全文获取类型
收费全文 | 1694篇 |
免费 | 87篇 |
国内免费 | 26篇 |
专业分类
化学 | 1075篇 |
晶体学 | 4篇 |
力学 | 89篇 |
数学 | 261篇 |
物理学 | 378篇 |
出版年
2024年 | 4篇 |
2023年 | 13篇 |
2022年 | 56篇 |
2021年 | 44篇 |
2020年 | 84篇 |
2019年 | 53篇 |
2018年 | 56篇 |
2017年 | 49篇 |
2016年 | 105篇 |
2015年 | 82篇 |
2014年 | 101篇 |
2013年 | 138篇 |
2012年 | 142篇 |
2011年 | 158篇 |
2010年 | 108篇 |
2009年 | 73篇 |
2008年 | 69篇 |
2007年 | 79篇 |
2006年 | 77篇 |
2005年 | 49篇 |
2004年 | 52篇 |
2003年 | 40篇 |
2002年 | 39篇 |
2001年 | 20篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1925年 | 1篇 |
排序方式: 共有1807条查询结果,搜索用时 15 毫秒
141.
142.
143.
Mansoureh Mohammadi Marzieh Kamankesh Zahra Hadian Amir Mohammad Mortazavian Abdorreza Mohammadi 《Chromatographia》2017,80(1):119-126
Microwave-assisted extraction and dispersive liquid–liquid microextraction followed by gas chromatography–mass spectrometry as a sensitive and efficient method was applied to extract and determine four biogenic amines (BAs) in Iranian Lighvan cheese samples. Carrez solutions were used for the sedimentation of proteins. Effective factors on the performance of microextraction were studied and optimized. The proposed method showed good linear ranges from 5 to 500 ng mL?1, with the coefficients of determination higher than 0.9929. Average recoveries were between 97 and 103%. Limits of detection for all analyzed BAs ranged from 5.9 to 14.0 ng g?1, and limits of quantitation ranged between 19.7 and 46.2 ng g?1. Compared with previous methods, the proposed method is simple, fast, accurate, and precise and gives low detection limits for investigating trace amounts of BAs in Iranian Lighvan cheese samples. The levels of four BAs were determined in five Lighvan cheese samples. Cadaverine was found as prevailing amine in the cheese samples. Putrescine, tyramine, and histamine were present at the second, third, and fourth highest levels, respectively. 相似文献
144.
S. Amir Aslanzadeh 《Journal of Structural Chemistry》2017,58(3):479-488
The effects of doping heteroatoms on the structure, electronic and adsorption properties of graphene are investigated using density functional theory calculations. Six different doped graphenes (with Al, B, Si, N, P, and S) are considered, and to obtain the interaction and adsorption properties, three sulfur-containing molecules (H2S, SO2, and thiophene) were interacted with selected graphenes. The adsorption energies (E ad) in the gas phase and solvents show the exothermic interaction for all complexes. The maximum E ad values are observed for aluminum doped graphene (AG) and silicon doped graphene (SiG), and adsorption energies in the solvent are not so different from those in the gas phase. NBO calculations show that the AG and SiG complexes have the highest E (2) interaction energies and simple graphene (G) and nitrogen doped graphene (NG) have the least E (2) energies. Population analyses show that doping heteroatoms change the energy gap. This gap changes more during the interaction and these changes make these structures useful in sensor devices. All calculated data confirm better adsorption of SO2 by graphenes versus H2S and thiophene. Among all graphenes, AG and then SiG are the best adsorbents for these structures. 相似文献
145.
Yakup Budak Meliha Burcu Gürdere Kezban Özcan Parham Taslimi İlhami Gülçin 《合成通讯》2017,47(24):2313-2323
Novel 4,5-dihydropyrazole derivatives (3a–i), 3-(4-((3aR,4S,7R,7aS)-1,3-dioxo-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindol-2(3H)-yl)phenyl)-5-phenyl-4,5-dihydro-1H-pyrazole-1-carbothio amide, were obtained by the addition of thiosemicarbazide (2) to the chalcones (1a–i). The addition–cyclization of 2,4′-dibromoacetophenone (4) to pyrazole derivatives (3a–i) gave the new pyrazolyl-thiazole derivatives (5a–i), (3aR,4S,7R,7aS)-2-(4-(1-(4-(4-bromophenyl)thiazol-2-yl)-5-phenyl-4,5-dihydro-1H-pyrazol-3-yl)phenyl)-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindole-1,3(2H)-dione. Antibacterial and acetylcholinesterase (AChE) enzyme and human carbonic anhydrase (hCA) I, and II isoform inhibitory activities of the compounds 3a–i and 5a–i were investigated. Some of the compounds showed promising antibacterial activity. In addition, the hCA II and I were effectively inhibited by the lately synthesized derivatives, with Ki values in the range of 18.90?±?2.37 ?58.25?±?13.62?nM for hCA II and 5.72?±?0.98 ?37.67?±?5.54?nM for hCA I. Also, the Ki parameters of these compounds for AChE were obtained in the range of 25.47?±?11.11???255.74?±?82.20?nM. Also, acetazolamide, clinical molecule, was used as a CA standard inhibitor that showed Ki value of 70.55?±?12.30?nM against hCA II, and 67.17?±?9.1?nM against hCA I, and tacrine inhibited AChE showed Ki value of 263.67?±?91.95. 相似文献
146.
The capability of six popular local composition models including electrolyte NRTL, modified electrolyte NRTL, electrolyte NRTL–NRF, electrolyte Wilson, modified electrolyte Wilson, and electrolyte NRF–Wilson models to predict the activity coefficients of ionic liquids in aqueous solutions was examined by correlating the experimental data of 16 ionic liquids available in the literature. The adjustable parameters and standard deviations of the fit were estimated for all of these models. Results indicate that the modified electrolyte Wilson model represents the activity coefficients with higher precision. 相似文献
147.
Somayeh Hosseini Amir Charkhi Abdolhamid Minuchehr Seyyed Javad Ahmadi 《Chemical Papers》2017,71(6):1143-1153
Pervaporation (PV) separation of water–acetonitrile mixture using sodium alginate (NaAlg) based mixed matrix membranes (MMM) comprising different amounts of nano NaA zeolite (10, 20 and 30 wt%) is investigated in various concentrations of water and temperatures. The prepared membranes are modified by sulfosuccinic acid (SSA) as a crosslinking agent. NaAlg-NaA/SSA membranes are synthesized by a solution casting technique. The process and membrane performance including separation factor, flux and activation energy of permeation are determined. Results reveal that adding of nano zeolite may lead to an increase in the flux and the separation factor of sodium alginate membrane up to 123 and 169%. In addition, using MMM in dehydration of a feed containing 30 wt% of water shows much better performance than alginate membrane. Furthermore, the activation energy of water permeation through MMM is predicted lower than sodium alginate membrane which reflects the facilitated permeation of water through MMM. 相似文献
148.
Takjoo Reza Takjoo Rozita Yazdanbakhsh Mohammad Aghaei kaju Amir Chen Yaguang 《中国化学》2010,28(2):221-228
A new mixed ligand palladium(II) complex with bidentate NS‐donor chelate, [PdCl(PPh3)L] (L: S‐allyl β‐N‐(benzylidene)dithiocarbazate), has been prepared and characterized using single crystal X‐ray diffraction and spectroscopic (electronic, IR, 1H NMR and 13C NMR) techniques. The shorter Pd? P bond distance, 2.255(7) Å, than the sum of the single bond radii for palladium and phosphorus (2.41 Å), showed partial double bond character. Visualizing and exploring the crystal structure using Hirshfeld surface analysis showed the presence of π··· π, N··· π, C? H··· π, Cl···H and weak C? H···S interactions as most important intermolecular interactions in the crystal lattice, which are responsible to extension of the supramolecular network of the compound and stabilization of the crystal structure. 相似文献
149.
Mohsen Nikpour Hossein Eshtiagh-Hosseini Masoud Mirzaei Amir Aghaei Kaju Soroush Zarinabadi 《中国化学快报》2010,21(4):501-505
<正>A unique ionic hybrid material[C_6H_(12)N_5O]_3[(PO_4)W__(12)O_(36)]·5H_2O has been synthesized from the reaction of((1H-tetrazole-5- yl)methyl)morpholine andα-H_3[(PO_4)W_(12)O_(36)]·21H_2O.It has successfully been characterized by elemental analysis,IR and ~1H NMR spectroscopies,TGA and single-crystal X-ray diffraction method.The title compound is constructed from the three [C_6H_(12)N_5O]~+ cations andα-Keggin[(PO_4)W_(12)O_(36)]~(3-) polyoxoanion.The most remarkable structural feature of this hybrid can be described as two-dimensional inorganic infinite plane-like(2D/∞)[(PO_4)W_(12)O_(36)]~(3-)) which forming via van der Waals interactions along the x axis. 相似文献
150.
Saleha S. Khan Afsar Khan Amir Ahmed Viqar U. Ahmad Umar Farooq Saima Arshad Sadia Bader Aqib Zahoor Imran N. Siddiqui Bilge Sener Nurgun Erdemoglu 《Helvetica chimica acta》2010,93(10):2070-2074
From the aerial parts of Zygophyllum fabago, two new monosodium salts of sulfated derivatives of ursolic acid, along with two known quinovic acid glycosides were isolated. The structures of the new compounds were determined as (3β,4α)‐3,23,30‐trihydroxyurs‐20‐en‐28‐al 3,23‐di(sulfate) sodium salt (1 : 1) ( 1 ) and of (3β,4α)‐3,23,28‐trihydroxyurs‐20‐en‐30‐yl β‐D ‐glucopyranoside 3,23‐di(sulfate) sodium salt (1 : 1) ( 2 ) with the molecular formula C30H47NaO10S2 and C36H59NaO15S2, respectively. The structures of the known compounds were 3‐O‐(2‐O‐sulfo‐β‐D ‐quinovopyranosyl)quinovic acid 28‐β‐D ‐glucopyranosyl ester ( 3 ) and 3‐O‐(β‐D ‐glucopyranosyl)quinovic acid 28‐β‐D ‐glucopyranosyl ester ( 4 ) (quinovic acid=(3β)‐3‐hydroxyurs‐12‐ene‐27,28‐dioic acid). The structures of all these compounds were determined by using 1D‐ and 2D‐NMR spectroscopic techniques. 相似文献