1. Ph.D Theses
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Item Design and Development of Chalcogenide based Superstrate Thin Film Solar Cells(National Institute of Technology Karnataka, Surathkal, 2021) Varadharajaperumal S.; Satynarayan, M N.; Hegde, Gopal Krishna.Thin film solar cells (substrate and superstrate) are promising approaches for various photovoltaic applications, which offer a wide variety of choices in terms of device design and fabrication. At the same time, for modern research development a novel, exotic, innovative and modest materials with simple manufacturing processes need to be pursued in a focussed manner. Present thesis work explores the design and development of superstrate type binary (CdTe) and quaternary (CZTS) thin film solar cells and their photovoltaic performance. Also, this thesis mainly focuses on the effect of combining two-hole transport layers in superstrate CdS/CdTe solar cells and the nanostructured (1D & 3D) metal oxides (ZnO and TiO2) as window or electron transporting layers (along with or without CdS (buffer) layer) in stoichiometry adjusted superstrate CZTS thin film solar cells. Before the fabrication of photovoltaic devices, the structural, morphological, optical and electrical properties of deposited photoanodes, absorbing layers and transporting (electron and hole) layers, using different techniques were respectively characterized.Item Photo-Physical Studies and Bandgap Engineering on Transition Metal Chalcogenides for Applications in Photocatalysis(National Institute of Technology Karnataka, Surathkal, 2021) Shenoy, Sulakshana.; Tarafder, Kartick.; Sridharan, Kishore.Two-dimensional (2D) transition metal chalcogenides (TMCs) based photocatalysts have recently attracted significant research attention for addressing the current worldwide challenges of energy shortage and environmental pollution. This thesis is mainly focused on the design and development of visible-light-driven TMCs based photocatalytic systems that are useful for both the generation of clean energy through solar water-splitting reaction and also towards the degradation of harmful organic pollutants present in water. The influence of structure-to-photocatalytic property relationship (size and shape effects) of semiconductor nanostructures are determined by systematic modifications in the synthesis methods to obtain photocatalysts of different size and morphology and their role in enhancement of the photocatalytic activity is studied. Significant attention is paid on building heterojunctions between two semiconductors having well-aligned band structures and possessing intimately contacted interfaces that are propitious to the effective separation and transfer of photogenerated charge carriers, bringing an excellent performance. Furthermore, firstprinciples calculation based on density functional theory (DFT) are used to investigate the structural, electronic (band structure and density of states) and optical properties of the TMCs-based photocatalysts. Besides, band edge positions of the semiconductor and the band alignment with respect to the normal hydrogen electrode is determined theoretically. It is anticipated that this work will provide a better understanding of the fundamental photocatalytic mechanism, assisted by the development of advanced photocatalysts and studying their photocatalytic performance towards both environmental remediation and production of clean energy.Item Theoretical Study of Functionalized Two-Dimensional Materials towards their Application in Supercapacitors(National Institute of Technology Karnataka, Surathkal, 2021) T, Sruthi.; Tarafder, Kartick.This thesis investigates possible roots to enhance the quantum capacitance(CQ) of two-dimensional materials based electrodes for supercapacitor applications through density functional theory(DFT) calculations. In this work, various two-dimensional materials such as graphene, molybdenum disulfide(MoS2), and hexagonal boron nitride(h- BN) have been considered, subsequently, chemical functionalization of these systems has been performed to manifest the high quantum capacitance. The quantum capacitance of functionalized systems was estimated from the precise electronic band structures of the system obtained by using DFT calculations. It has been observed that ad-atom functionalization of graphene can significantly enhance the quantum capacitance of the system. Therefore, in the first stage, the quantum capacitance of ad-atom doped graphene with a varying doping concentration has been systematically studied. The effect of temperature on quantum capacitance has also been investigated. The temperature-dependent study of CQ for functionalized graphene shows that the CQ remains very high in a broad range of temperatures close to room temperature. In the second stage, the graphene functionalization has been done by doping with different aliphatic and aromatic molecules and their radicals. Our theoretical investigation reveals that aromatic and aliphatic radicals introduce localized density of states near the Fermi level of the functionalized systems, due to a charge localization which in turn significantly enhances the quantum capacitance of the system. The effects of atomic dislocation on graphene during functionalization has also been incorporated in our investigation. In the third stage, we have carried out our investigation in other two-dimensional materials such as MoS2 and h-BN. Attempts have been made to enhance the quantum capacitance of these systems by introducing defects as well as performing chemical fictionalizations. The detailed study in this thesis suggests an efficient way to produce functionalized materials using two-dimensional materials that could be very suitable electrode materials of highly efficient supercapacitors.Item Excitation Wave Dynamics and their Interaction with External Fields(National Institute of Technology Karnataka, Surathkal, 2021) Shreyas; Shajahan, T K.Rotating spiral waves of excitation are common in many physical, chemical and biological systems. In physiological systems like the heart, such waves anchor to unexcitable tissue (an obstacle), become stable pinned waves and cause life-threatening cardiac arrhythmias. The traditional high voltage defibrillation techniques used to treat arrhythmias are known to have pro-arrhythmic effects. Therefore, it is crucial to develop low energy methods to unpin and eliminate them. This thesis investigates two kinds of low voltage electric fields to unpin the pinned spiral waves. In the first method using pulsed electric fields, the spiral wave will be unpinned only when the pulse is delivered inside a narrow time interval called the unpinning window of the spiral. In experiments with cardiac monolayers, we found that other obstacles situated near the spiral’s pinning centre can facilitate unpinning. In numerical simulations, we found that the unpinning window can change depending on the location, orientation and distance between the pinning centre and the obstacle. The second method involves unpinning the spiral using circularly polarised electric fields (CPEF). Here, we show that the spiral can always be unpinned below a threshold time period of CPEF for a given obstacle size. Our analytical formulation accurately predicts the threshold and explains the absence of the traditional unpinning window. We also show that the unpinning always happens within the first rotation of the electric field. Previous unpinning studies using two-dimensional experimental and numerical models show that the width of the unpinning window is very narrow. This could be due to the presence of multiple obstacles as our results suggests. The absence of unpinning window with CPEF eliminates the problem of timing the pulses and guarantees unpinning of the spiral below a certain threshold time period. We hope that the results discussed in this thesis regarding the spatial arrangement of the obstacles and its interactions with the electric fields will open new ways towards low-energy therapies of the cardiac arrhythmias.Item Phase Transitions and Microstructures of AdS Black Holes(National Institute of Technology Karnataka, Surathkal, 2021) A, Naveena Kumara.; M, Ajith K.The thesis is aimed to understand the aspects of the black hole phase transitions and the underlying microstructures in antide Sitter spacetime. In contrast to the conventional black hole thermodynamics, there exist the thermodynamic variables pressure and volume in the extended thermodynamics approach, which arise from the dynamic cosmological constant. With this utility, we have researched the following things: (i) We used Landau continuous phase transition theory to discuss the van der Waals like critical phenomena of the black hole. The wellknown interpretation of the phase transition of an AdS black hole as being a large and small black hole transition is reinterpreted as being a transition between a high potential phase and a low potential phase. (ii) We probed the phase structure of the regular AdS black holes using the null geodesics. The radius of photon orbit and minimum impact parameter shows a nonmonotonous behaviour below the critical values of the temperature and the pressure, corresponding to the phase transition in extended phase space. The respective differences of the radius of unstable circular orbit and the minimum impact parameter can be seen as the order parameter for the smalllarge black hole phase transition, with a critical exponent 1/2. Our study shows that there exists a close relationship between gravity and thermodynamics for the regular AdS black holes. (iii) We studied the interaction between the microstructures of the HaywardAdS black hole using Ruppeiner geometry. Our investigation shows that the dominant interaction between the black hole molecules is attractive in most part of the parametric space of temperature and volume, as in the van der Waals system. However, in contrast to the van der Waals fluid, there exists a weak dominant repulsive interaction for the small black hole phase in some parameter range. This result clearly distinguishes the interactions in a magnetically charged black hole from that of van der Waals fluid. (iv) By employing a novel Ruppeiner geometry method in the parameter space of temperature and volume, we investigated the microstructure of BornInfeld AdS black hole via the phase transition study, which includes standard and reentrant phase transition. We found that the microstructures of the black hole that lead to standard and reentrant phase transitions are distinct in nature. The critical phenomenon is observed from the curvature scalar, including the signature of the reentrant phase transition.Item Strain Dependent Properties, Intercalation and Adsorption Studies of Graphene Like Two Dimensional SiC and SiB(National Institute of Technology Karnataka, Surathkal, 2021) S, Manju M.; M, Ajith K.This thesis reports the variation in properties of graphene like two dimensional materials SiC and SiB with the application of strain and their properties in being the anode of rechargeable Li-ion batteries. The material was modelled and the calculations were carried out using Density Functional Theory (DFT) using Vienna Ab-initio Simulation Package (VASP) and Quantum Espresso packages. SiC in its pristine case showed isotropic behaviour but the inducing of stress changed it to anisotropic behaviour. On the other hand, SiB was anisotropic in its pristine form and anisotropic behaviour increased with the application of strain. Both the structures were mechanically stable satisfying the Born criteria. The mechanical behaviours showed highly non-linear nature for the case of SiB and the ultimate stress were much higher than their counterpart silicene. The application of strain specifically biaxial onto SiB generated a new structure having a planar architecture very similar to graphene. The experimental synthesis of SiC urged to check its potential in being the anode of Li-ion batteries. Pristine SiC was a poor candidate and was introduced with defects to check for improvement in properties for an anode. Bi vacancy defective configurations showed an interesting characteristic upon optimization. There was a transition from point to topological type of defects forming 5-8-5 rings in the case of C-bi vacancy and Si-C bi vacancy and it was a Li mediated transition in the case of Si-bi vacancy configuration. Among the various defective configurations, Stone-Wales (SW) defective configuration was found to be the best candidate having lowest value of binding energy compared to all the other configurations. The characteristics of SW configurations are specific capacity 501 mAh/g, open circuit voltage 0.11 V, diffusion barrier 0.57 eV which is in accordance with the electrochemical characteristics in being the anode of Li-ion batteries. Therefore, SW defective configuration was proposed to be a prospective candidate for the next generation Li-ion batteries.Item Critical Phenomena in Anti-de Sitter Black Holes(National Institute of Technology Karnataka, Surathkal, 2021) Ahmed, Rizwan C.L.; Vaid, Deepak.This thesis investigates thermodynamic phase transitions in anti-de Sitter (AdS) black holes. Motivated by the inconsistency of Smarr relation and the first law of black hole thermodynamics in AdS spacetime, cosmological constant L is given a status of thermodynamic variable pressure. This modification has led to the addition of pressurevolume term in the first law of black hole thermodynamics. In this extended phase space, black hole phase behavior is found analogous to everyday physical phenomena. We have focussed our studies on the van der Waals (vdW) like phase transitions and their manifestations, including Joule-Thomson expansion and heat engine in asymptotically AdS black holes. We have chosen charged black hole with a global monopole and a regular Bardeen black hole in AdS spacetime for our study. A detailed study of thermodynamical properties through the Hawking temperature, mass, entropy, heat capacity, and Gibbs free energy is conducted. A first-order phase transition similar to the liquid-gas phase transition is observed between small black hole (SBH) and large black hole (LBH) phases. Critical exponents calculated near the critical point match those of the van der Waals fluid. Further, we investigate the effect of global monopole on the Joule-Thomson expansion (JT) of charged AdS-black holes. We have calculated an exact expression for the JT coefficient, which determines the cooling and heating in the final phase. Using the JT coefficient, we have analyzed the effect of the monopole parameter in the inversion temperature and isenthalpic curves. Similarly, we have extended our studies to regular Bardeen black holes. In another aspect of our study, a heat engine constructed using a black hole as a working substance. The heat engine efficiency is calculated via thermodynamic cycle in the PV plane, which receives and ejects heat. It is observed that the heat engine efficiency is improved by adding a quintessence field. In the second part of the thesis, we probe the phase structure of the black hole using thermodynamic geometry. Constructing a thermodynamic metric in the phase space, we studied the critical behavior and microstructure of the black hole. Utilizing the analogy with vdW fluid, the thermodynamic geometry of a charged Bardeen AdS black hole is analyzed.Item Spin Manipulation in Functional Materials: Study from first Principles(National Institute of Technology Karnataka, Surathkal, 2021) Reddy, Indukuru Ramesh.; Tarafder, Kartick.The thesis investigates an intriguing phenomenon, namely the Spin Crossover (SCO) that has recently been observed in many functional materials. A detailed theoretical investigation of SCO phenomena in newly synthesized materials has been carried out by employing first-principles density functional theory +U calculations. The spin state switching of a class of square-planar magnetic molecules and their interactions to the metal surfaces has been investigated. The SCO triggered by an electric polarization was observed in the perovskite Sr2CoO3F (SCOF) system. In a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectric BaTiO3 (BTO) layers, the spin state of the Co atom in SCOF can be switched systematically from a high-spin to a low-spin by altering the polarization direction of the BTO with respect to SCOF. A giant magnetoelectric coupling has also been observed in this system. Pressure-driven SCO has been observed in Hofmann clathrate, namely (Fe{OS(CH3)2}2{Ag(CN)2}2), while applying the hydrostatic pressure. The study shows that under a relatively low isotropic hydrostatic pressure, the complex exhibits a reversible spin switching. The investigation reveals that the system undergoes a structural phase transition when the pressure is anisotropic. The transition pressure for the spin-state transition and structural transformation has been estimated from firstprinciples calculations. In the final stage of this work, the spin crossover in metalorganic molecules and their interactions with magnetic metal substrates have been investigated. The structural, electronic, and magnetic properties of Ni-quinonoid and Ni-dinuclear molecules have been studied upon adsorption on Co(001) substrate. The study shows that these molecules undergo a spin state switching when they adsorbed on the Co(001) surface. The exchange couplings between the magnetic centers are carefully investigated. Further, the spin state and magnetic anisotropy energy of Ni atom in the Ni-dinuclear molecule adsorbed on a Co(001) substrate has tailored by using adatom as an axial ligand to the central transition metal (TM) atoms in the molecule.Item Preparation and Characterization of Zinc Tungstate and Composite as Anode Material for Lithium-Ion Battery(National Institute of Technology Karnataka, Surathkal, 2021) K, Brijesh.; Nagaraja, H S.The thesis entitled “preparation and characterization of zinc tungstate and composite as anode material for lithium-ion battery” cover the preparation, characterization and electrochemical analysis as anode material for Lithium-ion battery (LIB) of zinc tungstate and their composites (ZnWO4/SnO2, ZnWO4/GeO2 and ZnWO4/SiO2) via solvothermal and microwave method. The structural, elemental and morphological properties of the prepared samples are characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), high-resolution transmission electron microscopy (HR-TEM), Brunauer-Emmett-Teller (BET) measurements, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. Prepared ZnWO4 and ZnWO4@r-GO nanocomposite delivered initial discharge capacity of 746 and 1158 mAh g-1 respectively at current density of 100 mA g-1 and potential window 0.02 - 3 V versus Li/Li+ at room temperature. Further, ZnWO4/SnO2 and ZnWO4/GeO2 are tested as anode material for LIB. Increasing percentage of SnO2 increases the specific capacity of the ZnWO4/SnO2 composite and GO further boosts the specific capacity of the composite. The capacity of the first discharge curve of ZWSN-5, ZWSN-10 and ZWSN-10/GO nanocomposite is noticed as 882, 1316 and 1486 mAh g-1 respectively. While in the case of ZnWO4/GeO2, the initial discharge capacity of ZWGE5, ZWGE10 and ZWGEC nanocomposites were 761, 825, and 930 mAh g−1, respectively. Further, CNT boosts the performance of the ZnWO4/GeO2 composite. ZnWO4/SiO2 is prepared via microwave method and used as an anode material for LIB. The initial charge discharge capacity of the ZWSO5 nanocomposites is 570 and 314 mAh g-1 respectively at 10 mA g-1. The discharge capacity of the ZW, ZWSO1, ZWSO2, ZWSO3 and ZWSO4 are 145, 265, 278, 363 and 453 mAh g−1 respectively. The increasing amount of SiO2 in the ZnWO4/SiO2 composite increases the overall performance of the ZnWO4/SiO2 composite.Item Studies on Corrosion, Mechanical and Wetting Properties of the Thermal Sprayed Coatings on Low Carbon Steel(National Institute of Technology Karnataka, Surathkal, 2021) A, Amudha.; Shashikala, H D.; Nagaraja, H S.Low-carbon or mild carbon steels are very attractive materials throughout the industrialized world in diverse applications but are susceptible to corrosion. This problem can be mitigated and the service lifetime of the low carbon steel can be increased by the application of protective coatings with good mechanical properties while reducing maintenance costs. In the present work, different corrosion-resistant materials like metal alloy (Inconel-625), ceramic-metal composite 25(NiCr)-75Cr3C2 and ceramic-graphene oxide nanoplatelets(GNP) composite (Al2O3-GNP and ZrO2-GNP) were coated using thermal spray techniques like weld overlay, High-Velocity Oxyfuel (HVOF), and Atmospheric Plasma Spray (APS) techniques respectively. The structural, morphological and compositional studies were carried out by XRD, FTIR, Raman Spectroscopy, XPS, FESEM-EDAX, TEM, and BET characterization techniques. The corrosion studies were conducted using the three-electrode electrochemical system. The stability of the coatings was studied using immersion tests upto 14 days. The mechanical and wetting properties of samples were studied using Vicker’s microhardness tester and contact angle measurements respectively. ANSYS FEA simulation showed that alternate skip weld overlay of SS-309Mo as the buffer layer by GTAW and Inconel-625 as final layer by SMAW process for the 6 mm thick low carbon steel substrate preheated to 100°C, to be the best model with 18 MPa surface residual stress among twelve combinations. Using the conditions of the best model, SS-309Mo and Inconel-625 have been coated on low carbon steel. The weld overlay coated Inconel-625 had nearly the same corrosion resistance as that of bulk Inconel-625 with increased microhardness. 25(NiCr)-75Cr3C2 cermet coating on low carbon steel using HVOF process showed hydrophobic behaviour with improved microhardness and corrosion resistance. The α-Al2O3- (X wt. %GNP) and ZrO2-(X wt. %GNP) (where X= 0, 0.5, 1.0, 1.5 and 2) composite coatings by APS process were successful in the retention of GNPs in the composite. The surface corrosion resistance increased by six orders of magnitude when coated with 2.0 wt.% GNP reinforced α-Al2O3 nanocomposite, in comparison with bare Al2O3 coating. The increase in corrosion resistance is due to the hydrophobic nature of in-situ reduced GNP. In addition, the mechanical properties have improved with the addition of GNP. The corrosion rate of ZrO2-2 wt. % GNP coating is 130 times lesser than that of ZrO2. Further, the mechanical and wetting properties of the coatings showed a similar trend as that of corrosion behaviour.