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31.
Single crystals of novel Strandberg type molybdophosphonate complex, (C5H7N2)6[Cu(H2O)3HP2Mo5O23]2·4H2O, are synthesized in aqueous solution and characterized by X-ray diffraction, spectroscopy (diffuse reflectance, UV–Vis and IR) and thermal analysis. Single crystal X-ray diffraction analysis reveals that this novel compound is composed of [Cu(H2O)3HP2Mo5O23]3? anions, three distinct 2-aminopyridinium cations as counter-ions and two distinct crystallization water molecules. The crystal packing is stabilized by H-bonds and π–π interactions, resulting in a 3D framework. In addition, the magnetic behavior of the related compound is measured. Magnetic measurements from 100 to 2 K indicate the presence of an antiferromagnetic coupling between the Cu (II) ions in (C5H7N2)6[Cu(H2O)3HP2Mo5O23]2·4H2O complex, resulting in a maximum of an antiferromagnetic–paramagnetic transition at TN = 7 K. Magnetic susceptibility data indicate an antiferromagnetic Curie–Weiss behavior in the studied temperature range, and molecular field theory gives the (J/kB) values of the nearest neighbor interactions between copper ions. 相似文献
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We present the structural, magnetic and electrical properties of some manganese oxides such as Ln0.5M0.5MnO3 (Ln = La, Pr and M = Ca, Sr, Ba) in which the average ionic radius <rA> of the A site cation is varied while keeping the Mn3+/Mn4+ ratio fixed to 1. All the studied samples have a perovskite structure with rhombohedral or orthorhombic distortion. This distortion was related to the A cation size. The lattice volume increases lineraly with ( r A) in all the range 1.179–1.3264 Å. The magnetic investigation shows that the magnetic transition temperature T t, and magnetisation saturation M s have similar behaviour by increasing and decreasing with (r A). The electrical measurements show conduction by small polarons attributed to a large canting angle 9 in all the range 4–300 K in the two ferromagnetic compounds La0.5Sr0.5MnO3 and La0.5Sr0.5MnO3. 相似文献
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Interference noising originating from the ultrasonic testing defect signal seriously influences the accuracy of the signal extraction and defect location. Time–frequency analysis methods are mainly used to improve the defects detection resolution. In fact, the S-transform, a hybrid of the Short time Fourier transform (STFT) and wavelet transform (WT), has a time frequency resolution which is far from ideal. In this paper, a new modified S-transform based on thresholding technique, which offers a better time frequency resolution compared to the original S-transform is proposed. The improvement is achieved by the introduction of a new scaling rule for the Gaussian window used in S-transform. Simulation results are presented and show correct time frequency information of multiple Gaussian echoes under low signal-to-noise ratio (SNR) environment. In addition, experimental results demonstrate better and reliable detection of close echoes drowned in the noise. 相似文献
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Letters in Mathematical Physics - We study the quantum open system evolution described by a Gorini–Kossakowski–Sudarshan–Lindblad generator with creation and annihilation... 相似文献
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Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution
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Second‐order rate constants have been measured spectrophotometrically for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine 1 with 4‐X‐substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ≠H and Δ≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e , whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (?4.03 to ?3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron‐donating group supports the proposal that the reactions proceed through a single‐electron transfer mechanism. 相似文献
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Complex perovskite oxides Ca2?xPrxMnO4 (x = 0–0.5) compounds were synthesized by a solid-state reaction technique. A tetra–ortho structural transition was observed. Impedance spectroscopy was used to study the electrical behavior in the frequency range 40 Hz–1 MHz and in the temperature range 80–350 K. Frequency-dependent conductivity spectra were found to obey the Jonscher's power law. Complex impedance plane plots have indicated that the dielectric response is mainly intrinsic. Materials bulk response was found to be dominated by non-localized or localized conduction, depending on temperature and frequency. 相似文献
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Ahmed Selmi Sobhi Hcini Hedi Rahmouni Aref Omri Mohamed Lamjed Bouazizi Abdessalem Dhahri 《Phase Transitions》2017,90(10):942-954
Spinel ferrite having composition Ni0.4Co0.4Mg0.2Fe2O4 was prepared by sol-gel method. X-ray diffraction result indicates that the ferrite sample has a cubic spinel type structure. FT-IR showed two absorption bands (ν1 and ν2) that are attributed to the stretching vibration of tetrahedral and octahedral sites. Complex impedance properties have been investigated in 200–420 K temperature range with varying frequency between 40 and 107 Hz. Frequency and temperature dependency of imaginary part of permittivity (?″) and dielectric loss (tanδ) has been discussed in terms of hopping of charge carriers between Fe2+ and Fe3+ ions. Activation energy has been estimated from both temperature dependency of dc conductivity and relaxation time data, which indicates that the relaxation process and conductivity have the same origin. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results. 相似文献
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La0.8Ca0.2MnO3 manganese oxides have been prepared by the conventional solid-state reaction. The samples crystallize in the orthorhombic structure with Pnma space group. A first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature that is found to be around 241?K. The magnetic and magnetocaloric properties have been simulated using a phenomenological model. The maximum of magnetic entropy change is of order 8.2?J/kg?K for an applied magnetic field of 5?T, and the adiabatic temperature change is found to be of 4?K under a same magnetic field; this high value gives the sample the possibility of technologic application in the magnetic refrigeration area. The simulated results of magnetization and magnetic entropy change are compared with the experimental measurements. Finely, we found a good agreement between experimental and theoretical results. 相似文献
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Polycrystalline perovskite manganites La0.7−xEuxBa0.3MnO3(x=0.05, 0.1 and 0.15) were prepared by sol-gel method. The prepared samples remain single phase with a perovskite structure, revealed by X-ray diffraction. The structure refinement of La0.7−xEuxBa0.3MnO3(x=0.05, 0.1 and 0.15) samples was performed in the hexagonal setting of the R3¯c space group. The dependence of magnetization M on applied magnetic field H and temperature T was measured carefully near the Curie temperature TC for all the samples. With the increasing Eu content, both the unit cell volume and Curie temperature TC of 298 K has been detected with a maximum of magnetic entropy |ΔSMmax| for the La0.7−xEuxBa0.3MnO3 with x=0.15, reaching a value of 2.3 J/kg K when a magnetic field of 10 kOe was applied and the relative cooling power (RCP) is 46 J/kg. These results suggest that the material may be a suitable candidate as working substance in magnetic refrigeration near room temperature. 相似文献