全文获取类型
收费全文 | 3251篇 |
免费 | 85篇 |
国内免费 | 21篇 |
专业分类
化学 | 1927篇 |
晶体学 | 28篇 |
力学 | 86篇 |
数学 | 473篇 |
物理学 | 501篇 |
无线电 | 342篇 |
出版年
2022年 | 23篇 |
2021年 | 35篇 |
2020年 | 43篇 |
2019年 | 35篇 |
2018年 | 28篇 |
2017年 | 27篇 |
2016年 | 54篇 |
2015年 | 52篇 |
2014年 | 55篇 |
2013年 | 142篇 |
2012年 | 151篇 |
2011年 | 158篇 |
2010年 | 90篇 |
2009年 | 79篇 |
2008年 | 172篇 |
2007年 | 150篇 |
2006年 | 175篇 |
2005年 | 149篇 |
2004年 | 116篇 |
2003年 | 100篇 |
2002年 | 112篇 |
2001年 | 45篇 |
2000年 | 39篇 |
1999年 | 44篇 |
1998年 | 49篇 |
1997年 | 39篇 |
1996年 | 32篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 40篇 |
1992年 | 33篇 |
1991年 | 36篇 |
1990年 | 31篇 |
1989年 | 52篇 |
1988年 | 38篇 |
1987年 | 34篇 |
1986年 | 32篇 |
1985年 | 55篇 |
1984年 | 54篇 |
1983年 | 43篇 |
1982年 | 52篇 |
1981年 | 59篇 |
1980年 | 37篇 |
1979年 | 27篇 |
1978年 | 45篇 |
1977年 | 36篇 |
1976年 | 51篇 |
1975年 | 39篇 |
1974年 | 37篇 |
1973年 | 33篇 |
排序方式: 共有3357条查询结果,搜索用时 15 毫秒
61.
The oxidation of uric acid has been studied by thin-layer spectroelectrochemistry using an optically transparent gold minigrid electrode and enzymatically using peroxidase. Under both enzymatic and electrochemical conditions a u.v.-absorbing intermediate (λmax=302–304 nm between pH 7–9.3) is observed. This intermediate is proposed to be an imine-alcohol species formed by rapid hydration of the primary reaction product which is a diimine. Under the conditions employed in both the enzymatic electrochemical studies the imine-alcohol intermediate is hydrated in a first order reaction with an observed rate constant of 3.5×10?3s?1 between pH 7–9.3. This study establishes the similarity between the electrochemical and enzymatic oxidation of uric acid and indicates that electrochemical investigations of the redox behavior of purines can provide unique insights into their biological redox properties. 相似文献
62.
Jakub Kołacz Henry G. Gotjen Robel Y. Bekele Jason D. Myers Jesse A. Frantz Michael Ziemkiewicz 《Liquid crystals》2020,47(4):531-539
ABSTRACTIn a planar dielectric waveguide, weak confinement of a propagating mode in a high index core leads to a measurable evanescent interaction with the cladding. In this work, we study the effect of a reorientable anisotropic cladding on the behaviour of Transverse Electric (TE) and Transverse Magnetic (TM) mode polarisations using a liquid crystal (LC)-clad waveguide architecture. The polarised evanescent field of a guided mode interacts with a voltage-tunable birefringent LC cladding to deflect an out-coupled beam. Experimental measurements are coupled with a theoretical framework and show good consistency with simulation results. We isolate the effect of mode confinement by changing the thickness of the high index core. Interactions between the LC index ellipsoid and the mode polarisation are probed by changing the initial alignment of the LC. Finally, we examine the difference in deflection between TE and TM modes, which incorporates both a change in mode confinement and a difference in LC index components. 相似文献
63.
A flame photometric method of determining calcium in phosphate, carbonate, and silicate locks has been developed Aluminum and phosphate interference was overcome by the addition of a large excess of magnesium. The method is rapid and suitable for routine analysis Results obtained are within ± 2% of the calcium oxide content. 相似文献
64.
Clifford Henry Taubes 《Communications in Mathematical Physics》1985,97(4):473-540
In each monopole sector there exist an infinite number of finite energy solutions to the Prasad-Sommerfield limit of the SU(2) Yang-Mills-Higgs equations on 3 whose energy is greater than any finite number.National Science Foundation Postdoctoral Fellow in Mathematics 相似文献
65.
M. Chatry M. Henry M. In C. Sanchez J. Livage 《Journal of Sol-Gel Science and Technology》1994,1(3):233-240
The hydrolysis and condensation of zirconium n-propoxide in n-propanol have been chemically controlled via the complexation of the zirconium precursor with acetylacetone. The size of the zirconium oxide-based particles is mainly controlled by the complexation ratio x=[acac]/[Zr]. the mean size increases from nanometric to submicronic range when x decreases from 1 to 0.1. Amorphous colloidal particles are obtained at room temperature. They result from a competitive growth/termination mechanism of zirconium-oxo species in the presence of acac surface capping agents. However non-aggregated nanocrystalline particles of tetragonal zirconia, about 2 nm in diameter are formed upon aging at 60°C when hydrolysis is performed in the presence of paratoluene sulfonic acid (PTSA). 相似文献
66.
67.
Summary A numer of bonded stationary phases were investigated for the separation of Cd2+, Cu2+, Fe3+, Mn2+, Ni2+, Pb2+, and Zn2+ by thin-layer chromatography. The effect of structure, surface loading, type of silica gel and mobile phase concentration was considered. 相似文献
68.
Hedi Hannachi Naoual Anoune Christian Arnaud Pierre Lant ri R mi Longeray Henry Chermette 《Journal of Molecular Structure》1998,434(1-3):183-191
The Baeyer-Villiger reaction of polymethoxybenzaldehydes with peroxysuccinic acid (PSA) is studied theoretically. The semi-empirical SCF-MO/PM3 method has been used to study the reaction and to calculate the energy profiles. In addition, heat of formation computations and bond order analyses of the transition species along the reaction coordinates have been performed and are interpreted qualitatively in terms of electron flow through the system during aryl migration. Our results support the hypothesis that the second step is a concerted reaction. Our data support a definite effect of the substituent on the aryl migration. The theoretical predictions are in good agreement with experimental results. 相似文献
69.
Inverse gas chromatography using water probing at 110°C was carried out to characterize rayon yarns after prior heating at 200 and 250°C in both inert (nitrogen) and oxidative (dry air) atmospheres for successively longer time intervals. During the early stages of heating in either atmosphere, the affinity of the rayon for water drops rapidly. However, whereas continued heating in N2 results in a further loss of activity, data have been obtained showing that after an initial period yarns heated in air at 250°C exhibit an increasing attraction for water even while losing more weight. 相似文献
70.
Richardson NA Wesolowski SS Schaefer HF 《Journal of the American Chemical Society》2002,124(34):10163-10170
The adiabatic electron affinity (AEA) for the Watson-Crick guanine-cytosine (GC) DNA base pair is predicted using a range of density functional methods with double- and triple-zeta plus polarization plus diffuse (DZP++ and TZ2P++) basis sets in an effort to bracket the true electron affinity. The methods used have been calibrated against a comprehensive tabulation of experimental electron affinities (Chem.Rev. 2002, 102, 231). Optimized structures for GC and the GC anion are compared to the neutral and anionic forms of the individual bases as well as Rich's 1976 X-ray structure for sodium guanylyl-3',5'-cytidine nonahydrate, GpC.9H(2)O. Structural distortions and natural population (NPA) charge distributions of the GC anion indicate that the unpaired electron is localized primarily on the cytosine moiety. Unlike treatments using second-order perturbation theory (MP2), density functional theory consistently predicts a substantial positive adiabatic electron affinity for the GC pair (e.g., TZ2P++/B3LYP: +0.48 eV). The stabilization of C(-) via three hydrogen bonds to guanine is sufficient to facilitate adiabatic binding of an electron to GC and is also consistent with the positive experimental electron affinities obtained by photoelectron spectroscopy of cytosine anions incrementally microsolvated with water molecules. The pairing (dissociation) energy for GC(-) (35.6 kcal/mol) is determined with inclusion of electron correlation and shows the anion to have greater thermodynamic stability; the pairing energy for neutral GC (TZ2P++/B3LYP 23.9 kcal/mol) compares favorably to previous MP2/6-31G (23.4 kcal/mol) results and a debated experiment (21.0 kcal/mol). 相似文献