全文获取类型
收费全文 | 14294篇 |
免费 | 2761篇 |
国内免费 | 2204篇 |
专业分类
化学 | 10122篇 |
晶体学 | 231篇 |
力学 | 900篇 |
综合类 | 161篇 |
数学 | 1599篇 |
物理学 | 6246篇 |
出版年
2024年 | 56篇 |
2023年 | 285篇 |
2022年 | 520篇 |
2021年 | 575篇 |
2020年 | 694篇 |
2019年 | 643篇 |
2018年 | 584篇 |
2017年 | 540篇 |
2016年 | 788篇 |
2015年 | 787篇 |
2014年 | 926篇 |
2013年 | 1148篇 |
2012年 | 1465篇 |
2011年 | 1416篇 |
2010年 | 1031篇 |
2009年 | 1019篇 |
2008年 | 1085篇 |
2007年 | 917篇 |
2006年 | 775篇 |
2005年 | 648篇 |
2004年 | 508篇 |
2003年 | 399篇 |
2002年 | 396篇 |
2001年 | 333篇 |
2000年 | 272篇 |
1999年 | 233篇 |
1998年 | 163篇 |
1997年 | 143篇 |
1996年 | 140篇 |
1995年 | 107篇 |
1994年 | 112篇 |
1993年 | 76篇 |
1992年 | 82篇 |
1991年 | 73篇 |
1990年 | 76篇 |
1989年 | 48篇 |
1988年 | 35篇 |
1987年 | 23篇 |
1986年 | 25篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 16篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1977年 | 2篇 |
1974年 | 5篇 |
1972年 | 2篇 |
1966年 | 2篇 |
1959年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
61.
62.
The basic theory of symplectic algorithm was introduced. A comparison between Runge-Kutta method and symplectic integration method was preformed in the simulation of the long time behavior of H + H2 system on BKMP potential energy surface. Our results reveal a dis-sipative behavior in the integral of ordinary differential equation by the fourth order Runge-Kutta method, which causes incorrect simulation results in QCT calculations. However, when the symplectic integration method is applied, the dissipative behavior is not found in the same system. When the initial state is the same, the energy deviation of fourth order symplectic integral method is almost one percent of that of fourth order Runge-Kutta method in a 60000-step simulation, and that of sixth order symplectic integral method is much less. These results show that the symplectic integral methods are always the better choice in the integral calculation of the long time behavior in maintaining energy conservation. 相似文献
63.
功能化基团对C60-β-丙氨酸衍生物清除超氧阴离子自由基O-2·能力的影响 总被引:4,自引:0,他引:4
自Chiang等[1]发现富勒醇可以清除*OH以来, C60衍生物作为自由基清除剂的研究备受关注[2,3]. 超氧阴离子自由基O-*2是生物体内一类重要的氧自由基, 可以经过一系列反应生成其它自由基, 从而引发一系列疾病[4]. 及时清除过剩的O-*2具有重要意义. 相似文献
64.
Xiangke Wang Jinzhou Du Zuyi Tao Zhongxiu Fan 《Journal of Radioanalytical and Nuclear Chemistry》2003,258(1):133-138
The migration of radionuclides 85+89Sr2+, 134Cs+, 125I- and 75SeO3
2- in calcareous soil from Yuzhong county of Gansu Province (China) at pH 7.8±0.2 and 152+154Eu(III) in red earth from Yingtan county of Jiangxi Province (China) at pH 4.6±0.2, in presence of CaCl2 was studied using column experiments. Results indicate that the negative anions, iodide and selenide are mobile nuclides while the migration of positive cations is related to the sorption capacity for the element. Iodide can be assumed to be non-reactive in calcareous soil. The breakthrough curves (BTCs) were fitted to the analytical solution or numerical solution of one-dimensional convection-dispersion transport models. Good agreements were obtained between the measured and predicted concentration profiles. 相似文献
65.
66.
Hong Tao Gao Dong Mei Dai Tong Hua Li 《中国化学快报》2007,18(4):495-498
Non-negative matrix factorization(NMF)is a technique for dimensionality reduction by placing non-negativity constraints onthe matrix.Based on the PARAFAC model,NMF was extended for three-dimension data decomposition.The three-dimension non-negative matrix factorization(NMF3)algorithm,which was concise and easy to implement,was given in this paper.The NMF3algorithm implementation was based on elements but not on vectors.It could decompose a data array directly without unfolding,which was not similar to that the traditional algorithms do.It has been applied to the simulated data array decomposition andobtained reasonable results.It showed that NMF3 could be introduced for curve resolution in chemometrics. 相似文献
67.
Margaret C. Gerthoffer Sikai Wu Bo Chen Tao Wang Steven Huss Shalisa M. Oburn Vincent H. Crespi John V. Badding Elizabeth Elacqua 《Chemical science》2020,11(42):11419
Limited supramolecular strategies have been utilized to synthesize sequence-defined polymers, despite the prominence of noncovalent interactions in materials design. Herein, we illustrate the utility of ‘sacrificial’ aryl-perfluoroaryl supramolecular synthons to synthesize sp3-hybridized nanothreads from sp2-enriched reactants. Our strategy features A–B reactant pairs in the form of a phenol:pentafluorophenol co-crystal that is preorganized for an electronically-biased and sequence-defined polymerization. The polymerization, initiated at 12 GPa, affords an alternating copolymer featuring exogenous –OH functionalities. The external substitution is confirmed through IR spectroscopy. Importantly, the inclusion of the functional unit provides the first experimental glimpse at reaction mechanism: keto–enol tautomerization that can only occur during cycloaddition is observed through IR spectroscopy. Our approach realizes the first example of a functionalized nanothread and attains sequence definition through sacrificial supramolecular preorganization and presents a further approach for de novo design of complex nanothreads.Supramolecular synthons are exploited to synthesize –OH functionalized sp3-rich sequence-defined nanothreads using pressure-induced polymerization of a phenol:pentafluorophenol co-crystal. 相似文献
68.
Using intercalated, covalently bound daunomycin as a redox probe, ground state charge transport in DNA films with a perturbation in base pair stacking was examined in comparison with breaks in the sugar-phosphate backbone. While the introduction of one or even two nicks in the sugar-phosphate backbone yields no detectable effect on electron transfer, a CA mismatch significantly attenuates the electron transfer yield. These results confirm that the base pair stack is the pathway for DNA-mediated charge transfer, not the sugar-phosphate backbone. 相似文献
69.
The covalent binding of acrylonitrile (CH(2)=CH-C triple bond N) and the formation of a C=C-C=N structure on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. For chemisorbed acrylonitrile, the absence of nu(C triple bond N) at 2245 cm(-1) and the appearance of nu(C=N) at 1669 cm(-1) demonstrate that the cyano group directly participates in the interaction with Si(100), which is further supported by XPS and UPS observations. Our experimental results and DFT calculations unambiguously demonstrate a [2 + 2] cycloaddition mechanism for acrylonitrile chemisorption on Si(100) through the binding of C triple bond N to Si dimers. The resulting chemisorbed monolayer with a C=C-C=N skeleton can serve as a precursor for further chemical syntheses of multilayer organic thin films in a vacuum and surface functionalization for in situ device fabrication. 相似文献
70.
Yan-Qing Wang Hong-Mei Zhang Gen-Cheng Zhang Wei-Hua Tao Shu-He Tang 《Journal of Molecular Structure》2007,830(1-3):40-45
The feature of brucine binding to human serum albumin (HSA) was investigated via fluorescence and UV/vis absorption spectroscopy. The results revealed that brucine caused the fluorescence quenching of HSA by the formation of brucine–HSA complex. The hydrophobic interaction plays a major role in stabilizing the complex; the binding site number n and apparent binding constant KA, corresponding thermodynamic parameters the free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) at different temperatures were calculated. The distance r between donor (HSA) and acceptor (brucine) was obtained according to fluorescence resonance energy transfer. The effect of brucine on the conformation of HSA was analyzed using synchronous fluorescence spectroscopy and UV/vis absorption spectroscopy. 相似文献