全文获取类型
收费全文 | 24344篇 |
免费 | 2378篇 |
国内免费 | 3411篇 |
专业分类
化学 | 4479篇 |
晶体学 | 174篇 |
力学 | 646篇 |
综合类 | 336篇 |
数学 | 1500篇 |
物理学 | 4416篇 |
综合类 | 18582篇 |
出版年
2024年 | 135篇 |
2023年 | 451篇 |
2022年 | 588篇 |
2021年 | 582篇 |
2020年 | 479篇 |
2019年 | 613篇 |
2018年 | 586篇 |
2017年 | 417篇 |
2016年 | 457篇 |
2015年 | 618篇 |
2014年 | 1361篇 |
2013年 | 945篇 |
2012年 | 1037篇 |
2011年 | 1120篇 |
2010年 | 1190篇 |
2009年 | 1247篇 |
2008年 | 1356篇 |
2007年 | 1365篇 |
2006年 | 1229篇 |
2005年 | 1071篇 |
2004年 | 1016篇 |
2003年 | 934篇 |
2002年 | 812篇 |
2001年 | 812篇 |
2000年 | 879篇 |
1999年 | 969篇 |
1998年 | 807篇 |
1997年 | 838篇 |
1996年 | 740篇 |
1995年 | 719篇 |
1994年 | 685篇 |
1993年 | 622篇 |
1992年 | 569篇 |
1991年 | 534篇 |
1990年 | 456篇 |
1989年 | 385篇 |
1988年 | 207篇 |
1987年 | 160篇 |
1986年 | 159篇 |
1985年 | 145篇 |
1984年 | 151篇 |
1983年 | 122篇 |
1982年 | 125篇 |
1981年 | 91篇 |
1980年 | 80篇 |
1979年 | 38篇 |
1978年 | 30篇 |
1965年 | 23篇 |
1964年 | 16篇 |
1957年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
931.
We have established a caesium double magneto-optical trap (MOT) system for
cavity-QED experiment, and demonstrated the continuous transfer of cold caesium
atoms from the vapour-cell MOT with a pressure of ~ 1×10-6 Pa to
the ultra-high-vacuum (UHV) MOT with a pressure of ~ 8×10-8 Pa via a
focused continuous-wave transfer laser beam. The effect of frequency detuning as
well as the intensity of the transfer beam is systematically investigated, which
makes the transverse cooling adequate before the atoms leak out of the vapour-cell
MOT to reduce divergence of the cold atomic beam. The typical cold atomic flux got
from vapour-cell MOT is ~2×107 atoms/s. About 5×106
caesium atoms are recaptured in the UHV MOT. 相似文献
932.
运用舍时密度泛函理论和局域密度近似方法,计算了水分子H2O在速度为12.5a0/fs的重离子C^+和C^2+作用下产生的各种电荷态的水分子离子的几率,平均逃逸电子敷和偶极矩的变化随时间的演化,计算结果表明,在重离子势最大时,电偶极矩的变化最大;重离子远离分子时,重离子的电荷态越高,产生高电荷态分子的几率反而越小。 相似文献
933.
934.
随着光刻特征尺寸的不断减小,硅片表面不平度对光刻性能的影响越来越显著.该文提出了一种新的硅片表面不平度的原位检测技术本文在分析特殊测试标记成像规律的基础上,讨论了测试标记的对准位置偏移量与硅片表面起伏高度的变化规律,提出了一种新的硅片表面不平度原位检测技术.实验表明,该技术可实现硅片表面不平度及硅片表面形貌的高准确度原位测量.该技术考虑了光刻机承片台吸附力的非均匀性对硅片表面不平度的影响,更真实反映曝光工作状态下的硅片表面不平度大小.与现有的原位检测方法相比,硅片表面不平度的测量空间分辨率提高了1.67%倍,可实现硅片表面形貌的原位检测. 相似文献
935.
A deuterium cluster jet produced in the supersonic expansion into vacuum of deuterium gas at liquid nitrogen temperature and moderate backing pressures are studied by Rayleigh scattering techniques. The experimental results show that deuterium clusters can be created at moderate gas backing pressures ranging from 8 to 23 bar, and a maximum average cluster size of 350 atoms per cluster is estimated. The temporal evolution of the cluster jet generated at the backing pressure of 20 bar demonstrates a two-plateau structure. The possible mechanism responsible for this structure is discussed. The former plateau with higher average atom and cluster densities is more suitable for the general laser-cluster interaction experiments. 相似文献
936.
Simple Strehl ratio based method for pupil phase mask''''s optimization in wavefront coding system 总被引:1,自引:0,他引:1
By applying the wavefront coding technique to an optical system, the depth of focus can be greatly increased. Several complicated methods have already been taken to optimize for the best pupil phase mask in ideal condition. Here a simple Strehl ratio based method with only the standard deviation method used to evaluate the Strehl ratio stability over the depth of focus is applied to optimize for the best coefficients of pupil phase mask in practical optical systems. Results of imaging simulations for optical systems with and without pupil phase mask are presented, and the sharpness of image is calculated for comparison. The optimized pupil phase mask shows good results in extending the depth of focus. 相似文献
937.
938.
In the presence of an applied static and uniform magnetic field, a cylindrical Kadomtsev-Petviashivili equation is derived for a relativistic electromagnetic solitary wave propagating in collisionless plasma consisting electrons, positrons, and ions in the case of weak relativistic limit. This equation is solved in a stationary frame to obtain explicit expression for the velocity, amplitude and width of solitons. The amplitude of the solitary wave has a maximum value at a critical αc of the ratio of the ion equilibrium density to the electron one, and it increases as the applied magnetic field becomes larger. 相似文献
939.
We experimentally studied the in-plane thermal and electrical properties of a suspended platinum nanofilm in thickness of 15 nm. The measured results show that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient of the studied nanofilm are much less than those of the bulk material, while the Lorenz number is greater than the bulk value. Comparing with the results reported previously for the platinum nanofilm in thickness of 28 nm, we further find that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient decrease with the decreasing thickness of the nanotilm, while the Lorenz number increases with the decreasing thickness of the nanofilm. These results indicate that strong size effects exist on the in-plane thermal and electrical properties of platinum nanofilms. 相似文献
940.
Permeability is one of the most important properties of porous media. It is considerably difficult to calculate reservoir permeability precisely by using single well-logging response and simple formula because reservoir is of serious heterogeneity, and well-logging response curves are badly affected by many complicated factors underground. We propose a neural network method to calculate permeability of porous media. By improving the algorithm of the back-propagation neural network, convergence speed is enhanced and better results can be achieved. A four-layer back-propagation network is constructed to effectively calculate permeability from well log data. Spontaneous potential, resistivity of deep lateral log, resistivity of micro-gradient log, resistivity of micro-normal log, Interval transit time of acoustic log and resistivity of shallow lateral log are selected as the inputs, and permeability is selected as the output. There are 35 and 40 units used in the two hidden layers, respectively. During the training course, the correlation coefficient between the calculated permeability and the standard pattern is as high as 0.9937, the average absolute error between them is 0.046μm^2 and the average relative error is only 1.93%. for practical applications, the average relative error between the calculated permeability and actual permeability is also as low as about 10.0%. 相似文献