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   220929篇
  免费   2698篇
  国内免费   1324篇
化学   122729篇
晶体学   3231篇
力学   8888篇
综合类   61篇
数学   24365篇
物理学   65677篇
  2020年   1754篇
  2019年   1896篇
  2018年   2483篇
  2017年   2531篇
  2016年   3645篇
  2015年   2424篇
  2014年   3537篇
  2013年   8872篇
  2012年   7686篇
  2011年   9397篇
  2010年   6649篇
  2009年   6451篇
  2008年   8793篇
  2007年   8900篇
  2006年   8386篇
  2005年   7705篇
  2004年   6871篇
  2003年   6054篇
  2002年   5986篇
  2001年   6703篇
  2000年   5067篇
  1999年   3669篇
  1998年   3038篇
  1997年   3008篇
  1996年   3012篇
  1995年   2618篇
  1994年   2718篇
  1993年   2581篇
  1992年   2829篇
  1991年   2894篇
  1990年   2703篇
  1989年   2616篇
  1988年   2557篇
  1987年   2498篇
  1986年   2520篇
  1985年   3300篇
  1984年   3359篇
  1983年   2804篇
  1982年   3046篇
  1981年   2811篇
  1980年   2607篇
  1979年   2790篇
  1978年   2996篇
  1977年   3032篇
  1976年   3059篇
  1975年   2793篇
  1974年   2906篇
  1973年   2956篇
  1972年   2318篇
  1971年   1845篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The Auger electron spectrum of water vapour has been recorded and analyzed. For the analysis, an approximate formula for calculating the intensities of the Auger electron lines is derived. It is shown, that the calculated intensities along with theoretical energies of the Auger transitions account well for the observed spectrum. In particular, new assignments in terms of transitions to triplet final states are suggested.  相似文献   
972.
The corresponding 2-(2-furyl)naphth[1,2-d]imidazoles were obtained by heating Schiff bases, prepared from 1,2-naphthalenediamine and furfural and 5-bromo- and 5-nitrofurfurals, in nitrobenzene. 2-[-(2-Furyl)vinyl]naphth[1,2-d]imidazoles were synthesized by condensation of 2-methylnaphth[1,2-d]imidazole with furfural and 5-bromo- and 5-nitrofurfural. The methylation, nitration, and acetylation of the compounds obtained were studied, and the replacement of the bromine atom in the furan ring by a nitro group was also investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1555–1557, November, 1972.  相似文献   
973.
The polymerization of acrylonitrile with the homogeneous catalyst system of VCl4–AlEt3 in acetonitrile at 40°C has been investigated. The rate of polymerization is found to be first-order with respect to monomer and inversely proportional to the catalyst concentration. The overall activation energy for this catalyst system is 10.97 kcal/mole. The inverse proportionality of rate of polymerization with the catalyst concentration is attributed to the permanent complex formation between the catalyst complex and acrylonitrile, and a reaction scheme is proposed.  相似文献   
974.
Summary The reduction of complex esters and carbonyl compounds in hydrocarbon media by aluminum hydrides of the alkali metals results in the formation of the corresponding alcohol in high yield. The reduction of nitriles proceeds with the formation of primary amines.  相似文献   
975.
Summary A method has been devised for the synthesis of -lacrams from amides of -ha!ocarboxylic acids. Some new azetidinones with residues containing carboxyl groups on the nitrogen have been piepated. The method permits the conversion of $-amino acids trio difficulty accessible mixed (secondary) amino acids.This paper is publised under the regulations governing Brief Communications.  相似文献   
976.
Summary Tokaj wines (Szamorodni and Aszu wines) of Hungarian origin were investigated on the basis of free amino acids and biogenic amines. The separation and determination of these compounds was performed by an amino acid analyser equipped with an ion-exchange resin column. The total amount of free amino acids and biogenic amines was higher in Aszu wines than in Szamorodni wines. The main amino acids were proline and arginine, while the major biogenic amines were tyramine and putrescine. The free amino acid and biogenic amine content of Aszu wines depended on the vineyards the wines originated from. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001  相似文献   
977.
Variously cross-linked poly(methylmethacrylates) (PMMAs) are synthesized with three additives incorporated at theoretically 1000 microg of the additive per gram of prepared polymer. The additives are Irganox 1010, Irganox 1076, and Irgafos 168. The in-house" synthesized polyacrylates are then subjected to supercritical fluid extraction (SFE) to determine if additive recovery is a function of percent cross-linking. Although considerable work in this regard has been performed with non-cross-linked polyolefins, the literature is lacking regarding polyacrylates. Some additive degradation apparently occurs during the synthesis, as judged by the increased complexity of the extract high-performance liquid chromatographic trace and the low percent recoveries observed especially for the Irganoxes. For low polymer cross-linking (1%), it appears that both PMMA synthetic reproducibility and readily observed polymer swelling during SFE are serious issues that adversely affect additive percent recovery and precision of results. Higher percent cross-linking yields more consistent analytical data than low percent cross-linking, even though the amount of additive extracted in all PMMA samples (regardless of cross-linking percentage) is essentially the same whether the extraction is via SFE or liquid-solid extraction with methylene chloride. Results for comparably cross-linked poly(ethylmethacrylate) and poly(butylmethacrylate) are similar to PMMA.  相似文献   
978.
4-Hydroxy-2-methyl-3-(2-chloro-2-propenyl)quinoline was prepared by O-allylation of 4-hydroxy-2-methylquinoline followed by the Claisen rearrangement of the 2-methyl-4-(2-chloro-2-propenyloxy)quinoline obtained. Chemical modifications of 4-hydroxy-2-methyl-3-(2-chloro-2-propenyl)quinoline resulted in the synthesis of 4-amino-2-methyl-3-(2-methyl-3-indolyl)quinoline.  相似文献   
979.
A new approach was proposed to fractionating soil lead species differing in mobility and biological availability. Ground solid samples were retained as a stationary phase in a rotating coiled column while continuously pumping aqueous solutions of salts and complexing agents necessary for the sequential leaching of different element species. It was shown that field-flow fractionation in a rotating coiled column is suitable for separating three lead species in an automatic mode within 4 h, as compared to several days for conventional methods of sequential extraction. The study of lead elution curves at different fractionation steps provides additional information about its mobilization under changing environmental conditions.  相似文献   
980.
The electrochemical behavior of formaldehyde (CH2O) at solid electrodes made of platinum, gold, silver, cobalt, nickel, copper, and graphite was studied. The working surface of the electrodes was renewed by cutting a thin layer (0.5 m) immediately in the test solution. It was found that, in alkaline solutions, CH2O was oxidized at all electrodes other than cobalt and graphite ones while scanning the potential to the anode and cathode regions. The peaks of CH2O oxidation at platinum and gold electrodes using potential scanning in the anode and cathode directions, as well as at nickel, copper, and silver electrodes using potential scanning in the anodic direction, are suitable for analytical purposes.  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