Papers by Author: Mansor Hashim

Abstract: The vulnerability of varistor ceramics to electrical degradation during operation not only affects their nonlinear properties but also leads to shortening of device’s lifetime by increasing the risk of failures including melting, fire or even explosions. In this study, ZnO-Pr₆O₁₁-Cr₂O₃ varistor ceramics were prepared through modified citrate gel method and solid state-sintering at 1250 °C for 1 hour. The stability of their nonlinear properties under prolonged application of DC electrical field in high temperature ambient was investigated. Degradation process was accelerated by applying direct current (DC) electrical field of 15 % below the breakdown field point of ceramics for 54 hours consecutively at temperature of 30 to 125 °C. The findings indicate that ZnO-Pr₆O₁₁-Cr₂O₃ ceramics exhibited its susceptibility to electrical degradation after prolonged electrical and thermal stresses application. Its nonlinear coefficient has reduced by 4.4 % reduction, the breakdown field has reduced by 9.9 % and the leakage current density increased by 13.7 % in comparison to its initial value. Degradation process in varistor ceramics of ZnO-Pr₆O₁₁-Cr₂O₃ was a direct response to collapsed Double Schottky Barrier at ZnO grain boundary.

Abstract: In this paper, nanosized La_0.85Na_0.15MnO₃ (LNMO) has been synthesized via sol-gel method by involving two major steps, first the complexation of citric acid (CA) with metal ions (MI) and second the polyesterification between CA and ethylene glycol (EG). The effect of molar ratio CA:MI varying from 2-4 on structure, microstructure and electrical transport properties of LNMO have been investigated by constant the amount of EG. All samples show single perovskite phase with hexagonal structure and space group R3c after sintering at 800°C for 10h. Sample of molar ratio 2.5 is observed to possess smallest grain sizes which yield high resistivity value compared with others, is suggested to originate from the increase of tunneling barriers (grain boundaries). The large low field magnetoresistance (LFMR) of about ~ -16% at 0.1T and low temperature confirmed the important role of grain boundaries in the nanosized LNMO.

Abstract: Prior to their use for microwave absorption, different compositions of Ni_xZn_1-xFe₂O₄ (x = 0.5, 0.6, 0.7 and 0.8) were prepared via mechanical alloying and sintering. X-ray diffractometry (XRD) was used to investigate the crystalline phase formation. Scanning transmission electron microscopy (STEM) and field emission electron microscopy (FeSEM) were used to investigate the particle size and surface morphology respectively. The complex-permeability components, μʹ and μʺ, were also measured using an Agilent 4291B material analyzer from 1 MHz to 1 GHz. From the XRD results it is shown that at 900oC the full phase of nickel zinc ferrite was formed. The μʺ values suggest that the ferrite sample can absorb well microwave energy for frequencies 1 MHz to 1 GHz and higher. This is proved by microwave absorption measurements carried out up to 12 GHz.Keywords: NiZn-ferrite, microwave absorption, magnetic materials

Abstract: The effect addition of manganites compound on the superconducting properties of YBCO was investigated through X-Ray diffraction (XRD), resistance-temperature (R-T) measurement, AC Susceptometer, and Scanning Electron Microscope (SEM). Samples with addition of (La_0.67Ca_0.33MnO₃)x (LCMO) (x = 0.0, 0.8, 1.0) in YBa₂Cu₃O_7-δ (YBCO) superconductor were prepared via solid state method. Addition of LCMO in YBCO superconductor changed the properties of samples. XRD analysis showed the addition of LCMO (0.8 wt% - 1.0 wt%) formed three new peaks of LCMO. The crystal structure is orthorhombic with space group Pmmm for pure Y123 but after addition 0.8 wt% and addition 1.0 wt% of LCMO the crystal structure was changed to tetragonal and space group P4/mmm. The temperature dependent of electrical resistance showed the samples were metallic above the transition temperature even though after addition. The T_c-on superconducting transition temperature decreased from 86 K to 84 K while the T_c-off decreased from 76 K to 70 K. Diamagnetic onset transition temperature, T_c-on(dia) decreased from 90.2 K to 89.8 K. This suppression was due to the magnetic behaviour of the LCMO addition. Morphology observation showed that the grain size and grains boundary are decreased while the porosity was increased as the addition of LCMO increases.

Abstract: Ni_0.4Zn_0.6Fe₂O₄-polypropylene (PP) composites were investigated for their dielectric property which constitutes the dielectric constant , and dielectric loss factor . Nickel zinc ferrites (NZF) were prepared via the conventional solid state method and sintered at 1250 °C for 10 hours. Ni_0.4Zn_0.6Fe₂O₄ acts as filler while PP is the matrix. 5% and 25% NZF were added into the PP and blended to form NZF-PP composites. Dielectric measurements were made from room temperature to 120 °C at 20 °C intervals using the HP 4284A Precision LCR Meter at frequencies 240 Hz to 1 MHz. is almost constant and independent of frequency and temperature below 100 KHz before decreasing showing dispersion probably due to space charge or interfacial polarization. Distinct relaxation loss peaks can be seen emerging at about 100 KHz shifting towards the higher frequency with increasing temperature. Generally, increasing the content of filler improve the dielectric strength of the composite. Thus, addition of NZF enhances the dielectric properties of NZF-PP composite.

Abstract: The polycrystalline Yttrium Iron Garnet (YIG) powder with the chemical formula Y3Fe5O12 has been synthesized by using High Energy Ball Milling technique. The effect of various preparation parameters on the crystallinity, morphology and complex permeability of YIG, which includes milling time and annealing temperature were studied respectively by using XRD, SEM and Impedance Material Analyzer. The frequency dependence of complex permeability namely real permeability, µ’ and magnetic loss, µ’’ were measured at room temperature for samples sintered from 600°C to 1400°C, in the frequency range 10 MHz to 1 GHz. The results showed that milling time plays a role in determining the crystallinity of the milled powder where higher milling time results in better crystallinity due to high reactivity of the particles. From complex permeability measurement, it was observed that the initial permeability and magnetic loss increased with increasing grain size. The permeability values increased with annealing temperature and the absolute values of permeability decreased after attaining the natural resonance frequency of the material.

Abstract: SrTiO₃ sample has been successfully prepared by mechanical alloying (MA) method. The effect of milling time on microstructure, crystallite size and dielectric properties of SrTiO₃ were studied. The results revealed that the mean crystallite size of milled powders decreased from 84.56 to 12.87 nm with increasing milling time. However, the average lattice strain of milled powders increased from 0.2 to 0.93% with increasing milling time. A single phase SrTiO₃ could not be formed with milling alone and required annealing process. A transformation of anatase-TiO₂ to rutile-TiO₂ was observed at 16 h of milling. After the milled powders were subjected to sintering process at 1200°C, formation of single-phase SrTiO₃-type cubic (Pm-3m) perovskite structure was observed. The peak intensities of the sintered SrTiO₃ samples decreased as the milling time was increased. For microstructural observations, the average grain size of the sintered SrTiO₃ sample milled for 8 h showed the largest. For dielectric measurements, the dielectric constant of the sintered SrTiO₃ sample milled for 8 h showed the highest among others. This could be due to the largest grain size obtained for sintered SrTiO₃ sample milled for 8 h. The decrease in the grain size with increasing milling time resulted to the decrease in dielectric constant.