全文获取类型
收费全文 | 3401篇 |
免费 | 82篇 |
国内免费 | 17篇 |
专业分类
化学 | 2284篇 |
晶体学 | 21篇 |
力学 | 128篇 |
数学 | 551篇 |
物理学 | 516篇 |
出版年
2022年 | 23篇 |
2021年 | 33篇 |
2020年 | 40篇 |
2019年 | 50篇 |
2018年 | 32篇 |
2017年 | 36篇 |
2016年 | 70篇 |
2015年 | 49篇 |
2014年 | 74篇 |
2013年 | 181篇 |
2012年 | 155篇 |
2011年 | 184篇 |
2010年 | 124篇 |
2009年 | 117篇 |
2008年 | 191篇 |
2007年 | 173篇 |
2006年 | 182篇 |
2005年 | 199篇 |
2004年 | 176篇 |
2003年 | 137篇 |
2002年 | 152篇 |
2001年 | 55篇 |
2000年 | 45篇 |
1999年 | 47篇 |
1998年 | 50篇 |
1997年 | 54篇 |
1996年 | 39篇 |
1995年 | 27篇 |
1994年 | 45篇 |
1993年 | 49篇 |
1992年 | 37篇 |
1991年 | 35篇 |
1990年 | 22篇 |
1989年 | 26篇 |
1988年 | 14篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 37篇 |
1984年 | 36篇 |
1983年 | 22篇 |
1982年 | 32篇 |
1981年 | 35篇 |
1980年 | 40篇 |
1979年 | 29篇 |
1978年 | 25篇 |
1977年 | 38篇 |
1976年 | 37篇 |
1975年 | 36篇 |
1974年 | 26篇 |
1973年 | 15篇 |
排序方式: 共有3500条查询结果,搜索用时 15 毫秒
101.
Alain Gioan 《Applied Mathematics and Optimization》1982,8(1):59-67
LetV andW be two Banach spaces, withV reflexive, a bounded convex set ofV, A a linear mapping fromV intoW, and letF be a convex functional onW. We minimizeJ(v)=F(Av) on using hypotheses about particular sequences in IfV is uniformly convex, we prove existence and uniqueness of a solution of minimal norm minimizingJ. In the Hilbert space case, withF defined byF(w)=w–f
2,f given inW, we get existence and uniqueness of the projection off on A(), which generalizes the case where A() is a closed set ofW (taking closed andA continuous). Finally, we give examples, and we study an unbounded operator case. 相似文献
102.
Stéphane Pellet-RostaingFrédéric Chitry Jean-Alexis SpitzAntoine Sorin Alain Favre-RéguillonMarc Lemaire 《Tetrahedron》2003,59(51):10313-10324
Calix-bis(benzocrown-6) 6 and 7 were converted into the water-soluble receptors 9, 10, 12 and 15 by introducing hydroxy, carboxy, sulfato or diethanolamino groups at the para position of the phenolic ring and/or on the benzo-ether moieties. The complexation properties of these ionophores were studied for all alkali cations in methanolic and aqueous media. Stability constants were calculated by UV-Vis spectroscopy. All ligands showed a more or less affinity for the larger cations, depending on the nature and the position of the substituents grafted on the benzo-ether chain only or both on the calixarene ring and the benzo-ether loop. For selective Cs+/Na+ separation, the efficiency of the ligands was evaluated by means of a nanofiltration system. In comparison with the known tetrahydroxylated bis-crown-6 calix[4]arene 1, compounds 9, 12 and 15 represent the most selective ligands for the Cs+ cation in a moderate salted medium ([NaNO3]=85 g/L). 相似文献
103.
Frank A Dunand Alain Borel André E Merbach 《Journal of the American Chemical Society》2002,124(4):710-716
Taking advantage of the Curie contribution to the relaxation of the protons in the Tb(III) complex, and the quadrupolar relaxation of the 17O and 2H nuclei on the Eu(III) complex, the effect of the internal motion of the water molecule bound to [Ln(DOTAM)(H2O)]3+ complexes was quantified. The determination of the quadrupolar coupling constant of the bound water oxygen chi(Omicron)(1 + eta(Omicron)2/3)1/2 = 5.2 +/- 0.5 MHz allows a new analysis of the 17O and 1H NMR data of the [Gd(DOTA)(H2O)]- complex with different rotational correlation times for the Gd(III)-O(water) and Gd(III)-H(water) vectors. The ratio of the rotational correlation times for the Ln(III)-H(water) vector and the overall rotational correlation time is calculated tau(RH)/tau(RO) = 0.65 +/- 0.2. This could have negative consequences on the water proton relaxivity, which we discuss in particular for macromolecular systems. It appears that the final effect is actually attenuated and should be around 10% for such large systems undergoing local motion of the chelating groups. 相似文献
104.
(Z)-Ethyl-3-perfluoroalkyl-3-magnesiated crotonates were prepared from (Z)-ethyl-3-perfluoroalkyl-3-iodo enoates by iodine-magnesium exchange reaction with isopropylmagnesium bromide in THF at -78 degrees C. These new reagents reacted with a range of electrophiles, leading to polyfunctional products bearing a perfluoroalkyl group. 相似文献
105.
Alain Valla Benoist Valla Régis Le Guillou Dominique Cartier Laurent Dufossé Roger Labia 《Helvetica chimica acta》2007,90(3):512-520
Since the C15 β‐end‐group aldehyde 10 ((β‐ionylidene)acetaldehyde), an excellent intermediate in the syntheses of retinoids, can be synthesized in many ways from β‐ionone, and since the corresponding acyclic C15 ψ‐end‐group aldehyde 5 can easily be synthesized from citral ( 1 ) (Scheme 3), we applied the C15+C5 route to the syntheses of γ‐retinal ((all‐E)‐ 8 ) (Scheme 3) and retinal ((all‐E)‐ 13 ) (Scheme 4), and therefore, by coupling (2×C20→C40), to the preparation of lycopene ( 14 ) and β‐carotene ( 15 ) (Scheme 5). Our new syntheses of retinal ((all‐E)‐ 13 ) and γ‐retinal ((all‐E)‐ 8 use an extended aldol reaction with a C6 building block that incorporates a C5 unit after decarboxylation. 相似文献
106.
Alain Glangetas Fazil O. Gülacar Jean M. J. Tronchet Armand Buchs 《Helvetica chimica acta》1980,63(6):1740-1747
Electron Impact Mass Spectrometry of the 3-Desoxy-1,2: 5,6-di-O-isopropylidene-3-methylidene-δ-D -hexofuranose and Some C(3′)-Substituted Analogues The mass spectra of the 3-desoxy-1,2:5, 6-di-O-isopropylidene-3-methylidene- α-D -ribo-hexofuranose and of some C(3′)-mono- and -disubstituted derivatives have been investigated. Deuterium labelled molecules allow fragmentation modes to be proposed. 相似文献
107.
Philippe Andrianary Gérard Jenner Alain Kiennemann 《Journal of organometallic chemistry》1983,252(2):209-220
The CO/H2 homologation of methanol to acetaldehyde and subsequently to its dimethyl acetal in the presence of cobalt acetate promoted by iodine was examined under various conditions. Temperature and pressure were found as critical parameters. High pressures (140 MPa) and low temperatures (160–170°C) give optimal yields and selectivity to acetaldehyde. According to pressure, temperature, contact time, gas ratio and ligand/catalyst ratio, the reaction is oriented towards acetaldehyde, its dimethyl acetal or methyl acetate. 相似文献
108.
Trombotto S Danel M Fitremann J Bouchu A Queneau Y 《The Journal of organic chemistry》2003,68(17):6672-6678
The base-catalyzed reaction of carboxymethyl 3,4,6-tri-O-acetyl-alpha-D-glucopyranoside 2-O-lactone (prepared from isomaltulose) with amino acids and fatty amines under basic catalysis gave a series of new pseudoglucopeptides, nonionic amphiphiles, and polymerizable derivatives. The same reaction applied to alcohols provided the corresponding 2-(alpha-D-glucopyranosyloxy)acetyl esters with either basic or acidic catalysts. 相似文献
109.
Osei-Twum EY Litorja LA Darkwa J Maisela LL Lesimple A Mamer O 《Journal of the American Society for Mass Spectrometry》2005,16(1):94-99
The mass spectral behavior of a number of organometallic complexes containing the Group 10 metals Ni, Pd, and Pt, together with various thiolate ligands were studied. For Pd, two main types of complexes, differing by the substituents on the phosphorus atom were studied. Types I and II were substituted with bis(diphenylphosphino)ferrocene and bis(diisopropylphosphino)ferrocene ligands, respectively. The Ni complexes, except for one, and the Pd Type I complexes had no molecular radical cations (M(+.)) in their EI spectra. On the other hand, all the Pt complexes showed intense M(+.) ions in their EI spectra indicating that these complexes were more stable as radical cations than those of Ni and Pd. The FAB and MALDI spectra of all the complexes displayed intense quasi-molecular ions (MH(+)) and the fragmentations in both modes were similar. The MALDI spectra of several complexes displayed only M(+.) ions while one gave evidence of both MH(+) and M(+.) ions. Several Pd Type II complexes yielded intense M(+.) in their EI spectra. 相似文献
110.
Valérie Chevalier-Billosta Jean-Paul Joseleau Alain Cochaux Katia Ruel 《Cellulose (London, England)》2007,14(2):141-152
The composition and ultrastructural arrangement of cell wall polymers in wood fibres have determining influence on the properties
of wood derived materials. It is therefore important to improve our understanding of the relationship between fibres organisation,
the modifications induced by pulping treatments, and the resulting paper sheet mechanical properties. The different treatments
to which fibres are subjected during the manufacturing of pulps and papers induce morphological and micro-structural alterations
due to the removal of wall constituents and of microfibrillar elements. The impact of pulping processes on fibres was investigated
at the ultrastructural scale of transmission electron microscopy. Particular attention was given to the effects of beating
in refiners at various intensities on the ultrastructure of fibres. The most characteristic effects consisted of delaminations,
microfibril disorganisation, and even fractures, of varying importance depending on the intensity of the mechanical refining.
The consequences of internal alterations and surface modifications of the fibres were examined in relation to the paper sheet
mechanical properties. Correlations between the type of alteration observed in the fibres and its possible impact on a given
paper mechanical property are suggested. With similar approaches, the effects of drying and recycling were studied. 相似文献