全文获取类型
收费全文 | 4037篇 |
免费 | 726篇 |
国内免费 | 371篇 |
专业分类
化学 | 2665篇 |
晶体学 | 54篇 |
力学 | 296篇 |
综合类 | 17篇 |
数学 | 416篇 |
物理学 | 1686篇 |
出版年
2024年 | 17篇 |
2023年 | 109篇 |
2022年 | 144篇 |
2021年 | 170篇 |
2020年 | 163篇 |
2019年 | 159篇 |
2018年 | 160篇 |
2017年 | 127篇 |
2016年 | 218篇 |
2015年 | 202篇 |
2014年 | 232篇 |
2013年 | 318篇 |
2012年 | 386篇 |
2011年 | 375篇 |
2010年 | 253篇 |
2009年 | 218篇 |
2008年 | 245篇 |
2007年 | 227篇 |
2006年 | 221篇 |
2005年 | 183篇 |
2004年 | 134篇 |
2003年 | 111篇 |
2002年 | 84篇 |
2001年 | 72篇 |
2000年 | 83篇 |
1999年 | 70篇 |
1998年 | 67篇 |
1997年 | 54篇 |
1996年 | 58篇 |
1995年 | 55篇 |
1994年 | 30篇 |
1993年 | 40篇 |
1992年 | 40篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 11篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 1篇 |
1983年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有5134条查询结果,搜索用时 15 毫秒
961.
Nishikimi A Uruno T Duan X Cao Q Okamura Y Saitoh T Saito N Sakaoka S Du Y Suenaga A Kukimoto-Niino M Miyano K Gotoh K Okabe T Sanematsu F Tanaka Y Sumimoto H Honma T Yokoyama S Nagano T Kohda D Kanai M Fukui Y 《Chemistry & biology》2012,19(4):488-497
- Download : Download high-res image (273KB)
- Download : Download full-size image
962.
The asymmetric epoxidation of 2-cyclohexen-1-one with aqueous H(2)O(2) as oxidant, 1,2-diaminocyclohexane as catalyst, and a Br?nsted acid trifluoroacetic acid (TFA) as cocatalyst has been studied by performing density functional theory calculations. It is confirmed that the catalyzed epoxidation proceeds via sequential nucleophilic addition and ring-closure processes involving a ketiminium intermediate. Four possible pathways associated with two Z isomers and two E isomers of ketiminium have been explored in detail. Our calculation indicates that these four pathways have high barriers and a small energy gap between two more favorable R and S pathways. We have analyzed the effects of the TFA anion and H(2)O on the activity and enantioselectivity of catalytic epoxidation. It is found that the TFA anion acts as a counterion to stabilize the transition states of the catalytic epoxidation by hydrogen-bond acceptance, leading to decreases in the barriers of the nucleophilic addition and ring-closure processes. The most significant decrease occurred in the ring-closure step of the Z-R-pathway, resulting in H-bond-induced enantioselectivity. Our calculations also show that water cooperates with TFA to further increase the reaction rate significantly. 相似文献
963.
A numerical model based on a hybrid finite element method is developed that seeks to join sound pressure fields in interior and exterior regions. The hybrid method is applied to the analysis of sound radiation from open pipes, or ducts, and uses mode matching to couple a finite element discretization of the region surrounding the open end of the duct to wave based modal expansions for adjoining interior and exterior regions. The hybrid method facilitates the analysis of ducts of arbitrary but uniform cross section as well the study of conical flanges and here a modal expansion based on spherical harmonics is applied. Predictions are benchmarked against analytic solutions for the limiting cases of flanged and unflanged circular ducts and excellent agreement between the two methods is observed. Predictions are also presented for flanged and unflanged rectangular ducts, and because the hybrid method retains the sparse banded and symmetric matrices of the traditional finite element method, it is shown that predictions can be obtained within an acceptable time frame even for a three dimensional problem. 相似文献
964.
965.
966.
There have been extensive studies on the large time behavior of solutions to systems on gas motions, such as the Navier-Stokes
equations and the Boltzmann equation. Recently, an approach is introduced by combining the energy method and the spectral
analysis to the study of the optimal rates of convergence to the asymptotic profiles. In this paper, we will first illustrate
this method by using some simple model and then we will present some recent results on the Navier-Stokes equations and the
Boltzmann equation. Precisely, we prove the stability of the non-trivial steady state for the Navier-Stokes equations with
potential forces and also obtain the optimal rate of convergence of solutions toward the steady state. The same issue was
also studied for the Boltzmann equation in the presence of the general time-space dependent forces. It is expected that this
approach can also be applied to other dissipative systems in fluid dynamics and kinetic models such as the model system of
radiating gas and the Vlasov-Poisson-Boltzmann system.
相似文献
967.
Jinqiao DUAN 《Frontiers of Mathematics in China》2009,4(3):425-436
Model uncertainties or simulation uncertainties occur in mathematical modeling of multiscale complex systems, since some mechanisms
or scales are not represented (i.e., ‘unresolved’) due to a lack in our understanding of these mechanisms or limitations in
computational power. The impact of these unresolved scales on the resolved scales needs to be parameterized or taken into
account. A stochastic scheme is devised to take the effects of unresolved scales into account, in the context of solving nonlinear
partial differential equations. An example is presented to demonstrate this strategy.
Dedicated to Professor Peter E. Kloeden on the occasion of his 60th birthday 相似文献
968.
969.
The electronic structures and optical properties of rocksalt indium nitride (InN) under pres-sure were studied using the first-principles calculation by considering the exchange and cor-relation potentials with the generalized gradient approximation. The calculated lattice con-stant shows good agreement with the experimental value. It is interestingly found that the band gap energy Eg at the Γ or X point remarkably increases with increasing pressure, but Eg at the L point does not increase obviously. The pressure coefficient of Eg is calculated to be 44 meV/GPa at the Γ point. Moreover, the optical properties of rocksalt InN were calculated and discussed based on the calculated band structures and electronic density of states. 相似文献
970.
By using the molecular dynamic simulation method with a
fourth-order Runge--Kutta algorithm, a two-dimensional dc- and
ac-driven Frenkel--Kontorova (FK) model with a square symmetry
substrate potential for a square lattice layer has been investigated
in this paper. For this system, the effects of many different
parameters on the average velocity and the static friction force
have been studied. It is found that not only the amplitude and
frequency of ac-driven force, but also the direction of the external
driving force and the misfit angle between two layers have some
strong influences on the static friction force. It can be concluded
that the superlubricity phenomenon appears easily with a larger ac
amplitude and lower ac frequency for some special direction of the
external force and misfit angle. 相似文献