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   176986篇
  免费   1835篇
  国内免费   551篇
化学   92305篇
晶体学   3093篇
力学   7336篇
综合类   3篇
数学   20367篇
物理学   56268篇
  2021年   1373篇
  2020年   1502篇
  2019年   1761篇
  2018年   2144篇
  2017年   2186篇
  2016年   3436篇
  2015年   2060篇
  2014年   3396篇
  2013年   8167篇
  2012年   6073篇
  2011年   7250篇
  2010年   5278篇
  2009年   5230篇
  2008年   6802篇
  2007年   6695篇
  2006年   6343篇
  2005年   5732篇
  2004年   5292篇
  2003年   4772篇
  2002年   4663篇
  2001年   5302篇
  2000年   3977篇
  1999年   3034篇
  1998年   2525篇
  1997年   2436篇
  1996年   2285篇
  1995年   2154篇
  1994年   2131篇
  1993年   2042篇
  1992年   2343篇
  1991年   2271篇
  1990年   2211篇
  1989年   2150篇
  1988年   2152篇
  1987年   2143篇
  1986年   2038篇
  1985年   2662篇
  1984年   2712篇
  1983年   2402篇
  1982年   2512篇
  1981年   2337篇
  1980年   2299篇
  1979年   2442篇
  1978年   2476篇
  1977年   2397篇
  1976年   2448篇
  1975年   2378篇
  1974年   2335篇
  1973年   2512篇
  1972年   1626篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
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   
932.
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   
933.
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   
934.
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   
935.
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   
936.
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   
937.
938.
939.
940.
It is generally believed that the uncertainty relation q p1/2, where q and p are standard deviations, is the precise mathematical expression of the uncertainty principle for position and momentum in quantum mechanics. We show that actually it is not possible to derive from this relation two central claims of the uncertainty principle, namely, the impossibility of an arbitrarily sharp specification of both position and momentum (as in the single-slit diffraction experiment), and the impossibility of the determination of the path of a particle in an interference experiment (such as the double-slit experiment).The failure of the uncertainty relation to produce these results is not a question of the interpretation of the formalism; it is a mathematical fact which follows from general considerations about the widths of wave functions.To express the uncertainty principle, one must distinguish two aspects of the spread of a wave function: its extent and its fine structure. We define the overall widthW and the mean peak width w of a general wave function and show that the productW w is bounded from below if is the Fourier transform of . It is shown that this relation expresses the uncertainty principle as it is used in the single- and double-slit experiments.  相似文献   
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