全文获取类型
收费全文 | 1179篇 |
免费 | 108篇 |
国内免费 | 116篇 |
专业分类
化学 | 684篇 |
晶体学 | 6篇 |
力学 | 43篇 |
综合类 | 7篇 |
数学 | 205篇 |
物理学 | 458篇 |
出版年
2024年 | 2篇 |
2023年 | 43篇 |
2022年 | 20篇 |
2021年 | 29篇 |
2020年 | 38篇 |
2019年 | 27篇 |
2018年 | 32篇 |
2017年 | 40篇 |
2016年 | 49篇 |
2015年 | 44篇 |
2014年 | 76篇 |
2013年 | 103篇 |
2012年 | 67篇 |
2011年 | 98篇 |
2010年 | 62篇 |
2009年 | 69篇 |
2008年 | 63篇 |
2007年 | 79篇 |
2006年 | 71篇 |
2005年 | 52篇 |
2004年 | 51篇 |
2003年 | 42篇 |
2002年 | 40篇 |
2001年 | 33篇 |
2000年 | 15篇 |
1999年 | 24篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 9篇 |
1992年 | 13篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1403条查询结果,搜索用时 31 毫秒
21.
22.
Paolo Calvini Andrea Gorassini Giorgio Luciano Enrico Franceschi 《Vibrational Spectroscopy》2006,40(2):177-183
Cellulose paper (Whatman no.1, chromatographic grade) was oxidised with 0.1 M sodium metaperiodate at different oxidation levels (0–11 days at room temperature), and analysed with FTIR before and after chemical treatments (reduction, further oxidation, alkaline hydrolysis). The deconvolution of infrared data allowed us to verify that periodate oxidises cellulose in isolated domains, leading to the decrease of crystallinity, in agreement with the results of wide angle X-ray scattering (WAXS). In particular, the alkaline hydrolysis (β-alkoxy fragmentation) followed by mild acid treatment removed the oxidised groups and recovered most of the crystallinity of cellulose, as determined by the FTIR crystallinity index. 相似文献
23.
Yoshitaka Gotoh Yutaka Kohno Wakichi Fukuda Masao Tomoi 《Journal of polymer science. Part A, Polymer chemistry》1994,32(8):1543-1550
Soluble polystyrenes with crown ether structures and hydroxyl groups adjacent to the macrorings were prepared by the reaction of epoxide-containing polystyrenes with monoaza-15-crown-5 or monoaza-18-crown-6. Rate of formation of the polymer-bound alkali cation-crown alkoxide ion pair from the soluble polystyrenes and aqueous NaOH or KOH depended on the loading of crown ether and hydroxyl units and on the size of the macroring. The elimination of HCl from less reactive 2-chloroethylbenzene with aqueous NaOH or KOH in the presence of the soluble polystyrene catalysts under two-phase conditions was limited mainly by the intrinsic reaction rate. The elimination of HBr from more reactive 2-bromoethylbenzene in the presence of the less (11%) ring-substituted polymer catalyst with 18-crown unit was limited by the alkoxide formation rate. © 1994 John Wiley & Sons, Inc. 相似文献
24.
25.
A variety of diaryl acetylenes were obtained in good yields when lithium hexamethyldisilazide was added to a solution of arylmethyl sulfone, aryl aldehyde, and chlorodiethylphosphate in THF. In this one‐shot process, a number of transformations such as aldol reaction, phosphorylation of aldolate, and double elimination of the resulting β‐substituted sulfone proceeded successively to afford the desired acetylenes. The one‐shot process was accelerated by the substitution of halogen atoms on the phenyl groups, and unsymmetrically substituted diaryl acetylenes were obtained without contamination of the dehalogenated products. Diaryl acetylenes with other substituents such as CF3, ethoxycarbonyl, dimethylamino, TMS‐acetylene groups, as well as pyridinyl and thienyl moieties were also accessible with this method. However, methoxy‐substituted compounds were obtained in moderate yields under the same conditions, but the yields were increased when lithium diisopropylamide was used instead. 相似文献
26.
The reaction of cyclobutanone with an alkyne in the presence of a nickel(0) catalyst formally achieves intermolecular alkyne insertion between the carbonyl carbon and the α-carbon of a cyclobutanone, providing a six-membered carbocyclic skeleton. 相似文献
27.
Taking into account the thiophilic properties of iodine, a very simple methodology to achieve 2′,3′-didehydro-2′,3′-dideoxy-β-nucleosides in high yield was performed, using mild, and inexpensive conditions, by means of the treatment of 2′-deoxy-3′,5′-dibenzoyl-2′-iodo-β-nucleoside derivatives with NaHS. The process has shown to be highly dependent of the relative geometry between the iodine atom and the adjacent leaving group. In this way, different essays carried out with pyranose derivatives have concluded in no reaction when the vicinal groups to eliminate do not adopt a trans-diaxial disposition. In addition, the treatment of 2-iodomethyl-cyclopropane-1,1-dicarboxylic acid diethyl ester under the same conditions softly and readily leads to the obtention of a mixture of the expected 2-allyl-malonic acid diethyl ester (as the minor product) and the thiirane derivative 2-thiiranylmethyl-malonic acid diethyl ester (as the major product). In this case, the responsible of the reaction progress are the nucleophilic properties of the sulfur atom rather than the thiophilic character of the iodine atom. 相似文献
28.
Poly(p-chloromethylstyrene-ethyleneglycoldimethacrylate) polymeric microbeads, poly(p-CMS-EGDMA), were synthesized and 1,5,9,13-tetrathiacyclohexadecane-3,11-diol (S4HD) was attached chemically onto the polymeric microbeads. Characterization of all microbeads was done by Fourier transform-infrared spectrometry (FT-IR) and elemental analyzer. The amount of attached 1,5,9,13-tetrathiacyclohexadecane-3,11-diol to the polymer was found to be 2.23 mmol g−1 polymers. The ligand attached microbeads, poly(p-CMS-EGDMA-S4HD), were used to examine the adsorption capacity of Pb(II), Cd(II), Hg(II) and Cr(II) ions for recovery, pre-concentration and the matrix elimination by changing the pH and the initial metal ion concentrations and also adsorption kinetics of the studied metal ions was determined. Pre-concentration factors for the studied toxic metal ions were found to be more than 500-fold and recovery was between 92 and 106%. In the drinking, lake, tap and sea-water samples from water lands, ultra-trace toxic metal ion concentrations were determined easily by using ligand modified microbeads after pre-concentration because of the high pre-concentration factor and easily matrix elimination using ligand modified microbeads. Reference sea-water material was used for the validation of the method and it was found that recovery, pre-concentration and the matrix elimination were performed perfectly. For the desorption of the toxic metal ions, 3 M HCl containing 0.8 M thiourea was used and desorption ratio was obtained more than 96%. 相似文献
29.
The effects of M3O3 (M = Al, Sc etc.) in Cu-ZnO-M2O3 catalysts on methanol synthesis at low pressure were studied with ESR, XPS and TPR spectroscopy. The results of ESR showed that the generation of monovalent cationic defects was because the valence state and electronic charge on the ZnO lattice lost their balance as M3+ doped into ZnO. The induced effect by Sc3+ is stronger than that by Al3+. The results of XPS and TPR indicated that the amount and stabilization of Cu+ on the surface of reduced copper-based catalyst and its catalytic activity were affected by the monovalent cationic defects on the surface of ZnO. 相似文献
30.
The performance of a PTV injector operated in the solvent venting mode with Tenax in the glass insert was evaluated with synthetic mixtures containing 5 to 50 ng of n-alkanes, ethyl esters, n-alcohols, and carboxylic acids. The influence of the sampled amount, the injected volume, and the nature of the solvent on accuracy and precision were studied. Coefficients of variation of relative (normalized) peak areas and absolute peak area ratios of each compound to the standard are of the order of 5 %. 相似文献