全文获取类型
收费全文 | 667篇 |
免费 | 18篇 |
国内免费 | 3篇 |
专业分类
化学 | 440篇 |
晶体学 | 7篇 |
力学 | 19篇 |
数学 | 76篇 |
物理学 | 146篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 24篇 |
2020年 | 19篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 31篇 |
2016年 | 33篇 |
2015年 | 14篇 |
2014年 | 31篇 |
2013年 | 70篇 |
2012年 | 42篇 |
2011年 | 76篇 |
2010年 | 37篇 |
2009年 | 24篇 |
2008年 | 44篇 |
2007年 | 38篇 |
2006年 | 28篇 |
2005年 | 24篇 |
2004年 | 17篇 |
2003年 | 20篇 |
2002年 | 11篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有688条查询结果,搜索用时 421 毫秒
71.
Ayhan S. Demir Thomas Dünnwald Hans Iding Martina Pohl Michael Müller 《Tetrahedron: Asymmetry》1999,10(24):162-4774
Aromatic aldehydes are converted into benzoins by benzoylformate decarboxylase catalyzed C–C bond formation. The reaction affords (R)-benzoins with high enantiomeric excess and in good chemical yields. A broad range of aromatic aldehydes can be used as substrates in aqueous buffer or buffer/DMSO-solutions. 相似文献
72.
Allyl alcohol (AA), acrylonitrile (AN), methyl methacrylate (MMA), monomers and monomer mixtures AA+AN, AA+MMA were used to conserve and consolidate Beech and Spruce. After impregnation, copolymerisation and polymerisation were accomplished by gamma irradiation. The fine structure of wood+polymer(copolymer) composites was investigated by Scanning Electron Microscopy (SEM). It was observed that copolymer obtained from AA+MMA monomer mixture showed the optimum compatibility. The compressional strength and Brinell Hardness Numbers determined for untreated and treated wood samples indicated that the mechanical strength of wood+copolymer composites was increased. It was found that the mechanical strength of the wood samples containing the AA+MMA copolymer was higher than the others. In the presence of P(AA/MMA), at highest conversion, the compressive strength perpendicular to the fibres in Beech and Spruce increased approximately 100 times. The water uptake capacity of wood+copolymer composites was observed to decrease by more than 50% relative to the original samples, and biodegradation did not take place. 相似文献
73.
Erol Gelenbe
Andreas Stafylopatis
《Applied Mathematical Modelling》1991,15(10):534-541We define a simple form of homogeneous neural network model whose characteristics are expressed in terms of probabilistic assumptions. The networks considered operate in an asynchronous manner and receive the influence of the environment in the form of external stimulations. The operation of the network is described by means of a Markovian process whose steady-statesolution yields several global measures of the network's activity. Three different types of external stimulations are investigated, which represent possible input mechanisms. The analytical results obtained concern the macroscopic viewpoint and provide a quick insight into the structure of the network's behavior. 相似文献
74.
Alemdar Hasanov Ali Demir Arzu Erdem 《Journal of Mathematical Analysis and Applications》2007,335(2):1434-1451
This article presents a mathematical analysis of input-output mappings in inverse coefficient and source problems for the linear parabolic equation ut=(kx(x)ux)+F(x,t), (x,t)∈ΩT:=(0,1)×(0,T]. The most experimentally feasible boundary measured data, the Neumann output (flux) data f(t):=−k(0)ux(0,t), is used at the boundary x=0. For each inverse problems structure of the input-output mappings is analyzed based on maximum principle and corresponding adjoint problems. Derived integral identities between the solutions of forward problems and corresponding adjoint problems, permit one to prove the monotonicity and invertibility of the input-output mappings. Some numerical applications are presented. 相似文献
75.
Dilek ÇökelilerMehmet Mutlu 《Analytica chimica acta》2002,469(2):217-223
A single-layer alcohol electrode was prepared by plasma polymerization technique. Ethylenediamine was used to incorporate amino groups on tract-etched polycarbonate membranes in glow discharge reactor. In order to determine the plasma polymerization parameters (discharged power, monomer flow rate, exposure time) on membrane permeability, hydrogen peroxide was used as tracer. The single-layer alcohol electrode that was produced by 0.6% (w/v) of alcohol oxidase (AOx) solution on the polycarbonate membrane, which was modified at 30 W, 20 ml/min monomer flow rate and 15 min exposure time, was selected for optimum performance. Sensitivity, linearity and response time of that particular layer were 5.6 nA/mM, 2 mM and 50 s, respectively. The performance of the amperometric alcohol electrode was tested on commercial alcoholic beverages. 相似文献
76.
The behavior of the bending modulus kappa of bilayers in lamellar phases was studied by Small Angle X-ray Scattering technique for various nonionic C(i)E(j) surfactants. The bilayers are either unswollen and dispersed in water or swollen by water and dispersed in dodecane. For unswollen bilayers, the values of kappa decrease with both an increase in the area per surfactant molecule and in the polar head length. They increase when the aliphatic chain length increases at constant area per surfactant molecule. Whereas for water-swollen membranes, the values of kappa decrease as the content of water increases converging to the value of the single monolayer bending modulus. Such a behavior results from the decoupling of the fluctuations of the two surfactant membrane monolayers. Our results emphasize the determinant contribution of the surfactant conformation to kappa. 相似文献
77.
The mathematical models representing machine tool chatter dynamics have been cast as differential equations with delay. In this paper, non-linear delay differential equations with periodic delays which model the machine tool chatter with continuously modulated spindle speed are studied. The explicit time-dependent delay terms, due to spindle speed modulation, are replaced by state-dependent delay terms by augmenting the original equations. The augmented system of equations is autonomous and has two pairs of pure imaginary eigenvalues without resonance. The reduced bifurcation equation is obtained by making use of Lyapunov-Schmidt Reduction method. By using the reduced bifurcation equations, the periodic solutions are determined to analyze the tool motion. Analytical results show both modest increase of stability and existence of periodic solutions near the new stability boundary. 相似文献
78.
79.
Ertuğrul Gazi Sağlam Sevcan Erden Özgür Tutsak Dilek Eskiköy Bayraktepe Zehra Yazan Durmuş Hakan Dal 《Phosphorus, sulfur, and silicon and the related elements》2017,192(3):322-329
Some 1,3-dithiadiphosphetane 2,4-disulfides (X2P2S4, X: Fc, FcLR; X: CH3O?C6H4?, LR) were allowed to react with alcohols to obtain dithiophosphonic acids (X(OR)PS2H). These were converted to the corresponding ammonium salts. The salts were of the structures [Fc(OR)PS2]?[NH4]+, R: 3-methyl-1-butyl- for I; 1-phenyl-1-propyl- for II; 3-pentyl- for III; 3-phenyl-1-propyl- for IV and [CH3O?C6H4(OR)PS2]?[NH4]+, R: 3-methyl-1-butyl- for V and 1-phenyl-1-propyl- for VI. To the best of our knowledge, all the compounds except V were prepared for the first time.The compounds synthesized were characterized by elemental analysis, NMR (1H, 13C, 31P), MS, FTIR, and Raman spectroscopies. Electrochemical behaviors of I–VI at disposable pencil graphite electrode (PGE) were investigated by using cyclic voltammetry (CV) and square-wave voltammetry (SWV). Adsorption and diffusion patterns of all the compounds on the PGE were also studied.Two electroactive groups were identified in the compounds I–IV and only one in V and VI. The ferrocenyl groups of I-IV were oxidized at around 0.4 V. The same compounds display a second, more intense CV band at 0.8 V. The corresponding band for the compounds V–VI appears at around 0.6 V with a much weaker intensity. It is suggested that the ferrocenyl group introduced into the structures stabilizes the radical species formed as the product of the oxidation of the dithiophosphonato group. 相似文献
80.
Today, the surface imprinted polymers emerge in various fields as synthetic adsorbents gaining attention in a variety of application areas. In this study, Cu(II) ion surface imprinted poly(2-hydroxyethyl methacrylate-glycidyl methacrylate), poly(HEMA-GMA), cryogels were synthesized via modified two-step polymerization which is different from given in literature and the adsorption of Cu(II) ion from aqueous solution was investigated batch wise. In this respect, the method applied in this study is new in the literature despite heavy metal removal studies reported. The polyethyleneimine (PEI) molecule was used in polymeric structure as a ligand. The poly(HEMA-GMA) cryogels prepared was characterized via Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma optical emission spectrometry (ICP-OES), elemental analysis, scanning electron microscopy (SEM) and the micro-computed tomography (μCT). 相似文献