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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3

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    Determination of thermal conductivity of eutectic Al-Cu compounds utilizing experiments, molecular dynamics simulations and machine learning
    (IOP Publishing Ltd, 2023) 0000-0001-9961-7702; 0000-0001-6763-5770; 0000-0002-8430-6941; Fromm, A. C.; Klose, C.; Maier, H. J.; Department of Mechanical Engineering; N/A; N/A; Canadinç, Demircan; Özdemir, Hüseyin Can; Nazarahari, Alireza; Faculty Member; PhD Student; PhD Student; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 23433; N/A; N/A
    In this study, the thermal conductivity (kappa) of Al-Cu eutectics were investigated by experimental and computational methods to shed light on the role of these compounds in thermal properties of Al-Cu connections in compound casting. Specifically, the nonequilibrium molecular dynamics (MD) method was utilized to simulate the lattice thermal conductivity (kappa(l)) of six compositions of Al-Cu with 5-30 at.% Cu. To extend the results of the MD simulations to bulk materials, instead of using conventional linear extrapolation methods, a machine learning approach was developed for the dataset acquired from the MD simulations. The bootstrapping approach was utilized to find the most suitable method among the support vector machine (SVM) with polynomial and radial basis function (RBF) kernels and the random forest method. The results showed that the SVM model with RBF kernel performed the best, and thus was used to predict the bulk kappa(l). Subsequently, the chosen compositions were produced by induction casting and their electrical conductivities were measured via eddy current method for calculating the electronic contribution of kappa using the Wiedemann-Franz law. Finally, the actual kappa of the alloys were measured using the xenon flash method and the results were compared with the computational values. It was shown that the MD method is capable of successfully simulating the thermal conductivity of this system. In addition, the experimental results demonstrated that the IS of Al-Cu eutectics decreases almost linearly with formation of the Al2Cu phase due to increase in the Cu content. Overall, the current findings can be used to enhance the IS of cooling devices made via Al-Cu compound casting.
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    Development and characterization of skin substitutes from electrospun polycaprolactone/silk fibroin
    (Sage Publications Ltd, 2023) Yildiz, Gulsah; Arslan, Yavuz Emre; Derkus, Burak; Menceloglu, Yusuf Ziya; Bayar, Gurkan Rasit; Sezgin, Billur; School of Medicine
    Tissue-engineered skin substitutes have great potential to treat chronic wounds and high-degree burns. Existing solutions, such as Integra Dermal Template, are extensively used for skin defects. However, these templates are still lacking in terms of recreating the functionality of the native tissue and providing scarless healing. In this study, polycaprolactone/silk fibroin (PCL/SF)-based nanofibers with varying blends were fabricated and characterized to develop a novel skin substitute. Morphological analysis showed that the nanofiber distribution of each sample was homogenous without showing any beads. In terms of mechanical properties, all the samples other than SF showed sufficient mechanical strength. It was observed that adding a specific amount of SF into the PCL nanofiber improves the tensile strength of the samples due to the introduction of intermolecular interactions from the functional groups of SF. In addition, incorporating SF into PCL improved Young's modulus of the PCL nanofibers since SF provides stiffness and structural integrity to the overall structure. Water contact angle analysis was performed as the hydrophilicity of a biomaterial is a significant factor in cell functionality. Each sample had a contact angle between 33 degrees and 48 degrees, indicating the adequate hydrophilicity of nanofibers for advanced cell proliferation other than PCL. Cell proliferation and viability studies were conducted with the seeding of primary human keratinocytes on the samples. It was examined that scaffolds containing blends of PCL and SF resulted in higher cell proliferation and viability after 7 days compared to pure PCL and SF nanofibers.
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    Society isn't designed for us to win: trauma and sexual healthcare experiences among transgender women in the Southeastern United States
    (WILEY-V C H VERLAG GMBH, 2024) Van Gerwen, Olivia T.; Muzny, Christina A.; Siwakoti, Krishmita; Batey, D. Scott; Department of Psychology; Department of Psychology; Turan, Bülent; College of Social Sciences and Humanities
    Transgender women (TGW) are disproportionately affected by sexually transmitted infections (STIs). Experienced trauma threatens TGW's commitment to sexual healthcare. Trauma-informed approaches to sexual healthcare can improve engagement. This study aimed to characterize the trauma experienced by TGW in the Southeastern US, especially related to sexual health. TGW completed in-depth individual interviews guided by the Modified Social Ecological Model (MSEM) and Minority Stress framework. Interviews explored the nature of trauma, sexual health, and their intersection in TGW's lives, and a thematic analysis was performed. Between August 2022 and January 2023, 13 TGW enrolled (69% Black, 31% White). Mental illness was common (77%). Five participants (38%) were HIV+, and seven (54%) reported lifetime STI history. Themes reflected societal (barriers to healthcare, anti-transgender legislation), community (misgendering/deadnaming), network (limited support), and individual (dysphoria, sex work, fear for physical safety, stigma, mental health conditions, race) stressors. The physical/sexual violence and resilience themes pervaded all stressor levels. Medical mistrust was exacerbated by past traumatic experiences within sexual healthcare settings. Resilience and transgender representation in healthcare settings were protective. Various experienced traumas and their cumulative effects were pervasive in the daily lives of TGW. Development of targeted interventions to improve sexual health engagement of TGW must optimize personal strengths and provide holistic support. Transgender women experience sexual and mental health disparities. Understanding how trauma related to these issues affects this population and, ultimately, impacts their engagement in sexual and reproductive healthcare is necessary. Societal, community, network, and individual factors influence the trauma they experience. A better understanding of trauma can inform the development of trauma-informed care initiatives that can improve sexual health outcomes.
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    An investigation of the electromechanical coupling and broadband shunt damping in composite plates with integrated piezo-patches
    (Sage Publications Ltd, 2019) N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Gözüm, Mehmet Murat; Aghakhani, Amirreza; Başdoğan, İpek; PhD Student; PhD Student; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 179940
    The popularity of laminated composite plate-like structures is increasing in various engineering applications. Piezoelectric patches with electrical circuit elements can be integrated into these structures for shunt damping applications. for analyzing the shunt damping performance of these systems, precise modeling tools are required, which consider the two-way electromechanical coupling between the piezo-patches and the host plate. This study aims to identify the system parameters which affect the electromechanical coupling coefficient, A metric for measuring the effectiveness of mechanical-to-electrical energy conversion. for that purpose, A thorough investigation is performed to determine the critical system parameters and their combined effects on the electromechanical coupling coefficient of laminated composite plates with surface-bonded piezo-patches. First, the first four natural frequencies of the electromechanical system are obtained using the Rayleigh-Ritz method for various patch sizes. then, the electromechanical coupling coefficient variations for a different set of system parameters are presented. Later, to demonstrate the applicability of the developed methodology for a broader frequency range, four independently shunted piezo-pairs are attached to the plate. the contours of electromechanical coupling coefficient values with respect to ply angle and patch-pair size are presented for the first four modes. Finally, the vibration amplitudes are successfully reduced for these modes using the optimal system parameters.
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    High-temperature oxidation of stellite 12 hardfacings: effect of mo on characteristics of oxide scale
    (Springer, 2019) Dilawary, Shaikh Asad Ali; Atar, Erdem; Çimenoğlu, Hüseyin; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    This study aims to understand the oxide scale formation on the surfaces of commercial and Mo (10wt.%)-modified Stellite 12 hardfacings deposited on 4140 steel. For this purpose, samples of the hardfacings were isothermally oxidized in air at various temperatures in between 500 and 900 degrees C for 30 h. As a result of changes in the microstructure upon Mo addition, about 50% higher mass gain at all oxidation temperatures was observed. The characteristics of the developed oxide scales were examined by utilizing Raman spectroscopy, scanning electron microscopy and x-ray diffraction analysis. The oxide scale formed over Stellite 12 consisted of CoO-, Co3O4- and Cr2O3-type oxides, while additionally CoMoO4-type complex oxide was detected on Mo-modified hardfacing, more likely at the alloy-scale interface. On the basis of findings of isothermal oxidation tests, the formation mechanisms of this complex oxide imposing better wear resistance as reported in our previous studies were explained during high-temperature wear testing, i.e., dynamic oxidation conditions.
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    Random vibration energy harvesting on thin plates using multiple piezopatches
    (Sage Publications Ltd, 2016) Ertürk, Alper; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Arıdoğan, Mustafa Uğur; Başdoğan, İpek; Phd Student; Faculty Member; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940
    Vibrational energy harvesting using piezoelectric cantilever beams has received significant attention over the past decade. When compared to piezoelectric cantilever-based harvesters, piezopatch energy harvesters integrated on plate-like thin structures can be a more efficient and compact option to supply electrical power for wireless structural health and condition monitoring systems. In this article, electroelastic modeling, analytical and numerical solutions, and experimental validations of piezopatch-based energy harvesting from stationary random vibrations of thin plates are presented. Electroelastic models for the series and parallel connected multiple piezopatches are given based on a distributed-parameter modeling approach for a thin host plate excited by a transverse point force. The analytical and numerical solutions for the mean power output and the mean-square shunted vibration response are then derived. The experimental measurements are carried out by employing a fully clamped thin plate with three piezopatches connected in series. It is shown that the analytical and numerical model predictions for the mean power output and the mean-square velocity response are in very good agreement with the experimental measurements. The electroelastic modeling framework and solution methods presented in this work can be used for design, performance analysis, and optimization of piezoelectric energy harvesting from stationary random vibration of thin plates.
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    Dynamics of spacing adjustment and recovery mechanisms of ABAC-type growth pattern in ternary eutectic systems
    (Elsevier, 2017) N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888
    In directionally solidified 2D samples at ternary eutectic compositions, the stable three-phase pattern is established to be lamellar structure with ABAC stacking, where A, B, and C are crystalline phases. Beyond the stability limits of the ABAC pattern, the system uses various spacing adjustment mechanisms to revert to the stable regime. In this study, the dynamics of spacing adjustment and recovery mechanisms of isotropic ABAC patterns were investigated using three-phase In-Bi-Sn alloy. Unidirectional solidification experiments were performed on 23.0 and 62.7 mu m-thick samples, where solidification front was monitored in real-time from both sides of the sample using a particular microscopy system. At these thicknesses, the pattern was found to be 2D during steady-state growth, i.e. both top and bottom microstructures were the same. However, during spacing adjustment and recovery mechanisms, 3D features were observed. Dynamics of two major instabilities, lamellae branching and elimination, were quantified. After these instabilities, two key ABAC pattern recovery mechanisms, namely, phase invasion and phase exchange processes, were identified and analyzed. After elimination, ABAC pattern is recovered by either continuous eliminations of all phases or by phase exchange. After branching, the recovery mechanisms are established to be phase invasion and phase exchange.
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    Silicon microspheres for near-IR communication applications
    (IOP Publishing Ltd, 2008) Demir, Abdullah; Department of Physics; Department of Physics; Serpengüzel, Ali; Faculty Member; College of Sciences; 27855
    We have performed transverse electric and transverse magnetic polarized elastic light scattering calculations at 90 degrees and 0 degrees in the o-band at 1.3 mu m for a 15 mu m radius silicon microsphere with a refractive index of 3.5. The quality factors are on the order of 10(7) and the mode/channel spacing is 7 nm, which correlate well with the refractive index and the optical size of the microsphere. The 90 degrees elastic light scattering can be used to monitor a dropped channel (drop port), whereas the 0 degrees elastic scattering can be used to monitor the transmission channel (through port). The optical resonances of the silicon microspheres provide the necessary narrow linewidths that are needed for high-resolution optical communication applications. Potential telecommunication applications include filters, modulators, switches, wavelength converters, detectors, amplifiers and light sources. Silicon microspheres show promise as potential building blocks for silicon-based electrophotonic integration.
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    Nanoscratch behavior of metallic glass/crystalline nanolayered composites
    (Springer, 2019) Abboud, Mohammad; Verma, Nisha; Özerinç, Sezer; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    Nanolayered metallic glass/crystalline composites provide an effective structure for improving the ductility of metallic glasses while maintaining their outstanding strength. The combination of high strength and ductility make these nanocomposites promising materials as wear-resistant coatings. In this work, we experimentally investigated the mechanical properties and nanoscratch behavior of CuZr/Zr metallic glass/crystalline nanolayers. The scratch resistance was highest for the monolithic CuZr, and diminished with decreasing layer thickness for nanolayered coatings, although hardness and elastic modulus were independent of layer thickness. The nanocomposite with a layer thickness of 10nm did not show any signs of failure in spite of compressive strain exceeding 80%. The low shear strength of the CuZr/Zr interface and strain hardening of Zr layers can explain the layer thickness-dependent scratch resistance and outstanding damage tolerance observed. Layered metallic glass/crystalline nanocomposites combine high hardness and resistance to fracture, providing a new design space for the development of effective wear-resistant coatings.
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    Fragmentation dynamics of ne-3(+) clusters: a classical trajectory study
    (2006) Sebastianelli, F; Gianturco, F. A.; Department of Chemistry; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; College of Sciences; 7129
    A new, ab initio potential energy surface for the ionic neon trimer [12] is adopted for the study the low and intermediate energy fragmentation dynamics of the Ne+3 system. The process is initiated from three of the most stable trimer structures and a very large number of classical trajectory calculations is employed to extract various fragmentation dynamics indicators. The system is found to eject one neon atom on a very fast time scale of ∼0.15 ps, a process that leaves behind an excited ionic dimer, which in turn undergoes full dissociation on a larger time scale.