Hassan K. Juwhari

The effect of powder preparation processes on the luminescent properties of yttrium oxide based phosphor materials

Eu and Tb3+ doped Y2O3, Y2SiO5 and Y3Al5O12(YAG) phosphor materials were prepared by different routes to investigate their effect on luminescent characteristics in micron and submicron particle size powders. Both amorphous low temperature co-precipitate reacted and crystalline high temperature solid state reacted (reference) powders were further crystallized in steam(S) and fluoriferous steam (SF) atmospheres at 1000\ghC for 8 hours, respectively. In spite of sharp XRD peaks in S, SF treated samples, all SEM images exhibited “soft” aggregates of about 3 μm in average diameter consisting of 0.2–0.3 μm small particles. The powder samples characterized by XRD, SEM and luminescence spectra indicated that low temperature reacted samples which were further crystallized in steam or fluoriferous steam have greater improvements in the luminescent intensity than solid state reacted Y2O3, Y2SiO5 based phosphors after S, SF treatments.

Effects of Heat Treatment on the Phase Evolution, Structural, and Magnetic Properties of Mo-Zn Doped M-Type Hexaferrites

In this article we report on the structural and magnetic properties of BaFe12-4xMoxZn3xO19 hexaferrites with Mo-Zn substitution for Fe ions. The starting materials were commensurate with the BaM stoichiometry, and the Mo:Zn ratio was 1:3. The powder precursors were prepared by high energy ball milling, and subsequently sintered at temperatures from 1100 to 1300° C. The structural analyses indicated that all samples sintered at 1100° C were dominated by a major M-type hexaferrite phase. The relative abundance of the BaMoO4 and Zn-spinel secondary phases increased with increasing the concentration of the substituents, resulting in a decrease of the saturation magnetization from about 67 emu/g (for x = 0.0) to 55 emu/g (for x = 0.3). The coercivity also decreased from 3275 Oe (for x = 0.0) to 900 Oe (for x = 0.3), demonstrating the ability to tune the coercivity to the range useful for magnetic recording by the substitution process. The saturation magnetization improved significantly with sintering at T > 1100° C, and the coercivity decreased significantly, signaling the transformation of the samples to soft magnetic materials. These magnetic changes were due to the high-temperature reaction of the spinel phase with the BaM phase to produce the W-type hexaferrite phase on the one hand, and to the growth of the particles on the other hand. The magnetic phases were further investigated using Mössbauer spectroscopy and thermomagnetic measurements. Our study indicated that the sample with x = 0.2 has the highest saturation magnetization (74 emu/g at sintering temperature of 1300° C) and a tunable coercivity between 2100 Oe and 450 Oe.

Effects of molybdenum concentration and valence state on the structural and magnetic properties of BaFe 11.6 Mo x Zn 0.4− x O 19 hexaferrites

Barium hexaferrites BaFe11.6MoxZn0.4−xO19 (x= 0.1, 0.2, 0.4) were prepared by precipitation of the precursors using wet chemical mixture method and then sintering the dried powders at 1100 °C. The properties of the prepared samples were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer, and Mössbauer spectroscopy. XRD patterns revealed that all prepared samples had BaFe12O19 hexaferrite structure as a majority phase. SEM images demonstated that the samples consisted mainly of hexagonal platelet-like grains with diameters ranging from 100 to 500 nm. Mössbauer spectra revealed that Zn2+ ions occupy 4f1 sites leading to the splitting of the 12k component. However Mo6+ ions occupy 2b sites while Mo4+ prefer 4f1 and 12k sites. For the sample with x = 0.4,Mo6+ and Mo4+ ions were found to have preference for 2b and 12k sites, respectively, and to induce the development of Fe2+ ions in the hexaferrite, leading to noticeable changes in the magnetic properties of the system. The observed magnetic properties were found to be consistent with the preferential site occupation of metal ions, and the hyperfine fields derived from Mössbauer spectra of these samples.

Optoelectrical properties of epoxy/silica nanocomposites

Abstract Nanosilica/epoxy composites were prepared with different silica concentrations (0, 3, 5, 10 and 15 wt-%). The optical and electrical properties of the prepared composites were investigated as a function of filler content, temperature and applied electric field frequency. The obtained optical data were analysed in terms of absorption formula for non-crystalline materials. Both the optical energy gaps and the width of the energy tails of the localised states have exhibited silica concentration dependence. It has been observed that electrical quantities such as the AC conductivity, measured using the impedance spectroscopy and the dielectric constants, vary with the applied frequency, silica concentrations and temperature. In addition, the relaxation time and activation energy for the thermal conduction processes were calculated and reported.

A study on the DC-electrical and thermal conductivities of epoxy/ZnO composites doped with carbon black

Thermo-electrical characterizations of hybrid polymer composites, made of epoxy matrix filled with various zinc oxide (ZnO) concentrations (0, 4.9, 9.9, 14.9, and 19.9 wt%), and reinforced with conductive carbon black (CB) nanoparticles (0.1 wt%), have been investigated as a function of ZnO concentration and temperature. Both the measured DC-electrical and thermal conductivities showed ZnO concentration and temperature dependencies. Increasing the temperature and filler concentrations were reflected in a negative temperature coefficient of resistivity and enhancement of the electrical conductivity as well. The observed increase in the DC conductivity and decrease in the determined activation energy were explained based on the concept of existing paths and connections between the ZnO particles and the conductive CB nanoparticles. Alteration of ZnO concentration with a fixed content of CB nanoparticles and/or temperature was found to be crucial in the thermal conductivity behavior. The addition of CB nanoparticles to the epoxy/ZnO matrix was found to enhance the electrical conduction resulting from the electronic and impurity contributions. Also, the thermal conductivity enhancement was mostly attributed to the heat transferred by phonons and electrons hopping to higher energy levels throughout the thermal processes. Scanning electron microscopy and energy-dispersive spectroscopy were used to observe the morphology and elements’ distribution in the composites. The observed thermal conductivity behavior was found to correlate well with that of the DC-electrical conductivity as a function of the ZnO content. The overall enhancements in both the measured DC- and thermal conductivities of the prepared hybrid composites are mainly produced through mutual interactions between the filling conductive particles and also from electrons tunneling in the composites bulk as well.

Purification and biochemical characterization of photo-active membrane protein bacteriorhodopsin from Haloarcula marismortui, an extreme halophile from the Dead Sea

Bacteriorhodopsin (BR) is an exciting photo-active retinal protein with many potential industrial applications. In this study, BR from the extremely halophilic archaeon Haloarcula marismortui (HmBR) was purified successfully using aqueous two phase extraction method. Absorption spectroscopy analysis showed maximum absorption peak of HmBR retinal protein (λmax) at 415 nm. The purified HmBR was visualized by SDS-PAGE, with a subunit molecular mass of 27 kDa, and its identity was confirmed by resonance Raman spectroscopy, Fourier transform infrared spectroscopy and atomic force microscopy. The effect of pH and salt concentration on the absorption spectrum of HmBR was evaluated. Red-shifted in λmax of HmBR was recorded at acidic condition (pH 5) and HmBR showed remarkable optical activity under high salinity condition. The photoelectric activity of HmBR was evaluated by measuring the DC-voltage generated from HmBR coated on indium tin oxide (ITO) glass when light illumination was applied.

Hydrothermal Synthesis, Crystal Structure, and Photoluminescent Properties of Li[UO 2 (CH 3 COO) 3 ] 3 [Co(H 2 O) 6 ]

Single crystal of Li[UO2(CH3COO)3]3[Co(H2O)6] was prepared and found to crystallize in the monoclinic crystal system in the sp. gr. C2/c, with Z = 2, and unit cell parameters a = 22.1857(15) Å, b = 13.6477(8) Å, c = 15.6921(10) Å, β = 117.842(9)°, V = 4201.3(4) Å3. The crystal was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The single crystal X-ray diffraction analysis revealed that the crystal has a lamellar structure in which a cobalt hydrate is sandwiched within the Li[UO2(CH3COO)3]32- chains. Furthermore, the room temperature photoluminescence spectrum of the complex was investigated in the solid state.

Optical dispersion functions of Co2−xEuxVSn using ab-initio calculations

The magnetic, electronic and optical properties of Co2−xEuxVSn alloys for selected concentrations (x = 0, 0.25, 0.50, 0.75 and 1.0) were investigated by means of density functional theory (DFT) calculations utilizing full potential linearized augmented plane wave (FP-LAPW) method. It was found that doping the Co2VSn alloy with rare-earth ions like Eu generates the nonstoichiometric Co2−xEuxVSn and changes its original behavior from half ferromagnetic material to a metallic one. In addition, the total magnetic moment was found to increase with increasing dopant concentration with maximum local magnetic contributions on the Eu-sites. The optical dielectric functions as well as their static value for all the above alloys were also investigated. Moreover, the absorption coefficient, reflectivity and refractive indices were calculated. All the optical calculations were found to agree well with the band structure calculations when determining the alloys’ half-metallic behavior. Finally, the nonstoichiometric me...