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ItemOpen Access
Modelling a janus particle activated by an optical potential
(Bilkent University, 2023-09)
The flow around a Janus particle under an optical potential is investigated. The thermal effects are majorly overlooked while studying the Janus particle although fluid heating caused by optical potential might negligible but the fluid heating caused due to Janus particle acting as a heating source is not negligible. The stream function approach is used to model the behavior of an activated Janus particle. The explicit finite difference method (FDM) is used to numerically obtain the solution. BIL-FLOW, an in-house FDM code is developed and validated for flow around a Janus particle under an optical potential. Additionally, The BIL-OP, an in-house optical module based on geometric ray approximation is developed and validated. BIL-OP utilizes linear algebra to calculate three dimensional (3D) optical force.
Visible light positioning in presence of malicious LED transmitters or intelligent reflecting surfaces
(Bilkent University, 2023-09) Kökdoğan, Furkan; Gezici, Sinan
Visible light positioning (VLP) is a recent solution to the localization problem in indoor environments which involves the use of light emitting diodes (LEDs) as transmitters and photodetectors (PDs) as receivers. VLP systems have in-creasingly been popular as LEDs are employed for illumination purposes over conventional light bulbs nowadays due to their various advantages. In this the-sis, we develop VLP algorithms for two main scenarios. In the first scenario, we assume that the system is not completely secure, meaning that the transmit power of some LEDs can be controlled by a third unknown party, i.e., hijacked, to degrade the positioning accuracy. In the second scenario, we assume the de-ployment of intelligent reflecting surfaces (IRSs) into the system to improve the positioning accuracy in the absence of line-of-sight (LOS) signals from of a subset of LED transmitters. First, we consider a VLP system in which a receiver performs position estimation based on signals emitted from a number of LED transmitters. Each LED transmitter can be malicious and transmit at an unknown power level with a certain probability. A maximum likelihood (ML) position estimator is derived based on the knowledge of probabilities that LED transmitters can be malicious. In addition, in the presence of training measurements, decision rules are designed for detection of malicious LED transmitters, and based on detection results, various ML based location estimators are proposed. To evaluate the performance of the proposed estimators, Cram´er-Rao lower bounds (CRLBs) are derived for position estimation in scenarios with and without a training phase. Moreover, an ML estimator is derived when the probabilities that the LED transmitters can be malicious are unknown. The performances of all the proposed estimators are evaluated via numerical examples and compared against the CRLBs. Second, we formulate and analyze a received power based position estimation problem for VLP systems in the presence of IRSs. In the proposed problem formulation, a visible light communication (VLC) receiver collects signals from a number of LED transmitters via LOS paths and/or via reflections from IRSs. We derive the CRLB expression and the ML estimator for generic three-dimensional positioning in the presence of IRSs with arbitrary configurations. In addition, we consider the problem of optimizing the orientations of IRSs when LOS paths are blocked, and propose an optimal adjustment approach for maximizing the received powers from IRSs based on analytic expressions, which can be solved in closed form or numerically. Since the optimal IRS orientations depend on the actual position of the VLC receiver, an N-step localization algorithm is proposed to perform adjustment of IRS orientations in the absence of any prior knowledge about the position of the VLC receiver. Performance of the proposed approach is evaluated via simulations and compared against the CRLB. It is deduced that although IRSs do no provide critical improvements in positioning accuracy in the presence of LOS signals from a sufficient number of LED transmitters, they can be very important in achieving accurate positioning when all or most of LOS paths are blocked.
Electrocatalytic water splitting with Prussian blue analogues under external stimuli
(Bilkent University, 2023-09) Ahmad, Waqar; Karadaş, Ferdi
The development of long-lasting and efficient catalysts for water splitting is crucial for the advancement of a carbon emission-free world. A well-known class of compounds called Prussian blue analogues (PBAs) offers several advantages such as high stability, diversity, and simple synthesis for the development of sustainable water-splitting devices. This thesis investigates the construction of PBA-based overall water-splitting electrolytic cells assisted with external stimuli. Alsac et al. investigated the oxygen evolution reaction (OER) efficiency of various PBAs and concluded that Co-Co exhibits the best performance as an OER catalyst among the Co-M PBAs. Ahmad et al. studied the hydrogen evolution reaction (HER) performance of various PBAs and observed that Co-Ni stands out in performance. Furthermore, Chalil Oglou et al. elucidated the effect of the magnetic field on the OER catalytic activity of Co-Fe PBA electrodeposited on the surface of the FTO. His findings unveiled an enhanced catalytic activity under the influence of a magnetic field. To further explore these concepts, we aim to move one step ahead and combine all these studies to investigate overall water splitting (OWS) under the influence of magnetic field and solar light irradiation. In this thesis, [Co-Co] was used for the OER reaction, while [Co-Ni] was utilized for the HER reaction. Both electrodes were prepared involving a two-step electrodeposition method and comprehensively characterized with SEM, EDAX, P-XRD, XPS, and ATR-FTIR. SEM images unveiled threat-like and needle-like grown particles with uniform sizes of 1-2 µm for [Co-Co] and [Co-Ni] formed on the fluorine-doped tin oxide (FTO) electrode respectively. The oxidation states of the pristine and post-catalytic electrodes and the stability during the electrocatalytic process were confirmed with XPS and FTIR studies. The electrochemical characterization of these catalysts was thoroughly investigated with linear sweep voltammetry (LSV), chronoamperometry (CA), and cyclic voltammetry (CV) profiles. The electrochemical performance was investigated in three chapters; OER, HER, and overall water splitting under magnetic and solar light irradiation. (i) OER performance of FTO/[Co-Co] was evaluated with LSV, which shows prominent enhancement peaks under the influence of external stimuli. Under the influence of the magnetic field, it illustrated an enhancement of 11.9% with an overpotential of 949 mV, while in the presence of solar light, it showed an augmentation of 10.7% with an overpotential of 949 mV. CA profiles, recorded under magnetic field showed that there is a direct relation between magnetic field strength and the enhancement in the current density. On the contrary, an opposite trend is observed with the CA profiles under solar light irradiation, which suggests that the origin of the enhancement under the magnetic field is different from the one under solar light irradiation. (ii) Similar to OER studies, HER activity of FTO/[Co-Ni] was investigated under the effect of solar light irradiation and magnetic field. The LSV profile showed enhancement only in the case of solar light, while no significant enhancement was observed under the magnetic field, contrary to the previous studies. Similar to OER, the CA profiles of FTO/[Co-Ni] illustrated the opposite trend with respect to overpotential applied. In the case of HER, CA under a magnetic field showed a small enhancement (1.4%) with an overpotential of 300 mV, which was attributed to the magnetohydrodynamic effect. (iii) Two and three-electrode systems were used to conduct the investigation into overall water splitting. To achieve a current density of 1 mA/cm2 in the two-electrode having FTO/[Co-Co] on the working/working sense electrode (W/WS) and FTO/[Co-Ni] on the counter/reference electrode R/C configuration, the system required an overpotential of roughly 1013 mV. The subsequent analysis of each electrode's unique voltage contributions helped explain this observation. OER takes around 1.3 V while it is 0.6 V for the HER side. On the other hand, in the three-electrode configuration, the working electrode was FTO/[Co-Co], the counter electrode was FTO/[Co-Ni], and the reference electrode was Ag/AgCl. The observed profile notably showed significant improvement seen when solar light and magnetic fields were present. Overall, this study indicates that there is still plenty of room for enhancement in catalysis, with slight modification in reaction conditions from another perspective i.e., external stimulus. This thesis takes a progressive step by raising the bar and adding a new dimension to the challenge of using PBAs in catalytic applications, building on earlier efforts.
Pivoting on the church of St. John Exoteichos: from the empire to modern Trabzon
(Bilkent University, 2023-09) Demet, Gönülnur; Özcan, Burcu Şenyapılı
Presently, the quantity of Byzantine and post-Byzantine churches that remained within the city center of Trabzon is quite limited. Additionally, aside from significant exceptions like the Hagia Sophia of Trebizond, numerous structures remain unfamiliar to the local community and have been repurposed for various uses. Byzantine and post-Byzantine studies in Turkey tend to focus on other centers and the heritage of Trebizond, a peripheral site with exceptional characteristics, receives less attention. The present research focuses on St. John Exoteichos, earlier known as Agios Ioannes tes Petra, and its neighborhood, Sotka, with a diachronic approach. Built in the 13th/14th century, St. John Exoteichos was rebuilt by Metropolitan Konstantios in the 19th century. From the early 20th century onward, the church, once part of a complex with an adjacent Greek school building, briefly functioned as a warehouse during the Republican period. Subsequently, it transformed into a versatile hall for the neighboring primary school. The study focuses on three different periods, and it illustrates the continuities, changes, and transformations in the design and use of the church building throughout, regarding the region's political, architectural, and social dynamics. This thesis aims at contributing academic literature by exploring the church's historical, architectural, and urban dimensions. The study of St. John Exoteichos, particularly its 19th-century version, offers a novel perspective within Trabzon's Byzantine and post-Byzantine heritage.
Density functional theory investigation of linear carbon chains
(Bilkent University, 2023-09) Salepci, Efe Dorukhan; Gülseren, Oğuz
In this thesis the structural and electronic properties of linear carbon chains are investigated using density functional theory. Polyyne structure of alternating single and triple bonds was shown to be energetically favored structure compared to successive double-double bonded cumulene structure. Band calculations showed that polyyne is a semiconductor whereas cumulene is a metal. Phonon calculations showed that cumulene is unstable. When put in a hexagonal formation these chains are found to form three possibly stable structures, one tightly bound hexagonal tube, and two loosely bound structures one which can be described as a hexagonal assembly of polyyne chains and one which can be considered stacks of hexagonal carbon flakes. Electronic band structure calculations showed that all three structures are semiconductors. Charge density profile showed strong chemical bonds both in vertical and horizontal directions for the first structure, whereas second structure of polyyne chains had no strong bonds between chains and third structure of hexagon flakes showed no strong bond between hexagon flakes. It is also found that as hexagon size shrinks the favored structure of chains changes from polyyne to cumulene and a band structure calculation showed that a semiconductor to metal transition happens.