全文获取类型
收费全文 | 741篇 |
免费 | 68篇 |
专业分类
化学 | 574篇 |
晶体学 | 5篇 |
力学 | 15篇 |
数学 | 67篇 |
物理学 | 148篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 17篇 |
2020年 | 4篇 |
2019年 | 17篇 |
2018年 | 9篇 |
2017年 | 5篇 |
2016年 | 34篇 |
2015年 | 21篇 |
2014年 | 31篇 |
2013年 | 54篇 |
2012年 | 82篇 |
2011年 | 82篇 |
2010年 | 40篇 |
2009年 | 34篇 |
2008年 | 43篇 |
2007年 | 42篇 |
2006年 | 49篇 |
2005年 | 30篇 |
2004年 | 32篇 |
2003年 | 19篇 |
2002年 | 17篇 |
2001年 | 3篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1975年 | 4篇 |
1961年 | 3篇 |
1956年 | 2篇 |
1954年 | 3篇 |
1927年 | 2篇 |
1890年 | 4篇 |
1887年 | 3篇 |
1871年 | 1篇 |
排序方式: 共有809条查询结果,搜索用时 312 毫秒
81.
Diaz F Lee MS Rejeaunier X Lehoucq G Goudail F Loiseaux B Bansropun S Rollin J Debes E Mils P 《Optics letters》2011,36(3):418-420
Imaging systems that combine a phase mask in the pupil and digital postprocessing may have better performance than conventional ones. We have built such a system to enhance the depth of field of an uncooled thermal camera. The phase masks are binary, their structures are optimized thanks to an image quality criterion, and they have been realized with three different technologies that give equivalent results. The deconvolution postprocessing is performed in real time with a graphics processing unit. A significant increase of the depth of field of a factor 3 has been obtained. 相似文献
82.
9-Membered rings are readily constructed through a radical addition-cyclisation sequence on a 1,6-dien-3-ol system followed by a Grob-type fragmentation. 相似文献
83.
Cuauhtémoc Pozos vázquez Russell Tayouo Christine Joly‐Duhamel Bernard Boutevin 《Journal of polymer science. Part A, Polymer chemistry》2010,48(10):2123-2134
The synthesis and characterization of poly(dimethylsiloxanes) bearing maleimides end‐groups (PDMSM) were carried out through imidization of maleic anhydride with three poly(dimethylsiloxanes) diamines of different molecular weights. Self‐photopolymerization of PDMSM was studied by Real‐Time Fourier Transform infrared spectroscopy (RT‐FTIR) and was possible even without photoinitiator (Darocur 1173). The reaction was found to proceed within seconds upon exposure to ultraviolet (UV) radiation to generate highly crosslinked polymer networks. The results indicated that these polymerizations were less sensitive to oxygen inhibition than the radical processes carried out on conventional UV‐curable acrylate resins. The thermal and mechanical properties of these resulting materials were studied starting from PDMS precursors with different molecular weights. These materials exhibit a low glass transition temperature (100 °C) and a high degradation temperature (400 °C) depending on the density of crosslinking points. These PDMSM are suitable as hydrophobic additives in a coating formulation based on a UV cured polypropylene glycol bismaleimide. Only a very small amount of silicone additives (0.3 wt %) is needed to change the wettability on the surface of the films. The migration of the additive was clearly shown before irradiation by scanning electron microscopy EDX and surface energy measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2123–2134, 2010 相似文献
84.
Remigiusz Zurawinski Christine Lepetit Yves Canac Laure Vendier Marian Mikolajczyk Remi Chauvin 《Tetrahedron: Asymmetry》2010,21(13-14):1777-1787
The synthesis and structure of Rh(I) and Pd(II) complexes of chiral P,C-chelating phosphino-(α-sulfinylalkyl)phosphonium ylide ligands with a trisubstituted asymmetric ylidic center P+–C1R(S1(O)p-Tol)–M (R = alkyl group) have been investigated, and compared to those of the analogous disubstituted ylide complexes (R = H). Reaction of the ethyl onium ylide of o-bis(diphenylphosphino)benzene with (?)-menthyl-(S)-p-tolylsulfinate afforded the corresponding racemic erythro phosphino-(α-sulfinylethyl)phosphonium in 90% de (R = Me). The racemization process is interpreted by a Berry-like pseudorotation mechanism driven by the steric repulsion between the α-methyl substituent and the bulky menthyloxy S-substituent or sulfur lone pair in the intermediate ylide-sulfinyl adduct. The ylide of phosphino-(α-sulfinylethyl)phosphonium reacts with [Rh(cod)2][PF6] and PdCl2(MeCN)2 to afford the corresponding P,C1-chelated threo-Rh(I) and erythro-Pd(II) mononuclear complexes in 70% yield and total diastereoselectivity. These respective complexes act as efficient catalytic precursors for the hydrogenation of (Z)-α-acetamidocinnamic acid and allylic substitution of 3-acetoxy-1,3-diphenyl-1-propene with sodium dimethyl malonate. The bonding features of the erythro-Pd(II) complex exhibiting a sulfinyl O?Pd interaction are studied theoretically at the DFT level using ELF and MESP analyses. The η2-P,C haptomeric form of the ylide ligand is estimated to compete at 19% with the η1-C haptomeric form dominating at 81%. 相似文献
85.
86.
In this work, we are interested in the numerical approximation of an eigenvalue problem arising from the computation of guided modes in integrated optics waveguides which are particular cases of open waveguides. 相似文献
87.
In this paper we show, by using dyadic approximations, the existence of a geometric rough path associated with a fractional
Brownian motion with Hurst parameter greater than 1/4. Using the integral representation of fractional Brownian motions, we
furthermore obtain a Skohorod integral representation of the geometric rough path we constructed. By the results in [Ly1],
a stochastic integration theory may be established for fractional Brownian motions, and strong solutions and a Wong-Zakai
type limit theorem for stochastic differential equations driven by fractional Brownian motions can be deduced accordingly.
The method can actually be applied to a larger class of Gaussian processes with covariance functions satisfying a simple decay
condition.
Received: 11 May 2000 / Revised version: 20 March 2001 / Published online: 11 December 2001 相似文献
88.
Marie‐Laure Louillat‐Habermeyer Rongwei Jin Prof. Dr. Frederic W. Patureau 《Angewandte Chemie (International ed. in English)》2015,54(13):4102-4104
A method was developed for the direct dehydrogenative construction of C? N bonds between unprotected phenols and a series of cyclic anilines without resorting to any kind of metal activation of either substrate and without the use of halides. The resulting process relies on the exclusively organic activation of molecular oxygen and the subsequent oxidation of the aniline substrate. This allows the coupling of ubiquitous phenols, thus furnishing aminophenols through an atom‐economical and most sustainable dehydrogenative amination method. This new reactivity, which relies on the intrinsic organic reactivity of cumene in what can be seen as a modified Hock activation process of oxygen, is expected to have a large impact on the formation of C? N bonds in organic synthesis. 相似文献
89.
A critique on the structural analysis of lignins and application of novel tandem mass spectrometric strategies to determine lignin sequencing
下载免费PDF全文
![点击此处可从《Journal of mass spectrometry : JMS》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Joseph Banoub Guo‐Hua Delmas Jr. Nicolas Joly Grahame Mackenzie Nadja Cachet Bouchra Benjelloun‐Mlayah Michel Delmas 《Journal of mass spectrometry : JMS》2015,50(1):5-48
This review is devoted to the application of MS using soft ionization methods with a special emphasis on electrospray ionization, atmospheric pressure photoionization and matrix‐assisted laser desorption/ionization MS and tandem MS (MS/MS) for the elucidation of the chemical structure of native and modified lignins. We describe and critically evaluate how these soft ionization methods have contributed to the present‐day knowledge of the structure of lignins. Herein, we will introduce new nomenclature concerning the chemical state of lignins, namely, virgin released lignins (VRLs) and processed modified lignins (PML). VRLs are obtained by liberation of lignins through degradation of vegetable matter by either chemical hydrolysis and/or enzymatic hydrolysis. PMLs are produced by subjecting the VRL to a series of further chemical transformations and purifications that are likely to alter their original chemical structures. We are proposing that native lignin polymers, present in the lignocellulosic biomass, are not made of macromolecules linked to cellulose fibres as has been frequently reported. Instead, we propose that the lignins are composed of vast series of linear related oligomers, having different lengths that are covalently linked in a criss‐cross pattern to cellulose and hemicellulose fibres forming the network of vegetal matter. Consequently, structural elucidation of VRLs, which presumably have not been purified and processed by any other type of additional chemical treatment and purification, may reflect the structure of the native lignin. In this review, we present an introduction to a MS/MS top–down concept of lignin sequencing and how this technique may be used to address the challenge of characterizing the structure of VRLs. Finally, we offer the case that although lignins have been reported to have very high or high molecular weights, they might not exist on the basis that such polymers have never been identified by the mild ionizing techniques used in modern MS. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
90.