https://scholar.gist.ac.kr/handle/local/7961 Sat, 13 Dec 2025 05:25:53 GMT 2025-12-13T05:25:53Z https://scholar.gist.ac.kr/handle/local/19868 Title: Ultrathin Epitaxial Lift-off GaAs Solar Cell Array with Highly Flexible and Lightweight Characteristics Author(s): Sungbum Cho Abstract: Recent interests increasing in ultraflexible and ultralight solar cells with reliable and high efficiency for untethered power supply to electronic devices such as attachable, implantable, and miniaturized drones, etc. However, the structural limitation of material obstructs the coexistence between the ultraflexible characteristics and the high electrical performance. This dissertation proposes an ultrathin, ultralight solar cell array with high efficiency on an ultrathin polymer film. Framing of 1.4 μm thick ultrathin film and a series of processes of cold-welding, epitaxial lift-off (ELO), and microfabrication gives the GaAs solar cell array ultraflexible and ultralight characteristics. The mechanical characteristics of the ultrathin GaAs photovoltaic (PV) array were analyzed through finite element analysis and planar compression test. The PV array operates under extreme deformation and demonstrates the connectivity to diverse unconventional surfaces. Lastly, the ultralight characteristic was ensured with a specific power of 5.44 W/g. Fri, 31 Dec 2021 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19868 2021-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19855 Title: Transfer-Printed Optoelectronics for Cylindrical Concentrating Photovoltaics and Vascular Hemodynamics Monitoring Author(s): 이태연 Abstract: The transfer printing method, which selectively separates optoelectronic elements based on compound semiconductor thin films epitaxially-grown on wafers and integrates them onto foreign substrates, offers applicability in various fields such as photovoltaic energy harvesting, wearable and implantable sensors. Optoelectronic elements transfer-printed onto flexible film substrates overcome the structural and functional limitations of conventional rigid elements, enabling applications such as curved surface-attached flexible solar cells, vertically stacking the light emitting diodes (LEDs), and the heterogeneous integration of photodetector and LEDs. The main focus of this dissertation is to design a system that meets the required performance through systematic analysis, and to fabricate devices by integrating optoelectronic elements using transfer printing method, demonstrate the applications in various fields, including concentrating photovoltaics (CPV) and hemodynamics monitoring. In this doctoral dissertation, a planar-type cylindrical CPV module design to save energy required for solar tracking and a vertically stacked LED structure for localized hemodynamics monitoring are proposed. Notably, red and blue-colored LEDs were vertically stacked on a foreign substrate using the transfer printing method, and a photodetector capable of measuring the light emitted from these LEDs was heterogeneously integrated on the same substrate, allowing it to function as a hemodynamic sensor. Thus, transfer-printed optoelectronics is a key technology that can expand the functionality of traditional semiconductors Tue, 31 Dec 2024 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19855 2024-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19845 Title: Topology Design Optimization and Data-aided Analysis by Physics Informed Neural Networks for Additive Manufactured Structure Author(s): Dongjin Kim Abstract: This dissertation addresses methodologies for the design and analysis related to additive manufacturing. First, the design methodology is proposed to optimize the structural shape for stiffness maximization, and to generate the CAD file for its additive manufacturing. Specifically, open-source MATLAB code is developed for topology optimization of three-dimensional arbitrary design domains. In addition, the post-processing procedure is built to generate STL format file for the additive manufacturing of topology optimization result. The developed program enables the practical design process, which includes the loading of three-dimensional CAD file for setting the design domain, topology design optimization, and the additive manufacturing of design result. The effectiveness of the developed code is validated through various design examples, including a simply supported beam, a bridge, and an airplane bearing bracket. Subsequently, the data-aided analysis methodology is developed, which aims to reconcile the discrepancies between the analysis results by Computer-Aided Engineering (CAE), and the actual behavior of additive manufactured structures. Here, the discrepancies come from uncertainty in additive manufacturing processes. To overcome this problem, the distribution of material properties is included as the additional state variable, and the measured physical variables are added as the data. To treat material property as unknown variable and contain the data in the structural analysis, the physics-informed neural networks (PINNs) are utilized in this dissertation. For the stable analysis of additive manufactured structure with complex shapes, energy functional targeting loss function is proposed, and its effectiveness is validated by comparing with the result of existing PINN loss functions. In addition, the effect of data, i.e. data type among displacement and strain, and data domains, on the analysis result is investigated how to improve the accuracy of data-aided analysis results. The limitation of this work is as follows. The developed code for topology design optimization contains inaccuracies in geometry representation due to the use of an external, and discrepancies between the CAE model and the actual structure caused using isotropic properties of materials. The proposed data-aided analysis treats simplified two-dimensional geometries, and utilizes virtual data instead of the actual measured data of addictive manufactured structure. Sat, 31 Dec 2022 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19845 2022-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19824 Title: Thermal characterization in electric double layer capacitor and graphite Author(s): Yeongcheol Park Abstract: As technology advances rapidly, industries are adapting to global challenges such as the energy crisis by embracing electrification and renewable energy sources. This transition emphasizes the crucial role of thermal energy in our everyday lives. From harnessing solar power to managing industrial heat, thermal energy is essential for sustainability and progress. Through thermal energy harvesting, we extract useful energy from heat to reduce reliance on fossil fuels and move towards a more environmentally friendly future. Additionally, effective thermal management is vital for optimizing the performance of systems such as electronics, highlighting the importance of both thermal energy harvesting and management. Throughout my Ph.D. program, I've been deeply interested in exploring thermal challenges in two engineering fields: thermal energy harvesting and thermal management. Firstly, I initiated the topic of low-grade heat harvesting as my first project. This thermal harvesting was conducted based on electrochemical concepts using electrochemically capacitive systems called supercapacitor. As part of this project, the electric-field dependent heat capacity change was investigated. To do this, the thermal properties of aqueous electric double layer capacitors (EDLCs) with KOH electrolytes were measured using the in situ and operando 3ω hot-wire method. The reversible entropy and free energy changes in the EDLCs caused dynamic changes in the effective heat capacity of the electrodes, which were monitored real-time during charge and discharge operations. In an EDLC with a 6 M KOH electrolyte, the effective heat capacities of the positive and negative electrodes with a varying voltage from 0 to 1 V were estimated to decrease by approximately 9.14% and 3.91%, respectively. This polarization dependence may be attributed to the capacitance capability, which is determined by the size of the adsorbed hydrated ions. Regarding the effect of KOH concentration, the maximal capacitance and change in effective heat capacity were observed in the vicinity of 8 M concentration, and attributed to changes in the thermodynamic state by the capacitance effect. For the second project, I focused on studying thermal conduction in graphite, which is a widely-used thermal management material due to its superb thermal properties. Thermal conduction is classified into diffusive, hydrodynamic, and ballistic thermal transport modes. Among them, phonon hydrodynamic transport has been relatively underexplored because of the stringent material conditions required for its emergence and the experimental challenges associated with its observation. Phonon hydrodynamics show the collective motion of phonons, similar to a molecular fluid flow, which is caused by the dominance of momentum-conserving normal (N) scattering over resistive scatterings (e.g., isotope, boundary, and Umklapp (U) scatterings). In common materials, the hydrodynamic phonon transport only occurs in the extremely low and narrow cryogenic temperature ranges. Recent calculations have predicted that graphitic materials such as graphene and graphite are potential candidates to possess phonon hydrodynamic phenomena even at near-room temperature due to the extraordinarily prevailing N-scattering over U-scattering. Second sound, which is a temperature wave analogous to a pressure sound wave in fluid dynamics, is a phenomenological evidence to characterize the phonon hydrodynamic transport in the transient aspect. Up to date, second sound in bulk graphite was observed in a relatively high temperature range of 100–200 K through optical pump-probe techniques, which agreed with previous computational findings. Compared to bulk graphite, thin graphite may be more favorable to have a strong second sound because of the more significant ZA phonon contribution and the absence of cross-plane heat conduction. Also, the thickness effect on basal-plane phonon transport in thin graphite remains a controversial issue, necessitating further experimental and theoretical investigation to clearly elucidate its relationship. For second sound measurements, a modified time-domain thermoreflectance method was employed with spatially separated pump and probe beams. Furthermore, the experimental results were analyzed with calculations based on the inviscid heat equation model. Although the propagation speed of observed thermal wave peak was closely similar to the theoretical value of the second sound speed, the thermal wave could not be definitely classified as either a diffusive or second sound. Therefore, further careful investigation is necessary for the analysis of peaks related to ballistic, hydrodynamic, and diffusive modes. Sun, 31 Dec 2023 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19824 2023-12-31T15:00:00Z