全文获取类型
收费全文 | 28947篇 |
免费 | 3934篇 |
国内免费 | 5186篇 |
专业分类
化学 | 16249篇 |
晶体学 | 264篇 |
力学 | 2985篇 |
综合类 | 111篇 |
数学 | 7174篇 |
物理学 | 11284篇 |
出版年
2024年 | 73篇 |
2023年 | 825篇 |
2022年 | 1026篇 |
2021年 | 1148篇 |
2020年 | 1321篇 |
2019年 | 939篇 |
2018年 | 1110篇 |
2017年 | 1237篇 |
2016年 | 1386篇 |
2015年 | 1496篇 |
2014年 | 1927篇 |
2013年 | 2598篇 |
2012年 | 2523篇 |
2011年 | 2505篇 |
2010年 | 1987篇 |
2009年 | 1991篇 |
2008年 | 1644篇 |
2007年 | 1687篇 |
2006年 | 1615篇 |
2005年 | 1133篇 |
2004年 | 848篇 |
2003年 | 751篇 |
2002年 | 673篇 |
2001年 | 676篇 |
2000年 | 498篇 |
1999年 | 628篇 |
1998年 | 458篇 |
1997年 | 407篇 |
1996年 | 329篇 |
1995年 | 276篇 |
1994年 | 229篇 |
1993年 | 218篇 |
1992年 | 256篇 |
1991年 | 182篇 |
1990年 | 156篇 |
1989年 | 193篇 |
1988年 | 415篇 |
1987年 | 393篇 |
1986年 | 80篇 |
1985年 | 65篇 |
1984年 | 41篇 |
1983年 | 29篇 |
1982年 | 45篇 |
1981年 | 18篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1971年 | 4篇 |
1959年 | 3篇 |
1957年 | 7篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Kang Hai TAN Xiao Ping YANG 《数学学报(英文版)》2006,22(3):701-710
In this paper we give a geometric interpretation of the notion of the horizontal mean curvature which is introduced by Danielli Garofalo-Nhieu and Pauls who recently introduced sub- Riemannian minimal surfaces in Carnot groups. This will be done by introducing a natural nonholonomic connection which is the restriction (projection) of the natural Riemannian connection on the horizontal bundle. For this nonholonomic connection and (intrinsic) regular hypersurfaces we introduce the notions of the horizontal second fundamental form and the horizontal shape operator. It turns out that the horizontal mean curvature is the trace of the horizontal shape operator. 相似文献
152.
An analytical method is presented to investigate the bending-torsion vibration characteristics of a cylinder with an arbitrary cross-section and partially submerged in water. The compressibility and the free surface waves of the water are considered simultaneously in the analysis. The exact solution of structure–water interaction is obtained mathematically. Firstly, the analytical expression of the velocity potential of the water is derived by using the method of separation of variables. The unknown coefficients in the velocity potential are determined by the longitudinal and circumferential Fourier expansions along the outer surface of the cylinder and are expressed in the form of integral equations including the unknown dynamic bending deflection and torsional angle of the cylinder. Secondly, the force and torque acting on the cylinder per unit length, provided by the water, are obtained by integrating the water dynamic pressure along the circumference of the cylinder. The general solution of bending-torsion vibration of the cylinder under the water dynamic pressure is derived analytically. The integral equations included in the velocity potential of the water can be solved exactly. Finally, the eigenfrequency equation of cylinder–water interaction is obtained by means of the boundary conditions of the cylinder. Some numerical examples for elliptical columns partially submerged in water are provided to show the application of the present method. 相似文献
153.
本文根据1978—2003年中国农村居民现金消费支出数据,综合考虑影响消费的主要因素来研究中国农村居民的现金消费行为。与以往研究不同的是,对于影响消费支出的不确定性因素,本文不是采用某个具体变量进行近似替代,而是采用刻画不确定性的条件异方差来描述。文章的结论是农村居民现金消费支出的主要影响因素并非收入,而是利率和不确定性因素,不确定性因素在1996年后表现得尤为突出。 相似文献
154.
The generalized Mycielskians (also known as cones over graphs) are the natural generalization of the Mycielski graphs (which were first introduced by Mycielski in 1955). Given a graph G and any integer m?0, one can transform G into a new graph μm(G), the generalized Mycielskian of G. This paper investigates circular clique number, total domination number, open packing number, fractional open packing number, vertex cover number, determinant, spectrum, and biclique partition number of μm(G). 相似文献
155.
A Novel Analytical Approach for Multi-Layer Diaphragm-Based Optical Microelectromechanical-System Pressure Sensors 下载免费PDF全文
An optical microelectromechanical-system (MEMS) pressure sensor based on multi-layer circular diaphragm is described and analysed by using the proposed novel analytical approach and the traditional transfer matrix method. The analytical expressions of the deflection of multi-layer diaphragm and absolute optical reflectance are derived respectively. The influence of residual stress on the deflection of diaphragm is also analysed. Simulation results given by the finite element method are consistent with the ones which are analysed by using the analytical approach. The analytical approach will be helpful to design and fabricate the optical MEMS pressure sensors with multi-layer diaphragm based on Fabry-Perot interferometry. 相似文献
156.
本文用石蜡模拟油垢,用^60Co放射源的1.25MeV平均能量的γ射线垂直入射,以一定的散射角和不同的探测立体角接收散射γ光子敫.发现散射γ计数与被测石蜡厚度之间存在不同的线性关系.结果发现,放射源与探测器之间屏蔽的好坏和合理的探测距离对提高线性度有很大关系.本实验测得最佳线性相关系数平方值为0.9981,平均测量精度为1.26mm.这对研究散射法检测油垢厚度和选择合适的探测距离提供了实验基础. 相似文献
157.
X.P. Hu X. Wang Z. Yan H.X. Li J.L. He S.N. Zhu 《Applied physics. B, Lasers and optics》2007,86(2):265-268
High power red light was generated from a periodically-poled stoichiometric LiTaO3 (PPSLT) by single-pass frequency doubling of a diode-side-pumped, Q-switched Nd:YAG laser at 1319 nm. An average power of
2.4 W of the 660 nm red light was obtained at the fundamental power of ∼5.4 W with the conversion efficiency up to 44.4% and
with low fluctuation down to 2%. The high efficiency and stability at the red output indicate that it is a practical method
to construct a reliable compact red laser.
PACS 42.70.Mp; 42.79.Nv; 42.55.Xi 相似文献
158.
159.
160.
Raman spectra of ceramic Sr2Bi4Ti5O18 (SBTi5) are reported to consist of four different Raman bands. Temperature-dependent spectra reveal the relationship between the lattice vibration and the material's structure. There appears a relatively large change in structure of the material at about 273K, The anharmonic potential of the material has a great influence on its phonon mode full width at half maximum (FWHM), which can be expressed by a function of temperature. Theoretical fittings of the FWHMs for the two modes at around 312 cm^-1 and 464cm^-1 indicate that the latter phonon mode is more anharmonic than the former one. 相似文献