全文获取类型
收费全文 | 869篇 |
免费 | 35篇 |
国内免费 | 48篇 |
专业分类
化学 | 347篇 |
晶体学 | 5篇 |
力学 | 51篇 |
数学 | 140篇 |
物理学 | 409篇 |
出版年
2023年 | 7篇 |
2022年 | 15篇 |
2021年 | 10篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 26篇 |
2016年 | 28篇 |
2015年 | 32篇 |
2014年 | 55篇 |
2013年 | 64篇 |
2012年 | 29篇 |
2011年 | 60篇 |
2010年 | 43篇 |
2009年 | 62篇 |
2008年 | 53篇 |
2007年 | 55篇 |
2006年 | 48篇 |
2005年 | 36篇 |
2004年 | 23篇 |
2003年 | 28篇 |
2002年 | 23篇 |
2001年 | 20篇 |
2000年 | 12篇 |
1999年 | 16篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 9篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 1篇 |
排序方式: 共有952条查询结果,搜索用时 78 毫秒
71.
Experimental liquid–liquid equilibrium (LLE) of the water–acetic acid–sec-butyl acetate ternary system was investigated at 298.15, 303.15, 308.15 and 313.15 K and at atmospheric pressure. Separation factors were also evaluated for the immiscibility region. The NRTL and UNIQUAC models were applied to fit the experimental data for the ternary system. The binary interaction parameters obtained from both models were found to be successfully correlated with the equilibrium compositions. The UNIFAC group contribution method was employed to predict the observed ternary LLE data. It was found that four types of the UNIFAC model (UNIFAC, UNIFAC-LL, UNIFAC-DMD, and UNIFAC-LBY) did not provide a good prediction of the LLE data for this ternary system. 相似文献
72.
73.
By expanding the Debye theory into the tight focusing of partially coherent field, the intensity and spatial correlation properties of partially coherent radially polarized vortex beams are studied. Expressions are derived for the intensity distribution and the spectral degree of coherence in the focal region. It is found that the intensity and the transverse and longitudinal coherence degrees in the focal region change with the variation of the topological charge and coherence length of the vortex beam. In addition, the degree of coherence is shown to exhibit phase singularities. 相似文献
74.
考虑初态He原子电子间的关联效应、末态敲出电子与剩余束缚电子间的关联效应和末态屏蔽效应, 计算和分析了在共面、大能量损失和小动量转移这一特殊几何条件下730eV电子离化He原子的三重微分截面. 所得结果与Stevenson等人的相对实验数据和CCC理论计算结果进行比较发现: 所得到的理论曲线能够反映实验数据给出的三重微分截面的结构特征. 相似文献
75.
We show how the spontaneous emission rate of an excited two-level atom placed in a trapped Bose-Einstein condensate of ground-state atoms is enhanced by bosonic stimulation. This stimulation depends on the overlap of the excited matter-wave packet with the macroscopically occupied condensate wave function, and provides a probe of the spatial coherence of the Bose gas. The effect can be used to amplify the distance-dependent decay rate of an excited atom near an interface. 相似文献
76.
Mehmet Ozger 《Physica A》2011,390(6):981-989
Fluctuations in the significant wave height can be quantified by using scaling statistics. In this paper, the scaling properties of the significant wave height were explored by using a large data set of hourly series from 25 monitoring stations located off the west coast of the US. Detrended fluctuation analysis (DFA) was used to investigate the scaling properties of the series. DFA is a robust technique that can be used to detect long-range correlations in nonstationary time series. The significant wave height data was analyzed by using scales from hourly to monthly. It was found that a common scaling behavior can be observed for all stations. A breakpoint in the scaling region around 4-5 days was apparent. Spectral analysis confirms this result. This breakpoint divided the scaling region into two distinct parts. The first part was for finer scales (up to 4 days) which exhibited Brown noise characteristics, while the second one showed 1/f noise behavior at coarser scales (5 days to 1 month). The first order and the second order DFA (DFA1 and DFA2) were used to check the effect of seasonality. It was found that there were no differences between DFA1 and DFA2 results, indicating that there is no effect of trends in the wave height time series. The resulting scaling coefficients range from 0.696 to 0.890 indicating that the wave height exhibits long-term persistence. There were no coherent spatial variations in the scaling coefficients. 相似文献
77.
78.
The stochastic solution to diffusion equations with polynomial coefficients is called a Pearson diffusion. If the time derivative is replaced by a distributed order fractional derivative, the stochastic solution is called a distributed order fractional Pearson diffusion. This paper develops a formula for the covariance function of distributed order fractional Pearson diffusion in the steady state, in terms of generalized Mittag-Leffler functions. The correlation function decays like a power law. That formula shows that distributed order fractional Pearson diffusions exhibits long range dependence. 相似文献
79.
Apparent correlations are found between the N-H chemical shifts of congeneric series of compounds and the dipole moments of the molecules, such as lactams and thiolactams, cyclic ureas and thioureas. When there is a high degree of correlation, either the N-H chemical shift or the dipole moment of the molecule can be used as an electronic parameter in correlating the biological activity with the chemical structure. In a series of substituted salicylaldehydes the Hammett σ constant gives better correlation with the biological activity than the O-H chemical shift. This is probably due to the anisotropic effect of the substituent besides the electronic effect. Other factors affecting the N-H chemical shift, e.g. intramolecular hydrogen-bonding of nitrosoureas and the deshielding effect of a benzene ring, in o-phenyleneureas are also presented. In spite of the limitations, the chemical shifts of many compounds can be obtained very easily, therefore, their uses in structure-activity correlations deserve further investigation. 相似文献
80.
Density and viscosity were determined for the binary mixtures containing the ionic liquid 1-butyl-3-methylimidazolium thiocyanate
([BMIM][SCN]) and 1-alcohols (1-butanol, 1-pentanol and 1-hexanol) at six temperatures (298.15, 308.15, 318.15, 328.15, 338.15
and 348.15) K and ambient pressure. The density and viscosity correlation for these systems was tested by an empirical second-order
polynomial and by the Vogel-Fucher-Tammann equation in wide ranges of temperatures. Excess molar volumes were described by
the Redlich-Kister polynomial expansions. The density and viscosity variations with composition were described by polynomials.
A qualitative analysis of the trend of the properties with solvent and temperature was performed. The obtained results indicate
that ionic liquid interactions with 1-alcohols are strongly dependent on the special trend of packing of this ionic liquid
into hydroxylic solvents. As previously observed, an increase of the 1-alcohol carbon-chain length leads to lower interactions
on mixing.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献