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 0 毫秒
41.
Somayeh KhazaeiMohammad Khazaei Hosein CheraghchiVahid Daadmehr Yoshiyuki Kawazoe 《Physica B: Condensed Matter》2011,406(20):3885-3890
We have used a non-equilibrium surface Green's function matching formalism combined with a tight-binding Hamiltonian to consider the effect of different arrangements of pentagon rings on localization of density of states at the tip regions of semi-infinite capped carbon nanotubes. The transfer matrixes are obtained by an iterative procedure. The results demonstrate that the positions of the peaks near Fermi energy are remarkably affected by the relative locations of pentagons. It is observed that in thin nanotubes, carbon atoms belonging two neighboring pentagon rings have significant contribution in the localized states near fermi energy. From our calculations, it turns out that the metallic or semiconducting behavior of capped nanotubes in the tip regions depends on the metallic or semiconducting nature of their nanotube stems. 相似文献
42.
Juan M. Salas Abderrahman Rahmani Marìa A. Romero Miguel Quiròs Edward R. T. Tiekink 《Journal of chemical crystallography》1994,24(10):669-673
The crystal and molecular structure of the title compound, ·[Pd(S=C(NH2)2)4]Cl2·2dmtp·2H2O. has been determined and refined to a finalR=0.034. The cation is centrosymmetric with the Pd atom existing in a square planar geometry with two disparate Pd–S bond distances of 2.3129(9) and 2.292(1)Å. Interatomic parameters are reported for the first non-coordinated dmtp molecule; these suggest the predominance of a particular canonical form in the solid state. The lattice is stabilized via a series H-bonding contacts involving the thiourea. Cl– and dmtp species. Crystals of [Pd(S=C(NH2)2)4]Cl2 · 2dmtp · 2H2O are monoclinic with space groupP21/c, and unit cell dimensionsa=15.129(1),b=8.512(1),c=12.663(1) Å, =104.05(1)°. 相似文献
43.
Safar Irandoust-Pakchin Somayeh Abdi-Mazraeh Ali Khani 《Computational Mathematics and Mathematical Physics》2017,57(12):2047-2056
In this paper, a variable-order fractional derivative nonlinear cable equation is considered. It is commonly accepted that fractional differential equations play an important role in the explanation of many physical phenomena. For this reason we need a reliable and efficient technique for the solution of fractional differential equations. This paper deals with the numerical solution of class of fractional partial differential equation with variable coefficient of fractional differential equation in various continues functions of spatial and time orders. Our main aim is to generalize the Chebyshev cardinal operational matrix to the fractional calculus. Finally, illustrative examples are included to demonstrate the validity and applicability of the presented technique. 相似文献
44.
The one‐pot multicomponent reaction of alkyl isocyanide, alkylidene‐substituted Meldrum's acid, and arylcarboxylic acids affords new derivatives of iminofuranone in fair yields. The structure of the products was deduced from their spectroscopic data. Two equivalents of the respective isocyanides participate in this reaction. 相似文献
45.
Hossaini Z Rostami-Charati F Firoziyan S Sabbaghan M Khalilzadeh MA 《Combinatorial chemistry & high throughput screening》2012,15(6):503-508
An efficient synthesis of dialkyl pyrrolo[2,1-a]isoquinoline-2,3-dicarboxylates, pyrrolo[1,2-a]quinoline-1,2- dicarboxylates and indolizines is described via one-pot reactions of isoquinoline, quinoline or pyridine and phenacyl bromids with dialkyl acetylenedicarboxylates or diaryloylacetylene under solvent-free conditions at 50°C. The mild reaction conditions and high yields of the products exhibit the good synthetic advantage of these methods. 相似文献
46.
Rezaee Elham Amrolah Somayeh Minaei Nazari Maryam Tabatabai Sayyed Abbas 《Molecular diversity》2021,25(1):45-53
Molecular Diversity - Soluble epoxide hydrolase (sEH) enzyme plays an important role in the metabolism of endogenous chemical mediators, epoxyeicosatrienoic acids, which are involved in the... 相似文献
47.
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
48.
A mild and efficient method for the synthesis of 1H-chromeno[2,3-d]pyrimidine-5-carboxamide derivatives via a one-pot, three-component reaction of an isocyanide, barbituric acid, and a salicylaldehyde in the presence of acetic acid in ethanol/water mixture at 75 °C is reported. This high atom economy reaction led to the construction of one benzopyran ring, and one amide group in a single synthetic step. 相似文献
49.
Synthesis of a graphene‐based nanocomposite for the dispersive solid‐phase extraction of vancomycin from biological samples 下载免费PDF全文
Faezeh Khalilian Somayeh Amiri Hanzaki Mohammad Yousefi 《Journal of separation science》2015,38(6):975-981
The approach of this work was to study the capability of graphene‐based materials in the field of biological sample preparation. A polypyrrole/graphene composite was synthesized and characterized. The potential of the nanocomposite was investigated as a sorbent in dispersive solid‐phase extraction followed by high‐performance liquid chromatography with UV detection for vancomycin as a model drug. The effect of different parameters influencing extraction efficiency such as sample pH and sample volume, ionic strength, extraction time, type, and volume of desorption solvent and desorption time were investigated. A comparison study was also conducted between polypyrrole/graphene and some different novel and classic sorbents. Under optimized conditions, the calibration curve for vancomycin showed linearity in the range of 0.05–10 μg/mL. In addition, limits of detection, and quantification were 0.003 and 0.01 μg/mL, respectively. The intraday and interday relative standard deviations at a concentration of 0.05 μg/mL (n = 3) were 1.6 and 2.1%, respectively. Furthermore, the proposed method was successfully applied for the determination of vancomycin in plasma and urine samples. The relative recoveries indicated the feasibility of graphene‐based sorbents in biological sample analysis. 相似文献
50.
Copper/Graphene/Clay Nanohybrid: A Highly Efficient Heterogeneous Nanocatalyst for the Synthesis of Novel 1,2,3‐Triazolyl Carboacyclic Nucleosides via ‘Click’ Huisgen 1,3‐Dipolar Cycloaddition 下载免费PDF全文
Mohammad Navid Soltani Rad Somayeh Behrouz S. Jafar Hoseini Hasan Nasrabadi Maryam Saberi Zare 《Helvetica chimica acta》2015,98(9):1210-1224
A very mild and highly efficient synthesis of some novel 1H‐1,2,3‐triazolyl carboacyclic nucleosides via a ‘Click’ Huisgen cycloaddition of N‐propargyl nucleobases and azido alcohols using Cu/aminoclay/reduced graphene oxide nanohybrid (Cu/AC/r‐GO nanohybrid) as nanocatalyst is described. The preparation and characterization of Cu/AC/r‐GO nanohybrid are discussed. This catalyst was characterized by X‐ray diffraction, FT‐IR, TEM, and energy‐dispersive analysis of X‐ray techniques. Cu/AC/r‐GO nanohybrid is a stable and highly efficient heterogeneous nanocatalyst that can be easily prepared, used, and restored from the reaction mixture by simple filtration, and reused for many consecutive trials without significant decrease in activity. 相似文献