全文获取类型
收费全文 | 385篇 |
免费 | 15篇 |
国内免费 | 4篇 |
专业分类
化学 | 235篇 |
晶体学 | 2篇 |
力学 | 51篇 |
数学 | 37篇 |
物理学 | 79篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 22篇 |
2021年 | 19篇 |
2020年 | 21篇 |
2019年 | 30篇 |
2018年 | 29篇 |
2017年 | 26篇 |
2016年 | 17篇 |
2015年 | 16篇 |
2014年 | 30篇 |
2013年 | 30篇 |
2012年 | 43篇 |
2011年 | 39篇 |
2010年 | 20篇 |
2009年 | 13篇 |
2008年 | 12篇 |
2007年 | 13篇 |
2006年 | 4篇 |
2005年 | 2篇 |
2004年 | 5篇 |
2003年 | 3篇 |
2002年 | 1篇 |
1996年 | 3篇 |
1970年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有404条查询结果,搜索用时 15 毫秒
111.
二氧化钛纳米粒子催化合成Hantzsch酯和多氢喹啉衍生物(英文) 总被引:2,自引:0,他引:2
Mahmood TAJBAKHSH Ehsan ALAEE Heshmatollah ALINEZHAD Mohammad KHANIAN Fatemeh JAHANI Samad KHAKSAR Parizad REZAEE Mahgol TAJBAKHSH 《催化学报》2012,(9):1517-1522
1,4-Dihydropyridine and polyhydroquinoline derivatives have been prepared efficiently in a one-pot synthesis via Hantzsch condensation using nanosized titanium dioxide as a heterogeneous catalyst.The present methodology offers several advantages such as excellent yields,short reaction times (30-120 min),environmentally benign,and mild reaction conditions.The catalyst can be readily separated from the reaction products and recovered in excellent purity for direct reuse. 相似文献
112.
Dr. Ehsan Ullah Mughal Jens Eberhard Prof. Dr. Dietmar Kuck 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(47):16029-16035
The linear extension of the rigid, C3v‐symmetrical carbon framework of tribenzotriquinacene (TBTQ) along its three wings is reported. The key step of the extension procedure consists of a Diels–Alder reaction of three ortho‐quinodimethane units generated in situ at the triquinacene core. The use of 1,4‐naphthoquinone provides a facile and particularly efficient access to tris(tetraceno)‐annellated triquinacenes. The steady‐state photophysical properties of these new oligotetracenes bearing three mutually orthogonal chomophores are determined and analyzed by DFT calculations. 相似文献
113.
Molecular structure, vibration analysis, and natural bond orbital study of four derivatives of tetraketonate ligand were investigated in three different solvents using density functional theory B3LYP/6-31++G** method. The solution phase studies were carried out using an Onsager model. According to the obtained results the enol form of the ligands is more stable than keto form, even in solvents with high dielectric constant such as DMSO. This result is also confirmed experimentally using NMR studies for tetraacetylethane. Their stabilities are due to the presence of hydrogen bonding in the enol tautomers. A comparison among different possible enol forms of the substituted tetraketonate ligands demonstrated that three factors control the stability of the compounds as hydrogen bonding, steric hindrance, and charge distribution. The effectiveness of each of these factors on the stability of ligands depends on the nature of the substituent attached to the ligand. 相似文献
114.
Mohammad Maleki Mohammad Hasan Naei Ehsan Hosseinian 《International Journal of Solids and Structures》2012,49(17):2230-2238
In this paper, exact three-dimensional analysis for torsion of multilayer piezoelectric transducers is presented to highlight the remarkable effects of electrode and size effects on interface stresses and deformation. A generalized formulation is introduced for torsion of an arbitrarily layered cross section. In order to reach an advanced and better device, multilayer piezoelectric materials with different properties are formulated while axis of torsion and the polarization axes of each layer make different orientations with respect to each other. An exact formulation for n-layer piezoelectric device with rectangular cross section is presented to compensate considerable deviation of previous studies’ results from exact solution due to the assumption of linear distribution for the electric potential. In order to improve the performance of the transducers, being utilized in industry, several case studies are presented in order to investigate the influence of different parameters, e.g. thickness and material properties of electrodes over shear and peel stresses. Besides, a correction factor is introduced to completely compensate the effect of ignoring electrodes. 相似文献
115.
The compressibility factor of natural gas is an important parameter in many gas and petroleum engineering calculations. This study presents a new empirical model for quick calculation of natural gas compressibility factors. The model was derived from 5844 experimental data of compressibility factors for a range of pseudo reduced pressures from 0.01 to 15 and pseudo reduced temperatures from 1 to 3. The accuracy of the new empirical correlation has been compared with commonly used existing methods. The comparison indicates the superiority of the new empirical model over the other methods used to calculate compressibility factor of natural gas with average absolute relative deviation percent (AARD%) of 0.6535. 相似文献
116.
Acrylic-based asymmetric and variable couplers have been developed using a single structured Y-branch design with a high-index-contrast
waveguide taper and a void structure for fiber attenuation using the lateral displacement of two fibers. Device fabrication
is performed by producing the device structure on an acrylic block using a computer numerical control (CNC) machine tool.
The fabricated device has an excess loss of 5.85 dB, while the coupling ratios are 56.86 and 43.14% when the device is operated
as a 3 dB coupler. In the asymmetric coupler mode, the coupling ratio ranges from 44.84 to 8.01% for port 1 and 55.16 to 91.99%
for port 2. The excess loss of this device varies from 5.42 to 7.64 dB. In the variable coupler mode, the coupling ratio ranges
from 10.09 to 32.88% for port 1 and from 89.91 to 67.12% for port 2. The excess loss of the device varies from 5.85 to 8.49
dB. 相似文献
117.
Using density functional theory, we present a model to illustrate that under a transverse electric field the overall amount of hydrogen storage can be increased on the SiC nanotube. Due to the cylindrical shape of the nanotube, an electric field does not have the similar effects on the different adsorption sites. Although it has the desired effects on some sites, the electric field may lead the binding energy to decrease on some other sites. We demonstrate that the binding energy decreases slightly just on the two small areas and increases significantly on the largest part of the nanotube surface. 相似文献
118.
Elahe Sedghamiz Zahra Halfinezhad Abolfazl Shiroudi Ehsan Zahedi 《Research on Chemical Intermediates》2013,39(8):3843-3857
Adsorption of nitrogen dioxide in three different configurations on the exterior surface of C30B15N15 is studied using density functional theory calculations. To this end, we optimized the structures of raw C30B15N15 and nine NO2–C30B15N15 complexes at the B3LYP/6-31G* level of theory and then calculated chemical shielding (CS) tensors at the GIAO-B3LYP/6-311G** level for the optimized structures. The calculated chemical shielding isotropy (CSI), chemical shielding anisotropy (CSA), and orientation of CS tensors (Euler angles) reveal that the adsorption configurations (nitro, trans-nitrite, and cis-nitrite) have different effects on the electronic structure of C30B15N15. Natural atomic charges based on natural population analysis (NPA) were used to justify the changes in CSI values after gas sorption. 相似文献
119.
120.
Hassan Mallaekeh Alireza Shams Mohammad Shaker Ehsan Bahramzadeh Donya Arefi 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(12):2081-2086
In this paper the complexation reaction of the 5,5′-(2E,2′E)-2,2′-(ethane-1,2-diylidene)bis(hydrazine-1-yl-2-ylidene)bis(4-amino-4H-1,2,4-triazole-3-thiol) ligand (HYT) with Cd2+ education was studied in some binary mixtures of methanol (MeOH), n-propanol (PrOH) and dimethyl-formamide (DMF) at different temperatures using the conductometry and spectrophotometry. The stability constants of the complex was determined using a GENPLOT computer program. The conductance data and absorbance-mole ratio plots show that in all solvent systems, the stoichiometry of the complex formed between (HYT) and Cd2+ cation is 1: 1. The obtained results show that the stability of (HYT)-Cd complex is sensitive to the mixed solvents composition. The values of thermodynamic parameters (ΔG °, ΔH °, and ΔS °) for formation of (HYT)-Cd complex were obtained from temperature dependence of the stability constant using the van’t Hoff plots. The results show that in most cases, the complex are enthalpy destabilized but entropy stabilized and the complex formation is affected by pH, time, temperature and the nature of the solvent. 相似文献