全文获取类型
收费全文 | 35154篇 |
免费 | 6129篇 |
国内免费 | 4366篇 |
专业分类
化学 | 25132篇 |
晶体学 | 401篇 |
力学 | 2403篇 |
综合类 | 324篇 |
数学 | 3860篇 |
物理学 | 13529篇 |
出版年
2024年 | 95篇 |
2023年 | 700篇 |
2022年 | 1171篇 |
2021年 | 1294篇 |
2020年 | 1501篇 |
2019年 | 1471篇 |
2018年 | 1241篇 |
2017年 | 1137篇 |
2016年 | 1712篇 |
2015年 | 1769篇 |
2014年 | 2040篇 |
2013年 | 2631篇 |
2012年 | 3167篇 |
2011年 | 3338篇 |
2010年 | 2179篇 |
2009年 | 2204篇 |
2008年 | 2311篇 |
2007年 | 2022篇 |
2006年 | 1957篇 |
2005年 | 1637篇 |
2004年 | 1343篇 |
2003年 | 1003篇 |
2002年 | 926篇 |
2001年 | 799篇 |
2000年 | 694篇 |
1999年 | 671篇 |
1998年 | 572篇 |
1997年 | 509篇 |
1996年 | 490篇 |
1995年 | 467篇 |
1994年 | 412篇 |
1993年 | 348篇 |
1992年 | 343篇 |
1991年 | 286篇 |
1990年 | 241篇 |
1989年 | 194篇 |
1988年 | 132篇 |
1987年 | 122篇 |
1986年 | 103篇 |
1985年 | 105篇 |
1984年 | 58篇 |
1983年 | 64篇 |
1982年 | 43篇 |
1981年 | 31篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1977年 | 9篇 |
1975年 | 11篇 |
1971年 | 6篇 |
1957年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
有机-无机纳米复合材料的研究在当前纳米科学技术的发展中占有重要地位,开创了材料与催化科学研究的新纪元。自从10年前首次合成纳米孔无机材料MCM-41至今,该领域研究不断深入,已展现出广阔的应用前景。当客体有机分子分散于纳米孔道内部时,其分子性质将产生明显变化。本文结合作者近期研究成果,力图从实验与理论两方面阐明在纳米复合体系中客体分子性质变化的原因。 相似文献
992.
The high-performance liquid chromatographic behavior of some basic drugs was studied on a n-octadecylphosphonic acid modified magnesia-zirconia (C18PZM) stationary phase. The effect of mobile phase variables such as methanol content, ionic strength, and pH on their chromatographic behavior was investigated. The retention mechanism of basic drugs on the stationary phase was elucidated. The results indicate that both hydrophobic and cation-exchange interactions contribute to solute retention under most chromatographic conditions. The inherent Br?nsted-acid sites and also the adsorbed Lewis base anionic buffer constituents on accessible ZM surface Lewis acid sites play a role in the retention of ionized solutes by cation-exchange interaction. However, especially at high mobile phase pH, the retention of basic drugs depends mainly on hydrophobic interactions between solutes and support. Separations of the basic drugs on the C18PZM phase by a predominantly reversed-phase retention mode were very promising. The mixed-mode retention feature on this phase, as a result of the adsorbed Lewis base anionic buffer constituents acting as sites for cation-exchange, could also be very useful, e.g. for enhancing the chromatographic selectivity of such analytes. The C18PZM seems to be an excellent alternative to silica-based reversed-phase stationary phase for the separation of strongly basic solutes. 相似文献
993.
冠醚配位作用的热力学性质——水液溶中1,4,7,10,13,16-六氧杂环十八烷与若干钾盐配位作用的负离子效应 总被引:3,自引:0,他引:3
本文用量热滴定法测定了1,4,7,10,13,16-六氧杂环十八烷(以下称18-冠-6)与KCl,KBr,KI, KClO_4,KNO_3,KBrO_3及K_2S_2O_3共八种钾盐在水溶液中, 25 ℃时的配位作用, 直接计算了配合物稳定常数及配位反应热。实验结果表明, 18-冠-6与KNO_3,K_2S_2O_3形成的配位化合物, 其稳定常数最高。讨论了阴离子对配位稳定性的影响。 相似文献
994.
995.
996.
Xu Q Tanaka K Mori M Helaleh MI Hu W Hasebe K Toada H 《Journal of chromatography. A》2003,997(1-2):183-190
A monolithic ODS-silica gel column modified by saturating it with lithium dodecylsulfate (Li-DS) was used to demonstrate the high-speed separation of H+ from other mono- and divalent cations, such as Na+, NH4+, K+, Mg2+ and Ca2+ using ion chromatography (IC). Using a 5 mM EDTA-2K solution containing 0.10 mM Li-DS (pH 4.80) as eluent, H+ was eluted with a sharp and symmetrical peak within 1.0 min before other cations at a flow-rate of 1.5 ml min(-1). The rapid elution of H+ and its conductimetric detection could be attributed to the presence of EDTA (HY2-), which can convert H+ ions as anions. i.e. H(+) + H2Y(2-) --> H3Y(-). The acidity of rainwater and deionized water samples was determined using this IC system with satisfactory results. 相似文献
997.
A high-performance liquid chromatographic (HPLC) method with refractive-index detection has been developed for simultaneous analysis of glycerol, dihydroxyacetone (DHA), 3-hydroxypropionaldehyde (3-HPA), and 1,3-propanediol (1,3-PD), four key substances in the metabolic pathway for production of 1,3-PD from glycerol by microorganism fermentation. The compounds were separated on a 300 mm × 7.8 mm ion-exclusion column with a 65:35 (v/v) mixture of deionized water and acetonitrile, containing 0.0005 M H2SO4, as mobile phase. The flow rate was 0.5 mL min?1. Under these conditions the retention times of 3-HPA, DHA, glycerol, and 1,3-PD were 6.87, 14.63, 16.37, and 18.50 min, respectively. Relative standard deviations and average recoveries were between 0.42 and 0.63% and between 96.7 and 103.1%, respectively; detection limits were between 0.017 and 0.038 g L?1. The method enabled separation of these compounds. 相似文献
998.
Anyun Zhang Jingxin Hu Xianye Zhang Fangding Wang 《Journal of Radioanalytical and Nuclear Chemistry》2002,253(1):107-113
N,Ndiethylhydroxylamine (DEHAN) can rapidly reduce Pu(IV) and Np(VI) extractable with trinbutyl phosphate (TBP) to Pu(III) and Np(V) unextractable with TBP in nitric acid solution. In order to apply DEHAN in the purification cycle step of U (noted as contactor 2D) of the secondcycle of the Purex Process to separate Np and Pu from U, the reduction of Np(VI) and the stability of Np(V) with DEHAN and the singlestage reduction extraction and backextraction of Np(VI) have been studied according to the experimental conditions of contactor 2D. The results show that more than 99% of Np(VI) can entirely be reduced to Np(V) with DEHAN within a few minutes either in aqueous or in organic phase containing uranium and without containing uranium, and more than 99% of Np(VI) can be backextracted from the organic to the aqueous phase with DEHAN as a reductant. More than 99% of Np(V) exists in nitric acid solution at least for 8 hours in presence of 0.01 mol/l DEHAN. These results are of benefit to the cascade extractionseparation of U/Np in contactor 2D in order to decontaminate Np from U. 相似文献
999.
Rapid determination of volatile constituents of Michelia alba flowers by gas chromatography-mass spectrometry with solid-phase microextraction 总被引:12,自引:0,他引:12
The volatile constituents of Michelia alba flowers, including fresh flowers, frozen flowers and withered flowers, were investigated by GC-MS. The volatiles in a simulated natural environment were sampled by solid-phase microextraction (SPME), with a 100 microm polydimethylsiloxane fiber at 25+/-5 degrees C for 4 h. The fibers were desorbed in a GC injection liner at 250 degrees C for 3 min. With headspace SPME-GC-MS analysis, 61 peaks were separated. The main compounds in headspace of fresh Michelia alba flowers included alpha-myrcene, (S)-limonene, (R)-fenchone, linalool, camphor, caryophyllene, germacrene D, etc., a greater number of compounds than for frozen flowers and withered flowers. At the same time, the biomarkers of fresh flowers were compared with the frozen flowers and withered flowers. In this study, headspace SPME-GC-MS afforded a simple and more sensitive sampling method for fresh Michelia alba flowers and other fresh flowers. 相似文献
1000.
Jia‐Xiang Yang Yu‐Peng Tian Qing‐Liang Liu Yong‐Shu Xie Hoong‐Kun Fun Suchada Chantrapromma Ibrahim Abdul Razak 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(1):m43-m44
In the title compound, [Fe(C5H5)(C16H12N3OS)], the 8‐aminoquinoline and acylthiourea moieties are almost planar. There are two perpendicular arrangements of the molecules in the crystal with slightly different conformations. The two cyclopentadienyl rings in each molecule are parallel and eclipsed. 相似文献