全文获取类型
收费全文 | 17773篇 |
免费 | 2739篇 |
国内免费 | 2473篇 |
专业分类
化学 | 13650篇 |
晶体学 | 308篇 |
力学 | 695篇 |
综合类 | 181篇 |
数学 | 2059篇 |
物理学 | 6092篇 |
出版年
2024年 | 33篇 |
2023年 | 274篇 |
2022年 | 510篇 |
2021年 | 577篇 |
2020年 | 649篇 |
2019年 | 672篇 |
2018年 | 545篇 |
2017年 | 581篇 |
2016年 | 824篇 |
2015年 | 846篇 |
2014年 | 1072篇 |
2013年 | 1308篇 |
2012年 | 1569篇 |
2011年 | 1702篇 |
2010年 | 1219篇 |
2009年 | 1174篇 |
2008年 | 1367篇 |
2007年 | 1149篇 |
2006年 | 1100篇 |
2005年 | 909篇 |
2004年 | 722篇 |
2003年 | 600篇 |
2002年 | 697篇 |
2001年 | 498篇 |
2000年 | 377篇 |
1999年 | 354篇 |
1998年 | 280篇 |
1997年 | 204篇 |
1996年 | 204篇 |
1995年 | 146篇 |
1994年 | 112篇 |
1993年 | 115篇 |
1992年 | 94篇 |
1991年 | 123篇 |
1990年 | 63篇 |
1989年 | 66篇 |
1988年 | 37篇 |
1987年 | 42篇 |
1986年 | 39篇 |
1985年 | 39篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 781 毫秒
81.
运用一种新的动力学突变检测方法——排列熵(permutation entropy,PE)算法,计算并分析了中国华北地区52个站点1960年—2000年逐日平均气温资料的排列熵演化情况,发现中国华北地区气温在20世纪70年代中期、80年代初均发生了较大突变;进一步用经验模态分解(empirical mode decomposition,EMD)方法对排列熵序列进行逐级平稳化处理,结果发现这一地区的气温突变与准10年这一年代际时间尺度的周期变率密切相关,其原因与太阳黑子活动有着密切联系.
关键词:
华北
突变
排列熵算法
经验模态分解 相似文献
82.
提出了一种基于微悬臂梁传感技术研究大分子折叠/构象转变的新方法.通过分子自组装的方法将热敏性的聚N-异丙基丙烯酰胺(PNIPAM)分子链修饰到微悬臂梁的单侧表面,用光杠杆技术检测温度在20—40℃之间变化时由于微悬臂梁上的PNIPAM分子在水中的构象转变所引起的微悬臂梁变形.实验结果显示:在升温过程中,微悬臂梁的表面应力发生了变化并且导致微悬臂梁产生了弯曲变形,这个过程对应着微悬臂梁上的PNIPAM分子从无规线团构象到塌缩小球构象的构象转变.在降温过程中,微悬臂梁发生了反方向的弯曲变形,这对应着PNIPA
关键词:
构象转变
聚N-异丙基丙烯酰胺分子链
表面应力
微悬臂梁 相似文献
83.
Flat-top interleavers with chromatic dispersion compensator based on phase dispersive free space Mach-Zehnder interferometer 总被引:2,自引:0,他引:2
In this paper, novel interleavers using circular cavities (CC) in a Mach-Zehnder interferometer (MZI) has been presented and demonstrated for the first time, in which CCs act as phase dispersive mirrors which exhibit a periodic dependence on the frequency of light. Three implementation schemes have been proposed and investigated. Theoretical analysis shows the spectral characteristics of each scheme in a 50-GHz channel spacing application. Furthermore, the chromatic dispersion (CD) of each output comb can be flattened within passband by appending an additional CC. The result shows that the proposed designs with novel interferometer technique can simultaneously provide flat top passbands, high isolation stopband and low CD value as well. 相似文献
84.
In this paper, an interior point algorithm based on trust region techniques is proposed for solving nonlinear optimization problems with linear equality constraints and nonnegative variables. Unlike those existing interior-point trust region methods, this proposed method does not require that a general quadratic subproblem with a trust region bound be solved at each iteration. Instead, a system of linear equations is solved to get a search direction, and then a linesearch of Armijo type is performed in this direction to obtain a new iteration point. From a computational point of view, this approach may in general reduce a computational effort, and thus improve the computational efficiency. Under suitable conditions, it is proven that any accumulation of the sequence generated by the algorithm satisfies the first-order optimality condition. 相似文献
85.
86.
87.
Xie Ruixiang Wang Min Duan Changchun Yan Yihua R. A. Sych A. T. Altyntsev 《中国科学A辑(英文版)》2002,45(1):97-105
A complex solar radio moving type IV burst was observed on 23 September 1998 with the broadband (1.0–2.0 GHz and 2.6–3.8 GHz) spectrometers with high temporal and spectral resolutions at National Astronomical Observatories of China (NAOC). Comparing to the high spatial resolution data of Siberian Solar Radio Telescope (SSRT), we find that this burst is a rare type of moving type IV burst which is caused by the expanding arches, and the spatial structure oscillations of the radio sources are related with the time structure pulsations of the radio emission. Furthermore, the burst is associated with the multiple quasi-periodic long-term pulsations, and this suggests the existence of multi-scale magnetic structures in a large expanding coronal arch. We think the moving type IV burst is due to the synchrotron emission of the energetic electrons trapped in the expanding arch, and the multiple quasi-periodic pulsations are due to the second harmonic plasma emission. 相似文献
88.
89.
90.
We propose a new lens scheme to focus cold atoms by using a controllable inhomogeneous magnetic field from a square current-carrying wire fabricated on a chip. The spatial distributions of the magnetic field are calculated, and the results show that the generated magnetic field is a two-dimensional (2D) quadrupole one and can be used to focus cold atoms or a cold atomic beam. The dynamic processes of cold atoms passing through our square wire layout and its focusing properties are studied by using Monte Carlo simulations. Our study shows that the atomic clouds can be focused effectively by our magnetic lens scheme, and the focal length of the atomic lens and its radius of focused spot can be continuously changed by adjusting the current in the wires. 相似文献