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   202745篇
  免费   1982篇
  国内免费   504篇
化学   110840篇
晶体学   3648篇
力学   8013篇
综合类   11篇
数学   19258篇
物理学   63461篇
  2020年   1738篇
  2019年   1938篇
  2018年   2251篇
  2017年   2364篇
  2016年   3539篇
  2015年   2210篇
  2014年   3574篇
  2013年   9116篇
  2012年   6549篇
  2011年   8056篇
  2010年   5716篇
  2009年   5677篇
  2008年   7179篇
  2007年   7088篇
  2006年   6704篇
  2005年   6072篇
  2004年   5537篇
  2003年   5106篇
  2002年   4879篇
  2001年   6148篇
  2000年   4629篇
  1999年   3523篇
  1998年   2749篇
  1997年   2706篇
  1996年   2594篇
  1995年   2445篇
  1994年   2336篇
  1993年   2169篇
  1992年   2826篇
  1991年   2730篇
  1990年   2672篇
  1989年   2692篇
  1988年   2714篇
  1987年   2722篇
  1986年   2591篇
  1985年   3305篇
  1984年   3316篇
  1983年   2598篇
  1982年   2712篇
  1981年   2766篇
  1980年   2538篇
  1979年   2876篇
  1978年   2860篇
  1977年   2981篇
  1976年   2818篇
  1975年   2554篇
  1974年   2511篇
  1973年   2467篇
  1972年   1703篇
  1968年   1703篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Hydrated iron oxide or amorphous-Fe2O3·3.5 H2O (HFeO), hydrated titanium oxide (HTiO) and hydrated thorium oxide (HThO) were synthesized and their applicability for the decontamination of intermediate level liquid wastes (ILLW) was tested. The sorption of a few actinides like plutonium and americium on HFeO, 137Cs and 106Ru on HTiO and 90Sr on HThO was investigated as a function of pH, time and loading capacity of the hydrous oxide with metal ions. The influence of the total dissolved salt content was also monitored. Some of these parameters influenced the sorption behavior significantly. The radiation stability of these inorganic sorbents were studied by irradiating them up to 48 Mrad. Adsorbed actinides and fission products were successfully eluted from HFeO and from the mix-bed of HTiO and HThO by 0.5M nitric acid.The authors wish to thank Shri R. D. Changarani, Chief Superintendent NRG Facilities and Shri P. K. Dey, Head FRD for their valuable advice and constant support.  相似文献   
992.
Standard papers regulating the quality of waters of different types are considered. Procedures for determining water quality indexes are briefly reviewed.__________Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 8, 2005, pp. 886–893.Original Russian Text Copyright © 2005 by Kutseva, Kartashova, Tchamaev.  相似文献   
993.
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   
994.
We present a density functional scheme for calculating the frequency-dependent linear response of superconductors. The central result is a set of integral equations determining the linear response of the normal and anomalous densities to external perturbations. Analytic solutions of these integral equations are obtained for homogeneous systems with separable effective interactions. For inhomogeneous superconductors, the formalism leads to a scheme for calculating the critical temperature without explicitly solving the gap equation. © 1995 John Wiley & Sons, Inc.  相似文献   
995.
Asymmetric carbocyanine dyes, the solutions of which are characterized by the presence of several absorption bands in the visible and near-IR regions, were synthesized from 4,5-dimethyl-1-oxo-2-phenyl-1,5-dihydrothiazolo[3,2-a]quinoxalin-3-ium perchlorate. Their deep color is due both to a more effective length of the condensed heterocyclic system and to the strong interaction of the electronic transitions, localized on the polymethine chain and on the heterocyclic ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1113–1116, August, 1992.  相似文献   
996.
Information is given on vegetable tanning substances, their classification, distribution in the plant world, and structure, and the dependence of their tanning properties on their composition, and also on methods of enriching tanning extracts. The review of literature sources includes publications from 1950 to 1989.A. S. Sadykov Institute of Bioorganic Chemistry, Uzbek SSR Academy of Sciences, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 293–307, May–June, 1990.  相似文献   
997.
The structure of the abnormal product 1a formed in the Knoevenagel condensation of 2-carbethoxycyclohexanone and malononitrile has been further confirmed. Oxidation of the tetrahydroisoquinoline 3b using Na2Cr2O-AcOH-H2SO4 gave the keto isoquinoline 3d and the isoquinoline-1-carboxylic acid 5a. The acid chloride of 5a was condensed with diethyl ethoxymagnesiomalonate to afford after decarbethoxylation the methyl ketone 5d which on Baeyer-Villiger oxidation gave a mixture of the acetate 1g and the title compound 1b. The unambiguous synthesis of 1b confirms the structure assigned earlier to the title compound also formed during the partial hydrolysis of the diethoxy compound 1c. Condensation of 2-acetylcyclohexane-1,3-dione with malononitrile gave the quinoline derivative 4c which on ethylation yielded the ketoquinoline 4d. The present studies have confirmed that the quinoline compound 4a is also formed in the condensation of 2-acetylcyclohexanone and cyanoacetamide.  相似文献   
998.
The title compounds, which contain six-membered chelate rings locked in the chair conformation, have been prepared by the reaction of (C6H5)3P with the appropriate tetracarbonyl derivative in refluxing mesitylene.  相似文献   
999.
Pharmaceutical counterfeiting   总被引:2,自引:0,他引:2  
Deisingh AK 《The Analyst》2005,130(3):271-279
Pharmaceutical counterfeiting is becoming a serious problem both in developed and developing countries. This paper considers the extent of the problem and provides several examples of drugs which have been counterfeited. Additionally, the effects of counterfeit products on consumers, health care providers, drug manufacturers and governments are discussed. Several of the currently used methods of detection are described and these include near-infrared spectroscopy, Raman spectroscopy, isotopic characterization, tensiography, chromatographic and mass spectrometric approaches. Finally, anti-counterfeiting measures such as the use of holograms, tracers and taggants and electronic tracking are summarized.  相似文献   
1000.
For singularly perturbed systems of ODE's satisfying a certain stability assumption the existence of an asymptotically stable (unstable) invariant manifold is proved. This invariant manifold is -close to the so-called reduced manifold of such a system. As an illustrative example a 3-dimensional autonomous system describing a model in biochemistry is considered.
Zusammenfassung Für singulär gestörte Differentialgleichungssysteme, die einer gewissen Stabilitätsbedingung genügen, wird die Existenz einer asymptotisch stabilen (instabilen) invarianten Mannigfaltigkeit nachgewiesen, die in einer -Umgebung der sogenannten reduzierten Mannigfaltigkeit eines solchen Systems liegt. Als Anschauungsbeispiel wird ein dreidimensionales autonomes System betrachtet, welches ein biochemisches Modell beschreibt.
  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