全文获取类型
收费全文 | 2168篇 |
免费 | 69篇 |
国内免费 | 6篇 |
专业分类
化学 | 1557篇 |
晶体学 | 112篇 |
力学 | 55篇 |
数学 | 92篇 |
物理学 | 427篇 |
出版年
2023年 | 16篇 |
2022年 | 48篇 |
2021年 | 40篇 |
2020年 | 38篇 |
2019年 | 46篇 |
2018年 | 35篇 |
2017年 | 59篇 |
2016年 | 84篇 |
2015年 | 42篇 |
2014年 | 58篇 |
2013年 | 197篇 |
2012年 | 111篇 |
2011年 | 135篇 |
2010年 | 88篇 |
2009年 | 73篇 |
2008年 | 117篇 |
2007年 | 96篇 |
2006年 | 96篇 |
2005年 | 73篇 |
2004年 | 59篇 |
2003年 | 50篇 |
2002年 | 52篇 |
2001年 | 28篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 14篇 |
1997年 | 15篇 |
1996年 | 19篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 24篇 |
1991年 | 24篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 23篇 |
1985年 | 17篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 28篇 |
1981年 | 30篇 |
1980年 | 22篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 18篇 |
1973年 | 16篇 |
排序方式: 共有2243条查询结果,搜索用时 31 毫秒
991.
[structure: see text] Attractive interactions between substituted arenes in the parallel displaced configuration have been quantitatively studied using triptycene-derived molecular conformational reporters. Charge-transfer bands are observed for models where the interactions are between strong donor and acceptors. Substituent effects on the strength of the aromatic interaction follow opposite trends for strongly electron-deficient arenes and mildly perturbed arenes. The free energy of interactions for models with strong electron donors and acceptors does not follow a linear correlation in the Hammett plot. Electrostatic models alone do not account for the nonlinearity of the free energy-substituents plot. 相似文献
992.
Handegard NO Patel R Hjellvik V 《The Journal of the Acoustical Society of America》2005,118(4):2210-2223
Tracking of individual fish targets using a split-beam echosounder is a common method for investigating fish behavior. When mounted on a floating platform like a ship or a buoy, the transducer movement often complicates the process. This paper presents a framework for tracking single targets from such a platform. A filter based on the correlated fish movements between pings is developed to estimate the platform movement, and an extended Kalman filter is used to combine the split-beam measurements and the platform-position estimates. Different methods for gating and data association are implemented and tested with respect to data-association errors, using manually tracked data from a free-floating buoy as a reference. The data association was improved by utilizing the estimated velocity for each track to predict the location of the next observation. The data association was more robust when estimates of platform tilt/roll were used. Other techniques to estimate position and velocity, like linear regression and smoothing splines, were implemented and tested on a simulated data set. The platform-state estimation improved the estimates for methods like the Kalman filter and a smoothing spline with cross validation, but not for robust methods like linear regression and smoothing spline with a fixed degree of smoothing. 相似文献
993.
Rosner JL Adam NE Alexander JP Berkelman K Cassel DG Crede V Duboscq JE Ecklund KM Ehrlich R Fields L Galik RS Gibbons L Gittelman B Gray R Gray SW Hartill DL Heltsley BK Hertz D Jones CD Kandaswamy J Kreinick DL Kuznetsov VE Mahlke-Krüger H Meyer TO Onyisi PU Patterson JR Peterson D Phillips EA Pivarski J Riley D Ryd A Sadoff AJ Schwarthoff H Shi X Shepherd MR Stroiney S Sun WM Urner D Wilksen T Weaver KM Weinberger M Athar SB Avery P Breva-Newell L Patel R Potlia V Stoeck H Yelton J Rubin P 《Physical review letters》2005,95(10):102003
The h(c)((1)P(1)) state of charmonium has been observed in the reaction psi(2S) --> pi(0)h(c) --> (gammagamma)(gammaeta(c)) using 3.08 x10(6) psi(2S) decays recorded in the CLEO detector. Data have been analyzed both for the inclusive reaction, where the decay products of the eta(c) are not identified, and for exclusive reactions, in which eta(c) decays are reconstructed in seven hadronic decay channels. We find M(h(c)) = 3524.4 +/- 0.6 +/- 0.4 MeV which corresponds to a hyperfine splitting DeltaM(hf)(1P) triple-bond pi(0)h(c)) x B(h(c) --> gammaeta(c)) = (4.0 +/- 0.8 +/- 0.7) x 10(-4). 相似文献
994.
The complexation reactions of iron(III) with 2-pyridine carboxylic acia (picolinic acid) and 2,6-pyridine dicarboxylic acid (dipicolinic acid) in aqueous solutions have been studied by spectrophotometric and stopped flow techniques. Equilibrium constants were determined for the 1 : 1 complexes at temperatures between 25 and 80°C. The values obtained are: Picolinic Acid (HL): Fe3++ H2L+? FeHL3++H+(K1 = 2.8,ΔH = 2 kcal mole?1 at 25°C, μ = 2.67 M) Dipicolinic Acid (H2D): Fe3++H2D? FeD++2H+(K1K1A= 227 M, ΔH = 3.4 kcal mole?1 at 25°C,μ = 1.0 M). The rate constants for the formation of these complexes are also given. The results are used to evaluate the effects of these two acids upon the rate of dissolution of iron(III) from its oxides. 相似文献
995.
996.
997.
998.
Harasit Kumar Mandal Biman Kumar Patel Somnath Dasmandal 《Journal of Dispersion Science and Technology》2018,39(4):552-559
The kinetic investigations on the alkaline hydrolysis of tris-(1,10–phenanthroline)iron(II) has been explored spectrophotometrically in microheterogeneous environment at 301?K and ionic strength of 0.13?mol?L?1. Guar gum, cationic amphiphiles, and their mixtures are used as the reaction environments to carry out the reaction. Guar gum decreases the rate of reaction, which indicates that Fe(II) complex may be trapped in the hydrophobic region of gum. Cationic amphiphile decreases the rate in the presence of guar gum. The extent of interaction between guar gum and amphiphile increases with the hydrophobic carbon chain length. The critical aggregation concentration (CAC) and critical micelle concentration (CMC) of the amphiphiles (cetyl trimenthyl ammonium bromide (CTAB), tetradecyl trimenthyl ammonium bromide (TTAB), dodecyl trimenthyl ammonium bromide (DTAB)) in the presence of guar gum have been determined with conductometry and tensiometry. All observations support either weak or strong interaction of cationic amphiphiles with guar gum. Activation parameters of the reaction in different environments have been determined which corroborate the rate data. 相似文献
999.
1000.
Phenacyl bromides are used as a building blocks in synthetic organic chemistry, which serves as a key model for the development of various biologically important heterocyclic compounds and other industrially important scaffolds. This review covers recent synthetic advances in the preparation of heterocyclic compounds utilizing various phenacyl bromides. A wide range of five- and six-membered heterocyclic compounds as well as fused heterocyclic compounds were synthesized efficiently from the phenacyl bromide precursors through one-pot multicomponent reactions. 相似文献