全文获取类型
收费全文 | 26020篇 |
免费 | 4305篇 |
国内免费 | 3026篇 |
专业分类
化学 | 12978篇 |
晶体学 | 254篇 |
力学 | 1224篇 |
综合类 | 187篇 |
数学 | 1961篇 |
物理学 | 8056篇 |
无线电 | 8691篇 |
出版年
2024年 | 121篇 |
2023年 | 597篇 |
2022年 | 823篇 |
2021年 | 1100篇 |
2020年 | 1026篇 |
2019年 | 927篇 |
2018年 | 856篇 |
2017年 | 888篇 |
2016年 | 1170篇 |
2015年 | 1262篇 |
2014年 | 1550篇 |
2013年 | 1933篇 |
2012年 | 2174篇 |
2011年 | 2288篇 |
2010年 | 1686篇 |
2009年 | 1687篇 |
2008年 | 1808篇 |
2007年 | 1645篇 |
2006年 | 1555篇 |
2005年 | 1314篇 |
2004年 | 956篇 |
2003年 | 809篇 |
2002年 | 744篇 |
2001年 | 623篇 |
2000年 | 565篇 |
1999年 | 468篇 |
1998年 | 393篇 |
1997年 | 370篇 |
1996年 | 320篇 |
1995年 | 327篇 |
1994年 | 278篇 |
1993年 | 239篇 |
1992年 | 168篇 |
1991年 | 140篇 |
1990年 | 137篇 |
1989年 | 97篇 |
1988年 | 64篇 |
1987年 | 53篇 |
1986年 | 48篇 |
1985年 | 35篇 |
1984年 | 22篇 |
1983年 | 26篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1971年 | 2篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
101.
Jin Luo Shu Li Qiongwen Kang Yi Sun 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):1180-1186
AbstractSixteen novel 3-methylthio-5-substituted benzamido-6-arylamino-1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4 (5H)-one derivatives (4a–p) were successfully synthesized from iminophosphoranes, aryl isocyanate, and substituted benzoylhydrazine. The structures of the title compounds were elucidated by FT-IR, 1H NMR, 13C NMR, and HRMS. Herbicidal activity of the compounds 4a–p against Brassica napus (rape), Echinochloa crusgalli (barnyard grass), Cucumis sativus (cucumber), and Triticum aestivum (wheat) were determined. The results showed that 5-(2-chlorobenzamido)-6-phenylamino-3-methylthio-1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4 (5H)-one (4c) displayed remarkable inhibition activity against the stalk and root of rape with 100% inhibition rate at the dosages of 10?mg/L and 100?mg/L, and 5-(4-nitrobenzamido)-6-phenylamino-3-methylthio-1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4 (5H) -one (4d) exhibited excellent activity against the stalk and root of barnyard grass with 100% inhibition rate at the same dosages. 相似文献
102.
This article presents an original work aimed at rationally designing molecularly imprinted polymer (MIP) toward a high specific adsorbent. Assembling with cobalt as the pivot, the MIP was prepared by coordinating polymerizable monomers around an inducible template. The use of pivot obviously plays a positive role on increasing the specificity of MIP, so as to adsorb more for the template and less for its analogue. Related studies indicate that these may be a result of increasing specific interaction, which makes the MIP capable of recognizing the imprint species. Further information from thermodynamic analysis reveals that the increasing specific interaction, in logic, can be due to a higher fidelity of imprint, which specifically allures the template to bind. 相似文献
103.
104.
Yan Yang Liu-Ting Yan Xu-Jian Luo Rong-Huan Qin Wen-Gui Duan 《Supramolecular chemistry》2013,25(11):810-818
Three novel coordination polymers, [Co(NH2-Aip)(H2Bibim)] n (1), [Co(NH2-Aip)(HBibimop)] n √ nH2O (2) and [Mn(NH2-Aip) √ H2O] n √ 2nH2O (3) with NH2-Aip and similar benzimidazole derivative ligands (NH2-Aip = 5-aminoisophthalic acid, H2Bibim = 2,2′-bibenzimidazole and HBibimop = 1,3-bis(benzimidazol-2-yl)-2-oxapropane), have been synthesised under hydrothermal conditions and structurally characterised by single-crystal X-ray diffraction analysis, elemental analysis and IR spectroscopy. 5-Aminoisophthalic acid ligand adopts μ2-, μ3- and μ4-bridge coordination fashion and benzimidazole derivatives exist as terminal- and bridge-mode in complexes 1–3. The intricate hydrogen bonds and π–π stacking interactions in supramolecular framework are discussed. Using the combination of ultraviolet–visible absorption titration and fluorescence spectra, the experimental results show that complexes 1 and 2 bind to DNA in an intercalative mode and their DNA-binding constants (K b ) are also found. 相似文献
105.
106.
2β,3α-Dihydroxyurs-12-en-28-oic acid(6) is a naturally occurring diastereoisomer of corosolic acid with glycogen phosphorylase inhibitory activity.A new strategy for the semi-synthesis of 6 was developed.Using the commercially available ursolic acid(1) as the starting materials,6 was synthesized through five facile reactions with a high stereoselectivity and an overall yield of 47.3%.The structure of 6 was confirmed by optical rotation.ESI-MS,1H NMR and 13C NMR data. 相似文献
107.
Qixin Dong Jiajun Zhu Qiang Sui Chao Tang Xiaomei Wang Yunqiu Yu 《Journal of separation science》2013,36(7):1200-1208
In this study, the objective was to investigate the degradation behavior of Esomeprazole under different recommended stress conditions according to International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use [1] by HPLC. Our research showed that the effect of mobile phase species on separation was significant for the determination of Esomeprazole and its related compounds. Successful separation of the drug from its related impurities and degradation products formed under different stress conditions was achieved using ammonium acetate buffer/ACN by a gradient elution. Compared with phosphate buffer/ACN, ammonium acetate buffer/ACN under same pH and gradient showed a great improvement in resolution due to the change of elution order. The drug was subjected to stress conditions including acidic, alkaline, oxidative, photolytic, and thermal conditions. Extensive degradation occurred in acidic and oxidative conditions, while mild degradation was observed in alkaline and photolytic conditions. Besides, it turned out the drug was extremely stable under thermal condition. The stability‐indicating LC–UV method was validated with respect to linearity, precision, accuracy, specificity, and robustness. The LC–MS method was also adopted for the characterization of degradation products. Based on the m/z values and fragmentation patterns, the degradation pathway of the drug has been proposed. 相似文献
108.
109.
Song Yun Weizhong Qian Chaojie Cui Yuntao Yu Chao Zheng Yi Liu Qiang Zhang Fei Wei 《Journal of Energy Chemistry》2013,22(4):567-572
For the synthesis of single-walled carbon nanotubes (SWCNTs) from CH4 over a Fe/MgO catalyst, we proposed a coupled Downer-turbulent fluidized-bed (TFB) reactor to enhance the selectivity and yield (or production rate) of SWCNTs. By controlling a very short catalyst residence time (1–3 s) in the Downer, only part of Fe oxides can be reduced to form Fe nano particles (NPs) available for the growth of SWCNTs. The percentage of unreduced Fe oxides increased and the yield of SWCNTs decreased accordingly with the increase of catalyst feeding rate in Downer. SWCNTs were preferentially grown on the catalyst surface and inhibited the sintering of the Fe crystallites which would be formed thereafter in the downstream TFB, evidenced by TEM, Raman and TGA. The coupled Downer-turbulent fluidized-bed reactor technology allowed higher selectivity and higher production rate of SWCNTs as compared to TFB alone. 相似文献
110.
A small library of isocoumarin derivatives has been synthesized via K2CO3-catalyzed domino reactions of 2-carboxybenzaldehyde and ω-bromoacetophenones under microwave irradiation (MW). This strategy offers a simple, environmentally acceptable route to synthesize isocoumarin derivatives in moderate to excellent yields. All new compounds were characterized by 1H NMR, 13C NMR, infrared, and high-resolution mass spectroscopic techniques. 相似文献