Materials Science Forum Vol. 846

Abstract: Activated carbon monoliths (ACMs) electrodes for supercapacitor application were prepared from the green monoliths (GMs) containing KOH treated self-adhesive carbon grains (SACG) added with KOH treated graphene at its weight percentages of 0, 2, 4, 6, 8 and 10 %, respectively. The SACG were prepared from fibers of oil palm empty fruit bunches by a low carbonization temperature method. The ACMs were produced by the carbonization and activation of the GMs. The surface area, structure and specific capacitance of the ACMs electrodes were found affected by the graphene addition. The highest surface area of the ACMs electrode was observed for the addition of 6% graphene, which corresponds to the carbon turbostratic structure of the ACMs electrodes with the values of its crystallites interlayers spacing (d₀₀₂ and d₁₀₀) at 0.352 nm and 0.205 nm, and its crystallites stack-width (L_a) and stack-height (L_c) at 43.21 nm and 10.06 nm, respectively. The specific capacitance of the cell using this electrode was 112 F / g.

Abstract: In this work, we analyze the electronic structure and charge distribution of the YBCO family compounds Y-123 and Y-124 bulk using the density functional theory. The band structures and the density of states of the compounds are shown. The charge transfers between the atoms in the two compounds are compared and discussed in detail. The lattice parameters obtained by Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm are compared with the experimental parameters. The critical differences of the charge distribution for these two compounds are discussed.

Abstract: High temperature superconductor ceramic of low density Bi_1.6Pb_0.4Sr₂(Ca_2-xEu_x)Cu₃O_y both doped and non-doped, have been produced by the method of conventional solid state reaction synthesis. The effects of rare earth doping on the low density superconducting compound have been studied by considering Eu substitution at Ca site to stoichiometric of Bi (Pb)-2223 with x ranging from 0.000 to 0.2000. The experimental works in this study consist of direct-current (DC) electrical resistance-temperature and critical current density measurements for both electrical and superconducting properties, and powder X-ray diffraction (XRD) for phase analyses and lattice parameters while Field Emission Scanning Electron Microscopy (FESEM) for surface morphology. All the Eu-doped samples consist of both low-Tc Bi (Pb)-2212 and High-Tc Bi (Pb)-2223 phases. The substitution of Eu at the Ca site induced the decrease of volume fraction for the Bi (Pb)-2223 phase and the increase of the volume fraction for Bi (Pb)-2212 phase. Eu doping does not enhance the Tc zero but gradually decrease its value and become semiconducting at higher dopant concentration with x=0.200. From FESEM investigation, the grain size started to degrade from x=0.000 to x=0.0100 and form smaller plate-like grain which belong to Bi (Pb)-2212 phase and the grain particles are seem to be coagulated to form clusters for x=0.200.

Abstract: Effect of Ca doped by rare-earth elements, Nd and Er (x=0.10) on the structural and transport properties of low density Bi_1.6Pb_0.4Sr₂Ca_1,9RE_0.1Cu₃O_δ (RE=Nd,Er) samples have been investigated. The samples have been prepared via co-precipitation (COP) method from precipitate of saturated solutions of metal acetates, iso-propanol solution and oxalic acid. The metal oxalates powder was subjected to thermal treatment of 24 hours calcination at 845oC and mixed with 0.10 g sucrose C₁₂H₂₂O₁₁ which is used as supplementary filler. The pelletized powder was heated for 2 hours at 400 oC and sintered for at 850oC for 48 hours. All samples show a single step transition in the R-T curves. The critical current density (J_c) and transition temperature (T_{c zero}) for both Nd and Er doped with x = 0.10 were found to be lower than standard samples. T_{c zero} obtained varies between 98 K and 66 K. XRD data showed the decrease of volume fraction for the Bi-2223 phase and increase of the volume fraction Bi-2212 phase with the doping of Nd and Er.

Abstract: The influences of cerium doped on low density Bi-2223 superconductor have been studied and this study examines the significant changes in the structural and superconducting properties. The nominal compositions of low density Bi_1.6Pb_0.4Sr₂Ca_2-xCe_xCu₃O_y (where x =0.000, 0.025, 0.05, 0.1 and 0.2) ceramic superconductor were prepared by solid state method. The effects of different Ce doping levels on the superconductor structure and superconducting properties has been investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction analysis (XRD), electrical resistance and transport critical current density (J_C) measurements. The results show that T_{C zero} and J_C of the samples reduced gradually with the increase in the Ce addition. It has been observed that, the J_C of low density Bi-2223 for Ce-free was measured to be 12.120 A/cm² and 0.448 A/cm² in x=0.025 at 60 K under zero magnetic field. As for XRD analysis showed the doping of Ce reduced the volume fraction of the 2223 phase and increased the volume fraction of the 2212 phase. FESEM results showed the surface morphology not homogenous and weak links between superconducting grains of the samples due to increase of the Ce doping.

Abstract: The effect of Ca substitution on the electrical and structural properties in high and low density Y(Ba_1-xCa_x)₂Cu₃O_δ where x = 0.00, 0.10, 0.20 and 0.30 via solid state reaction method has been investigated. The electrical properties, elemental analysis, and structural identification were measured by the four-point probe technique, energy dispersive x-ray (EDX) and X-ray diffraction (XRD) respectively. The electrical properties such as critical temperature (T_c) and critical current density (J_c) were found to be strongly dependent in both high and low densities Y(Ba_1-xCa_x)₂Cu₃O_7-δ. These parameters were decreased monotonously with the increasing of Ca substitution. An obvious results of the Ca-doped samples can be seen in x = 0.20 where T_{c zero} of high density sample is 77 K, which is higher than that of the low density sample that occurred at 73 K. Meanwhile, J_c at 60 K for high density is 1.842 A/cm² compared to 1.410 A/cm² in low density sample. EDX analysis confirmed the existence of Ca in all doped samples. The crystallographic structure remained orthorhombic and the volume of unit cell increased towards further increased of Ca concentration.

Abstract: PbBa₂Y_1-xCa_xCu₃O_7+δ (x = 0.00, 0.15, 0.20, 0.25 and 0.50) samples were prepared via solid state reaction. All samples except x = 0.50 exhibited various critical temperatures T_c. The X-ray diffraction patterns show all samples were in mixed phase. Longitudinal and shear ultrasonic velocities in these samples were measured using the pulsed-echo-overlap method with frequency 5-10 MHz in the temperature range 80-300 K. There are no obvious elastic anomalies which are usually observed in most high temperature superconductors were observed in any of the samples in both longitudinal and shear modes. Furthermore, there is no significant variation in the percentage of velocity change in both longitudinal and shear modes with the different content of x doped for all samples.

Abstract: This paper presented a theoretical study of structural, electronic, and optical properties of the ternary mixed chalcogenides Topological Insulators with a formula M₂X₂Y (M = Bi, X = Te and Y= Se, S) using density functional theory (DFT) within the local density approximation (LDA). From the calculation, we have evaluated the bulk modulus and its corresponding pressure derivatives of these compounds. The linear photon-energy dependent of dielectric functions, some optical properties such as reflectivity, refraction index, conductivity function, and energy-loss spectra, have also been obtained and analyzed within the electronic band structures and density of states of these compounds.

Abstract: SiO₂ compound was introduced into zinc ferrite using formula (1-x)ZnFe₂O₄ – xSiO₂ synthesized by sol-gel method where distilled water and citric acid was used as a solvent and binder agent. The produced samples were annealed at 450 °C for 1h. The AFM result analysis show that the average surface roughness and particle size decreased as the compositions of x was increased. XRD results analysis confirmed the formation of spinel structures with crystallite size within range 11.27 – 4.72 nm. UV-Vis analysis to determine the energy band gap of the (1-x)ZnFe₂O₄ – xSiO₂ samples. It shows that the energy band gap increased as the composition of SiO₂ was increased due to the dielectric properties of silicon dioxide. FTIR results analysis exhibit common band in the range of 400 – 4000 cm^-1. The observed band near 2350 cm^-1 shows the presence of oxygen-oxygen bond in the face centered cubic (fcc) crystal lattice of oxygen atoms. The addition of SiO₂ into ZnFe₂O₄ will help to enhance the morphological structures and optical properties. This new proposed dielectric material can be used as dielectric substrate microstrip patch antenna.

Abstract: Polarized infrared (IR) reflectance measurement was carried out to investigate the optical phonon modes of wurtzite structure In_0.92Ga_0.08N thin film grown by molecular beam epitaxy. Composition dependence of IR reststrahlen features was observed. Theoretical polarized IR reflectance spectrum was simulated using the standard multilayer optics technique with a multi-oscillator dielectric function model. By obtaining the best fit of experimental and theoretical spectrum, the Brillouin zone center E₁ optical phonon modes together with the dielectric constant, layer thickness, free carriers concentration and mobility were extracted non-destructively. The extracted E₁ optical phonon modes were compared with those generated from modified random element isodisplacement (MREI) model.