全文获取类型
收费全文 | 24947篇 |
免费 | 458篇 |
国内免费 | 99篇 |
专业分类
化学 | 13874篇 |
晶体学 | 154篇 |
力学 | 761篇 |
综合类 | 9篇 |
数学 | 3437篇 |
物理学 | 7269篇 |
出版年
2023年 | 114篇 |
2022年 | 317篇 |
2021年 | 392篇 |
2020年 | 358篇 |
2019年 | 313篇 |
2018年 | 406篇 |
2017年 | 364篇 |
2016年 | 622篇 |
2015年 | 517篇 |
2014年 | 595篇 |
2013年 | 967篇 |
2012年 | 1251篇 |
2011年 | 1427篇 |
2010年 | 792篇 |
2009年 | 751篇 |
2008年 | 1202篇 |
2007年 | 1174篇 |
2006年 | 1060篇 |
2005年 | 1490篇 |
2004年 | 1526篇 |
2003年 | 1075篇 |
2002年 | 716篇 |
2001年 | 598篇 |
2000年 | 557篇 |
1999年 | 345篇 |
1998年 | 289篇 |
1997年 | 288篇 |
1996年 | 355篇 |
1995年 | 313篇 |
1994年 | 306篇 |
1993年 | 329篇 |
1992年 | 321篇 |
1991年 | 263篇 |
1990年 | 216篇 |
1989年 | 203篇 |
1988年 | 179篇 |
1987年 | 175篇 |
1986年 | 175篇 |
1985年 | 231篇 |
1984年 | 214篇 |
1983年 | 184篇 |
1982年 | 186篇 |
1981年 | 183篇 |
1980年 | 168篇 |
1979年 | 150篇 |
1978年 | 176篇 |
1977年 | 138篇 |
1976年 | 134篇 |
1974年 | 132篇 |
1973年 | 122篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
921.
Carlos Victor Montefusco-Pereira Maria José de Carvalho Ana Paula de Araújo Boleti Lorisa Simas Teixeira Humberto Reis Matos Emerson Silva Lima 《Applied biochemistry and biotechnology》2013,170(6):1367-1378
Diabetes mellitus is a metabolic disease characterized by abnormally high plasma glucose levels, leading to major complications, such as insulin resistance, obesity, hyperlipidemia, and hypertension, also with alterations in the immune and neuronal systems. Brazilian plants have been studied as important sources for new molecules with medicinal properties. The genus Passiflora known as “Maracujá” has been used as a traditional folk medicine for a long time, so an investigation was performed regarding an endemic kind of passion fruit (Passiflora nitida Kunth) from Amazonas, Brazil. Here, we aimed to determine its potential biological activity against metabolic syndrome, oxidative stress, pain, and inflammation. The hydroethanol leaf extract revealed an in vitro α-glucosidase inhibitory activity of 50 % inhibitory concentration (IC50)?=?6.78?±?0.31 μg/mL and an α-amylase inhibition of IC50?=?93.36?±?4.37. In vivo, experiments of different saccharide tolerance resulted in significant glycemia control and, with alloxan-diabetic mice, resulted in a decrease of total cholesterol, a hypoglycemic effect, and an antioxidant activity by thiobarbituric acid-reactive substances measurement. Also, it decreased the carrageenan-induced edema volume and the rate of writhing as a nociceptive response. These results indicate positive effects of P. nitida extract and its potential to inhibit metabolic syndrome. 相似文献
922.
Fernanda Cortez Lopes Deise Michele Tichota Jamile Queiroz Pereira Jéferson Segalin Alessandro de Oliveira Rios Adriano Brandelli 《Applied biochemistry and biotechnology》2013,171(3):616-625
The search for new sources of natural pigments has increased, mainly because of the toxic effects caused by synthetic dyes used in food, pharmaceutical, textile, and cosmetic industries. Fungi provide a readily available alternative source of natural pigments. In this context, the fungi Penicillium chrysogenum IFL1 and IFL2, Fusarium graminearum IFL3, Monascus purpureus NRRL 1992, and Penicillium vasconiae IFL4 were selected as pigments producers. The fungal identification was performed using ITS and part of the β-tubulin gene sequencing. Almost all fungi were able to grow and produce water-soluble pigments on agro-industrial residues, with the exception of P. vasconiae that produced pigments only on potato dextrose broth. The production of yellow pigments was predominant and the two strains of P. chrysogenum were the largest producers. In addition, the production of pigments and mycotoxins were evaluated in potato dextrose agar using TOF-MS and TOF-MS/MS. Metabolites as roquefortine C, chrysogine were found in both extracts of P. chrysogenum, as well fusarenone X, diacetoxyscirpenol, and neosolaniol in F. graminearum extract. In the M. purpureus extract, the pigments monascorubrin, rubropunctatin, and the mycotoxin citrinin were found. The crude filtrates have potential to be used in the textile industry; nevertheless, additional pigment purification is required for food and pharmaceutical applications. 相似文献
923.
Ch. Jesús F. Socorro M. Rodríguez de Rivera 《Journal of Thermal Analysis and Calorimetry》2013,113(3):1003-1007
A calorimetric sensor has been developed for local and direct measurement of calorific dissipations in different parts of the human body. The constructed prototype has a detection surface of 36 cm2. The calibration of the sensor is based on a semi-empirical model that permits to simulate the operation of the device, making easier an operational functioning method. The device is modeled as a system with two inputs and two outputs. The inputs are the calorific power (W) that is intended to be measured and the power (W pid) that dissipates a resistance, keeping constant the thermostat temperature through the use of a PID controller. The outputs are the thermostat temperature (T pid) and the calorimetric signal (y) that provides the thermopile that is in contact with the body. 相似文献
924.
A methodology for kinetic modeling of conversion processes is presented.The proposed approach allows to overcome the lack of molecular detail of the petroleum fractions and to simulate the reactions of the process by means of a two-step procedure.In the first step,a synthetic mixture of molecules representing the feedstock is generated via a molecular reconstruction method,termed SR-REM molecular reconstruction.In the second step,a kinetic Monte Carlo method,termed stochastic simulation algorithm(SSA),is used to simulate the effect of the conversion reactions on the mixture of molecules.The resulting methodology is applied to the Athabasca vacuum residue hydrocracking.An adequate molecular representation of the vacuum residue is obtained using the SR-REM algorithm.The reaction simulations present a good agreement with the laboratory data for Athabasca vacuum residue conversion.In addition,the proposed methodology provides the molecular detail of the vacuum residue conversion throughout the reactions simulations. 相似文献
925.
Jorge Pisonero Nerea Bordel Claudia Gonzalez de Vega Beatriz Fernández Rosario Pereiro Alfredo Sanz-Medel 《Analytical and bioanalytical chemistry》2013,405(17):5655-5662
The combination of radiofrequency pulsed glow discharge (RF-PGD) analytical plasmas with time-of-flight mass spectrometry (TOFMS) has promoted the applicability of this ion source to direct analysis of innovative materials. In this sense, this emerging technique enables multi-elemental depth profiling with high depth resolution and sensitivity, and simultaneous production of elemental, structural, and molecular information. The analytical potential and trends of this technique are critically presented, including comparison with other complementary and well-established techniques (e.g. SIMS, GD–OES, etc.). An overview of recent applications of RF-PGD–TOFMS is given, including analysis of nano-structured materials, coated-glasses, photovoltaic materials, and polymer coatings 相似文献
926.
Michelle Gonçalves Mothé Cheila Gonçalves Mothé Carlos Henrique Monteiro de Carvalho Marcia Cristina Khalil de Oliveira 《Journal of Thermal Analysis and Calorimetry》2013,113(2):525-531
Present study investigates thermal behavior of two heavy crude oils with different °API values by simultaneous thermogravimetry–differential scanning calorimetry–fourier transform infrared spectroscopy (TG–DSC–FTIR), and an evaluation of the chemical element levels present in the oils’ ashes was done by energy dispersive X-ray fluorescence spectrometry. TG and DSC curves were obtained for two samples in nitrogen atmosphere. Among all inorganic components evaluated, the highest concentration in the two oils was SO3. Thus this study may contribute to a better understanding of the thermal behavior of heavy crude oils and their composition. 相似文献
927.
Jean-Pierre Djukic Michel Pfeffer Nathalie Gruber-Kyritsakas André de Cian 《Journal of organometallic chemistry》2011,696(20):3268-3273
A Mn(I) tricarbonyl complex of a 1,3-diferrocenyl-1-oxopentadienyl ligand was synthesised adventitiously by what seems to be an in-situ aldol-like condensation of two acetylferrocene units promoted by benzyl-Mn(CO)5. X-ray structural analysis of this unexpected product confirms the η5 coordination of the 1,3-diferrocenyl-1-oxopentadienyl ligand to the Mn(CO)3 moiety. The nature of the metal-ligand bonding relationship was studied by theoretical calculations; it outlines the charge unbalance (polarisation) at the oxopentadienyl moiety as well as the lack of ketone character of the latter Mn-bound ligand. 相似文献
928.
Kelly L. dos Santos CastroPaulo G. de Lima Leandro S.M. e MirandaRodrigo O.M.A. de Souza 《Tetrahedron letters》2011,52(32):4168-4171
A solvent mixture containing the ionic liquid derived from 4-picoline (C4MPyBF4) and water was tested for the Suzuki-Miyaura reaction under microwave irradiation. This solvent system led to very high conversions and clean reactions. When compared with the other more common ionic liquid, such as C4MImBF4, C4MPyBF4 proved to be superior. 相似文献
929.
Dirk T.S. Rijkers Fernando de Prada LópezRob M.J. Liskamp François Diederich 《Tetrahedron letters》2011,52(51):6963-6967
We report on the chemoselective coupling between colorless peptide fragments functionalized with a mutually reactive electron-rich Nα-(4-ethynylphenyl)-Nα-(methyl)-glycyl- and an electron-deficient [4-(2,2-dicyanovinyl)]benzoyl moiety. The resulting donor-substituted 1,1-dicyanobuta-1,3-dienes represent a new class of orange-red colored (λmax = 450-500 nm, with molar extinction coefficients (ε) above 5,000 mol−1 dm3 cm−1) peptide-based imaging chromophores. 相似文献
930.
de Vargas-Sansalvador IM Fay C Phelan T Fernández-Ramos MD Capitán-Vallvey LF Diamond D Benito-Lopez F 《Analytica chimica acta》2011,(2):216-222
A new system for CO2 measurement (0–100%) based on a paired emitter–detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO2 sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO2-insensitive luminophore by an acid–base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid–base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter–detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO2 response in terms of sensitivity. 相似文献