Parametric optimization of NiFe2O4 nanoparticles synthesized by mechanical alloying

In this study, the Taguchi robust design method is used for optimizing ball milling parameters including milling time, rotation speed and ball to powder weight ratio in the planetary ball milling of nanostructured nickel ferrite powder. In fact, the current work deals with NiFe₂O₄ nanoparticles mechanochemically synthesized from NiO and Fe₂O₃ powders. The Taguchi robust design technique of system optimization with the L9 orthogonal array is performed to verify the best experimental levels and contribution percentages (% ρ) of each parameter. Particle size measurement using SEM gives the average particle size value in the range of 59–67 nm. X-ray diffraction using Cu Kα radiation is also carried out to identify the formation of NiFe₂O₄ single phase. The XRD results suggest that NiFe₂O₄ with a crystallite size of about 12 nm is present in 30 h activated specimens. Furthermore, based on the results of the Taguchi approach the greatest effect on particle size (42.10 %) is found to be due to rotation speed followed by milling time (37.08 %) while ball to powder weight ratio exhibits the least influence.     

Magnetic M–H loops family characteristics in the microstructure evolution of BaFe12O19

A polycrystalline magnetic materials series produced from the same batch of high-reactivity powder by multi-sample sintering in a range of increasing temperatures can supposedly reveal its property and microstructural evolution through magnetic property measurement and clear morphological changes. Hence, in our work, we attempt to find out the variation of magnetic properties of BaFe₁₂O₁₉ against its evolving microstructure. Hexagonal BaFe₁₂O₁₉ nanometer-sized powder with the M-type structure was synthesized by the mechanical alloying method. The crystal structure, grain size and magnetic properties were studied by means of XRD, field emission scanning electron microscopy and a vibrating sample magnetometer. The ferrite materials were obtained from a mixture of barium carbonate and iron oxide by mixing them using conventional ball milling (12 h) and then assisted by high energy ball milling for 6 h. The comminuted powder was divided into several batches, moulded in pellet shape and sintered at different temperatures ranging from 400 to 1400 °C for a constant sintering time of 10 h in a static air atmosphere. Effects of sintering temperature on the formation, crystallite size, morphology and magnetic properties were systematically studied. In the sintering temperature range from 800 to 1200 °C, the coercivity (Hc) gradually increased due to the effect of rapid grain growth. This is because the fine starting powder was so reactive that rapid grain growth easily occurred. In the grains, strong interaction of magnetic moments within domains due to exchange forces led to strong anisotropy. It was also observed that three M–H-loop groups each belonging to a particular range of grain sizes exhibited different strengths of magnetism. It is believe of similar results have never been reported before and should be useful to the permanent-magnet industry.     

Structural,morphological and magnetic properties of hydrothermally synthesized ZnFe2O4 nanoparticles

Zinc ferrite (ZnFe₂O₄) nanoparticles were synthesized by a surfactant assisted hydrothermal method using different concentrations of ethylamine (EA) namely, 2, 4, 6, 8 and 10 ml. The powder X-ray diffraction measurements revealed that the amount of EA plays an important role in the formation of single phase ZnFe₂O₄ nanoparticles. The amount of 2 and 4 ml of EA yielded mixed phases of α-Fe₂O₃ and ZnFe₂O₄ whereas 6 ml of EA produced well crystalline and single phase ZnFe₂O₄ with regular spinel structures. Field emission scanning electron microscopy images revealed that ZnFe₂O₄ possess spherical shape, irrespective of the concentrations of EA. Magnetic characterizations revealed that the synthesized samples with EA concentrations 6, 8, 10 ml were superparamagnetic in nature at room temperature.     

Synthesis of superparamagnetic ZnFe2O4 nanoparticle by surfactant assisted hydrothermal method

Zinc ferrite (ZnFe₂O₄) ultrafine powder was synthesized by hydrothermal method using various amounts of cetyltrymethylammonium bromide (CTAB) as a surfactant. The phase purity, thermal stability, morphology, size and magnetic properties of the final products were studied. All the synthesized products were possessing normal spinel structure without any impurities. The crystalline size of synthesized products decreased with the increasing amounts of surfactant. Particles had narrow size distribution with an average particle size of 6.5 nm for 1.5 g of CTAB. Magnetic characterizations revealed that the synthesized products were superparamagnetic in nature.     

Effect of microwave sintering on microstructural and magnetic properties of strontium hexaferrite using sol–gel technique

The sol–gel method is used to prepared hexaferrite using d-Fructose as a fuel. The effect of sintering temperature on the microstructure of SrFe₁₂O₁₉ ceramics is analyzed. The observed XRD results indicate a well-formed crystalline phase of dense hexagonal SrFe₁₂O₁₉ ceramics. From this analysis, no secondary phases are identified. The microstructure of the sintered single phase M-type ferrites ceramics displays a hexagonal-platelet like morphology. Sintering temperature can markedly affect the grains in sintered ferrite. The sintered product is shown to be dense microstructure with relatively small grains. The maximum sintered density 95 % was obtained at lower temperature of 1,150 °C. In addition, saturation magnetization (50.43 emu/g) and the coercivity (H_c) 5,594.53 Gauss were observed.     

Reactivity Aspects of SBA15-Based Doped Supported Catalysts: H2O2 Direct Synthesis and Disproportionation Reactions

Pd and PdAu catalysts supported on SBA15 and SiO₂ were prepared and investigated for H₂O₂ direct synthesis in a batch autoclave (10 °C and 17.5 bar) and in the absence of halides and acids. The SiO₂ supported catalysts exhibited inferior performances compared to the mesoporous ordered SBA15. A good control of both the catalysts dispersion and nanoparticle stability was achieved using SBA15. Catalysts were doped with bromine, a promoter in the H₂O₂ direct synthesis. Productivity and selectivity decreased when bromine was incorporated in the catalysts, thus indicating a possible poisoning due to the grafting process. A synergetic effect between Pd and Au was observed both in presence and absence of bromopropylsilane grafting on the catalyst surface. Three modifiers of the SBA15 support (Al, CeO₂ and Ti) were chosen to elucidate the influence of the surface properties on metal dispersion and catalytic performance. Higher productivity and selectivity were achieved incorporating Al into the SBA15 framework, whereas neither Ti nor CeO₂ improved H₂O₂ yields. The enhanced performance observed for the Prau/Al–SBA15 catalysts was attributed to the increased number of Brønsted acid sites. A modification of this catalyst with bromine was confirmed to impair both productivity and selectivity, possibly due to the broader particle size distribution and the poor stability of the metal nanoparticles, as demonstrate by transmission electron microscopy (TEM) images. H₂O₂ disproportionation was also investigated. A much slower reaction rate was observed compared to the H₂O₂ production, suggesting that the major contributor in the process of H₂O₂ destruction must be connected to the hydrogenation reaction.     

Study of structural,morphological and magnetic properties of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Fe<Subscript>12</Subscript>O<Subscript>19</Subscript>

Single-phase barium Strontium hexaferrite (Ba_0.5Sr_0.5Fe₁₂O₁₉—BSF) was synthesized by sol–gel method using metal nitrates as source and d-Fructose as a fuel. The phase formation, surface morphology and magnetic properties of the samples were analyzed by X-ray Diffraction, High-resolution Scanning Electron Microscope (HR-SEM) and Vibrating Sample Magnetometer (VSM). X-ray analysis indicates that the sintered samples were remained in hexagonal structure. The densities of the sintered samples at 1,150 °C were found to be 93% of theoretical density. HR-SEM and VSM studies reveal that the sintered samples were resulted in hexagonal structure with good magnetic properties. The average diagonal of the grains varies from 0.95 to 1.7 μm. The thermal treatment effects the growth of the hexagonal grains of ferrites.     

Rapid Hydrogen Generation from the Reaction of Aluminum Powders and Water Using Synthesized Aluminum Hydroxide Catalysts

A synthesized and nano-sized Al(OH)₃ powder that promotes the generation of hydrogen from a Al/water reaction is demonstrated. In this study, aluminum hydroxides are synthesized using sodium aluminate NaAlO₂, distilled water and ethanol. The mole ratio of ethanol/water and the concentration of sodium aluminate in solution affect the crystal structure, morphology and sizes of the Al(OH)₃ powders significantly. These Al(OH)₃ powders contain both gibbsite and bayerite phases and exhibit excellent catalytic power on the hydrogen generation of Al/water system. It is proposed that two major characteristics of Al(OH)₃ powders dominate the catalytic power. That is, the surface area and the high-energy sites of Al(OH)₃. When mole ratio of ethanol/water is between 0.3–0.6 and the concentration of NaAlO₂ is higher than 0.0167 g/ml, the synthesized Al(OH)₃ powders are in a more gibbsite-oriented and plate-like structure. Other than above conditions result in a more bayerite-oriented and particulate-like structure. The plate-like structure exhibits strong catalytic power due to the existence of high-energy sites on the edge of plates even its surface area is not so high. The particulate-like structure may also have strong catalytic power when it has a high surface area. By taking advantage of the exothermic reaction, ~?100% yield of hydrogen can be produced from 1 g Al/10 g water system within 30 s using 3 g synthesized Al(OH)₃. A aluminum waste scrap can also react with water using these effective catalysts and generate?~?95% yield of hydrogen within 8 min.