Geetha, P; Anantharaman, M R(Cochin University of Science and Technology, October , 2014)
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Abstract:
Magnetism and magnetic materials have been playing a lead role in improving
the quality of life. They are increasingly being used in a wide variety of applications ranging from compasses to modern technological devices. Metallic glasses occupy an important position among magnetic materials. They assume importance both from a scientific and an application point of view since they represent an amorphous form of condensed matter with significant deviation from thermodynamic equilibrium. Metallic glasses having good soft magnetic properties are widely used in tape recorder heads, cores of high-power transformers and metallic shields. Superconducting metallic glasses are being used to produce high magnetic fields and magnetic levitation effect. Upon heat treatment, they undergo structural relaxation leading to subtle rearrangements of constituent atoms. This leads to densification of amorphous phase and subsequent nanocrystallisation. The short-range structural relaxation phenomenon gives rise to significant variations in physical, mechanical and magnetic properties. Magnetic amorphous alloys of Co-Fe exhibit excellent soft magnetic properties which make them promising candidates for applications as transformer cores, sensors, and actuators. With the advent of microminiaturization and nanotechnology, thin film forms of these alloys are sought after for soft under layers for perpendicular recording media. The thin film forms of these alloys can also be used for fabrication of magnetic micro electro mechanical systems (magnetic MEMS). In bulk, they are drawn in the form of ribbons, often by melt spinning. The main constituents of these alloys are Co, Fe, Ni, Si, Mo and B. Mo acts as the grain growth inhibitor and Si and B facilitate the amorphous nature in the alloy structure. The ferromagnetic phases such as Co-Fe and Fe-Ni in the alloy composition determine the soft magnetic properties. The grain correlation length, a measure of the grain size, often determines the soft magnetic properties of these alloys. Amorphous alloys could be restructured in to their nanocrystalline counterparts by different techniques. The structure of nanocrystalline material consists of nanosized ferromagnetic crystallites embedded in an amorphous matrix. When the amorphous phase is ferromagnetic, they facilitate exchange coupling between nanocrystallites. This exchange coupling results in the vanishing of magnetocrystalline anisotropy which improves the soft magnetic properties. From a fundamental perspective, exchange correlation length and grain size are the deciding factors that determine the magnetic properties of these nanocrystalline materials. In thin films, surfaces and interfaces predominantly decides the bulk property and hence tailoring the surface roughness and morphology of the film could result in modified magnetic properties. Surface modifications can be achieved by thermal annealing at various temperatures. Ion irradiation is an alternative tool to modify the surface/structural properties. The surface evolution of a thin film under swift heavy ion (SHI) irradiation is an outcome of different competing mechanism. It could be sputtering induced by SHI followed by surface roughening process and the material transport induced smoothening process. The impingement of ions with different fluence on the alloy is bound to produce systematic microstructural changes and this could effectively be used for tailoring magnetic parameters namely coercivity, saturation magnetization, magnetic permeability and remanence of these materials. Swift heavy ion irradiation is a novel and an ingenious tool for surface modification which eventually will lead to changes in the bulk as well as surface magnetic property. SHI has been widely used as a method for the creation of latent tracks in thin films. The bombardment of SHI modifies the surfaces or interfaces or creates defects, which induces strain in the film. These changes will have profound influence on the magnetic anisotropy and the magnetisation of the specimen. Thus inducing structural and morphological changes by thermal annealing and swift heavy ion irradiation, which in turn induce changes in the magnetic properties of these alloys, is one of the motivation of this study. Multiferroic and magneto-electrics is a class of functional materials with wide application potential and are of great interest to material scientists and engineers. Magnetoelectric materials combine both magnetic as well as ferroelectric properties in a single specimen. The dielectric properties of such materials can be controlled by the application of an external magnetic field and the magnetic properties by an electric field. Composites with magnetic and piezo/ferroelectric individual phases are found to have strong magnetoelectric (ME) response at room temperature and hence are preferred to single phasic multiferroic materials. Currently research in this class of
materials is towards optimization of the ME coupling by tailoring the piezoelectric
and magnetostrictive properties of the two individual components of ME composites. The magnetoelectric coupling constant (MECC) (_ ME) is the parameter that decides the extent of interdependence of magnetic and electric response of the composite structure. Extensive investigates have been carried out in bulk composites possessing on giant ME coupling. These materials are fabricated by either gluing the individual components to each other or mixing the magnetic material to a piezoelectric matrix. The most extensively investigated material combinations are Lead Zirconate Titanate (PZT) or Lead Magnesium Niobate-Lead Titanate (PMNPT) as the piezoelectric, and Terfenol-D as the magnetostrictive phase and the coupling is measured in different configurations like transverse, longitudinal and inplane longitudinal. Fabrication of a lead free multiferroic composite with a strong ME response is the need of the hour from a device application point of view. The multilayer structure is expected to be far superior to bulk composites in terms of ME coupling since the piezoelectric (PE) layer can easily be poled electrically to enhance the piezoelectricity and hence the ME effect. The giant magnetostriction reported in the Co-Fe thin films makes it an ideal candidate for the ferromagnetic component and BaTiO3 which is a well known ferroelectric material with improved piezoelectric properties as the ferroelectric component. The multilayer structure of BaTiO3- CoFe- BaTiO3 is an ideal system to understand the underlying fundamental physics behind the ME coupling mechanism. Giant magnetoelectric coupling coefficient is anticipated for these multilayer structures of BaTiO3-CoFe-BaTiO3. This makes it an ideal candidate for cantilever applications in magnetic MEMS/NEMS devices. SrTiO3 is an incipient ferroelectric material which is paraelectric up to 0K in its pure unstressed form. Recently few studies showed that ferroelectricity can be induced by application of stress or by chemical / isotopic substitution. The search for room temperature magnetoelectric coupling in SrTiO3-CoFe-SrTiO3 multilayer structures is of fundamental interest. Yet another motivation of the present work is
to fabricate multilayer structures consisting of CoFe/ BaTiO3 and CoFe/ SrTiO3 for possible giant ME coupling coefficient (MECC) values. These are lead free and hence promising candidates for MEMS applications. The elucidation of mechanism for the giant MECC also will be the part of the objective of this investigation.
Jiby, K. Gopinath; Dr. K. Sreekumar(Cochin University of Science & Technology, July 30, 2015)
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Abstract:
Polymers with conjugated π-electron backbone display unusual
electronic properties such as low energy optical transition, low ionization
potentials, and high electron affinities. The properties that make these
materials attractive include a wide range of electrical conductivity,
mechanical flexibility and thermal stability. Some of the potential
applications of these conjugated polymers are in sensors, solar cells, field
effect transistors, field emission and electrochromic displays, supercapacitors
and energy storage. With recent advances in the stability of
conjugated polymer materials, and improved control of properties, a
growing number of applications are currently being explored. Some of the
important applications of conducting polymers include: they are used in
electrostatic materials, conducting adhesives, shielding against
electromagnetic interference (EMI), artificial nerves, aircraft structures,
diodes, and transistors.
Rosabella,K Puthur; Sebastian, K L; Sugunan, S(Cochin University of Science and Technology, October , 2001)
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The thesis presents the dynamics of a polymer chain under tension. It includes existing theories of polymer fracture, important theories of reaction rates, the rate using multidimensional transition state theory and apply it to the case of polyethylene etc. The main findings of the study are; the life time of the bond is somewhat sensitive to the potential lead to rather different answers, for a given potential a rough estimate of the rate can be obtained by a simples approximation that considers the dynamics of only the bond that breaks and neglects the coupling to neighboring bonds. Dynamics of neighboring bonds would decrease the rate, but usually not more than by one order of magnitude, for the breaking of polyethylene, quantum effects are important only for temperatures below 150K, the lifetime strongly depends on the strain and as the strain varies over a narrow range, the life varies rapidly from 105 seconds to 10_5 seconds, if we change one unit of the polymer by a foreign atom, say by one sulphure atom, in the main chain itself, by a weaker bond, the rate is found to increase by orders of magnitude etc.
Sunandakumari,V M; Dr.Ramanujam, N(Cochin University of Science And Technology, May , 1988)
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Abstract:
During recent years, the theory of differential inequalities has been extensively used to discuss singular perturbation problems and method of lines to partial differential equations. The present thesis deals with some differential inequality theorems and their applications to singularly perturbed initial value problems, boundary value problems for ordinary differential equations
in Banach space and initial boundary value problems for parabolic differential equations. The method of lines to parabolic and elliptic differential equations are also dealt The thesis is organised into nine chapters
Description:
Department of mathematics, Cochin University of Science And Technology
Challa Ravi Kiran; Dr Sundresan, A(Cochin University of Science and Technology, June , 2015)
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Abstract:
Deep fat frying process is one of the widely followed cooking practices throughout the
world. Cooking oils serve as a medium for frying food for transferring heat and makes fried
food tasty and palatable. Frying process is a most complex process involving numerous
physicochemical changes which are complicated to understand. Frying leads to thermal
degradation of oil through thermo-oxidation, hydrolysis, and polymerization. Hydrolysis
results in formation of free fatty acids whereas oxidation process produces hydroperoxides
and small molecular carbonyl compounds. This whole process leads to the formation of polar
compounds and degradation of antioxidants that further degrades frying oil. Eventually,
through mass transfer process these degradation products accumulate into fried food and
reduce the nutritional quality of both oil and food. Thus, the frying process is of research
interest calls for detailed systematic study which is chosen for the present study. The primary
objective of this study is to understand the mechanism of degradation and characterization ofdegraded products which helps in arriving at the limits for frying oil utilization in terms of
number of frying cycles. The mechanistic studies and the knowledge on the degraded
products help to understand the way to retard the deterioration of oil for stability and
enhancement of frying cycles. The study also explores the formation of the predominant
polar compounds and their structural elucidation through mass spectrometry. Oxidation of oil
is another important factor that ignites the degradation phenomena. One of the best ways to
increase thermal stability of any oil is addition of potent antioxidants. But, most of the natural
and synthetic antioxidants are unstable and ineffective at frying temperatures. Therefore, it is
necessary to screen alternative antioxidants for their activity in the refined oils which are devoid of any added antioxidants. In this context, this study discussed the efficacy of several
natural and synthetic antioxidants to retard the formation of polar compounds and thermooxidation
during prolonged frying conditions. Similarly, the advantage of blending of two
different oils to improve the thermal stability was explored. The present study brings out the
total picture on the type of degradation products formed during frying and the ways of
retarding the determination to improve upon the stability of the oil and enhancement of frying
cycles.
Subin, Joseph P; Dr.Jathavedan, M(Cochin University of Science and Technology, January , 2006)
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Abstract:
In this thesis we are studying possible invariants in hydrodynamics and hydromagnetics.
The concept of flux preservation and line preservation of vector fields, especially
vorticity vector fields, have been studied from the very beginning of the study of fluid
mechanics by Helmholtz and others. In ideal magnetohydrodynamic flows the magnetic
fields satisfy the same conservation laws as that of vorticity field in ideal hydrodynamic
flows. Apart from these there are many other fields also in ideal hydrodynamic and magnetohydrodynamic flows which preserves flux across a surface or whose vector lines
are preserved.
A general study using this analogy had not been made for a long time. Moreover
there are other physical quantities which are also invariant under the flow, such as
Ertel invariant. Using the calculus of differential forms Tur and Yanovsky classified
the possible invariants in hydrodynamics. This mathematical abstraction of physical
quantities to topological objects is needed for an elegant and complete analysis of
invariants.Many authors used a four dimensional space-time manifold for analysing fluid flows.
We have also used such a space-time manifold in obtaining invariants in the usual three
dimensional flows.In chapter one we have discussed the invariants related to vorticity field using
vorticity field two form w2 in E4. Corresponding to the invariance of four form w2 ^ w2
we have got the invariance of the quantity E. w. We have shown that in an isentropic
flow this quantity is an invariant over an arbitrary volume.In chapter three we have extended this method to any divergence-free frozen-in
field. In a four dimensional space-time manifold we have defined a closed differential
two form and its potential one from corresponding to such a frozen-in field. Using this
potential one form w1
, it is possible to define the forms dw1 , w1 ^ dw1 and dw1 ^ dw1
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Corresponding to the invariance of the four form we have got an additional invariant
in the usual hydrodynamic flows, which can not be obtained by considering three
dimensional space.In chapter four we have classified the possible integral invariants associated with
the physical quantities which can be expressed using one form or two form in a three
dimensional flow. After deriving some general results which hold for an arbitrary dimensional
manifold we have illustrated them in the context of flows in three dimensional
Euclidean space JR3. If the Lie derivative of a differential p-form w is not vanishing,then the surface integral of w over all p-surfaces need not be constant of flow. Even
then there exist some special p-surfaces over which the integral is a constant of motion,
if the Lie derivative of w satisfies certain conditions. Such surfaces can be utilised
for investigating the qualitative properties of a flow in the absence of invariance over
all p-surfaces. We have also discussed the conditions for line preservation and surface
preservation of vector fields. We see that the surface preservation need not imply the
line preservation. We have given some examples which illustrate the above results.
The study given in this thesis is a continuation of that started by Vedan et.el. As
mentioned earlier, they have used a four dimensional space-time manifold to obtain
invariants of flow from variational formulation and application of Noether's theorem.
This was from the point of view of hydrodynamic stability studies using Arnold's
method.
The use of a four dimensional manifold has great significance in the study of knots
and links. In the context of hydrodynamics, helicity is a measure of knottedness of
vortex lines. We are interested in the use of differential forms in E4 in the study of
vortex knots and links. The knowledge of surface invariants given in chapter 4 may
also be utilised for the analysis of vortex and magnetic reconnections.
Description:
Department of Mathematics,
Cochin University of Science and Technology
Sethulakshmi, N; Dr. Anantharaman, M. R.(Cochin University of Science and Technology, September , 2014)
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Abstract:
The present thesis work focuses on hole doped lanthanum manganites and their thin film forms. Hole doped lanthanum manganites with higher substitutions of sodium are seldom reported in literature. Such high sodium substituted lanthanum manganites are synthesized and a detailed investigation on their structural and magnetic properties is carried out. Magnetic nature of these materials near room
temperature is investigated explicitly. Magneto caloric application potential of these materials are also investigated. After a thorough investigation of the bulk samples, thin films of the bulk counterparts are also investigated. A magnetoelectric composite with ferroelectric and ferromagnetic components is developed using pulsed laser deposition and the variation in the magnetic and electric properties are investigated. It is established that such a composite could be realized as a potential field effect device. The central theme of this thesis is also on manganites and is with the twin objectives of a material study leading to the demonstration of a device. This is taken up for investigation. Sincere efforts are made to synthesize phase pure compounds. Their structural evaluation, compositional verification and evaluation of ferroelectric and ferromagnetic properties are also taken up. Thus the focus of this investigation is related to the investigation of a magnetoelectric and magnetocaloric application potentials of doped lanthanum manganites with sodium substitution. Bulk samples of sodium substituted lanthanum manganites. Bulk samples of sodium substituted lanthanum manganites with Na substitution ranging from 50 percent to 90 percent were synthesized using a modified citrate gel method and were found to be orthorhombic in structure belonging to a pbnm spacegroup. The variation in lattice parameters and unit cell volume with sodium concentration were also dealt with. Magnetic measurements revealed that magnetization decreased with increase in sodium concentrations.