全文获取类型
收费全文 | 3378篇 |
免费 | 762篇 |
国内免费 | 184篇 |
专业分类
化学 | 761篇 |
晶体学 | 25篇 |
力学 | 327篇 |
综合类 | 29篇 |
数学 | 261篇 |
物理学 | 2921篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 97篇 |
2021年 | 85篇 |
2020年 | 97篇 |
2019年 | 75篇 |
2018年 | 77篇 |
2017年 | 127篇 |
2016年 | 117篇 |
2015年 | 107篇 |
2014年 | 213篇 |
2013年 | 256篇 |
2012年 | 194篇 |
2011年 | 243篇 |
2010年 | 198篇 |
2009年 | 219篇 |
2008年 | 241篇 |
2007年 | 238篇 |
2006年 | 231篇 |
2005年 | 164篇 |
2004年 | 184篇 |
2003年 | 154篇 |
2002年 | 130篇 |
2001年 | 116篇 |
2000年 | 117篇 |
1999年 | 101篇 |
1998年 | 91篇 |
1997年 | 74篇 |
1996年 | 54篇 |
1995年 | 54篇 |
1994年 | 32篇 |
1993年 | 32篇 |
1992年 | 37篇 |
1991年 | 19篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 16篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有4324条查询结果,搜索用时 15 毫秒
1.
The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by 1H- and 13C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C2 symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a Cs transition state with a calculated relative free energy of 11.0 kcal mol−1. The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol−1 at 298 K. 相似文献
2.
We proposed an electro-optic modulator with two-bus one-ring (TBOR) structure to improve the extinction ratio and reduce insert loss. It has a dual output compared with one-bus one-ring structure. In addition, double-layer graphene makes it possible for the modulation in the visible to mid-infrared wavelength range. It shows that this new electro-optic modulator can present two switching states well with low insertion loss, high absorption and high extinction ratio. At , when the switching states are based on the chemical potential, and , the insertion losses of both output ports are less than 2 dB, the absorption of the output port coupled via a micro-ring reaches 45 dB and the extinction ratio reaches 14 dB. When the refractive index of the dielectric material is 4.2, the applied voltage will be less than 1.2 V, thus can be used in low-voltage CMOS technology. 相似文献
3.
用密耦方法计算了非对称同位素替代分子与氦原子碰撞(He-HD,HT,DT)转动激发,当入射能量E=0.3 eV时,得到了上述碰撞体系的微分散射截面或角分布.基于上述理论计算,讨论了原子与双原子分子碰撞的同位素效应. 相似文献
4.
In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement. 相似文献
5.
气垫导轨在刚体转动惯量测定实验中的应用 总被引:1,自引:0,他引:1
本文介绍了一种测量刚体转动惯量的新方法。即把气垫导轨和刚体转动仪结合起来来测量刚体的转动惯量,使传统的气垫导轨实验和刚体转动惯量测定实验得以更新,提高了实验测量的精度,拓展了实验内容。 相似文献
6.
J.R. Cooper 《Journal of Molecular Spectroscopy》2006,239(1):51-58
The ν9 fundamental band of ethane occurs in the 12 μm region. It is the strongest band of ethane in a terrestrial window and is commonly used for the identification of ethane in the Jovian planets. The ν9 + ν4 − ν4 band occurs in the same region; neither can be analysed as an isolated band, since both are embedded in the torsional bath of the ground vibrational state. We report here two global fit models including data from both of these bands as well as the ν3 fundamental and the ν4, 2ν4 − ν4, and 3ν4 torsional transitions. The first is restricted to −5 ? KΔK ? 15 in the hot band and gives an excellent fit to the included data. Three resonant interactions are identified in this fit—a Coriolis interaction with two resonant cases between the ν9 torsional stack and that of the ground vibrational state (gs) and a resonant Fermi interaction between the ν3 fundamental and the gs. Hot band lines with KΔK < −5 are influenced by a fourth perturbation, with a crossing at −11 < KΔK < −10, which has been attributed to an interaction with the ν12 fundamental. A second fit, demonstrating a promising treatment of this interaction, is also presented. 相似文献
7.
8.
Zhang Xinliang Huang Dexiu Sun Junqiang Liu Deming 《Optical and Quantum Electronics》2004,36(7):627-634
A simple scheme for single to multi-channel wavelength conversion based on cross-gain modulation of amplified spontaneous emission (ASE) spectrum in semiconductor optical amplifier (SOA) is described. Single to 16-channel wavelength conversion at 10 Gb/s is first demonstrated without any additional probe lights, the modulation information carried by input signal could be converted into arbitrary many channels if only the demultiplexer with enough channels is exploited. Output performance and pattern effects are investigated experimentally. 相似文献
9.
R. Bairava Ganesh Hitoshi Matsuo Takahiro Kawamura Yoshihiro Kangawa Koji Arafune Yoshio Ohshita Masafumi Yamaguchi Koichi Kakimoto 《Journal of Crystal Growth》2008,310(11):2697-2701
Multicrystalline silicon was grown by unidirectional solidification method using the accelerated crucible rotation technique. The application of the accelerated crucible rotation technique in unidirectional solidification method induced growth striations across the axial direction of the grown crystal. This striation pattern was observed from carbon concentration distribution, obtained by using Fourier transform infrared spectroscopy. The generated striation pattern was found to be weak and discontinuous. Some striations were absent, probably due to back melting, caused during each crucible rotation. From the growth striations and applied time period in crucible rotation, the growth rate was estimated by using Fourier transformation analysis. 相似文献
10.
Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in H NMR in vivo
Leif Schrder Christian Schmitz Peter Bachert 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):69
Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (An system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this “pseudo-strong” system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time. 相似文献