全文获取类型
收费全文 | 24051篇 |
免费 | 3120篇 |
国内免费 | 2319篇 |
专业分类
化学 | 12144篇 |
晶体学 | 203篇 |
力学 | 1185篇 |
综合类 | 68篇 |
数学 | 1803篇 |
物理学 | 6291篇 |
综合类 | 7796篇 |
出版年
2024年 | 94篇 |
2023年 | 448篇 |
2022年 | 632篇 |
2021年 | 749篇 |
2020年 | 725篇 |
2019年 | 644篇 |
2018年 | 619篇 |
2017年 | 582篇 |
2016年 | 979篇 |
2015年 | 922篇 |
2014年 | 1236篇 |
2013年 | 1435篇 |
2012年 | 1787篇 |
2011年 | 1780篇 |
2010年 | 1271篇 |
2009年 | 1256篇 |
2008年 | 1450篇 |
2007年 | 1277篇 |
2006年 | 1153篇 |
2005年 | 1041篇 |
2004年 | 798篇 |
2003年 | 656篇 |
2002年 | 672篇 |
2001年 | 620篇 |
2000年 | 570篇 |
1999年 | 723篇 |
1998年 | 654篇 |
1997年 | 610篇 |
1996年 | 621篇 |
1995年 | 583篇 |
1994年 | 487篇 |
1993年 | 403篇 |
1992年 | 386篇 |
1991年 | 324篇 |
1990年 | 308篇 |
1989年 | 254篇 |
1988年 | 207篇 |
1987年 | 154篇 |
1986年 | 104篇 |
1985年 | 70篇 |
1984年 | 44篇 |
1983年 | 28篇 |
1982年 | 29篇 |
1981年 | 25篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1974年 | 7篇 |
1971年 | 6篇 |
1968年 | 6篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
61.
高智民 《纯粹数学与应用数学》1994,10(2):1-5
本文证明了:若X是K-半分层空间,f是闭映射,则f(X)是K-半分层空间,这一结果推广改进了Lutzer与高国士的有关结果。 相似文献
62.
试验表明,“926”植物生长调节剂对植物种子活力、根系活力、植物生长发育具有明显的促进作用。用“926”浸稻种和玉米种,可提高发芽势、发芽率,增加次生根数和次生根长。苗期施药,同样可促进幼苗生长发育、促进根系发育、增加分蘖、分枝,提高成穗率。其它生育期施药,可分别提高成稳率、穗粒数、粒重,显著提高作物产量。 相似文献
63.
对T型管节点的疲劳试验进行分析,研究疲劳裂纹的扩展规律,用“柔度法”计算应力强度因子范围ΔK,确定材料常数c,m,提出并运用“直线-曲线模式法”估算管节点疲劳寿命,结果与试验吻合,具有一定的实用参考价值。 相似文献
64.
Observation of spontaneous pattern with six-fold symmetry in disk-shaped ZnO complex microstructures
L.W. Yang Y.J. Gao X.L. Wu Y.M. Yang G.S. Huang Z.Y. Zhang P.K. Chu 《Applied Physics A: Materials Science & Processing》2007,89(1):173-176
Using a simple vapor phase transport technique, we have fabricated unique complex disk-shaped ZnO microstructures comprising
a small disk coaxially grown on a large one and observed spatially perfect six-fold symmetric patterns. The observed results
can be explained based on the spontaneous nanoindentation (NI) effect under the geometric constraints and the explanation
can be extended to fathom the growth mechanism of other highly symmetrical ZnO nanostructures. Our results indicate that NI
not only can elucidate the mechanical properties of surfaces and thin films but also is an effective approach to fabricate
ordered nanostructures with high precision on the location of the building blocks.
PACS 81.16.Rf; 81.07.-b; 81.05.Dz 相似文献
65.
W. Gao M. Li R. Klie E.I. Altman 《Journal of Electron Spectroscopy and Related Phenomena》2006,150(2-3):136-149
Oxide catalysts are frequently used to convert toxic species to environmentally benign molecules, and to prevent the formation of toxic species in the first place. In this paper, growth and characterization of model oxide systems employed in both approaches is discussed. An example of the former approach is the selective catalytic reduction (SCR) of NO emitted from power plants by NH3, which employs tungsten and vanadium oxides supported on the anatase polymorph of TiO2. To model SCR catalysts, epitaxial titanium, vanadium and tungsten oxide films were grown using molecular beam epitaxy and magnetron sputtering. Two different anatase orientations were grown on LaAlO3 substrates and their interactions with vanadia were characterized. On LaAlO3 (0 0 1), anatase exposed a (4 × 1) reconstructed (0 0 1) surface. Vanadia lifted the reconstruction and at 1 ML a (1 × 1) surface with mostly V5+ was observed. Continued V2O5 growth led to loss of order, but at high temperatures epitaxial VO2 could be grown; vanadia behaved similarly on anatase films on LaAlO3 (1 1 0). Results suggested that the monolayer is pseudomorphic with O adsorption oxidizing the surface V to 5+, since the anatase structure cannot accommodate more bulk oxygen, only a monolayer can be pseudomorphic and have only V5+. Thus the vanadia monolayer has unique structural and chemical properties that can help explain why vanadia monolayers on TiO2 are much more active than bulk V2O5. For WO3, a series of added row reconstructions were observed as the epitaxial films were reduced. The effect of these structures on surface chemistry was characterized by studying 1-propanol adsorption. The results indicated that the structure of the WO3 surface did not alter its catalytic function but had a strong effect on reaction kinetics. As an example of a system where catalysts prevent the formation of toxic species, the reactivity of oxidized Pd surfaces used in CH4 catalytic combustion were studied. An ordered PdO-like monolayer was found to be less reactive towards CO than adsorbed O on Pd. On the other hand, the PdO layer favored a lower activation energy C3H6 oxidation pathway. The results indicated that Pd oxidation reduces the sticking coefficient of reactive species but once molecules adsorb, the oxide surface can reduce the activation energy for subsequent reaction. 相似文献
66.
67.
Hongrui Zhang Mingju Chao Mingyi Gao Liwen Zhang Jianquan Yao 《Optics & Laser Technology》2003,35(6):573-449
A fiber-coupled diode-single-end-pumped Nd:YVO4 laser with an Nd:YVO4 crystal of 0.3 at% doping concentration and 3×3×10 mm3 dimensions was reported. 14.850 W of continuous-wave output power in an M2 factor of 1.12 was obtained under pump power of 27.365 W, with an optical conversion efficiency of 60.49%, and a slope efficiency of 64.5%. 相似文献
68.
69.
70.
Controlling the gain shape of Er^3+—doped fluorozirconate fibre amplifier by a coherent field 下载免费PDF全文
We proposed a four-level system to explore the quantum interference effects on optical amplification.We found that the gain of the probe.including positions and values of gain peaks,can be adjusted by changing the coherent field and the incoherent pumping.At the same time,we can also modify the absorption profile of electromagnetically induced transparency by the incoherent pump.The results provide a method to flatten the gain of Erbium-doped fibre amplifiers. 相似文献