全文获取类型
收费全文 | 3799篇 |
免费 | 464篇 |
国内免费 | 478篇 |
专业分类
化学 | 3170篇 |
晶体学 | 49篇 |
力学 | 173篇 |
综合类 | 20篇 |
数学 | 448篇 |
物理学 | 881篇 |
出版年
2024年 | 1篇 |
2023年 | 58篇 |
2022年 | 67篇 |
2021年 | 88篇 |
2020年 | 136篇 |
2019年 | 139篇 |
2018年 | 117篇 |
2017年 | 80篇 |
2016年 | 164篇 |
2015年 | 133篇 |
2014年 | 202篇 |
2013年 | 252篇 |
2012年 | 337篇 |
2011年 | 327篇 |
2010年 | 218篇 |
2009年 | 194篇 |
2008年 | 225篇 |
2007年 | 213篇 |
2006年 | 207篇 |
2005年 | 179篇 |
2004年 | 152篇 |
2003年 | 155篇 |
2002年 | 161篇 |
2001年 | 134篇 |
2000年 | 103篇 |
1999年 | 130篇 |
1998年 | 79篇 |
1997年 | 62篇 |
1996年 | 89篇 |
1995年 | 70篇 |
1994年 | 35篇 |
1993年 | 45篇 |
1992年 | 28篇 |
1991年 | 31篇 |
1990年 | 30篇 |
1989年 | 22篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1970年 | 1篇 |
1963年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有4741条查询结果,搜索用时 31 毫秒
991.
Wang Y Cheng P Song Y Liao DZ Yan SP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(29):8131-8138
By using tridentate ligand 4-(3-pyridinyl)-1,2,4-triazole (pytrz), cage-like complexes of {[Cu(mu2-pytrz)2](ClO4)(SO4)0.5C2H5OH.0.25 H2O}6 (1), {[Cu3(mu3-pytrz)4(mu2-Cl)2(H2O)2](ClO4)2Cl(2).2 H2O}n (2), and {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)2.5(BF4)(1.5)5.25 H2O}n (3) have been synthesized with different copper(II) salts. Complex 1 represents the second example of a M6L12 metal-organic octahedron with an overall Th symmetry. Complex 2 is constructed from a 3(8) cage-building unit (CBU) and each CBU connects six neighboring cages to give the first 3D metal-organic framework (MOF) based on octahedral M6L12. Complex 3 is built from Cu24(pytrz)12 CBUs with the trinuclear copper clusters serving as second building units (SBUs) and decorating each corner of the M24L12 polyhedron. The Cu24(pytrz)12 building unit is linked by extra ligands to give an extended 3D framework that has the formula Cu24(pytrz)24 and possesses a CaB6 topology. The mixed anions ClO4- and BF4- in 3 are both included in the inner cavity of the cage and can be completely exchanged by ClO4- through the open windows of the cage, as evidenced by the crystal structure of the 3D MOF {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)(4)4.5 H2O}n (4). Complex 4 can also be synthesized when employing 1 as a precursor in an extensive study of the anion-exchange reaction. This represents the first successful conversion of a discrete cage into a 3D coordination network based on a cage structure. Complex 2 remains invariable during anion-exchange reactions because uncoordinated Cl- ions are located in the comparatively small inner cavity. 相似文献
992.
聚合物摩擦系数的周期性降低现象 总被引:5,自引:0,他引:5
考察了聚醚醚酮(PEEK),低密度聚乙烯(LDPE)及20%CF增强PEEK等3种聚合物材料的摩擦系数的变化,发现其存在周期性降低现象,低熔点的LDPE的磨损量也呈现周期性的平台现象,发生此类现象的原因在于摩擦热作用下聚合物摩擦表面由玻璃态转变为粘流态而造成其软化及流动。 相似文献
993.
Zhongbo Yang Lei Zhang Hongjun You Zhiyuan Li Jixiang Fang 《Particle & Particle Systems Characterization》2014,31(3):390-397
In this study, we demonstrate an easy particle‐mediated protocol using the specific structure of mesocrystal Ag2O sacrificial templates to synthesize highly rough‐cubic Ag mesocages. To the best of our knowledge, the mesocrystal particles are reported for the first time as sacrificial templates for synthesizing metal particles. The obtained Ag mesocages show high surface‐enhanced Raman scattering (SERS) sensitivity because of the highly rough topography formed by arrays of uniform individual Ag nanoparticles. Abundant “hot spots” with greatly enhanced local electromagnetic field are promoted densely on the mesocage surface by the plenty of deep and narrow gaps and the hollow structure. The single‐particle SERS signal generated by the Ag mesocage has an enhancement factor of approximately 109, which is approximately four times higher than the Ag mesocage synthesized using single‐crystal Ag2O particle as a template. Meanwhile, this signal displays a linear dependence on the detected analyte concentration, sensitively down to 1.0 × 10?12 m . 相似文献
994.
In general, the design and analysis of ultrasonic motor and motor’s control strategy are based on mathematical model. The academic model is widely used in the analysis of traveling wave ultrasonic motor (TWUSM). But the dispersive characteristic of piezoelectric ceramics and other complicated process, such as the friction, make the model’s precision not so accurate. On the other hand, identification modeling method, which is built based on the tested data, has obtained increasing application in the study of ultrasonic motor’s control technology. Based on the identification model, many control strategies can be designed easily. But the identification model is an approximate model, so if a more accurate model of ultrasonic motor can be obtained, the analysis and design of motor control system will be more effective. Characteristic model is a kind of identification model which can accurately describe the characteristics of TWUSM. Based on the tested data, this paper proposes the modeling method of ultrasonic motor’s characteristic model. The paper also makes a comparison of the effectiveness of different identification algorithms. Aiming at the speed control of ultrasonic motor, the influence of the parameter’s initial values on the precision of model is discussed. The calculating results indicate the availability of this characteristic model. 相似文献
995.
Pair Production of the Doubly Charged Leptons via Electroweak Vector Boson Fusion at the Large Hadron Collider 下载免费PDF全文
Pair production of the doubly charged leptons X±± via vector boson fusion (γγ, Zγ and WW) at the large hadron collider is studied. Our numerical results show that the cross section via Zγ fusion is very small. For the center-of-mass energy √s= 14 TeV and Mx = 200 650 GeV, the values of the cross sections σγγ and σww are in the ranges of 5.2 fb-0.04 fb and 20.2 fb 1.2 fb, respectively. 相似文献
996.
997.
实验研究了一种基于谐波拟合产生周期性三角形光脉冲串的方法,方案首先利用马赫曾德调制器的载波抑制调制,获得具有周期性起伏的连续光强度信号,然后利用光纤色散所致的射频功率衰落效应,对光强度表达式中四次谐波分量进行抑制,调节调制深度后,光强度表达式将向理想三角形傅里叶展开式的前三项进行逼近,最后以谐波拟合的方式获得重复频率为射频调制频率二倍的周期性三角形光脉冲串.结合实验,在9.862 GHz和7.678 GHz射频调制频率下,获得了脉冲重复频率19.724 Gb/s(脉冲全宽约50.7 ps)和15.356Gb/s(脉冲全宽约65.1 ps)的稳定三角形光脉冲串输出,改变色散量并反向调节调制频率,可进一步改变脉冲的重复频率,所获得的实验结果与理论预期基本符合. 相似文献
998.
In this study, the binding mode of nobiletin (NOB) with pepsin was investigated by spectroscopic and molecular docking methods. NOB can interact with pepsin to form a NOB-pepsin complex. The binding constant, number of binding sites and thermodynamic parameters were measured, which indicated that NOB could spontaneously bind with pepsin through hydrophobic and electrostatic forces with one binding site. Molecular docking results revealed that NOB bound into the pepsin cavity. Synchronous and three-dimensional fluorescence spectra results provide data concerning conformational and some micro-environmental changes of pepsin. Furthermore, the binding of NOB can inhibit pepsin activity in vitro. The present study provides direct evidence at a molecular level to show that NOB could induce changes in the enzyme pepsin structure and function. 相似文献
999.
Based on Gaussian decomposition method, we present a theoretical investigation on the astigmatic Gaussian beam Z-scan when materials possess the simultaneous third- and fifth-order nonlinear refraction effects, and obtain an analytic expression of normalized transmittance of the Z-scan trace for the case of pinhole. The influences of ellipticity and waist separation on the Z-scan curves are analyzed. We find that normalized transmittance difference between the peak and the valley in Z-scan curves decreases gradually with the increment of the waist separation or with the decrement of the ellipticity. 相似文献