全文获取类型
收费全文 | 50994篇 |
免费 | 7062篇 |
国内免费 | 5766篇 |
专业分类
化学 | 32363篇 |
晶体学 | 917篇 |
力学 | 3230篇 |
综合类 | 398篇 |
数学 | 5431篇 |
物理学 | 21483篇 |
出版年
2024年 | 79篇 |
2023年 | 683篇 |
2022年 | 1454篇 |
2021年 | 1438篇 |
2020年 | 1508篇 |
2019年 | 1591篇 |
2018年 | 1459篇 |
2017年 | 1471篇 |
2016年 | 1991篇 |
2015年 | 2079篇 |
2014年 | 2637篇 |
2013年 | 3577篇 |
2012年 | 4264篇 |
2011年 | 4636篇 |
2010年 | 3411篇 |
2009年 | 3455篇 |
2008年 | 3783篇 |
2007年 | 3296篇 |
2006年 | 3143篇 |
2005年 | 2525篇 |
2004年 | 1991篇 |
2003年 | 1625篇 |
2002年 | 1622篇 |
2001年 | 1812篇 |
2000年 | 1344篇 |
1999年 | 1112篇 |
1998年 | 763篇 |
1997年 | 736篇 |
1996年 | 609篇 |
1995年 | 499篇 |
1994年 | 494篇 |
1993年 | 382篇 |
1992年 | 349篇 |
1991年 | 298篇 |
1990年 | 282篇 |
1989年 | 207篇 |
1988年 | 177篇 |
1987年 | 148篇 |
1986年 | 141篇 |
1985年 | 124篇 |
1984年 | 81篇 |
1983年 | 65篇 |
1982年 | 40篇 |
1981年 | 42篇 |
1980年 | 26篇 |
1979年 | 35篇 |
1977年 | 29篇 |
1976年 | 32篇 |
1975年 | 33篇 |
1974年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
81.
82.
83.
We investigate the origin of ultraviolet (UV) emission from Mg0.12 Zn0.88 O alloy thin films with a wurtzite structure fabricated on c-plane Al2O3 substrates by plasma assisted molecular beam epitaxy. At room temperature, the absorption edge and UV emission band of the Mg0.12Zn0.88O film shift to high-energy side compared with ZnO films. Temperature dependence of the photoluminescence spectra shows that the UV emission is composed of free exciton and neutral donor bound exciton emissions. Two-step dissociation processes of the UV emission are observed with the increasing temperature. The thermal quenching mechanism is attributed to the dissociation of the free exciton from the neutral donor bound exciton in the low temperature region and the dissociation of free electron and hole from the free exciton in the high temperature region. 相似文献
84.
Mo2Ni3Si/NiSi metal silicide composite coatings with a fine microstructure consisting of Mo2Ni3Si primary dendrites and the interdendritic Mo2Ni3Si/NiSi eutectics were fabricated on austenitic stainless steel AISI 321 by laser cladding process. Small amplitude reciprocating sliding wear resistance of the coatings is evaluated as functions of normal load and slip amplitude and the wear mechanisms were discussed based on worn surface morphology observations. Results showed that the Mo2Ni3Si/NiSi coatings have excellent small amplitude reciprocating sliding wear resistance. 相似文献
85.
The concept of super hamiltonian semigroup is introduced. As a result, the structure theorems obtained by A. Cherubini and A. Varisco on quasi commutative semi-groups and quasi hamiltonian semigroups respectively are extended to super hamiltonian semigroups. 相似文献
86.
家用吊扇的核心部分是单相异步电动机,此绕组如图1,有一主绕组(工作绕组)与一串联电容器C的副绕组(启动绕组)相并联.这样做可使二绕组的电流在相位上相差90°,从而形成旋转磁场.转子的转动过程与三相异步电动机相似. 相似文献
87.
The title compound (C2H7NO·C7H4N2O6) has been obtained by the reaction of etha- nolamine with 3,5-dinitrobenzoic acid in deionized water at room temperature. The crystal crystal- lizes in orthorhombic, space group P212121 with a = 6.048(2), b = 9.146(3), c = 21.955(7)(A), C9H11N3O7, Mr = 273.21, Z = 4, V = 1214.3(7)(A)3, Dc = 1.494 g/cm3, F(000) = 568, μ(MoKα) = 0.131 mm-1, R1 = 0.0338 and wR2 = 0.0497. The new organic adduct is composed of one ethanolamine and one 3,5-dinitrobenzoic acid, which are linked up by O-H…O and N-H…O types of hydrogen bonds to form a nine-membered ring and an eleven-membered ring, extending into a one-dimensional network structure. 相似文献
88.
Pascher W. Den Besten J. H. Caprioli D. Leijtens X. Smit M. van Dijk R. 《Optical and Quantum Electronics》2003,35(4-5):453-464
Based on a rigorous vectorial analysis, a fast travelling-wave Mach–Zehnder modulator is modelled and designed. The cross-section of the semiconductor layer stack and the lossy electrodes are carefully modelled using the method of lines in order to investigate propagation characteristics, velocity and losses. This yields an accurate microwave and optical field distribution to explain the behaviour of the component. In order to enhance the modulation efficiency, design curves are derived and the cross-sectional dimensions for minimum microwave loss are determined. The loss of the optimized modulator agrees very well with small-signal measurements up to 40 GHz and HFSS simulations. The layerstack of the fabricated device is suitable for integration with InP multi-wavelength lasers. 相似文献
89.
双氯芬酸钠与牛血清白蛋白结合反应的特征 总被引:2,自引:0,他引:2
在模拟人体生理条件下,采用荧光光谱和紫外 可见吸收光谱法研究了双氯芬酸钠与牛血清白蛋白的结合反应.实验结果显示,双氯芬酸钠对牛血清白蛋白有较强的荧光猝灭作用,该猝灭过程主要为静态荧光猝灭过程;二者结合常数KLB=2.167×105mol·L-1;二者的结合位置距212位色氨酸2.13nm;同时,由结合反应的热力学参数得出二者作用力主要是氢键或VanderWall's力. 相似文献
90.
D. Liu N.Q. Ngo X.Y. Dong S.C. Tjin P. Shum 《Applied physics. B, Lasers and optics》2005,81(6):807-811
We propose and experimentally demonstrate a stable dual-wavelength erbium-doped polarization-maintaining (PM) fiber laser
with tunable wavelength spacing using an all-PM linear cavity that makes use of two reflection peaks from the PM fiber Bragg
grating (PM-FBG). Experimental results show stable dual lasing lines with a wavelength separation of ∼0.22 nm and a large
optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. By applying axial strain to the PM-FBG, the center
wavelengths of the two lasing lines can be tuned over several nanometers and the wavelength separation between the lasing
lines can also be tuned to as small as 0.05 nm, which, to our knowledge, is the smallest wavelength spacing ever obtained
from a stable room-temperature dual-wavelength fiber laser. The proposed laser configuration has the advantages of simple
structure, low loss, stable dual-wavelength operation and a very small lasing linewidth of ∼5 kHz .
PACS 42.55.Wd; 42.81.-i; 42.81.Gs 相似文献