全文获取类型
收费全文 | 15195篇 |
免费 | 1290篇 |
国内免费 | 876篇 |
专业分类
化学 | 8511篇 |
晶体学 | 431篇 |
力学 | 216篇 |
综合类 | 51篇 |
数学 | 4963篇 |
物理学 | 3189篇 |
出版年
2023年 | 46篇 |
2022年 | 75篇 |
2021年 | 114篇 |
2020年 | 196篇 |
2019年 | 299篇 |
2018年 | 315篇 |
2017年 | 328篇 |
2016年 | 325篇 |
2015年 | 487篇 |
2014年 | 564篇 |
2013年 | 886篇 |
2012年 | 707篇 |
2011年 | 1010篇 |
2010年 | 879篇 |
2009年 | 1115篇 |
2008年 | 989篇 |
2007年 | 1010篇 |
2006年 | 851篇 |
2005年 | 718篇 |
2004年 | 817篇 |
2003年 | 745篇 |
2002年 | 608篇 |
2001年 | 533篇 |
2000年 | 462篇 |
1999年 | 373篇 |
1998年 | 306篇 |
1997年 | 285篇 |
1996年 | 279篇 |
1995年 | 249篇 |
1994年 | 281篇 |
1993年 | 225篇 |
1992年 | 174篇 |
1991年 | 112篇 |
1990年 | 91篇 |
1989年 | 88篇 |
1988年 | 68篇 |
1987年 | 83篇 |
1986年 | 71篇 |
1985年 | 65篇 |
1984年 | 56篇 |
1983年 | 22篇 |
1982年 | 50篇 |
1981年 | 48篇 |
1980年 | 62篇 |
1979年 | 75篇 |
1978年 | 60篇 |
1977年 | 56篇 |
1976年 | 45篇 |
1974年 | 15篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 176 毫秒
1.
3.
In the view of substrate availability, atomic efficiency and cost, directly using arenols as coupling partners in cross‐coupling, would be one of the most attractive goals. Up to date, many efforts have been made to activate the C—O bond of phenols with different strategies, for example, through in‐situ formed intermediates, through a catalytic reductive dearomatization‐condensation‐rearomatization sequence or catalytic deoxygenation. In this review, we summarized recent advances in cross‐couplings of arenols as the electrophiles via C—O activation. 相似文献
4.
Lubomír Havlk Jan Fbry Margarida Henriques Michal Duek 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(7):623-630
The title structures of KScS2 (potassium scandium sulfide), RbScS2 (rubidium scandium sulfide) and KLnS2 [Ln = Nd (potassium neodymium sufide), Sm (potassium samarium sulfide), Tb (potassium terbium sulfide), Dy (potassium dysprosium sulfide), Ho (potassium holmium sulfide), Er (potassium erbium sulfide), Tm (potassium thulium sulfide) and Yb (potassium ytterbium sulfide)] are either newly determined (KScS2, RbScS2 and KTbS2) or redetermined. All of them belong to the α‐NaFeO2 structure type in agreement with the ratio of the ionic radii r3+/r+. KScS2, the member of this structural family with the smallest trivalent cation, is an extreme representative of these structures with rare earth trivalent cations. The title structures are compared with isostructural alkali rare earth sulfides in plots showing the dependence of several relevant parameters on the trivalent cation crystal radius; the parameters thus compared are c, a and c/a, the thicknesses of the S—S layers which contain the respective constituent cations, the sulfur fractional coordinates z(S2−) and the bond‐valence sums. 相似文献
5.
Peng-Yan Gui Xiao-Hui Yan Wen-Juan Yuan Yuan-Liang Ma Xiao Ding Shuai Liu 《Natural product research》2019,33(19):2830-2836
A new A, D-seco limonoid, named 12-acetyloxyperforatin (1), along with three known ones, were isolated from the leaves of Harrisonia perforata. Their structures were elucidated on the basis of spectroscopic analysis, including extensive NMR techniques and computational modelling. These compounds showed no inhibitory activity against the 11β-HSD1 enzyme. 相似文献
6.
A.G. Rousskikh A.V. Fedyunin A.P. Artyomov A.S. Zhigalin V.I. Oreshkin 《Current Applied Physics》2019,19(6):704-708
The aim of the study presented in this paper was to characterize quantitatively the erosion of aluminum cathodes in high-current vacuum arcs. The experimental setup comprised two current generators. The first one, capable of generating a current of amplitude up to 350 kA, was used to produce a plasma jet, that is, the object to be investigated. The second generator was used to produce a source of probe radiation for imaging the object under investigation in soft x rays of energy ?ν ≈ 0.5–3 keV. The findings of the study are based on experimental data obtained by electrophysical and radiographic methods. It has been shown that the cathode erosion rate in a high-current vacuum arc is a function of the charge passed through the cathode. 相似文献
7.
非线性互补问题的一种全局收敛的显式光滑Newton方法 总被引:2,自引:0,他引:2
本针对Po函数非线性互补问题,给出了一种显式光滑Newton方法,该方法将光滑参数μ进行显式迭代而不依赖于Newton方向的搜索过程,并在适当的假设条件下,证明了算法的全局收敛性。 相似文献
8.
XU Jun YAN Ai-Zhen XU Qin-Hua Department of Chemistry Nanjing University Nanjing Jiangsu China 《中国化学》1995,(2)
The strength of basic sites has been measured by pyrrole-IR on alkali metal cation exchanged β and X zeolites, as well as NaOH loaded Naβ. The influence of cation type and the structure of zeolites on their basicity has been studied. The acidic and basic properties of the samples were investigated by NH3-TPD and isopropanol reaction. It was shown that the strength of basic sites on samples could be characterized by the shift of vNH band in the pyrrole-IR spectra. The framework oxygen charges were calculated from the Sanderson electronegativity. The changes in basic properties with various alkali metal cation are consistent with the changes of local oxygen charges of the zeolite framework. 相似文献
9.
In circuit-switched networks call streams are characterized by their mean and peakedness (two-moment method). The GI/M/C/0 system is used to model a single link, where the GI-stream is determined by fitting moments appropriately. For the moments of the overflow traffic of a GI/M/C/0 system there are efficient numerical algorithms available. However, for the moments of the freed carried traffic, defined as the moments of a virtual link of infinite capacity to which the process of calls accepted by the link (carried arrival process) is virtually directed and where the virtual calls get fresh exponential i.i.d. holding times, only complex numerical algorithms are available. This is the reason why the concept of the freed carried traffic is not used. The main result of this paper is a numerically stable and efficient algorithm for computing the moments of freed carried traffic, in particular an explicit formula for its peakedness. This result offers a unified handling of both overflow and carried traffics in networks. Furthermore, some refined characteristics for the overflow and freed carried streams are derived. 相似文献
10.
杂多酸催化合成乙酸乙酯 总被引:14,自引:0,他引:14
回流吸附法制备的负载型12-钨磷酸(HPW/C)是合成乙酸乙酯的高效催化剂。最佳反应条件下乙醇转化率达96%,酯化选择性100%。催化剂的高活性是由于HPW在活性炭上高度分散,使活性位增多,有效酸量增大。HPW/C还具有热稳定性高和不易流失等优点。 相似文献