全文获取类型
收费全文 | 5480篇 |
免费 | 166篇 |
国内免费 | 9篇 |
专业分类
化学 | 3886篇 |
晶体学 | 42篇 |
力学 | 115篇 |
数学 | 512篇 |
物理学 | 1100篇 |
出版年
2023年 | 42篇 |
2022年 | 76篇 |
2021年 | 193篇 |
2020年 | 142篇 |
2019年 | 153篇 |
2018年 | 133篇 |
2017年 | 95篇 |
2016年 | 219篇 |
2015年 | 182篇 |
2014年 | 233篇 |
2013年 | 334篇 |
2012年 | 375篇 |
2011年 | 429篇 |
2010年 | 281篇 |
2009年 | 285篇 |
2008年 | 365篇 |
2007年 | 305篇 |
2006年 | 273篇 |
2005年 | 223篇 |
2004年 | 190篇 |
2003年 | 144篇 |
2002年 | 119篇 |
2001年 | 95篇 |
2000年 | 86篇 |
1999年 | 57篇 |
1998年 | 50篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 39篇 |
1994年 | 44篇 |
1993年 | 47篇 |
1992年 | 47篇 |
1991年 | 25篇 |
1990年 | 28篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 19篇 |
1982年 | 10篇 |
1981年 | 22篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1973年 | 8篇 |
排序方式: 共有5655条查询结果,搜索用时 46 毫秒
991.
Manuel A.V. Ribeiro da Silva Ana I.M.C. Lobo Ferreira Joana I.T.A. Cabral Ana Filipa L.O.M. Santos Ana Rita G. Moreno Tiago L.P. Galvão Inês M. Rocha Paula M.V. Fernandes Sílvia Q. Salgueiro Vanessa A.F. de Moura Isabel M.S.C. Oliveira Paula C. Cotelo Mariana R.A. Ribeiro 《The Journal of chemical thermodynamics》2009,41(9):984-991
992.
A digital image-based flame emission spectrometric (DIB-FES) method for the quantitative chemical analysis is proposed here for the first time. The DIB-FES method employs a webcam to capture the digital images which are associated to a radiation emitted by the analyte into an air-butane flame. Since the detection by webcam is based on the RGB (red-green-blue) colour system, a novel mathematical model was developed in order to build DIB-FES analytical curves and estimate figures of merit for the proposed method. In this approach, each image is retrieved in the three R, G and B individual components and their values were used to define a position vector in RGB three-dimensional space. The norm of this vector is then adopted as the RGB-based value (analytical response) and it has revealed to be linearly related to the analyte concentration. The feasibility of the DIB-FES method is illustrated in three applications involving the determination of lithium, sodium and calcium in anti-depressive drug, physiological serum and water, respectively. In comparison with the traditional flame emission spectrometry (trad-FES), no statistic difference has been observed between the results by applying the paired t-test at the 95% confidence level. However, the DIB-FES method has offered the largest sensitivities and precision, as well as the smallest limits of detection and quantification for the three analytes. These advantageous characteristics are attributed to the trivariate nature of the detection by webcam. 相似文献
993.
Marcelo Anselmo Oseas da Silva 《Talanta》2009,77(3):985-442
An aqueous two-phase system, based on the cloud point phenomenon, was proposed for blood plasma albumin depletion prior to gel electrophoresis (SDS-PAGE) analysis. Albumin removal was evaluated using a mixture of Triton® X-114 and SDS. Variables affecting phase separation and the partition coefficient (K), such as pH (from 5.0 to 7.6) and salt composition, were first optimized to carry out phase separation at room temperature and to promote albumin removal (15 min with ca. 95% efficiency) to the surfactant-rich phase, in a global process that results in reduction of 12% α-helix structures, confirmed through circular dichroism analysis. Gel electrophoresis analyses were carried out to compare crude human plasma composition with human plasma submitted to the cloud point procedure as well as the same plasma submitted to the separation of albumin using a commercially available system based on a chromatographic affinity column. The cloud point extraction procedure allowed identification of 18 protein bands present in the surfactant-poor phase, resulting in a more detailed electrophoretic profile, when compared with crude plasma (10 protein bands). 相似文献
994.
An optical fiber biosensor has been developed for the determination of catecholamines (dopamine, norepinephrine and epinephrine) based on the recognition capacity of the enzyme laccase. In this study, a glass tube constituted by a fused silica fiber coated with a film of polystyrene/divinylbenzene resin (PS/DVB) was used for catecholamines separation. Firstly, the analyzer was tested for calibration and its analytical performance for catecholamines detection was compared with a classical analytical method, namely high performance liquid chromatography-electrochemical detector (HPLC-ED). The developed analytical device shows a high potential for catecholamines quantification with a detection limit of 2.1, 2.6 and 3.4 pg mL−1 for dopamine, norepinephrine and epinephrine, respectively. The analytical sensitivity, inferred from the slope of the calibration curves established for a range of concentrations between 5 and 125 pg mL−1, was found to be 0.344, 0.252 and 0.140 dB/pg mL−1 for dopamine, norepinephrine and epinephrine, respectively. Furthermore, catecholamines speciation with the PS/DVB fiber was completely achieved in 3 min. The analytical performance of the reported sensor was also evaluated and found adequate for catecholamines determination in human urine and plasma samples. 相似文献
995.
André Romero da Silva Natalia Mayumi Inada Daniel Rettori Mariana Ozello Baratti Anibal Eugenio Vercesi Renato Atilio Jorge 《Journal of photochemistry and photobiology. B, Biology》2009,94(2):101-112
In(III)-meso-tetraphenylporphyrin (InTPP) was encapsulated into nanoparticles (smaller than 200 nm) of poly(d,l-lactide-co-glycolide) (PLGA) using the emulsification–evaporation technique. The photodynamic efficacy of InTPP-loaded nanoparticles and its cellular uptake was investigated with LNCaP prostate tumour cells, in comparison with the free InTPP. The effects of incubation time (1–3 h), drug concentration (1.8–7.7 μmol/L) and incident light dose (15–45 J/cm2) with both encapsulated and free InTPP were studied. The type of cell death induced by the photochemical process using both encapsulated and free InTPP was also investigated. Cell viability was reduced more significantly with increasing values of these effects for InTPP-loaded nanoparticles than with the free drug. The cellular death induced by both encapsulated and free InTPP was preponderantly apoptotic. Confocal laser scanning microscopy data showed that the InTPP-loaded nanoparticles, as well free InTPP, were localized in the cells, and always in the perinuclear region. Encapsulated InTPP was measured by the intensity of fluorescence intensity of cell extracts and was three times more internalized into the cells than was the free InTPP. Electron paramagnetic resonance experiments corroborated the participation of singlet oxygen in the photocytotoxic effect of nanoparticles loaded with InTPP. 相似文献
996.
M. A. V. Ribeiro da Silva Ana Filipa L. O. M. Santos 《Journal of Thermal Analysis and Calorimetry》2009,95(2):333-344
The present work is part of a broader research program on the energetics of formation of heterocycles, aiming the study of
the enthalpic effects of the introduction of different substituents into heterocycles. In this work we present the results
of the thermochemical research on sulphur heterocycles of the type substituted thiophenes with different kind of substituents,
mainly alkyl, ester, acetyl, carboxamide, acetamide, carbonitrile and carboxaldehyde.
The standard (p
o=0.1 MPa) molar enthalpies of formation, in the condensed phase, at T=298.15 K, of a large number of substituted thiophenes, were derived from their standard massic energies of combustion, measured
by rotating-bomb combustion calorimetry, while the standard molar enthalpies of vaporization or sublimation of those compounds
were obtained either by high temperature Calvet Microcalorimetry, or by the temperature dependence of their vapour pressures
determined by the Knudsen effusion technique. The standard molar enthalpies of formation, of the studied sulphur heterocycles
in the gaseous phase, were then derived. The results are interpreted in terms of structural contributions to the energetics
of the substituted thiophenes, the internal consistency of the results is discussed and, whenever appropriate and possible,
empirical correlations are suggested for the estimation of standard molar enthalpies of formation, at T=298.15 K, of substituted thiophenes. A Table of enthalpic increments for different group substituents in positions 2 or 3
of the thiophene ring has been established. 相似文献
997.
A. L. M. de Oliveira J. M. Ferreira Márcia R. S. Silva Soraia C. de Souza F. T. G. Vieira E. Longo A. G. Souza Iêda M. G. Santos 《Journal of Thermal Analysis and Calorimetry》2009,97(1):167-172
NiWO4 and ZnWO4 were synthesized by the polymeric precursor method at low temperatures with zinc or nickel carbonate as secondary phase.
The materials were characterized by thermal analysis (TG/DTA), infrared spectroscopy, UV–Vis spectroscopy and X-ray diffraction.
NiWO4 was crystalline after calcination at 350 °C/12 h while ZnWO4 only crystallized after calcination at 400 °C for 2 h. Thermal decomposition of the powder precursor of NiWO4 heat treated for 12 h had one exothermic transition, while the precursor heat treated for 24 h had one more step between
600 and 800 °C with a small mass gain. Powder precursor of ZnWO4 presented three exothermic transitions, with peak temperatures and mass losses higher than NiWO4 has indicating that nickel made carbon elimination easier. 相似文献
998.
L. S. Bernardi P. R. Oliveira F. S. Murakami M. A. S. Silva S. H. M. Borgmann S. G. Cardoso 《Journal of Thermal Analysis and Calorimetry》2009,97(2):729-733
The thermal analytical study of venlafaxine hydrochloride, a third generation antidepressant, was investigated using thermogravimetry
(TG) and differential scanning calorimetry (DSC). The DSC curves have shown a sharp endothermic event at 211 °C and TG demonstrated
a single stage of mass loss between 254 and 283 °C. Solid-state characterization was carried out by DRIFT, SEM, and XRPD demonstrating
the drug physicochemical properties including crystallinity. Drug-excipient compatibility studies investigated by DSC have
shown a possible physical interaction of the drug with magnesium stearate, microcrystalline cellulose and starch. Nevertheless,
these results where not confirmed by DRIFT and SEM analyses. 相似文献
999.
T. M. Carvalho R. A. G. Silva A. T. Adorno A. G. Magdalena 《Journal of Thermal Analysis and Calorimetry》2009,97(1):53-56
The Ag-rich phase precipitation in the Cu-9 mass% Al was studied using differential scanning calorimetry (DSC), X-ray diffractometry
(XRD) and scanning electron microscopy (SEM). The results indicated that Ag additions did not interfere on the metastable
transitions sequence of the Cu-mass% Al alloy but Ag precipitation disturbs the β phase formation reaction and the martensitic
phase decomposition reaction. 相似文献
1000.
Christian Piontek Dr. Daniel Varón Silva Dr. Christian Heinlein Claudia Pöhner Stefano Mezzato Dr. Petra Ring Andreas Martin Dr. Franz Xaver Schmid Prof. Carlo Unverzagt Prof. 《Angewandte Chemie (International ed. in English)》2009,48(11):1941-1945
Active RNase glycoprotein from three pieces : The glycoprotein enzyme ribonuclease C, which contains a complex saccharide N‐glycan, was synthesized by sequential native chemical ligation. An optimized ligation and isolation protocol allowed the efficient assembly and refolding of the 124 amino acid enzyme.