全文获取类型
收费全文 | 310篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 148篇 |
晶体学 | 3篇 |
力学 | 11篇 |
数学 | 57篇 |
物理学 | 33篇 |
无线电 | 74篇 |
出版年
2023年 | 1篇 |
2022年 | 12篇 |
2021年 | 8篇 |
2020年 | 17篇 |
2019年 | 13篇 |
2018年 | 10篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 12篇 |
2014年 | 13篇 |
2013年 | 24篇 |
2012年 | 16篇 |
2011年 | 19篇 |
2010年 | 15篇 |
2009年 | 15篇 |
2008年 | 14篇 |
2007年 | 13篇 |
2006年 | 7篇 |
2005年 | 10篇 |
2004年 | 16篇 |
2003年 | 12篇 |
2002年 | 10篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有326条查询结果,搜索用时 31 毫秒
131.
132.
Carole C. Harrison Mounir A. Malati Nigel B. Smetham 《Journal of solution chemistry》1996,25(5):505-514
The rate of decomposition of NH4NO2 solutions, at pH 5–7, equals k[NH3] [HNO2]2 or k[NH
4
+
] [NO
2
–
][HNO2]. A plausible mechanism involves a ratedetermining attack of N2O3, derived from HNO2, on NH3. H++ and S++ are 82 kJ-mol–1 and –27 J-mol–1-K–1, respectively. On partially replacing the solvent water by methanol or ethanol, the change G++, coupled with the calculated standard Gibbs energy of transfer of the reactants from water to the mixed solvent indicated that, in the latter, there is a greater destabilization of the transition state compared to that of the reactants. This can be explained by assuming two hydrogen bonds from the same water molecule to the transition state and hence a loss of hydrogen bond energy in the mixed solvent compared to the aqueous solution. The rate constant for the reaction of ND4NO2 in D2O compared to the reaction of NH4NO2 in water, gave a composite isotope effect involving two acid-base equilibria, suggested in the proposed mechanism; in addition to primary isotope effects in the equilibrium: 2 HNO2N2O3+H2O. 相似文献
133.
Caroline Rmond Mounir Ferchichi Nathalie Aubry Richard Plantier-Royon Charles Portella Michael J. O'Donohue 《Tetrahedron letters》2002,43(52)
A thermostable α-l-arabinofuranosidase was tested for its ability to perform transglycosylation with different alcohol acceptors. Reactions were characterized by high rates with optimal synthesis being obtained within 10 min. Both primary and secondary alcohols could act as acceptors in transarabinosylation but yields of alkyl arabinosides decreased with increasing alkyl chain length. 相似文献
134.
Howari FM 《Annali di chimica》2005,95(9-10):667-675
Dubai is developing rapidly and many developmental activities are concentrated around its Creek. The present study reports the lateral distribution of heavy metals and compares it with local historical record of heavy metal concentrations. For this purpose surface sediment samples were collected and analyzed for metal contents, total organic carbon content (TOC), mineralogy and grain size. The percentages of the different grain size fraction of the collected sediments were as follow 65% for sand size, 15% for silt size fraction, and the rest accounted for clay size fraction. The microscopic analyses indicate that the sediment composed mainly from carbonate and quartz with traces of rock fragments. Such mineral composition is not believed to be a potential source of heavy metal. The study found that the average recorded heavy metal concentrations in the collected sediment samples were 87, 96, 127, 38.5, and 279 ppm for Cr, Cu, Ni, Pb, and Zn, respectively. Those values were slightly higher than metal concentrations recoded in 2001 with 1.22 (Cr), 2.5 (Cu), 2.87 (Ni), 0.69 (Pb), and 2.1 (Zn) folds. However, in 2001 and 2003 the measured metal contents, along the creek, were lower than those of the average earth crust. Along the Creek most metals recorded the highest concentrations in the upper reach of the Creek. The distribution of the measured heavy metals was not affected significantly with the TOC values. The present study also documented obvious related point sources of pollution. 相似文献
135.
Maaloum M Muller P Krafft MP 《Langmuir : the ACS journal of surfaces and colloids》2004,20(6):2261-2264
It has recently been found that monodisperse surface micelles (hemimicelles) were formed in Langmuir monolayers of the semifluorinated alkane C8F17C16H33 (F8H16) after transfer onto silicon wafers. Grazing incidence X-ray diffraction studies have demonstrated that compression of mixed Langmuir monolayers made from combinations of dipalmitoyl phosphatidylethanolamine (DPPE) and diblock F8H16 in various molar ratios resulted in the complete expulsion of the diblock molecule at high surface pressure. F8H16 then formed a second layer on top of a DPPE-only monolayer, demonstrating a novel type of reversible, pressure-induced, vertical phase separation. Using atomic force microscopy and X-ray reflectivity, we show now that mixed DPPE/F8H16 (1:1.3) Langmuir-Blodgett films transferred onto silicon wafers below 10 mN m(-1) are laterally phase separated and consist of domains of F8H16 surface micelles in coexistence with a monolayer of DPPE. The density of the network of F8H16 surface micelles increases when the surface pressure of transfer increases. Around 10 mN m(-1), the F8H16 surface micelles start to glide on the DPPE monolayer, progressively overlying it, until total coverage is achieved. 相似文献
136.
Synthesis and properties of 7-formyl-8-hydroxyquinoline and its transition metal complexes 总被引:4,自引:0,他引:4
Ahmed A. El-Asmy Adel Z. El-Sonbati Abdulla A. Ba-Issa Mohamed Mounir 《Transition Metal Chemistry》1990,15(3):222-225
Summary CoII, NiII, CuII, CdII and HgII complexes of 7-formyl-8-hydroxyquinoline (HFHQ) have been prepared, and characterized by elemental analysis, molar conductivities, electronic and i.r. spectra, and magnetic measurements. It was found that the ligand acts as a neutral monodentate or a monobasic bidentate donor. The CoII, NiII and CuII complexes possess, respectively, tetrahedral, octahedral and square-planar structures based on their magnetic and electronic spectral data. The electrical conductivities of HFHQ and its metal complexes were measured at different temperatures and their activation energies were calculated. The values obtained for the CoII, NiII, CdII and HgII complexes agree fairly well with those reported for semiconductors. The ligand was tested as a corrosion inhibitor for aluminium; the limiting concentration of HFHQ to give maximum efficiency (99.2%) is 10–3 mole dm–3 at 22°C. The metal-ligand ratios and apparent formation constants for the species generated in ethanol solution were determined spectrophotometrically. 相似文献
137.
Mounir Afilal Tarek Merabtene Karim Rhofir Abdelaziz Soufyane 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2016,67(5):119
We consider two Cauchy problems for the one-dimensional thermoelastic Bresse model. Using the energy method in the Fourier space, we show that for the first model, the \({L^{2}}\)-norm of the solution decays with the rate \({(1+t)^{-1/12}}\). In addition, the same decay rate has been obtained for the second model. 相似文献
138.
Dr. Emilie Moulin Dr. Gad Fuks Prof. Dr. Mounir Maaloum Prof. Dr. Eric Buhler Prof. Dr. Nicolas Giuseppone 《Angewandte Chemie (International ed. in English)》2016,55(2):703-707
An acid–base switchable [c2]daisy chain rotaxane terminated with two 2,6‐diacetylamino pyridine units has been self‐assembled with a bis(uracil) linker. The complementary hydrogen‐bond recognition patterns, together with lateral van der Waals aggregations, result in the hierarchical formation of unidimensional supramolecular polymers associated in bundles of muscle‐like fibers. Microscopic and scattering techniques reveal that the mesoscopic structure of these bundles depends on the extended or contracted states that the rotaxanes show within individual polymer chains. The observed local dynamics span over several length scales because of a combination of supramolecular and mechanical bonds. This work illustrates the possibility to modify the hierarchical mesoscopic structuring of large polymeric systems by the integrated actuation of individual molecular machines. 相似文献
139.
Blanco P Polyakov P Bou-Ali MM Wiegand S 《The journal of physical chemistry. B》2008,112(28):8340-8345
In the present work we studied the thermal diffusion behavior of n-decane in various alkanes by thermogravitational column (TC) technique and the thermal diffusion forced Rayleigh scattering (TDFRS) method. The investigated lighter alkanes compared to n-decane are n-pentane, n-hexane, n-heptane, n-octane, and the heavier ones are n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, and n-eicosane. The binary mixture n-decane/ n-pentane we investigated at several different concentrations; all other mixtures were only investigated at a mass fraction of 50%. Even for the volatile n-pentane/ n-decane mixture the deviations between the thermal diffusion coefficients determined by the different methods agreed within the error bars. Typically the agreement between the two methods was in the order of 5%. In comparison to recently published TC and TDFRS data we found deviations in the order of 30% up to 40%. We analyze and discuss the possible reasons for the discrepancies for the present and the past publications. 相似文献
140.
Gating motions in voltage-gated potassium channels revealed by coarse-grained molecular dynamics simulations 总被引:1,自引:0,他引:1
Voltage-gated potassium (Kv) channels are ubiquitous transmembrane proteins involved in electric signaling of excitable tissues. A fundamental property of these channels is the ability to open or close in response to changes in the membrane potential. To date, their structure-based activation mechanism remains unclear, and there is a large controversy on how these gates function at the molecular level, in particular, how movements of the voltage sensor domain are coupled to channel gating. So far, all mechanisms proposed for this coupling are based on the crystal structure of the open voltage-gated Kv1.2 channel and structural models of the closed form based on electrophysiology experiments. Here, we use coarse-grain (CG) molecular dynamics simulations that allow conformational changes from the open to the closed form of the channel (embedded in its membrane environment) to be followed. Despite the low specificity of the CG force field, the obtained closed structure satisfies several experimental constraints. The overall results suggest a gating mechanism in which a lateral displacement the S4-S5 linker leads to a closing of the gate. Only a small up-down movement of the S4 helices is noticed. Additionally, the study suggests a peculiar upward motion of the intracellular tetramerization domain of the channel, hence providing a molecular view on how this domain may further regulate conduction in Kv channels. 相似文献