全文获取类型
收费全文 | 33879篇 |
免费 | 261篇 |
国内免费 | 289篇 |
专业分类
化学 | 14144篇 |
晶体学 | 282篇 |
力学 | 1575篇 |
综合类 | 12篇 |
数学 | 9733篇 |
物理学 | 8683篇 |
出版年
2022年 | 47篇 |
2021年 | 92篇 |
2020年 | 103篇 |
2019年 | 106篇 |
2018年 | 1259篇 |
2017年 | 1490篇 |
2016年 | 845篇 |
2015年 | 645篇 |
2014年 | 603篇 |
2013年 | 923篇 |
2012年 | 3419篇 |
2011年 | 2665篇 |
2010年 | 1967篇 |
2009年 | 1722篇 |
2008年 | 861篇 |
2007年 | 946篇 |
2006年 | 926篇 |
2005年 | 4765篇 |
2004年 | 4172篇 |
2003年 | 2541篇 |
2002年 | 660篇 |
2001年 | 361篇 |
2000年 | 166篇 |
1999年 | 249篇 |
1998年 | 158篇 |
1997年 | 111篇 |
1996年 | 107篇 |
1995年 | 105篇 |
1994年 | 93篇 |
1993年 | 64篇 |
1992年 | 211篇 |
1991年 | 187篇 |
1990年 | 171篇 |
1989年 | 139篇 |
1988年 | 123篇 |
1987年 | 80篇 |
1986年 | 55篇 |
1985年 | 76篇 |
1984年 | 62篇 |
1983年 | 48篇 |
1982年 | 50篇 |
1980年 | 47篇 |
1979年 | 63篇 |
1978年 | 55篇 |
1977年 | 48篇 |
1976年 | 118篇 |
1975年 | 70篇 |
1974年 | 64篇 |
1973年 | 74篇 |
1972年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Novel shape descriptors for molecular graphs. 总被引:2,自引:0,他引:2
M Randi? 《Journal of chemical information and computer sciences》2001,41(3):607-613
We report on novel graph theoretical indices which are sensitive to the shapes of molecular graphs. In contrast to the Kier's kappa shape indices which were based on a comparison of a molecular graph with graphs representing the extreme shapes, the linear graph and the "star" graph, the new shape indices are obtained by considering for all atoms the number of paths and the number of walks within a graph and then making the quotients of the number of paths and the number of walks the same length. The new shape indices show much higher discrimination among isomers when compared to the kappa shape indices. We report the new shape indices for smaller alkanes and several cyclic structures and illustrate their use in structure-property correlations. The new indices offer regressions of high quality for diverse physicochemical properties of octanes. They also have lead to a novel classification of physicochemical properties of alkanes. 相似文献
102.
103.
The separation of racemic benoxaprofen into the two benoxaprofen enantiomers by preparative high-performance liquid chromatography and the application of the activated enantiomers as derivatization reagents for the simultaneous stereoselective determination of chiral amines in biological material is described. Activated (+)- and (-)-benoxaprofen are both shown to be very sensitive and stable chiral fluorescence markers, applicable to thin-layer chromatography as well as to high-performance liquid chromatography. 相似文献
104.
本文研究了二茂钛含氟长链有机酸衍生物(1~3)和具有环状结构的新型二茂钛全氟有机酸(4~7)的电子轰击质谱(EIMS)和负离子化学电离质谱(NICIMS)。在EI谱中出现弱的或不出现M~+,1~7均以CpTiF_2离子为基峰。高质量区的主要特征碎片离子有[M-·cp]~+、[M-R]~+(1~3)和氟重排离子。在低质量区出现一系列二茂钛及茂钛氟重排离子。 1、4~7的NICI(CH_4)质谱中,4、5出现强的M~-离子峰,1出现[M-1]-离子峰,有两个钛的环状结构化合物6和7出现双电荷M~=离子峰,而不出现M~-。 相似文献
105.
Biosensors for environmental pollutants and food contaminants 总被引:4,自引:0,他引:4
Baeumner AJ 《Analytical and bioanalytical chemistry》2003,377(3):434-445
This review article provides an overview of the most recent literature on biosensors for environmental pollutants and food contaminants. Due to the large number of publications, only papers published between 2000 and January 2003 were considered. Also, while not all of the published literature could be reviewed here, over 200 references are cited to provide a good overview of research undertaken in the last two years. Older publications are covered by a number of earlier review articles. This article provides an introduction into the field including specific consideration of the application areas, describes the typical biosensor assay format used, and is subsequently structured according to the biorecognition elements used (i.e., nucleic acids, enzymes, whole cells, tissue and whole organisms, antibodies and receptors, and biomimetic materials). In addition, a section on microbiosensing systems is provided. Since only very few microbiosensors with applications in environmental and food systems have been published, enabling technology is also covered in this article.Abbreviations BOD biological oxygen demand - cfu colony forming units - DNP dinitrophenol - GFP green fluorescent protein - NASBA nucleic acid sequence-based amplification - PCR polymerase chain reaction - ppt parts per trillion - QCM quartz crystal microbalance - SPR surface plasmon resonance 相似文献
106.
107.
Why carry out medicinal chemistry at a university, when it means competing with the billion-dollar research efforts of the pharmaceutical industry? In academic research, the race to get a drug to market is not the prime motivation. Instead, university-based medicinal chemistry is driven by the search for new knowledge and the opportunity to educate a new generation of chemists. Furthermore, academia can complement commercial efforts by addressing diseases neglected by private industry. 相似文献
108.
Maria Merces Marques C. Costa F. Lemos F. Ram?a Ribeiro A. R. Dias 《Reaction Kinetics and Catalysis Letters》1997,62(1):9-15
In this paper we present a kinetic approach to the analysis of steady-state homogeneous Ziegler-Natta polymerization activity
data. The influence of the number of monomeric species that are coordinated to the active site on the apparent rate law is
discussed and the equations are fitted to the experimental results. 相似文献
109.
V. Spěvá?ková J. Jhn M. Pra?ská 《Journal of Radioanalytical and Nuclear Chemistry》1983,80(1-2):115-120
The determination of uranium in natural waters by XRF analysis after its preconcentration with some organic precipitants is described.109Cd source is used for the excitation of L-series X-rays of uranium. Separation yield, kinetics of coprecipitation and the possibility of preparing thin-layer samples for XRF analysis were studied in detail for uranium concentration with α-nitroso-β-naphtol, methylene blue with ammonium thiocyanate, tannin with urotropine and other precipitants. A procedure using coprecipitation of uranium with α-nitroso-β-naphtol from 500 ml samples of surface or waste, waters is proposed. 相似文献
110.
Fausi?RassoulEmail author Heinz?L?ster Volker?Richter 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1501-1507
Summary. L-Carnitine, L-(−)-β-hydroxy-γ-trimethylaminobutyrate, plays an important role as a factor necessary for the transport of long-chain fatty acids into the mitochondria. In order to investigate the influence of L-carnitine on hyperlipidaemias, the experimental model of the sucrose-induced hypertriglyceridaemia of the rat was used. In these experiments L-carnitine in the dose of 11 mg per day and 100 g body weight (over the period of 1 week) was able to antagonize the sucrose-induced hypertriglyceridaemia and the increase of serum free fatty acid level in female rats of the Wistar strain. Carnitine administration did not change the activities of lipogenic enzymes and fatty acid synthesis in the liver. However, L-carnitine increases the rate of hepatic fatty acid oxidation. Our results indicate a hypotriglyceridemic and free fatty acid lowering effect of L-carnitine, and suggest the use of this compound in the therapy of hyperlipidaemias. 相似文献