首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   51354篇
  免费   4955篇
  国内免费   2516篇
化学   35162篇
晶体学   560篇
力学   2426篇
综合类   110篇
数学   5257篇
物理学   15310篇
  2023年   623篇
  2022年   901篇
  2021年   1294篇
  2020年   1423篇
  2019年   1414篇
  2018年   1129篇
  2017年   1016篇
  2016年   1864篇
  2015年   1725篇
  2014年   2075篇
  2013年   3232篇
  2012年   4243篇
  2011年   4488篇
  2010年   2852篇
  2009年   2606篇
  2008年   3325篇
  2007年   3119篇
  2006年   2868篇
  2005年   2564篇
  2004年   2081篇
  2003年   1601篇
  2002年   1564篇
  2001年   1107篇
  2000年   1008篇
  1999年   847篇
  1998年   676篇
  1997年   727篇
  1996年   777篇
  1995年   608篇
  1994年   576篇
  1993年   504篇
  1992年   497篇
  1991年   402篇
  1990年   322篇
  1989年   282篇
  1988年   237篇
  1987年   227篇
  1986年   182篇
  1985年   250篇
  1984年   159篇
  1983年   140篇
  1982年   157篇
  1981年   106篇
  1980年   90篇
  1978年   86篇
  1977年   88篇
  1976年   103篇
  1975年   112篇
  1974年   82篇
  1973年   105篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Summary A thin layer chromatographic method for a qualitative screening-test and a quantitative analysis of 2,4,6-TNT, biodegradation products, octogen and hexogen in ammunition wastes was developed using both polar and non-polar modified sorbents. For enrichment a solidphase extraction on LiChrolut® EN followed by removal with methanol/acetonitrile (11 v/v) was chosen. To imitate real samples, spiked tap water samples of known composition were used.  相似文献   
982.
Y. Shen  M. L. Lee 《Chromatographia》1996,43(7-8):373-379
Summary Polyethylene oxide (PEO)-based polymers with hydroxy, methoxy, and aminopropoxy terminal groups were coated on diol functionalized and hexamethyldisilazane end-capped silica particles. Proton-donor and proton-acceptor test solutes, including carboxylic acids, hydroxy-containing compounds, arylamines, and alkylamines were used to evaluate the chromatographic performances of these polymer coated particles under SFC conditions with neat CO2 as mobile phase. It was found that the particles coated with hydroxy-terminated PEO were suitable for the separation of proton-donor compounds such as hydroxy-containing compounds and carboxylic acids, and the particles coated with aminopropoxy-terminated PEO could be used for the separation of amines. That is, the proton-accepting stationary phase is suitable for the separation of proton accepting solutes, including strong basic alkylamines (pKb4), using neat CO2 as mobile phase, while the protondonating stationary phase is suitable for the separation of proton-donating compounds such as carboxylic acids (pKa4). Hydrogen bond basicity was found to be a critical factor for the chromatography of basic amines. Low volatility acidic and basic drugs were chromatographed using the new stationary phases. The stability of the PEO coated particles was determined by measuring the loss of organic carbon under SFC conditions. It was found that approximately 18 % of the coating (average molecular weight of 15,000) was washed out of the particles by supercritical CO2 after 7 h at 350 atm and 50°C  相似文献   
983.
984.
The formation and stereoselective quenching of 1-mannopyranosyl radicals by a tributyltin hydride-mediated intramolecular 1,5-hydrogen abstraction sequence is described. A competing process is 1,4-hydrogen atom abstraction leading principally to glucopyran-2-ulosides. Fragmentation of the anomeric radical resulting in the formation of ring opened products is a problem in certain series. The chemistry is dictated to a considerable extent by the nature of the protecting groups employed with the 4,6-benzylidene series and, for rhamnose, the Ley 3,4-dispiroketal, being particularly susceptible to the 1,4-hydrogen atom abstraction but less to the fragmentation. Photochemical conditions are described, in which these side reactions are practically eliminated, and applied to the inversion of an alpha- to a beta-mannoside in a disaccharide.  相似文献   
985.
986.
987.
Korenaga T  Sun F 《Talanta》1996,43(9):1471-1479
A highly sensitive detection system using a semiconductor laser (SCL) and a thin long flow-through cell (TLFCT) to match flow-based analysis, such as flow injection analysis (FIA), has been developed. The SCL was a GaAlAs semiconductor laser emitting at 780 nm (5 mW), and the TLFTC, which has a 100 mm thin long light path, was made of a poly(tetrafluoroethylene) (PTFE) rod. The sensitivity was essentially improved about 10-fold in comparison to usual FIA-spectrophotometry using a 10 mm conventional flow cell. When this detection system was applied to the determination of total phosphorus (P) in water by molybdenum-blue spectrophotometry, the relative standard deviation, detection limit and linear range were 1.0% (n = 10 at 20 mug P 1(-1)), 0.6 (3 sigma) and 1.0-50 mug P 1(-1), respectively, and the results for the samples were fairly consistent with those by the official method. It may also be possible to apply the present detection system to other flow-based analytical methods such as the liquid chromatography family.  相似文献   
988.
989.
990.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号