全文获取类型
收费全文 | 2212篇 |
免费 | 72篇 |
国内免费 | 17篇 |
专业分类
化学 | 1379篇 |
晶体学 | 20篇 |
力学 | 60篇 |
数学 | 340篇 |
物理学 | 502篇 |
出版年
2023年 | 12篇 |
2022年 | 27篇 |
2021年 | 31篇 |
2020年 | 31篇 |
2019年 | 32篇 |
2018年 | 33篇 |
2017年 | 21篇 |
2016年 | 61篇 |
2015年 | 68篇 |
2014年 | 61篇 |
2013年 | 108篇 |
2012年 | 149篇 |
2011年 | 149篇 |
2010年 | 93篇 |
2009年 | 70篇 |
2008年 | 101篇 |
2007年 | 121篇 |
2006年 | 107篇 |
2005年 | 77篇 |
2004年 | 69篇 |
2003年 | 58篇 |
2002年 | 57篇 |
2001年 | 25篇 |
2000年 | 35篇 |
1999年 | 21篇 |
1998年 | 17篇 |
1997年 | 28篇 |
1996年 | 30篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 30篇 |
1992年 | 31篇 |
1991年 | 16篇 |
1990年 | 26篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1985年 | 16篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 24篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 21篇 |
1978年 | 13篇 |
1977年 | 23篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 18篇 |
1973年 | 20篇 |
排序方式: 共有2301条查询结果,搜索用时 15 毫秒
71.
Rutile Ni
x
Ti1-3x
Sb2x
O2 solid solution nanoparticles were synthesized by a sol-gel route using propylene oxide as a gelation agent. Titanium oxide
nanopowder and 12% TiCl3 solution were used as the source for titanium to investigate the influence of the titanium precursors on the formation of
the target materials. It was found that the nanoparticles prepared using 12% TiCl3 solution showed a much lower phase formation temperature (700°C) as compared to those prepared from TiO2 nanoparticles (1000°C). This lower phase formation temperature allowed a substantial reduction of the aggregation of the
particles during calcination leading to the formation of nearly mono-dispersed nanoparticles of about 20 nm. The results of
this work show that the epoxide assisted sol-gel method is capable to produce titanium-based ternary oxide solid solution
nanoparticles, owing to the formation of a highly homogeneous precursor gel intermediate. 相似文献
72.
Carlos A. Escobar Oscar Donoso‐Tauda Ramiro Araya‐Maturana Andrs Vega 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(7):o426-o430
The 1,5‐benzodiazepine ring system exhibits a puckered boat‐like conformation for all four title compounds [4‐(2‐hydroxyphenyl)‐2‐phenyl‐2,3‐dihydro‐1H‐1,5‐benzodiazepine, C21H18N2O, (I), 2‐(2,3‐dimethoxyphenyl)‐4‐(2‐hydroxyphenyl)‐2,3‐dihydro‐1H‐1,5‐benzodiazepine, C23H22N2O3, (II), 2‐(3,4‐dimethoxyphenyl)‐4‐(2‐hydroxyphenyl)‐2,3‐dihydro‐1H‐1,5‐benzodiazepine, C23H22N2O3, (III), and 2‐(2,5‐dimethoxyphenyl)‐4‐(2‐hydroxyphenyl)‐2,3‐dihydro‐1H‐1,5‐benzodiazepine, C23H22N2O3, (IV)]. The stereochemical correlation of the two C6 aromatic groups with respect to the benzodiazepine ring system is pseudo‐equatorial–equatorial for compounds (I) (the phenyl group), (II) (the 2,3‐dimethoxyphenyl group) and (III) (the 3,4‐dimethoxyphenyl group), while for (IV) (the 2,5‐dimethoxyphenyl group) the system is pseudo‐axial–equatorial. An intramolecular hydrogen bond between the hydroxyl OH group and a benzodiazepine N atom is present for all four compounds and defines a six‐membered ring, whose geometry is constant across the series. Although the molecular structures are similar, the supramolecular packing is different; compounds (I) and (IV) form chains, while (II) forms dimeric units and (III) displays a layered structure. The packing seems to depend on at least two factors: (i) the nature of the atoms defining the hydrogen bond and (ii) the number of intermolecular interactions of the types O—H...O, N—H...O, N—H...π(arene) or C—H...π(arene). 相似文献
73.
Ramón Serrano Roberto Gaxiola Gabino Ríos Javier Forment Oscar Vicente Roc Ros 《Monatshefte für Chemie / Chemical Monthly》2003,11(3):1445-1464
We have performed functional genomics of salt stress by overexpression of gene libraries in yeast and selection for salt tolerance. Thirty halotolerance genes were isolated from yeast, Arabidopsis, and sugar beet. The results indicate that Na+ transport (uptake, efflux, and compartmentation), sulfate activation, RNA processing, and protein synthesis are crucial for salt tolerance. 相似文献
74.
Maria Ana Castro Zulema D. Chaia Oscar E. Piro Fabio D. Cukiernik Eduardo E. Castellano Marcia Rusjan 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(7):m393-m395
The title compound, tetrakis(μ‐3,4,5‐triethoxybenzoato‐κ2O:O′)bis[(pyrazine‐κN)rhodium(II)](Rh—Rh), [Rh2(C13H17O5)4(C4H4N2)2], crystallizes on an inversion centre in the triclinic space group . The equatorial carboxylate ligands bridge the two RhII atoms, giving a binuclear lantern‐like structure. The pyrazine molecules occupy the two axial coordination sites. The phenyl rings are tilted by ca 10° with respect to the attached carboxylate groups. The pyrazine planes have a torsion angle of ca 19° around the Rh—N bond with respect to the plane of the nearer carboxylate group and are not coplanar with the Rh—Rh bond. 相似文献
75.
Matthias Tacke Lorcan T. Allen William M. Gallagher Oscar Mendoza Franz-Josef K. Rehmann 《Journal of organometallic chemistry》2004,689(13):2242-2249
Starting from 6-(p−N,N-dimethylanilinyl)fulvene (1a) or 6-(pentamethylphenyl)fulvene (1b) [1,2-di(cyclopentadienyl)-1,2-di(p−N,N-dimethylaminophenyl)ethanediyl] titanium dichloride (2a) and [1,2-di(cyclopentadienyl)-1,2-bis(pentamethylphenyl)ethanediyl] titanium dichloride (2b) and their corresponding dithiocyanato complexes (3a, 3b) were synthesized. Titanocene 2b did not show a cytotoxic effect, but when 2a was tested against pig kidney carcinoma cells (LLC-PK) or human ovarian carcinoma cells (A2780/cp70) inhibitory concentrations (IC50) of 2.7 × 10−4 and 1.9 × 10−4 M, respectively, were observed. 相似文献
76.
A series of 4,4'-disilyl-substituted-2,2'-bipyridine ligands were prepared using a metathesis reaction of the dianion of 4,4'-dimethyl-2,2'-bipyridine with several trialkylsilyl chlorides: 4,4'bis(tert-butyldimethylsilylmethyl)-2,2'-bipyridine (dTBDMSbipy), 4,4'-bis(dimethylthexylsilylmethyl)-2,2'-bipyridine (dTHEXbipy), and 4,4'-bis(neophyldimethylsilylmethyl)-2-2'-bipyridine (dNEObipy). It was observed that the side chain length correlated with the ability of the ligand to form hydrocarbon soluble complexes of copper(I) bromide, with dNEObipy forming the most soluble and easily crystallized complexes. The atom transfer radical polymerization (ATRP) of styrene using dNEObipy as the ligand displayed molecular weight control equivalent to other ATRP systems in which solubilizing ligands, such as 4,4'-di-5-nonyl-2,2'-bipyridine or 4,4'-di-n-heptyl-2,2'-bipyridine, were used. The one-to-one complex of dNEObipy with CuBr was prepared and its crystal structure was determined. The resulting complex had the ionic formulation [(dNEObipy)2Cu]+[CuBr2]- and displayed similar activities in styrene ATRP as the standard 2 dNEObipy/CuBr catalyst system. These and other polymerization results in addition to NMR experiments suggest that the predominant copper(I) species formed in ATRP solutions is the 2-to-1 ligand-to-copper(I) cation, [(dNEObipy)2Cu]+, with either a dihalocuprate or halide counteranion, depending upon the conditions. 相似文献
77.
2-Acetoxy-3,4-di-O-acetyl-D-arabinal (6), similar to its D-xylo analogue 4, reacted with benzyl alcohol by the tin(IV) chloride-promoted glycosylation to produce optically active (S)-2-benzyloxy-2H-pyran-3(6H)-one (8a). The L-arabinal derivative (5) gave 9a, the dihydropyranone enantiomer of 8a. These results indicated that the configuration of the C-4 stereocenter in the starting glycal defines the configuration of the new chiral center in the resulting dihydropyranone. The influence of other catalysts (BF(3) or iodine) employed for the glycosylation on the optical purity of the dihydropyranone was studied. Enantiomerically pure dihydropyranones 8b and 9c were obtained using chiral alcohols ((R)- and (S)-2-octanol, respectively) as glycosylating agents. Compounds 8a,b and 9a,c proved to be reactive dienophiles in thermal and Lewis acid-promoted Diels-Alder reactions. The addition of 2,3-dimethylbutadiene, cyclopentadiene, and 1,3-cyclohexadiene to the beta-pyranones 8a,b led to the corresponding adducts 10a,b, 12a,b, and 16a,b as major products. Enantiomeric cycloadducts were synthesized from the alpha-pyranones 9a,c. The main products were formed by highly facial-diastereoselective addition of dienes to the pyranone ring, guided by the sterical hindrance of the alkoxy substituent of the C-2 stereocenter. As cycloadditions with cycloalkadienes were also highly endo diastereoselective, these reactions gave access to pure tetrahydrobenzopyranones that carry a multitude of stereogenic centers installed in a predictable way. 相似文献
78.
Amos Korin Moshe Levy David Vofsi 《Journal of polymer science. Part A, Polymer chemistry》1980,18(1):109-121
Vinyl fluoride was polymerized by photochemical initiation in a continuous-flow cylindrical reactor at room temperature and at pressures of up to 30 atm. Copolymers with vinyl acetate were prepared in order to improve the solubility and processability of poly(vinyl fluoride) (PVF). The copolymers were hydrolyzed to the corresponding vinyl alcohol copolymers and yielded hydrophilic films that are strong and flexible only when swollen by water. It was found that on hydrolysis the Tg, Tm, and heat of fusion as well as degree of crystallinity increased. It was suggested that PVF and the copolymers with vinyl alcohol are isomorphous. 相似文献
79.
Rosario Pardo Marcos Zayat David Levy 《Journal of Sol-Gel Science and Technology》2006,40(2-3):365-370
A photochromic naphthopyran derivative was embedded in sol-gel prepared thin ormosil films. The resulting samples show high
transparency and exhibit a strong red colouration upon irradiation with UV light. The photostability of the photochromic molecules
is strongly related to the nature of the embedding ormosil matrix. The introduction of organic functional groups into the
inner pore surface of the matrix allows tailoring the chemical environment where the dye molecules will be allocated, in terms
of the effectiveness of the interaction between the photochromic molecules and the Si-OH groups on the surface of the pores,
affecting the stability of the molecules upon prolonged exposition to UV light.
The photostability of the molecules was increased in matrices functionalized with larger organic groups, or with larger amount
of modifying groups. In this way the photodegradation of the photochromic molecules could be reduced by a factor of 5, as
compared with the photodegradation of the molecules in unfunctionalized silica matrix. 相似文献
80.