全文获取类型
收费全文 | 1890篇 |
免费 | 90篇 |
国内免费 | 42篇 |
专业分类
化学 | 1446篇 |
晶体学 | 11篇 |
力学 | 85篇 |
数学 | 189篇 |
物理学 | 291篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 32篇 |
2021年 | 71篇 |
2020年 | 101篇 |
2019年 | 91篇 |
2018年 | 119篇 |
2017年 | 91篇 |
2016年 | 146篇 |
2015年 | 97篇 |
2014年 | 139篇 |
2013年 | 232篇 |
2012年 | 150篇 |
2011年 | 146篇 |
2010年 | 128篇 |
2009年 | 109篇 |
2008年 | 110篇 |
2007年 | 69篇 |
2006年 | 39篇 |
2005年 | 26篇 |
2004年 | 25篇 |
2003年 | 23篇 |
2002年 | 18篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有2022条查询结果,搜索用时 15 毫秒
251.
Mohammad Saeid Hejazi Mohammad Hossein Pournaghi‐Azar Esmaeel Alipour Elaheh Dalir Abdolahinia Sanam Arami Hosnieh Navvah 《Electroanalysis》2011,23(2):503-511
In spite of the extensive attention paid on the development of various DNA detection strategies, very few studies have been reported regarding direct detection of DNA sequence and mutation in dsDNA. Here, we describe the feasibility of detection and discrimination of target DNA sequences and single base mutations (SBM) directly in double‐stranded oligonucleotides and PCR products without the need for denaturation of the target dsDNA samples. This goal was achieved by employing a peptide nucleic acid (PNA) chain, self‐assembled on the gold electrode as a probe, which binds to dsDNA and forms PNA‐dsDNA hybrid. 相似文献
252.
Hossein Behmadi Shima Naderipour Seyed Mahdi Saadati Mohammad Barghamadi Mohammad Shaker Niloofar Tavakoli‐Hoseini 《Journal of heterocyclic chemistry》2011,48(5):1117-1121
Some new 4‐(aryl)‐2,6‐di‐2‐naphthylpyridines and 4‐(aryl)‐2,6‐di‐2‐thienylpyridines have been prepared through three‐component condensation of 2‐acetylnaphthalene or 2‐acetylthiophene, aromatic aldehydes, and ammonium acetate in presence of 1‐(4‐sulfonylbutyl) pyridinium hydrogensulfate [(CH2)4SO3HPy][HSO4], a Brønsted acidic ionic liquid as a green and reusable catalyst in solvent‐free conditions. Also some new 4,4'‐(1,4‐phenylene)‐bis‐(2,6‐di‐aryl pyridine) was prepared. J. Heterocyclic Chem., (2011). 相似文献
253.
In this study the curing kinetics of epoxy based on the diglycidyl ether of bisphenol A (DGEBA), 4,4′‐diaminodiphenyl sulfone (DDS) as a hardener, and SiO2/TiO2 (70:30) as a nanofiller was investigated by nonisothermal differential scanning calorimetry (DSC). The effects of different weight contents of SiO2/TiO2 nanoparticles were studied using DSC in Dynamic Mode and the best value of the nanofiller was found to be 5 phr (parts per hundred). Dynamic measurements were used to obtain the total heat of reaction of the epoxy system as well as its activation energy (Ea) based on the isoconversional methods of Kissinger and Ozawa. The process revealed a dependence of the activation energy on conversion (α). The morphology of the cured system was investigated by scanning electron microscopy (SEM). It showed a strong cross‐linking between the resin and hardner and a relatively better dispersion of surface modified filler nanoparticles. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
254.
Kazem Jeddi Nader Taheri Qazvini Seyed Hassan Jafari Hossein Ali Khonakdar Javad Seyfi Uta Reuter 《Journal of Polymer Science.Polymer Physics》2011,49(4):318-326
Polymer–silicate nanocomposites based on poly (ethylene oxide), PEO, poly(methyl methacrylate), PMMA, and sodium montmorillonite clay were fabricated and characterized to investigate the effect of nanolayered silicates on segmental dynamics of PEO/PMMA blends. X‐ray results indicate the formation of an exfoliated morphology in the nanocomposites. At low silicate contents, an enhancement in segmental dynamics of blend nanocomposites and also PEO, minor component in blend, is observed at temperature region below blend glass transition. This result can be attributed to the improvement of the confinement effect of rigid PMMA matrix on the PEO chains by introducing a low amount of layered silicates. On the other hand, at high silicate contents, an enhancement in segmental dynamics of blend nanocomposites and PEO is observed at temperature region above blend glass transition. This behavior could be interpreted based on the reduction of monomeric friction between two polymer components, which can facilitate segmental motions of blend components in nanocomposite systems. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
255.
Hossein Tavakol 《International journal of quantum chemistry》2011,111(14):3717-3724
Nine tautomers and eleven possible tautomeric interconversions of N′‐acetyl formohydrazide have been studied at B3LYP/6‐311++G** level of theory. From these calculations, optimized geometries, molecular parameters, IR frequencies, NMR chemical shifts, and energetic results are obtained. In all tautomers except tautomers 4, E isomer is more stable than Z isomer. Energetic data were used to calculate the energy barriers of tautomeric interconversions and very high energy barriers were obtained for all tautomeric interconversions. Moreover, study of solvent effects on relative stabilities of tautomers and transition states showed that they are similar to those in the gas phase. In addition, intermolecular proton transfer with the assistance of one to three water molecules has been studied and the results showed that activation barriers in water‐assisted tautomerism are in general lower than those in the gas phase. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
256.
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
257.
Hossein Reza Darabi Abbas Darestani Farahani Mohammad Hashemi Karouei Kioumars Aghapoor Rohoullah Firouzi Rainer Herges 《Supramolecular chemistry》2013,25(9):653-657
A new E,E-stilbenophane was synthesised and characterised. The crystal structure of this cyclophane shows that this molecule has a cup-shaped structure, which hosts a phenyl ring of neighbouring molecule as guest in its cavity with a π–π distance of about 3.7 Å. Moreover, the NMR spectra and theoretical analysis (gauge-independent atomic orbitals (GIAO) and quantum theory of atoms in molecules (QTAIM)) suggest that the silver recognition by E,E-stilbenophane host molecules is based on cation–π interactions in which the π-electrons of the double bonds play a major role. 相似文献
258.
Hossein Anaraki-Ardakani Shirin Sadeghian Foroghossadat Rastegari Alireza Hassanabadi 《合成通讯》2013,43(12):1990-1999
The three-component reaction between dialkyl acetylenedicarboxylates and triphenylphosphine in the presence of arylsulfonyl hydrazides or aryl hydrazines produces highly functionalized, salt-free phosphorus ylides in excellent yields. 相似文献
259.
Ali Hossein Kianfar Sanam Asl Khademi Roghaye Hashemi Fath Mahmoud Roushani Mojtaba Shamsipur 《Journal of the Iranian Chemical Society》2013,10(2):347-355
The VO(IV) complexes of tridentate ONN Schiff ligands were synthesized and characterized by IR, UV–Vis and elemental analysis. The electrochemical properties of the vanadyl complexes were investigated by cyclic voltammetry. A good correlation was observed between the oxidation potentials and the electron-withdrawing character of the substituents on the Schiff base ligands, showing the following trend: MeO < H < Br < NO2. The thermogravimetry (TG) and differential thermoanalysis (DTA) of the VO(IV) complexes were carried out in the range of 20–700 °C. The VOL1(OH2) and VOL2(OH2) decomposed in three steps, whereas the VOL3(OH2) and VOL4(OH2) complexes decomposed in two steps. The thermal decomposition of these complexes is closely related to the nature of the Schiff base ligands and proceeds via first-order kinetics. The structures of compounds were determined by ab initio calculations. The optimized molecular geometry and atomic charges were calculated using MP2 method with 6-31G(d) basis. The results suggested that, in the complexes, V(IV) ion is in square-pyramid N2O3 coordination geometry. Also the bond lengths and angles were studied and compared. 相似文献
260.
Dissociation equilibria of 4‐(2‐pyridylazo) resorcinol (PAR) in aqueous micellar solutions were determined spectrophotometrically at 25 °C and at the constant ionic strength I = 0.1 M KNO3. For this purpose, the effect of nonionic (Brij‐35, Triton X‐100, Triton X‐114, Triton X‐405), and anionic (SDS) surfactants on the absorption spectra of PAR at different pH values was studied. Results show that the pKa values and pure spectra of each species of PAR are influenced by percentages of a neutral and an anionic surfactant such as Brij‐35, Triton X‐100, Triton X‐114, Triton X‐405 and SDS, respectively, added to the solution of this reagent. 相似文献