全文获取类型
收费全文 | 379篇 |
免费 | 24篇 |
专业分类
化学 | 305篇 |
力学 | 9篇 |
数学 | 71篇 |
物理学 | 18篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 5篇 |
2020年 | 15篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 22篇 |
2015年 | 18篇 |
2014年 | 10篇 |
2013年 | 23篇 |
2012年 | 35篇 |
2011年 | 40篇 |
2010年 | 18篇 |
2009年 | 19篇 |
2008年 | 23篇 |
2007年 | 25篇 |
2006年 | 25篇 |
2005年 | 22篇 |
2004年 | 17篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1991年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有403条查询结果,搜索用时 15 毫秒
71.
Luis Gómez‐Hortigüela Dr. Joaquín Pérez‐Pariente Prof. Furio Corà Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(6):1478-1490
A combination of experimental characterisation techniques and computational modelling has allowed us to gain insight into the molecular features governing structure direction in the synthesis of microporous aluminophosphates. The occlusion of three different structure‐directing agents (SDAs), triethylamine (TEA), benzylpyrrolidine (BP) and (S )‐(?)‐N‐benzylpyrrolidine‐2‐methanol (BPM), within the AFI structure during its crystallisation, together with the simultaneous incorporation of water, has been experimentally measured. We found a higher incorporation of organic molecules in the structure obtained with BPM, while a higher water (and lower organic) content is found for the ones obtained with TEA and BP as SDAs. The computational study provides a thermodynamic explanation for the observed behaviour in terms of the relative stabilisation energy of the SDAs and water molecules within the AFI framework compared with when they are in aqueous solution, and demonstrates that a competition for preferential occupation exists between water and organic SDAs, which is a function of the interaction with the inorganic framework. The lower interaction of TEA and BP molecules with the AFI structure promotes the simultaneous incorporation of water molecules in the 12‐membered‐ring (MR) channel, to increase the host–guest interaction energy and thus the thermodynamic stability. The presence of strongly interacting methanol groups in the BPM molecules leads to the incorporation of only organic molecules within the 12‐MR channels. Our results demonstrate the essential role that water molecules play in the stabilisation of hydrophilic microporous aluminophosphates; a minimum amount of organic SDA is, however, essential for a templating role of the microporous architecture. 相似文献
72.
Two new phenylmalonate-bridged copper(II) complexes with the formulas [Cu(4,4'-bpy)(Phmal)](n).2nH(2)O (1) and [Cu(2,4'-bpy)(Phmal)(H(2)O)](n)() (2) (Phmal = phenylmalonate dianion, 4,4'-bpy = 4,4'-bipyridine, 2,4'-bpy = 2,4'-bipyridine) have been synthesized and characterized by X-ray diffraction. Complex 1 crystallizes in monoclinic space group P2(1), Z = 4, with unit cell parameters of a = 9.0837(6) Angstroms, b = 9.3514(4) Angstroms, c = 11.0831(8) Angstroms, and beta = 107.807(6) degrees , whereas complex 2 crystallizes in orthorhombic space group C2cb, Z = 8, with unit cell parameters of a = 10.1579(7) Angstroms, b = 10.3640(8) Angstroms, and c = 33.313(4) Angstroms. The structures of 1 and 2 consist of layers of copper(II) ions with bridging bis-monodentate phenylmalonate (1 and 2) and 4,4'-bpy (1) ligands and terminal monodentate 2,4'-bpy (2) groups. Each layer in 1 contains rectangles with dimensions of 11.08 x 4.99 Angstroms(2), the edges being defined by the Phmal and 4,4'-bpy ligands. The intralayer copper-copper separations in 1 through the anti-syn equatorial-apical carboxylate-bridge and the 4,4'-bpy molecule are 4.9922(4) and 11.083(1) Angstroms, respectively. The anti-syn equatorial-equatorial carboxylate bridge links the copper(II) atoms in complex 2 within each layer with a mean copper-copper separation of 5.3709(8) Angstroms. The presence of 2,4'-bpy as a terminal ligand accounts for the large interlayer separation of 15.22 Angstroms. The copper(II) environment presents a static pseudo-Jahn-Teller disorder which has been studied by EPR and low-temperature X-ray diffraction. Magnetic susceptibility measurements of both compounds in the temperature range 2-290 K show the occurrence of weak antiferromagnetic [J = -0.59(1) cm(-1) (1)] and ferromagnetic [J = +0.77(1) cm(-1) (2)] interactions between the copper(II) ions. The conformation of the phenylmalonate-carboxylate bridge and other structural factors, such as the planarity of the exchange pathway in 1, account for the different nature of the magnetic interaction. 相似文献
73.
Joaquín Moraleda Javier Segurado Javier Llorca 《International Journal of Solids and Structures》2009,46(25-26):4287-4297
The influence of interface properties (strength and toughness) on the tensile behavior of fiber-reinforced elastomers deformed perpendicularly to the fibers was studied using computational micromechanics. Numerical simulations were performed by means of the finite element analysis of a representative volume element of the composite microstructure. The effect of finite deformations and of interface fracture was included in the simulations, the latter through a bidimensional and quadratic interface element inserted at the fiber/matrix interfaces. A parametrical study was carried out to assess the effect of interface strength and toughness on the tensile strength and damage micromechanisms. It was found that the onset of damage and tensile strength were controlled by interface strength while the evolution of damage depended on interface toughness. 相似文献
74.
Manuela Martín‐Zarco Santiago Toribio Joaquín C. García‐Martínez Julián Rodríguez‐López 《Journal of polymer science. Part A, Polymer chemistry》2009,47(23):6409-6419
A series of polyamido amine (PAMAM) dendrimers (Generations 2, 3, 4, and 6) fully functionalized at their periphery with first‐ and second‐generation poly (phenylenevinylene) (PPV) dendrons have been efficiently prepared. MALDI‐TOF mass spectrometry proved to be particularly useful for the characterization of the new hybrid dendrimers as well as for the estimation of the average number of PPV dendrons attached to the surface. The optical absorption and emission properties of these systems were studied. The materials display extremely high molar extinction coefficients and emit blue light with only slightly lower fluorescence quantum yields than the corresponding free dendrons. Self‐quenching interactions between PPV units were not observed in THF. However, the luminescence properties underwent a dramatic change when toluene was used as the solvent. The lower polarity of toluene caused shrinkage of the PAMAM structure and brought the PPV chromophores closer together, leading to self‐quenching interactions and excimer formation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6409–6419, 2009 相似文献
75.
Sandra Barriga Ana Méndez Joaquín Cámara Francisca Guerrero Gabriel Gascó 《Journal of Thermal Analysis and Calorimetry》2010,99(3):981-986
The objective of this study is to study the influence of de-inking paper sludge (DPS) and sewage sludge (SL) mixtures addition
at different rates (2, 4 and 8%) in two soils. Incubation experiments were performed during 60 days and the influence of treatments
in physical soil properties was determined by soil porosity and stability of aggregates. Differential thermal analysis (DTA)
of amended soils after incubation was performed. Experimental results show that amendment increased biological soil activity,
soil porosity and stability of aggregates. DTA analysis shows that the first exothermic peak generally increases with the
dosage of DPS:SL due to the addition of immature organic matter. Moreover, the second peak enlarges probably due to the humification
process during incubation. 相似文献
76.
77.
Jean‐Hubert Olivier Franck Camerel Dr. Joaquín Barberá Dr. Pascal Retailleau Dr. Raymond Ziessel Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(33):8163-8174
We have designed and synthesised a series of modular, mesogenic complexes based on anthracene‐2,6‐disulfonate and trialkoxybenzyl‐functionalised imidazolium cations. Each complex contains a central, rigid, dianionic anthracene core and two flexible monocations bearing paraffin chains anchored on imidazolium rings. Anthracene‐2,6‐disulfonate can be crystallised with various simple alkylammonium ions and, in the case of +N(CH3)2(C16H33)2, a crystal structure determination has shown that the long paraffinic chains are intercalated between the anthracene moieties. The dianion forms columnar mesophases with trialkoxybenzylimidazolium cations, as identified by polarising optical microscopy and X‐ray scattering measurements. Differential scanning calorimetry studies confirmed mesomorphic behaviour from room temperature to about 200 °C for alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of anthracene is maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The new functional materials described herein provide an easy access to stable and luminescent mesomorphic materials engineered by an ionic self‐assembly process. 相似文献
78.
79.
Mann JA Rodríguez-López J Abruña HD Dichtel WR 《Journal of the American Chemical Society》2011,133(44):17614-17617
Single-layer graphene is a newly available conductive material ideally suited for forming well-defined interfaces with electroactive compounds. Aromatic moieties typically interact with the graphene surface to maximize van der Waals interactions, predisposing most compounds to lie flat on its basal plane. Here we describe a tripodal motif that binds multivalently to graphene through three pyrene moieties and projects easily varied functionality away from the surface. The thermodynamic and kinetic binding parameters of a tripod bearing a redox-active Co(II) bis-terpyridyl complex were investigated electrochemically. The complex binds strongly to graphene and forms monolayers with a molecular footprint of 2.3 nm(2) and a ΔG(ads) = -38.8 ± 0.2 kJ mol(-1). Its monolayers are stable in fresh electrolyte for more than 12 h and desorb from graphene 1000 times more slowly than model compounds bearing a single aromatic binding group. Differences in the heterogeneous rate constants of electron transfer between the two compounds suggest that the tripod projects its redox couple away from the graphene surface. 相似文献
80.
Mixed-matrix membranes (MMMs) were prepared by combinations of two different kinds of porous fillers [metal-organic frameworks (MOFs) HKUST-1 and ZIF-8, and zeolite silicalite-1] and polysulfone. In the search for filler synergy, the MMMs were applied to the separation of CO(2)/N(2), CO(2)/CH(4), O(2)/N(2), and H(2)/CH(4) mixtures and we found important selectivity improvements with the HKUST-1-silicalite-1 system (CO(2)/CH(4) and CO(2)/N(2) separation factors of 22.4 and 38.0 with CO(2) permeabilities of 8.9 and 8.4 Barrer, respectively). 相似文献