Materials Science Forum Vol. 846

Abstract: This study evaluates the structural and magnetic properties of Ni-Mg substituted Cobalt ferrite samples prepared through the co-precipitation method. The nominal compositions Co_0.5Ni_0.5−xMg_x Fe₂O₄ in the range x = 0.1 have been synthesized and then was sintered at temperature at 700 and 1000°C in the furnace for 10 hour with a heating rate of 5°C/min. The prepared nanoferrites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and vibration sample magnetometer (VSM). XRD confirmed formation of single phase spinel ferrite with average crystalline size in the range of 27–33 nm. The lattice constant (a), cell volume (V) and X-ray density (ρ_x) are also calculated from XRD data. Lattice constant (a) decreases with an increase of sintering temperature. Further information about the structure and morphology of the nanoferrites was obtained from FESEM and results are in good agreement with XRD. Saturation magnetization showed increasing trend with sintering temperature from 700 to 1000°C.

Abstract: Porous GaN structures were formed from crystalline GaN on conducting AL₂O₃ substrate using Pt-assisted electroless etching in HF: CH₃OH: H₂O₂ = 1:4:4 under illumination of 500 W UV lamp. Scanning electron microscope (SEM) photoluminescence (PL) and Raman spectra measurements evidenced important features of the pore morphology, nanostructures and optical properties. According to the SEM micrographs, the three-dimensional ridge structure appears with the formation of porous material between the ridges. The porous layer exhibited a substantial PL intensity enhancement with red-shifted band-edge PL peaks associated with the relaxation of compressive stress. The shift of E₂(high) to the lower frequency in Raman spectra of the porous GaN films further confirms such a stress relaxation.

Abstract: In present work, the effect of changing microstructure on magnetic properties which evolves in parallel, in particular from amorphous-to-crystalline development, in yttrium iron garnet was investigated. 9 toroidal samples of polycrystalline yttrium iron garnets were prepared by using the mechanical alloying technique and sintered at low to high sintering temperature for microstructure-dependent-magnetic evolutions. A brief, yet revealing characterization of the samples were carried out by using an X-ray Diffraction, Field Emission Scanning Electron Microscopy, Impedance Material Analyzer, LCR-meter and, Picoammeter. It is believed that microstructural features such as amorphous phase, grain boundary, secondary phase and intergranular pores contribute significant additional magnetic anisotropy and demagnetizing fields, thus affecting the initial permeability accordingly. A scrutinizing observation of the permeability component results show that they tend to fall into three groups of magnetic permeability according to degree of magnetic behaviour dominance. The Curie temperature remained relatively stable and unaffected by the evolution, thus confirming its intrinsic character of being dependent only on the crystal structure and compositional stoichiometry. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicate improved phase purity and compositional stoichiometry.

Abstract: We have carried out a series of DFT calculations to investigate changes on the structural and electronic properties of Silicon (Si) quantum dots as a function of surface passivation. In particular, we have study non-polar passivation effect of hydrogen (H) and methyl (CH₃) at the surface of quantum dots. From geometry optimization result, we find that clusters with reconstructed surfaces a complete methyl passivation is possible and steric repulsion prevents full passivation of Si dots with unreconstructed surfaces. On the electronic properties point of view, it is noticed for small nanocrystals, the presence of mini-gaps are more pronounced which can limit the non-radiative relaxation of excitons. Obviously, methyl passivation weakly affects the band gap values of silicon quantum dots, while it substantially decreases the band gap and reduce mini-gap appearance compared to hydrogen passivation Si QDs. On the basis of our results we propose that methyl terminated quantum dots may be size selected taking advantage of the reduction on mini-gap and the localization of electron as a function of the cluster size.

Abstract: Quaternary chalcogenide semiconductor has attracted much attention as absorber-layer materials in solar cells. The absorber-layer material, Cu₂ZnSnSe₄ (CZTSe) which is a p-type semiconductor that has high absorption coefficient, had been synthesized by using solvothermal method. The variation of concentrations of the Copper (II) Chloride dihydrate have brought some effects towards producing the stoichiometry and non-stoichiometry copper zinc tin selenide that may lead to the improvement of efficiency of solar cells. The synthesized reaction was conducted at 250°C for 24 hours. Properties of the samples have been characterized by X-ray diffraction (XRD) to determine the crystal structure of the sample and X-ray Fluorescence (XRF) to determine the elemental composition of the sample. The electrical properties such as resistance and conductivity have been studied through Van der Pauw configuration. The CZTSe has been successfully synthesized at concentrations of 0.15 M with no impurities phases existed.

Abstract: The mechanical alloying technique was used to prepare barium hexaferrite (BaM) with 3, 5, 10 and 20 wt% cobalt oxide (Co₃O₄). In this work, steel waste flakes were cold-rolling steel mill for several hours to form a fine powder. The steel waste powder was purified by using magnetic separation to isolate the magnetic and non magnetic particles. The method was continued for Curie temperature separation technique to separate the magnetic ions by varied Curie temperature of the magnetic powder. The purified powder was then oxidize at 500 °C at 6 °C/mins to form hematite, Fe₂O₃. The steel waste-derived hematite was used as the raw material in preparing BaM ferrites. The BaCO₃, Fe₂O₃ and different percentages of Co₃O₄ (Co) were mixed and milled for several hours by using mechanical alloying. The powder were pelletised in 11 × 1 mm (diameter × height) and the sintered at 1200 °C for 10 hours. The addition of Co^2+/3+ ions to the BaM shows a varying in the magnetic properties of BaM. By increasing the Co doping, the remanence M_r was reduced from 17.6 emu/g to 6.2 emu/g. The coercivity H_c results varying magnitude from 102 Oe to 1079 Oe. The M_r and H_c of undoped BaM is obtain at 14.6 emu/g and 860 Oe, respectively. The grain size of BaM also increases with Co doping. The densities of the compounds are decreasing with increasing Co doping with a maximum value of 4.2 g/cm³.

Abstract: This work will focus on the preparation of yttrium iron garnet (Y₃Fe₅O₁₂, YIG) via mechanical alloying technique derive by steel waste product. The Fe₂O₃ powder derived from the steel waste purified by using magnetic and non-magnetic particles (MNM) and Curie temperature separation (CTS) technique. The purified powder was then oxidized in air at 500 °C for 9 hours in air. The Fe₂O₃ was mixed with Y₂O₃ using high energy ball milling for 9 hours. The mixed powder obtained was pressed and sintered at different temperature 500/600/700/800/900/1000/1100 °C. X-ray diffraction (XRD) shows the YIG is completely form at 1100 °C. The field emission scanning electron microscopy (FESEM) images shows the grain size increases as increase the sintering temperatures. The frequency dependence on the complex permeability, µ’ and magnetic loss, µ’’ in the frequency range 10 MHz to 1 GHz were measured in this study. The results showed that the highest μ΄ is 5.890 obtained from 1100 °C.

Abstract: Steel waste product had been used as the main source of raw material in the preparation of permanent magnets ferrites. Steel waste product is an impure material that contains the iron oxide and impurities. The steel waste product is a form of flakes is grinding for several hours to form a fine powder. The iron oxide powder is separated from magnetic and non-magnetic particle using magnetic particle separation. The magnetic particle was again been purified by using the Curie temperature separation technique. The magnetic powder was carried out from the purification and oxidize at 500 °C for 6 hours at 2 °C/ mins to form the hematite, Fe₂O₃, used as a raw powder to prepare SrFe₁₂O₁₉. Microstructure of Nd-doped strontium ferrites, Sr_1-xNd_xFe₁₂O₁₉, with x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5, were prepared through a mechanical alloying technique. Several characterizations have been done, such as X-ray Diffraction (XRD) and Field emission scanning electron microscopy (FESEM). The magnetic properties of coercivity (H_c) and the energy product BH_max of samples are carried out. The magnetic properties of samples were investigated with an expectation of enhancing the magnetic properties by substitutions of Nd³⁺ ions on Fe³⁺ ion basis sites. The saturation magnetization M_s revealed magnetic behavior with respect to Nd³⁺ doping concentration, showing a decrease. The coercivity H_c increased with increasing Nd³⁺ doping concentration.

Abstract: In this paper, the structural, magnetic and electrical properties of multiferroic BaFe₁₂O₁₉/MgFe₂O₄/BaTiO₃ composites have been studied. BaFe₁₂O₁₉/MgFe₂O₄/BaTiO₃ composites were synthesized by using the conventional solid state reaction technique and sintered at different temperatures. XRD analysis confirmed the existence of hexagonal BaFe₁₂O₁₉, cubic spinel MgFe₂O₄ and tetragonal BaTiO₃ for ferrites and ferroelectric phases, respectively. The suitable sintering temperature for preparing BaFe₁₂O₁₉/MgFe₂O₄/BaTiO₃ composites is between 1000 °C and 1050 °C. SEM analysis showed that as a whole the grain size increases and the pores is reduced with sintering temperature; thus the sample became denser. The coercive field and saturation magnetization decreases when sintering temperature is increased. Resistance of the samples decrease from 103 kW to 19 kW while the capacitance increases from 0.8 nF to 4.0 nF with sintering temperature.

Abstract: This paper reports some research findings on the parallel evolutions of microstructural properties and thermal diffusivity in strontium titanate. Strontium titanate samples have been prepared via the high energy ball milling technique and subsequently moulded by a hydraulic pressing and followed by cold isostatic pressing. Nanometer-sized compacted powder samples were sintered from 500 to 1400°C using 100°C increments. Strontium titanate formation was observed at as early as 500°C sintering temperature alongside secondary phases. The full formation of strontium titanate was observed at 800°C sintering temperature and above. Average grain sizes showed a fluctuating trend with increased sintering temperatures due to carbonate decomposition at lower sintering temperatures (500 to 800°C) and grain growth phenomenon at higher sintering temperatures (900 to 1400°C). Parallel characterization of evolving thermal diffusivity showed the same trend of fluctuation at low sintering temperatures as indirect relationship but increased with increased grain size due to a lesser amount of phonon scattering. However, thermal diffusivity values decreased with increased temperatures because of increased phonon-phonon scattering.