全文获取类型
收费全文 | 129066篇 |
免费 | 20844篇 |
国内免费 | 12833篇 |
专业分类
化学 | 87958篇 |
晶体学 | 1518篇 |
力学 | 8164篇 |
综合类 | 743篇 |
数学 | 15159篇 |
物理学 | 49201篇 |
出版年
2024年 | 421篇 |
2023年 | 2435篇 |
2022年 | 4221篇 |
2021年 | 4539篇 |
2020年 | 4900篇 |
2019年 | 4783篇 |
2018年 | 4156篇 |
2017年 | 3893篇 |
2016年 | 6016篇 |
2015年 | 5815篇 |
2014年 | 7063篇 |
2013年 | 9340篇 |
2012年 | 11201篇 |
2011年 | 11732篇 |
2010年 | 7858篇 |
2009年 | 7598篇 |
2008年 | 8159篇 |
2007年 | 7507篇 |
2006年 | 6805篇 |
2005年 | 5787篇 |
2004年 | 4324篇 |
2003年 | 3447篇 |
2002年 | 3184篇 |
2001年 | 2640篇 |
2000年 | 2307篇 |
1999年 | 2573篇 |
1998年 | 2282篇 |
1997年 | 2108篇 |
1996年 | 2305篇 |
1995年 | 1850篇 |
1994年 | 1781篇 |
1993年 | 1476篇 |
1992年 | 1345篇 |
1991年 | 1232篇 |
1990年 | 1004篇 |
1989年 | 738篇 |
1988年 | 621篇 |
1987年 | 495篇 |
1986年 | 499篇 |
1985年 | 461篇 |
1984年 | 333篇 |
1983年 | 213篇 |
1982年 | 209篇 |
1981年 | 159篇 |
1980年 | 126篇 |
1979年 | 102篇 |
1978年 | 72篇 |
1976年 | 72篇 |
1974年 | 85篇 |
1973年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
非线性光纤方向耦合器孤子动力特性分析 总被引:2,自引:0,他引:2
利用变分方法,导出了双曲正割型光孤子在非线性光纤耦合器中传输时满足的动力学方程组,分析了该方程组的平衡点性态,讨论了光孤子的开关特性.指出参数|β|<2π.是能实现完全的开关操作的必要条件. 相似文献
62.
Yoshiaki Murata Cheng-Huang Lin Totaro Imasaka 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):543-544
Summary A supersonic jet spectrum of 9,10-dichloroanthracene is measured by stimulated-emission-pumping fluorescence dip spectrometry and conventional fluorescence spectrometry. The performance obtained is compared for these spectrometric methods, providing same information concerned with the energy level of the ground state. The former is more preferential for measurement of a high-resolution spectrum, since the spectral resolution is determined by the linewidth of the dumping laser. On the other hand, the latter is more preferential for measurement with better sensitivity at the expense of the spectral resolution, since the fluorescence throughput can be improved by increasing the slit width of the monochromator.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
63.
Addition of functionalized zinc-copper reagents to the title complexes proceeds in a highly diastereoselective fashion to afford dienol complexes. The relative configurations of adducts 3d were determined by single X-ray diffraction analysis. 相似文献
64.
A short regio- and stereoselective synthesis of two carbocyclic 3′-deoxynucleoside analogues is described, the key step of which consists in the photosensitized addition of MeOH to a cyclopent-2-enone derivative. As in both cases functional groups capable to react with each other are present in the same molecule, the synthetic compounds can form polymers similar to oligonucleotides. 相似文献
65.
The theory of tree-growing (RECPAM approach) is developed for outcome variables which are distributed as the canonical exponential family. The general RECPAM approach (consisting of three steps: recursive partition, pruning and amalgamation), is reviewed. This is seen as constructing a partition with maximal information content about a parameter to be predicted, followed by simplification by the elimination of ‘negligible’ information. The measure of information is defined for an exponential family outcome as a deviance difference, and appropriate modifications of pruning and amalgamation rules are discussed. It is further shown how the proposed approach makes it possible to develop tree-growing for situations usually treated by generalized linear models (GLIM). In particular, Poisson and logistic regression can be tree-structured. Moreover, censored survival data can be treated, as in GLIM, by observing a formal equivalence of the likelihood under random censoring and an appropriate Poisson model. Three examples are given of application to Poisson, binary and censored survival data. 相似文献
66.
Surface texturing has been recognized as an effective means to improve the tribological performances of sliding surfaces. Usually, generation additional hydrodynamic pressure to increase the load carrying capacity is regarded as the most significant effect of surface texture. In the case of silicon carbide sliding against identical material in water, the experimental results indicate that surface texture is also helpful to improve the running-in progress to smooth the contact surfaces, showing another reason to result in low friction. Based on the consideration of enhancing the generation of hydrodynamic pressure and improving running-in progress, a surface texture pattern, which was combined with large (circle, 350 μm in diameter) and small (rectangular, 40 μm in length) dimples, was designed to maximize the texture effect on the load carrying capacity of SiC surfaces sliding in water. The friction coefficient of such textured surface was evaluated and compared with that of untextured and those only with large or small dimples only. The friction reduction mechanisms of the patterns with different dimples in size are discussed. 相似文献
67.
68.
69.
The object of this note is to prove the followingTheorem Let{a_n}and{b_n}be sequences of real numbers such that0<∑∑a_n~2<+∞and0<∑b_n~2<+∞.Then we have the inequalitysum from m=1 to∞sum from n=1 to∞a_mb_n/m+n<{sum from n=1 to∞(π-θ/n~(1/2)a_n~2}~1/2{sum from n=1 to∞(π-θ/n~(1/2)b_n~2}~1/2 (1)whereθ=3/2~(1/2)-1=1.121320343. 相似文献
70.
Gang Wang Xiulin Zhu Zhenping Cheng Jian Zhu 《Journal of polymer science. Part A, Polymer chemistry》2006,44(9):2912-2921
A series of (di)picolinic acids and their derivates are investigated as novel complexing tridentate or bidentate ligands in the iron‐mediated reverse atom transfer radical polymerization of methyl methacrylate in N,N‐dimethylformamide at 100 °C with 2,2′‐azobisisobutyrontrile as an initiator. The polymerization rates and polydispersity indices (1.32–1.8) of the resulting polymers are dependent on the structures of the ligands employed. Different iron complexes may be involved in iron‐mediated reverse atom transfer radical polymerization, depending on the type of acid used. 1H NMR spectroscopy has been used to study the structure of the resulting polymers. Chain‐extension reactions have been performed to further confirm the living nature of this catalytic system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2912–2921, 2006 相似文献