全文获取类型
收费全文 | 241篇 |
免费 | 10篇 |
专业分类
化学 | 124篇 |
晶体学 | 11篇 |
力学 | 18篇 |
数学 | 65篇 |
物理学 | 33篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 9篇 |
2020年 | 4篇 |
2019年 | 8篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 2篇 |
2014年 | 5篇 |
2013年 | 37篇 |
2012年 | 25篇 |
2011年 | 22篇 |
2010年 | 10篇 |
2009年 | 15篇 |
2008年 | 13篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 3篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有251条查询结果,搜索用时 15 毫秒
111.
A mixed finite element method is combined to finite volume schemes on structured and unstructured grids for the approximation of the solution of incompressible flow in heterogeneous porous media. A series of numerical examples demonstrates the effectiveness of the methodology for a coupled system which includes an elliptic equation and a nonlinear degenerate diffusion–convection equation arising in modeling of flow and transport in porous media. 相似文献
112.
Yves Leroux Driss El Manouni Brahim Ben Jaafar Ramon Burgada 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-3):205-208
Abstract - Addition of trivalent phosphorus compounds with α, β ethylenic ketones and esters leads to examples of prototropy or cyclisation or rearrangement with ring expansion from 5 to 7 atoms. When trivalent phosphorus compounds react with acetylenic ketones and esters, the 1, 3 dipolar species can be trapped with an electrophilic reagent (aldehydes or the original acetylenic compound) or a protic reagent (alcohol, acid, amide, phenol, etc…) Ylides, phosphoranes, spirophos-phoranes and phospholes can be obtained. 相似文献
113.
Bessem Brahim Sandra Alves Richard B. Cole Jean-Claude Tabet 《Journal of the American Society for Mass Spectrometry》2013,24(12):1988-1996
Charge enhancement of single-stranded oligonucleotide ions in negative ESI mode is investigated. The employed reagent, meta-nitrobenzyl alcohol (m-NBA), was found to improve total signal intensity (Itot), increase the highest observed charge states (zhigh), and raise the average charge states (zavg) of all tested oligonucleotides analyzed in negative ESI. To quantify these increases, signal enhancement ratios (SER1%) and charge enhancement coefficients (CEC1%) were introduced. The SER1%, (defined as the quotient of total oligonucleotide ion abundances with 1 % m-NBA divided by total oligonucleotide abundance without m-NBA) was found to be greater than unity for every oligonucleotide tested. The CEC1% values (defined as the average charge state in the presence of 1 % m-NBA minus the average charge state in the absence of m-NBA) were found to be uniformly positive. Upon close inspection, the degree of charge enhancement for longer oligonucleotides was found to be dependent upon thymine density (i.e., the number and the location of phospho-thymidine units). A correlation between the charge enhancement induced by the presence of m-NBA and the apparent gas-phase acidity (largely determined by the sequence of thymine units but also by the presence of protons on other nucleobases) of multiply deprotonated oligonucleotide species, was thus established. Ammonium cations appeared to be directly involved in the m-NBA supercharging mechanism, and their role seems to be consistent with previously postulated ESI mechanisms describing desorption/ionization of single-stranded DNA into the gas phase.
? 相似文献
114.
This article presents an electrochemical method for sulphur dioxide removal; the idea consists to couple the phenomenon of electrolysis upstream to the sulphuric acid plant. Otherwise, this proposition consists in modifying the hybrid cycle Westinghouse by the elimination of the sulphuric acid decomposition stage, which corresponds to 61% of the energy distribution of this cycle. The step of the decomposition is replaced by the exploitation of the quantity of sulphur dioxide emissions SO2 stemming from the process of sulphuric acid H2SO4 production. The industrial complex of Gabès in South Tunisia is selected to be the case study complex. In fact, most industries are chemical oriented, the fast growing numbers of H2SO4 factories have resulted in fairly serious air pollution, especially sulphur dioxide SO2 pollution. To confirm the efficiency of this process, a mass balance study was made on the sulphuric acid plant, which used a double contact/double absorption process; this study demonstrates that this new method can treat 8 t d?1 of sulphur dioxide, recover 12 t d?1 of sulphuric acid of title 50% and produce 6889 m3 d?1 of the hydrogen. This idea can achieve economical and environmental benefits. It is concluded that the usage of this electrochemical process can significantly reduce the SO2 air pollution in Gabès (South Tunisia) by the elimination of 46.94 t d?1 of SO2. The results obtained are ambitious for future application of the new process, which can succeed in the realisation of 3.92 million dollars of gain every day. 相似文献
115.
Brahim Chaguir Mohamed Brakta Véronique Bolitt Paul Lhoste Denis Sinou 《Journal of carbohydrate chemistry》2013,32(5):609-625
Abstract Phenyl 2, 3-dideoxy-4, 6-di-O-benzyl-D-erythro-hex-2-enopyranoside 1α (or 1β) is alkylated regio- and stereospecifically at the anomeric center by stabilized tertiary nucleo-philes in the presence of Pd(0) as a catalyst, leading to the C-glycoside of α- (or β-) configuration. The observed loss of stereoselectivity using secondary stabilized nucleophiles is mainly due to a retro Michael reaction. The assignment of the α- (or β-) configuration at the anomeric center was accomplished using 13C NMR and NOE experiments. 相似文献
116.
Brahim Brahimi Djamel Meraghni Abdelhakim Necir Ri?ardas Zitikis 《Insurance: Mathematics and Economics》2011,49(3):325-334
The distortion parameter reflects the amount of loading in insurance premiums. A specific value of a given premium determines a value of the distortion parameter, which depends on the underlying loss distribution. Estimating the parameter, therefore, becomes a statistical inferential problem, which has been initiated by Jones and Zitikis [Jones, B.L., Zitikis, R., 2007. Risk measures, distortion parameters, and their empirical estimation. Insurance: Mathematics and Economics, 41, 279–297] in the case of the distortion premium and tackled within the framework of the central limit theorem. Heavy-tailed losses do not fall into this framework as they rely on the extreme-value theory. In this paper, we concentrate on a special but important distortion premium, called the proportional-hazard premium, and propose an estimator for its distortion parameter in the case of heavy-tailed losses. We derive an asymptotic distribution of the estimator, construct a practically implementable confidence interval for the distortion parameter, and illustrate the performance of the interval in a simulation study. 相似文献
117.
Rahib Yassine Sarh Brahim Chaoufi Jamal Bonnamy Sylvie Elorf Abdallah 《Journal of Thermal Analysis and Calorimetry》2021,145(5):2405-2416
Journal of Thermal Analysis and Calorimetry - A detailed investigation of the physicochemical properties and thermal characteristics of argan fruit residues (AFRs) was carried out to identify their... 相似文献
118.
119.
Sidi Mohamed Abdallahi Dr. Ewies F. Ewies Mohamed El-Shazly Brahim Ould Elemine Abderrahmane Hadou Ján Moncol Dr. Ata Martin Lawson Prof. Adam Daich Dr. Mohamed Othman 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(62):15440-15449
Iron-based catalysts were applied in cascade-type reactions for the synthesis of different carbonyl compounds. The reactions proceeded by a new iron-catalyzed cascade of alkynylation/hydration by using both the σ- and π-Lewis acid properties of iron salts. The alkynylation reactions of several endo and exocyclic acetoxylactams were achieved with three different catalysts including FeCl3 ⋅ 6H2O, FeCl3, and Fe(OTf)3 showing the efficiency of σ-Lewis acidity of iron (III) salts in catalyzing the alkynylation reaction. We also demonstrated that the reaction sequence could be shortened by the direct use of hydroxylactams, leading to an environmentally friendly protocol, avoiding the need to perform unnecessary lengthy steps. A combination of the hard/soft iron Lewis acid properties was then used to implement an unprecedented tandem intermolecular alkynylation/intramolecular hydration sequence allowing expedient access to a new carbonyl structures from trivial materials. 相似文献
120.
The recent observation of higher quark combinations, tetraquarks and pentaquarks, is a strong indication of more exotic hadrons. Using Z2 and Z3 symmetries and standard model data, a general quark combination producing new hadronic states is proposed in terms of polygon geometries according to the Dynkin diagrams of  n affine Lie algebras. It has been shown that Z2,3 invariance is crucial in the determination of the mesonic or the baryonic nature of these states. The hexagonal geometry is considered in some details producing both mesonic and baryonic states. A general class of this family is also presented. 相似文献