Department of Physics

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A series of FeIII complexes of stoichiometry [FeLX3].nH2O have been synthesized, where X = Cl-, HCO-2 and L is a flexible bidentate ligand, 1,3(bis-benzimidazolyl) propane and 1,4(bis-benzimidazolyl)butane. Mössbauer data reveals that the isomer shift values lie in the range typically observed for high spin FeIII complexes, while the slightly large quadrupole splitting parameter indicates a rhombically distorted FeIII centre. Cyclic voltammetric studies reveal that the E1/2 for the FeIII/FeII couple for the formate complex shifts more cathodically than for the chloride complex; implying that HCO-2 is more effective at lowering the Lewis acidity of the FeIII centre.

World University root node

Transition Metal Chemistry

A Mössbauer and electrochemical study of mononuclear iron(III) complexes with benzimidazole-based flexible bidentate ligands

The anodic response of two Fe-Si-B-Cu-Nb alloys was investigated in buffered chloride and fluoride media through polarisation studies and corrosion product analysis. It was found that the anodic reactions and dissolution of the alloys were dependent on the alloy compositions as well as the state (amorphous or crystalline) of the alloy substrate. It was observed that crystallisation resulting in the formation of the ordered Fe80Si20 phase only, was beneficial to corrosion resistance in chloride media, however the presence of secondary intermetallic phases was detrimental. The anodic responses in the fluoride media were dominated by the formation of fluorosilicate. The anodic response of two Fe-Si-B-Cu-Nb alloys was investigated in buffered chloride and fluoride media through polarisation studies and corrosion product analysis. It was found that the anodic reactions and dissolution of the alloys were dependent on the alloy compositions as well as the state (amorphous or crystalline) of the alloy substrate. It was observed that crystallisation resulting in the formation of the ordered Fe80Si20 phase only, was beneficial to corrosion resistance in chloride media, however the presence of secondary intermetallic phases was detrimental. The anodic responses in the fluoride media were dominated by the formation of fluorosilicate.

Corrosion Science

Anodic reactions of amorphous and devitrified Fe-B-Si-Nb-Cu alloys in buffered chloride and fluoride

The room-temperature synthesis of nanocomposites of Mn-Zn spinel ferrites in a copolymer matrix was examined by X-ray diffraction analysis, Moessbauer spectroscopy, and infrared spectroscopy. The Moessbauer and X-ray diffraction results show that the concentration of Zn ions plays a very important role in controlling the size of magnetic particles and hence the magnetic properties of the nanocomposites. Heating the samples at 450 °C results in the formation of fine particles of Mn-Zn ferrites still retaining their nanometer size.

Nanostructured Materials

Room temperature synthesis of nanocomposites of Mn-Zn ferrites in a polymer matrix

Synthesis of nanocomposites of mixed oxides of Fe and Cr in a copolymer matrix of aniline and formaldehyde at room temperature along with IR, XRD and 57Fe Mossbauer studies on as synthesized as well as the samples on heating at different temperatures is described. The XRD and 57Fe Mossbauer studies show the formation of nanosized iron oxide particles. These studies further show the formation of nanosized particles of metal oxides and formation of solid solution of iron and chromium oxide on heating the samples at 700°C. Further, the IR studies show that the polymeric backbone is strongly influenced by different reaction conditions and lead to variable magnetic character in the heated samples as shown by their Mossbauer studies.

Fulltext

Bulletin of Materials Science

Magnetic properties of nanocomposites of mixed oxides of iron and chromium synthesized under different oxidative environments

Scripta Materialia

Changes in electrochemical responses of some Fe-B-Si-Cu-Nb alloys before and after devitrification

The crystallization and magnetic properties of Fe72.5Nb4.5Cu1Si10B12, Fe73.5Nb3Cu1Si13.5B9 and Fe72Nb4.5Cu1Si13.5B9 alloys have been studied. After primary crystallization, the α-Fe(Si) phase was formed for the alloy which has 10 at.% Si. The ordered Fe80Si20 phase was formed for alloys containing 13.5 at.% Si. The activation energy for the formation of the α-Fe(Si) phase was 67 kcal mol1, which is lower than that for the formation of the ordered phase. In the scannins temperature variation or resistivity measurement, a drop in resistivity was found during the formation of the α-Fe(Si) phase whereas a rise in resistivity was observed during primary crystallization for the other two alloys. Grain-boundary scattering and the low-temperature coefficient of resistivity were the causes of the rise in resistivity during the formation of the ordered phase. The Fe72Nb45Cu1Si13.5B9 alloy exhibited superior soft-magnetic properties among the measured alloys when annealed between 875 and 875 K for 30 min. The rapid deterioration in the magnetic properties at higher temperatures of the annealed alloys was primarily due to the formation of the Fe2B phase, which has a strong magnetocrystalline anisotropy energy. © 1998 Taylor &amp; Francis Group, LLC.

Philosophical Magazine B: Physics of Condensed Matter; Statistical Mechanics, Electronic, Optical and Magnetic Properties

Crystallization and magnetic behaviour of Fe-Nb-Cu-Si-B alloys

The dependence of magnetic Barkhausen emissions on magnetic field, frequency and applied tensile stress in positive magnetostrictive amorphous Fe82Si8B10 ribbon has been studied. The Barkhausen voltages (rms and peak-to-peak) and the number of events increased with increasing field amplitude and reached a constant saturation value at higher field amplitudes. This saturation voltage was a linearly increasing function of applied tensile stress over the range of stress studied. The Barkhausen voltage and number of events were also dependent on the frequency of the applied magnetic field. In these cases the number of Barkhausen events decreased with frequency, but the rms Barkhausen voltage increased linearly with frequency. A model theory has been developed assuming Barkhausen emissions originated from stochastic motion of domain walls, and it is shown that the instantaneous Barkhausen voltage is proportional to the number of Barkhausen events, the average size of the discontinuous change of magnetization, the rate of change of applied magnetic field and the irreversible differential susceptibility. The effect of stress on the Barkhausen parameters is explained assuming as-quenched amorphous alloy consists broadly of transverse and longitudinal domain orientations. An expression for the irreversible susceptibility, which is a function of stress, is presented.

Journal of Magnetism and Magnetic Materials

Magnetic Barkhausen emissions in as-quenched Fe-Si-B amorphous alloy

Micromagnetic Barkhausen emissions have been measured from a number of specimens of Cr-Mo steel in order to study the effects of creep. Creep causes changes in the microstructure of materials as a result of exposure to a high temperature and pressure. Scanning electron micrographs showed that all the specimens were at an early stage of creep. The number density of cavities in specimens which were taken from the outer surface of the pipe was lower than in specimens taken from the inner surface, indicating lower creep on the outer surface. Rms and peak-to-peak voltages of Barkhausen emissions were found to increase from the outside to the inside of the pipe, indicating that these emissions increase with increasing level of creep. Segregation of impurities, which takes place during creep, was considered to be the primary cause of the increase in Barkhausen emission signals. A model has been developed which showed that Barkhausen voltage should increase up to a critical value of the number density of cavities, beyond which it should decrease. As the specimens measured in this study were in the initial stage of creep, only an increase in Barkhausen voltage (rms and peak to peak) was observed with creep across the range of specimens investigated. In the experimental measurements an angular variation in rms voltage was also observed which was explained by the orientation of the easy magnetization axis due to the development of long-range residual stresses in a direction different from the pipe axis. © 1997 Taylor & Francis Ltd.

Philosophical Magazine A: Physics of Condensed Matter, Structure, Defects and Mechanical Properties

Micromagnetic barkhausen emissions in 2-25 wt% Cr-1 wt% Mo steel subjected to creep

The saturation magnetization and switching characteristics of amorphous Fe77.5Si7.5B15 fibres were investigated after hydrogen incorporating by the electrochemical method. The switching field (H *) increased from its as-received value (5.57 Am-1) to 18.62 Am-1 after charging with a current density (Ich) of 30 mA cm-2 for 2 h. The saturation magnetization (Ms) decreased with hydrogen content up to a critical concentration corresponding to Ich = 60 mA cm-2 after 2 h of charging. At this critical current density, Ms decreased from 1.57 T, its as-received value, to 0.89 T. The results are explained by considering various kinds of interaction of hydrogen with metals, such as electron exchange, volume dilation, etc.

Influence of hydrogen on the magnetic properties of amorphous Fe-Si-B fibres

A novel approach for in-situ room temperature synthesis of magnetic nanocomposite of γ-Fe2O3 is described. 57Fe Mössbauer spectroscopy has been used to study its magnetic behavior. Observation of a relaxed six line spectrum at room temperature and increase in hyperfinefield values at 77 K are typical characteristics of magnetic microcrystalline particles, interestingly, the studies at high temperature also show an increase in hyperfine magnetic field values, though it is quite unusual for small magnetic particles. This unusual behavior may, however, be attributed to some new kind of interaction between magnetic particles and the polymer matrix, or between magnetic particles themselves caused by conformational changes in the copolymer matrix at higher temperatures. Moreover, these studies clearly indicate that both the polymer matrix and zinc ions play definite roles in the clustering of γ-Fe2O3 particles, resulting in the phenomenon of relaxation over a wide range of temperatures. ©1998 Acta Metallurgica Inc.