全文获取类型
收费全文 | 309篇 |
免费 | 9篇 |
国内免费 | 3篇 |
专业分类
化学 | 201篇 |
晶体学 | 3篇 |
力学 | 9篇 |
数学 | 41篇 |
物理学 | 67篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 26篇 |
2018年 | 15篇 |
2017年 | 15篇 |
2016年 | 16篇 |
2015年 | 9篇 |
2014年 | 28篇 |
2013年 | 40篇 |
2012年 | 17篇 |
2011年 | 25篇 |
2010年 | 25篇 |
2009年 | 10篇 |
2008年 | 14篇 |
2007年 | 10篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1994年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有321条查询结果,搜索用时 15 毫秒
41.
Ion chromatography–inductively coupled plasma-mass spectrometry (IC–ICP-MS) was used for the identification and quantification of chromium species. Chromium(III) and chromium(VI) were separated and determined by IC–ICP-MS. The separation was achieved using an anion exchange column with 0.55?M HNO3 as mobile phase. It was a particular goal of this work to exclusively use nitric acid for elution in order to reduce interferences in the ICP-MS system. Analytical figures of merit were calculated under the optimum conditions by developing calibration plots in a concentration range of 0.50–250?µg/L for both species. The detection limits for Cr(III) and Cr(VI) were 0.09 and 0.03?µg/L, respectively. Spiked recovery tests were used to evaluate the applicability of the analytical method in environmental samples, and the recoveries ranged between 97 and 103% for both analytes. The accuracy of the method for total chromium content was validated through the analysis of a spring water-certified reference material (UME 1201), and the obtained results were in good agreement with the certified value. Lettuce seedlings were cultivated to evaluate the intake levels of these species. In addition, the bioaccessibility of Cr(III) and Cr(VI) from the lettuce seedlings in simulated gastric and intestinal fluids media was examined. 相似文献
42.
Selimefendigil Fatih Oztop Hakan F. Chamkha Ali J. 《Journal of Thermal Analysis and Calorimetry》2019,135(1):325-340
Journal of Thermal Analysis and Calorimetry - In this study, fluid flow and heat transfer in a vertical lid-driven CuO–water nanofluid filled square cavity with a flexible fin attached to its... 相似文献
43.
Control of four-wave mixing phase-matching condition using the Brillouin slow-light effect in fibers
All-optical control of the phase-matching condition in four-wave mixing (FWM) processes is demonstrated using the Brillouin slow-light effect in optical fibers. A counterpropagating stimulated Brillouin scattering (SBS) pump has been used to control the phase velocity of the FWM pump in a wavelength conversion scheme. Both experimental results and theoretical simulations show an SBS-controlled 20 dB difference on the wavelength conversion efficiency. 相似文献
44.
Aidaoui Lakhdar Lasbet Yahia Selimefendigil Fatih 《Journal of Thermal Analysis and Calorimetry》2022,147(10):5865-5882
Journal of Thermal Analysis and Calorimetry - This work aims to characterize the heat exchange system efficiency and entropy generation of nanofluid forced convection flow with chaotic... 相似文献
45.
Fatih Güleç Aysel Niftaliyeva Ali Karaduman 《Research on Chemical Intermediates》2018,44(12):7205-7218
2,6-Dimethylnaphthalene (2,6-DMN) is one of the crucial intermediates for the synthesis of polybutylenenaphthalate and polyethylene naphthalate (PEN). The complex synthesis procedure and the high cost of 2,6-DMN production significantly reduce the commercialisation of PEN even though PEN demonstrates superior properties compared with polyethylene terephthalate. 2,6-DMN can be produced by methylation of 2-methylnaphthalene (2-MN) and/or naphthalene, disproportionation of 2-MN, and/or isomerisation of dimethylnaphthalenes (DMNs). In this study, synthesis of 2,6-triad DMN isomers consisting of 2,6-DMN, 1,6-DMN, and 1,5-DMN have been investigated with the disproportionation of 2-MN over unmodified and Zr-modified mesoporous MCM-41 zeolite catalysts. In contrast to other DMN isomers, both 1,5-DMN and 1,6-DMN can be effectively isomerised to be profitable 2,6-DMN. The disproportionation of 2-MN experiments were carried out in a catalytic fixed-bed reactor in the presence of 1 g of catalyst at a temperature range of 350–500 °C and weight hourly space velocity between 1 to 3 h?1. The results demonstrated that mesoporous MCM-41 zeolite catalyst has a selective pore shape for 2,6-triad DMN isomers, which may allow a decrease in the production cost of 2,6-DMN. Additionally, 2,6-DMN was successfully synthesised by the disproportionation of 2-MN over MCM-41 zeolite catalyst. Furthermore, both the conversion of 2-MN and the selectivity of 2,6-DMN were considerably enhanced by the Zr impregnation on MCM-41. 相似文献
46.
An efficient procedure for the directed reductive amination of beta-hydroxy-ketones (3) for the stereoselective preparation of 1,3-syn-amino alcohols (6) is reported. The operationally simple protocol uses Ti(iOPr)4 for coordination of the intermediate imino alcohol (5) and PMHS as the reducing agent. The method was expanded to an asymmetric aldol reductive amination sequence to allow a highly convergent synthesis of the hydroxy-amine core of the HIV-protease inhibitors ritonavir and lopinavir. [reaction: see text]. 相似文献
47.
Fatih Yaar Tarik elik Bernd A. Berg Hagai Meirovitch 《Journal of computational chemistry》2000,21(14):1251-1261
The multicanonical (Muca) Monte Carlo method enables simulating a system over a wide range of temperatures and thus has become an efficient tool for studying spin glasses, first‐order phase transitions, the helix–coil transition of polypeptides, and protein folding. However, implementation of the method requires calculating the multicanonical weights by an iterative procedure that is not straightforward and is a stumbling block for newcomers. A recursive procedure that takes into account the statistical errors of all previous iterations and thus enables an automatic calculation of the weights without the need for human intervention after each iteration has been proposed. This procedure, which has already been tested successfully for lattice systems, is extended here to continuum models of peptides and proteins. The method is examined in detail and tested for models of the pentapeptide Leu‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Leu) described by the potential energy function ECEPP. Because of the great interest in the structural mapping of the low‐energy region of biomolecules, the energy of structures selected from the Muca trajectory is minimized. The extent of conformational coverage provided by the method is examined and found to be very satisfactory. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1251–1261, 2000 相似文献
48.
We have calculated the effects of the intense laser field on the total optical absorption coefficient (the linear and third-order nonlinear) for transition between two lower-lying electronic levels in the asymmetric parabolic \({\text{GaAs/ Ga}}_{{ 1 {\text{ - x}}}} {\text{Al}}_{\text{x}} {\text{As}}\) quantum well. Total absorption coefficient (linear and nonlinear absorption coefficient) for the transitions between any two electronic states was calculated by using density matrix formalism and the perturbation expansion method. Our results show that the effects of intense laser field and the well dimensions on the optical transitions are more pronounced. If well center is changed to be \({\text{L}}_{\text{c}} < 0 \, ({\text{L}}_{\text{c}} > 0)\), effective well width decreases (increases) and thus we can obtain the red or blue shift in the peak position of the absorption coefficient by changing the intensities of the non-resonant intense laser field as well as dimensions of the well. 相似文献
49.
Melek Pamuk Algi Zahide Oztas Seha Tirkeş Atilla Cihaner Fatih Algi 《Journal of fluorescence》2017,27(2):509-519
Two chemiluminescent compounds containing 2,5-di(thien-2-yl)pyrrole and pyridazine units, namely 5,7-di(thiophen-2-yl)-2,3-dihydro-1H-pyrrolo[3,4-d]pyridazine-1,4(6H)-dione (5) and 6-phenyl-5,7-di(thiophen-2-yl)-2,3-dihydro-1H-pyrrolo[3,4-d]pyridazine-1,4(6H)-dione (6), were successfully synthesized and electrochemically polymerized. The compounds have chemiluminescent properties and glow in the presence of hydrogen peroxide in basic medium. The intensity of the glow can be increased dramatically by using Fe3+ ions, hemin (1.0 ppm) or blood samples (1.0 ppm) as catalyst. The compounds 5 and 6 have one well-defined irreversible oxidation peak at 1.08 V and 1.33 V vs Ag/AgCl, respectively. Electrochemical polymerization of both 5 and 6 were carried out successfully by repeating potential scanning in the presence of BF3. Et2O in an electrolyte solution of 0.1 M LiClO4 dissolved in acetonitrile. The electronic band gaps (Eg) of the polymers P5 and P6 were found to be 2.02 eV and 2.16 eV, respectively. On the other hand, the corresponding polymers are electroactive and exhibited electrochromic features. 相似文献
50.
The diamagnetic susceptibility of a hydrogenic donor placed in Si, Ge and GaAs quantum wells with infinite confinement potential which have different effective mass anisotropy parameters (γ=m⊥/m∥) has been investigated as a function of the well sizes. The binding energies of the donor have also been computed using a trial wave function with two parameters in the framework of the effective mass approximation. It has been observed that the diamagnetic susceptibility of the donor in the anisotropic materials converges rapidly to the bulk limit as the well size increases. 相似文献