http://localhost:8080/xmlui/handle/123456789/389 2026-04-26T08:14:37Z http://localhost:8080/xmlui/handle/123456789/3509 Title: Understanding Phonon Transport in Extended Solids Using First Principles Calculations Authors: CHAND RAKESH ROSHAN, S Abstract: Materials with an extreme lattice thermal conductivity (L) are indispensable for thermal energy management applications. Therefore, microscopic understanding of phonon transport is critically important for designing functional materials. In the present thesis, a systematic investigation has been made for in-depth understanding of phonon transport in binary and ternary compounds using first principles calculations in combination with Boltzmann transport theory. In contrast to the expected trend based on their atomic mass, anomalous trends for L are observed in binary systems, namely Alkaline-earth chalcogenides and Alkali halides. It has been shown how atomic mass contrast can tune the contribution of optical phonons to L and its implications on scattering rates either enhancing or suppressing L. Ternary alkaline-earth halofluorides and Bismuth halooxides provide an avenue for designing functional materials with low L due to their intrinsic bonding heterogeneity. Investigation of iso-structural layered materials with varying average atomic mass is intriguing because they allow to make structure-property correlations by exploring the interplay between bonding heterogeneity and atomic mass and their implications on lattice dynamics thereby tailoring the phonon transport properties. Overall, the present thesis focused on understanding interplay amongst crystal structure, atomic mass, chemical bonding, mechanical properties, lone pair activity, and their role in phonon transport properties, which would aid in designing extremely low L materials. This is indispensable for the development of sustainable energy conversion devices for future thermal energy management applications. The Thesis consists of six chapters, the finer details are provided below. Chapter 1: It provides an introduction to the domain of phonon transport in extended solids and why low L plays a crucial role in thermal management applications. It also provides a comprehensive view of the various mechanisms affecting the phonon transport, both extrinsic and intrinsic with a comprehensive literature survey of the mechanisms to lower L. Chapter 2: This chapter provides the theoretical background for the current work and the computational methodology utilized for this work. iii The Density Functional Theory (DFT) formalism and an overview of first principles calculations are discussed. Both harmonic and anharmonic approximations concerning the phonons and phonon transport has been discussed followed by the Boltzmann transport theory for obtaining L, phonon-phonon scattering mechanism has been discussed, specifically three phonon scattering, as the same has been considered in the current work. The methodology employed in the present study, known as Temperature Dependent Effective Potential (TDEP), is elaborated upon, this is followed by an overview of list of packages utilized for the current work. Chapter 3: The first part of the chapter focuses on a detailed and comparative study on phonon transport of Alkaline Earth Chalcogenides (AEC’s) MCh (M = Mg, Ca, Sr, Ba and Ch = O, S, Se,Te) compounds in order to provide insights to achieve low L materials through phonon engineering. More light is shed on understanding lattice dynamics, phonon transport, and mechanical properties of 16 MCh (M = Mg, Ca, Sr, Ba and Ch = O, S, Se, Te) compounds. The second part of this chapter deals with another set of isostructural binary systems, Alkali Halides (AH’s), consisting of 20 MX ( M = Li, Na, K, Rb, Cs and X = F, Cl, Br, I) compounds and presented in comparison with the results obtained with AEC’s. This chapter provides an in-depth understanding of atomic mass and its effect on phonon transport properties of AH’s and AEC’s. Furthermore, this reveals that by manipulating the atomic masses, one can engineer materials with both high and low values of L, providing exciting possibilities for tailored thermal conductivity in various applications. Chapter 4: This chapter explores layered materials which are bonded through strong covalent/ionic bonds within the plane (in-plane) and coupled by weak van der Waals (vdW) interactions in the perpendicular (out-of-plane) direction i.e., bonding heterogeneity, thus resulting in a strong structural anisotropy. Therefore, through bonding heterogeneity, these layered materials provide an avenue for tailoring phonon transport properties. Investigation of iso-structural layered materials with varying average atomic mass is intriguing because they allow structure-property correlations by exploring the inter-play between bonding heterogeneity and atomic mass and their implications on lattice dynamics, thereby fine-tuning the phonon transport properties. Consequently, for layered materials, a microscopic understanding of crystal structure, iv bonding, anharmonic lattice dynamics, and phonon transport properties is of the utmost importance. Alkaline-earth halofluorides, MXF (M = Ca, Sr, Ba and X = Cl, Br, I) belong to the class of matlockite (PbClF)-type layered materials and they provide an avenue to ex plore the interplay between crystal structure, atomic mass, and bonding heterogeneity and thereby to fine tune their phonon transport properties. The outcomes of the chapter are that structural anisotropy and/or bonding plays a crucial role along with atomic mass in determining the L in these iso-structural MXF compounds. This study on MXF compounds provides an in-depth understanding on interplay among crystal structure, atomic mass and bonding heterogeneity, which would aid in designing extreme L materials by manipulating in-plane and out-of-plane bonding for future thermal energy management applications. Chapter 5: This chapter explores another family of layered materials known as Bismuth halooxides, BiXO (X= Cl, Br, I). BiXO is composed of a series of ionically bonded X-Bi-O-O-Bi-X layers stacked perpendicular to the c-axis. These layers are held together by weak van der Waals (vdW) interactions; consequently, these compounds exhibit bonding heterogeneity, featuring in-plane ionic bonding and out-of plane weak vdW bonding. The rattling mechanism owing to bonding heterogeneity that results in an ultralow L has been described. Chapter 6: This chapter summarizes the results that are obtained and consolidates the same by proposing few design principles for obtaining low L materials followed by the future scope of work. Description: NITW 2024-01-01T00:00:00Z http://localhost:8080/xmlui/handle/123456789/3508 Title: Studies on Coherent Population Trapping Resonance in Rubidium for Application in Atomic Clocks and Timekeeping Methodology for Satellite Navigation Authors: Rajaiah, Kaitha Abstract: Atomic clocks with superior frequency accuracy and stability are essential for timekeeping applications. The application of clocks in satellite navigation demands the accurate predictability of clock behaviour and their utility in space necessitates miniaturization. Coherent population trapping (CPT) is a promising technique for developing miniaturized atomic clocks. Two main objectives are pursued in this thesis. The first objective is to investigate the characteristics of CPT resonance and derive optimum operating parameters as well as system configuration for improved frequency stability of CPT based atomic clock. The second one is the real-time characterization of on-board clock behaviour and development of optimum timekeeping methodology to ensure uninterrupted navigation service to the user. An experimental investigation is carried out to study and optimize CPT resonance characteristics (quality figure and frequency shift) with respect to critical parameters such as dimension of Rubidium vapor cell, laser intensity, cell temperature, Radio Frequency (RF) power, buffer gas species and pressure inside the vapor cell. A theoretical model based on four level atomic system is developed to understand the laser-atom dynamics that governs the CPT phenomena in alkali atoms. A new empirical parameter is introduced in the model to account for the influence of cell size on resonance characteristics. Designed and implemented a methodology to solve the differential equation governing four level atomic system through the utilization of Runge-Kutta 4th order numerical integration method. Theoretical computation of quality figure is carried out for cells with different dimensions, buffer gas pressures, temperatures and compared with the experimental results. The outcome of this study helped in deriving optimum operating parameters and system configuration in order to achieve enhanced frequency stability of CPT based atomic clocks. The performance of atomic clocks used in navigation satellites plays a crucial role in providing the desired position accuracy for navigation users. Continuous monitoring and real time characterization of these clocks are necessary to provide uninterrupted navigation service to the users. In this regard, designed and developed a scheme for characterization and continuous monitoring of on-board clocks using one-way carrier phase measurements to sensitively detect the on-board clock anomalies. Devised and developed a single algorithm that detects anomalies present in the clock data. The characterization scheme developed in this study enables the timely generation and up-linking of clock correction parameters to ensure the user position accuracy within the specification. i In each navigation satellite, satellite clock offset is allowed to accumulate only up to the pre-allocated broadcast limit beyond which the range measurements are unreliable owing to significant error in user position. Hence, the clock offset needs to be maintained within the given broadcast limit, which is referred to as satellite timekeeping. A timekeeping methodology is established to maintain the satellite time optimally within the allocated broadcast limit. New mathematical model is derived to compute optimal frequency offset correction that provides maximum time interval between the two successive corrections to maintain the satellite clock offset always within the broadcast limit. The developed methodology facilitates the seamless navigation service to the user. Description: NITW 2024-01-01T00:00:00Z http://localhost:8080/xmlui/handle/123456789/3507 Title: Design and Development of ZnO Nanosheet based Nanogenerators to Drive Self-Powered Systems Authors: POTU, SUPRAJA Abstract: The never-ending quest for sustainable energy sources has led to the development of novel technology capable of harvesting ambient mechanical energy. Among these, piezoelectric and triboelectric nanogenerators have emerged as attractive opportunities for energy harvesting that are both efficient and environmentally beneficial. The investigation opens with a detailed examination of piezoelectric nanogenerators, clarifying the piezoelectric phenomenon and its application in transforming mechanical vibrations into electrical energy. Various materials are investigated for their applicability in improving the performance of piezoelectric nanogenerators. Following that, the thesis delves into triboelectric nanogenerators, which use the triboelectric effect to generate electricity through material friction. The detailed mechanism of the TENG that drives triboelectric charge generation is investigated, with a focus on the wide range of materials used, such as polymers, metals, and nanostructures. Innovative fabrication processes and surface engineering methodologies are being investigated to improve the output performance, scalability, and adaptability of the triboelectric nanogenerator. The synthesis process involves the direct growth of ZnO nanosheets on an aluminum substrate, and their structural, morphological, compositional, and surface potential analyses are conducted to gain insights into the material's properties. Further, discusses a detailed analysis of the open circuit voltage, short circuit current, and durability of the ZnO nanosheet-based nanogenerators (PENG/TENG). The devices exhibit promising results, showcasing their potential for efficient energy harvesting. Notably, the study explores various applications, demonstrating the device’s ability to charge capacitors and power small electronic devices such as light-emitting diodes (LEDs), digital thermometers, digital watches, etc, owing to the adaptability of PENGs and TENGs used in real-world applications, proving their adaptability in multiple domains. Chapter 1 provides an introduction to the field of energy harvesting and its diverse energy sources. It briefly outlines the various types of nanogenerators and delves into strategies and methods to improve the output performance of TENGs. Additionally, this chapter explores the selection of materials and potential applications for the PENG and TENG devices. Chapter 2 the process of synthesizing zinc oxide nanosheets (ZnO) using the hot plate-assisted hydrothermal method is detailed. This chapter discusses material characterization and justifies the usage of specialized approaches of the synthesized material, utilizing advanced tools for understanding its structural, morphological, compositional, and surface detail. The chapter also (vi) provides insights into the steps involved in creating piezoelectric and triboelectric nanogenerator devices for mechanical energy harvesting. Furthermore, it elucidates the techniques employed for characterizing the devices characterizing devices and using instrumentation to assess their properties. Chapter 3 deals with the synthesis of ZnO nanosheets, covering their characterization, results, and a detailed discussion. The chapter's focus includes optimizing the growth temperature and time for the synthesis of ZnO nanosheets. The study encompassed a range of temperatures from 50°C to 90°C, with a consistent growth duration of 4 hrs. Additionally, at a fixed growth temperature of 85°C, the growth time was varied from 1 hour to 6 hours. Surface morphology, crystalline properties, and microstructure analysis of the ZnO nanosheets were conducted using SEM, XRD, and TEM techniques. TEM revealed the 2D structure of the ZnO nanosheets, while EDAX confirmed their purity. Furthermore, PENG device was fabricated using the ZnO nanosheet film as the active piezoelectric layer, and ITO and aluminum served as electrodes. The chapter includes a comprehensive exploration of the device characterizations. Multiple PENG devices were tested, each employing ZnO nanosheets synthesized at varying growth temperatures and times. The growth conditions of 85°C for 4 hrs were optimized for the PENG device. The chapter also explores the switching polarity test and load characteristics of the PENG device. Notably, the maximum output voltage and power density for the PENG were recorded at 225 mV and 77 µW/m², respectively. The response of the PENG to different finger tapping forces was studied, demonstrating that increased pressure and deformation of the nanosheets resulted in higher output. This finding suggests potential applications in pressure and force sensing. Additionally, the chapter discusses the fabrication of a Triboelectric Nanogenerator (TENG) device utilizing ZnO nanosheets as the tribo-layer, ITO, and aluminum as electrodes. Under hand tapping force, the TENG yielded an output voltage of 2 V, a current of 7 µA, and a power density of 0.1 mW/m². The TENG was also used to charge various capacitors, achieving a maximum stored energy of 2.8 µJ. Notably, the TENG was capable of directly powering one LED under each hand-tapping force. Chapter 4 delves into the synthesis and the characteristic properties of ZnO nanosheets and PET (Polyethylene Terephthalate). X-ray Diffraction (XRD) analysis confirmed the polycrystalline nature of ZnO nanosheets, and their distinctive nanosheet structure was verified through SEM analysis. A notable achievement discussed in this chapter is the fabrication of a simple and cost-effective TENG using ZnO nanosheet film and PET as triboelectric layers for (vii) the first time. This TENG device demonstrated an impressive output, with an output voltage of 4.9 V, a current of 10 µA, and a power density of 1 mW/m². The chapter also explored the TENG's output response at different frequencies of hand tapping, ranging from 1 to 7 Hz, and varied the active areas of the devices for comprehensive study. The TENG's stability was rigorously tested, more than 1000 cycles, demonstrating its durability and reliability over numerous cycles. Furthermore, the chapter delved into the charging characteristics of various capacitors and examined stored voltage, charge, and energy concerning the load capacitor. The TENG achieved a maximum stored charge of 40 µC and stored energy of 16.9 µJ. Further, The TENG successfully powered three LEDs directly, and the stored energy in the capacitor enabled the activation of a digital watch and 24 LEDs. This underscores its potential as a promising solution for self-powered sensors and devices. Chapter 5 focuses shifts to the utilization of ZnO nanosheets in conjunction with Overhead Projector (OHP) sheets for TENG devices in various applications. An innovative addition to this chapter is the introduction of OHP sheets as a novel triboelectric layer. OHP sheets, composed of transparent PET material, possess pre-charged properties on one side, making them efficient at receiving ink particles. The existence of charge on the OHP sheets reduces the surface ion injection step. Comprehensive morphological, elemental, and structural analyses were conducted using SEM, EDX, and XRD techniques. Additionally, Atomic Force Microscopy (AFM) confirmed that the ZnO nanosheet film exhibited greater roughness compared to the OHP sheet. This rougher surface configuration forms a more effective contact area, contributing to the generation of high output power in the TENG. A high-performance TENG device was fabricated, leveraging ZnO nanosheets and OHP sheets as frictional layers. The TENG device exhibited notable characteristics, with an output voltage of 150 V, a current of 34.5 µA, and an impressive power density of 424 mW/m2. The stability of the TENG was rigorously tested, demonstrating its robust performance with over 4,300 cycles under hand tapping and 10,000 cycles under machine tapping. Moreover, the effects of different active areas of TENG devices, varying frequencies of applied forces, and different magnitudes of applied force were thoroughly investigated. Notably, the TENG device achieved remarkable results, with a maximum stored voltage of up to 9 V, a stored charge of 150 µC, and a stored energy of 160 µJ, observed under the optimal load capacitance of 47 µF. In a practical demonstration of its capabilities, the TENG directly powered 135 LEDs and a digital watch when subjected to hand-tapping forces. Additionally, the TENG was employed to power an electroluminescence device, which finds potential applications in e-paper and (viii) digital billboard technologies. Leveraging the TENG's high-output performance, it was integrated with Cholesteric liquid crystal (CLC) devices, opening the door to a range of real time applications. These applications include secure authentication devices, such as facial unlocking in mobile phones and QR code scanning. Furthermore, the TENG was used to drive smart windows and CLC displays, which capitalize on the CLC's ability to control transparency in the planar state and opacity in the focal conic state. This integration allowed for the display of letters and various information on CLC devices, expanding their utility. Chapter 6 A key innovation in this chapter was the surface modification of ZnO nanosheets was introduced to enhance the output of the TENG. We have synthesized ZnO nanosheets on aluminum substrate with any seed layer using hotplate assisted hydrothermal method. The surface of ZnO nanosheets was modified by the direct growth of Zeolitic imidazolate framework-8 (ZIF-8) crystals using the hydrothermal method. ZIF-8, belonging to the Metal Organic Framework (MOF) family, consists of metal ions (Zn2+) and organic linkers (imidazolate). Structural and morphological analyses of both ZnO and ZnO modified with ZIF 8 (ZnO@ZIF-8) were conducted. XRD and Raman spectra confirmed the successful formation of ZIF-8 on ZnO nanosheets. The TENG device employing ZnO@ZIF-8 and PMMA as triboelectric layers exhibited significant improvements, achieving an output voltage of 200 V, a current of 41.5 µA, and a substantial power density of 800 mW/m². This performance represented nearly a twofold increase compared to ZnO-based TENGs. To validate this enhancement, the work function of ZnO and ZnO@ZIF-8 films was studied. The work function difference was notably greater for ZnO@ZIF-8 and PMMA compared to ZnO and PMMA. A larger work function difference enhances electron transfer, contributing to the enhanced output performance of the TENG. Additionally, the chapter explored the characteristics of charging capacitors under hand-tapping forces, examined the TENG's stability over an extended duration, and tested it under machine tapping for approximately 20 minutes, equivalent 4800 cycles. Notably, the ZnO@ZIF-8 TENG demonstrated its ability to directly power various electronic devices, including an electronic watch, calculator, and thermometer. Impressively, it directly powered 180 LEDs without any storage element, highlighting its potential for real time self-powered devices and biomechanical energy harvesting applications. Chapter 7 discusses a high-performance TENG is achieved through a surface engineering method involving emery papers to modify the surface of one of the triboelectric layers. This innovative approach offers several advantages, notably its cost-effectiveness and simplicity. The methodology involves scratching the aluminum substrate surface with different grit sizes (ix) of emery papers, ranging from 80 to 400, in one direction multiple times, followed by the incorporation of ZnO nanosheets using a hydrothermal method. This process not only increases the surface area but also enhances contact points, resulting in improved output. Under an optical microscope, all the samples were examined, and it was observed that the use of P220 grit-size emery paper created optimum roughness and provided more contact points. SEM analysis of the ZnO nanosheets confirmed that the density of the nanosheets increased after the modification of the aluminum substrate, while PDMS exhibited a smoother surface. The work function of various samples was measured, and it was found that ZnO nanosheets incorporated on scratched aluminum with P220 grit size (ZnO-Al220) had a lower work function. Additionally, the work function difference was higher between PDMS and ZnO-Al220(T12). This TENG exhibited the potential for the highest output due to its enhanced electron transfer between the tribo-layers. Multiple TENG devices were fabricated and tested for their output responses, with the T12 TENG producing the highest output voltage, aligning with the work function measurements. The T12 TENG generated the highest output voltage, current, and power density, reaching 1442 V, 155 µA, and 10.8 W/m², respectively. To gain insights into the electrical potential distribution and charge transfer process of the TENG device, finite element simulations were conducted using COMSOL 6.0 software. The stability of the TENG was thoroughly tested through repeated applied forces over 13,000 cycles at a frequency of 3-4 Hz. The chapter also explored various experimental parameters affecting the TENG device (T12), including the active area, frequency of hand-tapping forces, spacing between triboelectric layers, and application of different hand-tapping forces. Impressively, the TENG powered 820 LEDs and activated various electronic gadgets. The TENG device's high sensitivity was further harnessed by attaching it to the human body's chest to monitor output voltage and current signals generated by different breathing patterns. This allowed for the detection of various respiration rates, including normal, deep, and rapid breathing. The TENG device holds significant promise for convenient and accurate breath monitoring, potentially impacting health management and optimizing athletic performance. Chapter 8 describes the general summary, conclusions, and scope for future work Description: NITW 2024-01-01T00:00:00Z http://localhost:8080/xmlui/handle/123456789/3469 Title: DESIGN AND OPTIMIZATION OF GRAPHITIC CARBON NITRIDE HYBRID NANOSTRUCTURES FOR ENHANCED PHOTOCATALYTIC ACTIVITY Authors: MADHURIMA, V P Abstract: Carbon nanomaterials (CNMs), ever since their discovery have gained widespread attention due to their remarkable properties like high tensile strength, Young’s modulus, surface area to volume ratio, thermal and electrical conductivities, carrier mobility and many more. Due to their phenomenal features, CNMs have catered to several fields of research ranging from aerospace to drug delivery applications. The present study was an effort to develop and understand the photocatalytic behaviour of CNMs-based graphitic carbon nitride (g-C3N4) hybrid photocatalyst materials for effective degradation of rhodamine B (RhB) dye. Hence, the overall objective of this thesis work was to develop g-C3N4 photocatalyst material by optimizing the synthesis parameters and study its photocatalytic dye degradation property. Further, to enhance the photocatalytic performance of g-C3N4 by synergistically coupling with the CNMs and finally to compare & study the photocatalytic property and eventually propose a degradation mechanism based on morphology of the developed photocatalyst materials. In this thesis work, 2D g-C3N4 was synthesized using a conventional thermal decomposition process using melamine as the precursor material. The synthesis parameters like reaction temperature and atmosphere were optimized to obtain the near-ideal g-C3N4 material. This optimized photocatalyst was further improved by incorporating ammonium chloride (NH4Cl) during the synthesis process. The role of NH4Cl was to act as a bubbling agent and provide porous and thin sheets of g-C3N4. The structure, morphology, and optical properties of the developed photocatalyst materials were investigated using XRD, SEM, TEM, UV-DRS, FTIR, PL, BET and XPS techniques. The photocatalytic behaviour was evaluated with the help of an in-house fabricated reactor taking RhB as the target dye. It was observed that the improved g-C3N4 removed 94% of the RhB dye (Conc. = 10 mg/L) in 30 minutes whereas the initially prepared g-C3N4 showed only 35% efficiency. Further, various CNMs like carbon nanotubes (CNTs) and carbon soot nanoparticles (CS) were prepared using the arc discharge technique. It is a conventional physical method of preparing CNMs where two graphite rods are brought near and a high current is applied in an inert atmosphere. The sublimation of one of the graphite electrodes and deposition on the cooler surfaces results in the formation of the CNTs and CS. Besides, graphene nanoplatelets (GNP) were synthesized through microwave exfoliation of graphite intercalated compound followed by sheer mixing in a solvent mixture (DMSO & DI water). The obtained CNMs were coupled v with g-C3N4 to attain higher degradation efficiencies. It was observed that CS coupled g C3N4hybrid photocatalyst showed 97% degradation of RhB (conc. = 20 mg/L) in 90 minutes however, pristine g-C3N4 showed 88% in similar conditions. Moreover, the CNT-coupled g C3N4 hybrid photocatalyst also showed 97% degradation in 90 minutes under visible light irradiation. Further, the 2D/2D hybrid photocatalysts showed the highest performance. Here, GNP-coupled g-C3N4hybrid photocatalyst showed 96% degradation in 60 minutes whereas pristine g-C3N4 showed 55% efficiency. The enhanced performance of CNMs coupled hybrid photocatalyst was essentially due to the effective charge separation of photogenerated charge carriers resulting in reduced recombination rates. It was discovered that the 2D/2D coupling showed faster degradation rates than 1D/2D and other coupling systems. This was further proved by developing another 2D/2D system using commercial hexagonal boron nitride (hBN) coupled g-C3N4. It was observed that 91% of RhB was removed in 60 minutes under the visible light source. Therefore, the morphology played a significant role in deciding the overall performance of the materials. Description: NITW 2023-01-01T00:00:00Z