全文获取类型
收费全文 | 179702篇 |
免费 | 866篇 |
国内免费 | 262篇 |
专业分类
化学 | 89717篇 |
晶体学 | 2635篇 |
力学 | 9377篇 |
综合类 | 7篇 |
数学 | 20939篇 |
物理学 | 58155篇 |
出版年
2020年 | 1849篇 |
2019年 | 2289篇 |
2018年 | 3397篇 |
2017年 | 3524篇 |
2016年 | 4159篇 |
2015年 | 1855篇 |
2014年 | 3589篇 |
2013年 | 7051篇 |
2012年 | 5540篇 |
2011年 | 6591篇 |
2010年 | 5511篇 |
2009年 | 5636篇 |
2008年 | 6366篇 |
2007年 | 6328篇 |
2006年 | 5624篇 |
2005年 | 4989篇 |
2004年 | 4827篇 |
2003年 | 4372篇 |
2002年 | 4530篇 |
2001年 | 5000篇 |
2000年 | 3696篇 |
1999年 | 2825篇 |
1998年 | 2606篇 |
1997年 | 2458篇 |
1996年 | 2258篇 |
1995年 | 2048篇 |
1994年 | 2006篇 |
1993年 | 2042篇 |
1992年 | 2076篇 |
1991年 | 2381篇 |
1990年 | 2317篇 |
1989年 | 2384篇 |
1988年 | 2266篇 |
1987年 | 2296篇 |
1986年 | 2125篇 |
1985年 | 2657篇 |
1984年 | 2671篇 |
1983年 | 2316篇 |
1982年 | 2373篇 |
1981年 | 2275篇 |
1980年 | 2077篇 |
1979年 | 2393篇 |
1978年 | 2547篇 |
1977年 | 2639篇 |
1976年 | 2729篇 |
1975年 | 2536篇 |
1974年 | 2467篇 |
1973年 | 2634篇 |
1972年 | 2152篇 |
1971年 | 1973篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
The coalescence of branches in the Y junctions of single-wall carbon nanotubes (10 nm long) is predicted to occur when the branches approach each other under the action of a load (~10 nN) applied to their ends. A transition to the new state with parallel branches bound by molecular interactions was simulated and the energy characteristics were calculated by the molecular dynamics method. The Y junctions with parallel branches are stable at temperatures up to 2000 K. It is established that there is a threshold distance between the branch ends, below which the branches exhibit spontaneous sticking under the action of molecular attraction forces. If the branches are unloaded before this threshold distance is reached, they oscillate (acting as a nanodimensional “tuning fork”) at a frequency of ~100 GHz. 相似文献
932.
The pseudopotential augmented-plane-wave method was used to study the surface structure of thin copper films with monolayers of Co, Ni, and Cs. Local densities of electron states, distributions of charge densities in the layers, and electron energy spectra are analyzed. The effect of adsorbates on the electronic properties of the Cu(001) surface and work function are discussed. Satisfactory agreement with available experimental data has been obtained. 相似文献
933.
934.
I T Vieira P R Harper A K Shahani V De Senna 《The Journal of the Operational Research Society》2003,54(7):713-722
Mother-to-child transmission (MTCT) of HIV is the most significant source of HIV infection in children below the age of 15 years. In 2000 alone, about 600 000 new infections occurred, the vast majority from mothers living in developing countries who were not aware of their HIV-positive status. To date, at least 4.3 million children have died from AIDS. In this paper, we discuss the development of an operational model that can be used to evaluate intervention options for the prevention of MTCT of HIV. The problem was approached by defining suitable pregnancy risk groups, prevention options and the required model outputs to evaluate different intervention strategies. The method chosen to approach the problem was a discrete-event, three-phase simulation, built in Visual Basic, with a stochastic semi-Markov structure. The developed model takes individual pregnant women through each stage of their pregnancy, labour and birth. Different intervention strategies may be modelled at any time, including short-course antiretroviral drugs and cessation of breastfeeding. The model is demonstrated with data from Botswana, which has one of the highest HIV-infection rates in the world. 相似文献
935.
936.
Bakirova I. N. Shagidullin R. R. Avvakumova L. V. Demchenko I. G. Zenitova L. A. 《Russian Journal of Applied Chemistry》2003,76(3):468-471
The activity of a polyether derived from ethylene and propylene oxides (Laprol 5003) and of N,N,N',N'-tetra(hydroxypropyl)ethylenediamine (Lapramol 294) in glycolytic degradation of elastic foamed polyurethane was studied. 相似文献
937.
Nonlinear processes caused by the propagation of low-frequency and high-frequency acoustic pulses in an unbounded medium and the propagation of continuous waves in a ring resonator are theoretically studied on the basis of two hysteretic equations of state for media with imperfect elasticity. The profiles and parameters of pulses, the resonance curve and the Q factor of the resonator, and the ratio of the nonlinear resonance frequency shift to the nonlinear damping decrement are determined. For nonlinear wave processes in such media, the distinctive features that allow one to choose an appropriate hysteretic equation of state for analytically describing the experimental data are revealed. 相似文献
938.
939.
Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ∼7 g aerosil in 1 cm3 of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10-2-109 Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-Fulcher-Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface). 相似文献
940.
Mechanisms of the self-organization of nanosystems at the condensation of the extremely weak steady flows of copper vapors
obtained in magnetron sputtering in an ultrapure argon are studied. It is shown that the free-energy minimization near equilibrium
condensation creates prerequisites for the self-organization of statistically homogeneous layers of nanoclusters and determines
the transition to the formation of nanosystems in the form of fractal networks.
Original Russian Text ? V.I. Perekrestov, A.S. Kornyushchenko, Yu.A. Kosminskaya, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ
i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 12, pp. 879–883. 相似文献