Theses and Dissertations
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Item Embargo MODIFICATION OF NANOMATERIALS WITH INTENSE PULSED ION BEAMS FOR PHOTOCATALYTIC APPLICATIONS(Nazarbayev University School of Sciences and Humanities, 2024-06-04) Abduvalov, AlshynThe world energy problem has been facing harsh challenges in the last few decades due to the increase in energy consumption of the growing world population and decreasing reserves of traditional carbon-based energy resources. Photoelectrochemistry (PEC) based solar water splitting is one of the potential paths for transforming renewable solar energy into green hydrogen fuel to meet energy expectations. In PEC water splitting, hydrogen is generated using semiconductor materials that can absorb sunlight and decompose water molecules into hydrogen and oxygen gases. Fabrication of highly effective, stable, and economically viable semiconductor photoelectrode materials, to increase their performance, and enhancing their photocatalytic activity has been proved to be a vital task for PEC solar water decomposition. Among all the suitable materials, WO3 has been reputed as a promising photoelectrode in the last decade due to the many criteria that it fits. The main current problems in WO3, based PEC water splitting systems are their low solar to hydrogen efficiencies and poor photocatalytic properties. Developing and applying new methods of surface modifications and engineering are favorable strategies to enhance photocatalytic properties of WO3. This thesis work is devoted to the study of surface modification of WO3 photoelectrodes with intense pulsed ion beam (IPIB) irradiation for photocatalytic properties enhancement and studying the effect of IPIB irradiation on solid state dewetting shape formation of plasmonic Ag nanoparticles (NPs). The thesis also reports strategies of combining plasmonic nanoparticles and downshifting photoluminescent NPs with WO3. IPIB irradiation is a method for modification of surfaces of materials with few hundreds keV energetic ions that penetrate deep into 1-2 micrometers. The outcomes of experiments show that surface engineering of WO3 photoelectrode with IPIB can enhance its photocatalytic properties and promote charge-carrier characteristics. The IPIB also gives opportunity to study shape formations of silver NPs and results exhibit huge influence of super-fast annealing on sphericity of the silver NPs. Simultaneous use of plasmonic NPs and fluorescent materials also proven to be effective method of photoactivity enhancement of WO3 thin films.Item Restricted AN EMERGENT COSMOLOGICAL MODEL FROM ASYMPTOTIC SAFETY(Nazarbayev University School of Sciences and Humanities, 2024-05-03) Zholdasbek, AknurThe modern cosmological models tell us that the early universe has gone through a phase of accelerated expansion, known as inflation, which solves the horizon and flatness problems. In this thesis, I present an inflationary cosmological model based on the Asymptotically Safe behavior of the Newton constant at Planckian energies. Unlike other models, the variability of G is given by a multiplicative coupling constant χ in the matter Lagrangian with a conserved energy-momentum tensor, and the specific functional form of χ is deduced from Asymptotically Safe gravity. In the beginning, the universe undergoes a quasi-de-Sitter phase and, then transitions to the conventional cosmological evolution after the Planck Era.Item Restricted OPTICAL AND PHOTOVOLTAIC PROPERTIES OF ORGANIC SOLAR CELLS VS BULK-HETEROJUNCTION MORPHOLOGY(Nazarbayev University School of Sciences and Humanities, 2024-04-30) Asanov, NurzhanThe performance of organic bulk-heterojunction solar cells is closely tied to the morphology of their active layer, a disordered mixture of organic donor and acceptor materials, where photoexcitation and charge generation occur. In this thesis work, optical mixing principles are utilized to model the common active layer structure PM6:Y6. By controllably varying the size and shape of acceptor inclusions within a donor matrix, I investigate how changes in morphology affect the optical properties of the resulting effective medium. This finding is further utilized in transfer matrix optical simulations in combination with the Hecht equation to analyze the effect of morphology on the photovoltaic properties of organic solar cells. The reported findings can guide the optimal fabrication of similar structures by providing valuable insights into which characteristics of the blend could potentially hinder or benefit the performance of photovoltaics.Item Open Access THE EFFECT OF CONVECTION ON CORE-COLLAPSE SUPERNOVAE(Nazarbayev University School of Sciences and Humanities, 2024-04) Telman, YerassylModern Astrophysics, and the study of Core-Collapse Supernovae in particular, require an intense computational power and convenient code for different machines. A portable versatile code IDEFIX is able to deal with these issues. This thesis demonstrates a simulation of Core-Collapse Supernova with stable shock using IDEFIX. The model has an outer accreting part and inner shock. The code provides an extensive possibilities to efficiently modify the model for specific needs. Throughout the work, we obtain standing accretion shock instability (SASI) with 3D simulations, as well as observe how an outer convective shell with velocity perturbations affect the shock and SASI for different perturbation profiles. Additionally, we provide a way of analyzing those accreting perturbations which hints towards the future research of this topic.Item Open Access CONVERSION OF NOX GASES TO NITRATE IONS BY OXIDATION WITH PERSULFATE SPECIES FOR AIR PURIFICATION(Nazarbayev University School of Sciences and Humanities, 2024-04-21) Abdukhairov, IlyarThe absorption and oxidation of NOx gases by potassium persulfate has been implemented to remove the NOx gases from the air by forced ventilation through wet silica gel beads soaked with K2S2O8 /K2CO3 and (NH4)2S2O8 /K2CO3 .The influence conditions: air flow, duration, cartridge packing, concentration of solution on the efficiency of NOx capturing was studied. The ability to absorb NOx gases and convert into nitrate ions was confirmed, however low efficiency ≅0.1% indicates that improvements of the method are required. The increase of duration decreases the efficiency, and increase of the packing increases the efficiency of absorption, however the exact magnitudes appear inconsistent.Item Open Access NOVEL ORGANIC TANDEM SOLAR CELLS(School of Sciences and Humanities, 2023) Asare, Ernest AdiyiahOrganic tandem solar cells have attracted significant attention as a promising technology for next-generation photovoltaics due to their potential to achieve high power conversion efficiencies. In this study, we present a simulation-based approach using transfer matrix modeling to investigate the optical and electrical properties of organic tandem solar cells. We investigate the influence of different device parameters such as the thickness of the active layers on the device performance. The simulation results demonstrate that the optimized thicknesses of the active layers involved in the tandem structure can significantly improve the short circuit current JSC thereby increasing the power conversion efficiency compared to the single-junction solar cells. Additionally, the transfer matrix model provides a comprehensive understanding of the optical behavior of the device, including the absorption and reflection of light at different interfaces. These results provide valuable insights into the design and optimization of high-performance organic tandem solar cells.Item Open Access ULTRA-SHARP ANGULAR FILTERING OF FAR-FIELD SIGNALS WITH GRADED-INDEX MULTILAYERS(School of Sciences and Humanities, 2023) Shukurov, SanatDielectric multilayers with graded index treat the propagating waves impinging at different angles very differently, due to total internal reflection. Such an ultra-sharp change in the response can be exploited for filtering, manipulation and tailoring of spatially distributed electromagnetic signals. Various utilities of the proposed filter such as power splitting, DC isolation, beam shifting and low-profile energy interaction are demonstrated, even in the presence of losses. The proposed setup can be vital as component in the operation of integrated photonic devices dealing with optical signal processing and analog information controlItem Restricted DISSIPATIVE COLLAPSE OF THE SPHERICAL STELLAR OBJECT(School of Sciences and Humanities, 2023) Aryngazin, AnsarIn this research we investigate the shear-free, non-viscous perfect fluid model describing stellar object that emits radiation that leads to dissipation. We study matching of the interior metric with the exterior outgoing Vaidya metric via junction conditions. We also impose regularity and energy conditions in order for the model to be physically realistic. We find solution to main Einstein field equations, analyze and emphasize the features of the solution, specifically the impact of dissipation of the density 𝜌, pressure 𝑝 and mass 𝑚 functions, and present the resulted analysis in plots. We compare obtained solution to the conventional OSD collapse model and to solutions found in previous works, and investigate the prospects of the implementation of obtained results on future research in modified GR.Item Open Access VORTEX RING RECONNECTIONS IN PARAXIAL LASER BEAMS(School of Sciences and Humanities, 2023) Kulchukova, ZhamilaIn recent years, optical vortex knots have been the subject of active research. The knots appear in various areas of physics and understanding the mechanism that is responsible for their formation is a problem that extends beyond optics. However, due to their spontaneous nature, it is impossible to predict their location. To unravel such a complex problem, it is important to understand simpler systems that may allow for the generation of vortex knots. In this thesis, I investigate the interference between the elliptical Gaussian beam and the plane wave, as a result of which vortex rings appear. The rings are basically the unknots, and the ellipticity allows for ring deformations. Consequently, at certain parameters, the rings are able to reconnect. I identify two types of ring reconnections: the reconnection that merges the rings together, and the self-crossing, where a loop-like structure is formed from a single ring crossing itself. This study has the potential to pinpoint specifics of the underlying mechanism that causes spontaneous knottings.Item Restricted GRAVITATIONAL COLLAPSE MODEL WITH DISSIPATION(School of Sciences and Humanities, 2023) Abilmazhinova, MadinaRadiating spherically-symmetric collapsing model is studied. Exterior Vaidya spacetime is matched with the interior of a perfect fluid with a heat flow. Junction conditions and field equations are derived, revealing that a nonzero pressure at the boundary is necessary to satisfy the boundary conditions. Solution for model with assumption that the fluid trajectories are geodesics is found. When heat flux is set to zero, it is demonstrated that the model becomes that of an Oppenheimer and Snyder.Item Open Access STAR CLUSTER MEMBERSHIP IDENTIFICATION BY SUPERVISED MACHINE LEARNING MODELS APPLIED TO N-BODY SIMULATIONS(School of Sciences and Humanities, 2023) Bissekenov, AbylayThis thesis investigates possible ways to apply supervised machine learning algorithms on N-body simulations. Because of the limitations of observational data, there is a motivation to research star clusters by the N-body simulations. The simulations used for the study are based on the Plummer model, and each has its star formation efficiency (SFE) and several random realizations. A random forest model was trained on the simulation with 15% star formation efficiency on a 20-100 Myr timeframe. The model was tested on the other N-body simulations with 17-25% SFEs and showed high classification accuracy throughout the whole dynamic evolution of tested simulations. The majority of mistakes of the model were the false positives (FP) that turned out to be within a 2 Jacobi radius, indicating that they might be gravitationally bounded to center of cluster. Framework and learning strategy can be considered effective and further applied for the mock observations of N-body simulations.Item Open Access Expectation Values of Singular Operators in Variational Calculations of Atomic P-States(Nazarbayev University School of Sciences and Humanities, 2020-05-05) Shomenov, ToreniyazExpectation values of singular operators evaluated in the framework of the Rayleigh-Ritz variational method in quantum mechanics may show slow convergence with increasing the number of basis functions, K. An example of such commonly used operator in the case of high-accuracy calculations of few-electron atoms and molecules is the Dirac delta function dependent on interparticle distances, δ(rij). One way to improve the convergence is to adopt the expectation value identities, in which the singular operator is replaced by an certain non-singular operator so that the expectation value is the same in the limit when the trial wave function approaches the exact solution to the Schrödinger equation. However, when the wave function is approximate, which takes place for any finite K, the convergence of the expectation value of this equivalent non-singular operator is usually improved, often by orders of magnitude. In this thesis, we provide the derivation of formulas for such expectation value identities and implement them into a computer code for the case of atomic P-states, whose wave function is expanded in terms of all-particle explicitly correlated Gaussian basis functions.Item Open Access Analysis of Individual Extensive Air Shower using Pulse Shape Information(Nazarbayev University School of Sciences and Humanities, 2018-04-23) Yessimbet, KamilyaStudy of Ultra-High energy Cosmic Rays (UHECR) coming to Earth from the deep cosmos has the potential to open the veil on some remaining mysteries in the field of High Energy Physics and Astrophysics. Horizon-T detector system is constructed to study the nature of UHECR by means of studying its interactions with the Earth atmosphere that result in so-called Extensive Air Showers (EAS). The detector system is based at Tien Shan High-altitude Science Station (TSHSS) of P.N. Lebedev Physical Institute of the Russian Academy of Sciences. It is located near the Almaty city, Kazakhstan, at ∼ 3400 meter above the sea level. About 2000 EAS were selected from the Physics Run 1 that has been detected during the period from January 2017 to April 2017 at Horizon-T detector system. The simulation activities indicate the existence of the "invariant" property of the standard EAS as defined by the CORSIKA simulator. In this work, I analyzed single-peaked EAS events in order to use this property as a new approach to EAS data analysis using the fine time resolution feature of Horizon-T detector. The definition of this invariant property, its test using data and the classification of the EAS events from Physics Run 1 are described in this thesis.Item Open Access Brick-By-Brick: A Construction of ’t Hooft’s Brick Wall(Nazarbayev University School of Sciences and Humanities, 2020) Gilman, NarynbekA pedagogical construction and review, aimed at the undergraduate level, are given of the brick wall model. The brick wall is a finite region just outside the event horizon of a black hole. It is introduced in order to calculate the entropy of the collapsed star. The statistical origin of the entropy is still unknown, but the brick wall model offers an elementary exercise in counting quantum field solutions to arrive at the famous entropy-area result. The brick wall is a cut-off, effectively regularizing an otherwise divergent result. The model encompasses many fields of physics, including general relativity, quantum theory and statistical mechanics. Its multidisciplinary approach hints at what a more sophisticated solution will look like to the problem of a statistical explanation of the entropy of black holes.