全文获取类型
收费全文 | 175篇 |
免费 | 9篇 |
国内免费 | 5篇 |
专业分类
化学 | 112篇 |
晶体学 | 2篇 |
力学 | 15篇 |
数学 | 21篇 |
物理学 | 39篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 8篇 |
2018年 | 13篇 |
2017年 | 6篇 |
2016年 | 21篇 |
2015年 | 10篇 |
2014年 | 11篇 |
2013年 | 10篇 |
2012年 | 9篇 |
2011年 | 17篇 |
2010年 | 13篇 |
2009年 | 8篇 |
2008年 | 14篇 |
2007年 | 6篇 |
2006年 | 7篇 |
2005年 | 7篇 |
2003年 | 1篇 |
2002年 | 4篇 |
1999年 | 3篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有189条查询结果,搜索用时 570 毫秒
41.
Farzad Didehvar 《Mathematical Logic Quarterly》1999,45(4):467-470
We define a class of so-called ∑(n)-sets as a natural closure of recursively enumerable sets Wn under the relation “∈” and study its properties. 相似文献
42.
Saeed Montazeri Moghadam Maryam Sadeghi Talarposhti Ali Niaty Farzad Towhidkhah Sajad Jafari 《Nonlinear dynamics》2018,93(3):1183-1199
Recent findings on the dynamical analysis of human locomotion characteristics such as stride length signal have shown that this process is intrinsically a chaotic behavior. The passive walking has been defined as walking down a shallow slope without using any muscular contraction as an active controller. Based on this definition, some knee-less models have been proposed to present the simplest possible models of human gait. To maintain stability, these simple passive models are compelled to show a wide range of different dynamics from order to chaos. Unfortunately, based on simplifications, for many years the cyclic period-one behavior of these models has been considered as the only stable response. This assumption is not in line with the findings about the nature of walking. Thus, this paper proposes a novel model to demonstrate that the knee-less passive dynamic models also have the ability to model the chaotic behavior of human locomotion with some modifications. The presented novel model can show chaotic behavior as a stable and acceptable answer using a chaotic function in heel-strike condition. The represented chaotic model is also able to simulate different types of motor deficits such as Parkinson’s disease only by manipulating the value of chaotic parameter. Our model has extensively examined in complexity and chaotic behavior using different analytical methods such as fractal dimension, bifurcation and largest Lyapunov exponent, and it was compared with conventional passive models and the stride signal of healthy subjects and Parkinson patients. 相似文献
43.
This article deals with the wave propagation analysis of single/double layered functionally graded (FG) size-dependent nanobeams in elastic medium and subjected to a longitudinal magnetic field employing nonlocal elasticity theory. Material properties of nanobeam change gradually according to the sigmoid function. Applying an analytical solution, the acoustical and optical dispersion relations are explored for various wave number, nonlocality parameter, material composition, elastic foundation constants, and magnetic field intensity. It is found that frequency and phase velocity of waves propagating in S-FGM nanobeam are significantly affected by these parameters. Also, presence of cut-off and escape frequencies in wave propagation analysis of embedded S-FGM nanobeams is investigated. 相似文献
44.
Wentao Fan Farzad Fathizadeh Matilde Marcolli 《Letters in Mathematical Physics》2018,108(12):2729-2747
We show that, when considering the anisotropic scaling factors and their derivatives as affine variables, the coefficients of the heat-kernel expansion of the Dirac–Laplacian on SU(2) Bianchi IX metrics are algebro-geometric periods of motives of complements in affine spaces of unions of quadrics and hyperplanes. We show that the motives are mixed Tate and we provide an explicit computation of their Grothendieck classes. 相似文献
45.
Farzad Tasbihgoo 《International Journal of Non》2007,42(1):99-117
A general procedure is presented for developing data-based, non-parametric models of non-linear multi-degree-of-freedom, non-conservative, dissipative systems. Two broad classes of methods are discussed: one relying on the representation of the system restoring forces in a polynomial-basis format, and the other using artificial neural networks to map the complex transformations relating the system state variables to the needed system outputs. A non-linear two-degree-of-freedom system is used to formulate the approach under discussion and to generate synthetic data for calibrating the efficiency of the two methods in capturing complex non-linear phenomena (such as dry friction, hysteresis, dead-space non-linearities, and polynomial-type non-linearities) that are widely encountered in the applied mechanics field. Subsequently, a reconfigurable test apparatus was used to generate experimental measurements from a physical non-linear “joint” involving two-dimensional motion (translation and rotation) and complicated interaction forces between the different motion axes, among its internal elements. Both the polynomial-basis approach and the neural network method were used to develop high-fidelity, non-parametric models of the physical test article. The ability of the identified models to accurately “generalize” the essential features of the non-linear system was verified by comparing the predictions of the models with experimental measurements from data sets corresponding to different excitations than those used for identification purposes. It is shown that the identification techniques under discussion can be useful tools for developing accurate simulation models of complex multi-dimensional non-linear systems under broadband excitation. 相似文献
46.
Farzad Iranzad Mohammad Gheibi 《International journal of environmental analytical chemistry》2018,98(1):16-30
In this research, magnetic Fe3O4 nanoparticles were synthesised by co-precipitation method and modified with polythiophene (PT) to produce Fe3O4-PT nanoparticles for preconcentration and determination of cadmium (??) ion followed by electrothermal atomic absorption spectrometry. The results of FT-IR spectroscopy, EDX analysis and SEM images show that Fe3O4-PT nanoparticles were synthesised successfully. Different parameters such as sample pH, amounts of adsorbent, sample volume, extraction time, type and concentration of eluent and desorption time were completely investigated and optimum conditions were selected.Under the optimum conditions, the calibration curve was linear in the range of 0.01–0.25 µg L?1 of cadmium (??). The relative standard deviation was 4.7% (n = 7, 0.10 µg L?1 Cd2+) and limit of detection was 3.30 ng L?1. The accuracy of the proposed method was verified by the analysis of a certified reference material and spike method. Finally, the proposed method was applied for the determination of ultra-trace levels of cadmium (??) in different water and food samples. 相似文献
47.
In this study, the effect of Ru and Gd promoters on 15Co/\(\gamma\)-Al2O3 catalyst in the Fischer–Tropsch synthesis is investigated. The catalysts were synthesized by dry impregnation method and characterized by XRD, adsorption/desorption of nitrogen, TPR, TEM, ICP and XPS analyses. Activity and selectivity of the catalysts were examined in a fixed bed reactor at 210–230 °C with a H2/CO ratio of 2 and atmospheric pressure. The results showed that the Ru-promoted catalyst has the highest activity and methane selectivity which reduce the chain growth probability. The Gd-promoted catalyst was shown smaller particle size and higher dispersion of cobalt particles in compared with unpromoted catalyst. The smaller particles have more interaction and thus show the lower catalyst reducibility. The presence of Gd in the catalyst cause higher chain growth probability compared to the unpromoted one. The Ru–Gd-promoted catalysts were shown a synergic effect in the catalyst reducibility. Based on the screening of the catalysts in the atmospheric pressure; the unpromoted, 0.1Ru/15Co/Al2O3, and 0.1Ru1Gd/15Co/Al2O3 catalysts were selected to test at high pressure conditions, which the 0.1Ru1Gd/15Co catalyst showed the highest C5 + selectivity (75%) compared with the 0.1Ru/15Co/Al2O3 and the unpromoted one. 相似文献
48.
Photocatalytic ring opening of α-epoxyketones by 2,4,6-triphenylpyrylium tetrafluoroborate in acetone resulted in the formation of 1,3-dioxolanes as major products through C–O bond cleavage and the formation of alcoholic by-products through C–C bond cleavage. The type and nature of the substituent affects the rate of ring opening. 相似文献
49.
AliReza Nowroozi Haidar Raissi Farzaneh Farzad 《Journal of Molecular Structure》2005,730(1-3):161-169
All the plausible conformations of β-aminoacrolein (AMAC) have been investigated by the Bekes-Lee-Yang-Parr (B3LYP) nonlocal density functional with extended 6-311++G** basis set for studying the stability order of conformers and the various possibilities of intramolecular hydrogen bonding formation. In general the ketoamine (KA) conformers of AMAC, by mean average, are more stable than the corresponding enolimine (EI) and ketoimine (KI) analogues and this stability is mainly due to the π-electron resonance in these conformers that established by NH2 functional group. The contribution of resonance to the stability of chelated KA conformers is about 75.6 kJ/mol, which is greater than that of the hydrogen bond energy (EHB=35.0 kJ/mol). The relative decreasing order of the various hydrogen bond energies was found to be: O–HNimine(strong)>Namine-HOketo (normal)>Nimine-HOhydroxyl (weak) > Nimine-HOketo (weak). Hydrogen bond energies for all systems were obtained from the method that we called related rotamers method (RRM). The topological properties of the electron density contributions for various type of intramolecular hydrogen bond have been analyzed in term of the Bader theory of atoms in molecules (AIM). The results of these calculations support the previous calculations, which obtained by the related rotamer methods. 相似文献
50.