http://hdl.handle.net/20.500.12228/299 Tue, 07 Apr 2026 06:56:04 GMT 2026-04-07T06:56:04Z http://hdl.handle.net/20.500.12228/450 Prediction of Formation and Tracks of Bay of Bengal Cyclones Alam, Dr. Md. Mahbub Rawindsonde data of 0000 UTC for standard isobaric surfaces at the surface, 850, 700, 500, 400, 300, 200, 150 and 100 hPa levels for the periods of different cyclones in the last decade were considered to study the different meteorological parameters. The vertical wind shear, the vertical variation of zonal wind speed and meridional wind speed and temperature were studied for different cyclones in the surroundings of the Bay of Bengal in relation to the movement of cyclones. The dry static energy, the latent heat energy, the moist static energy, the total energy, the meridional and zonal fluxes of moist static energy and their vertical distribution were studied in the surroundings of the Bay of Bengal in relation to the movement of cyclones and their ultimate landfall. A two dimensional model is developed for the prediction of cyclone tracks in the Bay of Bengal. The boundary value problem is then solved by using the complex potentials. The different cyclones are integrated in different time steps to find the relationship between the actual travel time and observed time by the models. The results obtained for cyclone tracks by this model are almost the same as the actual track. The cyclonic track data were collected from the Storm Warning Centre (SWC) of Bangladesh Meteorological Department (BMD). A simplified system of equations that can simulate the evolution and mature stages of tropical cyclones is presented. The study has been done using a three-dimensional numerical model constructed which produces circularly symmetric feature of the tropical cyclone. It has 5 layers in the vertical with the horizontal extent of 1200 km. The horizontal grid length of the model is 80 km. The model equations have been solved using the Leapfrog scheme. Numerical simulation shows that the model sensitivity to the vertical stability is similar to results obtained from other general models. Latent heat release associated with condensation is calculated from the moisture equation. The sensible heating parameter is added to the heating term. Initially the model atmosphere is assumed to be at rest and is then excited by a large scale heating to generate an initial vortex. The evolution of surface pressure drop, tangential wind, radial wind, heat released due to condensation, temperature anomaly, D-value, horizontal structure of vertical p-velocity, vertical structure of vertical p-velocity, vorticity and static stability parameter have been discussed based on the results obtained from the tropical cyclone modelling experiment. This thesis is submitted to the Department of Physics, Dhaka University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, December 1998.; Cataloged from PDF Version of Thesis.; Includes bibliographical references (pages 234-241). Tue, 01 Dec 1998 00:00:00 GMT http://hdl.handle.net/20.500.12228/450 1998-12-01T00:00:00Z http://hdl.handle.net/20.500.12228/319 Temperature Dependence of Magnetization and Induced Magnetic Anisotropy of Some Fe, Co and Ni-Based Amorphous Ribbons Sikder, Shibendra Shekher Ferromagnetic amorphous ribbons of Fe-Si-B, Ni-Fe-B and Co-Fe-B-Si series have been prepared by rapid quenching technique and the amorphousity of the samples has been confirmed by X-ray diffraction technique. The kinetics of glass formation and crystallization as affected by a slight change in composition is studied by differential thermal analysis (DTA). It is observed that the thermodynamics of the amorphous ribbons in respect of the formation of glassy state and its stability is affected by the complexity of the composition. The magnetic ordering of Fe-based, Ni-based and Co-based amorphous ribbons is studied by measuring A.C. and D.C. magnetization as functions of temperature. D.C. measurements are made by using a vibrating sample magnetometer and A.C. initial permeability is measured, using a furnace in which the heating wire is wound in accordance with the bifiller technique. Coercivity, remanence, saturation magnetization and maximum permeability have been determined as static magnetic properties for Fe-based, Ni-based and Co-based specimens from the hysteresis loops obtained by Ballistic method. The dynamic characteristic such as the real and imaginary components of the complex permeability in A.C. conditions have been measured as a function of the instantaneous value of a sine wave core current density by means of an adapted LCR bridge method. Initial permeability, frequency dependence of complex permeability and relative quality factor have been measured in the frequency range 0.5KHz to 13MHz for all the different compositions and for annealing effect of initial permeability. The measurements of saturation specific magnetization of Fe-based, Ni-based and Co-based amorphous ribbons have been measured by V.S.M. We have correlated the behaviour of magnetic moment in amorphous alloys containing 3d transition metals on the basis of rigid band model, assuming that the metalloid atoms contribute some of their S electrons and P electrons to fill the d band of the transition metal. This accounts for the reduction of magnetization of all the amorphous ribbons with increasing metalloid content. The temperature dependence of magnetization, of amorphous system decreases faster with increasing temperature as compared to crystalline materials. According to the mean field approximation, reduced magnetization versus reduced temperature graphs for different Fe-based, Ni-based and Co-based amorphous ribbons leads to fluctuation in the exchange interaction giving rise to a structural disorder. Induced magnetic anisotropy of amorphous ribbons having Fe-based, Ni-based and Co-based system have been measured for temperature and field dependence using torque magnetometer with proportional integrating differentiating compensation. The results of compositional dependence of initial permeability, coercive force and induced anisotropy are found to be mutually consistent for all the samples. The origin the uniaxial anisotropy in these amorphous materials is assumed to be the stress, induced during the preparation. The temperature dependence of uniaxial anisotropy is caused by stress, relieving and decreases with the increase of temperature. The corresponding curves of In [Ku(0)/ Ku(T)] versus In [σs(0)/ σs(T)] in amorphous ribbons with Fe-based, Ni-based and Co-based system follow straight lines arising from the disorder caused by randomly oriented easy axes. The values of the exponents are different for the different samples. All amorphous ribbons have been investigated using Mössbauer spectroscopy. The Mossbauer measurements were performed with a conventional constant acceleration spectrometer at room temperature in transmission geometry, using 57Co in a rhodium matrix. The velocity scale of a thin 57Fe sample and the isomer shifts are given relative to the centroid of the spectrum. The isomer shifts of Fe-atoms in all the Fe, Ni and Co-based samples behave as trivalent ions. The spectrum for the as prepared samples consists of six lines which are broadened as compered with the Mössbauer spectrum of a pure 57Fe sample in the form of a thin foil. The curve for the experimental spectrum is obtained by using the best fit of the distribution of hyperfine fields. The consistency between the average magnetization measured by V.S.M. and the magnetic moment of the Fe-atom as estimated from the hyperfine field distribution confirms the collinearity of the magnetic ordering of the sample. This thesis is submitted to the Department of Physics, Bangladesh University of Engineering & Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics, July 1999.; Cataloged from PDF Version of Thesis.; Includes bibliographical references (pages 224-232). Thu, 01 Jul 1999 00:00:00 GMT http://hdl.handle.net/20.500.12228/319 1999-07-01T00:00:00Z http://hdl.handle.net/20.500.12228/307 Electromagnetic Properties of Iron deficient Ni-Cu-Zn Ferrites and the influence of additives as sintering AID Khan, Md. Zakir Hossain The present work is focused on the influence of substitutions and sintering aids on electromagnetic properties of Fe-deficient Ni-Cu-Zn ferrite. Three series of ferrite samples of the compositions (Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x (where x=0.00, 0.02, 0.04, 0.06 & 0.08), Ni0.28Cu0.10+Zn0.62-x Fe1.98O4 (where x=0.00, 0.03, 0.06, 0.09 & 0.12) and Ni0.28Cu0.10Zn0.62Fe1.98O4 doped with V2O5 and Li2O were prepared by using the solid state reaction technique. The phase identification was carried out by using the X-ray diffraction. The X-ray diffraction analysis revealed that all the samples of the studied three series were found to crystallize in single-phase cubic spinel structure. A slight increase of Curie temperature, Tc and saturation magnetization, Ms of (Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x series with increasing iron deficiency were observed. Lattice parameter of Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x series slightly decreases with x content. Initial permeability was found to increase with increasing iron deficiency and also with increasing sintering temperature which may be attributed to enhanced density and reduced anisotropy energy as well as increase in grain size of the studied samples. The permeability spectrum with frequency follows the Snoek's limit. From hysteresis parameters it was revealed that optimum soft magnetic properties corresponds to the composition with x=0.08 sintered at Ts=1150°C having highest permeability of 358, maximum induction, minimum coercivity and hysteresis losses. From the detail study on the magnetic properties of Ni0.28Cu0.10+xZn0.62-xFe1.98O4 series, it appeared that optimum soft magnetic properties were found in the composition with x=0.09 with the manifestation of highest permeability of 586 with low coercivity of 0.75 Oe and hysteresis loss of 15.5 W/kg. Saturation magnetization, M increases with x content with high value of 61.8 emu/g for x0.09. Lattice parameter decreases linearly with increasing Cu content while Curie temperature increases linearly with Cu content. The decrease of Curie temperature may be attributed to the strengthening of A-B (JAB) exchange interaction. Initial permeability increases gradually with increasing Cu content and sintering temperature, Ts attaining a maximum value for x0.09 at Ts=1200°C. This enhancement of permeability may be correlated with improved microstructural features. The effect of dopants such as V2O5 (0.4 wt %) and Li2O (0.4 wt %) on the composition Ni0.28Cu0.10Zn0.62Fe1.98O4 with Ts=1150°C revealed that a substantial increase of initial permeability from 284 to 476 for V2O5 and 380 for Li2O is observed. This may be accounted for due to increased density and grain size. A noticeable decrease of coercivity and increase of magnetization of the doped samples have been observed resulting in high permeability. This thesis is submitted to the Department of Physics, Khulna University of Engineering & Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics, January 2013.; Cataloged from PDF Version of Thesis.; Includes bibliographical references (pages 171-182). Tue, 01 Jan 2013 00:00:00 GMT http://hdl.handle.net/20.500.12228/307 2013-01-01T00:00:00Z http://hdl.handle.net/20.500.12228/304 Study of Additive Effects on the Structural, Magnetic and Transport Properties of Cobalt Ferrites Noor, Saroaut A detailed study was carried out on four different series of spinel Co-based ferrites, with composition Co1-xCdxFe2O4 and Co1-xZnxFe2O4 (where x = 0.0 - 1.0 in steps of 0.1), rare-earth (RE) substituted diluted system Co0.2Cd0.8 Fe2-xRExO4 (RE = Ho and Sm; x = 0.05, 0.1) and Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu: x = 0.05, 0.1). The samples were prepared by conventional double sintering ceramic technique and were found to be single-phase cubic spinel structure of Co-Cd and Co-Zn series by X-ray diffraction technique, while all rare-earth (RE) doped samples showed additional peaks other than spinet. The lattice constant of Co1-xCdxFe2O4 and Co1-xZnxFe2O4 ferrites increase linearly with the increase of Cd and Zn content, followed Vegard's law. Bulk density and X-ray density increases significantly with the increase of Cd and Zn content. Curie temperature, Tc decreases almost linearly with increasing x content up to x =0.7. Samples with x > 0.8 show paramagnetic characteristic at T = 5K with no spontaneous magnetization that revealed through Arrot-Belov-Kouvel plots. The continuous decrease of Tc with the substitution of non-magnetic Cd and Zn in Co1-xCdxFe2O4 and Co1-x ZnxFe2O4 system is attributed to progressive weakening of the strength of A-B intersublattice exchange interaction, JAB. Small thermal Hysteresis of initial permeability, μ' is observed below Te. Saturation magnetization, Ms and magnetic moment are observed to increase upto x = 0.4 - 0.5 and decrease thereafter due to the spin canting in B-sites. The change of saturation magnetization with the increase of Cd/Zn substitution has been explained on the basis of Ned's collinear two-sublattice magnetization model and Yafet-Kittel's non-collinear magnetization model. The initial permeability is found to increase with the increase of Cd2 and Zn2 ions upto x = 0.6. Temperature dependence of low field DC magnetization in the field-cooled and zero-field cooled conditions are performed for the dilute Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 (x = 0.7, 0.8, 0.9, 1.0) spinet ferrites. Samples with x = 0.7 and x = 0.8 show re-entrant spin-glass behavior while the samples, x = 0.9 and x = I show the spin-glass behavior. Frequency dependent complex AC susceptibility measurement as a function of temperature as the samples CdFe2O4 and ZnFe2O4 indicates a spin-glass behavior with the manifestation of shift of spin freezing temperature. The samples also show a typical spin-glass behavior with a manifestation of non-equilibrium dynamics, such as aging, rejuvenation, and memory effects. Large magnetic hysteresis effect has been observed at low temperature for the diluted ferrite compositions. Low field B-H loops, at room temperature, were measured at constant frequency. The hysteresis behavior of Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 reveals the softer ferromagnetic nature of the studied materials with the increase of Cd and Zn content. DC electrical resistivity increases significantly with the increase of Cd and Zn content. The AC resistivity and dielectric constant, ε`, of the samples are found to decrease with increasing frequency, exhibiting normal ferrimagnetic behavior. Dielectric constant, c, decreases rapidly with the increase in frequency at lower frequencies and slowly at higher frequencies, which may be due to the Maxwell-Wagner interlacial polarization. The variation of electrical and dielectric properties is explained on the basis of Fe2+ /Fe3+ ionic concentration as well as the electronic hopping frequency between Fe2+ and Fe3+ ions. Rare-earth (RE) doping of Ho3+ and Sm3+ in Co0.2Cd0.8Fe2-xRExO4 ferrite shows some anomalous behavior. A minority second ferromagnetic phase having well defined Tc has been detected Ho3+ doped sample, where Tc is found to increase with Ho3+ content. This ferromagnetic phase may be assigned as ferromagnetic cluster containing Ho3+. Magnetization is found to increase with Ho3+ addition having higher magnetization for higher Ho content while Sm3+ addition reduces magnetization. The decrease of magnetization with Sm3 substitution for Fe3+ may be explained as due to lower free ion magnetic moment of Sm3+. Samples Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu) show substantial increase of magnetization with Gd3+ which attains higher value with higher content of Gd3+ while decreasing with Eu3+ doped sample. The decrease of magnetization for Eu3+ doping is due to nonmagnetic nature of Eu which has no magnetic moment of the free Eu3+ ion even at low temperature. This thesis is submitted to the Department of Physics, Khulna University of Engineering & Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics, April 2011.; Cataloged from PDF Version of Thesis.; Includes bibliographical references (pages 170-179). Fri, 01 Apr 2011 00:00:00 GMT http://hdl.handle.net/20.500.12228/304 2011-04-01T00:00:00Z