全文获取类型
收费全文 | 5631篇 |
免费 | 252篇 |
国内免费 | 54篇 |
专业分类
化学 | 3980篇 |
晶体学 | 70篇 |
力学 | 255篇 |
综合类 | 1篇 |
数学 | 615篇 |
物理学 | 1016篇 |
出版年
2024年 | 12篇 |
2023年 | 56篇 |
2022年 | 456篇 |
2021年 | 333篇 |
2020年 | 248篇 |
2019年 | 252篇 |
2018年 | 208篇 |
2017年 | 171篇 |
2016年 | 279篇 |
2015年 | 156篇 |
2014年 | 211篇 |
2013年 | 459篇 |
2012年 | 331篇 |
2011年 | 329篇 |
2010年 | 206篇 |
2009年 | 173篇 |
2008年 | 222篇 |
2007年 | 227篇 |
2006年 | 186篇 |
2005年 | 139篇 |
2004年 | 143篇 |
2003年 | 123篇 |
2002年 | 139篇 |
2001年 | 63篇 |
2000年 | 46篇 |
1999年 | 39篇 |
1998年 | 36篇 |
1997年 | 25篇 |
1996年 | 36篇 |
1995年 | 34篇 |
1994年 | 38篇 |
1993年 | 29篇 |
1992年 | 35篇 |
1991年 | 37篇 |
1990年 | 29篇 |
1989年 | 48篇 |
1988年 | 40篇 |
1987年 | 28篇 |
1986年 | 31篇 |
1985年 | 28篇 |
1984年 | 37篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 22篇 |
1979年 | 18篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1976年 | 14篇 |
1973年 | 11篇 |
排序方式: 共有5937条查询结果,搜索用时 15 毫秒
931.
In this paper, we derive the time dependent solution of the effective master equation for the reduced density matrix operator of a two-level atom driven by a strong classical field and damped into a “modelled” reservoir with non-flat density of modes. The effects of different parameters on the atomic inversion, the von Neumann entropy and the entropy squeezing are discussed. 相似文献
932.
Rongrong Zhang Ahmed Yousif Ghazal Yuzhu Liu Yan Zhang Bifeng Tang 《Optics Communications》2009,282(11):2169-6193
The photodissociation dynamics of iodocyclohexane (C6H11I) at 266 and 277 nm has been investigated by ion velocity imaging technique. The velocity distributions, angular distributions and relative quantum yields are obtained for I (2P3/2) (denoted I) and I (2P1/2) (denoted I*) fragments. The energy partitioning shows that about 70% of the available energy goes into the internal excitation of the photofragments for both dissociation channels. From the angular distributions, we found the value of the anisotropy parameter β for I* at the corresponding excitation wavelength was less than that for I. Based on the measured angular distributions and relative quantum yields, the relative fractions of each excited state to the products are determined. The curve crossing probabilities between the 3Q0 and 1Q1 states are determined 0.503 at 266 nm and 0.443 at 277 nm. 相似文献
933.
Naveed Ahmad Khan Osamah Ibrahim Khalaf Carlos Andrs Tavera Romero Muhammad Sulaiman Maharani A. Bakar 《Entropy (Basel, Switzerland)》2021,23(8)
In this study, a novel application of neurocomputing technique is presented for solving nonlinear heat transfer and natural convection porous fin problems arising in almost all areas of engineering and technology, especially in mechanical engineering. The mathematical models of the problems are exploited by the intelligent strength of Euler polynomials based Euler neural networks (ENN’s), optimized with a generalized normal distribution optimization (GNDO) algorithm and Interior point algorithm (IPA). In this scheme, ENN’s based differential equation models are constructed in an unsupervised manner, in which the neurons are trained by GNDO as an effective global search technique and IPA, which enhances the local search convergence. Moreover, a temperature distribution of heat transfer and natural convection porous fin are investigated by using an ENN-GNDO-IPA algorithm under the influence of variations in specific heat, thermal conductivity, internal heat generation, and heat transfer rate, respectively. A large number of executions are performed on the proposed technique for different cases to determine the reliability and effectiveness through various performance indicators including Nash–Sutcliffe efficiency (NSE), error in Nash–Sutcliffe efficiency (ENSE), mean absolute error (MAE), and Thiel’s inequality coefficient (TIC). Extensive graphical and statistical analysis shows the dominance of the proposed algorithm with state-of-the-art algorithms and numerical solver RK-4. 相似文献
934.
Ahmed El‐Shafei David Hinks Paul D. Boyle Harold S. Freeman 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(8):o569-o571
The title compounds, C18H22Cl2N2O2 and C20H28N2O2, respectively, are isomorphous. The molecules lie at general positions in the unit cell. In each structure, chemically equivalent but crystallographically inequivalent amine N atoms exhibit different degrees of pyramidalization. The structures exhibit weak N—H⋯N hydrogen bonding, which is influenced by the differences in hybridization around the amine N atoms. The torsion angles across the biphenyl linkage for the two compounds are 67.2 (2) and 68.3 (3)°. 相似文献
935.
Ahmad Umar M. Shaheer Akhtar Hassan Algadi Ahmed A. Ibrahim Mohsen A. M. Alhamami Sotirios Baskoutas 《Molecules (Basel, Switzerland)》2021,26(15)
Herein, we have developed a novel sensing electrode to detect the eco-toxic 4-nitrophenol (4-NP). Ag-doped-ZnO nanoflowers were synthesized by facile hydrothermal method and examined by several characterization techniques in order to understand the morphology, crystal structure, composition, and surface properties. Morphological results were confirmed by the formation of Ag-doped ZnO nanoflowers decorated with nanosheets. Ag-doped ZnO/glassy carbon electrode (GCE) electrode-material-matrix was used for electrochemical sensing of toxic 4-NP. Under optimized conditions, Ag-doped ZnO/GCE modified electrode exhibits high-sensitivity and selectivity compared to the bare GCE electrode. The Ag-doped ZnO/GCE modified electrode exhibits high electrocatalytic oxidation towards 4-NP. Anodic peak current of 4-NP is increased linearly by increasing the concentration of nitrophenol. Additionally, Ag-doped ZnO/GCE shows a wide range of sensitivity from 10 µM to 500 µM, and a linear calibration plot with a good detection limit of 3 µM (S/N = 3). The proposed Ag-doped ZnO/GCE modified electrode showed high sensing stability. In addition, the oxidation mechanism was studied. The obtained results revealed that the Ag-ZnO/GCE electrode could be the promising sensing electrode for 4-NP sensing. 相似文献
936.
Christof Schmitz Ahmed Mourran Helmut Keul Martin Möller Martina Keerl Walter Richtering 《Colloid and polymer science》2009,287(10):1183-1193
The effect of microstructure on the aggregation behaviour of symmetrical di- and triblock copolymers P(BMA)-b-P(MAA) and P(BMA)-b-P(BMA-co-MAA)-b-P(MAA) with a molecular weight of 40,000 g/mol was studied. The critical micelle concentration, hydrodynamic radius and morphology
of the micelles were determined by fluorescence spectroscopy, dynamic light scattering and scanning force microscopy (SFM).
Whereas no effect of the microstructure on the critical micelle concentration could be detected, the hydrodynamic radius decreased
from di- to triblock copolymer from 53 to 36 nm. The decrease of about 32% corresponds to the length of the random middle
block within the triblock copolymer so that the reduction in hydrodynamic radius was caused by a complete orientation of the
random middle block at the core corona interface. Finally, the SFM investigation showed that dehydration of micelles on a
substrate is accompanied by formation of a physisorbed monolayer with a thickness of 2 nm on which the micelles are deposited. 相似文献
937.
Nabil S. Youssef Eman A. El-Zahany Barsoum N. Barsoum Ahmed M. A. El-Seidy 《Transition Metal Chemistry》2009,34(8):905-914
New transition metal complexes of Co(II), Cu(II), Ni(II), and Fe(III) of the ligands 6,6′-(1E,1′E)-(4,5-dimethyl-1,2-phenylene)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one)
H2L1 and 6,6’-(1E,1′E)-cyclohexane-1,2-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one)
H2L2 have been prepared and characterized using physio-chemical and spectroscopic methods. The results obtained for the complexes
indicated that the geometries of the metal centres are either square planar or octahedral. Cyclopropanation reactions of unactivated
olefins by ethyldiazoacetate (EDA) in the presence of [L1Cu]·H2O, [L2Cu]·2H2O and [L2*Co]·2H2O as catalysts were examined. The results showed that only [L2*Co]·2H2O can act as a catalyst for the cyclopropanation reaction of unactivated olefins with very high selectivity (up to 99% based
on EDA). 相似文献
938.
Pummerer chemistry applied to 2-sulfinyl indoles promotes oxidative cyclization of pendant nucleophiles to furnish C(3) spirocyclic indolenine products. Use of tetrasubstituted silyl enol ether nucleophiles in this transform yields spirocycles featuring vicinal all-carbon quaternary centers in two cases, but fails when a nearby amine can intervene. 相似文献
939.
Treatment of 1,2,4-triazoles (1) with diethylmalonate in bromobenzene gave 1,2,4-triazolo-[1,5-a]pyridines 2. Chlorination of 2 using POCl3/DMF (Vilsmeier reagent) led to the isolation of 7-chloro-6-formyl-1,2,4-triazolo[1,5-a]pyridine derivative 4, which reacted with the stabilized ylid 5 to afford 6-ethoxycarbonylvinyl-1,2,4-triazolo[1,5-a]-pyridines 6. Azidation of 6 yielded the corresponding azido compound 7, (Scheme 2). Reduction of 7 with Na2S2O4 gave the corresponding 7-amino compound 8, which cyclized in boiling DMF to give the novel 1,2,4-triazolo[1,5-g][1,6]naphthyridines 9. On the other hand, reacting 7 with one equivalent of PPh3 (aza-Wittig reaction) in CH2Cl2 gave 7-imino-phosphorane derivative 10, and subsequent cyclization in boiling DMF afforded the new 1,2,4-triazolo[1,5-g][1,6]naphthyridine derivative 11 (Scheme 3). However, treatment of 10 with phenyl isothiocyanate in 1,2-dichlorobenzene at reflux temperature gave the new 1,2,4-triazolo[1,5-g][1,6]naphthyridine derivative 14 (Scheme 4). Refluxing 6 with excess of a primary amines 15a,b in absolute. EtOH yielded the corresponding 7-alkyl-amino-1,2,4-triazolo[1,5-a]pyridines 16a,b. These obtained amines 16a,b underwent intramolecular heterocyclization in boiling DMF to give the novel 9-alkyl-1,2,4-triazolo[1,5-g][1,6]-naphthyridines 17a,b, in excellent yields (Scheme 5). 相似文献
940.
Shaheen Faizi Fatima Farooqi Aneela Naz Farheen Ansari Shakeel Ahmed Khan 《Tetrahedron》2009,65(5):998-1004
A rare alkaloid, shahidine (1), having an unstable oxazoline core has been isolated as a major constituent from the fresh leaves of Aegle marmelos. It is moisture-sensitive, and found to be the parent compound of aegeline and other amides, however, it is stable in dimethyl sulfoxide. Its structure was established by spectroscopic analysis. Biogenetically, oxazolines may be considered as the precursor of hydroxy amides and oxazoles found in plants. Shahidine (1) showed activity against a few Gram-positive bacteria. 相似文献