全文获取类型
收费全文 | 367933篇 |
免费 | 4625篇 |
国内免费 | 2040篇 |
专业分类
化学 | 178816篇 |
晶体学 | 5455篇 |
力学 | 19343篇 |
综合类 | 97篇 |
数学 | 54123篇 |
物理学 | 116764篇 |
出版年
2020年 | 2585篇 |
2019年 | 2751篇 |
2018年 | 11517篇 |
2017年 | 12242篇 |
2016年 | 8759篇 |
2015年 | 4008篇 |
2014年 | 5071篇 |
2013年 | 11699篇 |
2012年 | 13090篇 |
2011年 | 22262篇 |
2010年 | 14347篇 |
2009年 | 14170篇 |
2008年 | 19644篇 |
2007年 | 22936篇 |
2006年 | 9718篇 |
2005年 | 14460篇 |
2004年 | 10844篇 |
2003年 | 10205篇 |
2002年 | 8245篇 |
2001年 | 7729篇 |
2000年 | 6186篇 |
1999年 | 4669篇 |
1998年 | 4152篇 |
1997年 | 4047篇 |
1996年 | 3962篇 |
1995年 | 3643篇 |
1994年 | 3761篇 |
1993年 | 3420篇 |
1992年 | 3857篇 |
1991年 | 3876篇 |
1990年 | 3812篇 |
1989年 | 3644篇 |
1988年 | 3744篇 |
1987年 | 3618篇 |
1986年 | 3418篇 |
1985年 | 4506篇 |
1984年 | 4757篇 |
1983年 | 3992篇 |
1982年 | 4115篇 |
1981年 | 4035篇 |
1980年 | 4003篇 |
1979年 | 3985篇 |
1978年 | 4301篇 |
1977年 | 4149篇 |
1976年 | 4233篇 |
1975年 | 3901篇 |
1974年 | 3935篇 |
1973年 | 4089篇 |
1972年 | 2857篇 |
1971年 | 2373篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
We present in this paper an efficient approach for solving the problem of planning the long-term (multiyear) operation of a multireservoir hydroelectric power system for the critical period with a monthly variable load. This load is equal to a certain percentage of the total generation at the end of the year, subject to satisfying a number of constraints on the hydrosystem, using the minimum norm formulation.The proposed method is efficient in computing time and in calculating the total expected benefits from the system during the critical period. Numerical results are reported for a real system in operation consisting of two rivers. Each river has two series reservoirs.This work was supported by the Natural Science and Engineering Research Council of Canada, Grant No. A4146. 相似文献
992.
Sajewicz Mieczysław Hauck Heinz-E. Drabik Gabriela Namysło Ewa Głód Bronisław Kowalska Teresa 《平面色谱法杂志一现代薄层色谱法》2006,19(4):278-281
JPC – Journal of Planar Chromatography – Modern TLC - We have previously described unexpected two-dimensionality in the thin-layer chromatographic separation of pairs of enantiomers of... 相似文献
993.
We give a review of general known results on problems concerning the structure of scales of computability potentials of n-element algebras. We also formulate a number of natural open problems.
__________
Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 9, No. 3, pp. 145–164, 2003. 相似文献
994.
Baowei Qiao Jie Feng Yun Ling Ting’ao Tang Bomy Chen 《Applied Surface Science》2006,252(24):8404-8409
The effects of Si doping on the structural and electrical properties of Ge2Sb2Te5 film are studied in detail. Electrical properties and thermal stability can be improved by doping small amount of Si in the Ge2Sb2Te5 film. The addition of Si in the Ge2Sb2Te5 film results in the increase of both crystallization temperature and phase-transition temperature from face-centered cubic (fcc) phase to hexagonal (hex) phase, however, decreases the melting point slightly. The crystallization activation energy reaches a maximum at 4.1 at.% and then decreases with increasing dopant concentration. The electrical conduction activation energy increases with the dopant concentration, which may be attributed to the increase of strong covalent bonds in the film. The resistivity of Ge2Sb2Te5 film shows a significant increase with Si doping. When doping 11.8 at.% of Si in the film, the resistivity after 460 °C annealing increases from 1 to 11 mΩ cm compared to the undoped Ge2Sb2Te5 film. Current-voltage (I-V) characteristics show Si doping may increase the dynamic resistance, which is helpful to writing current reduction of phase-change random access memory. 相似文献
995.
L. Torrisi A.M. Mezzasalma J. Badziak J. Wolowski G. Franco 《Applied Surface Science》2006,252(24):8533-8538
High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 106 J/cm2, the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis. 相似文献
996.
We calculate the loss induced in a single-mode rectangular optical waveguide by the presence of a second waveguide, perpendicular to the first, which crosses over the first waveguide at a variable distance d. Our calculation is applied to the analysis of several doped silica waveguides of practical importance for optical circuit design. 相似文献
997.
Panasyugin A. S. Rat'ko A. I. Bondareva G. V. 《Russian Journal of Applied Chemistry》2002,75(11):1824-1827
The sorption properties of mixed adsorbent-catalyst based on high-silica zeolite and montmorillonite modified with mixed oxides of Cr(III) and Cu(II) with respect to organic compounds of various classes were studied, and the optimal dynamic conditions of its operation were determined. 相似文献
998.
The exact solution is constructed to a torsion problem for a circular elastic shaft in a medium referred to a spherical coordinate system. One end of the shaft is rigidly fixed and the other is subjected to either tangential forces or a torque. New integral transforms are obtained to solve the problem. 相似文献
999.
1000.
S. Neukirch G.H.M. van der HeijdenJ.M.T. Thompson 《Journal of the mechanics and physics of solids》2002,50(6):1175-1191
We use three different approaches to describe the static spatial configurations of a twisted rod as well as its stability during rigid loading experiments. The first approach considers the rod as infinite in length and predicts an instability causing a jump to self-contact at a certain point of the experiment. Semi-finite corrections, taken into account as a second approach, reveal some possible experiments in which the configuration of a very long rod will be stable through out. Finally, in a third approach, we consider a rod of real finite length and we show that another type of instability may occur, leading to possible hysteresis behavior. As we go from infinite to finite length, we compare the different information given by the three approaches on the possible equilibrium configurations of the rod and their stability. These finite size effects studied here in a 1D elasticity problem could help us guess what are the stability features of other more complicated (2D elastic shells for example) problems for which only the infinite length approach is understood. 相似文献