全文获取类型
收费全文 | 18237篇 |
免费 | 1236篇 |
国内免费 | 1073篇 |
专业分类
化学 | 7772篇 |
晶体学 | 55篇 |
力学 | 787篇 |
综合类 | 92篇 |
数学 | 762篇 |
物理学 | 2941篇 |
综合类 | 8137篇 |
出版年
2024年 | 49篇 |
2023年 | 167篇 |
2022年 | 362篇 |
2021年 | 402篇 |
2020年 | 492篇 |
2019年 | 414篇 |
2018年 | 427篇 |
2017年 | 542篇 |
2016年 | 615篇 |
2015年 | 675篇 |
2014年 | 791篇 |
2013年 | 1025篇 |
2012年 | 1157篇 |
2011年 | 1047篇 |
2010年 | 802篇 |
2009年 | 829篇 |
2008年 | 840篇 |
2007年 | 1042篇 |
2006年 | 927篇 |
2005年 | 804篇 |
2004年 | 752篇 |
2003年 | 681篇 |
2002年 | 634篇 |
2001年 | 523篇 |
2000年 | 566篇 |
1999年 | 529篇 |
1998年 | 424篇 |
1997年 | 465篇 |
1996年 | 356篇 |
1995年 | 367篇 |
1994年 | 287篇 |
1993年 | 220篇 |
1992年 | 234篇 |
1991年 | 214篇 |
1990年 | 186篇 |
1989年 | 165篇 |
1988年 | 149篇 |
1987年 | 96篇 |
1986年 | 59篇 |
1985年 | 43篇 |
1984年 | 46篇 |
1983年 | 21篇 |
1982年 | 29篇 |
1981年 | 16篇 |
1980年 | 19篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
41.
Semi-empirical molecular orbital calculations were carried out for the compounds (C2H5)3As, (C2H5)3Ga and RAsH2 (R = C2H5, i-C3H7, i-C4H9, and t-C4H9) by using the CNDO/2-U program, and their capability of β-elimination reaction is compared on the basis of the torsion energy to the transition state, electrostatic interactions and orbital overlapping between the central atom and the β-hydrogen, and bond order of the metal-carbon, and carbon-hydrogen bond. In the comparison of (C2H5)3As with (C2H5)3Ga, we found that the β-elimination of (C2H5)3As could hardly be expected to take place in the thermal decomposition. The capability of β-elimination would be smaller in C2H5AsH2 than that in (C2H5)3As. Moreover when the ethyl group is replaced by a t-butyl group in RAsH2, the β-elimination reaction appears to become more difficult and a large possibility for a radical process is suggested. 相似文献
42.
余学义 《西安科技大学学报》1996,(4)
通过分析地下开采的影响,黄土覆盖层变形裂缝规律,提出以开采方法和工艺方式来控制或减弱地表变形裂缝破坏程度,并在保护地表建筑物的前提下,最大限度回收“三下”呆滞煤炭 相似文献
43.
We prove that, for any given vertex ν* in a series-parallel graph G, its edge set can be partitioned into κ = min{κ′(G) + 1, δ(G)} subsets such that each subset covers all the vertices of G possibly except for ν*, where δ(G) is the minimum degree of G and κ′(G) is the edge-connectivity of G. In addition, we show that the results in this paper are best possible and a polynomial time algorithm can be obtained for
actually finding such a partition by our proof. 相似文献
44.
R. Németh 《Journal of statistical physics》1991,63(1-2):419-424
We calculate the site occupation probabilities of one-dimensional lattice gas models within the canonical and grand canonical ensembles. The appearing differences do not vanish if we increase the system size keeping the site energies discrete. In this way one can explain the surprising numerical results of Barszczak and Kutner. This effect in the single-site occupation number disappears in higher dimensions. 相似文献
45.
Modulated
DSC for gas hydrates analysis 总被引:1,自引:0,他引:1
C. Giavarini F. Maccioni Maria Laura Santarelli 《Journal of Thermal Analysis and Calorimetry》2006,84(2):419-424
Modulated DSC has been applied to the study of methane,
ethane and propane hydrates at different hydrate and ice concentrations. The
reversing component of the TMDSC curves, makes it possible to characterize
such hydrates.
Methane and ethane hydrates show the melting-decomposition
peak at a temperatures higher than the ice contained in the sample, while
propane hydrate melts and decomposes at lower temperature than the ice present
in the sample. The hydrate peaks tend to disappear if the hydrate is stored
at atmospheric pressure. Guest size and cavity occupation fix the heat of
dissociation and stability of the hydrates, as confirmed by parallel tests
on tetrahydrofurane hydrates. 相似文献
46.
Summary The gas chromatographic behaviour of crosslinked macroporous 2-hydroxyethyl methacrylate grafted with 2-methyloxazoline was
investigated and the range of applicability of the copolymer was assessed. Retention index values were measured on this copolymer
and compared with those obtained on the terpolymer 2-hydroxyethyl methacrylate-ethylene dimethacrylate-acrylonitrile. Rohrschneider
constants and efficiency data for selected sorbates are given. The use of this type of polymer in the separation of various
types of compounds is documented by various examples. 相似文献
47.
48.
49.
Yu. N. Grigoryev I. V. Ershov 《Journal of Applied Mechanics and Technical Physics》2003,44(4):471-481
The influence of thermal excitation on a finite-amplitude vortex disturbance in a shear flow of a molecular gas is studied in a model problem. The evolution of such vortex structures is typical of both the nonlinear stage of the laminar–turbulent transition and for developed turbulence. Since the excitation level was assumed to be comparatively low, full Navier–Stokes equations for a compressible heat-conducting gas were used in calculations; nonequilibrium was taken into account by the coefficient of bulk viscosity. As the bulk viscosity increases in the range of realistic values, the disturbance-energy damping rate in a weakly compressible flow increases approximately by 10%. The increase in the Mach number enhances the effect of disturbance suppression. 相似文献
50.
Summary The solubility parameters of cationic surfactants were obtained using the inverse gas chromatographic technique. The surfactants
didodecyl dimethyl ammonium bromide, dioctadecyl dimethyl ammonium bromide and dodecyl pyridinium chloride were used as stationary
phase and retention data of different probe solutes were measured at different temperatures. The results were analysed by
the combination of Flory-Huggins and Hildebrand theories, and the solubility parameters of the surfactants were obtained in
a range of temperatures between 80–120°C. 相似文献