全文获取类型
收费全文 | 604629篇 |
免费 | 5410篇 |
国内免费 | 1740篇 |
专业分类
化学 | 273718篇 |
晶体学 | 8382篇 |
力学 | 25273篇 |
综合类 | 16篇 |
数学 | 64623篇 |
物理学 | 167451篇 |
无线电 | 72316篇 |
出版年
2021年 | 4776篇 |
2020年 | 5275篇 |
2019年 | 5925篇 |
2018年 | 7951篇 |
2017年 | 7990篇 |
2016年 | 11347篇 |
2015年 | 6402篇 |
2014年 | 10876篇 |
2013年 | 25452篇 |
2012年 | 19165篇 |
2011年 | 23180篇 |
2010年 | 17336篇 |
2009年 | 18056篇 |
2008年 | 22762篇 |
2007年 | 23530篇 |
2006年 | 21920篇 |
2005年 | 19604篇 |
2004年 | 18184篇 |
2003年 | 16473篇 |
2002年 | 16328篇 |
2001年 | 18222篇 |
2000年 | 14508篇 |
1999年 | 11596篇 |
1998年 | 10263篇 |
1997年 | 10105篇 |
1996年 | 9359篇 |
1995年 | 8498篇 |
1994年 | 8524篇 |
1993年 | 8114篇 |
1992年 | 8835篇 |
1991年 | 8942篇 |
1990年 | 8579篇 |
1989年 | 8172篇 |
1988年 | 7880篇 |
1987年 | 7406篇 |
1986年 | 6979篇 |
1985年 | 8861篇 |
1984年 | 9149篇 |
1983年 | 7659篇 |
1982年 | 7718篇 |
1981年 | 7390篇 |
1980年 | 7261篇 |
1979年 | 7427篇 |
1978年 | 7751篇 |
1977年 | 7620篇 |
1976年 | 7616篇 |
1975年 | 7070篇 |
1974年 | 7118篇 |
1973年 | 7313篇 |
1972年 | 5224篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
F. Ati S. El-Aoufi A. Chergui H. Y. Aboul-Enein B. Maouche 《Journal of the Iranian Chemical Society》2008,5(3):506-513
This paper describes khellin hydrolysis mechanism using semiemperical PM3 implemented in Gaussian 03 package. The calculations show that in the presence of an acidic media, an enolate molecular ion leads directly to ω-acetokhellinone while in the basic media it leads to khellinone. 相似文献
962.
Specific features of the formation of submicron (70–300 nm) inclusions in Ti:sapphire (Al2O3:Ti) grown in a carbon-containing medium have been investigated. These inclusions are caused by deviation from the melt stoichiometry and are formed during the melt-crystal phase transition. These defects are submicropores containing excess aluminum and its suboxides; they can be destroyed by thermal loading of a crystal. 相似文献
963.
We consider a finite element algorithm intended to study the dynamic behavior of an elastic cylindrical shell filled with an immovable or flowing fluid. To describe the fluid, we use the perturbed velocity potential whose equations with the corresponding boundary conditions are solved by the Bubnov-Galerkin method. To describe the shell, we use the variation principle, which includes the linearized Bernoulli equation for calculating the hydrodynamic pressure acting on the shell on the side of the fluid. Solving the problem is reduced to calculating and analyzing the eigenvalues of the coupled system of equations obtained as a result of combining the equations for the perturbed velocity potential and the shell displacements. We consider several test problems in which, along with the comparison of the computational results with the earlier published experimental, analytic, and numerical data, we also study the dynamic behavior of the “shell-fluid” system for various boundary conditions for the perturbed velocity potential. 相似文献
964.
965.
Ashwani K. Padthe Nalin A. Chaturvedi Dennis S. Bernstein Sanjay P. Bhat Anthony M. Waas 《International Journal of Non》2008,43(4):277-291
We study a linearly damped preloaded two-bar linkage that exhibits hysteresis due to the presence of multiple attracting equilibria. The dynamics at the unstable equilibrium, through which a snap-through buckle occurs, are not linearizable due to a solution-dependent singularity. We stabilize the unstable equilibrium using two distinct non-linear controllers. The feedback-linearization controller requires knowledge of the linkage parameters, whereas the robust version of the intrinsic non-linear proportional-derivative controller requires only an upper bound on the stiffness. 相似文献
966.
The paper outlines a procedure to identify the space-and time-dependent external nonstationary load acting on a closed circular
cylindrical shell of medium thickness. Time-dependent deflections at several points of the shell are used as input data to
solve the inverse problem. Examples of numerical identification of various nonstationary loads, including moving ones are
presented. The relationship between the external load and the stress-strain state of the shell is described by the Volterra
equation of the first kind. The identification problem is solved using Tikhonov's regularization method and Apartsin's h-regularization method
__________
Translated from Prikladnaya Mekhanika, Vol. 44, No. 7, pp. 91–100, July 2008. 相似文献
967.
968.
969.
970.
Zhi‐Yong Huang Jin‐Can Shen Zhi‐Xia Zhuang Xiao‐Ru Wang Frank S. C. Lee 《应用有机金属化学》2004,18(6):255-261
Cinnabar, as one of the most widely used mineral drugs in traditional Chinese medicines, has been proven to have prominent curative effects in clinical use for more than 2000 years. But the safety and toxicity of the drug has been under constant debate in clinic usage. Metallothionein (MT) contains about 30% of cysteine in the molecule, and plays an important detoxification role against heavy metals. In this study, it was used as a biomarker to assess mercurial accumulation in rats fed orally with cinnabar. After feeding rats with cinnabar by gastric gavage at different dosages and at different times, the distribution of heavy metals (including mercury, copper and zinc) and MT was investigated among rat tissues, including liver, kidney, heart, brain, testis and blood. Metals and MT determinations were carried out using inductively coupled plasma mass spectrometry (ICP‐MS) and a modified mercury saturation assay technique respectively. The results indicated that mercury was easily accumulated in the tissues of rats exposed to cinnabar, especially in kidney. For example: at a feeding dosage of 5 g kg?1 (bw) for 4 weeks, the mercury concentrations in kidney were 13, 8.7, 21.6 and 26 times those in liver, testis, brain and heart respectively; and at 2.5 g kg?1 (bw) for 2 weeks, the mercury concentrations in kidney were 21, 2.1, 3 and 21 times those in liver, testis, brain and heart respectively. In addition, mercury in kidney and liver of all cinnabar groups was significantly higher than that of the control group (P < 0.01). A high positive correlation observed between MT concentrations and mercury levels in both liver and kidney (R2 = 0.9299, P < 0.02 for liver; R2 = 0.9923, P < 0.0008 for kidney) indicated that MT could be used as a biomarker for mercury in tissues. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献