全文获取类型
收费全文 | 359篇 |
免费 | 5篇 |
国内免费 | 2篇 |
专业分类
化学 | 278篇 |
晶体学 | 1篇 |
力学 | 3篇 |
综合类 | 1篇 |
数学 | 20篇 |
物理学 | 63篇 |
出版年
2023年 | 5篇 |
2022年 | 34篇 |
2021年 | 27篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 15篇 |
2017年 | 11篇 |
2016年 | 20篇 |
2015年 | 5篇 |
2014年 | 11篇 |
2013年 | 22篇 |
2012年 | 27篇 |
2011年 | 34篇 |
2010年 | 20篇 |
2009年 | 8篇 |
2008年 | 7篇 |
2007年 | 5篇 |
2006年 | 8篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1991年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
1903年 | 1篇 |
排序方式: 共有366条查询结果,搜索用时 15 毫秒
91.
92.
93.
Synthesis of N-aryl-γ-amino-γ-lactams by oxidative coupling of aromatic amines with 2-pyrrolidinone using catalytic amount of CuO nanoparticles is described. The procedure has broad substrate scope, good yields, complete regioselectivity and catalyst recyclability. 相似文献
94.
Two copper/cobalt metal complexes, [Cu(l-cys)(2,2′-bpy)(H2O)] (1) and {[Co(l-cys)(4,4′-bpy)(H2O)]·H2O}n (2), with the N-donor ligands 2,2′-bipyridyl/4,4′-bipyridyl and the l-cysteate dianion (l-cys) have been synthesized by different reaction methods and structurally characterized. Compound 1 exists as a discrete monomeric unit in which the metal ions possess a distorted square-pyramidal coordination environment provided by nitrogen atoms from the 2,2′-bpy ligand and the amino-carboxylate group of l-cys in a chelated coordination, constituting the square base, and with a water molecule occupying the axial coordination site to complete the penta coordination. Packing and hydrogen bonding interactions of 1 reveal that the screw related monomeric units are involved in intermolecular hydrogen bonding with the formation of helical bilayers via O–H?O and N–H?O interactions. These hydrogen bonded bilayered helical nets are involved in stacking and C–H?O interactions which generate a two dimensional hydrogen bonded network in the bc-plane. Complex 2 is a two dimensional coordination polymer which is insoluble in common polar and non-polar solvents. The coordination around the metal center possesses a distorted octahedral geometry. The adjacent metal centers are bridged via the carboxylate group of the l-cys moiety in a syn–anti fashion, generating a one dimensional helical network along the b-axis. Adjacent helical chains are further pillared by the 4,4′-bpy ligand through the terminal nitrogen atoms, generating a two dimensional square grid type coordination network. Both the complexes are characterized well by various physico-chemical techniques such as CHN analysis, IR spectroscopy, PXRD and CD analysis. 相似文献
95.
Amal Al-Kahlout Diogo Vieira César O. Avellaneda Edson R. Leite Michel A. Aegerter Agnieszka Pawlicka 《Ionics》2010,16(1):13-19
Proton-conducting gel polymer electrolytes based on gelatin plasticized with glycerol and containing acetic acid were investigated,
characterized, and applied to electrochromic window. For glycerol contents varying from 7% to 48%, the conductivity of the
uniform and predominantly amorphous gel electrolyte was found to follow a Vogel–Tamman–Fulcher behavior with the temperature.
Typically, for the electrolyte chosen to make 7 × 2 cm2 electrochromic smart window with the configuration: glass/fluor-doped tin oxide (FTO)/WO3/gelatin electrolyte/CeO2–TiO2/FTO/glass and containing 28% of glycerol, the conductivities were found to be of the order of 5 × 10−5 S/cm at room temperature and 3.6 × 10−4 S/cm at 80 °C. The device was characterized by spectroelectrochemical techniques and was tested up to 10,000 cycles showing
a fast coloring/bleaching behavior, where the coloring process was achieved in 10 s and the bleaching in 2 s. The transmission
variation at the wavelength of 550 nm was about 15%. The cyclic voltammograms showed a very good reversibility of the cathodic/anodic
processes, and the charge density was about 3.5 mC/cm2. The memory tests showed that the transmittance in the colored state increased by 8% in 90 min after removing the potential. 相似文献
96.
Amal H. Ismail Nadeen H. Nsouli Michael H. Dickman Jakša Knez Ulrich Kortz 《Journal of Cluster Science》2009,20(2):453-465
The 20-tungsto-4-tellurate(IV) polyanion [H2Te4W20O80]22? (1) has been synthesized and isolated in the form of its hydrated sodium salt Na22[H2Te4W20O80]·64H2O (Na-1). Na-1 has been characterized in the solid state by FTIR and TGA. Single-crystal X-ray diffraction analysis revealed that Na-1 crystallizes in the triclinic space group P ${\bar 1}The 20-tungsto-4-tellurate(IV) polyanion [H2Te4W20O80]22− (1) has been synthesized and isolated in the form of its hydrated sodium salt Na22[H2Te4W20O80]·64H2O (Na-1). Na-1 has been characterized in the solid state by FTIR and TGA. Single-crystal X-ray diffraction analysis revealed that Na-1 crystallizes in the triclinic space group P
, with a = 12.741(2) ?, b = 16.842(3) ?, c = 17.197(4) ?, α = 93.961(10)°, β = 107.466(8)°, γ = 108.548(8)°, V = 3281.5(11) ?3 and Z = 1. The polyanion 1 comprises two [HTe2W10O40]11− fragments, connected through Te−O–W μ2-oxo bridges, and each containing a pair of face-shared WO6 octahedra. Furthermore, the 15-tungstotellurate(IV) polyanion [NaTeW15O54]13− (2) was synthesized and crystallized as a sodium salt in the orthorhombic space group Pccn, with a = 11.8841(5) ?, b = 20.8934(11) ?, c = 31.801(2) ?, V = 7896.3(7) ?3 and Z = 4.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Dedicated to Professor C. N. R. Rao on the occasion of his 75th birthday 相似文献
97.
The electrochemical behavior of sulfaguanidine was investigated in PBS buffer aqueous solutions. Cyclic voltammograms have shown that (1) the Sg provided a well-defined irreversible oxidation peak (2) the signal-to-background current ratio is 3 times higher at COOH-MWCNT coated GCE than that of bare GCE and (3) the modifying GCE surface by COOH-MWCNT led to a significant improvement (2.7 folds) of the electrochemical response. It has been shown that Sg oxidizes according to a diffusion-controlled mechanism. A linear calibration curve was obtained for the oxidation of Sg at 10–70 μM. The COOH-MWCNT coated GCE has also been successfully used for the determination of Sg in real samples. 相似文献
98.
In this paper, we study a general discrete‐time model representing the dynamics of a contest competition species with constant effort exploitation. In particular, we consider the difference equation x n +1=x n f (x n ?k )?h x n where h >0, k ∈{0,1}, and the density dependent function f satisfies certain conditions that are typical of a contest competition. The harvesting parameter h is considered as the main parameter, and its effect on the general dynamics of the model is investigated. In the absence of delay in the recruitment (k =0), we show the effect of h on the stability, the maximum sustainable yield, the persistence of solutions, and how the intraspecific competition change from contest to scramble competition. When the delay in recruitment is 1 (k =1), we show that a Neimark‐Sacker bifurcation occurs, and the obtained invariant curve is supercritical. Furthermore, we give a characterization of the persistent set. 相似文献
99.
This paper presents the dynamic characteristics of functionally graded beam with material graduation in axially or transversally through the thickness based on the power law. The present model is more effective for replacing the non-uniform geometrical beam with axially or transversally uniform geometrical graded beam. The system of equations of motion is derived by using the principle of virtual work under the assumptions of the Euler–Bernoulli beam theory. The finite element method is employed to discretize the model and obtain a numerical approximation of the motion equation. The model has been verified with the previously published works and found a good agreement with them. Numerical results are presented in both tabular and graphical forms to figure out the effects of different material distribution, slenderness ratios, and boundary conditions on the dynamic characteristics of the beam. The above mention effects play very important role on the dynamic behavior of the beam. 相似文献
100.
Amal Halder Dulal C. Mukherjee Sumanta Bhattacharya 《Journal of solution chemistry》2010,39(9):1327-1340
Ground state non-covalent interactions between a macrocyclic receptor, C-methylcalix[4]resorcinarene (1) and fullerenes (C60 and C70) have been studied in benzonitrile by an absorption spectrophotometric method. Absorption bands are located in the visible
region due to the charge transfer (CT) transition between 1 and various electron acceptors (including fullerenes), namely, 2,3-dichloro-5,6-dicyano-p-benzoquinone, tetracyanoquinodimethane and p-chloranil. Utilizing the CT absorption bands, various important physicochemical parameters, including oscillator strength,
resonance energy, transition dipole strength of all the acceptor-1 complexes and vertical ionization potential of 1 are determined. Job’s method of continuous variation reveals 1:1 stoichiometry between fullerenes and 1. The most fascinating feature of the present study is that 1 binds selectively to C70 compared to C60 as obtained from binding constant (K) data of C60-1 (KC60-1K_{\mathrm{C}60\mbox{-}\mathbf{1}}) and C70-1 (KC70-1K_{\mathrm{C}70\mbox{-}\mathbf{1}}) complexes, i.e., KC60-1=190K_{\mathrm{C}60\mbox{-}\mathbf{1}}=190 dm3⋅mol−1 and KC70-1=5,800K_{\mathrm{C}70\mbox{-}\mathbf{1}}=5{,}800 dm3⋅mol−1 and selectivity (KC70-1
/KC60-1
) ∼30. Quantum chemical calculations based on hybrid density functional theory estimate the enthalpies of formation of the
fullerene-1 complexes in vacuo and provide very good support for selectivity in the K values of the C70 and C60 complexes of 1. The exchange and correlation energies have been calculated using a hybrid DFT functional method. We have opted to use the
hybrid DFT functional over the Hartree-Fock method, as it can account for correlation effects also. Molecular electrostatic
potential map calculations give a clear picture on the electronic structures of the fullerene-1 complexes. 相似文献