全文获取类型
收费全文 | 64372篇 |
免费 | 5195篇 |
国内免费 | 5021篇 |
专业分类
化学 | 38557篇 |
晶体学 | 433篇 |
力学 | 2691篇 |
综合类 | 860篇 |
数学 | 9888篇 |
物理学 | 22159篇 |
出版年
2023年 | 536篇 |
2022年 | 1666篇 |
2021年 | 2074篇 |
2020年 | 1568篇 |
2019年 | 1566篇 |
2018年 | 1277篇 |
2017年 | 1410篇 |
2016年 | 1908篇 |
2015年 | 1964篇 |
2014年 | 2576篇 |
2013年 | 4140篇 |
2012年 | 2922篇 |
2011年 | 3689篇 |
2010年 | 3094篇 |
2009年 | 3812篇 |
2008年 | 3939篇 |
2007年 | 4025篇 |
2006年 | 3475篇 |
2005年 | 2517篇 |
2004年 | 2310篇 |
2003年 | 2127篇 |
2002年 | 4380篇 |
2001年 | 1941篇 |
2000年 | 1516篇 |
1999年 | 1276篇 |
1998年 | 1253篇 |
1997年 | 917篇 |
1996年 | 936篇 |
1995年 | 855篇 |
1994年 | 763篇 |
1993年 | 773篇 |
1992年 | 728篇 |
1991年 | 527篇 |
1990年 | 461篇 |
1989年 | 378篇 |
1988年 | 404篇 |
1987年 | 280篇 |
1986年 | 291篇 |
1985年 | 395篇 |
1984年 | 303篇 |
1983年 | 182篇 |
1982年 | 346篇 |
1981年 | 509篇 |
1980年 | 458篇 |
1979年 | 501篇 |
1978年 | 397篇 |
1977年 | 305篇 |
1976年 | 260篇 |
1974年 | 116篇 |
1973年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Several 2- and 4-alkylcyclohexadienones were prepared and shown to accept electrons to produce ketyl radical anions that dissociated rapidly at room temperature to release carbon-centered radicals and an aromatic phenoxide type anion. In the PET process with benzyl-substituted cyclohexadienones, initiated with triethylamine, the benzyl radicals dimerised or abstracted an H-atom from solvent. In electrochemical reductions, and in reductions with alkali metals in liquid ammonia, the benzyl radicals were further reduced to anions. 相似文献
162.
Aldehydes undergo smooth conjugate addition to α,β-unsaturated ketones in the presence of 5-(2-hydroxyethyl)-1,3-thiazolium halides and DBU adsorbed onto the surface of basic alumina under microwave irradiation and solvent-free conditions to afford 1,4-diketones in enhanced yields and reduced reaction times compared to conventional methods. 相似文献
163.
In this paper, we prove convergence rates for spherical spline Hermite interpolation on the sphere Sd−1 via an error estimate given in a technical report by Luo and Levesley. The functionals in the Hermite interpolation are either point evaluations of pseudodifferential operators or rotational differential operators, the desirable feature of these operators being that they map polynomials to polynomials. Convergence rates for certain derivatives are given in terms of maximum point separation. 相似文献
164.
两相同部件温贮备可修的人机系统解的性质分析 总被引:5,自引:1,他引:4
郭卫华 《数学的实践与认识》2003,33(7):88-95
本文首先用强连续算子半群理论证明了两相同部件温贮备可修的人机系统动态非负解的存在唯一性 ,然后证明了 0是系统主算子的本征值 ,并得到 0本征值对应的本征向量是正的 ,从而系统存在稳态正解 . 相似文献
165.
B. Szafran P. S
p J. Adamowski S. Bednarek 《Physica E: Low-dimensional Systems and Nanostructures》2003,18(4):523-529
We present a theoretical study of the charging spectra in natural and artificial atoms. We apply a model electrostatic potential created by a homogenously charged sphere. This model potential allows for a continuous passage from the Coulomb potential of the nucleus to parabolic confinement potential of quantum dots. We consider electron systems with N=1,…,10 electrons with the use of the Hartree–Fock method. We discuss the qualitative similarities and differences between the chemical potential spectrum of electron systems bound to nucleus and confined in quantum dots. 相似文献
166.
J. T. Hoeft M. Polcik D. I. Sayago M. Kittel R. Terborg R. L. Toomes J. Robinson D. P. Woodruff M. Pascal G. Nisbet C. L. A. Lamont 《Surface science》2003,540(2-3):441-456
The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding. 相似文献
167.
François Ledrappier Mark Pollicott 《Bulletin of the Brazilian Mathematical Society》2005,36(2):143-176
In this paper we study the ergodic properties of the linear action of lattices Γ of SL(2,ℚp) on ℚp × ℚp and distribution results for orbits of Γ. Following Serre, one can define a “geodesic flow” for an associated tree (actually
associated to GL(2,ℚp)). The approach we use is based on an extension of this approach to “frame flows” which are a natural compact group extension
of the geodesic flow. 相似文献
168.
The content of residual monomers is one of the most important characteristics of polymer dispersions. As a result of the similar physicochemical parameters of ethyl acrylate and methyl methacrylate, it is very difficult to determine the residual monomers in acrylate dispersions obtained by emulsion polymerization of both monomers. Gas chromatography with capillary columns, however, permits separation of these monomers and their quantitative determination in acrylate dispersions. 相似文献
169.
DEEB MARJI KHAMIS ABBAS 《Journal of inclusion phenomena and macrocyclic chemistry》1997,27(4):269-277
The effect of 18-crown-6 and kryptofix (2,2,1) on the oxidation of triethylamine byaqueous ferricyanide ion has been studied at pH 4, 7, and 9.The macrocycle retarded the normal oxidation process at all pH values,the effect depending on the macrocyclic ligand concentration Kryptofix (2,2,1) had a greater effect than 18-crown-6 at all pHvalues in this study. For 18-crown-6, retardation decreases in the order pH 9 > 4 > 7 while forkryptofix it follows the order pH 9 > 7 > 4. 相似文献
170.
It is shown that the potential perturbation that shifts a chosen standing wave in space is a block of potential barrier and well for every wave bump between neighbouring knots. The algorithms shifting the range of the primary localization of a chosen bound state in a potential well of finite width are as well applicable to the scattering functions if states of the continuous spectrum are considered as bound states normalized to unity but distributed on an infinite interval with vanishing density. The potential perturbations of the same type on the half-axis concentrate the scattering wave at the origin, thus creating a bound state embedded into the continuous spectrum (zero width resonance). 相似文献