全文获取类型
收费全文 | 3382篇 |
免费 | 181篇 |
国内免费 | 96篇 |
专业分类
化学 | 2108篇 |
晶体学 | 43篇 |
力学 | 162篇 |
综合类 | 10篇 |
数学 | 703篇 |
物理学 | 633篇 |
出版年
2023年 | 52篇 |
2022年 | 45篇 |
2021年 | 83篇 |
2020年 | 132篇 |
2019年 | 114篇 |
2018年 | 114篇 |
2017年 | 80篇 |
2016年 | 132篇 |
2015年 | 108篇 |
2014年 | 141篇 |
2013年 | 277篇 |
2012年 | 236篇 |
2011年 | 274篇 |
2010年 | 165篇 |
2009年 | 155篇 |
2008年 | 222篇 |
2007年 | 203篇 |
2006年 | 184篇 |
2005年 | 177篇 |
2004年 | 139篇 |
2003年 | 118篇 |
2002年 | 98篇 |
2001年 | 55篇 |
2000年 | 32篇 |
1999年 | 34篇 |
1998年 | 30篇 |
1997年 | 31篇 |
1996年 | 19篇 |
1995年 | 22篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1940年 | 2篇 |
排序方式: 共有3659条查询结果,搜索用时 15 毫秒
1.
Science China Chemistry - Secrecy has received tremendous attention in modern information society. Innovative polymer-based fluorescent materials with multiple mode emission are quite desirable to... 相似文献
2.
We investigate a diffusive, stage-structured epidemic model with the maturation delay and freelymoving delay. Choosing delays and diffusive rates as bifurcation parameters, the only possible way to destabilize the endemic equilibrium is through Hopf bifurcation. The normal forms of Hopf bifurcations on the center manifold are calculated, and explicit formulae determining the criticality of bifurcations are derived. There are two different kinds of stable oscillations near the first bifurcation: ... 相似文献
3.
Abdullah S. AlRamadan Moez Ben Houidi Julien Sotton Marc Bellenoue Bengt Johansson S. Mani Sarathy 《Proceedings of the Combustion Institute》2021,38(4):5539-5548
One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture. 相似文献
4.
Influences of nanoparticles and chain length on thermodynamic and electrical behavior of fluorine liquid crystals 下载免费PDF全文
Ines Ben Amor Lotfi Saadaoui Abdulaziz N. Alharbi Talal M. Althagafi and Taoufik Soltani 《中国物理 B》2022,31(10):104202-104202
Hydrogen-bonded polar nematic liquid crystal series with the general formula nOBAF (n = 7—12) is studied. The mesomorphic characterization is demonstrated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The complexes with short alkyl chains (n=7, 8) present a wide nematic range and monotropic smectic F mesophase, whereas the longer alkyl chain (n=10—12) analogues show high melting and low clearing mesomorphic liquid crystals. The thermal range of the mesophase and the birefringence increase with chain length decreasing. Furthermore, the effect of the nanoparticles (LiNbO3) on the thermal and the electrical behavior of 8OBAF are investigated. The presence of LiNbO3 nanoparticles increases the conductivity and reduces the resistivity of the complex. 相似文献
5.
Stability analysis of internally damped rotating composite shafts using a finite element formulation
Safa Ben Arab José Dias Rodrigues Slim Bouaziz Mohamed Haddar 《Comptes Rendus Mecanique》2018,346(4):291-307
This paper deals with the stability analysis of internally damped rotating composite shafts. An Euler–Bernoulli shaft finite element formulation based on Equivalent Single Layer Theory (ESLT), including the hysteretic internal damping of composite material and transverse shear effects, is introduced and then used to evaluate the influence of various parameters: stacking sequences, fiber orientations and bearing properties on natural frequencies, critical speeds, and instability thresholds. The obtained results are compared with those available in the literature using different theories. The agreement in the obtained results show that the developed Euler–Bernoulli finite element based on ESLT including hysteretic internal damping and shear transverse effects can be effectively used for the stability analysis of internally damped rotating composite shafts. Furthermore, the results revealed that rotor stability is sensitive to the laminate parameters and to the properties of the bearings. 相似文献
6.
Ben Jadi S. El Guerraf A. Bazzaoui E.A. Wang R. Martins J.I. Bazzaoui M. 《Journal of Solid State Electrochemistry》2019,23(8):2423-2433
Journal of Solid State Electrochemistry - Due to their distinctive chemical, electronic, and environmental properties, polypyrrole is used as a blocking barrier for methanol leakage in direct... 相似文献
7.
Marwa Chaabene Soumaya Agren Abdul‐Rahman Allouche Mohamed Lahcinie Rafik Ben Chabane Mohamed Hassen V. Baouab 《应用有机金属化学》2019,33(11)
The novel compounds (E)‐2‐(((4‐hydroxyphenyl)imino)methyl)phenol, Tetraphenyl (hydroxyl) imidazole and their corresponding Boron difluoride complexes were synthesized and characterized by spectroscopic techniques. Density functional theory calculations at B3LYP‐D3/6–311++G (d, p) level of theory were performed for the geometric parameters. The MEP surface studies were used to understand the behavior of molecules in terms of charge transfer and to determine how these molecules interact. We used the GIAO and the B3LYP‐D3 with a 6–311++ G (d, p) basis set to simulate the (1H‐NMR and 19F‐NMR) and the IR spectra, respectively. The corresponding calculated results are in good agreement with the experimental data. The stability of the molecule arising from hyperconjugation interaction and charge delocalization were analyzed using NBO analysis. FMOs revealed the occurrence of charge transfer within the molecule. The complexation using BF3.Et2O was also found to have remarkable effects on the electrochemical properties of the studied molecules, where (b) and (d) present lower chemical stability, higher reactivity and higher polarizability than (a) and (c), respectively. Moreover, the energy gap of (a) and (c) decreased after complexation using BF3.Et2O, indicating the reliability of the electrochemical evaluation of LUMO and HOMO energy levels. These values are the factors explaining the possible charge transfer interaction within the molecule. The absorption and emission spectra of the model compound were also simulated and compared to experimental observations in the DMF solvent. The results of DFT calculations supported the structural and spectroscopic data and confirmed the structure modification of frontier molecular orbitals for BF2 complexes as well as tunable potentials and energy levels. 相似文献
8.
Analysis Mathematica - We introduce and study the k-Hankel Gabor transform. We investigate the localization operators for this transform. In particular, we study their trace class properties and we... 相似文献
9.
10.