全文获取类型
收费全文 | 581篇 |
免费 | 11篇 |
国内免费 | 68篇 |
专业分类
化学 | 563篇 |
晶体学 | 4篇 |
力学 | 4篇 |
物理学 | 89篇 |
出版年
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 6篇 |
2020年 | 14篇 |
2019年 | 11篇 |
2018年 | 23篇 |
2017年 | 16篇 |
2016年 | 14篇 |
2015年 | 10篇 |
2014年 | 30篇 |
2013年 | 78篇 |
2012年 | 30篇 |
2011年 | 28篇 |
2010年 | 23篇 |
2009年 | 24篇 |
2008年 | 44篇 |
2007年 | 49篇 |
2006年 | 37篇 |
2005年 | 27篇 |
2004年 | 31篇 |
2003年 | 24篇 |
2002年 | 16篇 |
2001年 | 11篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1979年 | 1篇 |
1976年 | 2篇 |
排序方式: 共有660条查询结果,搜索用时 734 毫秒
641.
α,β‐Unsaturated aldehydes reacted with diimide (diazene) in the presence of optically active ammonium salt 1 as a catalyst to give the corresponding saturated aldehydes in excellent yields and up to 98 : 2 er. Attractive features of the asymmetric transfer hydrogenation are its high yields, and chemo‐, and enantioselectivities. 相似文献
642.
Synergistic effect between expandable graphite and ammonium polyphosphate on flame retarded polylactide 总被引:3,自引:0,他引:3
Hongfang ZhuQiliang Zhu Juan Li Kang TaoLixin Xue Qing Yan 《Polymer Degradation and Stability》2011,96(2):183-189
Synergistic effect was observed between expandable graphite (EG) and ammonium polyphosphate (APP) on flame retarded polylactide (PLA) in this paper using limiting oxygen index (LOI), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and X-ray spectroscopy (XPS) and cone calorimeter tests etc. In the experiments, PLA composites with 15 wt% of APP/EG(1:3) combinations showed a LOI value of 36.5 and V-0 rating in UL-94 tests, greatly improved flame retardant properties from composites with APP or EG alone. Results from TGA and cone calorimeter demonstrated that APP/EG combination could retard the degradation of polymeric materials above the temperature of 520 °C by promoting the formation of a compact char layer. This char layer protects the matrix effectively from heat penetrating inside and prevents its further degradation, resulting in lower weight loss rate and better flame retarded performance. 相似文献
643.
A fast, economic and sensitive chemiluminescence (CL) method has been developed for the analysis of cetrizine hydrochloride (CET) in pharmaceutical formulations and in biological fluids. The CL method is based on the oxidation of tris(2,2′-bipyridyl)ruthenium(II) (Ru (bipy)32+) by peroxydisulphate in a two-chip device. Up to 180 samples can be analysed per hour, consuming only minute quantities of reagents. Three instrumental setups were tested to find the most economical, sensitive and high throughput setup. In the first setup, a continuous flow of sample and CL reagents was used, whereas in the second setup, a fixed volume (2 μL) of (Ru (bipy)32+) was introduced into a continuous infusion of peroxydisulphate and the sample. In the third design, a fixed volume of sample (2 μL) was injected while the CL reagents were continuously infused. Compared to the first setup, a 200% signal enhancement was observed in the third setup. Various parameters that influence the CL signal intensity, including pH, flow rates and reagent concentrations, were optimized. A linear response was observed over the range of 50 μg L−1 to 6400 μg L−1 (R2 = 0.9959) with RSD values of 1.1% (n = 15) for 1000 μg L−1. The detection limit was found to be 15 μg L−1 (S/N = 3). The amount of consumed sample was only 2 μL, from which the detected amount of CET was found to be 6.5 × 10−14 mol. This procedure was successfully applied to the analysis of CET in pharmaceutical formulations and biological fluids. 相似文献
644.
A new design of a membraneless gas-diffusion (MGD) unit coupled to a multisyringe flow injection system is proposed. The spectrophotometric determination of ammonium using an acid-base indicator was chosen to show the feasibility of this approach. Hence, in alkaline medium, ammonium ions are transformed into ammonia (donor channel) which diffuses through the headspace into the acceptor stream (bromothymol blue solution), causing a pH change and subsequently a colour change. The exploitation of the enhanced potentialities of this re-designed MGD device was the main purpose of the present work. Hence, several strategies concerning flow management were studied seeking to characterize and improve the analytical features of the methodology and moreover, untreated environmental samples were analysed without previous filtration. Consequently, stopped flow in acceptor channel with continuous flow in donor channel was chosen for the application to wastewater and spiked river water samples. A linear concentration range between 10.0 and 50.0 mg L−1 of NH4+, a limit of detection of 2.20 mg L−1 and a determination frequency of 11 h−1 were obtained. 相似文献
645.
A strong post chemiluminescence (PCL) phenomenon was observed when ammonium was injected into the reaction mixture after the finish of CL reaction of N-bromosuccinimide (NBS) and dichlorofluorescein. Based on this, a sensitive flow injection PCL method was established for the determination of ammonium. The possible CL mechanism of the reaction was proposed based on a series of experiments. The PCL intensity responded linearly to the concentration of ammonium in the range 3.0 × 10−11-1.0 × 10−7 g mL−1 with a detection limit of 1 × 10−11 g mL−1. The relative standard deviation (R.S.D.) was 1.4% for 1.0 × 10−9 g mL−1 ammonium (n = 11). This method had been applied to the determination of ammonium in samples of mineral water, tap water and river water. 相似文献
646.
Coordination properties of axially unfixed chiral dipyridine ligands towards metal and ammonium ions
Kiu-Chor ShamChi-Sing Lee Ka-Yee ChanShek-Man Yiu Wing-Tak WongHoi-Lun Kwong 《Polyhedron》2011,30(6):1149-1156
New chiral dipyridine ligands with an axially unfixed 1,1′-biphenyl bridge were prepared via homocoupling of bromophenyl pyridines. The conformeric ratios of the free ligands in solution and their coordination properties towards metal ions were studied by NMR spectroscopy. X-Ray crystallography of the silver(I) and copper(I) complexes showed 1:1 metal to ligand complexes and S planar chirality. Interestingly, the biphenyl ligands show a 1:2 stepwise binding towards most ammonium ions tested with strong fluorescence enhancement, but a selectively 1:1 binding towards l-ornithine methyl ester hydrochloride with no fluorescence enhancement. 相似文献
647.
Carlos M.C. Infante 《Microchemical Journal》2011,98(1):97-102
This paper describes the optimization and use of a Sequential Injection Analysis (SIA) procedure for ammonium determination in waters. Response Surface Methodology (RSM) was used as a tool for optimization of a procedure based on the modified Berthelot reaction. The SIA system was designed to (i) prepare the reaction media by injecting an air-segmented zone containing the reagents in a mixing chamber, (ii) to aspirate the mixture back to the holding coil after homogenization, (iii) drive it to a thermostated reaction coil, where the flow is stopped for a previously established time, and (iv) to pump the mixture toward the detector flow cell for the spectrophotometric measurements. Using a 100 μmol L− 1 ammonium solution, the following factors were considered for optimization: reaction temperature (25 - 45 °C), reaction time (30 - 90 s), hypochlorite concentration (20 - 40 mmol L− 1), nitroprusside concentration (10 - 40 mmol L− 1) and salicylate concentration (0.1 - 0.3 mol L− 1). The proposed system fed the statistical program with absorbance data for fast construction of response surface plots. After optimization of the method, figures of merit were evaluated, as well as the ammonium concentration in some water samples. No evidence of statistical difference was observed in the results obtained by the proposed method in comparison to those obtained by a reference method based on the phenol reaction. 相似文献
648.
Sphere- and pod-like α-Fe2O3 particles have been selectively synthesized using NH3·H2O and NaOH solution to adjust the pH value of the designed synthetic system, respectively. The sphere-like α-Fe2O3 particles with diameter about 25 nm on average were encapsulated into carbon shells to fabricate a novel core-shell composite (α-Fe2O3@C) through the coating experiments. The catalytic performance of the products on the thermal decomposition of ammonium perchlorate (AP) was investigated by thermal gravimetric analyzer (TG) and differential thermal analysis (DTA). The thermal decomposition temperatures of AP in the presence of pod-like α-Fe2O3, sphere-like α-Fe2O3 and α-Fe2O3@C are reduced by 72, 81 and 109 °C, respectively, which show that α-Fe2O3@C core-shell composites have higher catalytic activity than that of α-Fe2O3. 相似文献
649.
Syed Shahed Ali 《中国化学快报》2011,22(7):793-796
Various 2-phenylimidazo[4,5-f][1,10]phenanthrolines were synthesized from potassium ferro-cyanide trihydrate(KFCT) complex catalyzed three component reaction of 1,10-phenanthroiine-5,6-dione,aromatic aldehydes and ammonium acetate at room temperature in excellent isolated yield.This is a simple and straight forward,high yielding,not involving any hazardous or expensive catalyst method. 相似文献
650.