全文获取类型
收费全文 | 663篇 |
免费 | 40篇 |
国内免费 | 15篇 |
专业分类
化学 | 431篇 |
力学 | 56篇 |
数学 | 92篇 |
物理学 | 139篇 |
出版年
2024年 | 3篇 |
2023年 | 1篇 |
2022年 | 14篇 |
2021年 | 35篇 |
2020年 | 30篇 |
2019年 | 31篇 |
2018年 | 35篇 |
2017年 | 36篇 |
2016年 | 44篇 |
2015年 | 25篇 |
2014年 | 49篇 |
2013年 | 84篇 |
2012年 | 69篇 |
2011年 | 58篇 |
2010年 | 45篇 |
2009年 | 29篇 |
2008年 | 18篇 |
2007年 | 26篇 |
2006年 | 11篇 |
2005年 | 10篇 |
2004年 | 5篇 |
2003年 | 6篇 |
2002年 | 4篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有718条查询结果,搜索用时 31 毫秒
31.
Ali Ramazani Ali Morsali Bijan Ganjeie Ali Reza Kazemizadeh Ebrahim Ahmadi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2439-2442
Abstract Selenourea reacts with dialkyl acetylenedicarboxylates in acetone to form 1:1 adducts, which undergo a cyclization reaction to produce alkyl Z-2-(2-amino-4-oxo-1,3-selenazol-5(4H)-yliden) acetates in fairly good yields. The reaction is completely stereoselective. 相似文献
32.
Ali Ramazani Ali Reza Kazemizadeh Bijan Ganjeie Ebrahim Ahmadi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2569-2572
Tiourea reacts with dialkyl acetylenedicarboxylates in solventless conditions to form 1:1 adducts, which undergo a cyclization reaction to produce alkyl Z-2-(2-amino-4-oxo-1,3-thiazol-5(4H)-yliden)acetates in fairly good yields. The stereochemistry of the ethyl Z-2-(2-amino-4-oxo-1,3-thiazol-5(4H)-yliden)acetate was established by the use of X-ray single crystal structure analysis. The reaction is completely stereoselective. 相似文献
33.
A. Mobinikhaledi N. Foroughifar B. Ahmadi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(9):1769-1775
Oxazolopyrimidine compounds 2(a–c) and 3(a–e) were synthesized by a simple one-pot condensation reaction of the pyrimidine derivative 1 with 1,2-dibromoethane and 2-bromopropanoic acid, respectively. In a similar way the oxazepinopyrimidine compounds 4(a–b) were synthesized by reaction of 1 and 1,4-dichlorobutane in dioxane under reflux condition. The yields of products following recrystallization were of the order of 55–85%. 相似文献
34.
Chemical functionalization of C60 fullerene with one to six carbene (CH2) molecule(s) has been investigated using density functional theory. We have found that the reaction is regioselective so that a CH2 molecule prefers to be adsorbed atop a C–C bond which is shared between two hexagonal rings of the C60, releasing energy of ?3.95 eV. Singly occupied molecular orbital (SOMO) of the CH2 interacts with LUMO of the C60 via a [2 + 1] cycloaddition reaction. Energy of the reaction and work function of the system are decreased by increasing the number of adsorbed CH2 molecules. The HOMO/LUMO energy gap of C60 is slightly changed and the electron emission from its surface is facilitated upon the functionalization. 相似文献
35.
36.
Reza Takjoo Alireza Akbari Mehdi Ahmadi Hadi Amiri Rudbari Giuseppe Bruno 《Central European Journal of Chemistry》2013,11(11):1844-1851
A new Schiff base ligand of 5-bromo-2-hydroxybenzaldehyde S-ethyl-isothiosemicarbazone (H2L) was synthesized and its mixed-ligand Cu(II) complex was also prepared by reaction of Cu(NO3)2·3H2O with H2L and imidazole. Their structures were fully characterized by elemental analysis, FT-IR, molar conductivity and UV-Vis methods. The analytical data suggest that the metal, H2L and imidazole ratios in the Schiff base complex are 1:1:1. Single crystal diffraction was also used to better understand the molecular structure of the Cu(II) complex. The results of physico-chemical analyses of the Schiff base complex reveal the coordination geometry around the central atom is square planar. The H2L ligand (NNO donor) is coordinated to the metal center as a tridentate bionegatively agent. Another position of the square planar geometry is occupied by the imidazole ligand. Furthermore, computational studies of the new complex were performed by carrying out DFT calculations. Geometry optimization and natural band analysis of the complex is discussed in further detail. 相似文献
37.
Mohammad Navid Soltani Rad Somayeh Behrouz Abdollah Movahedian Mohammad Mahdi Doroodmand Younes Ghasemi Sara Rasoul‐Amini Abdol‐Rasoul Ahmadi Gandomani Ramin Rezaie 《Helvetica chimica acta》2013,96(4):688-701
A facile and simple protocol for the 1,3‐dipolar cycloaddition of organic azides with terminal alkynes catalyzed by doped nano‐sized Cu2O on melamine? formaldehyde resin (nano‐Cu2O? MFR) as a new and convenient heterogeneous catalyst is described. In this method, ‘click’ cycloaddition of various structurally diverse β‐azido alcohols and alkynes in the presence of nano‐Cu2O? MFR in H2O/THF 1 : 2 furnished the corresponding 1,4‐disubstituted 1H‐1,2,3‐triazole adducts 1a – 1o in good to excellent yields at room temperature (Scheme and Table 3). The nano‐Cu2O? MFR was characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), inductively coupled plasma (ICP) analysis, and FT‐IR. The nano‐Cu2O? MFR could be easily recovered and recycled from the reaction mixture and reused for many consecutive trials without significant decrease in activity (Table 4). The in vitro antibacterial activities of all synthesized compounds were tested on several Gram‐positive and/or Gram‐negative bacteria (Table 5). The results demonstrate the promising antibacterial activity for some of the synthesized compounds. 相似文献
38.
Carbonyl derivatives of phenylhydrazine, 2-nitrophenylhydrazine, 2, 4-dinitrophenazine, semicarbozone, and hydroxylamine are converted to their corresponding carbonyl compounds with tetrakis(pyridine)silver dichromate in benzene. Selectivity is also observed for benzylic C = N cleavage. 相似文献
39.
Saeed Balalaie Morteza Bararjanian Masoumeh Sheikh‐Ahmadi Shohreh Hekmat Peyman Salehi 《合成通讯》2013,43(7):1097-1108
Diammonium hydrogen phosphate was used as a mild, efficient, neutral, and cheap catalyst for the synthesis of various 4H‐benzo[b]pyran derivatives via a one‐pot, three‐component condensation of aromatic aldehydes, active methylene compounds, and dimedone in aqueous media. 相似文献
40.
S. Zeinali Heris F. Ahmadi Omid Mahian 《Journal of Dispersion Science and Technology》2013,34(10):1368-1375
An experimental study is performed to determine the pressure drop and performance characteristics of Al2O3/water and CuO/water nanofluids in a triangular duct under constant heat flux where the flow is laminar. The effects of adding nanoparticles to the base fluid on the pressure drop and friction factor are investigated at different Reynolds numbers. The results show that at a specified Reynolds number, using the nanofluids can lead to an increase in the pressure drop by 35%. It is also found that with increases in the Reynolds number, the rate of increase in the friction factor with the volume fraction of nanoparticles is reduced. Finally, the performance characteristics of the two nanofluids are investigated using the data of pressure drop and convective heat transfer coefficient. The results show that the use of Al2O3/water nanofluid with volume fractions of 1.5% and 2% is not helpful in the triangular duct. It is also concluded that at the same volume fraction of nanoparticles, using Al2O3 nanoparticles is more beneficial than CuO nanoparticles based on the performance index. 相似文献