全文获取类型
收费全文 | 993篇 |
免费 | 41篇 |
国内免费 | 8篇 |
专业分类
化学 | 820篇 |
晶体学 | 3篇 |
力学 | 24篇 |
数学 | 78篇 |
物理学 | 117篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 53篇 |
2020年 | 38篇 |
2019年 | 33篇 |
2018年 | 27篇 |
2017年 | 18篇 |
2016年 | 36篇 |
2015年 | 27篇 |
2014年 | 36篇 |
2013年 | 68篇 |
2012年 | 68篇 |
2011年 | 80篇 |
2010年 | 58篇 |
2009年 | 54篇 |
2008年 | 60篇 |
2007年 | 69篇 |
2006年 | 51篇 |
2005年 | 60篇 |
2004年 | 44篇 |
2003年 | 29篇 |
2002年 | 23篇 |
2001年 | 9篇 |
2000年 | 16篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 1篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1042条查询结果,搜索用时 109 毫秒
91.
92.
M. Beatriz Vázquez-Santos 《Polymer Degradation and Stability》2009,94(1):7-12
The main objective of this work was to investigate porosity development in carbonaceous materials formed by thermal degradation of the high thermal stability polymer, poly(p-phenylene benzobisoxazole) (PBO). Two varieties of PBO, as-spun (AS) and high-modulus (HM) were studied. Chars obtained at various polymer decomposition temperatures (selected on the basis of thermogravimetric and differential thermal analysis results) were characterized by elemental analysis, infrared spectroscopy, X-ray diffraction and CO2 adsorption at 273 K. The obtained adsorbents essentially contained narrow micropores, and maximum pore development took place at ca. 1000 K, when the polymer was not yet completely decomposed. At higher temperatures, there is a certain pore widening along with a displacement towards smaller pore sizes, and the pore volume significantly decreases. 相似文献
93.
M. B. Dantas A. R. Albuquerque L. E. B. Soledade N. Queiroz A. S. Maia I. M. G. Santos A. L. Souza E. H. S. Cavalcanti A. K. Barro A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2011,106(2):607-611
Even not being described in the EN 14112 standard, PDSC has been used for the determination of the biodiesel oxidative stability,
by OIT and OT measurements. In this study, biodiesel blends were obtained by mixing soybean (BES) and castor (BEM) ethyl esters
and its induction periods were measured by Rancimat and PDSC. The blends (BSM
X
) showed intermediate values of OSI, OT, and OIT, compared with BES and BEM. Although, the molar fraction of the components
varied linearly in BSM
X
, OSI, OT, and OIT values increased exponentially in relation to the castor biodiesel amount in the blends. Introduction of
castor oil biodiesel increased the blend stability, so the BSM30 blend reached the OSI limit of 6 h. OSI, OIT, and OT showed a high-linear correlation, pointing out that PDSC can be used
in the analysis of this kind of biodiesel, with a smaller sample and analysis time, as compared to Rancimat. The use of biodiesel
blends was a good alternative in the correction of the oxidative stability of the final product without the need of antioxidant
addition. 相似文献
94.
Maria Rambla-Alegre Juan Peris-Vicente Sergio Marco-Peiró Beatriz Beltrán-Martinavarro Josep Esteve-Romero 《Talanta》2010,81(3):894-274
Melamine is a toxic triazine, illegally used as an additive in milk to apparently increase the amount of protein. A chromatographic procedure using a C18 column and a micellar mobile phase of sodium dodecyl sulphate (0.05 M) and propanol (7.5%), buffered at pH 3, and a detection set by absorbance at 210 nm, was reported for the resolution and quantification of melamine in liquid and powdered milk samples. In this work, samples were diluted with a SDS solution and were directly injected, thus avoiding long extraction and experimental procedures. Melamine was eluted in nearly 9.3 min without overlapping the protein band or other endogeneous compounds. The optimal mobile phase composition was taken using a chemometrical approach that considers the retention factor, efficiency and peak shape. Validation was performed following the European Commission's indications (European Decision 2002/657/EC), and the main analytical parameters studied were: linearity (0.02-100 ppm; r2 = 0.999), limit of detection (5 ppb), intra- and inter-day precision (R.S.D. <7.6% and <9.7%, respectively) and robustness (R.S.D. <7.4% for retention time and <5.0% for area). Sensitivity was adequate to detect melamine under the safety limits proposed by the US FDA. Finally, recoveries for several milk samples were found in the 85-109% range. 相似文献
95.
Georgina Bassani Pablo Fuciños Guillermo Picó Beatriz Farruggia 《Colloids and surfaces. B, Biointerfaces》2010,75(2):532-537
The interaction between a lipase from Candida rugosa (Lip1) and polyethyleneglycols of different molecular masses was studied using fluorescence and circular dichroism approaches in order to be applied to the analysis of the enzyme partition mechanism in aqueous two-phase systems of polyethyleneglycol–potassium phosphate. The decrease of the partition coefficients with the polyethyleneglycol molecular mass showed that the enzyme partition is driven by the excluded volume effect and not by the enzyme–polymer interaction. The polymer did not affect the secondary and tertiary structure of the enzyme nor its biological activity. The lipase from Candida rugosa lyophilizate was partitioned in favour of the polyethyleneglycol rich phase; PEG 2000 being the system which showed the better enzyme recovery (78.26%) with a purification factor of 2.3. This method could be applied as a first step to isolate the enzyme from a culture medium with good recovery and without modifying the enzymatic capacity and the molecular structure. 相似文献
96.
Josiane Courtois Jean-Paul Seguin Sophie Declomesnil Alain Heyraud Philippe Colin-Morel Luciana Dantas 《Journal of carbohydrate chemistry》2013,32(4-5):441-448
Abstract A mutant of the R. meliloti M5N1 strain has been selected. This strain, R. meliloti M5N1 CS (NCIMB 40472), excretes an extracellular material composed of 2-O-Ac-β-GlcpA, 3-O-Ac-β-GlcpA, 2,3-di-O-Ac-β-GlcpA and three species of β-GlcpA residues 1→4 linked. For the culture conditions used, the weight average molecular weight of the polymer varied in the range of 6 × 104 < Mw < 4 × 105. High molecular weight glucuronate forms thermoreversible gels at 5 g L?l. In the presence of divalent cation such as Ca2+ or trivalent cations such as Cr3+ or Fe3 +, cross linking of the polymer occurs. This polysaccharide is the first exocellular (1→4)-β-D-glucuronan produced by a R. meliloti strain. 相似文献
97.
Jussara de Araújo Gonçalves André Luis Dantas Ramos Layla L. L. Rocha Anderson Kurunczi Domingos Robson S. Monteiro José Sotolongo Peres Nelson C. Furtado Carlton A. Taft Donato A. G. Aranda 《Journal of Physical Organic Chemistry》2011,24(1):54-64
Biodiesel can be obtained from fatty acid raw materials through esterification. The reactivity of lauric, palmitic, stearic, oleic, and linoleic fatty acids with methanol using powdered niobic acid as a heterogeneous catalyst was investigated in this work, both experimentally (in a batch reactor) and theoretically. A 23 experimental design was used, with methanol/fatty acid molar ratio, catalyst concentration, and temperature as main factors. An empirical model demonstrated that temperature is the most important variable. Fourteen heterogeneous and 56 homogeneous‐like kinetic models were tested. A homogeneous‐like model considering zero order for all species and inhibition by water was the most adequate for experiments without catalyst. A homogeneous‐like model considering a second‐order reaction in relation to the fatty acid and no water inhibition was the most adequate for niobic acid catalyzed reaction. Molecular modeling confirmed the experimental results showing that the reactivity is directly related to the increase of unsaturated bonds and the reduction of carbon chain length. The polarity of the fatty acid is determinant in the reactivity. At the molecular level, reaction occurs between the HOMO orbital of methanol and LUMO orbitals of fatty acids and reactivity is higher when the energy difference between these orbitals is lower. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
98.
A reverse flow injection spectrophotometric enzymatic method is proposed to quantify total phenols in urine samples. The polyphenol oxidase (PPO; EC 1.14.18.1) obtained as a crude extract from sweet potato root (Ipomoea batatas) was used as enzymatic catalyze. The detection limit, the sample throughput and relative standard deviation were 7.7 mg l−1 of total phenols, 49 h−1 and 0.9%, respectively. The method was applied to real samples and a recovery study was carried out in order to its validation. 相似文献
99.
Márcio José Horta Dantas José Manoel Balthazar 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2007,28(1):940-958
In this work, motivated by non-ideal mechanical systems, we investigate the following O.D.E.
[(x)\dot] = f (x) + eg (x, t) + e2[^(g)] (x, t, e){\dot{x} = f (x) + \varepsilon g (x, t) + \varepsilon^{2}\widehat{g} (x, t, \varepsilon)}
, where
x ? W ì \mathbbRn{x \in \Omega \subset \mathbb{R}^n}
,
g,[^(g)]{g,\widehat{g}}
are T periodic functions of t and there is a
0 ∈ Ω such that f ( a
0) = 0 and f ′( a
0) is a nilpotent matrix. When n = 3 and f (x) = (0, q (x
3) , 0) we get results on existence and stability of periodic orbits. We apply these results in a non ideal mechanical system:
the Centrifugal Vibrator. We make a stability analysis of this dynamical system and get a characterization of the Sommerfeld
Effect as a bifurcation of periodic orbits. 相似文献
100.
57Fe Mössbauer spectroscopy is used to study the interactions, geometry, and the coordination characteristics of the Fe-complexes of biopolymers such as chitosan, glucosamine, and chondritin sulfate. In addition, a computational effort is undertaken for predicting the geometries and energies of the metal complexes by the Density Functional Theory (DFT) methods as implemented in the Gaussian 2003 quantum mechanical program. Both experimental and computational results suggest that the structure of the metal complexes resemble closely the structure of the active sites of metalloenzymes in 2+ or 3+ oxidation states and is at least tetracoordinated and can possibly have six ligands. 相似文献