全文获取类型
收费全文 | 520篇 |
免费 | 39篇 |
国内免费 | 7篇 |
专业分类
化学 | 320篇 |
晶体学 | 4篇 |
力学 | 15篇 |
数学 | 55篇 |
物理学 | 119篇 |
无线电 | 53篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 31篇 |
2021年 | 17篇 |
2020年 | 24篇 |
2019年 | 42篇 |
2018年 | 37篇 |
2017年 | 30篇 |
2016年 | 49篇 |
2015年 | 29篇 |
2014年 | 45篇 |
2013年 | 69篇 |
2012年 | 41篇 |
2011年 | 38篇 |
2010年 | 22篇 |
2009年 | 16篇 |
2008年 | 13篇 |
2007年 | 7篇 |
2006年 | 12篇 |
2005年 | 8篇 |
2004年 | 6篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1996年 | 2篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1987年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有566条查询结果,搜索用时 27 毫秒
91.
Ali Reza Fakhari Davood Nematollahi Somayeh Makarem Abdolali Alizadeh 《Tetrahedron》2007,63(18):3894-3898
Electrochemical oxidation of catechols (1a-c) has been studied in the presence of methyl acetoacetate (2a) and ethyl acetoacetate (2b) as nucleophiles in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from catechols (1a-c) participate in Michael addition reactions with 2a and 2b to form the corresponding benzofuran derivatives (3a-f). The electrochemical synthesis of 3a-f has been successfully performed in an undivided cell in good yield and purity. The oxidation mechanism was deduced from voltammetric data and by coulometry at controlled potential. The products have been characterized after purification by IR, 1H NMR, 13C NMR, MS, and single crystal X-ray diffraction. 相似文献
92.
A modified glassy carbon electrode has been constructed using a 1-benzyl-4-ferrocenyl-1H-[1,2,3]-triazole along with multiwalled carbon nanotubes. The electrochemical behaviour of modified electrode has been investigated by cyclic voltammetry. Electrocatalytic activity of the modified electrode was investigated for the oxidation of hydroxylamine in 0.1 M phosphate-buffered solution of pH 8. The modified electrode showed electrocatalytic response to the oxidation of hydroxylamine at the potential of 330 mV. The linear range and detection limit for the detection of hydroxylamine in the optimum condition were found to be 4.0?×?10?7 to 6.75?×?10?4 M and 28.0?±?1.0 nM, respectively. Finally, the method was employed for the determination of hydroxylamine in water samples. 相似文献
93.
Dihydropyrimidinone derivatives were prepared by tri-component reaction of ethyl aceto acetate, aldehydes and thiourea in the presence of modified montmorillonite nanostructure as a catalyst and used as key intermediates for the synthesis of ethyl-5-(aryl)-2-(2-alkokxy-2-oxoethylidene)-7-methyl-3-oxo-3,5-dihydro-2H-thiazolo[3,2-a]pyri midine-6-carboxylate derivatives with use of diethyl and dimethyl acetylene dicarboxylate by two methods: (a) in methanol as a solvent under ultrasonic irradiation at ambient temperature (b) in methanol as a solvent at ambient temperature (conventional magnetic stirring). Ultrasound-assisted synthesis provides excellent yields in short reaction times (15–25 min) at room temperature. 相似文献
94.
Somayeh Ahmadi Alessio Roccon Francesco Zonta Alfredo Soldati 《Flow, Turbulence and Combustion》2018,100(4):979-993
In this work we study the turbulence modulation in a viscosity-stratified two-phase flow using Direct Numerical Simulation (DNS) of turbulence and the Phase Field Method (PFM) to simulate the interfacial phenomena. Specifically we consider the case of two immiscible fluid layers driven in a closed rectangular channel by an imposed mean pressure gradient. The present problem, which may mimic the behaviour of an oil flowing under a thin layer of different oil, thickness ratio h2/h1 =?9, is described by three main flow parameters: the shear Reynolds number Re τ (which quantifies the importance of inertia compared to viscous effects), the Weber number We (which quantifies surface tension effects) and the viscosity ratio λ = ν1/ν2 between the two fluids. For this first study, the density ratio of the two fluid layers is the same (ρ2 = ρ1), we keep Re τ and We constant, but we consider three different values for the viscosity ratio: λ =?1, λ =?0.875 and λ =?0.75. Compared to a single phase flow at the same shear Reynolds number (Re τ =?100), in the two phase flow case we observe a decrease of the wall-shear stress and a strong turbulence modulation in particular in the proximity of the interface. Interestingly, we observe that the modulation of turbulence by the liquid-liquid interface extends up to the top wall (i.e. the closest to the interface) and produces local shear stress inversions and flow recirculation regions. The observed results depend primarily on the interface deformability and on the viscosity ratio between the two fluids (λ). 相似文献
95.
We present the results of ab initio density functional theory calculations on the energetic, and geometric and electronic structure of Li-intercalated (6,6) silicon carbide nanotube (SiCNT) bundles. Our results show that intercalation of lithium leads to the significant changes in the geometrical structure. The most prominent effect of Li intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from lithium to the SiCNTs. All the Li-intercalated (6,6) SiCNT bundles are predicted to be metallic representing a substantial change in electronic properties relative to the undoped bundle, which is a wide band gap semiconductor. Both inside of the nanotube and the interstitial space are susceptible for intercalation. The present calculations suggest that the SiCNT bundle is a promising candidate for the anode material in battery applications. 相似文献
96.
In this work, we study the relativistic oscillators in a noncommutative space and in a magnetic field. It is shown that the effect of the magnetic field may compete withthat of the noncommutative space and that is able to vanish the effect of the noncommutative space. 相似文献
97.
In this paper, we establish exact solutions for five complex nonlinear Schrödinger equations. The semi-inverse variational principle (SVP) is used to construct exact soliton solutions of five complex nonlinear Schrödinger equations. Many new families of exact soliton solutions of five complex nonlinear Schrödinger equations are successfully obtained. 相似文献
98.
Behrooz Mirza Somayeh Soleimani‐Amiri Maziar Mirza 《Journal of Physical Organic Chemistry》2018,31(2)
Density functional theory (DFT) calculations introduced triplet ground states for [6]n SiC‐cyclacenes and ‐acenes with alternate silabenzene rings including silicon atoms in 2 opposite edges (n = 6, 8, 10, 12). The singlet‐triplet energy gap (ΔE(S‐T)), binding energy per atom (BE/n), and NBO calculation with very small band gap (ΔELUMO‐HOMO) confirmed the triplet ground states. In contrast to polyacenes, the singlet [6]n SiC‐cyclacenes displayed more stability improvement than triplets, through n increasing. This may open the way for synthesis of larger stable [6]n SiC‐cyclacenes. The ΔE(S‐T), BE/n, and the strain energy through homodesmic equations indicated more stability for larger [6]n SiC‐cyclacenes, which was more noticeable in singlet states. Cyclacenes and acenes with high conductivity and full point charge were introduced as suitable candidates for hydrogen storage. 相似文献
99.
Mohsen Emami-Razavi Marian Kowalski Somayeh Asgary 《International Journal of Theoretical Physics》2018,57(10):2989-3004
The variational method within the Hamiltonian formalism of quantum field theory has been used in order to investigate the effect of virtual pairs for four-body scalar systems consisting of two particles and two antiparticles of the same mass. The scalar particles and antiparticles interact via a massive or massless mediating scalar field. The ground state energy solutions of Fock-space variational trial states (\(|N{\bar {N}}N{\bar {N}}{ \rangle }+|N{\bar {N}}N{\bar {N}}N{\bar {N}\rangle }\)) of the relativistic wave equations have been studied. We have compared these results with the previous work of four-body system (variational trial states of the form \(|N{ \bar {N}}N{\bar {N}}{\rangle }\)) and we have shown that the inclusion of virtual pairs has a noticeable effect at low coupling and at high coupling the energy of the system is changed by an important amount. In other words, the calculations show that the inclusion of virtual pairs augments the binding energy of the four-body system by a substantial amount at strong coupling. This study can pave the way for some new ideas in order to investigate the effect of virtual pairs, for example, for a bound-states quark-antiquark or tetraquark systems in future. 相似文献
100.
Mandani Somayeh Sarbishaei Mohsen Javidan Kurosh 《International Journal of Theoretical Physics》2017,56(10):3244-3256
We have investigated the dynamics of a four-level N-type atom in cavity QED with consideration to the Kerr effect. The non-Markovianity of the system has been studied using the Breuer-Laine-Piilo (BLP) measure (N B L P ). Moreover the effects of system parameters like temperature and atom-field coupling have also been discussed. The evolution equation of the system has been derived using the time convolution-less(TCL) master equation. Some interesting behaviour of the system and their reasons are discussed.
相似文献