全文获取类型
收费全文 | 171881篇 |
免费 | 1557篇 |
国内免费 | 510篇 |
专业分类
化学 | 88571篇 |
晶体学 | 3040篇 |
力学 | 7167篇 |
综合类 | 3篇 |
数学 | 19573篇 |
物理学 | 55594篇 |
出版年
2020年 | 1402篇 |
2019年 | 1555篇 |
2018年 | 1985篇 |
2017年 | 2069篇 |
2016年 | 3194篇 |
2015年 | 1893篇 |
2014年 | 3124篇 |
2013年 | 7681篇 |
2012年 | 5776篇 |
2011年 | 6908篇 |
2010年 | 5030篇 |
2009年 | 5009篇 |
2008年 | 6559篇 |
2007年 | 6459篇 |
2006年 | 6109篇 |
2005年 | 5536篇 |
2004年 | 5115篇 |
2003年 | 4615篇 |
2002年 | 4467篇 |
2001年 | 5231篇 |
2000年 | 3869篇 |
1999年 | 2976篇 |
1998年 | 2485篇 |
1997年 | 2388篇 |
1996年 | 2250篇 |
1995年 | 2122篇 |
1994年 | 2100篇 |
1993年 | 2011篇 |
1992年 | 2318篇 |
1991年 | 2257篇 |
1990年 | 2201篇 |
1989年 | 2128篇 |
1988年 | 2128篇 |
1987年 | 2132篇 |
1986年 | 2020篇 |
1985年 | 2640篇 |
1984年 | 2689篇 |
1983年 | 2387篇 |
1982年 | 2493篇 |
1981年 | 2311篇 |
1980年 | 2287篇 |
1979年 | 2433篇 |
1978年 | 2468篇 |
1977年 | 2395篇 |
1976年 | 2441篇 |
1975年 | 2364篇 |
1974年 | 2331篇 |
1973年 | 2509篇 |
1972年 | 1627篇 |
1971年 | 1341篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
971.
A gas chromatographic investigation of the volatile part of a pineapple concentrate led to the identification of 59 substances, 35 of which were not previously found in pineapple. The identifications and syntheses of some esters (Δ3- and Δ4-unsaturated, β-hydroxy, β-acetoxy, and δ-acetoxy) and of a new sulfur compound are given. 相似文献
972.
Reaction products of N-carbazolylacetyl chloride under Friedel-Crafts reaction conditions include carbon monoxide, the reaction product from carbazole and formaldehyde (CF condensate), and carbazole. It is postulated that decarbonylation of N-carbazolylacetyl chloride involves intermediate formation of a N-carbazolylmethyl cation. 相似文献
973.
The general features of a new physical process for the qualitative and quantitative routine analysis of rare earth mixtures are given. The characteristic sharp lines in the magneto-optical rotation spectra of these elements are utilized. The recording photoelectric spectropolarimeter used is described briefly. A comparison is made with two other physical processes-absorption and X-ray fluorescence spectrometry. The proposed method appears to be the best when a mixture with more than 3 or 4 elements must be analysed. The sensitivity and accuracy are given for 9 rare earths. Improvements of the process are suggested. 相似文献
974.
De Wasch K Le Bizec B De Brabander H André F Impens S 《Rapid communications in mass spectrometry : RCM》2001,15(16):1442-1447
In previous work (Le Bizec et al., Rapid Commun. Mass Spectrom. 2000; 14: 1058), it was demonstrated that a boar meal intake could lead to possible false accusations of abuse of 17beta-nortestosterone in antidoping control. The aim of the present study was to identify and quantify endogenous 19-norsteroids in boar edible tissue at concentrations that can alter the steroid urinary profile in humans, and lead to excretion of 19-norandrosterone (19-NA) and 19-noretiocholanolone (19-NE). The samples were analysed in two laboratories. The methodologies used for extraction and detection (GC/MS(EI) and LC/MS/MS(APCI+)) are compared and discussed. 19-Norandrostenedione (NAED), 17beta- and 17alpha-nortestosterone (bNT, aNT), and 17beta- and 17alpha-testosterone (bT, aT) were quantified. The largest concentrations of NAED and bNT were observed in testicles (83 and 172 microg/kg), liver (17 and 63 microg/kg) and kidney (45 and 38 microg/kg). A correlation between the bNT and NAED content of a typical meal prepared with boar parts and the excreted concentrations of 19-NA and 19-NE in human urine was demonstrated. 相似文献
975.
For the determination of trace elements in neutron irradiated, selenium by gamma-ray spectrometry the separation of the matrix activity is often necessary. In model experiments the decontamination of cation and anion impurities from the matrix solution was investigated by the counter current ion migration. After a processing of 3 hrs the trace activities of Co, Cr, Ga, Na and Zn were decontaminated from Se with a factor of >103. For the trace elements As and Te representing the anionic constituents, a decontamination factor of 3·102 was obtained. 相似文献
976.
Walfred S. Saari Wasyl Halczenko Mark B. Freedman Byron H. Arison 《Journal of heterocyclic chemistry》1982,19(4):837-840
Reaction of phenyl N-(8-quinolinyl)carbamate with sodium borohydride afforded 1,2-dihydro-4H-imidazo-[5,4,1-ij]quinolin-2-one, 2a . The 5,6-double bond of 2a was functionalized by reaction with nitrosyl chloride to give the nitroolefin 4 and by reaction with hypobromous acid to give the trans-bromohydrin 5 . Reaction of 5 with sodium azide led to the rearranged trans-6-azido-5-ol 7 , the structure of which was determined by 1H nmr studies. 相似文献
977.
978.
979.
Regulation of polar cap formation in the life cycle of Escherichia coli 总被引:48,自引:0,他引:48
980.
Prof. Dr. B. Korte Prof. Dr. L. Lovász 《Mathematical Methods of Operations Research》1985,29(7):249-267
In previous papers many different classes and constructions of greedoids have been defined and studied. This paper documents inclusion relations among all subclasses of greedoids which are known so far. It will be shown that all inclusion relations are proper and that all but one subclasses of interval greedoids are distinct.
Supported by the joint research project Algorithmic Aspects of Combinatorial Optimization of the Hungarian Academy of Sciences (Magyar Tudományos Akadémia) and the German Research Association (Deutsche Forschungsgemeinschaft, SFB 21). Part of this research was done while the first author was visiting the Department of Combinatorics and Optimization, University of Waterloo. 相似文献
Zusammenfassung In früheren Arbeiten wurden viele verschiedene Klassen und Konstruktionen für Greedoide eingeführt und studiert. In dieser Arbeit werden alle bekannten Inklusionsbeziehungen zwischen Unterklassen von Greedoiden dokumentiert. Es wird gezeigt, daß alle Inklusionsbeziehungen echt und alle Unterklassen mit einer Ausnahme tatsächlich verschieden sind.
Supported by the joint research project Algorithmic Aspects of Combinatorial Optimization of the Hungarian Academy of Sciences (Magyar Tudományos Akadémia) and the German Research Association (Deutsche Forschungsgemeinschaft, SFB 21). Part of this research was done while the first author was visiting the Department of Combinatorics and Optimization, University of Waterloo. 相似文献