全文获取类型
收费全文 | 586093篇 |
免费 | 7884篇 |
国内免费 | 2453篇 |
专业分类
化学 | 315739篇 |
晶体学 | 8657篇 |
力学 | 25636篇 |
综合类 | 72篇 |
数学 | 70572篇 |
物理学 | 175754篇 |
出版年
2021年 | 4720篇 |
2020年 | 5336篇 |
2019年 | 5778篇 |
2018年 | 7472篇 |
2017年 | 7390篇 |
2016年 | 11206篇 |
2015年 | 7171篇 |
2014年 | 10926篇 |
2013年 | 26459篇 |
2012年 | 20889篇 |
2011年 | 25515篇 |
2010年 | 17956篇 |
2009年 | 17772篇 |
2008年 | 23622篇 |
2007年 | 23592篇 |
2006年 | 21923篇 |
2005年 | 19688篇 |
2004年 | 18054篇 |
2003年 | 15891篇 |
2002年 | 15681篇 |
2001年 | 17573篇 |
2000年 | 13339篇 |
1999年 | 10267篇 |
1998年 | 8623篇 |
1997年 | 8379篇 |
1996年 | 8177篇 |
1995年 | 7271篇 |
1994年 | 7250篇 |
1993年 | 7029篇 |
1992年 | 7777篇 |
1991年 | 7851篇 |
1990年 | 7503篇 |
1989年 | 7269篇 |
1988年 | 7178篇 |
1987年 | 7052篇 |
1986年 | 6767篇 |
1985年 | 8895篇 |
1984年 | 9032篇 |
1983年 | 7332篇 |
1982年 | 7562篇 |
1981年 | 7360篇 |
1980年 | 6811篇 |
1979年 | 7340篇 |
1978年 | 7678篇 |
1977年 | 7595篇 |
1976年 | 7528篇 |
1975年 | 6989篇 |
1974年 | 7075篇 |
1973年 | 7042篇 |
1972年 | 5165篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
971.
972.
K. K. Turgunov B. Tashkhodzhaev L. V. Molchanov Kh. M. Shakhidoyatov 《Journal of Structural Chemistry》2004,45(5):912-916
The structure of bis-(2,3-pentamethylene-3,4-dihydro-4-oxoquinazolinium) tetrachlorocuprate (II) sesquihydrate, (C13H15N2O)2[CuCl4]·1.5H2O, was determined by single crystal X-ray diffraction. In contrast to the previously studied analogs, the compound contains crystallization water molecules.Original Russian Text Copyright © 2004 by K. K. Turgunov, B. Tashkhodzhaev, L. V. Molchanov, and Kh. M. Shakhidoyatov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 955–959, September–October, 2004. 相似文献
973.
Lígia M. Rodrigues 《Tetrahedron》2004,60(40):8929-8936
Tetrapeptides containing one of a set of four different α,α-dialkyl glycines at the C-terminus were synthesized by conventional methods in solution and their conformational behavior investigated by 1H NMR spectroscopy in connection with molecular mechanics calculations. The results were consistent with conformations stabilized by a γ-turn in the case of compounds with alkyl groups larger than methyl, while the corresponding Aib derivative did not exhibit intramolecular hydrogen bonding. 相似文献
974.
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
975.
Using radical chemistry novel highly condensed heterocycles have been synthesized. The mechanism for their formation is discussed. 相似文献
976.
Mechatronic subsystems have been extensively developed in automotive applications as they increase performance compared to passive components. The Hydractive CRONE suspension based on fractional differentiation is presented as a hybrid system undergoing an infinite sequence of autonomous switchings. The modelling and operation of Hydractive CRONE suspension as a hybrid system are presented. The stability of Hydractive CRONE suspension is demonstrated using the maximum principle extended to hybrid systems. Finally, comfort mode synthesis is proposed using the third generation CRONE control system design. 相似文献
977.
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
978.
A preconditioning approach based on the artificial compressibility formulation is extended to solve the governing equations for unsteady turbulent reactive flows with heat release, at low Mach numbers, on an unstructured hybrid grid context. Premixed reactants are considered and a flamelet approach for combustion modelling is adopted using a continuous quenched mean reaction rate. An overlapped cell‐vertex finite volume method is adopted as a discretisation scheme. Artificial dissipation terms for hybrid grids are explicitly added to ensure a stable, discretised set of equations. A second‐order, explicit, hybrid Runge–Kutta scheme is applied for the time marching in pseudo‐time. A time derivative of the dependent variable is added to recover the time accuracy of the preconditioned set of equations. This derivative is discretised by an implicit, second‐order scheme. The resulting scheme is applied to the calculation of an infinite planar (one‐dimensional) turbulent premixed flame propagating freely in reactants whose turbulence is supposed to be frozen, homogeneous and isotropic. The accuracy of the results obtained with the proposed method proves to be excellent when compared to the data available in the literature. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
979.
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
980.
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