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   147316篇
  免费   1298篇
  国内免费   397篇
化学   80456篇
晶体学   2389篇
力学   6068篇
综合类   3篇
数学   13580篇
物理学   46515篇
  2021年   1033篇
  2020年   1186篇
  2019年   1252篇
  2018年   1640篇
  2017年   1693篇
  2016年   2544篇
  2015年   1561篇
  2014年   2351篇
  2013年   6055篇
  2012年   4849篇
  2011年   5942篇
  2010年   4183篇
  2009年   4214篇
  2008年   5741篇
  2007年   5529篇
  2006年   5398篇
  2005年   4967篇
  2004年   4467篇
  2003年   4135篇
  2002年   4018篇
  2001年   5549篇
  2000年   3976篇
  1999年   3078篇
  1998年   2114篇
  1997年   2210篇
  1996年   2071篇
  1995年   1917篇
  1994年   1914篇
  1993年   1725篇
  1992年   2224篇
  1991年   2301篇
  1990年   2138篇
  1989年   2104篇
  1988年   2086篇
  1987年   2093篇
  1986年   1913篇
  1985年   2376篇
  1984年   2395篇
  1983年   1912篇
  1982年   1916篇
  1981年   1754篇
  1980年   1744篇
  1979年   2031篇
  1978年   2101篇
  1977年   2049篇
  1976年   1976篇
  1975年   1861篇
  1974年   1845篇
  1973年   1875篇
  1972年   1238篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
The catalytic dehydrocyclization of 3-isopropyl-2,6-diphenylpyridine proceeds in two directions: conversion to 10-methyl-3-phenyl-4-azaphenanthrene and conversion to 9,9-dimethyl-4-azafluorene. C(9)-Unsubstituted 4-azafluorenes were obtained in the dehydrocyclization of pyridine bases that contain a phenyl group in the position and an n-propyl group in the position.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 648–650, May, 1984.  相似文献   
952.
The proceeding of the IAEA symposium held in 1978 on nuclear activation techniques in the life sciences are reviewed. A total of 56 papers are reviewed on methodology, analytical quality control, comparisons between neutron activation analysis and other methods, and applications of activation analysis in biology and medicine (including in-vivo activation analysis) and in public and environmental health. The materials analysed in the papers are adjusted in connection with the elements determined and the purposes of the study. Those elements are tabulated together with the analytical methods used for the element determination. The standard reference materials analysed for the check of the reliability of analysis are also arranged. Proc. Symp. Nuclear Activation Techniques in the Life Sciences, IAEA, Vienna, 1978.  相似文献   
953.
954.
Nitropyrazoles     
A method for the synthesis of bi-, ter-, and quaterpyrazoles by cyclization ofC-(diformylmethyl)nitropyrazoles with hydrazine has been elaborated. Some of theirC- andN-derivatives have been obtained. The compounds prepared have been characterized by1H,13C,14N, and15N NMR spectra.For communication 6, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1941–1945, November, 1993.  相似文献   
955.
Desialylated fetuin (asialofetuin) was adopted as a carrier for introducing drugs in parenchymal liver cells. Mitomycin C, as a model guest-compound, was covalently attached to asialofetuin through a spacer of the succinyl group. The asialofetuin-mitomycin C conjugate contained 4.4 w/w% of mitomycin C and liberated it gradually at physiological conditions (t1/2 = 37 h). The survival time of the conjugate in rat blood circulation was significantly smaller than that of the non-desialylated fetuin conjugate; the elimination half-life was 7.3 min after intravenous injection. At 30 min after injection of the conjugate in rats, 40% of the dose was present in the liver. Parenchymal cells in the liver selectively took up the conjugate, which was highly distributed to the lysosomal fraction in the cell. The greater uptake of the conjugate by hepatocytes reflected the increased excretion in the bile; totally 10.4% of the dose was recovered.  相似文献   
956.
The monitoring of the physiological and functional state of personnel from dangerous industries is very important for the ensuring of ecological security. Usually, this involves testing for the consumption of alcohol and drugs of abuse. During the last several years the use of legal psychotropic drugs has increased and the control over these drugs has became an actual problem. As is well known, the long-term history of drugs present in the body is accessible through hair analysis. This is why the aim of our research is to create a screening procedure based on coupling high-performance thin-layer chromatography with microliquid extraction as a psychotropic drug testing method for hair. Some widely distributed antidepressants, neuroleptics, and sedative drugs are chosen for research. The optimal experimental conditions for all of the consequent steps for the screening detection of the model samples are determined. The visual and densitometric detection limits allow for the employment of the proposed technique for a fast and cost-effective analysis of the drugs of abuse.  相似文献   
957.
Ammonium salts containing phenacyl group in combination with 4-allyloxy(phehoxy)-2-butynyl group, when treated with a suspension of potassium hydroxide in benzene, undergo the 3,2-Stevens rearrangement with the formation of allenic amino ketones which under the reaction conditions give the furan derivatives.  相似文献   
958.
Progressive reaction networks as frequently arise in chemistry are naturally identifiable as "partially ordered sets" (or posets). Here the direction of the reaction identifies the partial ordering of the set of molecular species. The possibility that different properties are similarly ordered is a further natural consideration and is here investigated for a suite of over 30 properties for (methyl and chloro) substituted benzenes. Such a posetic correlation is favorably demonstrated for these substituted benzenes, and it is illustrated how suitable properties may be simply predicted in an interpolative parameter-free (albeit not model-free) fashion through the use of the reaction poset. Some numerical model-quality indicators are identified, and the simple approach is deemed quite reasonable.  相似文献   
959.
New polymethine dyes with an indolo[3,2-d]thiazole residue are described, and their spectral properties are discussed. Replacement of the vinylene group in the naphtho[1,2-d]thiazole residue by an NH group leads to a considerably greater bathochromic shift of the absorption maximum of carbo- and merocyanines as compared with replacement by a sulfur atom. The basicity of the indolo[3,2-d]thiazole residue is higher than the basicity of the naphtho[1,2-d]-thiazole and thionaphtheno[3,2-d]thiazole residues.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1606–1610, December, 1972.  相似文献   
960.
The kinetics of oxidation of isopropanol (IPA) by IO 4 in the presence of Os(VIII), Ru(III) and mixture of Os(VIII) and Ru(III) has been studied. The catalytic effect was found to be in the order Os(VIII)
IO 4 Os(VIII), Ru(III) Os(VIII) Ru(III), . : Os(VIII)  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