全文获取类型
收费全文 | 3229篇 |
免费 | 144篇 |
国内免费 | 13篇 |
专业分类
化学 | 2457篇 |
晶体学 | 38篇 |
力学 | 50篇 |
数学 | 362篇 |
物理学 | 479篇 |
出版年
2023年 | 16篇 |
2022年 | 14篇 |
2021年 | 61篇 |
2020年 | 65篇 |
2019年 | 72篇 |
2018年 | 44篇 |
2017年 | 37篇 |
2016年 | 108篇 |
2015年 | 85篇 |
2014年 | 104篇 |
2013年 | 176篇 |
2012年 | 226篇 |
2011年 | 234篇 |
2010年 | 165篇 |
2009年 | 172篇 |
2008年 | 206篇 |
2007年 | 219篇 |
2006年 | 197篇 |
2005年 | 172篇 |
2004年 | 147篇 |
2003年 | 125篇 |
2002年 | 116篇 |
2001年 | 109篇 |
2000年 | 90篇 |
1999年 | 66篇 |
1998年 | 48篇 |
1997年 | 32篇 |
1996年 | 42篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有3386条查询结果,搜索用时 15 毫秒
1.
Verdecho María-José Alarcón-Valero Faustino Pérez-Perales David Alfaro-Saiz Juan-José Rodríguez-Rodríguez Raúl 《Central European Journal of Operations Research》2021,29(4):1231-1251
Central European Journal of Operations Research - Sustainability practice within supply chains remains in an early development phase. Enterprises still need tools that support the integration of... 相似文献
2.
Dr. Antonio M. Rodríguez Dr. Pilar Prieto Prof. Antonio de la Hoz Prof. Ángel Díaz-Ortiz D. Raúl Martín Prof. José I. García 《ChemistryOpen》2015,4(3):308-317
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation. 相似文献
3.
4.
Karen Beatriz Méndez-Rodríguez Nicté Figueroa-Vega César Arturo Ilizaliturri-Hernandez Mónica Cardona-Alvarado Jaime Antonio Borjas-García Carlos Kornhauser Juan Manuel Malacara Rogelio Flores-Ramírez Francisco Javier Pérez-Vázquez 《Biomedical chromatography : BMC》2020,34(12):e4956
Metabolomics is a potential tool for the discovery of new biomarkers in the early diagnosis of diseases. An ultra-fast gas chromatography system equipped to an electronic nose detector (FGC eNose) was used to identify the metabolomic profile of Volatile Organic Compounds (VOCs) in type 2 diabetes (T2D) urine from Mexican population. A cross-sectional, comparative, and clinical study with translational approach was performed. We recruited twenty T2D patients and twenty-one healthy subjects. Urine samples were taken and analyzed by FGC eNose. Eighty-eight compounds were identified through Kovats's indexes. A natural variation of 30% between the metabolites, expressed by study groups, was observed in Principal Component 1 and 2 with a significant difference (p < 0.001). The model, performed through a Canonical Analysis of Principal coordinated (CAP), allowed a correct classification of 84.6% between healthy and T2D patients, with a 15.4% error. The metabolites 2-propenal, 2-propanol, butane- 2,3-dione and 2-methylpropanal, were increased in patients with T2D, and they were strongly correlated with discrimination between clinically healthy people and T2D patients. This study identified metabolites in urine through FGC eNose that can be used as biomarkers in the identification of T2D patients. However, more studies are needed for its implementation in clinical practice. 相似文献
5.
Design and development of a two‐dimensional system based on hydrophilic and reversed‐phase liquid chromatography with on‐line sample treatment for the simultaneous separation of excreted xenobiotics and endogenous metabolites in urine 下载免费PDF全文
Diego García‐Gómez Encarnación Rodríguez‐Gonzalo Rita Carabias‐Martínez 《Biomedical chromatography : BMC》2015,29(8):1190-1196
In the present work we describe a two‐dimensional liquid chromatographic system (2D‐LC) with detection by mass spectrometry (MS) for the simultaneous separation of endogenous metabolites of clinical interest and excreted xenobiotics deriving from exposure to toxic compounds. The 2D‐LC system involves two orthogonal chromatographic modes, hydrophilic interaction liquid chromatography (HILIC) to separate polar endogenous metabolites and reversed‐phase (RP) chromatography to separate excreted xenobiotics of low and intermediate polarity. Additionally, the present proposal has the novelty of incorporating an on‐line sample treatment based on the use of restricted access materials (RAMs), which permits the direct injection of urine samples into the system. The work is focused on the instrumental coupling, studying all possible options and attempting to circumvent the problems of solvent incompatibility between the RAM device and the two chromatographic columns, HILIC and RP. The instrumental configuration developed, RAM‐HILIC‐RPLC‐MS/MS, allows the simultaneous assessment of urinary metabolites of clinical interest and excreted compounds derived from exposure to toxic agents with minimal sample manipulation. Thus, it may be of interest in areas such as occupational and environmental toxicology in order to explore the possible relationship between the two types of compounds. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
6.
7.
8.
Capillary Electrophoresis Chiral Separation for Enantiomeric Purity Determination of a Basic Drug in Pharmaceutical Formulations 总被引:1,自引:0,他引:1
Capillary electrophoresis using a running buffer composed of β-cyclodextrin as the chiral selector and ethanolamine mesylate at pH 9.6 is being used to monitor the stereochemical stability of a hydrophobic drug, containing two chiral centers, in two different formulated self-emulsifying drug delivery system (SEDDS) products. The separation takes place in less than 25 min. Strategies for enhancing the method reproducibility and detection sensitivity in the lower potency formulation are presented. The results demonstrating the specificity, assay precision, recovery, linearity and range achieved during the method validation experiments are presented in this paper.Presented at: CE in the Biotechnology and Pharmaceutical Industries: 7th Symposium on the Practical Applications for the Analysis of Proteins, Nucleotides and Small Molecules, Montreal, Canada, August 12–16, 2005. 相似文献
9.
Jerónimo Rodríguez 《Comptes Rendus Mathematique》2004,339(6):445-450
In the present Note we introduce an extension of the conservative space–time mesh refinement method presented by Fouquet et al. We also propose a post-treatment of the solution that reduces the spurious phenomena due to the non-conformity between the time meshes. A reinterpretation of the equations in terms of new unknowns leads to a new scheme with second order consistent coupling equations. Numerical experiments in 2D and a plane wave analysis for the 1D model show that the method is second order accurate for an arbitrary refinement. To cite this article: J. Rodríguez, C. R. Acad. Sci. Paris, Ser. I 339 (2004). 相似文献
10.
Ricaurte Rodríguez Manuel Nogueras Justo Cobo John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(7):o376-o378
Molecules of the title compound, C12H13ClN4, are linked by two independent N—H...N hydrogen bonds into a chain of edge‐fused R22(8) rings. The significance of this study lies in its attempt to rationalize the patterns of supramolecular aggregation in the title compound and in a range of analogous 4,6‐disubstituted 2‐aminopyrimidines. 相似文献