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Publication Metadata only Al-Sm alloys under far-from-equilibrium conditions(Springer International Publishing AG, 2021) Okuyucu, Can; Kaygusuz, Burçin; Işıksaçan, Cemil; Meydanoğlu, Onur; Özerinç, Sezer; Kalay, Yunus Eren; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/ATraditional Al alloys have shown tremendous potential in the aerospace industry due to their attractive properties such as ductility, fracture toughness, and fatigue resistance. However, modern aerospace applications call for next-generation Al alloys with a stringent combination of properties such as high strength, low density, and excellent environmental stability. In that sense, we studied highly driven Al-Rare-Earth (RE) alloys under far-from-equilibrium conditions to investigate the possible effects of cooling rate on the expected microstructure, thus mechanical properties. Al94Sm6 was produced using a copper wheel melt spinner. XRD analysis showed the Sm is entirely trapped within the Al matrix. The heat-treated specimens resulted in the formation of the nanocrystalline Al4Sm phase embedded in the Al matrix, with a two-step precipitation sequence. The hardness values determined by nanoindentation shows that the initial supersaturated solid solution has 3.83 GPa hardness, while the heat-treated ones have 3.34 GPa. The mechanisms behind this extreme strength and ductility through solute trapping, and subsequent heat-treatments were discussed in detail using a combined study of micromechanical characterization, nanoindentation , electron microscopy, XRD, and DSC.Publication Open Access Effect of cerium doping on morphology and physical properties of alpha-Fe2O3 films prepared by hydrothermal electrodeposition(Electrochemical Society (ECS), 2014) Department of Chemistry; Akkaya, Ceren Yılmaz; Ünal, Uğur; PhD Student; Faculty Member; Department of Chemistry; Graduate School of Sciences and EngineeringIn this study, electrochemical growth of Ce-doped alpha-Fe2O3 films is reported. Electrodeposition at 75 degrees C and also a combined hydrothermal-electrochemical method to obtain hematite are presented. Both acidic and neutral plating solutions were used for the depositions. Ferric oxyhydroxide is produced by electrodeposition at 75 degrees C and converted to hematite phase after annealing. Hydrothermal-electrodeposition at 130 degrees C results in formation of hematite phase directly. The effect of temperature, deposition solution and cerium ion amount on the morphology and physical properties of hematite films are characterized.Publication Metadata only Fabrication of 1D ZNO nanostructures on mems cantilever for VOC sensor application(Elsevier, 2014) Kosemen, Arif; Öztürk, Sadullah; Yerli, Yusuf; Öztürk, Zafer Ziya; Department of Electrical and Electronics Engineering; N/A; Department of Mechanical Engineering; Department of Electrical and Electronics Engineering; Kılınç, Necmettin; Çakmak, Onur; Ermek, Erhan; Ürey, Hakan; Researcher; PhD Student; Other; Faculty Member; Department of Mechanical Engineering; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; 59959; N/A; N/A; 8579This study reports the fabrication method and sensing performance for novel 1D zinc oxide (ZnO) nanorods and nanotubes grown on nickel MEMS cantilevers. The fabrication of the nanostructures and the cantilevers are simple and low-cost using standard lithography, electrodeposition, and hydrothermal etching processes. 1D ZnO nanostructures increase the total sensitive area for biological and chemical sensor applications. We performed experiments with various VOCs with a real-time sensor system developed in our laboratory. While Ni microcantilevers produced no signal, ZnO nanostructure coated microcantilevers showed good sensitivity and repeatable changes. Furthermore, the nanotube coated microcantilevers showed more than 10 fold increase in sensitivity compared to the nanorod coated microcantilevers which can be explained to the fact that ZnO nanotubes have higher surface area and subsurface oxygen vacancies and these provide a larger effective surface area with higher surface-to-volume ratio as compared to ZnO nanorods. The tests are performed using dynamic mode of operation near resonant frequency using magnetic actuation and optical sensing. The phase stability and the limit of detection of ZnO nanotube coated microcantilevers exposed to diethylamine (DEA) were 0.02 degrees and lower than 10 ppm, respectively. ZnO nanostructure coated microcantilevers have good potential for VOC sensor applications especially for amine groups.Publication Open Access FR4-based electromagnetic energy harvester for wireless tyre sensor nodes(Elsevier, 2009) Department of Electrical and Electronics Engineering; Hatipoğlu, Gökhan; Ürey, Hakan; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; N/A; 8579An electromagnetic (EM) power generator having 46 Hz resonance frequency is designed to scavenge mechanical vibrations occurring in tyres due to lyre-road contact. The major innovation is the use of FR4 as a structural spring material as well as utilizing a spacer and stopper mechanism increasing the shock resistance by limiting the maximum deflection. The novel magnet assembly and spacer design provide high power density. The tangential acceleration waveforms of typical tyre rotation is used as an input in the experiments and 0.4 mW power is obtained over a 100 Omega load resistance for 15g peak-to-peak amplitude at 22,83 Hz, corresponding to about 150 kph vehicle speed. Maximum acceleration is limited with the shaker, larger power values are expected in actual operation. The performance is obtained off-resonance and superior to resonant Silicon MEMS based scavengers.Publication Open Access Highly sensitive optical sensor for hydrogen gas based on a polymer microcylinder ring resonator(Elsevier, 2020) Eryürek, Mustafa; Department of Physics; Department of Chemistry; Department of Electrical and Electronics Engineering; Bavili, Nima; Balkan, Timuçin; Morova, Berna; Uysallı, Yiğit; Kaya, Sarp; Kiraz, Alper; Researcher; Researcher; PhD Student; Faculty Member; Faculty Member; Department of Physics; Department of Chemistry; Department of Electrical and Electronics Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; N/A; N/A; N/A; N/A; 116541; 22542A highly sensitive platform is demonstrated for hydrogen gas (H-2) sensing based on a polymer microcylinder ring resonator (PMRR) obtained by an optical fiber coated with an inner nanofilm of amorphous palladium (Pd) and an outer polymer layer of polydimethylsiloxane (PDMS) permeable to H-2. The sensing scheme is based on monitoring the spectral shifts of high-quality optical resonances called whispering gallery modes (WGMs) that propagate in the vicinity of the outer rim of the PDMS layer without being affected by the absorption and scattering losses caused by the Pd nanofilm. WGMs are excited by a single-mode tapered optical fiber evanescently coupled to the PMRR. The observed reversible spectral shifts of the WGMs are induced by changes in the diameter of the PDMS layer caused by expansion or contraction of the Pd nanofilm exposed to varying concentrations of H-2. Maximum spectral shift sensitivity of 140 pm/% H-2, a minimum response time of 95 s, and minimum limit of detection of similar to 60 ppm were measured for sensors prepared with different thicknesses of the amorphous Pd nanofilm and tested in the H-2 concentration range up to 1%, having nitrogen gas (N-2) as a carrier. Experiments were also conducted with Pd nanofilms annealed in air or N-2 atmosphere after the deposition. In both cases, smaller sensitivities were observed due to the formation of larger grains within the film, resulting in slower diffusion and reduced solubility of H in the Pd layer. The impacts of oxygen gas and humidity on sensor performance were also studied.Publication Metadata only Optical sensor for hydrogen gas based on a palladium-coated polymer microresonator(Elsevier, 2015) Karadag, Yasin; Tasaltin, Nevin; Kilinc, Necmettin; N/A; Department of Physics; Eryürek, Mustafa; Kiraz, Alper; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 22542We report an integrated optical sensor of hydrogen (H-2) gas employing an SU-8 polymer microdisk resonator coated with a palladium (Pd) layer and coupled to a single-mode optical waveguide. The sensing mechanism relies on the expansion in the Pd lattice due to palladium hydride formation in the presence of H-2. Strain induced in the microresonator then causes a red shift of the spectral positions of the resonator whispering gallery modes (WGMs) which is monitored using a tunable laser coupled to the waveguide. H-2 concentrations below the flammable limit (4%) down to 0.3% could be detected in nitrogen atmosphere at room temperature. For H-2 concentrations between 0.3 and 1%, WGM spectral positions shifted linearly with H-2 concentration at a rate of 32 pm/% H-2. Average response time of the devices was measured to be 50 s for 1% H-2. The proposed device concept can also be used to detect different chemical gases by using appropriate sensing layers. (C) 2015 Elsevier B.V. All rights reserved.