全文获取类型
收费全文 | 884篇 |
免费 | 16篇 |
国内免费 | 11篇 |
专业分类
化学 | 545篇 |
晶体学 | 26篇 |
力学 | 47篇 |
数学 | 179篇 |
物理学 | 114篇 |
出版年
2023年 | 8篇 |
2022年 | 19篇 |
2021年 | 24篇 |
2020年 | 22篇 |
2019年 | 11篇 |
2018年 | 18篇 |
2017年 | 10篇 |
2016年 | 36篇 |
2015年 | 12篇 |
2014年 | 33篇 |
2013年 | 64篇 |
2012年 | 43篇 |
2011年 | 42篇 |
2010年 | 34篇 |
2009年 | 41篇 |
2008年 | 57篇 |
2007年 | 35篇 |
2006年 | 42篇 |
2005年 | 30篇 |
2004年 | 29篇 |
2003年 | 30篇 |
2002年 | 35篇 |
2001年 | 11篇 |
2000年 | 18篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 10篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1965年 | 2篇 |
1916年 | 4篇 |
排序方式: 共有911条查询结果,搜索用时 15 毫秒
901.
902.
We consider a class of autonomous Hamiltonian systems subject to small, time-periodic perturbations. When the perturbation parameter is set to zero, the energy of the system is preserved. This is no longer the case when the perturbation parameter is non-zero. We describe a topological method to establish orbits which diffuse in energy for every suitably small perturbation parameter . The method yields quantitative estimates:
- (i) the existence of orbits along which the energy drifts by an amount independent of ε; the time required by such orbits to drift is ;
- (ii) the existence of orbits along which the energy makes chaotic excursions;
- (iii) explicit estimates for the Hausdorff dimension of the set of such chaotic orbits;
- (iv) the existence of orbits along which the time evolution of energy approaches a stopped diffusion process (Brownian motion with drift), as ε tends to 0. For each ε fixed, the set of initial conditions of the orbits that yield the diffusion process has positive Lebesgue measure, and in the limit the measure of these sets approaches 0. Moreover, we can obtain any desired values of the drift and variance for the limiting Brownian motion for appropriate sets of initial conditions.
903.
Marcin Kozanecki Jacek Ulaski Piotr Wojciechowski Marian Kryszewski Micheline Boudeulle Eugen Duval 《Macromolecular Symposia》1999,141(1):185-195
Novel anisotropic polymer composites obtained by the photopolymerisation of acrylic acid (AA) in lyotropic solutions of (2-hydroxypropyl)cellulose (HPC) were investigated. Raman spectroscopy was used in the study of hydrogen bonding formed by carboxylic groups of poly(acrylic acid) (pAA) between themselves and with the hydroxyl groups of HPC macromolecules. These investigations show that the supramolecular structure of pAA obtained by photopolymerisation contains mainly inner oligomeric forms and cyclic dimer forms, as a consequence of the “matrix polymerisation” of AA. The presence of HPC affects the process of the photopolymerisation of AA, by a redistribution of hydrogen bonding within the system, leading to a different supramolecular structure of pAA. With increasing amounts of HPC the inner oligomeric forms characteristic of bulk pAA disappear. In the composites, the pAA chains probably form the terminal oligomeric structure, with some free COOH groups, as well as hydrogen bonds with HPC. 相似文献
904.
Joanna Pietrasik Marian Zaborski Gisele Boiteux Olivier Gain 《Macromolecular Symposia》2003,194(1):321-328
We tried to synthesise silica's fillers in polar and nonpolar elastomers. Di-, tri- and tetraalkoxysilanes were used as a precursor of silica. Such functionality allowed us to obtain a filler with different ratio of crosslinking. We could expect some interactions between elastomer and silanes, which contained additional functional groups. We investigated vulcanizates by using the following methods: crosslink density in toluene and 10% solution of ethylenediamine in toluene, mechanical properties, IR microscopy, DMA. It appeared that alkoxysilanes influenced advantageously on elastomers and properties. 相似文献
905.
M. Sc. Marian J. Menke Dr. Pascal Schneider Dr. Christoffel P. S. Badenhorst Dr. Andreas Kunzendorf B. Sc. Florian Heinz Dr. Mark Dörr Dr. Martin A. Hayes Prof. Uwe T. Bornscheuer 《Angewandte Chemie (International ed. in English)》2023,62(51):e202313912
Enzyme-catalyzed late-stage functionalization (LSF), such as methylation of drug molecules and lead structures, enables direct access to more potent active pharmaceutical ingredients (API). S-adenosyl-l -methionine-dependent methyltransferases (MTs) can play a key role in the development of new APIs, as they catalyze the chemo- and regioselective methylation of O-, N-, S- and C-atoms, being superior to traditional chemical routes. To identify suitable MTs, we developed a continuous fluorescence-based, high-throughput assay for SAM-dependent methyltransferases, which facilitates screening using E. coli cell lysates. This assay involves two enzymatic steps for the conversion of S-adenosyl-l -homocysteine into H2S to result in a selective fluorescence readout via reduction of an azidocoumarin sulfide probe. Investigation of two O-MTs and an N-MT confirmed that this assay is suitable for the determination of methyltransferase activity in E. coli cell lysates. 相似文献
906.
Statistics of nucleation of chiral forms of sodium chlorate from unstirred, achiral solutions was studied. The trimodal distribution of enantiomers was obtained in which apart from the peak for the same probability of the creation of L and D crystals, two other maxima for the large enantiomeric excesses of L and D forms were present. The first nucleated crystals govern the handedness of the secondary crystals and the formation of pure enantiomers and high enantiomeric excesses in crystallization from unstirred, unseeded solutions are caused by the process of chiral symmetry breaking rather than statistical fluctuations. 相似文献
907.
The subject of this research work was an effect of liquid crystals on the properties of filled elastomer systems. Both commercially available and own synthesized liquid crystals were used. The elastomer was ethylene-propylene rubber, while precipitated silica and calcium carbonate were used as fillers. The liquid crystals were deposited on the surface of the fillers from solution. Their effect on the mechanical properties of the vulcanizates was studied. 相似文献
908.
Ciplea Sorina Anamaria Lungu Nicolaie Marian Daniela Rassias Themistocles M. 《Aequationes Mathematicae》2023,97(4):649-657
Aequationes mathematicae - In this paper we will study Hyers-Ulam stability for a general linear partial differential equation of first order in a Banach space. 相似文献
909.
Dr. Yu-Fei Ao Shuxin Pei Dr. Chao Xiang Marian J. Menke Prof. Lin Shen Dr. Chenghai Sun Dr. Mark Dörr Dr. Stefan Born Prof. Matthias Höhne Prof. Uwe T. Bornscheuer 《Angewandte Chemie (International ed. in English)》2023,62(23):e202301660
Amine transaminases (ATAs) are powerful biocatalysts for the stereoselective synthesis of chiral amines. Machine learning provides a promising approach for protein engineering, but activity prediction models for ATAs remain elusive due to the difficulty of obtaining high-quality training data. Thus, we first created variants of the ATA from Ruegeria sp. (3FCR) with improved catalytic activity (up to 2000-fold) as well as reversed stereoselectivity by a structure-dependent rational design and collected a high-quality dataset in this process. Subsequently, we designed a modified one-hot code to describe steric and electronic effects of substrates and residues within ATAs. Finally, we built a gradient boosting regression tree predictor for catalytic activity and stereoselectivity, and applied this for the data-driven design of optimized variants which then showed improved activity (up to 3-fold compared to the best variants previously identified). We also demonstrated that the model can predict the catalytic activity for ATA variants of another origin by retraining with a small set of additional data. 相似文献
910.
To characterize an activity of silica toward elastomers usually the parameters of the filler structure and a possibility of its interactions with macromolecules of a matrix are taken into consideration [1,2,3]. As a rule, low molecular weight substances of different polarity are dissolved in a rubber mix (e.g. accelerators, activators, crosslinking substances, etc). They undergo adsorption on the surface of active fillers what exerts an effect on a structure of a network formed and influences mechanical properties of vulcanizates at the same time. 相似文献