全文获取类型
收费全文 | 451021篇 |
免费 | 3128篇 |
国内免费 | 1043篇 |
专业分类
化学 | 221835篇 |
晶体学 | 6774篇 |
力学 | 23366篇 |
综合类 | 16篇 |
数学 | 64013篇 |
物理学 | 139188篇 |
出版年
2020年 | 3497篇 |
2019年 | 4179篇 |
2018年 | 11991篇 |
2017年 | 12496篇 |
2016年 | 10891篇 |
2015年 | 4779篇 |
2014年 | 7277篇 |
2013年 | 16314篇 |
2012年 | 15598篇 |
2011年 | 24058篇 |
2010年 | 16310篇 |
2009年 | 16551篇 |
2008年 | 22106篇 |
2007年 | 24705篇 |
2006年 | 13019篇 |
2005年 | 16092篇 |
2004年 | 12993篇 |
2003年 | 12373篇 |
2002年 | 10710篇 |
2001年 | 10544篇 |
2000年 | 8344篇 |
1999年 | 6115篇 |
1998年 | 5253篇 |
1997年 | 5346篇 |
1996年 | 5085篇 |
1995年 | 4302篇 |
1994年 | 4473篇 |
1993年 | 4421篇 |
1992年 | 4830篇 |
1991年 | 4961篇 |
1990年 | 4752篇 |
1989年 | 4813篇 |
1988年 | 4686篇 |
1987年 | 4659篇 |
1986年 | 4450篇 |
1985年 | 5724篇 |
1984年 | 5924篇 |
1983年 | 4851篇 |
1982年 | 5109篇 |
1981年 | 4969篇 |
1980年 | 4576篇 |
1979年 | 5144篇 |
1978年 | 5256篇 |
1977年 | 5323篇 |
1976年 | 5368篇 |
1975年 | 5019篇 |
1974年 | 4933篇 |
1973年 | 5197篇 |
1972年 | 3702篇 |
1971年 | 3363篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
841.
Jonathan L. Sessler Patricia J. Melfi Anne E.V. Gorden Doris K. Ford C. Drew Tait 《Tetrahedron》2004,60(49):11089-11097
Hexaphyrin(1.0.1.0.0.0) (isoamethyrin) undergoes a significant color change in the presence of UO22+, PuO22+, and NpO22+. The complexation of the first of these dioxo actinide cations was studied in semi-quantitative fashion in 1:1 MeOH-CH2Cl2. Under these conditions, the detection limit for UO22+ was found to be ca. 5.8 ppm by naked eye monitoring and <28 ppb by UV-vis spectroscopy. Isoamethyrin does not undergo a color change in the presence of most transition metals or when exposed to Gd(III). Isoamethyrin thus constitutes an attractive alternative to 2,2′(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)-bisbenzenarsonic acid (AzIII) and 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (BrPADAP), systems currently used as actinide cation sensors. 相似文献
842.
A variety of aryl nitriles were prepared in excellent yields from the palladium acetate catalyzed coupling of aryl halides with Zn(CN)2 using polymer-supported triphenyl phosphine as the ligand and dimethylformamide as solvent under microwave irradiation conditions. 相似文献
843.
In basin modelling the thermodynamics of a multicomponent multiphase fluid flux are computationally too expensive when derived from an equation of state and the Gibbs equality constraints. In this article we present a novel implicit molar mass formulation technique using binary mixture thermodynamics. The two proposed solution methods, with and without cross derivative terms between components, are based on a preconditioned Newton‐GMRES scheme for each time‐step with analytical computation of the derivatives. These new algorithms reduce significantly the numerical effort for the computation of the molar masses, and we illustrate the behavior of these methods with numerical computations. Copyright © 2004 John Wiley & Sons Ltd. 相似文献
844.
This paper presents an evaluation of the capability of turbulence models available in the commercial CFD code FLUENT 6.0 for their application to hydrofoil turbulent boundary layer separation flow at high Reynolds numbers. Four widely applied two‐equation RANS turbulence models were assessed through comparison with experimental data at Reynolds numbers of 8.284×106 and 1.657×107. They were the standard k–εmodel, the realizable k–εmodel, the standard k–ωmodel and the shear‐stress‐transport (SST) k–ωmodel. It has found that the realizable k–εturbulence model used with enhanced wall functions and near‐wall modelling techniques, consistently provides superior performance in predicting the flow characteristics around the hydrofoil. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
845.
K.B. Welter Q. Wu Y. You K. Abel D. McCreary S.M. Bajorek J.N. Reyes Jr 《International Journal of Multiphase Flow》2004,30(12):407-1484
This article describes a comprehensive literature review of liquid entrainment in horizontal pipes with vertical-up branches. Deficiencies in the available data and correlations were identified. The Air–water Test Loop for Advanced Thermal–hydraulic Studies (ATLATS) was constructed and entrainment onset and steady-state data were collected for a wide range of flow conditions. Using new insights gained from experimental testing, the authors developed a model for predicting the onset of entrainment and steady-state entrainment rate. Previously published correlations, along with the new model, are compared against all available data. The new model shows very good agreement with the onset data, but is not very good at predicting branch quality at high liquid flow rates. 相似文献
846.
A technique is described for the measurement of all components of mean velocity and Reynolds stresses, in a complex turbulent flow where achieving coincidence data acquisition is difficult. The method is based on data recorded using four orientations of the laser probe. It is shown that the measurement errors are not the same for all the components of the Reynolds tensor, but they are sufficiently small to give a good accuracy. An application to a turbomachinery flow is given to illustrate the method. 相似文献
847.
848.
849.
850.
V. Babin P. Bohacek E. Bender A. Krasnikov E. Mihokova M. Nikl N. Senguttuvan A. Stolovits Y. Usuki S. Zazubovich 《Radiation measurements》2004,38(4-6):533-537
Luminescence characteristics of a number of undoped and variously doped PbWO4 crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO4,CdWO4 and PbMoO4 crystals. Two types of green emission centres are detected in PbWO4 crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb+-WO3 centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated. 相似文献