全文获取类型
收费全文 | 3347篇 |
免费 | 154篇 |
国内免费 | 83篇 |
专业分类
化学 | 2494篇 |
晶体学 | 15篇 |
力学 | 177篇 |
数学 | 393篇 |
物理学 | 505篇 |
出版年
2024年 | 1篇 |
2023年 | 18篇 |
2022年 | 66篇 |
2021年 | 89篇 |
2020年 | 116篇 |
2019年 | 146篇 |
2018年 | 202篇 |
2017年 | 150篇 |
2016年 | 270篇 |
2015年 | 167篇 |
2014年 | 237篇 |
2013年 | 449篇 |
2012年 | 284篇 |
2011年 | 254篇 |
2010年 | 203篇 |
2009年 | 169篇 |
2008年 | 183篇 |
2007年 | 137篇 |
2006年 | 74篇 |
2005年 | 70篇 |
2004年 | 74篇 |
2003年 | 52篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 17篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有3584条查询结果,搜索用时 0 毫秒
91.
Mohammad Ali Zolfigol Maryam Kiany-borazjani Majid M. Sadeghi Hamid Reza Memarian Iraj Mohammadpoor-baltork 《合成通讯》2013,43(16):2945-2950
A combination of NaHSO4.H2O and NaNO2 in the presence of wet SiO2 was used as an effective oxidizing agent for the oxidation of dihydropyridines to their corresponding pyridine derivatives at room temperature with excellent yields. 相似文献
92.
Shahnaz Rostamizadeh Ali Mohammad Amani Reza Aryan Hamid Reza Ghaieni Nasrin Shadjou 《合成通讯》2013,43(20):3567-3576
A simple, inexpensive, and efficient one-pot synthesis of 2,3-dihydroquinazoline-4(1H)-one derivatives under solvent-free conditions using a catalytic amount of iodine with excellent product yields is reported. This methodology provides easy, quantitative access to various 2,3-dihydroquinazoline-4(1H)-one derivatives, using commercially available iodine as a catalyst. 相似文献
93.
A series of three-component reactions has been carried out using HClO4-SiO2 as a versatile heterogeneous catalyst. A series of new and novel N-protected 1-aminoalkyl-2-naphthol derivatives have been prepared under thermal solvent-free reaction conditions. In all cases, the reaction conditions were very simple and high-yielding. 相似文献
94.
Marzieh Saeedi Maryam Anafcheh Reza Ghafouri Nasser L. Hadipour 《Structural chemistry》2013,24(2):681-689
Density functionla theory (DFT) calculations are performed to characterize geometric and electronic features of the octahedral Al n N n and Al n P n cages (n = 12, 16, 28, 32, and 48). Toward this aim, 15N, 27Al, and 31P chemical shielding (CS) tensors as well as natural charge analyses are calculated for the optimized structures. CS parameters detect three distinct electronic environments for atoms within the Al n N n and Al n P n cages. The chemical shifts of N2 sites belonging to a hexagon and surrounded by three hexagons and a square obtained are different from those of N3 sites belonging to a hexagon that is surrounded only by hexagons—due to different curvatures exerted at the sites with different local structures. In addition, there is an increasing tendency in the Δσ values of the three local structures, Δσ (N1) > Δσ (N2) > Δσ (N3), N1 sites belonging to four-membered rings. The chemical shieldings of those Al and P sites belonging to a hexagon that is surrounded only by hexagons in the cages (360.7–366.7 and 496.5–514.7 ppm) are close to those previously reported for AlP nanotubes. Three distinct electrostatic environments around the N, Al, and P nuclei are also confirmed by the calculated natural charges. It should be noted that the positively charged Al atoms on the cages turn out to be the available sites for adsorption of H2 molecules. 相似文献
95.
Mahmoud Reza Sohrabi Zahra Matbouie Ali Akbar Asgharinezhad Ali Dehghani 《Mikrochimica acta》2013,180(7-8):589-597
We describe a novel magnetic metal-organic framework (MOF) for the preconcentration of Cd(II) and Pb(II) ions. The MOF was prepared from the Fe3O4-pyridine conjugate and the copper(II) complex of trimesic acid. The MOF was characterized by IR spectroscopy, elemental analysis, SEM and XRD. A Box-Behnken design through response surface methodology and experimental design was used to identify the optimal parameters for preconcentration. Extraction time, amount of magnetic MOF and pH value were found to be critical factors for uptake, while type, volume, concentration of eluent, and elution time are critical in the elution step. The ions were then determined by FAAS. The limits of detection are 0.2 and 1.1 μg?L?1 for Cd(II), and Pb(II) ions, respectively, relative standard deviations are <4.5% (for five replicates at 50 μg?L?1 of Cd(II) and Pb(II) ions), and the enrichment capacity of the MOF is at around 190 mg?g?1 for both ions which is higher than the conventional Fe3O4-pyridine material. The magnetic MOF was successfully applied to the rapid extraction of trace quantities of Cd(II) and Pb(II) ions in fish, sediment, and water samples. Figure
Schematic illustration of synthesized magnetic MOF-pyridine nanocomposite 相似文献
96.
Saied Saeed Hosseiny Davarani Hamid Reza Moazami Ali Reza Keshtkar Mohammad Hossein Banitaba Saeed Nojavan 《Analytica chimica acta》2013
A novel method for the selective electromembrane extraction (EME) of U6+ prior to fluorometric determination has been proposed. The effect of extraction conditions including supported liquid membrane (SLM) composition, extraction time and extraction voltage were investigated. An SLM composition of 1% di-2-ethyl hexyl phosphonic acid in nitrophenyl octyl ether (NPOE) showed good selectivity, recovery and enrichment factor. The best performance was achieved at an extraction potential of 80 volts and an extraction time of 14 minutes Under the optimized conditions, a linear range from 1 to 1000 ng mL−1 and LOD of 0.1 ng mL−1 were obtained for the determination of U6+. The EME method showed good performance in sample cleanup and the reduction of the interfering effects of Mn2+, Zn2+, Cd2+, Ni2+, Fe3+, Co2+, Cu2+, Cl− and PO43− ions during fluorometric determination of uranium in real water samples. The recoveries above 54% and enrichment factors above 64.7 were obtained by the proposed method for real sample analysis. 相似文献
97.
Hindered internal rotation about the C‐N single bonds joining the thiuram disulfide was studied by 1H NMR complete line‐shaped analysis in different dimethyl sulfoxide‐chloroform (DMSO‐CDCl3) mixtures. From the temperature dependence of methyls proton spectra, activation parameters (Ea, ΔH≠, ΔS≠, and ΔG≠) were obtained. The Arrhenius plots showed a distinct isokinetic temperature at about 35 °C at which the exchange rate is more or less independent of the solvent composition. The resulting ΔH≠ against TΔS≠ plot showed a firmly good linear correlation, indicating the existence of an enthalpy‐entropy composition in an exchange process. 相似文献
98.
A kinetic spectrophotometric method for determination of trace nitrite in two dynamic ranges (2–100 and 100–500 ng/mL) based on its catalytic effect on the reaction between methylthymol blue and potassium bromate in acidic (sulfuric acid) media is described. The reaction was monitored spectrophotometrically by measuring the decreasing color of methylthymol blue at 437 nm by the fixed‐time method of 4.0 min at 30°C. The detection limit is 0.6 ng/mL, and the relative standard deviations for 50.0 and 250.0 ng/mL nitrite are 1.6% and 1.3%, respectively. The method was used for the determination of nitrite in water samples. 相似文献
99.
This paper describes a new method for prediction of the Chapman-Jouguet detonation pressures of CHNOFCl explosives using the heat of detonation, Qdet, the number of moles of gaseous products of detonation per gram of explosive, α, and the average molecular weight of gaseous products, M. The equation has the form: PCJ=15.88α(MQdet)1/2ρ02−11.17, where PCJ is the Chapman-Jouguet detonation pressure and ρ0 the loading density. Calculated PCJ by this procedure show good result with respect to measured detonation pressure for any pure or mixture of ideal and some of less ideal CHNOFCl explosives at ρ0>0.8 g/cm3. 相似文献
100.