全文获取类型
收费全文 | 783406篇 |
免费 | 8552篇 |
国内免费 | 2758篇 |
专业分类
化学 | 357187篇 |
晶体学 | 10037篇 |
力学 | 30571篇 |
综合类 | 20篇 |
数学 | 83874篇 |
物理学 | 205451篇 |
无线电 | 107576篇 |
出版年
2021年 | 5712篇 |
2020年 | 6285篇 |
2019年 | 6762篇 |
2018年 | 8249篇 |
2017年 | 8235篇 |
2016年 | 12946篇 |
2015年 | 8353篇 |
2014年 | 13046篇 |
2013年 | 33722篇 |
2012年 | 25430篇 |
2011年 | 31074篇 |
2010年 | 21902篇 |
2009年 | 23037篇 |
2008年 | 30487篇 |
2007年 | 31911篇 |
2006年 | 30048篇 |
2005年 | 27311篇 |
2004年 | 25215篇 |
2003年 | 22538篇 |
2002年 | 22217篇 |
2001年 | 24326篇 |
2000年 | 19527篇 |
1999年 | 15950篇 |
1998年 | 13687篇 |
1997年 | 13378篇 |
1996年 | 12894篇 |
1995年 | 11783篇 |
1994年 | 11370篇 |
1993年 | 11113篇 |
1992年 | 11875篇 |
1991年 | 11897篇 |
1990年 | 11124篇 |
1989年 | 10633篇 |
1988年 | 10498篇 |
1987年 | 9291篇 |
1986年 | 8854篇 |
1985年 | 11879篇 |
1984年 | 11937篇 |
1983年 | 9900篇 |
1982年 | 10320篇 |
1981年 | 9984篇 |
1980年 | 9664篇 |
1979年 | 9841篇 |
1978年 | 9958篇 |
1977年 | 9845篇 |
1976年 | 9720篇 |
1975年 | 9401篇 |
1974年 | 9106篇 |
1973年 | 9347篇 |
1972年 | 6150篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
861.
J.-U. Lbus 《Mathematische Nachrichten》1991,152(1):229-245
862.
In this paper we analyse numerical models for time-dependent Boussinesq equations. These equations arise when so-called Boussinesq terms are introduced into the shallow water equations. We use the Boussinesq terms proposed by Katapodes and Dingemans. These terms generalize the constant depth terms given by Broer. The shallow water equations are discretized by using fourth-order finite difference formulae for the space derivatives and a fourth-order explicit time integrator. The effect on the stability and accuracy of various discrete Boussinesq terms is investigated. Numerical experiments are presented in the case of a fourth-order Runge-Kutta time integrator. 相似文献
863.
M. S. Nikol’skii 《Computational Mathematics and Modeling》2007,18(4):420-431
We consider the behavior of bundles of optimal controls when the initial state of the system goes to some given vector. We
investigate three types of optimization problems: problems with fixed process length and a free right endpoint; problems with
fixed process length and a fixed right endpoint; time-optimal problems. The article is a review of recent results obtained
by the author.
__________
Translated from Nelineinaya Dinamika i Upravlenie, No. 4, pp. 301–314, 2004. 相似文献
864.
T. Ramanathan S. Stankovich D. A. Dikin H. Liu H. Shen S. T. Nguyen L. C. Brinson 《Journal of Polymer Science.Polymer Physics》2007,45(15):2097-2112
Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ~10 nm. Solution‐based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007 相似文献
865.
N. S. Oliveira J. Dorgan J. A. P. Coutinho A. Ferreira J. L. Daridon I. M. Marrucho 《Journal of Polymer Science.Polymer Physics》2007,45(5):616-625
The sorption of carbon dioxide in glassy Poly(lactic acid) (PLA) films was studied by quartz crystal microbalance (QCM) at high pressures. Two thermal treatments, melted and quenched, were performed in PLA with two different L:D contents, 80:20 and 98:2, films and compared with a third thermal protocol, annealed, and used in a previous work. The results obtained show that for pressures higher than 2 MPa, the carbon dioxide solubility is larger in PLA 80:20 than in PLA 98:2, indicating that the L:D plays a dominant role on this property. The thermal treatments only affect the gas solubility in PLA 98:2. Sorption isotherms at temperatures 303, 313, and 323 K, below the glass transition temperature of the polymer, and pressures up to 5 MPa were measured and analyzed with three different models, the dual‐mode sorption model, the Flory–Huggins equation, and a modified dual‐mode sorption model where the Henry's law term was substituted by the Flory–Huggins equation. This last model performs especially well for CO2 in PLA 80:20, due to the convex upward curvature of the solubility isotherms for that system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 616–625, 2007 相似文献
866.
The tradeoff between common-emitter current gain (β) and Early voltage (V A) in heterojunction bipolar transistors (HBTs) where the bandgap varies across the base has been studied. The Early voltage depends exponentially on the difference between the bandgap at the collector side of the base and the largest bandgap in the base, allowing very high Early voltages with only very thin narrow bandgap regions. Using Si/Si1-xGex/Si HBTs with a two-layer stepped base, βV A products of over 100000 V have been achieved for devices with a cutoff frequency expected to be about 30 GHz 相似文献
867.
The effect of a secondary path signal on the detectability of the symbol rate spectral line of a binary phase-shift keyed (BPSK) signal by a delay and multiply receiver is investigated. The measure of detectability used is the rate line signal-to-noise ratio. The parameters that define the secondary path signal are initially modeled as deterministic quantities; the phase of the secondary path signal is then taken to be a random variable independent and uniform on (0, 2π). It is found that, with deterministic path parameters, the presence of a secondary path can seriously affect the detectability of BPSK rate lines. When the phase is taken to be uniform on (0, 2π) over the receiver observation time, then the delay and multiply receiver performs as if no multipath is present. In this case, a noncoherent power addition of the primary and secondary path signals occurs 相似文献
868.
869.
M. S. Kalinin M. B. Krainev 《Bulletin of the Russian Academy of Sciences: Physics》2007,71(7):984-987
The behavior of the intensity of anomalous and galactic cosmic rays in the outer heliosphere is compared. The previous hypothesis, according to which the lowest energy galactic cosmic rays affected by large-scale electric fields far beyond the termination shock wave of the solar wind may contribute to the behavior of the anomalous component intensity, is analyzed. 相似文献
870.
Doping the waveguide core (p=2/spl times/10/sup 17/ cm/sup -1/) in asymmetric-waveguide InGaAs/InGaAsP, two-quantum-well diode lasers (/spl lambda/=980 nm) raises the injection efficiency to 90% and decreases the threshold-current density, J/sub th/. For 2 mm long, 100 /spl mu/m wide stripe, uncoated chips J/sub th/ decreases from /spl sim/188 A/cm/sup 2/ to /spl sim/150 A/cm/sup 2/. High characteristic temperatures for J/sub th/ and the slope efficiency are obtained: T/sub 0/=215K and T/sub 1/=600K. 相似文献