全文获取类型
收费全文 | 437315篇 |
免费 | 3836篇 |
国内免费 | 1323篇 |
专业分类
化学 | 212322篇 |
晶体学 | 6204篇 |
力学 | 15545篇 |
综合类 | 8篇 |
数学 | 40354篇 |
物理学 | 115424篇 |
无线电 | 52617篇 |
出版年
2019年 | 3622篇 |
2018年 | 4725篇 |
2017年 | 4532篇 |
2016年 | 6808篇 |
2015年 | 4155篇 |
2014年 | 6506篇 |
2013年 | 17245篇 |
2012年 | 12228篇 |
2011年 | 15074篇 |
2010年 | 10968篇 |
2009年 | 11313篇 |
2008年 | 15162篇 |
2007年 | 15810篇 |
2006年 | 15022篇 |
2005年 | 13613篇 |
2004年 | 12440篇 |
2003年 | 11424篇 |
2002年 | 11189篇 |
2001年 | 12880篇 |
2000年 | 10446篇 |
1999年 | 8264篇 |
1998年 | 7105篇 |
1997年 | 7158篇 |
1996年 | 6777篇 |
1995年 | 6007篇 |
1994年 | 6036篇 |
1993年 | 5857篇 |
1992年 | 6437篇 |
1991年 | 6459篇 |
1990年 | 6149篇 |
1989年 | 6121篇 |
1988年 | 5830篇 |
1987年 | 5263篇 |
1986年 | 5005篇 |
1985年 | 6521篇 |
1984年 | 6612篇 |
1983年 | 5498篇 |
1982年 | 5831篇 |
1981年 | 5613篇 |
1980年 | 5318篇 |
1979年 | 5643篇 |
1978年 | 5924篇 |
1977年 | 5781篇 |
1976年 | 5667篇 |
1975年 | 5343篇 |
1974年 | 5313篇 |
1973年 | 5419篇 |
1972年 | 3691篇 |
1968年 | 3550篇 |
1967年 | 3928篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
71.
V. N. Jmerik A. M. Mizerov T. V. Shubina A. V. Sakharov A. A. Sitnikova P. S. Kop’ev S. V. Ivanov E. V. Lutsenko A. V. Danilchyk N. V. Rzheutskii G. P. Yablonskii 《Semiconductors》2008,42(12):1420-1426
Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the Al x Ga1 ? x N/Al y Ga1 ? y N quantum wells. Structural and optical properties of the Al x Ga1 ? x N layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al2O3 substrates, heterostructures for light-emitting diodes with Al x Ga1 ? x N/Al y Ga1 ? y N quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm. 相似文献
72.
73.
74.
75.
A one-dimensional bulk reaction model for the oxidation of nickeltitanium is formulated, with preferential oxidation of titaniumbeing included. The modelling is directed at the better understandingof the dominant mechanisms involved in the oxidation processand their significance for the biocompatibility of the alloy.Two different regimes for the relative diffusivities of oxygenand the metals are investigated. By assuming fast bulk reactions,different asymptotic structures emerge in different parameterregimes and the resulting models take the form of moving boundaryproblems. Different profiles of nickel concentration are obtained:in particular a nickel-rich layer (observed in practice) ispresent below the oxide/metal interface for the case when oxygenand the metals diffuse at comparable rates. 相似文献
76.
D. Amutha Rani Y. Yamamoto S. Mohri M. Sivakumar Y. Tsujita H. Yoshimizu 《Journal of Polymer Science.Polymer Physics》2003,41(3):269-273
In this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003 相似文献
77.
The optical properties of bismuth telluride crystals doped with donor-and acceptor-type impurities are studied. The fact that energy corresponding to the resonance frequency of plasma oscillations of free charge carriers (plasmons) approaches the band-gap energy is detected in the infrared spectral region, where the main elementary excitations in the electronic system of these materials are observed. The mentioned approach of energies varies the intensity of electron-plasmon interaction, which affects the recombination processes in the materials widely used for the fabrication of thermoelectric energy converters. 相似文献
78.
We present closed-form expressions for the average bit error probability (ABEP) of BPSK, QPSK and M-QAM of an amplify-and-forward average power scaling dual-hop relay transmission, over non-identical Nakagami-m fading channels, with integer values of m. Additionally, we evaluate in closed-form the ABEP under sufficiently large signal-to-noise ratio for the source-relay link, valid for arbitrary rn. Numerical and simulation results show the validity of the proposed mathematical analysis and point out the effect of the two hops unbalanced fading conditions on the error performance. 相似文献
79.
M. M. Zverev N. A. Gamov D. V. Peregoudov V. B. Studionov E. V. Zdanova I. V. Sedova S. V. Gronin S. V. Sorokin S. V. Ivanov P. S. Kop’ev 《Semiconductors》2008,42(12):1440-1444
Emission characteristics of an electron-beam-pumped Cd(Zn)Se/ZnMgSSe semiconductor laser are studied. The laser’s active region consists of a set of ten equidistant ZnSe quantum wells containing fractional-monolayer CdSe quantum-dot inserts and a waveguide formed by a short-period superlattice with the net thickness of ~0.65 μm. Lasing occurs at room temperature at a wavelength of 542 nm. Pulsed power as high as 12 W per cavity face and an unprecedentedly high efficiency of ~8.5% are attained for the electron-beam energy of 23 keV. 相似文献
80.
S. K. Chen A. Serquis G. Serrano K. A. Yates M. G. Blamire D. Guthrie J. Cooper H. Wang S. Margadonna J. L. MacManus‐Driscoll 《Advanced functional materials》2008,18(1):113-120
By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in MgxB2 are found. The non‐stoichiometry produces enhanced in‐field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ~ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ~ 17 T (4.2 K) for near‐stoichiometric samples. 相似文献