全文获取类型
收费全文 | 782篇 |
免费 | 22篇 |
国内免费 | 6篇 |
专业分类
化学 | 492篇 |
晶体学 | 7篇 |
力学 | 38篇 |
数学 | 163篇 |
物理学 | 110篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 9篇 |
2021年 | 33篇 |
2020年 | 28篇 |
2019年 | 25篇 |
2018年 | 22篇 |
2017年 | 14篇 |
2016年 | 46篇 |
2015年 | 22篇 |
2014年 | 37篇 |
2013年 | 65篇 |
2012年 | 48篇 |
2011年 | 48篇 |
2010年 | 29篇 |
2009年 | 31篇 |
2008年 | 32篇 |
2007年 | 26篇 |
2006年 | 39篇 |
2005年 | 35篇 |
2004年 | 20篇 |
2003年 | 26篇 |
2002年 | 8篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 10篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1971年 | 2篇 |
1965年 | 1篇 |
排序方式: 共有810条查询结果,搜索用时 31 毫秒
41.
The present work focused on the utilization of Egyptian red clay (ERC) without any treatment as a highly efficient, selective, and eco-friendly catalyst for synthesis of bio-ethyl acetate in the gas phase. The best conditions used in conventional fixed bed reactor were 130 cm3/min total flow rate of air with 1.6 and 2.1 vol% bio-ethanol and bio-acetic acid, respectively, 1 g clay catalyst calcined at 300 and 225°C reaction temperature. Under these conditions, the conversions to ester achieved were about 80 and 98% after 5 and 30 min, respectively, from the initial admission of the reactants into the reactor. The amount and type of surface acidity were identified by isopropanol reaction and chemisorption of basic probes. The results demonstrated that the majority of intermediate strength of BrØnsted acid sites are the key role for the dehydration reaction toward ester formation. The virgin and annealed clay solids were characterized using X-ray fluorescence (XRF), thermogravimetry (TG) and differential thermal analysis (DTA), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and N2 sorption. 相似文献
42.
Mohamed El MESSOUDI Aicha BOUKHRISS Laila SADALLAH Laila SAJID M'hamed El KOUALI Said GMOUH 《高等学校化学研究》2020,36(5):877-884
In this paper, phosphate nanoparticles were coated on cotton(CO) and polyester(PES) textile surfaces by sol-gel method using tetraethylorthosilicate(TEOS) and chloropropyltriethoxysilane(CPTS) as silica precursors. The deposited nanoparticles were observed with scanning electronic microscopy energy-dispersive X-ray spectroscopy (SEM-EDX). The release kinetics of phosphorus(P) from these textiles into the aqueous medium(Aq), acid(Ac), and basic(Ba) artificial sweats were then studied. The released amount of phosphorus was evaluated by the inductively coupled plasma(ICP) according to ISO NF EN 16711-2 procedure. The results revealed that the release of P into the aqueous medium is lower than in the artificial sweat. The kinetics data[the phosphorus desorption amount(mg/g) as a function of time] were modeled according to five models:the first order, the second order, the third order, simple Elovich and parabolic diffusion. The suitable model was chosen based on the coefficient of determination(R2) and the calculation of the sum of the absolute errors(EABS), which describes the error between the theoretical and experimental values. SEM observations were also carried out on the fabrics after desorption in order to show the impact of desorption on their morphology. Furthermore, the impact of P release on the tensile strength of CO and PES fabrics was investigated using a uniaxial tensile test. The thermal stability of all samples before and after desorption was assessed by thermogravimetric analysis(ATG). 相似文献
43.
Laila SADALLAH Aicha BOUKHRIS Hassan HANNACHE Said GMOUH 《Turkish Journal of Chemistry》2020,44(1):142
Two organic fluorophores, fluorescein (F) and rhodamine B (Rd), were entrapped in calcium phosphate nanoparticles. The as-obtained nanoparticles can be used for biological release applications. For this aim, calcium phosphate nanoparticles were synthesized using the precipitation method. Structural analysis of these nanoparticles was performed using XRD, FTIR, and Raman spectroscopy, confirming that the synthesized nanoparticles were hydroxyapatite. TEM and SEM analyses demonstrated that these nanoparticles had a size of 20 nm and a well-defined morphology. F and Rd (about 0.5 wt.%) were entrapped in these nanoparticles and their release, as a function of time, was studied via UV-Vis spectroscopy. The obtained results showed that the release of both fluorophores was progressive over time. The trapping efficiencies of the fluorophores were 67.15% and 90.76% for F and Rd, respectively. 相似文献
44.
Searching new light‐absorbing materials to replace toxic lead halide in solar cells is very important and highly desirable. In this research, we firstly demonstrated that tellurium iodide (TeI4) could be used as a light‐absorbing material in solar cells due to its suitable optical band gap and the active lone‐pair electron orbital in Te4+. The best power conversion efficiency (PCE=3.56%) was achieved with a concentration of 0.9 M TeI4 in DMF:DMSO (4 : 1, v,v) without any heat treatment or antisolvent dripping. Our study indicates the promising potential of TeI4 for photovoltaic and optoelectronic applications. 相似文献
45.
46.
The present paper deals with a nonlinear viscoelastic equation having a dissipation term. With the equation some classical
and non classical boundary conditions are combined. Based on some a priori bounds, iteration processes and density arguments,
we simultaneously solve the nonlinear and the associated linear problems. 相似文献
47.
48.
49.
Mohamed M. M. Abd El-Wahab Abd El-Aziz A. Said Shar S. Al-Shihry 《Monatshefte für Chemie / Chemical Monthly》2004,135(4):357-370
Summary. A series of AlPO4-V2O5 (APV) systems with various vanadia amounts 1–30mol% were prepared by the impregnation method and calcinated at 400 and 600°C for 4h. The catalysts were characterized by TG/DTG, DSC, IR spectroscopy, XRD, N2 adsorption, and electrical conductivity measurements. The surface acidity and basicity of the catalysts were studied by the dehydration-dehydrogenation of isopropyl alcohol and the adsorption of pyridine. The catalytic gas phase esterification of acetic acid with ethyl alcohol was carried out at 210°C in a flow system at 1atm using air as a carrier gas. The results showed that the catalysts calcinated at 400°C were active and selective towards the formation of ethyl acetate whereas the calcination of samples at 600°C led to a drastic reduction in both activity and selectivity. Good correlations were obtained between catalytic activities towards ester formation and acidity of the prepared catalysts. 相似文献
50.
A procedure is described for constructing branch groups on the binary tree, which
yields in particular finitely generated branch groups with non-cyclic free subgroups. 相似文献