全文获取类型
收费全文 | 447篇 |
免费 | 17篇 |
国内免费 | 4篇 |
专业分类
化学 | 291篇 |
晶体学 | 1篇 |
力学 | 12篇 |
数学 | 54篇 |
物理学 | 110篇 |
出版年
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 11篇 |
2019年 | 8篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 14篇 |
2014年 | 11篇 |
2013年 | 27篇 |
2012年 | 25篇 |
2011年 | 33篇 |
2010年 | 17篇 |
2009年 | 17篇 |
2008年 | 27篇 |
2007年 | 19篇 |
2006年 | 13篇 |
2005年 | 23篇 |
2004年 | 14篇 |
2003年 | 14篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 20篇 |
1999年 | 15篇 |
1998年 | 10篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1988年 | 7篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1959年 | 1篇 |
排序方式: 共有468条查询结果,搜索用时 26 毫秒
461.
A polystyrene-b-poly-4-vinypyridine (PS-b-P4VP) diblock copolymer is modified with a gold precursor to obtain an organic–inorganic (hybrid) block copolymer in bulk
with gold nanoparticles selectively incorporated in the P4VP block. In the linear viscoelastic regime, temperature sweep tests
over a series of these hybrid block copolymer systems revealed consistent shifts (ΔT) in the glass transition temperatures (both T
g\text-PS_{\rm g\text{-}PS} and T
g\text-P4VP_{\rm g\text{-}P4VP}) of the hybrid materials in comparison to the pristine polymers. Studying different volume fractions of the pyridine block,
a level-off point was found for block copolymers with f
P4VP > 0.26, where the shifts in T
g\text-P4VP_{\rm g\text{-}P4VP} consistently increased up to ΔT = 25°C. By artificially increasing the volume fraction of the pyridine block, the nanoparticles reduce the transition regime
determined in master curves. At higher volume fractions of the pyridine block, crossover frequencies were not detected after
the entanglement regime, indicating that the material does not relax from topological constraints (entanglements and nanoparticles)
into the terminal regime. Above a specific volume fraction of nanoparticles (Φ
P = 0.05), the flow behaviour of the hybrid materials becomes increasingly elastic, exhibiting wall-slip from the geometry
at lower strain values in comparison to the pristine material. In the non-linear viscoelastic regime, Fourier-transformed
rheology was used to analyse the raw signals from strain sweep experiments. It was clearly demonstrated the nanoparticle effect
by following the second and third harmonic (I
2/1, I
3/1) of the stress response. Comparing the behaviour of the third and second harmonics provided an unambiguous fingerprint for
the effect of the nanoparticles. 相似文献
462.
463.
E. I. Klimova T. Klimova Berestneva J. M. Martínez Mendoza J. M. Méndez Stivalet R. A. Toscano M. Martínez García 《合成通讯》2013,43(14):2299-2315
2-Ferrocenylmethylidene-1,2-diphenylpropanedione (3), 2,4-dibenzoyl-3-ferrocenyl-1,5-diphenylpentane-1,5-dione (4), and 2,4-dibenzoyl-3-ferrocenyl-2-[(ferrocenyl)hydroxymethyl]-1,5-diphenylpentane-1,5-dione (5) react with MeLi to undergo fragmentation and 1,2-addition or only 1,2-addition at the carbonyl group. Dehydration of intermediate tertiary alcohols affords α-methylstyrene (6), 3-ferrocenyl-1-phenylprop-2-enone (7), 3,5-diferrocenyl-1-phenyl-4-(1-phenylvinyl)cyclohexene (8), 3-ferrocenylmethylidene-2,4-diphenylpenta-1,4-diene (9), 2-benzoyl-1-ferrocenyl-3-phenylbuta-1,3-diene (10), 2-benzoyl-1-ferrocenyl-3-methylindene (11), 4-ferrocenyl-2-methyl-2,6-diphenyl-3,4-dihydro-2H-pyran (19), and (Z,Z)-2,4-dibenzoyl-1,3-diferrocenyl-5-phenylhexa-1,4-diene (21), isolated by chromatography. The spatial structures of ferrocenyldihydropyran (19) and diferrocenylhexadiene (21) were established by X-ray diffraction analysis. 相似文献
464.
Pozzolans play an important role in the industry of cement and concrete. They increase the mechanical strength of cement matrices and can be used to decrease the amount of cement in concrete mixtures, thus decreasing the final economic and environmental cost of production; also, as some of them are byproducts of industrial processes (such as silica fume and fly ash) and their use can be seen as a solution for some residues, that otherwise would be disposed as a waste. Pozzolans fixate the Ca(OH)2 generated during cement’s hydration reactions to form calcium silicate hydrates (C–S–H), calcium aluminate hydrates (C–A–H), or calcium aluminosilicate hydrates (C–A–S–H), depending on the nature of the pozzolan. Traditionally, the pozzolanic activity is identified using the Ca(OH)2 fixation percentage which is quantified by thermogravimetric (TG) analysis, using the mass loss due to the Ca(OH)2 dehydroxylation around 500 °C. An alternative method to identify pozzolanic activity at lower temperatures using a standard issue moisture analyzer (MA) is presented in this paper, using the mass loss due to hydrate’s dehydration generated by pozzolans in the pozzolanic reaction. Samples of Ca(OH)2 blended with different pozzolans were prepared and tested at different hydration ages. Using TG analysis and an MA, a good correlation was found between the total mass loss of the same sample, using the two methods at the same temperature. It was concluded that the MA method can be considered a less expensive and less time-consuming alternative to identify pozzolanic activity of siliceous or aluminosiliceous materials. 相似文献
465.
Egla Y. Bivin‐Castro Sylvain Berns Jaime Escalante Guillermo Mendoza‐Díaz 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(5):m205-m207
In the cationic complex present in the title compound, chloro[2‐(4‐imidazolyl‐κN1)ethylamine‐κN](1,10‐phenanthroline‐κ2N,N′)copper(II) chloride monohydrate, [CuCl(C5H9N3)(C12H8N2)]Cl·H2O, the metal centre adopts a five‐coordinate geometry, ligated by the two phenanthroline N atoms, two amine N atoms of the histamine ligand (one aliphatic and one from the imidazole ring) and a chloro ligand. The geometry around the Cu atom is a distorted compressed trigonal bipyramid, with one phenanthroline N and one imidazole N atom in the axial positions, and the other phenanthroline N atom, the histamine amine N atom and the chloro ligand in the equatorial positions. The structure includes an uncoordinated water molecule, and a Cl− ion to complete the charge. The water molecule is hydrogen bonded to both Cl− ions (coordinated and uncoordinated), and exhibits a close Cu⋯H contact in the equatorial plane of the bipyramid. 相似文献
466.
We introduce an algorithm to generate two-dimensional diffusion-limited star-branched aggregates (DLSA)attaching bi-functional
monomers successively to a central colloidal particle with any desired number of reactive sites. The proposed algorithm produces
star-shaped aggregates that grow forever and show a power law polydispersity in the chemical length of the arms near the central
colloid. More interestingly, it gives rise to a number of arm selection consisting in that only a small number of arms (around
five) define the final structure at relatively large distances from the central colloid, independently of the initial number
of reactive sites and the size of the central colloid. We characterize the structure of the aggregates by means of the particle-particle
correlation function, analyze its scaling properties and obtain the fractal dimension. 相似文献
467.
468.
Ferran Planas Stefanie V. Kohlhepp Genping Huang Abraham Mendoza Fahmi Himo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(8):2767-2773
The dynamic equilibria of organomagnesium reagents are known to be very complex, and the relative reactivity of their components is poorly understood. Herein, a combination of DFT calculations and kinetic experiments is employed to investigate the detailed reaction mechanism of the Pummerer coupling between sulfoxides and turbo-organomagnesium amides. Among the various aggregates studied, unprecedented heterometallic open cubane structures are demonstrated to yield favorable barriers through a concerted anion-anion coupling/ S−O cleavage step. Beyond a structural curiosity, these results introduce open cubane organometallics as key reactive intermediates in turbo-organomagnesium amide mixtures. 相似文献