VNU Journal of Science: Mathematics - Physics

Laser beam diffraction by a patterned surface has been investigated theoretically and experimentally for the determination of the thermal expansion coefficient (a) of polymeric materials. By tracking the deviation of the first order diffraction mode from surface-patterned polymers, expansion coefficients in a range 10-7 to 10-4 K-1 can be measured by temperature changes less than 100 oC. A set-up of laser diffraction (SLD) was made, using a He-Ne laser (l = 632.8 nm) and thin film casting technique. The results of measurements on the SLD system for polymers like PVK polycarbonate, PDMS, organic complex (chitosan) and conducting polymer (P3HT) showed that SLD technique can be applied to determine thermal expansion coefficients of different polymeric materials with a considerably small volume. Especially, the fact that a of P3HT-composite films was found to be much lower than that of the pure P3HT suggests a potential application of polymeric composites for organic devices working at elevated temperature, for organic solar cells (OSC) in particular.Keywords: Laser beam diffraction; thermal expansion; diffraction grating; polymer.

Abstract: For fast developing materials science, the communication technology also introduces new applications especially in the field of application of new materials and devices. Meta-materials with repeating structures that exhibit the negative refractive index, negative dielectric constant, and negative magnetic permeability promise many benefits in the wireless communication using a new class of microwave antennas. These novel antennas overcome disadvantages of traditional microwave antennas while having smaller physical size, wider band-width and higher efficiency. In this paper we present the results of fabrication and test of meta-structured antennas. Keywords: Meta-material, antenna, meta-antenna, negative refractive index…

Abstract: We report the ab initio calculation of electronic structure of Eu-doped CaO. The obtained results appeared in a very good agreement with experimental data and predicted the existence of a ferromagnetic state for one doped compound. The light doping could induce the electron trapping property and the heavy doping the half-metallic ferromagnetic state.Keywords. CaO; DFT; electronic structure; ab initio; ferromagnetism.

Abstract: Magnetic memory effect was newly observed in hard magnetic FePd nanoparticles prepared by sonoelectrochemistry. The applied magnetic field was changed in defined protocols during the magnetic relaxation process and the magnetic memory effect occurs when the field changes are larger than the critical field change of around 25 Oe. This magnetic memory effect can be explained by the magnetic reversal processes in strong interaction systems. The width of the energy barrier distribution of these hard magnetic particles is related to the critical field change deduced from this study.

The nonlinear dynamic response and vibration of porous functionally graded cylindrical panel (PFGCP) subjected to the thermal load, mechanical load and resting on elastic foundations are determined by an analytical approach as the Reddy’s third order shear deformation theory, Ahry’s function… The study results for dynamic response of PFGCP present the effect of geometrical ratio, elastic foundations: Winkler foundation and Paskternak foundation; loads: mechanical load and thermal load; and the material properties and distribution type of porosity. The results are shown numerically and are determined by using Galerkin methods and Fourth-order Runge-Kutta method.

Abstract: We present first-principles studies of CO2 and NH3 adsorbed on graphene nanoribbons (GNRs). The electronic and transport properties are calculated based on density functional theory combined with non-equilibrium Green's function method. Absorption energy, density of states, electron density deformation, charge transfer, current-voltage characteristics, and transmission spectra were analyzed. It is found that CO2 and NH3 adsorbed on GNRs exhibit acceptor-like and donor-like behaviors, respectively. Both CO2 and NH3 molecules show physissorption on GNRs with low adsorption energies and small charge transfers. In other words, the interactions between CO2 and NH3 molecules and GRNs are very weak. The results suggest that the sensitivity and selectivity of GRN-based gas sensors could be improved by introducing the dopant, defect, or modification of electronic structures of graphene.Keywords: First-principles, Graphene Nanoribbons, CO2 Adsorbed on Graphene, NH3 Adsorbed on Graphene.

Zinc aluminate - alumina (ZnAl2O4/Al2O3) systems doped with Cr3+ different concentrations were synthesized by sol-gel method. The samples were characterized by X-ray diffraction (XRD), photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The results showed that Cr3+ dopant concentration had a significant effect on the lattice constants of ZnAl2O4/Al2O3 composite and their luminescence characteristics.

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Abstract: Effect of BaSnO3 (BSO) nanoparticle inclusion on the critical current density of GdBa2Cu3O7-x (GdBCO) thin films was investigated. BSO nanoparticles were prepared on a thin GdBCO seed layer of < 50 nm thick by using pulsed laser deposition (PLD) technique. The GdBCO superconducting layers were deposited on top of the BSO nanoparticles to produce films with the entire thickness of approximately 250 nm[O1] . The number of laser pulses was varied from 20 to 320 in order to obtain BSO nanoparticles with different sizes and densities. Magnetization data measured at 65 K showed that: at a small number of laser pulses, the slight enhancement in critical current density (Jc) up to 1.5 T was obtained. Further increasing the number of laser pulses, Jcs were found to be consistently decreased. The increase in Jc was attributed to the addition of BSO nanoparticles serving as artificial pinning centers (APCs) inside GdBCO films. The decreases in Jc was probably caused by to the crystallinity degradation of the BSO-doped GdBCO films which was confirmed by the larger FHWM values.Keywords: GdBCO thin film, critical current density, BaSnO3, artificial pinning centers. [O1]