全文获取类型
收费全文 | 2376篇 |
免费 | 37篇 |
国内免费 | 16篇 |
专业分类
化学 | 1393篇 |
晶体学 | 15篇 |
力学 | 40篇 |
数学 | 516篇 |
物理学 | 465篇 |
出版年
2021年 | 23篇 |
2020年 | 28篇 |
2019年 | 18篇 |
2018年 | 19篇 |
2017年 | 19篇 |
2016年 | 40篇 |
2015年 | 31篇 |
2014年 | 39篇 |
2013年 | 105篇 |
2012年 | 82篇 |
2011年 | 90篇 |
2010年 | 57篇 |
2009年 | 47篇 |
2008年 | 88篇 |
2007年 | 98篇 |
2006年 | 106篇 |
2005年 | 95篇 |
2004年 | 94篇 |
2003年 | 73篇 |
2002年 | 58篇 |
2001年 | 37篇 |
2000年 | 28篇 |
1998年 | 20篇 |
1997年 | 22篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 24篇 |
1993年 | 26篇 |
1992年 | 32篇 |
1991年 | 23篇 |
1990年 | 20篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 20篇 |
1986年 | 22篇 |
1985年 | 32篇 |
1984年 | 34篇 |
1983年 | 17篇 |
1982年 | 39篇 |
1981年 | 30篇 |
1980年 | 43篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 42篇 |
1976年 | 48篇 |
1975年 | 44篇 |
1974年 | 36篇 |
1973年 | 27篇 |
1972年 | 19篇 |
1971年 | 21篇 |
排序方式: 共有2429条查询结果,搜索用时 31 毫秒
61.
F. J. Coetzer T. C. Kotzé P. B. Kotzé F. J. Mostert P. van der Westhuizen 《Zeitschrift für Physik A Hadrons and Nuclei》1985,320(3):359-362
Detailed measurements on the 5p 4 S 3 2/0 and 5p~' 2 D 5 2/0 levels in Kr II are reported. Cascade effects were taken into account with the aid of the arbitrarily normalized decay curve (ANDC) technique. Experimental values are compared to values obtained by other investigators as well as to theoretical values. 相似文献
62.
Gutiérrez MC Sleegers A Simpson HD Alphand V Furstoss R 《Organic & biomolecular chemistry》2003,1(20):3500-3506
The first fluorogenic assay allowing for detection of microbial enzymes able to perform Baeyer-Villiger oxidation is described. This is based on the use of 4-oxopentyl umbelliferyl ether 1 as a fluorogenic substrate. When Baeyer-Villigerases active against this test ketone are present in the selected whole cells, 1 is transformed into 3-hydroxypropyl umbelliferyl ether 3, which, in a subsequent step, releases the fluorescent product umbelliferone. Different microorganisms, known to be endowed with Baeyer-Villigerase activity, were assayed. 相似文献
63.
Arthur J. Banister Anthony W. Luke 《Journal of polymer science. Part A, Polymer chemistry》1992,30(12):2653-2656
Mono-, bis- and tris-(1,3,2,4-dithiadiazolium) salts [R-(CNSNS +)n]n+[AsF-6]n (R = aryl, n = 1, 2, 3) were found to initiate the cationic ring-opening polymerization of tetrahydrofuran (THF) at room temperature to give clear gels from which the pure polymer was precipitated. 1,3,2,4-Dithiadiazolium cations associated with the hard [AsF6]- anion thus constitute a new class of cationic polymerization initiators. The poly(THF) formed by initiation with 1,3,2,4-dithiadiazolium cation was characterized by gel permeation chromatography, infrared spectrophotometry, and 13C-NMR spectroscopy. Number-average molecular weights of 198 700 g mol-1 (polydispersity 1.96) and 190 000 g mol-1 (polydispersity 1.61) were obtained using [PhCNSNS ] [AsF6] and [C6H3-1,3,5-(CNSNS )3][AsF6]3, respectively, as initiators. The use of multifunctional dithiadiazolium salts as initiators suggests that they may be useful in the preparation of starburst and dendritic polymers. © 1992 John Wiley & Sons, Inc. 相似文献
64.
A fundamental difficulty in theoretical physics is the dual and apparently incompatible interpretations of radiation as showing both continuous and extensive wave properties but also those of discrete atomic or smaller individual particles. Some of these contradictions are outlined. The explanation offered is of a quantized nature of time; units to=h/moc2 for a particle at rest, and of similar interval unit so when in relative motion, with conventional relativistic corrections.
For many purposes this form of quantization replaces the need for a wave concept which then appears as a mathematical approach, chosen to avoid the physical concept of an intrinsicc time for any particle, just as we have for its intrinsic mass, spin and electrical charge. to and so are directly related to its frequency, energy and mass. The uncertainty principle and interference relations follow directly from this model, without any physical wave concept. 相似文献
65.
Carbon-supported catalysts were phosphonated using 2-aminoethylphosphonic acid, and the resulting catalysts with largely enhanced proton conductivity performed substantially better than the untreated counterparts in proton-exchange membrane fuel cells. 相似文献
66.
B.M.Ratnayake Bandara Arthur J. Birch Brian Chauncy Lawrence F. Kelly 《Journal of organometallic chemistry》1981,208(2):C31-C35
Complexation commencing with some substituted 1,3- or 1,4-cyclohexadienes with Fe(CO)5 indicates that the steric direction is controlled by classical hindrance with groups such as alkyl, but probably involves transmission of the entering group through intermediate complexation with CO2Me. The results help to define methods of obtaining desired stereoisomers in the series. 相似文献
67.
Arthur J. Carty Shaun F. Malone Nicholas J. Taylor 《Journal of organometallic chemistry》1979,172(2):201-211
The synthesis of selenium bound selenourea complexes of methylmercury, [CH3HgSeC(NH2)2]X (X=Cl, Br, NO3, ClO4) are described. A single crystal X-Ray analysis of the nitrate salt has provided for the first time bond length data pertinent to the biologically important mercury-selenium interaction. Crystals of [CH3HgSeC(NH2)2] NO3 belong to the space group PnZic with a = 7.524(1), b = 11.204(2), c = 9.738(2)Å, and z = 4. The structure was solved and refined using 561 observed reflections measured on a Syntex P21 diffractometer to a final R value of 0.037. The mercury atom is linearly coordinated to the methyl group and the selenium atom of the selenourea with Hg-Se of 2.477(3)Å. Strong Hg-Se bonding is indicated by the X-Ray data and by the 1H - 199Hg nmr coupling constants. A comparison of spectroscopic data for analogous thio and selenourea complexes is presented. 相似文献
68.
Brightness reversion of softwood (SW) and hardwood (HW) bleached chemithermomechanical pulps (BCTMP) with a 300–400nm light source under argon, oxygen, and air were studied in the absence and presence of UV-screens using UV–Vis reflectance difference spectroscopy. The UV-Vis difference absorption spectra of control SW and HW BCTMP testsheets indicate that as the concentration of atmospheric oxygen is increased, the absorption at max 360nm increases and a blue shift occurs. In all of the cases studied, brightness reversion was observed to occur under an argon atmosphere. The addition of benzophenone and benzotriazole additives to BCTMP testsheets resulted in a significant decrease in formation of chromophores absorbed at 400nm and 360nm. The possible mechanisms contributing to these effects are discussed in terms of the photoformation of lignin chromophoric structures. 相似文献
69.
We study positive solutions of the Dirichlet problem: u(x)+f(u(x))=0,xD
n
,u(x)=0,xD
n
, whereD
n
is ann-ball. We find necessary and sufficient conditions for solutions to be nondegenerate. We also give some new existence and uniqueness theorems.Research supported in part by NSF Contract Number MCS 80-02337 相似文献
70.
Simple two-parameter Hückel and Pitzer equations were used for the calculation of the activity coefficients of aqueous hydrochloric acid at temperatures 0–60°C up to a molality of 2.0 mol-kg–1. The data obtained by Harned and Ehlers(2,3) on galvanic cells without a liquid junction were used in the parameter estimations of these equations. These data consist of sets of measurements at the temperature intervals of 5°C. It was observed that all estimated parameters follow very simple equations with respect to temperature. They are either constant or depend linearly on the temperature. The values for the activity coefficient parameters calculated by these simple equations are recommended here. The recommended parameter values were tested by predicting the data of Gupta, Hills, and Ives,(5) consisting of cell measurements from 5 to 45°C and molalities up to 1.0 mol-kg–1, and the data of Bates and Bower,(4) which extend to 95°C but measurements were only made on molalities less than about 0.1 mol-kg–1. The activity coefficients obtained by the new equations were also compared to those calculated by the Pitzer equations with the parameter values determined by Saluja, Pitzer, and Phutela(6) from calorimetric data. The agreement observed was excellent up to a molality of 1.5 mol-kg–1 at temperatures from 0 to 60°C. 相似文献