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Abstract
Nanocrystalline materials are showing great prospects in industry and technology. This is mainly because they have some unique properties which are not shown by the bulk crystalline materials. Nanomaterials represent almost the ultimate in increasing surface area. Substances with high surface area have enhanced chemical, mechanical, optical and magnetic properties, and this can be exploited for a variety of structural and non-structural applications. Preparation of nanocrystalline SHF SrFe12O19 1. Nano-crystallite strontium hexaferrite was prepared by self flash combustion of acetate precursors. It was found that the used mole ratio of the mixed iron acetate and strontium acetate affects the produced phase of strontium ferrite. Using (1 mole strontium acetate: 2 moles of iron acetate) a double phase of strontium monoferrite (50%) and strontium hexaferrite (50%) was obtained. Single phase strontium hexaferrite (100%) was obtained by using the precursor’s mole ratio (1 mole strontium acetate: 8 moles of iron acetate). 2. Magnetic properties were measured by vibrating sample magnetometer showing promising values for Hc, Br and Bs. Single phase strontium hexaferrite, SrFe12O19, prepared at 850 oC, showed the best magnetic properties, Bs = 57.16 emug-1, Br = 30.45 emug-1 and Hc = 4747 Oe . 3. The catalytic activity of the prepared strontium hexaferrite was revealed by studying its photocatalytic effect on the degradation of Toludine Blue dye in 200 wt tungsten lamp as a source of visible light. The complete photocatalytic degradation of the dye was achieved at 20 hrs using the single phase hexaferrite. While, the complete photocatalytic degradation of the dye was achieved at 10 hrs using the double phase monoferrite and hexaferrite. The photocatalytic activity was followed up by UV-VIS spectrophotometry. (2) Phase and conductivity dynamics of SHF nanocrystals in a hydrogen gas flow 1. The phase and electrical conductivity dynamics of self-flash combustion SHF nanocrystal compacts were studied in a hydrogen gas flow at atmospheric pressure and at different temperatures. Electric conductivity-temperature dependent measurements of SHF in air showed that the nanocrystal compacts have semiconductor behaviors with transition temperature in conduction that coincide with its magnetic Curie point. Electric conductivity measurements in a hydrogen gas flow showed that the material is n-type with electrons as majority carriers. During hydrogen gas exposure, oxygen from the surface as well as from the bulk of the SHF nanocrystal is removed as water molecules resulting in the formation of different phases of strontium–iron oxides, iron oxides and iron metal depending on the operating temperature. Formation of different oxide phases and transformation of these oxides from valance state to another increase the ionic conductivity as well as the electronic conductivity and hence the total conductivity is increased. 2. The temporal changes of conductivity as well as the formed phases at partial and complete reduction were found to be significantly affected by the operating temperature. The total reduction times of the SHF nanocrystals were found to depend inversely on the reduction temperature; it is shorter at higher temperature. Nanocrystals reduced at higher temperatures showed the formation of metallic iron with increasing sizes responsible for higher electric conductivity during reduction as shown in XRD patterns of partially reduced nanocrystals. 3. The curve of the conductivity change with time during hydrogen exposure showed three regions consistence with the desorption of the adsorbed oxygen gas at early reduction stage, followed by reduction of the surface layers of the nanocrystals, and finally the bulk interior of the nanocrystals is reduced at longer exposing time. For nanocrystals reduced at reduction temperature of 400 °C two reduction regions could be identified, whereas those reduced at higher temperatures showed three regions. Calculating the activation energy for each reduction region indicates that oxygen desorption region follows a chemical reaction controlled mechanism, while surface and bulk reduction regions are of combined gas diffusion and interfacial chemical reaction controlled mechanisms. Thermogravimetry measurements confirmed these activation energies and reduction controlled mechanisms for hydrogen-SHF nanocrystal interaction. (3) Kinetics of acetylene decomposition over reduced strontium hexaferrites catalyst for the production of carbon nanotubes 1. Catalyst of the composition 40Strontium hexaferrites SHF (SrFe12O19): 60Al2O3 was prepared by wet impregnation method. Carbon nanotubes were synthesized over the prepared catalysts by the catalytic decomposition of acetylene at different reaction conditions. 2. The kinetics of synthesis of CNTs were investigated through two types of experiments, the first was done at constant reaction time 30 min. and rate gas flow of 10 C2H2: 90 H2, samples were reduced at 500-650 oC and subjected to C2H2 flow at each temperature. The optimum conditions for the higher yield % were found to be 600 oC which gave 367 % 3. The second type of experiments is done at variable decomposition temperature 500-800 oC and constant reduction temperature (600 oC). The highest yield % was found at reduction and decomposition temperature 600 and 700 oC respectively. 4. The activation energies for the first and second experiments were found to be 26.3 and 5.2 kJmol-1 respectively, so the adsorption of acetylene on catalyst surface is physiosorption process. 5. from the adsorption isotherm, the catalyst is considered as type II isotherms. This type of isotherm represents unrestricted monolayer-multilayer adsorption, and it considers as type C of hysteresis loopswhich is produced by wedge-shaped pores with open ends. 6. The presence of catalytic nanoparticles at the tip of the produced CNTs suggests that the CNT production occurred via a tip-growth mechanism.