全文获取类型
收费全文 | 1786篇 |
免费 | 47篇 |
国内免费 | 7篇 |
专业分类
化学 | 1421篇 |
晶体学 | 9篇 |
力学 | 18篇 |
数学 | 154篇 |
物理学 | 238篇 |
出版年
2024年 | 1篇 |
2023年 | 11篇 |
2022年 | 77篇 |
2021年 | 101篇 |
2020年 | 40篇 |
2019年 | 60篇 |
2018年 | 74篇 |
2017年 | 49篇 |
2016年 | 70篇 |
2015年 | 76篇 |
2014年 | 58篇 |
2013年 | 163篇 |
2012年 | 82篇 |
2011年 | 107篇 |
2010年 | 92篇 |
2009年 | 76篇 |
2008年 | 106篇 |
2007年 | 120篇 |
2006年 | 82篇 |
2005年 | 88篇 |
2004年 | 56篇 |
2003年 | 63篇 |
2002年 | 47篇 |
2001年 | 15篇 |
2000年 | 11篇 |
1999年 | 23篇 |
1998年 | 13篇 |
1997年 | 17篇 |
1996年 | 23篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有1840条查询结果,搜索用时 15 毫秒
111.
Bernardo Duarte Renato Mamede Irina A. Duarte Isabel Caador Susanne E. Tanner Marisa Silva David Jacinto Teresa Cruz Vanessa F. Fonseca 《Molecules (Basel, Switzerland)》2022,27(4)
The stalked barnacle Pollicipes pollicipes is an abundant species on the very exposed rocky shore habitats of the Spanish and Portuguese coasts, constituting also an important economical resource, as a seafood item with high commercial value. Twenty-four elements were measured by untargeted total reflection X-ray fluorescence spectroscopy (TXRF) in the edible peduncle of stalked barnacles sampled in six sites along the Portuguese western coast, comprising a total of 90 individuals. The elemental profile of 90 individuals originated from several geographical sites (N = 15 per site), were analysed using several chemometric multivariate approaches (variable in importance partial least square discriminant analysis (VIP-PLS-DA), stepwise linear discriminant analysis (S-LDA), linear discriminant analysis (LDA), random forests (RF) and canonical analysis of principal components (CAP)), to evaluate the ability of each approach to trace the geographical origin of the animals collected. As a suspension feeder, this species introduces a high degree of background noise, leading to a comparatively lower classification of the chemometric approaches based on the complete elemental profile of the peduncle (canonical analysis of principal components and linear discriminant analysis). The application of variable selection approaches such as the VIP-PLS-DA and S-LDA significantly increased the classification accuracy (77.8% and 84.4%, respectively) of the samples according to their harvesting area, while reducing the number of elements needed for this classification, and thus the background noise. Moreover, the selected elements are similar to those selected by other random and non-random approaches, reinforcing the reliability of this selection. This untargeted analytical procedure also allowed to depict the degree of risk, in terms of human consumption of these animals, highlighting the geographical areas where these delicacies presented lower values for critical elements compared to the standard thresholds for human consumption. 相似文献
112.
Valentina S. Borovkova Yuriy N. Malyar Irina G. Sudakova Anna I. Chudina Andrey M. Skripnikov Olga Yu. Fetisova Alexander S. Kazachenko Angelina V. Miroshnikova Dmitriy V. Zimonin Vladislav A. Ionin Anastasia A. Seliverstova Ekaterina D. Samoylova Noureddine Issaoui 《Molecules (Basel, Switzerland)》2022,27(1)
Spruce (Picea abies) wood hemicelluloses have been obtained by the noncatalytic and catalytic oxidative delignification in the acetic acid-water-hydrogen peroxide medium in a processing time of 3–4 h and temperatures of 90–100 °C. In the catalytic process, the H2SO4, MnSO4, TiO2, and (NH4)6Mo7O24 catalysts have been used. A polysaccharide yield of up to 11.7 wt% has been found. The hemicellulose composition and structure have been studied by a complex of physicochemical methods, including gas and gel permeation chromatography, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The galactose:mannose:glucose:arabinose:xylose monomeric units in a ratio of 5:3:2:1:1 have been identified in the hemicelluloses by gas chromatography. Using gel permeation chromatography, the weight average molar mass Mw of hemicelluloses has been found to attain 47,654 g/mol in noncatalytic delignification and up to 42,793 g/mol in catalytic delignification. Based on the same technique, a method for determining the α and k parameters of the Mark–Kuhn–Houwink equation for hemicelluloses has been developed; it has been established that these parameters change between 0.33–1.01 and 1.57–472.17, respectively, depending on the catalyst concentration and process temperature and time. Moreover, the FTIR spectra of the hemicellulose samples contain all the bands characteristic of heteropolysaccharides, specifically, 1069 cm−1 (C–O–C and C–O–H), 1738 cm−1 (ester C=O), 1375 cm−1 (–C–CH3), 1243 cm−1 (–C–O–), etc. It has been determined by the thermogravimetric analysis that the hemicelluloses isolated from spruce wood are resistant to heating to temperatures of up to ~100 °C and, upon further heating, start destructing at an increasing rate. The antioxidant activity of the hemicelluloses has been examined using the compounds simulating the 2,2-diphenyl-2-picrylhydrazyl free radicals. 相似文献
113.
Alan R. Katritzky Irina V. Shcherbakova Robert D. Tack 《Journal of polymer science. Part A, Polymer chemistry》1996,34(9):1651-1655
Amidoalkylations of polybutenes and of ethene-propene copolymer, catalyzed by trifluoroacetic acid, result in 5,6-dihydro-4H,1,3-oxazino-substituted polyalkenes with the 2-position of the oxazine ring unsubstituted or occupied by a vinyl or a phenyl group. The vinyl-substituted dihydrooxazino polymers thus obtained readily add butylamine to the vinyl fragment yielding alkylamino-substituted derivatives. © 1996 John Wiley & Sons, Inc. 相似文献
114.
The influence of the die geometry and of the thermal conductivity of materials on the solid-liquid interface shape in Edge-defined Film-fed Growth has been studied. 相似文献
115.
Dr. Igor L. Dalinger Dr. Irina A. Vatsadze Dr. Tatyana K. Shkineva Dr. Alexandr V. Kormanov Dr. Marina I. Struchkova Dr. Kyrill Yu. Suponitsky Dr. Anatoly A. Bragin Dr. Konstantin A. Monogarov Dr. Valery P. Sinditskii Dr. Aleksei B. Sheremetev 《化学:亚洲杂志》2015,10(9):1987-1996
A new family of energetic compounds, nitropyrazoles bearing a trinitromethyl moiety at the nitrogen atom of the heterocycle, was designed. The desirable high‐energy dense oxidizers 3,4‐dinitro‐ and 3,5‐dinitro‐1‐(trinitromethyl)pyrazoles were synthesized in good yields by destructive nitration of the corresponding 1‐acetonylpyrazoles. All of the prepared compounds were fully characterized by multinuclear NMR and IR spectroscopy, as well as by elemental analysis. Single‐crystal X‐ray diffraction studies show remarkably high density. Impact sensitivity tests and thermal stability measurements were also performed. All of the pyrazoles possess positive calculated heats of formation and exhibit promising energetic performance that is the range of 1,3,5‐trinitroperhydro‐1,3,5‐triazine and pentaerythritol tetranitrate. The new pyrazoles exhibit positive oxygen balance and are promising candidates for new environmentally benign energetic materials. 相似文献
116.
Natalia P. Kuzmina Dmitry M. Tsymbarenko Igor E. Korsakov Zoya A. Starikova Kostantin A. Lysenko Olga V. Boytsova Andrey V. Mironov Irina P. Malkerova Andrey S. Alikhanyan 《Polyhedron》2008
The novel mixed ligand complexes [Ca(hfa)2(diglyme)(H2O)] (I), [Sr(hfa)2(diglyme)(H2O)] (II) and [Ba(hfa)2(diglyme)2] (III) (Hhfa = 1,1,1,5,5,5-hexafluoropentane-2,4-dione, diglyme = 2,5,8-trioxanonane) were synthesized by the reactions of the alkaline earth element (AEE) carbonates in n-hexane with a mixture of Hhfa and diglyme, and they were characterized by elemental analysis, 1H and 13C NMR, and FTIR spectroscopy. The crystal structures of I–III, consisting of mononuclear isolated molecules, have been determined. The thermal behavior and composition of the vapor phase have been studied for I–III by thermal analysis at low pressure and mass spectrometry using a Knudsen cell. The stability of the mixed ligand complexes [M(hfa)2(diglyme)n] to the removal of diglyme molecules under heating decreases in the row I > II ≈ III, and only I evaporates as the mixed ligand complex after water removal. 相似文献
117.
Kosyakov Dmitry S. Anikeenko Elena A. Ul’yanovskii Nikolay V. Khoroshev Oleg Yu Shavrina Irina S. Gorbova Natalya S. 《Analytical and bioanalytical chemistry》2018,410(28):7429-7439
Analytical and Bioanalytical Chemistry - The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the study of lignin is still extremely limited due to its low... 相似文献
118.
Yuliya V. Rassukana Ludmyla V. Bezgubenko Oleg V. Stanko Eduard B. Rusanov Irina B. Kulik Petro P. Onysko 《Tetrahedron: Asymmetry》2017,28(4):555-560
The cycloaddition reaction of (S)-(α-phenylethylimino)trifluoropropionate with diazomethane leads to a diastereomeric mixture (4.5:1) of 5-trifluoromethyl-1,2,3-triazoline-5-carboxylates. Enantiopure diastereomers were isolated by column chromatography and converted into their respective non-racemic 2-trifluoromethyl-aziridine-2-carboxylates and carboxylic acids. The absolute configuration of newly formed stereogenic centers was determined by XRD analysis. The stereoselective reaction between (S)-N-(α-phenylethyl)trifluoroacetimidoylphosphonate and diazomethane produces a diastereomeric mixture (2.5:1) of 5-trifluoromethyltriazoline-5-phosphonates readily separated by column chromatography in diastereomerically pure forms. 相似文献
119.
Zhihui Yi Luca Giacomo Bettini Gaia Tomasello Prajwal Kumar Paolo Piseri Irina Valitova Paolo Milani Francesca Soavi Fabio Cicoira 《Journal of Polymer Science.Polymer Physics》2017,55(1):96-103
Planar organic electrochemical transistors (OECTs) using PEDOT:PSS as the channel material and nanostructured carbon (nsC) as the gate electrode material and poly(sodium 4‐styrenesulfonate (PSSNa) gel as the electrolyte were fabricated on flexible polyethylene terephthalate (Mylar®) substrates. The nsC was deposited at room‐temperature by supersonic cluster beam deposition (SCBD). Interestingly, the OECT acts as a hybrid supercapacitor (to give a device that we indicate as transcap). The energy storage ability of transcaps has been studied with two cell configurations: one featuring PEDOT:PSS as the positive electrode and nsC as the negative electrode and another configuration with reversed electrode polarity. Potentiostatic charge/discharge studies show that both supercapacitors show good performance in terms of voltage retention, in particular, when PEDOT:PSS is used as the positive electrode. Galvanostatic charge–discharge characteristics show typical symmetric triangular shape, indicating a nearly ideal capacitive behavior with a high columbic efficiency (close to 100%). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 96–103 相似文献
120.
[reaction: see text] The first example of intermolecular hydrophosphination of styrenes catalyzed by Ni and Pd complexes is described. The reaction of Ph2PH with styrene, 4-vinylpyridine, 2-vinylpyridine, 4-methoxystyrene, 2-methoxystyrene, and 5-vinyl-2-methylpyridine in benzene under Ni[P(OEt)3]4 catalysis proceeds with high yield and selectivity to give only anti-Markovnikov product. 相似文献