全文获取类型
收费全文 | 13247篇 |
免费 | 1043篇 |
国内免费 | 561篇 |
专业分类
化学 | 9950篇 |
晶体学 | 116篇 |
力学 | 688篇 |
综合类 | 36篇 |
数学 | 1311篇 |
物理学 | 2750篇 |
出版年
2024年 | 33篇 |
2023年 | 133篇 |
2022年 | 285篇 |
2021年 | 588篇 |
2020年 | 567篇 |
2019年 | 589篇 |
2018年 | 593篇 |
2017年 | 532篇 |
2016年 | 791篇 |
2015年 | 602篇 |
2014年 | 762篇 |
2013年 | 1258篇 |
2012年 | 1088篇 |
2011年 | 1052篇 |
2010年 | 686篇 |
2009年 | 649篇 |
2008年 | 670篇 |
2007年 | 590篇 |
2006年 | 489篇 |
2005年 | 384篇 |
2004年 | 311篇 |
2003年 | 295篇 |
2002年 | 287篇 |
2001年 | 179篇 |
2000年 | 140篇 |
1999年 | 138篇 |
1998年 | 120篇 |
1997年 | 109篇 |
1996年 | 79篇 |
1995年 | 88篇 |
1994年 | 82篇 |
1993年 | 74篇 |
1992年 | 71篇 |
1991年 | 52篇 |
1990年 | 55篇 |
1989年 | 30篇 |
1988年 | 33篇 |
1987年 | 35篇 |
1986年 | 36篇 |
1985年 | 42篇 |
1984年 | 32篇 |
1983年 | 21篇 |
1982年 | 32篇 |
1981年 | 22篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 13篇 |
1977年 | 12篇 |
1975年 | 12篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Umie F.M. Haziz Rosenani A. Haque Shun-Ze Zhan Hassan H. Abdallah Mohd R. Razali 《应用有机金属化学》2020,34(10):e5818
The synthesis and characterizations for a series of dinuclear gold (I)-di-NHC complexes, 1–8 through the trans-metalation method of their respective silver (I)-di-NHC complexes, i–viii are reported (where NHC = N-heterocyclic carbene). The successful complexation of a series of unusual non-symmetrical and symmetrical di-NHC ligands, 3,3'-(ethane-1,2-diyl)-1-alkylbenzimidazolium-1'-butylbenzimidazolium (with alkyl = methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, benzyl) with the gold (I) ions are suggested by elemental analysis, Fourier transform-infrared, 1H- and 13C-NMR data. The 13C-NMR spectra of 1–8 show a singlet sharp peak in the range of 190.00–192.00 ppm, indicating the presence of a carbene carbon that bonded to the gold (I) ion. From single crystal X-ray diffraction data, the structure of complex 6 with the formula of [di-NHC-Au (I)]2·2PF6 is obtained [where NHC = 3,3'-(ethane-1,2-diyl)-1-hexylbenzimidazolium-1'-butylbenzimidazolium]. The photophysical study in solid state of 6 displays an intense photoluminescence with a strong emission maxima, λem = 480 nm, upon excitation at 340 nm at room temperature. Interestingly, the emission maximum at 77 K shows a structural character with a strong peak at 410 nm, a medium at 433 nm and a weak at 387 nm, accompanied by a tail band to about 500 nm. 相似文献
32.
33.
34.
Science China Mathematics - We prove that if a compact Riemannian 4-manifold with positive sectional curvature satisfies a strengthened Kato type inequality, then it is definite. We also discuss... 相似文献
35.
Efficient measurement of the performance index (the distance of a loading parameter from the voltage collapse point) is one of the key problems in power system operations and planning and such an index indicates the severity of a power system with regard to voltage collapse. There exist many interesting methods and ideas to compute this index. However, some successful methods are not yet mathematically justified while other mathematically sound methods are often proposed directly based on the bifurcation theory and they require the initial stationary state to be too close to the unknown turning point to make the underlying methods practical.This paper first gives a survey of several popular methods for estimating the fold bifurcation point including the continuation methods, bifurcation methods and the test function methods (Seydel's direct solution methods, the tangent vector methods and the reduced Jacobian method) and discuss their relative advantages and problems. Test functions are usually based on scaling of the determinant of the Jacobian matrix and it is generally not clear how to determine the behaviour of such functions. As the underlying nonlinear equations are of a particular type, this allows us to do a new analysis of the determinants of the Jacobian and its submatrices in this paper. Following the analysis, we demonstrate how to construct a class of test functions with a predictable analytical behaviour so that a suitable index can be produced. Finally, examples of two test functions from this class are proposed. For several standard IEEE test systems, promising numerical results have been achieved. 相似文献
36.
37.
In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement. 相似文献
38.
An admissible minimax estimator of a bounded scale-parameter in a subclass of the exponential family under scale-invariant squared-error loss 总被引:3,自引:0,他引:3
Mohammad Jafari Jozani Nader Nematollahi Khalil Shafie 《Statistics & probability letters》2002,60(4):437-444
A subclass of the scale-parameter exponential family is considered and for the rth power of the scale parameter, which is lower bounded, an admissible minimax estimator under scale-invariant squared-error loss is presented. Also, an admissible minimax estimator of a lower-bounded parameter in the family of transformed chi-square distributions is given. These estimators are the pointwise limits of a sequence of Bayes estimators. Some examples are given. 相似文献
39.
Azizollah Habibi Leyla Mousavifar Issa Yavari Mohammad R. Yazdanbakhsh 《Monatshefte für Chemie / Chemical Monthly》2007,4(8):603-606
Alkyl isocyanides undergo a smooth reaction with alkylidene Meldrum’s acids in the presence of arylhydroxylamines to produce N
1-alkyl-2-(3,5-dioxo-2-aryltetrahydro-4-isoxazolyl)alkanamides in high yields. 相似文献
40.
5-Perfluorophenyl 4,5-dihydro-1H-pyrazoles were synthesized from 1,3-dipolar cycloaddition reaction of perfluorobenyl 2,4,6-triisopropylbenzenesulfonylhydrazone and α,β-unsaturated carbonyl compounds or acrylonitrile in THF or water. It was worthy to note that better results were obtained when water was employed as the solvent, which was considered as an effective, economic and environmentally friendly method to synthesize these pyrazole derivatives. 相似文献