全文获取类型
收费全文 | 1380篇 |
免费 | 72篇 |
国内免费 | 26篇 |
专业分类
化学 | 1028篇 |
晶体学 | 5篇 |
力学 | 72篇 |
数学 | 154篇 |
物理学 | 219篇 |
出版年
2024年 | 4篇 |
2023年 | 9篇 |
2022年 | 40篇 |
2021年 | 61篇 |
2020年 | 83篇 |
2019年 | 69篇 |
2018年 | 113篇 |
2017年 | 63篇 |
2016年 | 111篇 |
2015年 | 83篇 |
2014年 | 92篇 |
2013年 | 187篇 |
2012年 | 103篇 |
2011年 | 116篇 |
2010年 | 72篇 |
2009年 | 74篇 |
2008年 | 61篇 |
2007年 | 41篇 |
2006年 | 20篇 |
2005年 | 22篇 |
2004年 | 7篇 |
2003年 | 14篇 |
2002年 | 10篇 |
2001年 | 4篇 |
1999年 | 4篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有1478条查询结果,搜索用时 15 毫秒
241.
In this paper, a direct adaptive fuzzy controller with compensation signal is presented to control and stabilize a class of fractional order systems with unknown nonlinearities. Based on a Lyapunov function candidate the global Mittag–Leffler stability is proved and a new fractional order adaptation law is derived. The adaptation law adjusts free parameters of the fuzzy controller and bounds them by utilizing a novel fractional order projection algorithm. Furthermore, due to the use of compensation term, the proposed approach does not demand suitable membership functions in the fuzzy system. In addition, the stability of the closed-loop system is guaranteed by utilizing a supervisory controller. Numerical simulations show the validity and effectiveness of the introduced scheme for various fractional order nonlinear models that perturbed by disturbance and uncertainty. 相似文献
242.
In this paper we report a time-delayed chameleon-like chaotic system which can belong to different families of chaotic attractors depending on the choices of parameters. Such a characteristic of self-excited and hidden chaotic flows in a simple 3D system with time delay has not been reported earlier. Dynamic analysis of the proposed time-delayed systems are analysed in time-delay space and parameter space. A novel adaptive modified functional projective lag synchronization algorithm is derived for synchronizing identical time-delayed chameleon systems with uncertain parameters. The proposed time-delayed systems and the synchronization algorithm with controllers and parameter estimates are then implemented in FPGA using hardware–software co-simulation and the results are presented. 相似文献
243.
Scaling behavior of earthquakes’ inter-events time series 总被引:1,自引:0,他引:1
Shahriar Shadkhoo Fakhteh Ghanbarnejad Gholam R. Jafari Mohammad R. R. Tabar 《Central European Journal of Physics》2009,7(3):620-623
In this paper, we investigate the statistical and scaling properties of the California earthquakes’ inter-events over a period
of the recent 40 years. To detect long-term correlations behavior, we apply detrended fluctuation analysis (DFA), which can
systematically detect and overcome nonstationarities in the data set at all time scales. We calculate for various earthquakes
with magnitudes larger than a given M. The results indicate that the Hurst exponent decreases with increasing M; characterized
by a Hurst exponent, which is given by, H = 0:34 + 1:53/M, indicating that for events with very large magnitudes M, the Hurst exponent decreases to 0:50, which is for independent
events.
相似文献
244.
Shadkhoo S. Jafari G. R. 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,72(4):679-683
The European Physical Journal B - We use Multi-Fractal Detrended Cross-Correlation Analysis (MF-DXA) method to investigate the cross-correlation of temporal and spatial inter-events seismic data,... 相似文献
245.
Seyfollah Toroghi Ahmad Khayat Jafari Ala Hashemi Golpayegani 《Optics & Laser Technology》2009,41(6):800-803
In this paper, the dependence of the absorbed power on the temperature in an end-pumped CW quasi-three-level Yb:YAG thin disk laser is calculated. Here, we have used the temperature-dependent form of the Boltzmann occupation factors, absorption cross-section and thermal conductivity of the Yb:YAG crystal. A Monte Carlo ray tracing code and a 2D finite element analysis (FEA) with the ANSYS package have been used to calculate the absorbed power and the temperature distribution inside the Yb:YAG thin disk laser, respectively. According to the model, the temperature-dependent absorbed power turns out to be 18% less than the temperature-independent absorbed power in the top center of the Yb:YAG thin disk laser for used parameters. 相似文献
246.
The present study reports the preparation and characterization of PbO nanocrystals obtained via a thermal decomposition route. The PbO nanocrystals were synthesized using lead oxalate powder as a precursor. Nanostructured products were investigated by means of XRD, TEM, FT-IR and XPS. The XRD results indicate that tetragonal phase β-PbO with a particle size of about 30–45 nm was obtained when the intermediate precipitate was calcined at 500 °C. 相似文献
247.
Heshmat Alinezhad Mahmood Tajbakhsh Fatemeh Salehian Kazem Fazli 《Tetrahedron letters》2009,50(6):659-6607
A simple and convenient procedure for reductive amination of aldehydes and ketones using N-methylpyrrolidine zinc borohydride (ZBHNMP) as a reducing agent is described. The reactions are carried out with 1 equiv of amine and 1 equiv of aldehyde or ketone using 1 equiv of ZBHNMP in methanol under neutral conditions at room temperature. 相似文献
248.
BALALAIE Saeed CHADEGANI Fatemeh DARVICHE Fatemeh BIJANZADEH Harnid Reza 《中国化学》2009,27(10):1953-1956
A new and efficient method for the synthesis of 1,8‐dioxo‐9‐aryl‐decahydroacridine derivatives was developed via a one‐pot three component reaction of dimedone, aromatic aldehydes and ammonium acetate in the presence of ammonium chloride, or Zn(OAc)2·2H2O or L‐proline separately in water in the short period of time and high yields. 相似文献
249.
HERAVI Majid M. BEHESHTIA Yahya S. KHORSHIDI Maliheh BAGHERNEJAD Bita BAMOHARRAM Fatemeh F. 《中国化学》2009,27(3):569-572
A simple, clean and environmentally benign route to the synthesis of 3-cyanopyridines is described via a one-pot multi-component reaction of 3,4-dimethoxyacetophenone, malonitrile or ethylcyanoacetate, aldehyde and ammonium acetate using heteropolyacids as heterogeneous and recyclable catalysts in very good yields. 相似文献
250.
Babak Mokhtarani Ali Sharifi Hamid Reza Mortaheb Mojtaba Mirzaei Morteza Mafi Fatemeh Sadeghian 《The Journal of chemical thermodynamics》2009,41(12):1432-1438
Densities and viscosities of 1-butyl-3-methylimidazolium nitrate [Bmim][NO3], and its binaries with alcohol (ethanol, 1-propanol, or 1-butanol) were measured at different temperatures. The densities and viscosities of pure ionic liquid were correlated successfully by empirical equations. The Vogel–Fulcher–Tammann equations can fit the experimental data of viscosities for pure IL. Excess molar volume and viscosity deviations were calculated for the binaries. The excess molar volumes have negative deviations from the ideal solution. 相似文献