首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   408篇
  免费   17篇
  国内免费   9篇
化学   270篇
晶体学   4篇
力学   18篇
数学   59篇
物理学   83篇
  2023年   1篇
  2022年   8篇
  2021年   17篇
  2020年   14篇
  2019年   28篇
  2018年   31篇
  2017年   24篇
  2016年   28篇
  2015年   12篇
  2014年   25篇
  2013年   54篇
  2012年   37篇
  2011年   26篇
  2010年   12篇
  2009年   22篇
  2008年   19篇
  2007年   22篇
  2006年   13篇
  2005年   8篇
  2004年   5篇
  2003年   3篇
  2002年   1篇
  2001年   5篇
  1997年   2篇
  1994年   1篇
  1989年   2篇
  1985年   2篇
  1981年   1篇
  1980年   2篇
  1978年   4篇
  1977年   2篇
  1975年   1篇
  1974年   2篇
排序方式: 共有434条查询结果,搜索用时 0 毫秒
61.
This paper deals with the stability of multiobjective nonlinear programming problems with fuzzy parameters in the objectives and constraints functions. These fuzzy parameters are characterized by fuzzy numbers. The existing results concerning the qualitative analysis of the notions (solvability set, stability sets of the first kind and of the second kind) in parametric nonlinear programming problems are reformulated to study the stability of multiobjective nonlinear programming problems under the concept of α-pareto optimality. An algorithm for obtaining any subset of the parametric space which has the same corresponding α-pareto optimal solution is also presented. An illustrative example is given to clarify the obtained results.  相似文献   
62.
Chiral SU2 × SU2 breaking, Cabibbo angle, non-electromagnetic isospin breaking, η → 3π decay rate, and pion and kaon mass differences are related to one another in a nonlinear Lagrangian model of broken chiral SU3 × SU3 symmetry. The agreement with experiment is very good.  相似文献   
63.
Transmittance measurements were carried out for LiF thin films directly after preparation, then one day, one week, four weeks and six weeks later. The aging effect on the optical properties of the film was thus studied. This showed that the aged LiF films have refractive indices larger than those of fresh films.  相似文献   
64.
Novel semi-Interpenetrating Polymer Network (semi-IPN) hydrogels based on partially Hydrolyzed Polyacrylamide (HPAM) and Poly(Vinyl Alcohol) (PVA) were prepared by solution crosslinking using chromium triacetate. Effects of PVA content on the gelation process and swelling behavior in tap water and different electrolyte solutions were investigated. Study of the gelation behavior using dynamic rheometery showed that the limiting storage modulus of the semi-IPN gels decreased with increasing PVA content. It was also found that increasing the PVA content increases the loss factor, indicating that the viscous properties of this gelling system increase more strongly than the elastic properties. The swelling ratio of the semi-IPN gels in tap water decreased as the concentration of the PVA increased. However, the semi-IPN gels showed lower salt sensitivity factor in synthetic oil reservoir water as compared with HPAM gels. Therefore, they are potentially good candidates for enhanced oil recovery applications.  相似文献   
65.
66.
Novel $N$ -alkyl-2-(alkylthio)-1 $H$ -imidazole derivatives were synthesized in a single step by an efficient and simple method in high yields. Readily available starting materials, mild reaction conditions, operational simplicity and novelty are the key advantages of this method. Besides their novel structures, these compounds may have important biological activities and industrial applications.  相似文献   
67.
The behavior of Al2O3/Al composite coated Al electrodes fabricated by surface mechanical alloying ‘SMA’ was studied. The work was accomplished using Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques in alkaline media 2MKOH were done at room temperature. Results show hydroxyl ions accumulate on the surface due to Al deformation micro cavities filling with Al2O3 until full charge blockage reached. A barrier cover layer development causing an increase of both resistance and capacitance as it becomes more stable and thinner with exposure time increase. Migrating hydroxyl ion inside micro cavity changed its composition from Al2O3 to stable tetrahedral Al(OH)4? aluminate ions. Therefore future benefits could be reached by developing such surfaces having charge accumulation that enables environmental interaction.  相似文献   
68.
Two new cryptic 3,4-dihydronaphthalen-(2H)-1-one (1-tetralone) derivatives, aspvanicin A (1) and its epimer aspvanicin B (2), as well as several known cryptic metabolites (38), were obtained from the ethyl acetate extract of the co-culture of the endophytic fungus Aspergillus versicolor KU258497 with the bacterium Bacillus subtilis 168 trpC2 on solid rice medium. When A. versicolor was cultured axenically in liquid Wickerham medium supplemented with 3.5% DMSO, an additional three known secondary metabolites (911) were isolated that were lacking when the fungus was fermented on rice medium. The structures of the new compounds were elucidated using one- and two-dimensional NMR spectroscopy as well as HRESIMS. The relative and absolute configurations of 1 and 2 were determined by the combination of NMR and electronic circular dichroism (ECD) analysis aided by DFT conformational analysis and TDDFT-ECD calculations. The ECD calculations revealed that although the sign of the blue-shifted overlapping n-π1 ECD transition follows the helicity rule of cyclic aryl ketones, the calculation of low-energy conformers and ECD spectra was necessary to determine the stereochemistry. All metabolites were assessed for their antibacterial and cytotoxic activities; one of the new diastereomers, compound 2, showed moderate cytotoxic activity against the mouse lymphoma cell line L5178Y.  相似文献   
69.
Microchimica Acta - Accurate detection of temperature and oxygen concentration at the cellular level is important in tumor diagnosis. Multifunctional nanocomposites are described that consist of...  相似文献   
70.
In this article, the palladium (II) mixed-ligand complex synthesized with reduced graphene oxides containing tetraethoxysilane and menthone oxime was used as an efficient solid catalyst for the Heck coupling reaction. To maintain stability and catalytic activity in the C–C bond reaction, graphene was considered due to the available surface as the solid support. Then, the structure of new heterogeneous catalyst was investigated by FT-IR, UV–Vis DRS, FE-SEM, EDX, AFM, XRD, ICP-OES, Raman, and TGA. The newly synthesized nanocatalyst have beneficial properties, including product’s easy separation, the shorter time to react, purity products (yield 79–99%), and easier work-up procedure. Furthermore, the catalyst was reused six times without significant degradation in catalytic activity and performance.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号