全文获取类型
收费全文 | 331篇 |
免费 | 12篇 |
国内免费 | 2篇 |
专业分类
化学 | 255篇 |
晶体学 | 1篇 |
力学 | 6篇 |
综合类 | 1篇 |
数学 | 22篇 |
物理学 | 60篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 25篇 |
2020年 | 12篇 |
2019年 | 14篇 |
2018年 | 15篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 6篇 |
2014年 | 11篇 |
2013年 | 22篇 |
2012年 | 26篇 |
2011年 | 33篇 |
2010年 | 21篇 |
2009年 | 8篇 |
2008年 | 7篇 |
2007年 | 4篇 |
2006年 | 8篇 |
2005年 | 9篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1991年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
1903年 | 1篇 |
排序方式: 共有345条查询结果,搜索用时 218 毫秒
281.
282.
In this work the diagnosis control of the complex impedance of selected perovskite compounds versus artificial neural network model optimized with the Levenberg–Marquardt algorithm is performed as detection of aging and degradation of materials usually requires destructive testing. The artificial neural network optimized by the Levenberg–Marquardt algorithm used in this work allows us to monitor the materials (LaNd) SrMnCrO3 in a non-destructive manner. This intelligent control is done by calculating the complex impedance which reveals reliable information on the phenomenon of transport in materials. The method overcomes the problem of the lack of a mathematical expression between the input parameters (temperature, doping, and frequency) and the necessary parameters for computing the impedance (bulk resistance, grain boundary resistance, and the two parameters of the constant phase element impedance A0 and P). The robustness and performance of the artificial neural network model was verified by introducing additional noise and by using the root mean square error and the R-square. 相似文献
283.
Yaming Zhou Abdessamad Debbab Victor Wray WenHan Lin Barbara Schulz Rozenn Trepos Claire Pile Claire Hellio Peter Proksch Amal H. Aly 《Tetrahedron letters》2014
Chromatographic separation of a crude extract obtained from the fungus Aspergillus sp., isolated from the Mediterranean sponge Tethya aurantium, yielded a new tryptophan derived alkaloid, 3-((1-hydroxy-3-(2-methylbut-3-en-2-yl)-2-oxoindolin-3-yl)methyl)-1-methyl-3,4-dihydrobenzo[e][1,4]diazepine-2,5-dione (1), and a new meroterpenoid, austalide R (2), together with three known compounds (3–5). The structures of the new compounds were unambiguously elucidated on the basis of extensive one and two-dimensional NMR (1H, 13C, COSY, HMBC, and ROESY) and mass spectral analysis. Interestingly, the compounds exhibited antibacterial activity when tested against a panel of marine bacteria, with 1 selectively inhibiting Vibrio species and 2 showing a broad spectrum of activity. In contrast, no significant activity was observed against terrestrial bacterial strains and the murine cancer cell line L5178Y. 相似文献
284.
‘Lateritin’ (1), a morpholine-2,5-dione (depsipeptide), was reinvestigated for its structure and absolute configuration. On the basis of thorough 1D and 2D NMR and mass spectrometrical analyses, the structure of 1 was revised to be identical with beauvericin (8) and confirmed that beauvericin (8) is the trimeric lactone of ‘lateritin’ (1). The absolute configuration was determined by acidic hydrolysis, followed by application of Marfey’s method, menthyl ester derivatization, and GC–MS analysis. In addition, the specific optical rotation values of the hydrolysis products were compared with those of available standards. 相似文献
285.
Indranil Banerjee Kakali De Sankha Chattopadhyay Amal Kumar Bandyopadhyay Mridula Misra 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(2):837-843
Nanoparticle biodistribution study is of great importance in bringing nanomedicine to patients. In this article, solid lipid nanoparticle (SLN) with dimension less than 100 nm was successfully radiolabelled with 99mTc by using sodium borohydride as a reducing agent (instead of stannous salts). Paclitaxel (PTX) was used as a model anticancer drug for the preparation of drug loaded SLN (PSLN). PSLN was characterized by standard methods. Encapsulation efficiency for PTX in PSLN was estimated by HPLC. PTX (Taxol formulation) and PSLN were radiolabelled separately and subsequent characterizations of these complexes were performed. Greater than 95 % radiolabelling efficiency was achieved and the labelling efficiency was calculated to be more than 90 % upto 24 h. Both the above-mentioned complexes passed the in vitro stability tests. PSLN achieved more brain concentration than Taxol as determined by biodistribution studies. This type of radiolabelling technique can be useful in preclinical evaluation of drug loaded SLN. 相似文献
286.
An overview of the main simulation methods of fractional systems is presented. Based on Oustaloups recursive poles and zeros approximation of a fractional integrator in a frequency band, some improvements are proposed. They take into account boundary effects around outer frequency limits and simplify the synthesis of a rational approximation by eliminating arbitrarily chosen parameters. 相似文献
287.
West Douglas X. Nassar Amal A. El-Saied Fathy A. Ayad Mohamed I. 《Transition Metal Chemistry》1998,23(4):423-427
Mononuclear nickel(II) complexes with 2-aminoacetophenone thiosemicarbazones and three N(4)-substituted thiosemicarbazones have been prepared in EtOH solution and characterized by physical and spectral methods. I.r. and electronic spectra of the thiosemicarbazones and their complexes, along with physical properties of the complexes, have been obtained. The 2-aminoacetophenone thiosemicarbazones coordinate both as neutral and anionic ligands via the thiosemicarbazone moiety's azomethine nitrogen and thione/thiolato sulfur [on loss of the N(2) hydrogen]. 相似文献
288.
Mostafa M. Emara Roshdy A. Ali Amal S. Turky Mahmoud M. Elghayish 《Journal of Dispersion Science and Technology》2016,37(5):733-742
A series of Gemini borate surfactants were synthesized based on glucose molecule. Their chemical structures were confirmed using 1H-NMR,13C nuclear magnetic resonance (NMR), and mass spectroscopy. The surface activities of these Gemini amphiphiles were measured, including surface tension (γ), critical micelle concentration (CMC), effectiveness (IIcmc), efficiency (pC20), maximum surface excess (Γmax), and minimum surface area (Amin) at different temperatures 25, 35, and 45°C either in pure water or in water–ethanol mixture (10%). Also, thermodynamic data including free energy, entropy, and enthalpy changes (ΔS, TΔS, ΔH) for adsorption at the air–water interface and also for micellization in surfactant solutions were calculated. 相似文献
289.
Cannizzo A Blanco-Rodríguez AM El Nahhas A Sebera J Zális S Vlcek A Chergui M 《Journal of the American Chemical Society》2008,130(28):8967-8974
Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b (1)A' state of [Re(L)(CO) 3(bpy)] ( n ) (L = Cl, Br, I, n = 0; L = 4-Et-pyridine, n = 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addition, energies, characters, and molecular structures of the emitting states were calculated by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a (3)A" state at longer times. The analysis of the data allows us to identify three spectral components. The first two are characterized by decay times tau 1 = 85-150 fs and tau 2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b (3)A"), respectively. The third component corresponds to the long-lived phosphorescence from the lowest a (3)A" state. In addition, it is found that the fluorescence decay time (tau 1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. tau 2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, molecular orbitals, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling constant of the ligand L, but we find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime. 相似文献
290.
H. Maheswaran P.J. Amal Joseph K. Leon Prasanth S. Priyadarshini P. Satyanarayana Praveen R. Likhar M. Lakshmi Kantam 《Tetrahedron: Asymmetry》2010,21(17):2158-2166
The use of structurally well-defined chiral [CuCl2(sparteine)] 1 and [NiCl2(sparteine)] 2 complexes as catalysts under fluoride anion-promoted double catalytic activation (DCA) conditions cause enantioreversal in the asymmetric Mukaiyama aldol reaction of 1-phenyl-1-trimethylsiloxyethylene with various aromatic aldehydes. A similar enantioreversal also occurs in the direct aldol reaction between methyl vinyl ketone and various aromatic aldehydes under Et3N promoted DCA conditions. We have quantitatively analyzed using group theoretical analysis via symmetry deformation coordinates, the molecular structures of 1 and 2 from the X-ray structural data; the results from the study show that the innate stereochemical differences that are present in their molecular structures, form the basis for the enantioreversal phenomenon in the asymmetric C–C bond-forming aldol reactions. 相似文献