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   215437篇
  免费   1564篇
  国内免费   627篇
化学   117922篇
晶体学   3270篇
力学   8349篇
综合类   6篇
数学   23305篇
物理学   64776篇
  2020年   1970篇
  2019年   2263篇
  2018年   3205篇
  2017年   3113篇
  2016年   4267篇
  2015年   2371篇
  2014年   3818篇
  2013年   9260篇
  2012年   6891篇
  2011年   8434篇
  2010年   6071篇
  2009年   5921篇
  2008年   8136篇
  2007年   8207篇
  2006年   7629篇
  2005年   6936篇
  2004年   6166篇
  2003年   5641篇
  2002年   5497篇
  2001年   5939篇
  2000年   4684篇
  1999年   3470篇
  1998年   3007篇
  1997年   3033篇
  1996年   2823篇
  1995年   2319篇
  1994年   2419篇
  1993年   2450篇
  1992年   2595篇
  1991年   2657篇
  1990年   2559篇
  1989年   2533篇
  1988年   2463篇
  1987年   2434篇
  1986年   2357篇
  1985年   3037篇
  1984年   3194篇
  1983年   2559篇
  1982年   2827篇
  1981年   2703篇
  1980年   2503篇
  1979年   2718篇
  1978年   2833篇
  1977年   2856篇
  1976年   2832篇
  1975年   2687篇
  1974年   2665篇
  1973年   2810篇
  1972年   1875篇
  1967年   1830篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Single extraction tests are commonly used to study the eco-toxicity and mobility of metals in soils, e.g. to assess the bioavailable metal fraction (and thus to estimate the related phyto-toxic and nutritional deficiency effects) and the environmentally accessible trace metals upon disposal of e.g. sediment on to a soil (e.g. contamination of ground waters). However, the lack of uniformity in the different procedures does not allow the results to be compared worldwide nor the procedures to be validated. This paper describes the interlaboratory testing of EDTA- and DTPA-extraction procedures for soil analysis, followed by the preparation of a calcareous soil reference material (CRM 600), the homogeneity and stability studies and the analytical work performed for the certification of the EDTA- and DTPA- extractable contents of some trace metals (following the standardized extraction procedures). Received: 21 May 1997 / Revised: 4 July 1997 / Accepted: 9 July 1997  相似文献   
942.
Radiochemical techniques play an important role in nuclear waste management studies at the Centre for Analytical Research in the Environment. Current research includes the measurement of atmospheric emissions from a reprocessing plant, modelling of environmental pathways, and evaluation of reactor materials for the prediction of releases from ultimate disposal sites. A radiochemical separation procedure has been developed for129I, which has been applied to a study of emissions from the Sellafield nuclear reprocessing plant. Atmospheric emissions, rainfall, grass, milk and wild food are analysed to model the ultimate fate of129I. A new procedure has been developed for measuring sub-ppm concentrations of chlorine in archive steel samples as part of programme to produce an inventory of arisings of36Cl from nuclear power stations. The method has been extended to other reactor materials and this paper describes the application of the radiochemical method for Magnox alloys, mild steel and cast iron.  相似文献   
943.
The hydrolysis of bis(chloromethyl)phosphonic acid p-nitroanilide has been studied in the pH range 6.86–13.0 and the effects of ionic surfactant micelles on this process have also been examined. The nature of the micellar effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) on this reaction lead us to conclude that both the neutral and anionic forms of the substrate are reactive, and that in the range of pH > pKa hydrolysis of the ionic form of the anilide predominates, while at pH < pKa the reaction of the neutral form is predominant. The binding constants for both the neutral and anionic forms of bis(chloromethyl)phosphonic acid p-nitroanilide in micellar CTAB and SDS solutions have been determined using a kinetic method, as well as by measuring the changes in the acid-base properties of the substrate resulting from the influence of micelles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1718–1722, August, 1990.  相似文献   
944.
A destructive neutron activation analysis procedure was developed for determining trace aluminum content in bone. It was found that soil contamination can influence the aluninum bone levels in prehistoric bone specimens. These maximum aluninum content values for prehistoric bone are larger than those of modern bone and comparable to aluminum levels present in bone from renal patients.  相似文献   
945.
A method that utilizes the Vilsmeier reaction to form a pyrrole ring was developed for the preparation of the previously unknown 7-formyl-2-methyl-4-oxo-3,4-dihydropyrrolo[3,2-d]pyrimidine from 2,6-dimethyl-5-amino-4-oxo-3,4-dihydropyrimidine. The nitrile, which was converted to the amide, was obtained from the oxime of the synthesized aldehyde. 7-Unsubstituted pyrrolopyrimidine was synthesized by oxidation of the aldehyde to the corresponding acid and subsequent decarboxylation. The IR and PMR spectroscopic data are presented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1672–1674, December, 1977.  相似文献   
946.
947.
A three-step synthesis of bispidine ( 1 ) is described. Mannich condensation of N-benzyl-4-piperidone, formaldehyde, and benzylamine afforded N,N′-dibenzylbispidinone ( 5 ), which was reduced under modified Wolff-Kishner conditions to yield N,N′ -dibenzylbispidine ( 6 ). Catalytic debenzylation of this compound gave 1 .  相似文献   
948.
The condensation of 3,4-diamino 1,2,4-triazole with ethyl aceloacelate gave 6-methyl-8,9-dihydro(7H)-s-triazolo[4,3-b]-1,2,4-triazepin-8-one ( 2 ); 2 has been caracterized by nmr spectroscopy and by comparison with its methylated derivative prepared in an unambigous manner.  相似文献   
949.
950.
The specific features of free-radical polymerization of methyl methacrylate in the presence of a new initiating system, benzoyl peroxide-N,N-dimethyl-N-(methylferrocenyl)amine, are studied. Mutual influence of the ferrocene and amino groups on the kinetic parameters of polymerization and on the microstructure of the resulting polymers is revealed. The polymer formed in the presence of this initiating system is shown to have a predominantly syndiotactic structure.  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