Lakshminath Bezbaroa Central Library Digital Repository
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Recent Submissions
Expeditious Reactivity of Sulfoximines and o-Alkynylanilines Towards the Construction of C-C and C-Heteroatom Bonds
(2024) Chakraborty, Nikita
The contents embodied in this thesis is divided into five chapters including one introductory chapter based on experimental results obtained during the research period. The introductory chapter represents an overview of the utility of sulfoximines and o-alkynylanilines towards the construction of C-C and C-heteroatom bonds. This includes a brief discussion about all the possible reactive sites present in sulfoximines and their possibility of forming different functionalized molecular complexities. Similarly, the reactivity of o-alkynylanilines for the formation of diverse heterocyclic scaffolds has been discussed.
Design And Fabrication Of Solid State TiO2|Ag Structure For Developing Efficient Plasmonic Photo-Electric Conversion Device
(2024) Devi, Kshetrimayum Priyalakshmi
The global shift towards renewable energy sources, propelled by environmental concerns, has ignited a surge in research aimed at developing efficient solar energy technologies. This thesis is dedicated to the creation of a solid-state plasmonic energy harvesting device using low cost methodologies. Initial simulations compare various noble metal nanoparticles for their plasmonic resonance properties, with silver identified as particularly advantageous due to its sensitivity and electronic characteristics. Semiconductor substrates are synthesized through a simplified Sol-Gel technique, resulting in the production of TiO2 thin films tailored for solar applications. Subsequently, a solid-state energy harvesting device is fabricated, leveraging metal/semiconductor heterojunctions to achieve promising cell performances. Furthermore, the thesis delves into an environmentally friendly approach to synthesizing silver-graphene nanocomposites, which holds significant potential for enhancing device efficiency. These findings represent a significant stride forward in the design and implementation of efficient plasmonic energy harvesting devices, paving the way for sustainable advancements in solar power generation.
Snow/Glacier Dynamics of Himalayan Ranges and Associated Hazards
(2023) Mondal, Sandeep Kumar
The Himalayan cryosphere, also termed “the Third Pole, " acts as a natural storehouse to several mega river systems, such as the Ganges, Indus, and Brahmaputra, serving billions of people with fresh water. However, the high mountain ranges of the Himalayas are also susceptible to various categories of hazards, such as snow/rock-ice avalanches, landslides, earthquakes, and glacial lake outburst floods (GLOFs). The present study attempts to perform a series of investigations to better understand the dynamics of snow/glaciers in the Himalayan region and their contribution to perennial river streams. It also includes studying the major types of Himalayan hazards that have occurred in the recent past.
(A) numerical investigation on augmented heat flux in non-standard variants of turbulent Rayleigh-Bénard convection
(2023) Chand, Krishan
The present work investigates non-standard variants (roughness-aided and tilted convection) of Rayleigh-Bénard convection (RBC) to augment heat flux for a fixed working fluid (Prandtl number = 0.7) over a wide Rayleigh number range (106 ≤ Ra ≤ 1010). For both 2D and 3D, the study focuses on the coherent structures and heat transfer mechanisms in different configurations of Rayleigh-Bénard convection (RBC), considering thermal plumes, boundary layers, and large-scale rolls (LSR). In the smooth case, the absence of lateral direction results in the entrapped thermal plumes, which are subsequently emitted as thermal jets into the bulk. The Nusselt number (Nu) quantifies the heat carried by thermal plumes across the isothermal walls. A positive correlation between vertical velocity and temperature fluctuations is used to quantify thermal plumes. The impact of surface roughness on heat flux is investigated, highlighting the influence of irregular roughness geometries. The study identifies an onset of enhanced heat flux regime and explores the role of bulk-plume interaction and fluid mixing. With increasing Rayleigh number, transformation from a double-roll state to multiple-roll state is associated with the onset of enhanced heat flux regime for the taller configuration. On the other hand, presence of huge number of roughness elements is responsible for enhanced heat flux in the smaller configuration. Near-wall dynamics and the penetration of peaks into the thermal boundary layer are studied, revealing the significance of secondary vortices and the tendency of plume emission. The investigation extends to three-dimensional RBC with conical roughness configurations, emphasizing the role of coherent structures and intense thermal plumes in enhancing heat flux. The study provides insights into the influence of roughness on flow strength and the orientation of large-scale rolls. The effect of inclination angles in tilted RBC is examined, indicating shifts in heat transport effectiveness and early onset of turbulence with increased roughness height. In the smooth case, inclined convection (IC) enhances heat flux below Ra = 108, while above this value, normal RBC yields the highest heat flux. However, for rough surfaces, the effectiveness of IC to transport heat shifts to lower Ra as the roughness height increases, leading to an early onset of turbulence. The maximum heat flux in the smooth case is achieved at a tilt of 75° for Ra ≤ 108, while in roughness cases, it depends on both Ra and the roughness configurations. The study reports a maximum increase of 25% in Nusselt number for roughness-aided tilted convection. Additionally, as Ra increases, the onset of thermal stratification is delayed in the smooth case, while an increase in roughness height results in a similar delay in rough configurations, indicating an early onset of turbulence even at larger inclination angles.
Rare-earth elements based Perovskites: Bulk and Low Dimensional Superlattices
(2023) Das, Shaona
Perovskites have been a subject of extensive research due to their immense potential applications in magneto-electronics and photovoltaics, ever since the discovery of Calcium titanate (CaTiO3) in the Russian Ural Mountains by Gustav Rose in 1839. The mineral was named after the Russian mineralogist Lev Perovski. The present thesis delineates the investigation of thin films, including superlattices and bilayer, as well as polycrystalline perovskite oxides that comprise transition metal and rare earth elements. Bilayers comprising [La0.7Sr0.3MnO3(5 nm)/LaCoO3(15 nm)] were fabricated on SrTiO3 substrates using two different deposition sequences. Our findings indicate the presence of magnetic characteristics, specifically the pseudo antiferromagnetic (AFM) pinned character, in one bilayer, while the other exhibits solely ferromagnetic (FM) nature. Additionally, we observed that the lattice mismatch and lattice strain resulted in the suppression of the Curie temperature and other physical properties. The present study investigates the structural, morphological, electronic, optical, and magnetic properties of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices that were deposited through pulsed laser deposition (PLD) on SrTiO3-(001), (011), and (111) substrates. The study reveals that the mixed valence Ni2+/3+ and Mn3+/4+ electronic states are dominant at the core level. Furthermore, the relative intensity ratio of the Mn ions is found to be higher in the superlattices grown on (111) oriented SrTiO3 compared to the other two orientations. The calculated hopping energies, obtained from the variable range hopping mechanism, are of significant magnitude (≥ 40 meV). A noteworthy observation was made regarding the decrease in Curie temperature from 67 K to 110 K, coupled with a marked increase in the effective exchange interaction. Polycrystalline samples of Dy1-xCexCrO3 were prepared, where x ranged from 0.1 to 0.5. Our findings indicate that the tolerance factor increases while the octahedral distortion factor decreases with increasing Ce doping. This suggests that greater stability is achieved in samples with higher levels of Ce doping. The study also documented an increase in the Néel temperature from 156 K to 162 K in the heavily doped samples exhibiting G-type AFM character with 𝛤4(𝐺𝑥,𝐴𝑦,𝐹𝑧) spin-configuration. In contrast, samples with x = 0.2-0.5 demonstrated a phase-transition across TPC (< 𝑇N1) with 𝛤2(𝐹𝑥,𝐶𝑦,𝐺𝑧), while samples with x = 0.1-0.3 underwent another magnetic phase transition TSR (< TPC) with 𝛤25 (𝐹𝑥,𝐶𝑦,𝐺𝑧; 𝐹𝑥𝑅,𝐶𝑦𝑅,𝐺𝑥𝑅,𝐴𝑦𝑅). An increase in the magnetic entropy change (Δ𝑆𝑀) was observed in the DyCrO3 system with 10% Ce substitution and improved refrigerant capacity (RCP) of approximately 360 J/kg. This was measured at a temperature of 5 K and a magnetic field strength of 40 kOe, suggesting potential advancements in magnetic refrigeration. Previous studies on DyCrO3 under the same conditions reported a Δ𝑆𝑀 of 256 J/kg. The Gd1-xSmxCrO3 samples were synthesized with varying Sm concentrations, specifically x = 0.1 (GSO1), 0.5 (GSO5), and 0.9 (GSO9). Notably, GSO5 demonstrated multiple magnetization switching behavior across all three ZFCW, FCC, and FCW protocols, rendering it a promising candidate for magnetic switching applications. This phenomenon has not been previously reported in any perovskite oxide materials. The coexistence of metastable
magnetic phases Γ4 (𝐺𝑥 , 𝐴𝑦 , 𝐹𝑧) and Γ2 (𝐹𝑥 , 𝐶𝑦 , 𝐺𝑧) was observed in the GSO9 sample. This phenomenon resulted from the clustering of ferromagnetic islands within an antiferromagnetic matrix in the face-centered cubic case, which is referred to as a magnetic glass-like signature. The analysis of refined structural parameters obtained from X-ray diffraction indicates a fluctuation in the tilt angles,
which can be attributed to the quasi-harmonic effect resulting from the exchange interaction between Gd3+ and Cr3+ ions. This effect causes a reduction in the stiffness of the A1g (3) mode as the temperature increases.