全文获取类型
收费全文 | 222775篇 |
免费 | 3981篇 |
国内免费 | 2087篇 |
专业分类
化学 | 124099篇 |
晶体学 | 2653篇 |
力学 | 9243篇 |
综合类 | 94篇 |
数学 | 24970篇 |
物理学 | 67784篇 |
出版年
2020年 | 1969篇 |
2019年 | 2117篇 |
2018年 | 2640篇 |
2017年 | 2704篇 |
2016年 | 3896篇 |
2015年 | 2695篇 |
2014年 | 3812篇 |
2013年 | 9212篇 |
2012年 | 8180篇 |
2011年 | 9947篇 |
2010年 | 7030篇 |
2009年 | 6756篇 |
2008年 | 9192篇 |
2007年 | 9263篇 |
2006年 | 8712篇 |
2005年 | 8011篇 |
2004年 | 7116篇 |
2003年 | 6162篇 |
2002年 | 5698篇 |
2001年 | 6626篇 |
2000年 | 4875篇 |
1999年 | 3512篇 |
1998年 | 2896篇 |
1997年 | 2920篇 |
1996年 | 2905篇 |
1995年 | 2677篇 |
1994年 | 2744篇 |
1993年 | 2591篇 |
1992年 | 2821篇 |
1991年 | 2901篇 |
1990年 | 2699篇 |
1989年 | 2598篇 |
1988年 | 2555篇 |
1987年 | 2478篇 |
1986年 | 2498篇 |
1985年 | 3283篇 |
1984年 | 3325篇 |
1983年 | 2773篇 |
1982年 | 3017篇 |
1981年 | 2770篇 |
1980年 | 2580篇 |
1979年 | 2752篇 |
1978年 | 2972篇 |
1977年 | 3009篇 |
1976年 | 3030篇 |
1975年 | 2776篇 |
1974年 | 2887篇 |
1973年 | 2938篇 |
1972年 | 2301篇 |
1971年 | 1825篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In situ ion beam sputter deposition and X‐ray photoelectron spectroscopy (XPS) of multiple thin layers under computer control for combinatorial materials synthesis 下载免费PDF全文
Thomas A. Wilson Anders J. Barlow Michael L. Foster Mariela Bravo Sanchez Jose F. Portoles Naoko Sano Peter J. Cumpson Ian W. Fletcher 《Surface and interface analysis : SIA》2017,49(1):18-24
Deposition of ultra‐thin layers under computer control is a frequent requirement in studies of novel sensors, materials screening, heterogeneous catalysis, the probing of band offsets near semiconductor junctions and many other applications. Often large‐area samples are produced by magnetron sputtering from multiple targets or by atomic layer deposition (ALD). Samples can then be transferred to an analytical chamber for checking by X‐ray photoelectron spectroscopy (XPS) or other surface‐sensitive spectroscopies. The ‘wafer‐scale’ nature of these tools is often greater than is required in combinatorial studies, where a few square centimetres or even millimetres of sample is sufficient for each composition to be tested. The large size leads to increased capital cost, problems of registration as samples are transferred between deposition and analysis, and often makes the use of precious metals as sputter targets prohibitively expensive. Instead we have modified a commercial sample block designed to perform angle‐resolved XPS in a commercial XPS instrument. This now allows ion‐beam sputter deposition from up to six different targets under complete computer control. Ion beam deposition is an attractive technology for depositing ultra‐thin layers of great purity under ultra‐high vacuum conditions, but is generally a very expensive technology. Our new sample block allows ion beam sputtering using the ion gun normally used for sputter depth‐profiling of samples, greatly reducing the cost and allowing deposition to be done (and checked by XPS) in situ in a single instrument. Precious metals are deposited cheaply and efficiently by ion‐beam sputtering from thin metal foils. Samples can then be removed, studied and exposed to reactants or surface treatments before being returned to the XPS to examine and quantify the effects. Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd. 相似文献
992.
993.
L. Pedrazzetti L. Nobili L. Magagnin R. Bernasconi A. Lucotti P. Soltani A. Mezzi S. Kaciulis 《Surface and interface analysis : SIA》2017,49(11):1088-1094
Ultrathin carbon films were grown on different types of metallic substrates. Free‐standing foils of Cu and Ni were prepared by electroforming, and a pure Ni film was obtained by galvanic displacement on a Si wafer. Commercial foil of Ni 99.95% was used as a reference substrate. Carbon films were grown on these substrates by chemical vapour deposition in a CH4‐H2 atmosphere. Obtained films were characterized by Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and ultraviolet photoemission spectroscopy. The XPS at grazing collection angle was used to determine the thickness of carbon films. Depending on the deposition parameters, the films of graphene or graphite were obtained on the different substrates. The uniformity of graphene and its distribution over the sample area were investigated from Raman data, optical images, and XPS chemical maps. The presence of graphene or graphite in the films was determined from the Raman spectra and Auger peak of C KVV. For this purpose, the D parameter, which is a fingerprint of carbon allotropes, was determined from C KVV spectra acquired by using X‐rays and electron beam. A formation of an intermediate layer of metal hydroxide was revealed in the samples with graphene overlayer. 相似文献
994.
Luana Bettanin Sumbal Saba Fábio Z. Galetto Gustavo A. Mike Jamal Rafique Antonio L. Braga 《Tetrahedron letters》2017,58(50):4713-4716
We describe herein a simple, fast and inexpensive protocol for the oxidative coupling of thiols employing a stoichiometric amount of DMSO and iodine as the catalyst. Various aromatic disulfides were obtained in good to excellent yields in short reaction times at room temperature, while aliphatic disulfides were achieved in good yields when the reactions were conducted under microwave irradiation. 相似文献
995.
Carolina G. Dos Santos Daniela T. Marquez Charles-Oneil L. Crites Jose Carlos Netto-Ferreira Juan C. Scaiano 《Tetrahedron letters》2017,58(5):427-431
Two different hybrid materials composed of gold nanoparticles (AuNPs) supported on either commercial niobium oxide HY 340 or mesoporous niobium oxide catalyzed the Friedel-Crafts alkylation of anisole by benzyl chloride. Excitation of the surface plasmon of the supported AuNPs allowed the reaction to occur at lower temperatures by acting as an alternative heat source. The localized heating produced via plasmon excitation permitted the acid catalyzed reaction to occur - at the Lewis acid sites on the Nb2O5 support - at 80 °C while thermal-dark reactions using a conventional heat source, required temperatures of 120 °C or higher. The catalytic activity of the tested hybrid materials decreased with storage time. However, the deactivation showed to be reversible upon lyophilisation indicating that the nature of the deactivation could be due to water adsorption. 相似文献
996.
Patrick J. Fricke Jenna L. Stasko Dylan T. Robbins Alexander C. Gardner Jacqueline Stash Mark J. Ferraro Michael W. Fennie 《Tetrahedron letters》2017,58(48):4510-4513
Propargyl imidates derived from aromatic and aliphatic nitriles cyclize at room temperature in high yields when treated with a catalytic amount of copper (I) iodide. This 5-exo-dig process affords dihydrooxazoles which do not aromatize under the reaction conditions, and which are isolated without chromatography. Investigations of the reaction scope, subsequent functionalization of the reaction products, and preliminary mechanistic data are presented. 相似文献
997.
Bryan S. Holtzman Eric T. Roberts Nicholas S. Caminiti Jacob A. Fox Madison B. Goodstein Staci A. Hill Zitong B. Jia Isabelle N.-M. Leibler Michael L. Martini Gina M. Mendolia Sarah B. Nodder Molly S. Costanza-Robinson Richard C. Bunt 《Tetrahedron letters》2017,58(5):432-436
A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh3 and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs2CO3, but much less elimination to diene side products was observed with Cs2CO3. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs2CO3 crossover only began occurring after 100% completion had been reached. 相似文献
998.
999.
Rohit L. Vekariya 《Journal of Dispersion Science and Technology》2017,38(11):1594-1599
The interaction of ionic liquids (ILs) with non-ionic triblock copolymer, Pluronic® P123 in aqueous solutions has been investigated using DLS, surface tension, and viscosity measurements. The addition of ILs increased the Critical Micelle Concentration (CMC) of P123, which appears to be logistic. As the added IL enhances the solubility of PPO moiety (and PEO), which makes them to behave like a more hydrophilic block copolymer that would be micellized at high copolymer concentration. The DLS data is in good agreement to the results observed from surface tension measurements. Viscosity results show the propensity in micellar size reduction upon addition of ILs, and hence, intrinsic viscosity decreases as compared to pure P123 aqueous solution. The results are studied and discussed as a function of cationic head groups of N-octyl-pyridinium/imidazolium chloride based ILs. 相似文献
1000.
Networks are frequently studied algebraically through matrices. In this work, we show that networks may be studied in a more abstract level using results from the theory of matroids by establishing connections to networks by decomposition results of matroids. First, we present the implications of the decomposition of regular matroids to networks and related classes of matrices, and secondly we show that strongly unimodular matrices are closed under k-sums for \(k=1,2\) implying a decomposition into highly connected network-representing blocks, which are also shown to have a special structure. 相似文献