全文获取类型
收费全文 | 348219篇 |
免费 | 3350篇 |
国内免费 | 971篇 |
专业分类
化学 | 186109篇 |
晶体学 | 6124篇 |
力学 | 14410篇 |
综合类 | 12篇 |
数学 | 37487篇 |
物理学 | 108398篇 |
出版年
2020年 | 2941篇 |
2019年 | 3256篇 |
2018年 | 3930篇 |
2017年 | 4122篇 |
2016年 | 6247篇 |
2015年 | 3818篇 |
2014年 | 6168篇 |
2013年 | 15735篇 |
2012年 | 11396篇 |
2011年 | 13772篇 |
2010年 | 9917篇 |
2009年 | 9945篇 |
2008年 | 12707篇 |
2007年 | 12524篇 |
2006年 | 11844篇 |
2005年 | 10676篇 |
2004年 | 9788篇 |
2003年 | 8937篇 |
2002年 | 8631篇 |
2001年 | 10411篇 |
2000年 | 7838篇 |
1999年 | 6029篇 |
1998年 | 4889篇 |
1997年 | 4725篇 |
1996年 | 4543篇 |
1995年 | 4236篇 |
1994年 | 4120篇 |
1993年 | 3929篇 |
1992年 | 4792篇 |
1991年 | 4633篇 |
1990年 | 4522篇 |
1989年 | 4519篇 |
1988年 | 4505篇 |
1987年 | 4520篇 |
1986年 | 4298篇 |
1985年 | 5558篇 |
1984年 | 5628篇 |
1983年 | 4635篇 |
1982年 | 4827篇 |
1981年 | 4706篇 |
1980年 | 4476篇 |
1979年 | 4935篇 |
1978年 | 4979篇 |
1977年 | 5037篇 |
1976年 | 4912篇 |
1975年 | 4587篇 |
1974年 | 4522篇 |
1973年 | 4650篇 |
1972年 | 3123篇 |
1968年 | 2788篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
It is shown that the small-scale Rossby waves in a shallow rotating fluid placed in a gravitational field can generate large-scale
zonal flows by means of a two-stream-like mechanism. This mechanism is revealed under the conditions when the Lighthill instability
criterion is not satisfied.
The text was submitted by the authors in English. 相似文献
952.
Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr2O3 using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr2O3). A second phase, namely θ-(Al,Cr)2O3, was revealed by XRD in the sample containing 9.60 wt% Cr2O3. The structure of mullites was refined by the Rietveld method, location of Cr3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr2O3 content less than ∼5 wt%) Cr3+ ions were substituted for Al3+ in the AlO6 octahedra of the mullite structure (M1 site). For higher doping level, Cr3+ ions were additionally substituted for Al3+ in the AlO6 octahedra of the second phase [θ-(Al,Cr)2O3 at 1400 °C, or α-(Al,Cr)2O3 at 1600 °C] which segregated in the system. Substitution of Cr3+ for Al3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O6 octahedron and decrease of average distances in T*O4 tetrahedron, while average distances in TO4 tetrahedron stayed almost constant. 相似文献
953.
We analyze the statistical behavior of signals in nonlinear circuits with delayed feedback in the presence of external Markovian noise. For the special class of circuits with intense phase mixing we develop an approach for the computation of the probability distributions and multitime correlation functions based on the random phase approximation. Both Gaussian and Kubo-Andersen models of external noise statistics are analyzed and the existence of the stationary (asymptotic) random process in the long-time limit is shown. We demonstrate that a nonlinear system with chaotic behavior becomes a noise amplifier with specific statistical transformation properties. 相似文献
954.
A Haag-Ruelle Scattering Theory for Euclidean Lattice Field Theories is developed.Work supported by DAAD, Deutscher Akademischer Austauschdienst. Partially supported by CNPq 相似文献
955.
Gulis I. M. Komyak A. I. Saechnikov K. A. Tsvirko V. A. 《Journal of Applied Spectroscopy》1989,50(3):257-262
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 50, No. 3, pp. 385–391, March, 1989. 相似文献
956.
B. Yu. Furchin 《Ukrainian Mathematical Journal》1989,41(6):681-685
Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 41, No. 6, pp. 794–798, June, 1989. 相似文献
957.
High-average-power and high-energy lasers with good beam quality are fundamental tools for pumping high-peak and high-average-power ultrafast Ti:sapphire laser systems. Laser-diode (LD) pumped solid slate lasers with phase conjugation offer a unique combination of efficiency, reliability, and compactness. We present the design details and performance characteristics of a 360-W 1-kHz LD pumped Nd:YAG phase-conjugated laser. We also discuss the basic design aspects and present our preliminary experimental investigations on ultrashort-pulse laser systems. 相似文献
958.
Investigations of thickness induced percolation process in Ptξ-Al2O3 nano-composites were carried out using grazing angle X-ray scattering. It is shown that the morphological characteristics of the nano-scaled Pt particles change significantly within the growth transversal direction. The initially non percolated two phases Ptξ-Al2O3 nano-composites show a noteworthy tendency to a percolated morphology with a gradual transversal increase of Pt nano-particles' average diameter in addition to the increase of the Pt nano-particles' poly-dispersity. This transversal morphological evolution is followed by a percolation phenomenon of the Pt nano-particles for a thickness of about ∼119.5 nm. This transversal morphological evolution seems to corroborate far too much with a columnar growth process. 相似文献
959.
960.
Y Watanabe M Suda Y Matsumoto K Takayama M Matsumoto W Zhao 《Chemical & pharmaceutical bulletin》1991,39(9):2391-2394
The dissolution behavior of the aspirin enteric granule prepared using acylglycerols, glyceryl monostearate (GMS) and glyceryl trilaurate (GTL), was investigated in vitro and in human subjects in a fasting or non-fasting state. Aspirin was slowly released from the granule in vitro at pH 1.2. No acceleration of the aspirin dissolution rate in the medium without lipase and cholic acid was observed when the pH level of the medium increased to a neutral region (pH 6.4). However, the dissolution of aspirin was significantly increased by increasing the concentrations of lipase and cholic acid in the medium. Lipase appears to play an essential role in the dissolution process of aspirin granules. In human subjects, the average levels of the cumulative amount of total salicylate excreted in a urine-time curve, and the mean residence time (MRT) obtained after oral administration of a granule in the fasting state were markedly delayed in comparison with the results observed using an aqueous solution and a crystalline form of aspirin. In comparing the fasting condition with the non-fasting condition (after food ingestion), no significant difference was recognized in the total amount of salicylate excreted in urine to an infinite time (Ae(infinity)), whether the MRT was obtained by granule, crystalline form or aqueous solution. It can be concluded that aspirin granule prepared by GMS and GTL has a property of pancreatic lipase-sensitive dissolution, and its bioavailability is unaffected by food intake. 相似文献