全文获取类型
收费全文 | 623篇 |
免费 | 21篇 |
国内免费 | 10篇 |
专业分类
化学 | 509篇 |
晶体学 | 1篇 |
力学 | 22篇 |
数学 | 37篇 |
物理学 | 85篇 |
出版年
2024年 | 2篇 |
2022年 | 6篇 |
2021年 | 12篇 |
2020年 | 20篇 |
2019年 | 27篇 |
2018年 | 39篇 |
2017年 | 23篇 |
2016年 | 50篇 |
2015年 | 25篇 |
2014年 | 54篇 |
2013年 | 69篇 |
2012年 | 59篇 |
2011年 | 46篇 |
2010年 | 36篇 |
2009年 | 28篇 |
2008年 | 16篇 |
2007年 | 22篇 |
2006年 | 11篇 |
2005年 | 19篇 |
2004年 | 15篇 |
2003年 | 17篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1987年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有654条查询结果,搜索用时 11 毫秒
541.
The first study on the high efficiency of nano-structured lead dioxide as a new fiber for solid-phase microextraction (SPME) purposes has been reported. The size of the PbO2 particles was in the range of 34-136 nm. Lead dioxide-based fibers were prepared via electrochemical deposition on a platinum wire. The extraction properties of the fiber to benzene, toluene, ethylbenzene, and xylenes (BTEX) were examined using headspace solid-phase microextraction (HS-SPME) mode coupled to gas chromatography-flame ionization detection (GC-FID). The results obtained proved the suitability of proposed fibers for the sampling of organic compounds from water. The extraction procedure was optimized by selecting the appropriate extraction parameters, including preparation conditions of coating, salt concentration, time and temperature of adsorption and desorption and stirring rate. The calibration graphs were linear in a concentration range of 0.1-100 microg l(-1) (R2 > 0.994) with detection limits below 0.012 microg l(-1) level. Single fiber repeatability and fiber-to-fiber reproducibility were less than 10.0 and 12.5%, respectively. The PbO2 coating was proved to be very stable at relatively high temperatures (up to 300 degrees C) with a high extraction capacity and long lifespan (more than 50 times). Higher chemical resistance and lower cost are among the advantages of PbO2 fibers over commercially available SPME fibers. Good recoveries (81-108%) were obtained when environmental samples were analyzed. 相似文献
542.
Hosseiny Davarani SS Nematollahi D Shamsipur M Najafi NM Masoumi L Ramyar S 《The Journal of organic chemistry》2006,71(5):2139-2142
The electrooxidation of 2,3-dimethylhydroquinone (1) has been studied in the presence of 2-phenyl-1,3-indandione (3a), 3-hydroxy-1H-phenalen-1-one (3b), and 2-chloro-5,5-dimethyl-1,3-cyclohexanedione (3c) as CH acid nucleophiles in water/acetonitrile (85/15) solution, using cyclic voltammetry and controlled-potential coulometry. The results indicate that p-benzoquinone, generated by electrochemically driven oxidation of the 2,3-dimethylhydroquinone (1), is scavenged by 3a-c, to give related products (5a, 9b, 8c) via various electrochemical mechanisms. The electrochemical syntheses of 5a, 9b, and 8c have been successfully performed in one-pot in an undivided cell using an environmentally friendly method with high atomic economy. 相似文献
543.
Thiol‐functionalized fructose‐derived nanoporous carbon as a support for gold nanoparticles and its application for aerobic oxidation of alcohols in water 下载免费PDF全文
Gold nanoparticles supported on thiol‐functionalized fructose‐derived nanoporous carbon (AuNPs@thiol‐Fru‐d‐NPS) were found to be a simple bench‐top, biocompatible, recyclable and selective catalytic system for the aerobic oxidation of various types of alcohols into their corresponding aldehydes and ketones at room temperature under the environmentally friendly conditions with excellent yields. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
544.
Electrochemical Behavior of Glucose Oxidase Immobilized on Pd‐Nanoparticles Decorated Ionic Liquid Derived Fibrillated Mesoporous Carbon 下载免费PDF全文
Behzad Haghighi Babak Karimi Mojtaba Tavahodi Hesam Behzadneia 《Electroanalysis》2014,26(9):2010-2016
Pd nanoparticles with an average diameter of 5 nm were decorated on the surface of ionic liquid derived fibrillated mesoporous carbon (IFMC) to prepare a novel nano‐hybrid material (Pd@IFMC). Thereafter, glucose oxidase was immobilized on Pd@IFMC modified glassy carbon electrode to fabricate an enzymatic glucose biosensor. A pair of well‐defined redox peaks was recorded for direct electron transfer of the immobilized glucose oxidase at the formal potential of ? 0.418 V with a peak to peak separation of 25 mV. Electron transfer rate constant of was calculated to be 14.6 s?1. The response of fabricated biosensor was linear towards glucose concentration. 相似文献
545.
A novel flow injection analysis (FIA) system based on liquid-liquid microextraction and fluorimetric determination was developed for the determination of traces of the Zn2+ ion using 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L) as a sensitive and selective fluorimetric sensor, with λex = 373 nm and λem = 530 nm, and hexanol as the extracting organic solvent. In the designed FIA system, the phase separation takes place via gravitation forces in the absence of any segmenter. The influence of pH and ionic strength of the solution, amount of ligand, nature of counter ion, volume of organic solvent, extraction time and coil length was investigated. Under optimized experimental conditions, the calibration curve found to be liner over a concentration range of 0.025-4.53 μg mL−1 (R2 = 0.9951) with a limit of detection of 2.3 ng mL−1. The enrichment factor was 45 and relative standard deviation for 7 replicate determinations was 2.43%. The method is very fast and uses low levels of organic solvents. The proposed method was applied successfully to the determination of zinc(II) in human hair, human serum and two inorganic sludge samples. 相似文献
546.
Bakhshali Massoumi Mojtaba Abbasian Rahim Mohammad‐Rezaei Amir Farnudiyan‐Habibi Mehdi Jaymand 《先进技术聚合物》2019,30(6):1484-1492
A polystyrene‐modified epoxidized novolac resin/montmorillonite nanocomposite was fabricated and characterized successfully. For this purpose, novolac resin (NR) was epoxidized through the reaction of phenolic hydroxyl group with epichlorohydrin in super basic medium to produce epoxidized novolac resin (ENR). Afterward, a polystyrene was synthesized by atom transfer radical polymerization (ATRP) technique, and then brominated at the benzylic positions using N‐bromosuccinimide (NBS). The brominated polystyrene (PSt‐Br) was reacted with ethanolamine in basic medium in order to afford an amine‐functionalized polystyrene (PSt‐NH2). An organo‐modified montmorillonite (O‐MMT) was synthesized through the treatment of MMT with hexadecyl trimethyl ammonium chloride salt. Finally, ENR‐PSt/MMT nanocomposite was fabricated through curing a mixture of ENR (70 wt.%) and O‐MMT (5 wt.%) with PSt‐NH2 (25 wt.%). Transition electron microscopy (TEM) and powder X‐ray diffraction (XRD) analysis revealed that the fabricated nanocomposite has an exfoliated structure. Thermal property studies using thermogravimetric analysis (TGA) showed that the curing of ENR by PSt‐NH2, as well as incorporation of a small amount of MMT have synergistic effect on the thermal stability of the ENR resin. 相似文献
547.
Mohyeddin Safarzaei Malek Taher Maghsoodlou Ebrahim Mollashahi Nourallah Hazeri Mojtaba Lashkari 《中国化学会会志》2019,66(5):543-547
A novel one‐pot three‐component condensation reaction of an aldehyde, 2‐aminopyrimidine and 2‐naphthol to afford the corresponding 2‐aminopyrimidinomethylnaphtols in good yields. The remarkable features of this new procedure are high conversions, short reaction time, clean reaction profiles, and environmentally benign and simple work‐up procedures. 相似文献
548.
Mojtaba Alipour 《International journal of quantum chemistry》2019,119(11):e25898
During this contribution, we present a benchmark investigation on the applicability of several Minnesota functionals from various classes like local meta-generalized and meta-nonseparable gradient approximations, hybrids, and range-separated hybrids for describing the valence and Rydberg excitation energies of some organic compounds from different categories. Furthermore, the performances of Minnesota density functionals from density functional theory are also assessed against a wave function theory based approach in the context of excite states calculations, symmetry adapted cluster-configuration interaction (SAC-CI) method. Pragmatically, the singles and doubles linked excitation operators are considered in the SAC-CI wave functions. With more or less different accountabilities of the considered methods, it is shown that the M06-2X, M05-2X, and M11 functionals have the best performances for valence excited states. On the other hand, for Rydberg excited states although the SAC-CI method outperforms others, the statistical analyses reveal that the efficiency of some Minnesota functionals is also respectable. 相似文献
549.
Melamine trisulfunic acid is employed as a recyclable catalyst for the condensation reaction of aromatic aldehydes with 3‐methyl‐l‐phenyl‐2‐pyrazolin‐5‐one. This condensation reaction was performed in ethanol under refluxing conditions giving 4,4′‐(arylmethylene)‐bis‐(3‐methyl‐1‐phenyl‐1H‐pyrazol‐5‐ols) in 80‐96% yields. 相似文献
550.
A new and convenient method for the synthesis of biscoumarins based on the condensation of 4‐hydroxycoumarin and aromatic aldehydes in the presence of Mohr's salt hexahydrate (ferrous ammonium sulfate hexahydrate) as a novel catalyst on water is developed. This method has advantages such as the use of inexpensive and available reagent, avoidance of organic solvents, short reaction times, and high product yields. 相似文献