全文获取类型
收费全文 | 3726篇 |
免费 | 177篇 |
国内免费 | 36篇 |
专业分类
化学 | 2462篇 |
晶体学 | 27篇 |
力学 | 111篇 |
数学 | 689篇 |
物理学 | 650篇 |
出版年
2023年 | 22篇 |
2022年 | 38篇 |
2021年 | 70篇 |
2020年 | 70篇 |
2019年 | 91篇 |
2018年 | 83篇 |
2017年 | 56篇 |
2016年 | 132篇 |
2015年 | 126篇 |
2014年 | 167篇 |
2013年 | 233篇 |
2012年 | 288篇 |
2011年 | 336篇 |
2010年 | 183篇 |
2009年 | 168篇 |
2008年 | 250篇 |
2007年 | 230篇 |
2006年 | 211篇 |
2005年 | 195篇 |
2004年 | 167篇 |
2003年 | 155篇 |
2002年 | 120篇 |
2001年 | 71篇 |
2000年 | 56篇 |
1999年 | 34篇 |
1998年 | 36篇 |
1997年 | 30篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 33篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1961年 | 3篇 |
1942年 | 6篇 |
1940年 | 3篇 |
排序方式: 共有3939条查询结果,搜索用时 31 毫秒
991.
Maia Campos PM Gianeti MD Kanashiro A Lucisano-Valim YM Gaspar LR 《Photochemistry and photobiology》2006,82(3):683-688
A new tendency in cosmetic formulations is the association of botanical extracts and vitamins to improve skin conditions by synergic effects. The objective of this study was to determine the antioxidant activity of associated bioflavonoids, retinyl palmitate (RP), tocopheryl acetate (TA) and ascorbyl tetraisopalmitate (ATIP), as well as their photoprotective effects in preventing increased erythema, transepidermal water loss (TEWL) and sunburn cell formation in hairless mouse skin. The antioxidant activity of solutions containing the association or each substance separately was evaluated in vitro by a chemiluminescence assay. The photoprotective effect was evaluated by means of in vivo tests. Dorsal skin of hairless mice was treated daily by topical applications for 5 days with formulations containing or not containing (vehicle) the flavonoid-vitamins association (5%). The skin was irradiated (UVA/B) 15 minutes after the last application. The results showed that bioflavonoids had in vitro antioxidant properties and also that when they were associated with vitamins their antioxidant activity was more pronounced. On the other hand, erythema and UV damage to the permeability barrier function (TEWL) was not significantly reduced by previous treatment with the flavonoid-vitamin-association formulations, when compared to the irradiated vehicle-treated area. However, the treatment protected the skin from UV damage because it reduced the number of sunburn cells, when compared to the vehicle-treated area. Finally, the association of vitamins and bioflavonoids added to a dermocosmetic formulation showed a relevant biological activity in terms of photoprotection, because the association of bioflavonoids and vitamins acted by different mechanisms, such as antioxidation and absorption of UV radiation, which suggests its use in antiaging and photoprotective products. 相似文献
992.
Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between theta=13 and theta=1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on (111) surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results (i.e., scanning-tunneling microscopy images of atomic adlayers). 相似文献
993.
Reyes LH Encinar JR Marchante-Gayón JM Alonso JI Sanz-Medel A 《Journal of chromatography. A》2006,1110(1-2):108-116
In order to investigate the potentially bioavailable selenium-containing compounds in the selenized yeast candidate reference material SEAS 6, a two-dimensional (size exclusion-reversed phase) chromatography approach has been worked out. Electrospray tandem mass spectrometry (ESI Q-TOF MS) was then used for off-line identification of low molecular weigh selenocompounds generated during the gastrointestinal digestion. Selenomethionine (SeMet) was the major compound identified in the gastrointestinal extract while SeMet selenoxide was its main degradation product formed after medium and long-term sample storage, respectively. Total Se and SeMet were quantified in both the soluble extracts and the residue. Results showed that 89+/-3% of total Se was extracted after gastrointestinal digestion, but only 34+/-1% was surprisingly quantified as free SeMet. The rest of Se was present as many other low, medium and high molecular weight Se-species, which could be detected and further characterized by using the two-dimensional chromatography approach proposed here. Interestingly, most of Se-species seemed to be Se-peptides unspecifically produced by the gastrointestinal juice. These results show for the first time that while the efficiency of human gastrointestinal digestion to dissolve Se-containing proteins present in yeast may be high, its efficiency to convert them into free SeMet is much lower. Se-species present in the insoluble residue (not assimilated by the organism), accounting for 11+/-1% of the total Se in selenized yeast, were also studied. After treatment with SDS (denaturing agent) only 13+/-2% of this "insoluble" Se was solubilized, indicating that it was mainly non-protein bound and likely associated to other insoluble matrix components. 相似文献
994.
Bonincontro A Spigone E Ruiz Peña M Letizia C La Mesa C 《Journal of colloid and interface science》2006,304(2):342-347
Mixing aqueous sodium dodecylsulfate with cetyltrimethylammonium bromide solutions in mole ratios close to (1.7/1.0) allows the formation of cat-anionic vesicles with an excess of negative charges on the outer surface. The vesicular dispersions are mixed with lysozyme, and interact electrostatically with the positive charges on the protein, forming lipo-plexes. Dielectric relaxation, zeta-potential, and light scattering indicate the occurrence of interactions between vesicles and the protein. According to CD, the vesicle-adsorbed protein retains its native conformation. Binding and surface saturation, inferred by dielectric relaxation and zeta-potential, fulfil a charge neutralisation stoichiometry. Adsorbed lysozyme promotes the vesicle clustering and is concomitant with the lipo-plexes flocculation. Above the charge neutralisation threshold, lysozyme in excess remains dispersed in molecular form. Attempts were made to determine in what conditions protein release from the vesicles occurs. Accordingly, the full neutralisation of sodium dodecylsulfate in excess by cetyltrimethylammonium bromide ensures the lipo-plexes break-up, the precipitation of the mixed surfactants and the protein release in native form. 相似文献
995.
Martinez A Hemmert C Loup C Barré G Meunier B 《The Journal of organic chemistry》2006,71(4):1449-1457
We describe a general synthetic strategy for the preparation of a series of macrocyclic chiral manganese(III) salen complexes. The developed reaction pathway allows the modulation of the different key groups, namely, the chiral diimine, the bulky substituents in positions 3 and 3', and the linker used in the macrocyclization of the Schiff base. The different complexes presented here illustrate these readily available structural variations. The catalytic properties of the catalysts (5 mol %) were improved for the asymmetric epoxidation of 2,2'-dimethylchromene with NaOCl or H2O2 as oxygen atom donor. A large range of enantiomeric excesses was obtained (ee values from 30% to 96%), depending on the features and the stability of the complexes. The most efficient catalyst, in terms of stereoinduction (ee value = 96%), contains a diiminocyclohexyl moiety, ethyl groups in positions 3 and 3', and a short polyether junction arm. 相似文献
996.
Ilyn AP Loseva MV Vvedensky VY Putsykina EB Tkachenko SE Kravchenko DV Khvat AV Krasavin MY Ivachtchenko AV 《The Journal of organic chemistry》2006,71(7):2811-2819
We present a convenient synthesis of novel heteroaryl-fused 3-oxo-1,4-thiazepine-5-carboxamides and 5-oxo-1,4-thiazepine-3-carboxamides using a modification of four-component Ugi condensation. We demonstrate the usefulness and versatility of the developed approach for the synthesis of variously substituted compounds and discuss the scope and limitations of the chemistry involved. 相似文献
997.
Siemer AB Arnold AA Ritter C Westfeld T Ernst M Riek R Meier BH 《Journal of the American Chemical Society》2006,128(40):13224-13228
We report the observation of undetected (until now) residues of the prion protein fragment HET-s(218-289) which give rise to well-resolved (13)C, (15)N, and (1)H NMR resonances under high-resolution magic-angle spinning (HRMAS) conditions. The observed signals belong to large polymeric units as shown by measuring the lateral diffusion constants. The amino acids identified in the spectra are compatible with their localization in the segments of the protein which could not be detected in earlier solid-state NMR experiments. The observed chemical shifts indicate that these residues are in a random-coil conformation. Complementary experiments which detect only dynamic or static residues, respectively, strongly suggest that they belong to different parts of the same molecule. 相似文献
998.
999.
Méry D Plault L Ornelas C Ruiz J Nlate S Astruc D Blais JC Rodrigues J Cordier S Kirakci K Perrin C 《Inorganic chemistry》2006,45(3):1156-1167
Hexasubstitution of apical triflate ligands in the octahedral clusters [M]2[Mo6X8(CF3SO3)6] (M = n-Bu4N or Cs, X = Br or I) and monosubstitution in [n-Bu4N]2[Mo6Br13(CF3SO3)] was carried out in tetrahydrofuran at 60 degrees C with simple pyridines and then extended to organometallic pyridines, yielding cluster-cored stars, and to dendronic polyallyl- and polyferrocenylpyridines, yielding cluster-cored polyallyl and polyferrocenyl dendrimers and dendrons. The orange pyridine-substituted clusters, whose pyridine protons are deshielded in 1H NMR (a practical tool for characterization), are air-stable and thermally stable with simple pyridines, light- and air-sensitive with organometallic pyridines, and air-fragile and thermally fragile with large dendronized pyridines. 相似文献
1000.
Amo-Ochoa P Rodríguez-Tapiador MI Castillo O Olea D Guijarro A Alexandre SS Gómez-Herrero J Zamora F 《Inorganic chemistry》2006,45(19):7642-7650
The direct reaction between 6-mercaptopurine (6-MP) and Cd(II) under different conditions yields either [Cd2(6-MP)4(NO3)2](NO3)2 (1) or [Cd(6-MP-)2.2H2O]n (4). Compound 1 behaves as the building block of the polymer [Cd(6-MP2-)2]n[Ca(H2O)6]n (3), by deprotonation of 6-MP ligand. In the reaction of 1 to give 3, the dinuclear compound [Cd2(6-MP)4(H2O)2](NO3)4.2H2O (2) can be isolated as an intermediate. Polymers 3 and 4 convert into each other in water via deprotonation-protonation reactions. The structures of compounds 1-3 have been determined by X-ray diffraction. Given the small differences in the arrangement shown in the crystal structures of the polymer 4 and the polyanion of 3, the stabilities and energetics of the two arrangements have been examined by DFT calculations to determine the possibility of identifying new conformations of both polymers. In addition, the two polymers have been characterized on surfaces by means of AFM. The direct reaction between 6-MP and Cd(II) and the deprotonation of the polymer 4 have proven to be useful routes for the isolation of one-dimensional systems on surfaces. The development of new strategies to characterize these types of polymers on surfaces opens the possibility to perform nanoscale studies on their properties and their potential use as nanomaterials. 相似文献