科学G辑(英文版)》2004,47(6):694-701
Because of their large band-gap, large high-field electron velocity, large breakdownfield, and large thermal conductivity, GaN and its heterojunction with AlGaN and InGaNhave foreseeable potential in the applications of high-power/temperature electronics, andoptoelectronic devices operative in UV and visible wavelength. Polarization inducedelectric field can reach the magnitude of ~MV/cm[1,2]. For AlGaN/GaN based FETs theconcentration of sheet carrier induced by polarization in the cha… 相似文献