全文获取类型
收费全文 | 27967篇 |
免费 | 4711篇 |
国内免费 | 3492篇 |
专业分类
化学 | 19705篇 |
晶体学 | 317篇 |
力学 | 1904篇 |
综合类 | 288篇 |
数学 | 3206篇 |
物理学 | 10750篇 |
出版年
2024年 | 97篇 |
2023年 | 543篇 |
2022年 | 970篇 |
2021年 | 1069篇 |
2020年 | 1136篇 |
2019年 | 1104篇 |
2018年 | 967篇 |
2017年 | 855篇 |
2016年 | 1291篇 |
2015年 | 1431篇 |
2014年 | 1606篇 |
2013年 | 2018篇 |
2012年 | 2429篇 |
2011年 | 2666篇 |
2010年 | 1759篇 |
2009年 | 1606篇 |
2008年 | 1719篇 |
2007年 | 1578篇 |
2006年 | 1490篇 |
2005年 | 1258篇 |
2004年 | 1104篇 |
2003年 | 878篇 |
2002年 | 907篇 |
2001年 | 751篇 |
2000年 | 592篇 |
1999年 | 568篇 |
1998年 | 460篇 |
1997年 | 390篇 |
1996年 | 409篇 |
1995年 | 391篇 |
1994年 | 350篇 |
1993年 | 268篇 |
1992年 | 283篇 |
1991年 | 233篇 |
1990年 | 215篇 |
1989年 | 174篇 |
1988年 | 103篇 |
1987年 | 99篇 |
1986年 | 81篇 |
1985年 | 88篇 |
1984年 | 49篇 |
1983年 | 49篇 |
1982年 | 39篇 |
1981年 | 25篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1969年 | 4篇 |
1957年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
41.
本文使用变分方法证明了三维标准切触球的标准勒让德扭结,至少存在一个弦连结适应于标准切触结构的任何选定切触形式,这部分证明了著名的阿诺德弦猜想。 相似文献
42.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
43.
Tm3+/Yb3+-codoped germanate-niobic (GN) and germanium-bismuth (GB) glasses have been synthesized by conventional melting and quenching method. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions 1G4→3H6 and 1G4→3H4, respectively, were observed at room temperature. The possible up-conversion mechanisms are discussed and estimated. GN glass showed a weaker up-conversion emission than GB glass, which is inconsistent with the prediction from the difference of maximum phonon energy between GN and GB glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Compared with phonon side-band spectroscopy, Raman spectroscopy extracts more information including both phonon energy and phonon density. For the first time, our results reveal that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency. 相似文献
44.
45.
This paper delineates the first steps in a systematic quantitative study of the spacetime fluctuations induced by quantum
fields in an evaporating black hole. We explain how the stochastic gravity formalism can be a useful tool for that purpose
within a low-energy effective field theory approach to quantum gravity. As an explicit example we apply it to the study of
the spherically-symmetric sector of metric perturbations around an evaporating black hole background geometry. For macroscopic
black holes we find that those fluctuations grow and eventually become important when considering sufficiently long periods
of time (of the order of the evaporation time), but well before the Planckian regime is reached. In addition, the assumption
of a simple correlation between the fluctuations of the energy flux crossing the horizon and far from it, which was made in
earlier work on spherically-symmetric induced fluctuations, is carefully analyzed and found to be invalid. Our analysis suggests
the existence of an infinite amplitude for the fluctuations of the horizon as a three-dimensional hypersurface. We emphasize
the need for understanding and designing operational ways of probing quantum metric fluctuations near the horizon and extracting
physically meaningful information.
Dedicated to Rafael Sorkin on the occasion of his 60th birthday. 相似文献
46.
副品红共振光散射法测定脱氧核糖核酸 总被引:3,自引:0,他引:3
本文基于脱氧核糖核酸 (DNA)对有机染料副品红的共振光散射的增强效应 ,拟定了一种测定DNA的共振光散射法。在pH =0 5~ 1 5范围内 ,有机染料副品红于 35 5nm处的共振光散射强度被DNA强烈增强 ,且增强程度与DNA浓度呈线性关系 ,线性范围为 0 10~ 15 μg·mL- 1 ,检出限可达 36ng·mL- 1 。该方法简便、快速、具有较高的灵敏度和准确度 ,且线性范围较宽。将该方法用于混合样品中DNA的测定 ,结果令人满意。 相似文献
47.
在本文中引入了泛代数的二次扩张的概念,解决了TU-EC(A)和UT-EC(A)的存在、真类和基数问题,范畴TU-EC(A)(及UT-EC(A)),并得到了有关二次扩张的几个同构定理,还对一点二次扩张作了讨论。 相似文献
48.
Hua Jin Zhuangqi Hu Qingmin Liu Yunlong Ge Changxu Shi 《Applied Physics A: Materials Science & Processing》1992,54(5):399-403
A crystallization study has been carried out for rapidly solidified Bi2Pb0.5Sr2Ca4Cu5Ox glass. Glass transition temperature T
g, crystallized superconducting phases and microstructural changes were measured and analysed by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The crystallization mechanism of the three superconducting phases — (2201) 20 K phase, (2212) 80 K phase, and (2223) 110 K phase — has been discussed, and a time-temperature-transformation diagram for the glass has been constructed. 相似文献
49.
Traveling wave Ti:LiNbO3 Mach-Zehnder optical modulators with buried electrodes and etched grooves in the SiO2 buffer layer are analyzed by the finite element method. The tradeoff between the bandwidth BW and the half-wave voltage Vπ is discussed. The value of BW/Vπ is used to weight the total performance of the modulator. Taking a thick buffer layer and etching deep grooves in the buffer layer are demonstrated as two effective methods to improve the performance of the modulator. A 3-dB optical bandwidth of 18 GHz with half-wave voltage 5V at a wavelength of 1.55 pm could be obtained even though the electrode is not very thick. When the requirement of half-wave voltage is not very critical, a bandwidth of more than 100 GHz can be obtained. 相似文献
50.
生物小分子与铜锌超氧化物歧化酶相互作用的波谱学研究及其应用 总被引:1,自引:0,他引:1
本工作以ESR和NMR为主要手段,并结合其它生化方法,考察了氨基酸、核酸碱基、糖等生命基本物质,和抗坏血酸等生命必需物质与钢锌超氧化物歧化酶的相互作用,实验观测到氨基酸、核酸碱基和抗坏血酸在水溶液中可以与酶中的Cu2+作用而使其脱离活性部位,以小分子络合物形式游离在溶液中,同时使酶失活,脱离活性部位的Cu2+的比例和酶的失活程度取决于小分子配体的加入量及其与Cu2+的络合能力。此外,首次尝试使用ESR方法,并借助氨基酸与酶的作用,考察了铜锌超氧化物歧化酶在盐酸胍变性和热变性过程中的构象变化行为,结果表明这一方法是直观而有效的。 相似文献