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   16400篇
  免费   2455篇
  国内免费   1455篇
化学   18146篇
晶体学   208篇
力学   44篇
综合类   50篇
数学   15篇
物理学   1847篇
  2024年   52篇
  2023年   201篇
  2022年   611篇
  2021年   713篇
  2020年   1117篇
  2019年   801篇
  2018年   647篇
  2017年   553篇
  2016年   928篇
  2015年   877篇
  2014年   913篇
  2013年   1382篇
  2012年   1018篇
  2011年   927篇
  2010年   778篇
  2009年   789篇
  2008年   763篇
  2007年   827篇
  2006年   728篇
  2005年   685篇
  2004年   684篇
  2003年   580篇
  2002年   1205篇
  2001年   335篇
  2000年   289篇
  1999年   211篇
  1998年   254篇
  1997年   198篇
  1996年   169篇
  1995年   163篇
  1994年   125篇
  1993年   95篇
  1992年   112篇
  1991年   82篇
  1990年   84篇
  1989年   62篇
  1988年   54篇
  1987年   37篇
  1986年   32篇
  1985年   31篇
  1984年   48篇
  1983年   18篇
  1982年   28篇
  1981年   23篇
  1980年   13篇
  1979年   24篇
  1978年   10篇
  1975年   4篇
  1972年   5篇
  1969年   4篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
51.
Luminescence properties of porous anodic aluminum oxide films formed in a 0.6 M solution of citric acid and luminescence of paraterphenyl, perylene, coumarin 7, and rhodamine 6G dyes adsorbed by the films are investigated. The nature of emitting centers in anodic aluminum oxide is revealed. Intense photoluminescence of all tested dyes embedded into pores of anodic aluminum oxide has been found. A redshift of fluorescence spectra of dyes adsorbed by the matrix and emergence of an additional longwave band have been detected. Data obtained can be used in developing new thin-film luminescent coatings for future applications in optoelectronics and molecular electronics. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 4, pp. 483–488, July–August, 1997.  相似文献   
52.
Linear and nonlinear optical properties of racemic (±)2-(α-methylbenzylamino)-5-nitropyridine ((±)MBANP) single crystals have been comprehensively investigated and compared with those of the enantiomorph (–)2-(α-methylbenzylamino)-5-nitropyridine ((–)MBANP) crystals. (±)MBANP crystal exhibits very high chemical and physical stability, but relatively small nonlinear optical coefficients (d31 = 6.8 pm/V, d32 = 4.7 pm/V, d33 = 0.84 pm/V). A comparison between the nonlinear optical coefficients of (±)MBANP and (–)MBANP demonstrates the validity of the oriented-gas model in molecular crystals that neglects all the contributions from intermolecular interaction.  相似文献   
53.
To incorporate an acceptor type polythiophene segment onto a supramolecular block copolymer for potential light harvesting applications, effective synthetic routes for the end‐functionalized and acceptor‐substituted polythiophenes are critical. The Ullmann coupling reaction can be utilized to obtain electron‐deficient polythiophenes and to attach terminal thiophene units that carry functional groups. In this article, the reactions involving a 2,5‐dibromothiophene monomer containing an electron‐withdrawing fluorinated ester and 5‐bromo‐2‐thiophenecarboxaldehyde (the end‐capper) were studied in detail. It was found that the Ullmann coupling reaction of the dibromide is very fast (completed in a few minutes) and the terminal bromine group does not survive long under the reaction condition. These findings lead to the development of an effective procedure for aldehyde end‐capping of electron‐deficient polythiophenes. Polymers with molecular weights around 4000 Da are routinely obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 41–47, 2007  相似文献   
54.
4-Vinylphenols, useful compounds for industrial applications, were obtained by decarboxylation of 4-hydroxycinnamic acids under microwave irradiation in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base and basic aluminum oxide as solid support. The reactions were fast (15-30 min). The selective extraction of the final products with ethyl acetate avoids chromatographic purifications. The conversions are quantitative and the yields are satisfactory. Only the unstable 4-vinylcatechol was obtained in moderate yield. This procedure was successfully extended to a natural sample of ferulic acid extracted from wheat bran to get the corresponding 4-vinylguaiacol, a FEMA GRAS (Flavor and Extract Manufacturer's Association; General Regarded as Safe) approved flavoring agent.  相似文献   
55.
56.
新疆野生黑加仑与马林果实的营养成分分析   总被引:2,自引:0,他引:2  
通过对新疆野生黑加仑和野生马林的脂肪油,8种微量元素,17种氨基酸以及维生素C的含量进行测定得出:黑加仑脂肪油含量为17.9%,马林的脂肪油含量为22.3%,其中,黑加仑脂肪油中,r-亚麻酸的含量为19.02%,而马林中,其含量为9.40%,黑加仑中K,Na,Ca,Cu,Zn,Fe的含量比马林中的含量高,而Mn,Mg元素的含量却比马林含量低;马林的维生素C含量为16.22mg/g,黑加仑的含量为108mg/g,黑加仑的氨基酸的总量为1.63%,马林为1.14%。  相似文献   
57.
Information is presented on structure, composition, and response to enzymes of corn stover related to barriers for bioconversion to ethanol. Aromatic compounds occurred in most tissue cell walls. Ferulic acid esterase treatment before cellulase treatment significantly improved dry weight loss and release of phenolic acids and sugars in most fractions over cellulase alone. Leaf fractions were considerably higher in dry weight loss and released sugars with esterase treatment, but stem pith cells gave up the most phenolic acids. Results help identify plant fractions more appropriate for coproducts and bioconversion and those more suitable as residues for soil erosion control.  相似文献   
58.
59.
Following detoxification of the liquid hydrolysate produced in a corn stover pretreatment process, inhibitor levels are seen to increase with the re-addition of solids for the ensuing hydrolysis and fermentation processes. The solids that were separated from the slurry before detoxification of the liquor contain approx 60% (w/w) moisture, and contamination occurs owing to the diffusion of inhibitors from the moisture entrained in the porous structure of the corn stover solids into the bulk fluid. This evidence suggests the need for additional separation and detoxification steps to purge residual inhibitors entrained in the moisture in the solids. An overliming process to remove furans from the hydrolysate failed to reduce total organic acids concentration, so acids were removed by treatment with an activated carbon powder. Smaller carbon doses proved more efficient in removing organic acids in terms of grams of acid removed per gram of carbon powder. Sugar adsorption by the activated carbon powder was minimal.  相似文献   
60.
Landfill gas, cryotrapped on a loop fashioned from a length of a capillary gas chromatography (GC) column, was examined for volatile organometallic compounds (VOMCs) and for volatile organic compounds (VOCs) by using GC–mass spectrometry (MS). A large number of organic components were present and many were identified, but the only VOMCs present in high enough concentrations to be detected were trimethylstibine and tetramethyltin. The use of inductively coupled plasma (ICP)‐MS as an element‐specific detector allowed the identification of a number of other organometallic species in the landfill gas, including trimethylarsine and trimethylbismuth, and, for the first time, butyltrimethyltin and dibutyldimethyltin. The presence of molybdenum hexacarbonyl was confirmed. Gas from a large‐scale compost heap and from compost incubated in the laboratory contained iodomethane but no common VOMCs (GC–ICP‐MS). Only VOCs were present in forest air (GC–MS). Copyright © 2003 John Wiley & Sons, Ltd.  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