全文获取类型
收费全文 | 19445篇 |
免费 | 537篇 |
国内免费 | 345篇 |
专业分类
化学 | 12090篇 |
晶体学 | 146篇 |
力学 | 603篇 |
综合类 | 30篇 |
数学 | 3018篇 |
物理学 | 4440篇 |
出版年
2022年 | 281篇 |
2021年 | 355篇 |
2020年 | 276篇 |
2019年 | 299篇 |
2018年 | 263篇 |
2017年 | 204篇 |
2016年 | 495篇 |
2015年 | 458篇 |
2014年 | 515篇 |
2013年 | 1053篇 |
2012年 | 918篇 |
2011年 | 1061篇 |
2010年 | 726篇 |
2009年 | 668篇 |
2008年 | 944篇 |
2007年 | 930篇 |
2006年 | 816篇 |
2005年 | 830篇 |
2004年 | 769篇 |
2003年 | 747篇 |
2002年 | 768篇 |
2001年 | 495篇 |
2000年 | 440篇 |
1999年 | 352篇 |
1998年 | 255篇 |
1997年 | 246篇 |
1996年 | 277篇 |
1995年 | 274篇 |
1994年 | 267篇 |
1993年 | 212篇 |
1992年 | 190篇 |
1991年 | 197篇 |
1990年 | 218篇 |
1989年 | 171篇 |
1988年 | 156篇 |
1987年 | 149篇 |
1986年 | 134篇 |
1985年 | 245篇 |
1984年 | 254篇 |
1983年 | 183篇 |
1982年 | 202篇 |
1981年 | 170篇 |
1980年 | 186篇 |
1979年 | 168篇 |
1978年 | 132篇 |
1977年 | 170篇 |
1976年 | 157篇 |
1975年 | 131篇 |
1974年 | 116篇 |
1973年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
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
63.
In this work we consider computing and continuing connecting orbits in parameter dependent dynamical systems. We give details
of algorithms for computing connections between equilibria and periodic orbits, and between periodic orbits. The theoretical
foundation for these techniques is given by the seminal work of Beyn in 1994, “On well-posed problems for connecting orbits
in dynamical systems”, where a numerical technique is also proposed. Our algorithms consist of splitting the computation of
the connection from that of the periodic orbit(s). To set up appropriate boundary conditions, we follow the algorithmic approach
used by Demmel, Dieci, and Friedman, for the case of connecting orbits between equilibria, and we construct and exploit the
smooth block Schur decomposition of the monodromy matrices associated to the periodic orbits. Numerical examples illustrate
the performance of the algorithms.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
64.
The reaction pathway for the Ca3Al2O6 formation up to 1300°C, from mechanochemically treated mixtures of amorphous aluminum hydroxide and CaCO3, was studied in situ by differential thermal analysis, constant heating rate dilatometry and time-resolved neutron powder diffraction. The experiment was carried out, in an open system, on a sample with the nominal Ca3Al2O6 stoichiometry. The results obtained by neutron diffractometry and thermal analysis were in good agreement with the data obtained by scanning electron microscopy and X-ray diffraction on heat-treated and-quenched samples. The synthesis path implied the formation of cryptocrystalline Al2O3, crystalline CaO, CaAl2O4 and Ca12Al14O33 as transitory phases. Finally the nucleation and growth of the single phase Ca3Al2O6 took place at 1300°C and exhibited porous structure due to CO2 and H2O release. 相似文献
65.
Dielectric studies are presented of a banana-shaped compound that exhibits the antiferroelectric B2 phase. Upon application and subsequent removal of strong electric fields the textures and dielectric properties of the phase drastically change. Most notable is the huge increase of the low frequency permittivity. This behaviour would suggest the induction of ferroelectricity by the electric field. 相似文献
66.
The inhomogeneous Gevrey classes, defined in terms of Fourier transform, are a natural extension of the standard Gevrey classes. We find equivalent characterizations and discuss algebraic and topological properties. We therefore introduce the dual spaces, the inhomogeneous ultradistributions, giving some equivalent definitions and corresponding algebraic and topological properties; in particular, a version of the Paley-Wiener-Schwartz theorem is proved in our framework. Finally, as an important example, the multianisotropic Gevrey classes and ultradistributions are considered. 相似文献
67.
Valerio De Angelis 《Journal of Mathematical Analysis and Applications》2004,294(1):288-293
We prove that the solutions of the non-autonomous Lyness equation xn+1=(xn+bn)/xn−1, bn>0, x0>0, x1>0, are bounded away from zero and infinity if the sequence bn is monotonic. We also give an example to show that the solutions need not be bounded even when the sequence bn attains only two values. 相似文献
68.
Image potential resonances on the Sn/Ge(1 1 1) α-phase are investigated by two closely related methods: specular electron reflection and so-called selective electron scattering. Electrons from image resonances are detected on this surface at 120 and 300 K, i.e. below and above the phase transition at about 200 K. The dispersion of the image resonances reveals at these two temperatures equivalent effective electron masses, which are characteristic for this type of electronic surface states. The results of the two methods are consistent according to the similarity of the scattering processes. Changes in the loss peak intensity with the annealing temperature are assigned to the surface quality and are reflected by characteristic photoemission intensities. 相似文献
69.
70.
We consider a SOS type model of interfaces on a substrate which is both heterogeneous and rough. We first show that, for appropriate values of the parameters, the differential wall tension that governs wetting on such a substrate satisfies a generalized law which combines both Cassie and Wenzel laws. Then in the case of an homogeneous substrate, we show that this differential wall tension satisfies either the Wenzel's law or the Cassie's law, according to the values of the parameters. 相似文献