PhD Theses (Physics)

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    Studies on certain vanadium pentoxide based semiconducting oxide glasses exhibiting majority charge carrier reversal phenomenon
    (2002) Sharma, Bachaspatimayum Indrajit
    Vanadium Pentoxide (V2O5) based glasses form an important class of amorphous semiconductors. V2O5 based glasses are normally n-type semiconductors. The possibility of obtaining p-type semiconductors in some V2O5 based trenary glasses has been pointed out in the literature. After a careful study of the glass-forming region of known V2O5 based glassed systems coupled with the thermoelectric power (TEP) measurements, three glass systems, namely, x V2O5.20SnO.(80-x)TeO2 glasses (18<_ x<_50), x V2O5.40CaO. (60-x) P2O5 glasses (10<_x<_30) and x V2O5.40CaO.(60-x)B2O3 glasses (10<_x<_30) were identified for the present investigations. These three glass systems exhibit a reversal in the carrier type from n-type to p-type when the composition x was systematically varied. Bulk glass samples belonging to the three glass systems were prepared by melt quenching technique. The as-quenched samples were characterized using X-ray diffraction technique, TEP measurement and differential scanning calorimeter (DSC). TEP measurements revealed that the career reversal occurs near 23 mol%V2O5 in V2O5-SnO-TeO2 glasses. In V2O5-CaO-P2O5 and V2O5-CaO-B2O3 glasses, the career reversal was observed at 20 mol%V2O5.
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    Studies on bulk and thin films of lead (Pb) modified chalcogenide glasses
    (2002) Pattanaik, Akhyaya K
    Chalcogenide glasses contain group VI (A) elements of the periodic table such as Sulphur, Selenium or Tellurium as their major constituent. Chalcogenide glasses are known to be p-type semiconductors. It was reported that Ge-Se glasses containing appropriate amounts of Bismuth (Bi) or Lead (Pb) exhibited n-type conduction. Bi modified chalcogenide glasses have received more attention in the literature as compared to Pb modified chalcogenide glasses. In this thesis, a systematic study of the electrical, thermal and optical properties of bulk and thin films of Pb modified chalcogenide glasses is reported. The basic interest in chalcogenide glasses exhibiting p-type to n-type charge carrier reversal and the general lack of information on thin films of these glasses are the motivations behind this work.
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    Study of N=1 and N=2 supersymmetric integrable hierarchies and their soliton solutions
    (2002) Sarma, Debojit
    Integrable models have seen major advances in the last four decades bringing together various branches of mathematics and having diverse physical applications. Of great interest is the realization that integrable systems give rise to nondispersive, localized travelling wave solutions called solitons. The task of constructing such solutions have resulted in the development of many different techniques to solve these systems, which are non-linear, partial differential equations and difficult to solve analytically. Another characteristic of integrable models is that they are Hamiltonian systems and their Hamiltonian structures are closely related to the conformal algebras. This was made clear when it was shown that the second Hamiltonian structure of the KdV hierarchy is isomorphic to non-linear algebras known as the Wn algebras. Later on the symmetry structures associated with the KP hierarchy, which includes the KdV hierarchy have been investigated
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    Electrical Resistivity And Ac Susceptibility Studies In Perovskite Manganites
    (2004) Kar, Manoranjan
    Materials based on manganese oxides are generally known as manganites. The manganites RMnO3 (R= Trivalent rare earth elements) and AMnO3 (A= Divalent alkaline earth elements) are known to be insulating with antiferromagnetic (AFM) ordering. Even though the mixed valence manganites, R1-xAxMnO3 (x <= 0.50) are known to exhibit ferromagnetic (FM) to paramagnetic and metal-insulator transitions as early as 1950, the interest in this type of compounds was renewed only after the discovery of large negative magneto-resistivity in the vicinity of the transition temperature (Tc). The Phenomenon of large...
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    Multiple beams interferometry for array illuminator and its applications
    (2005) Patra, Ardhendu Sekhar
    The present work is aimed towards generation and characterization of the arrays using various interferometric configurations. Heterodyne interferometric configurations using polarized lights was developed and studied. The expressions for the intensity distribution of all configurations presented in the thesis were worked out and compered with the experimental observations. The applications of these optical arrays as a single step microlithography technique was proposed and demonstrated. Line integrated electron density in a pulsed spark gap was recorded using interferometric setup. Two interferometers in tendem were assembled for the generation of square and rectangular arrays of tiny light spots. the hexagonal arrays were produced using interference of eight beams. eight coherent beams were generated using one Mach-Zehnder and two Michelson interferometers in tandem. The beating ...
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    Novel configurations of atomic beam and laser beam for micro and nanolithography
    (2005) Alti, Kamlesh Mukundrao
    Present work mainly focus on two lithographic techniques viz. selective laser ablation lithography and atom lithography using dipole force. The work on atom lithography using dipole is on simulation of atomic trajectories under dipoleforce using semi-classical technique for various configuration of atomic beams and light fields to focus down the atomic beam at sub-micron level. Through these studies new configuration of light fields and atomic beams were proposed for their applications in the field of microlithography and nanolithography. We proposed the use of square arrays of multiple atomic lens produced by interference of four nearly collinear optical beams in atom lithography using dipole force. This configuration is useful in writing large number of micro-periodic structure in square arrays in a single step via atom lithography. A novel configuration of microscopic square arrays...
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    Atomistic simulation of point defects in ionic crystals
    (2005) Sahariah, Munima B
    In this thesis, we have presentrd a comprehensive study of modeling point defects in high and low symmetry crystals with Polarizable Point Ion (PPI) model. We worked out an alternate scheme to estimating energies of point defects through finite size calculations. This new method is tested for cubic NaCl crystal and is found to agree well with other existing results. Next, we studied in detail the effect of quadrupoles in the formation energy of vacancies in alkaline earth oxides using the approximate Mott-Littleton (ML) scheme. In agreement with earlier results on AgCl and AgBr, the quadrupoles are found to have substantial effect on defect energies raising a challenge to the existing dipolar models. We then worked out the PPI firmulations for low symmetry crystals, both in perfect and defect environment. A suitable set of short range potential parameters are deduced for monoclinic...
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    Excess conductivity,magneto-conductivy and AC susceptibility studies in La-Ba-Ca-Cu-O superconducturs
    (2006) Nayak, Pramoda Kumar
    The discovery of high transition temperature (Tc) superconductivity in (La-Ba)-Cu-O system with Tc = 30 K in1986 by Bednorz and Muller triggered the development of several high Tc Cu- O based superconductors. Among them, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems are widely studied in literature. Tl-Ba-Ca-Cu-O and Hg-Ba-Ca-Cu-O systems exhibit superconductivity with large transition temperatures such as 120 and 135 K respectively. However, these two series are toxic in nature. There are several reports on the enhancement of critical current density, Jc by novel preparation techniques including melt processing technique. The temperature variations of precise electrical resistivity and magneto-resistivity measured on most of the above high Tc superconductors have been studied in terms of thermodynamic fluctuations induced excess conductivity above Tc. As a result of thermodynamic fluctuations in the amplitude of order parameter, there is a finite probability of Cooper pair formation above Tc and this leads to excess conductivity. The temperature and magnetic field variations of ac susceptibility and magnetization of high Tc superconductors are widely analyzed in terms of several critical state models. According to critical state models, the penetrated super current flows with a density equal to critical current density. ...
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    Percolation Under Crossed Bias Fields: Criticallity and Scaling
    (2007) Sinha, Santanu
    A new site percolation model namely the directed spiral percolation (DSP) model is constructed imposing directional and rotational constraints simultaneously on the ordinary percolation model. the DSP model can be used to study the critical propcrties of disordered systems under crossed external bias fields. The bias fields could be of electric and magnetic in nature when the motion of charged particles are considered. In order ...
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    Role of interlayer couplings in bilayer Superconductors:A variational Monte Carlo Study
    (2008) Medhi, Amal
    The ground state superconducting and magnetic properties of bilayer cuprate superconductors are studied using a t-J model with interlayer couplings using the variational Monte Carlo (VMC) method. As a separate exercise, but relevant to the single layers and ladder compounds, a brief study of an impurity doped Hubbard model via the quantum Monte Carlo method is reported. To settle a crucial question regarding the symmetry of the superconducting order parameter, we investigated the stability of a number of different pairing symmetries for a t-J bilayer. For model parameter values that are suggested by experimental studies on bilayer superconductors, the favourable pairing symmetry of the superconducting state is found to be d-wave. Superconductivity coexists with antiferromagnetic long range order in the underdoped region. Superconducting and magnetic properties in the pure superconducting and the coexisting state are examined and it is convincingly shown that in the coexisting phase, both the antiferromagnetic and superconducting correlations are enhanced. Further, to compliment the phase diagrams of planar systems available in literature, we obtained a ground state phase diagram for a t-J bilayer as a function of interlayer couplings. The phase diagram shows rich features, such as, a crossover of superconducting correlations from being predominantly planar to strongly interplanar as interlayer exchange coupling is increased, a non-monotonic variation of the critical hole concentraion, upto which the d-wave superconducting phase remains stable, as a function of interplanar hopping parameter etc. In addition, there is a lowering of superconducting correlations as a function of interlayer hopping in the under and optimally doped phases, while these are enhanced in the overdoped region for moderate values of the coupling parameter. However the effect of interlayer coupling parameters on magnetic correlations are much less pronounced. Finally, to complete our discussion on bilayered superconductors, we studied the consequence of interlayer pair tunneling using a grand canonical VMC method, where the superconducting state of a bilayer is described by a variational wavefunction that is a product of two Gutzwiller projected d-wave BCS wavefunctions, one for each layer, with variable particle number. Our calculations show that the energy due to the tunneling of Cooper pairs across the layers has interesting bahaviour. However, the magnitude of energy is found to be too small to have any significant effect on the physical properties at least for realistic values of the tunneling parameter...
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    Electrical Resistivity and Magnetic Properties in (La, Ag)-Mn-O, (Nd, Ag)-Mn-O and (Y, Ce)-Mn-O Based Manganites
    (2008) Srivastava, Sandep Kumar
    One group of fascinating compounds with many potential applications is the mixed valence perovskite manganites. The versatility of perovskite manganites for electronic applications is demonstrated by the fact that, these materials exhibit wide range of conductivity ranging from highly conducting metallic state to insulating state. The discovery of large negative magnetoresistivity in these materials has led to the expectation of potential applications in magnetic storage, magnetic sensor and magnetic switches. These materials have also potential applications as catalysts for automobile exhausts, oxygen sensors and magnetic refrigeration. The number of applications is so wide that Physicists, Chemists and Materials scientists have shown a keen interest in these materials. There are several reports on La based divalent alkaline earth doped manganites and investigations on doping of Mn site with other magnetic and non-magnetic materials to understand the nature of magnetic interactions. The doping at Mn site in these mixed valent manganites by other transitions elements (T = Co, Cr, Cu, Fe, Ti, Ni, Ru etc.) or nonmagnetic impurities like Al gives rise to change in Mn3+/Mn4+ ratio, and the nearest neighbor environment. The magnetic and transport properties of such materials were found to be quite interesting. But there was lack of information on doping of these transition elements at Mn site of CMR materials based on mono-valent substitution in rare earth site. The mono-valent substitution based CMR materials with optimum Mn3+/Mn4+ concentration with relatively small lattice distortion is already known viz. La1-xAgxMnO3, La1-xKxMnO3, etc. Moreover the research work on electron doped manganites, where there is a possible double exchange interaction in Mn2+-O-Mn3+ networks is limited. In my research work, I have mainly chosen monovalent doped compounds. The monovalent doping has the advantage of creation of optimum concentration of Mn3+/Mn4+effect in Nd-Mn-O series. To understand the magnetic interactions in Mn2+/Mn3+ networks, electron doped (Y, Ce)-Mn-O were prepared and studied. I have prepared the following compounds for the present thesis work, 1. La0.85Ag0.15Mn1-yCoyO3 (y=0-0.50) 2. La0.85Ag0.15Mn1-yCryO3 (y=0-0.20) 3. La0.85Ag0.15Mn1-yAlyO3 (y=0-0.20) 4. LaMn1-yCuyO3 (y=0-0.30) 5. Nd1-xAgxMnO3 (x=0-0.20) 6. Y1-xCexMnO3 (x=0-0.15) The above samples were characterized by using X-ray diffraction (XRD), Scanning electron micrographs (SEM), Energy-dispersive spectrum (EDS) and chemical titration. To understand the magnetic properties and different magnetic interactions involved, I have carried out temperature and frequency variations of ac susceptibility and, dc magnetization as a function of temperature and field. To explore the electronic transport properties, I have carried out temperature variations of electrical resistivity and magneto-resistivity. The present thesis is divided into seven chapters, namely, (1) Introduction (2) Experimental Techniques (3) Mn site doped (La, Ag)-Mn-O Series (4) Cu doped La-Mn-O Series (5) Hole doped (Nd, Ag)-Mn-O Series (6) Electron doped (Y, Ce)-Mn-O Series (7) Conclusions. In chapter 1, different types of crystal structure of manganites are discussed. Various types of magnetic ordering, magnetic interaction and magnetic frustrations due to competing interactions are presented. Differen....
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    Preparation and Characterization of Co-Ni-Ga Ferromagnetic Shape Memory Alloys
    (2008) Sarma, Sidananda
    Shape memory alloys (SMA) have been engineered for various applications and devices since the first discovery of the shape memory effect in the 1930s. The advent of Nitinol (a Ni-Ti alloy) in 1962 established SMAs as a major area of research and development. . Unlike most conventional metals that recover less than 1% of the strain before plastic deformation, SMAs undergo a diffusionless thermo-elastic martensitic phase transformation that enable them to deform via the movement of twins or self accommodation process rather than by conventional dislocation slip mechanism and thereby allow recovery of strain as large as 8%. Diffusionless phase transformation in an SMA can be triggered by temperature change or application of stress or magnetic field. In conventional SMA, shape change or mechanical strain is achieved by applying a mechanical stress or by temperature variation. These inherently slow processes put an upper limit on the actuation speed of the SMA. Faster change of shape / volume or mechanical strain can be achieved in some magnetic alloys by applying an alternating magnetic field. Such alloys are called ferromagnetic shape memory alloys (FSMAs). Current research activities on FSMAs are mainly aimed at understanding the properties of FSMAs and developing FSMAs with properties desirable for actuator applications. Quite a few FSMAs had been developed and many of them have been proposed as potential candidates for sensor and actuator applications. But till now, no practical device has been reported with these FSMAs and the material is still being investigated intensively. Practical application of prototype Ni-Mn-Ga alloy is limited because of its extreme brittleness in polycrystalline state. Co-Ni-Ga solidifies in a peritectic reaction and forms a composite structure with fcc g-phase and bcc b matrix. Thus, controlled amount of -phase can be introduced in the -matrix by proper choice of composition and suitable heat treatment conditions. The hot workability and room temperature ductility of these alloys are significantly improved by the introduction of the -phase, which is a great advantage for practical applications. Understanding the evolution of various crystalline phase in FSMAs by different processing conditions and the resulting changes in properties of the alloys is crucial for evaluating these materials for actuator applications. Although the prototype FSMA, Ni- Mn-Ga has been well studied, other FSMAs such as Co-Ni-Ga, Ni-Fe-Ga etc. have not yet been investigated with so much rigour. In this thesis work, a systematic investigation of the processing conditions and physical properties of several Co-Ni-Ga alloys has been carried out. Three series of Co-Ni-Ga alloys have been prepared by a process consisting of arc melting technique followed by the homogenization at high temperature and quenching to low temperature. Care was taken to prepare alloys with ferromagnetic martensite near room temperature. The samples were characterized by powder X-ray diffractometer, energy dispersive spectrometer attached to a scanning electron microscope, optical microscope, differential scanning calorimeter, magnetic ac susceptometer, vibration sample magnetometer, Vickers microhardness tester, Universal testing machine (UTM) and strain gauge setup couple to an electromagnet, etc. The present thesis work has been presented in six chapters...
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    Structureand magnetic properties of Mechanically alloyed nanocrystalline Fe-Si (-M) [M=AI, B, Cr] Powders
    (2008) Kalita, Manas Pratim Chakrapani
    Fe-Si based alloys are traditionally well known soft magnetic materials which find applications in split-core current transformers, magnetic cores of electrical appliances, magneto-fluids, magnetic shielding, electromagnetic noise suppression, etc. Nanocrystalline materials prepared from melt-spun amorphous precursors with the average crystallite size less than the ferromagnetic exchange length exhibit ultra soft magnetic properties with very low coercivity ~10-3 Oe. Mechanical alloying (MA) is an alternate and commercially viable route for preparing nanocrystalline materials. Preparing mechanically alloyed soft magnetic nanocrystalline Fe-Si powders and understanding the correlation between the structure and magnetic properties of these materials are important from basic physics as well as application view points.This thesis work aims to understand (i) the correlation between the microstructure and magnetic properties of mechanically alloyed Fe-Si based nanocrystalline materials and (ii) to explore the possibility of improving their soft magnetic properties. The following atomic compositions were studied in the present work: Fe75Si25-xMx (x = 0, 5, 10; M = Al, B, Cr). The thesis work is presented in six chapters. The first chapter serves as a general introduction to the materials of interest to this thesis work, the theoretical formalisms relevant to them and the content of the thesis. The experimental techniques used in the preparation, processing and characterization of Fe-Si-M powders are discussed in the second chapter.In the third chapter, the results and discussion on Fe-Si and Fe-Si-M (M = Al, B, Cr) powders milled for various time periods have been presented. Milling was carried out for 80hours in a planetary ball mill. MA was found to produce non-equilibrium solid solution aDFe(Si) in case of Fe75Si25 powders with an average crystallite size of about 10 nm and dislocation density of 1018 m-2 after 80 hours of milling. The 80 hours milled powders showed coercivity of 128 Oe. An increase of coercivity with increasing milling time periods was observed, which has been mainly attributed to the introduction of dislocations and the reduction of average particle sizes in the course of the milling process. Domain wall pinning results increased coercivity in the milled powders. The Curie temperature (TC) of the powders milled for 80 hours was found to be higher than that of melt-spun ribbons of similar compositions. Such higher TC has been attributed to the lattice distortions caused by the presence of strain in the powders, induced during MA process. Similar studies were carried out on Fe75Si25-xMx (x = 5, 10) powder compositions, which also showed good correlations between the structure and magnetic properties. However, the structural and magnetic parameters were different for different alloy compositions. In the fourth chapter, the results and discussion on heat-treated nanocrystalline Fe-Si and Fe-Si-M (M = Al, B, Cr) powders have been detailed. Annealing has been found to have a significant effect on the microstructure and improvement of the soft magnetic properties of the powders. Annealing resulted in a gradual transformation of the disordered a-Fe(Si) phase of as-milled powders to ordered DO3 superlattice phase in the case of Fe75Si25 powders, as revealed by X-ray diffraction (XRD) studies. In a particula..
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    Investigations on Selected Bioactive Glasses and Glass-Ceramics Containing Iron Oxide
    (2009) Singh, Rajendra Kumar
    Magnetism and magnetic materials have a strong role to play in health care and biological application such as cell separation, drug delivery and magnetic intracellular hyperthermia treatment of cancer. Magnetic bioglass ceramics (MBC) are complex and multiphase biocompatible materials, usually fabricated from the original bioglass with the addition of Fe2O3 in the basic bioglass composition. MBCs exhibit bioactivity as well as magnetic properties. This Ph. D thesis work is focussed on the preparation and characterization of CaO, MgO and ZnO based glasses containing variable amounts of iron oxide and MBC derived from them. These investigations are aimed at (i) optimization of preparative conditions of the MBC, (ii) magnetic hysteresis loss and physical property evaluation of MBC, and (iii) evaluation of biocompatibility of the MBCs by treating them with SBF. Since preparation of bioactive glasses is the first stage in the preparation of the bioglassceramics, optimization and evaluation of the properties of the base glasses are also required in order to fully comprehend the MBCs derived from them. After a careful study of the bioactive based glass containing Fe2O3 and magnetic glass-ceramics obtained by controlled crystallization of the respective parent glasses, three glass systems, viz., 41CaO(52-x)SiO24P2O5xFe2O33Na2O, (x = 0, 2, 4, 6, 8 and 10 mole %), 4.5MgO(45-x)CaO34SiO216P2O50.5CaF2x Fe2O3 (x = 0, 5, 10, 15 and 20 wt.%) and x(ZnO,Fe2O3)(65-x)SiO2 20(CaO,P2O5)15Na2O (x = 6, 9, 12, 15, 18 and 21 mole % ) with Ca/P = 1.67 and Fe/Zn = 6.5 were identified for the present investigations. Bulk glass samples belonging to the three glass systems were prepared by melt quenching technique. Powder X-ray diffraction technique was used to verify the amorphous nature of the glasses and to record the X-ray diffraction (XRD) pattern of heat treated glasses. Differential scanning calorimeter (DSC) was employed to determine the glass transition temperature (Tg) and the crystallization temperature (Tc) of the glasses. Density of the glass samples was measured using ArchimedesD principle. A microhardness tester equipped with a Vickers pyramidal indenter was used for micro hardness measurements. Electron Paramagnetic Resonance (EPR) spectra were recorded at room temperature on powder samples using a Spectrometer in the X band for understanding the valence states and local structure of iron ions as a function of composition. Magnetization versus Temperature (M-T) curves and magnetic hysteresis (M-H) loops of the samples would be obtained using a Vibrating Sample Magnetometer (VSM) to elucidate the magnetic properties of the samples. In vitro bioactivity test was performed on the samples by immersion of bulk samples in SBF (KokuboDs solution) for different periods of time and monitoring the changes in the structure of the glasses / glass-ceramics by grazing incidence XRD, reflectance FT-IR spectroscopy and Scanning Electron Microscopy (SEM) . All the experimental data were analysed using existing theoretical formalisms. The thesis is arranged in six chapters. The first chapter serves as a brief introduction to bioglasses and glass-ceramics, the materials of interest to this thesis work. The motivation behind the thesis work is briefly stated at the end of this chapter. The second chapter discusses the experimental techniques used in the presen...
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    MAGNETIC PROPERTIES OF Co AND Mn SUBSTITUTED Fe–Zr–B ALLOYS PREPARED BY MELT SPINNING AND MECHANICAL ALLOYING PROCESSES
    (2009) Mishra, Debabrata
    Fe-Zr-B based amorphous and nanocrystalline alloys are well known soft magnetic materials which are characterized by high saturation magnetization (MS) and low coercivity (HC). These soft magnetic alloys find wide applications in highper formance magnetic parts such as in electronic devices, common mode choke-coils, power electronics, bead cores, electrical noise absorbers, magnetic switch cores, transformers, current sensors and electromagnetic shielding, magnetic refrigerants, magnetoimpedance devices etc. These materials are prepared through melt spinning and mechanical alloying (MA) routes by controlled crystallization of amorphous precursors and structural reduction of crystallites by the introduction of defects and dislocations, respectively. Conventional methods of improving the intrinsic and extrinsic soft magnetic properties focus on tailoring composition, control of microstructure with heat treatment under different environments, reduction of HC, increase of MS and control of intergranular exchange coupling in the amorphous and nanocrystalline materials. Hence, preparing Fe-Zr-B based amorphous and nanocrystalline alloys through different synthesis routes and understanding the correlation between the structural and magnetic properties of these materials over a wide composition range are very much important from basic physics as well as application view point. This thesis work aims to (i) understand the effect of substituting elements (B, Co, and Mn) on the stability of amorphous phase and temperature dependent magnetic properties of amorphous (a-)Fe-Zr based alloys prepared by melt spinning and MA routes, (ii) study the magnetocaloric effect (MCE) in amorphous ribbons, and (iii) investigate the effect of magnetic field annealing on the improvement of soft magnetic properties of the melt-spun ribbons in correlation with microstructure and magnetic domain structure. The thesis is presented in eight chapters. The first chapter serves as a general introduction to the materials of interest to this work, motivation behind the work with historical perspective from the literature and the objectives of the thesis work.
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    Statistical Field Spectral Calculations in Some Problems of Anisotropic Turbulence
    (2010) Dutta, Kishore
    In this research work we apply the standard perturbative methods to some anisotropic cases of fluid turbulence and obtain information about the effect of anisotropy within such frameworks. As a matter of fact, the anisotropy makes the problems much more complex than the corresponding isotropic cases, and we hope that such a study will have both academic and practical relevance. Homogeneous shear turbulence has been studied in great details in the last few decades. The cases of anisotropic turbulence have been mostly dealt with SO(3) decomposition [102]. However, we take alternative routes to the problems of anisotropic turbulence and hence we are interested in tackling directly the anisotropic terms appearing in the dynamical equations by means of standard perturbative methods. In practical applications, what one is interested in is the effect of turbulence on the mean flow and on transport, which is parametrized in terms of eddy-viscosities and eddy-diffusivities. In some cases, in order to calculate these quantities, information about the turbulent energy spectrum is necessary, and the universal Kolmogorov spectrum with scaling law E(k) = C 2/3k5/3 is usually assumed. Here k is the wavenumber, is the energy flux, and C the universal Kolmogorov constant...
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    (A) Theoretical Study of Optical Constants of Two Dimensional Carbon Based Materials
    (2010) Meera, V.
    The electromagnetic response of two dimensional carbon based systems is studied using basic classical tools. The most interesting and important carbon based system known till today is the by now well-known two-dimensional material DGrapheneD. This material is special in several ways, indeed, its very existence is an enigma. Graphene and a few of graphene based systems are studied using theoretical techniques. The key point of this work is the theoretical formulation of monolayer free-standing graphene. This formulation acts as the reference for the subsequent studies done in various graphene based systems. In this formulation monolayer free standing graphene is modelled as a conducting medium of one atom thickness such that the media is non-refracting. The emergent electric field of the system has been calculated by solving the fundamental equations of electromagnetism, DMaxwellDs EquationsD which consists of reflected and transmitted fields. Three physically important and experimentally measurable quantities are derived analytically, viz. coefficient of reflection, coefficient of transmission and polarization of reflection. The energy conservation theorem has been derived for the system using which the loss also has been calculated to make sure of the correctness of the formalism. The results obtained for some of these quantities with fixed values of various parameters show an excellent agreement with the experimental observations available in this field. For most of the device and technological applications, graphene has to be deposited on a substrate. For this reason, we next study a related system - substrate graphene. Theoretical modelling of this system has been done by regarding graphene as a conducting medium deposited on top of a purely dielectric material characterized by a dielectric constant. MaxwellDs equations for this combined system has been solved using the boundary conditions and derived equations for the two parts of emergent field, reflection and transmission which has been made use to study the optical properties of this system. The linearly polarized limit of incident light in this system is important because of a well-known phenomenon in optics, DBrewsterDs phenomenonD. The most interesting feature of BrewsterDs phenomenon related to substrate graphene is the azimuthal angle dependence of the Brewsters minimum. The next immediate system related to graphene is the combination of two single graphene sheets separated by a distance of a few Angstroms. With the monolayer results as reference, the optical constants of the bilayer system has been solved using series summation methods used in many fundamental optics books (such as [142]). Presence of an extra layer leads to multiple reflection phenomena....
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    Laser induced plasma in liquid/solid-liquid interface and implication on nucleation of nanoparticles
    (2011) Nath, Arpita
    The present work focuses on transient evolution of pulsed laser induced breakdown process in liquid and solid-liquid interface. The focusing of high power laser inside a liquid media leads to plasma formation and shockwave emission. When the same process takes place at a solid-liquid interface, the plasma from the solid target interacts with the surrounding liquid and under suitable condition results into formation of nanoparticulates. On a larger time scale, plasma is replaced by cavitation vapor bubbles which expands and collapses in order to maintain the pressure gradient with the surrounding liquid. In the present work, the complete dynamics of laser induced breakdown in water and metal-water interface is studied. For this various diagnostics; beam deflection set-up, shadowgraphic technique, electric probes and plasma spectroscopy were devloped. Beam deflection was employed to study the spatial and temporal evolution of shockwave velocity and cavitation bubble dynamics at the breakdown region. The shockwave velocity and cavitation bubble expansion velocity at the breakdown region were found to be in the range of 104 m/s and 102 m/s respectively. The multiple bubble hydrodynamics was studied via shadowgraphic technique to unleash the bubble evolution and bubblebubble interaction. The electric probe was used to record the transient conduction and corresponding thermal diffusivity of laser produced water plasma. The plasma spectroscopy was performed to detect the Hydrogen bonded OH, O2 and HO2* produced during laser induced breakdown of water. Laser induced breakdown at metal-water interface is used to synthesize Titanium breakdown is altered to tailor the size and structural properties of the nanoparticles. The measurement of pressure and temperature using beam deflection set-up and plasma spectroscopy during laser induced breakdown at solid-liquid interface is used to estimate the nucleation time, growth velocity and size of nanoparticles and compared with experimentally observed size of nanoparticles. The nanoparticles are characterized using TEM, Raman, UV-Vis and Photo Luminescence spectra. The applicability of the synthesized titanium oxide nanoparticles as photocatalyst and copper oxide nanoparticles as antibacterial agent are also reported....