全文获取类型
收费全文 | 33221篇 |
免费 | 5833篇 |
国内免费 | 4551篇 |
专业分类
化学 | 24964篇 |
晶体学 | 440篇 |
力学 | 1653篇 |
综合类 | 268篇 |
数学 | 3513篇 |
物理学 | 12767篇 |
出版年
2024年 | 41篇 |
2023年 | 661篇 |
2022年 | 853篇 |
2021年 | 1197篇 |
2020年 | 1361篇 |
2019年 | 1384篇 |
2018年 | 1259篇 |
2017年 | 1183篇 |
2016年 | 1739篇 |
2015年 | 1662篇 |
2014年 | 2089篇 |
2013年 | 2670篇 |
2012年 | 3263篇 |
2011年 | 3383篇 |
2010年 | 2383篇 |
2009年 | 2171篇 |
2008年 | 2389篇 |
2007年 | 2113篇 |
2006年 | 1967篇 |
2005年 | 1501篇 |
2004年 | 1119篇 |
2003年 | 832篇 |
2002年 | 884篇 |
2001年 | 746篇 |
2000年 | 559篇 |
1999年 | 661篇 |
1998年 | 432篇 |
1997年 | 421篇 |
1996年 | 370篇 |
1995年 | 328篇 |
1994年 | 275篇 |
1993年 | 289篇 |
1992年 | 192篇 |
1991年 | 197篇 |
1990年 | 155篇 |
1989年 | 136篇 |
1988年 | 105篇 |
1987年 | 93篇 |
1986年 | 89篇 |
1985年 | 67篇 |
1984年 | 61篇 |
1983年 | 34篇 |
1982年 | 36篇 |
1981年 | 26篇 |
1980年 | 28篇 |
1976年 | 17篇 |
1975年 | 15篇 |
1973年 | 15篇 |
1968年 | 14篇 |
1957年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
91.
92.
两段级联掺铒光纤放大器的优化研究 总被引:7,自引:4,他引:3
基于Giles模型,研究了980 nm和1480 nm泵浦的两段级联掺铒光纤放大器(EDFA)的设计,得出了掺铒光纤(EDF)的最佳长度和光隔离器的最佳位置随泵浦功率和信号功率的变化关系.比较980 nm和1480 nm泵浦的两段级联EDFA,可以发现,前者的最佳EDF长度短,而光隔离器最佳位置距EDFA输入端远. 相似文献
93.
It is a well-known feature of odd space-time dimensions d that there exist two inequivalent fundamental representations A and B of the Dirac gamma matrices. Moreover, the parity transformation swaps the fermion fields living in A and B. As a consequence, a parity-invariant Lagrangian can only be constructed by incorporating both the representations. Based upon these ideas and contrary to long-held belief, we show that in addition to a discrete exchange symmetry for the massless case, we can also define chiral symmetry provided the Lagrangian contains fields corresponding to both the inequivalent representations. We also study the transformation properties of the corresponding chiral currents under parity and charge-conjugation operations. We work explicitly in 2 + 1 dimensions and later show how some of these ideas generalize to an arbitrary number of odd dimensions. 相似文献
94.
The compression behaviour of Ni77P23 amorphous alloy is investigated at room temperature in a diamond-anvil cell instrument using in-situ high pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state is determined by fitting the experimental data according to the Birch-Murnaghan equation. It is found that the structure of Ni77P23 amorphous alloy is stable under pressures up to 30.5 GPa. Within the pressure range from zero to the experimental one, the pressure-induced structural relaxation is reversible. 相似文献
95.
Ricardo Mañé 《Bulletin of the Brazilian Mathematical Society》1997,28(2):141-153
The objective of this note is to present some results, to be proved in a forthcoming paper, about certain special solutions of the Euler-Lagrange equations on closed manifolds. Our main results extend to time dependent periodic Lagrangians with minor modifications.We have chosen the autonomous case because this formally simpler framework allows to reach more easily the core of our concepts and results. Moreover the autonomous case exhibits certain special features involving the energy as a first integral that deserve special attention. They are closely related to the link found by Carneiro [C] between the energy and Mather's action function [Ma].Reprinted by permission of Addison Wesley Longman Ltd. 相似文献
96.
97.
98.
Lizeng Wang Chengsong Ma Xiaoli Zhang Yibing Ren Yong Yu 《Fresenius' Journal of Analytical Chemistry》1995,351(7):689-691
The electrochemical behavior of the copper-tyrosine complex has been studied by linear-sweep adsorption voltammetry. In 0.02 mol/L Na2HPO4 buffer solution (pH=9.6), the complex can be adsorped on a hanging mercury drop electrode and reduced at a peak potential of about –0.42 V (vs. SCE). The secondary derivative peak height is linear proportional to the concentration of tyrosine in the range 1.0×10–7–5.0×10–5 mol/L. The detection limit is 5×10–8 mol/L.Project supported by the Provincial Science Foundation of Shandong Province 相似文献
99.
Tao Liu Zuxiong Xu Zhongtao Zhao Ruzhang Ma Tiandou Hu Yaning Xie Yinghuan Guo 《中国科学A辑(英文版)》1997,40(4):395-399
The role of magnetoelastic coupling effects in nanocrystalline ferromagnets is investigated by means of high-field magnetization and Doppler-broadening spectrum measurements. For the nanocrystalline Fe73 5Cu1Nb3Si13.5B9 alloy, the results show that the pinning effects resulting from the quasidislocation dipole intensely influence the movement of domain wall; by coupling with the magnetostriction the defects-induced stress fields determine the magnetic properties at the early stage of crystallization. In view of the effective anisotropy and magnetoelastic coupling energy the optimal annealing conditions of alloys are discussed. 相似文献
100.
The effects of toxicants on the populations for ann-dimensional food chain model in a polluted environment are studied. The thresholds between persistence and extinction of the populations are obtained as functions of model parameters and a time-varying chemical concentration. 相似文献