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Publication Metadata only A deformation-based approach to tuning of magnetic micromechanical resonators(2018) Yalçınkaya, Arda D.; Department of Mechanical Engineering; N/A; Department of Mechanical Engineering; Biçer, Mahmut; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; Researcher; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 115108Resonance frequency tuning in magnetic micromechanical resonators remains a primary field of study for frequency reference applications. The use of magnetic micromechanical resonators for innovative timing, oscillator and sensing applications necessitates a platform for the precise control of the resonance frequency. The present work addresses a deformation based technique for tuning the resonance frequency of nickel micromechanical resonators. Frequency response is measured through magnetic actuation and optical readout. The tuning approach is based on a combination of flexural deformation and uniaxial strain. The bending deformation is achieved by using a DC current through the microbeam. This magnetomotive mechanism reduces the resonance frequency by about 13% for a maximum DC current of 80 mA. A substrate bending method is used for applying uniaxial strain to increase the resonance frequency by about 8%. A bidirectional frequency modulation is thus demonstrated by utilizing both deformation techniques. The interpretation of results is carried out by finite element analysis and electromechanical analogy in an equivalent circuit. Using deformation techniques, this study provides a rigorous approach to control the resonance frequency of magnetic micromechanical resonators.Publication Open Access A new consistent hybrid algorithm for solution of the PDF equations of turbulent reactive flow(American Institute of Physics (AIP) Publishing, 2013) Department of Mechanical Engineering; Sheikhsarmast, Reza Mokhtarpoor; Inmas, Shabrina Virta; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 46561This paper presents a newly developed consistent hybrid finite-volume (FV)/particle algorithm for solution of joint PDF (JPDF) model equation of turbulent reacting flows. In this approach, the open source FV package of OpenFOAM is employed to solve the Favre-averaged mean mass and momentum equations using pressure-based PISO algorithm while a particle-based Monte Carlo algorithm is used to solve the fluctuating velocity-turbulence frequency-compositions JPDF transport equation. In the earlier hybrid method [2, 3], a density-based FV algorithm was used to solve the mean flow equations but it has been found to be too dissipative and yet not very robust for incompressible or nearly incompressible flows mainly due to stiffness of the compressible flow equations in the low Mach number limit. In the this work, the density-based FV algorithm is first replaced with a pressure-based PISO algorithm to tackle this problem and then applied for simulation of the Sydney swirl stabilized bluff-body flame SM1. All the equations solved by the FV and particle algorithms are directly derived from the modeled JPDF transport equation so the present method is completely consistent at the level of governing equations. The position and velocity correction algorithms [3] are used to enforce full constancy at the numerical solution level. The results are found to be in a good agreement with the available experimental data and the recent computational results of De Meester et al. [1].Publication Open Access Adaptive Q control for tapping-mode nanoscanning using a piezoactuated bimorph probe(American Institute of Physics (AIP) Publishing, 2007) Department of Mechanical Engineering; Günev, İhsan; Varol, Aydın; Karaman, Sertaç; Başdoğan, Çağatay; Master Student; Faculty Member; Department of Mechanical Engineering; College of Engineering; N/A; N/A; N/A; 125489A new approach, called adaptive Q control, for tapping-mode atomic force microscopy (AFM) is introduced and implemented on a homemade AFM setup utilizing a laser Doppler vibrometer and a piezoactuated bimorph probe. In standard Q control, the effective Q factor of the scanning probe is adjusted prior to the scanning depending on the application. However, there is a trade-off in setting the effective Q factor of an AFM probe. The Q factor is either increased to reduce the tapping forces or decreased to increase the maximum achievable scan speed. Realizing these two benefits simultaneously using standard Q control is not possible. In adaptive Q control, the Q factor of the probe is set to an initial value as in standard Q control, but then modified on the fly during scanning when necessary to achieve this goal. In this article, we present the basic theory behind adaptive Q control, the electronics enabling the online modification of the probe's effective Q factor, and the results of the experiments comparing three different methods: scanning (a) without Q control, (b) with standard Q control, and (c) with adaptive Q control. The results show that the performance of adaptive Q control is superior to the other two methods.Publication Open Access Al-doped MgB2 materials studied using electron paramagnetic resonance and Raman spectroscopy(American Institute of Physics (AIP) Publishing, 2016) Erdem, Emre; Repp, Sergej; Weber, Stefan; N/A; Department of Chemistry; Bateni, Ali; Somer, Mehmet Suat; PhD Student; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; N/A; 178882Undoped and aluminum (Al) doped magnesium diboride (MgB2) samples were synthesized using a high-temperature solid-state synthesis method. The microscopic defect structures of Al-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman spectroscopy, and electron paramagnetic resonance. It was found that Mg-vacancies are responsible for defect-induced peculiarities in MgB2. Above a certain level of Al doping, enhanced conductive properties of MgB2 disappear due to filling of vacancies or trapping of Al in Mg-related vacancy sites. Published by AIP Publishing.Publication Metadata only An electrochemical gelation method for patterning conductive PEDOT:PSS hydrogels(2019) Feig, Vivian Rachel; Tran, Helen; Lee, Minah; Liu, Kathy; Huang, Zhuojun; Mackanic, David G.; Bao, Zhenan; Department of Mechanical Engineering; Beker, Levent; Faculty Member; Department of Mechanical Engineering; College of Engineering; 308798Due to their high water content and macroscopic connectivity, hydrogels made from the conducting polymer PEDOT:PSS are a promising platform from which to fabricate a wide range of porous conductive materials that are increasingly of interest in applications as varied as bioelectronics, regen-erative medicine, and energy storage. Despite the promising properties of PEDOT:PSS-based porous materials, the ability to pattern PEDOT:PSS hydrogels is still required to enable their integration with multifunctional and multichannel electronic devices. In this work, a novel electrochemical gelation (“electrogelation”) method is presented for rapidly patterning PEDOT:PSS hydrogels on any conductive template, including curved and 3D surfaces. High spatial resolution is achieved through use of a sacrificial metal layer to generate the hydrogel pattern, thereby enabling high-performance conducting hydrogels and aerogels with desirable material properties to be introduced into increasingly complex device architecturesPublication Open Access Broadband and band-limited random vibration energy harvesting using a piezoelectric patch on a thin plate(Society of Photo-optical Instrumentation Engineers (SPIE), 2014) Erturk, Alper; Department of Mechanical Engineering; Arıdoğan, Mustafa Uğur; Başdoğan, İpek; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940This paper presents analytical modeling and case studies of broadband and band-limited random vibration energy harvesting using a piezoceramic patch attached on a thin plate. The literature of vibration-based energy harvesting has been mostly focused on resonant cantilevered structures. However, cantilevered beam-type harvesters have limited broadband vibration energy harvesting capabilities unless they are combined with a nonlinear component. Moreover, cantilever arrangements cannot always be mounted on thin structures (which are basic components of marine, aerospace, and ground transportation systems) without significantly affecting the host system's design and overall dynamics. A patch-based piezoelectric energy harvester structurally integrated to a thin plate can be a proper alternative to using resonant cantilevers for harvesting energy from thin structures. Besides, plate-like structures have more number of vibration modes compared to beam structures, offering better broadband performance characteristics. In this paper, we present analytical modelling of patch-based piezoelectric energy harvester attached on a thin plate for random vibrations. The analytical model is based on electromechanically-coupled distributed-parameter formulation and validated by comparing the electromechanical frequency response functions (FRFs) with experimental results. Example case studies are then presented to investigate the expected power output of a piezoceramic patch attached on an aluminum plate for the case of random force excitations. The effect of bandwidth of random excitation on the mean power and shunted mean-square vibration response are explored with a focus on the number of vibration modes covered in the frequency range of input power spectral density (PSD).Publication Open Access Broadly tunable continuous-wave solid-state red source based on intracavity-doubled Cr (4+): forsterite laser(Society of Photo-optical Instrumentation Engineers (SPIE), 2002) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851This work describes the development and characterization of a continuous-wave (cw) room-temperature intracavity-doubled Cr4+:forsterite laser which produces broadly tunable red radiation. Such a source is potentially important in spectroscopy, display technologies, and medical applications. In the experiments, a 2-cm-long Cr4+:forsterite crystal was placed in an astigmatically compensated x-cavity which was end-pumped by a 1064-nm Nd:YAG laser. The crystal which had a small-signal pump absorption of 68% was maintained at 20 degreesC. An intracavity Brewster-cut SF10 prism was used to tune the output of the laser. Intracavity frequency doubling was achieved by using a periodically poled lithium niobate (PPLN) crystal which had 8 different poling periods. The PPLN crystal was placed inside the resonator between a curved folding mirror and the curved output coupler. The transmission of the output coupler was 2.6% at 1260 nm. The PPLN temperature was maintained at 188 degreesC. By translating the PPLN crystal through sections with different poling periods, second harmonic generation was obtained in the wavelength region between 613 and 655 run. With an incident pump power of 6.8 W at 1064 rim, the Cr4+:forsterite laser produced 245 mW of cw output power at 1260 nm and intracavity frequency doubling yielded 45 mW at 630 nm.Publication Open Access Controlled photoluminescence in amorphous-silicon-nitride microcavities(American Institute of Physics (AIP) Publishing, 2001) Tanrıseven, S.; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Narrow-band and enhanced photoluminescence have been observed in hydrogenated amorphous-silicon-nitride microcavities. The distributed Bragg reflectors were fabricated using alternating layers of hydrogenated amorphous-silicon nitride and hydrogenated amorphous-silicon oxide. The microcavity resonance wavelength was designed to be at the maximum of the bulk hydrogenated amorphous-silicon-nitride luminescence spectrum. At the microcavity resonance, the photoluminescence amplitude is enhanced, while the photoluminescence linewidth is reduced with respect to the bulk hydrogenated amorphous-silicon nitride. (C) 2001 American Institute of Physics.Publication Open Access Correcting for electrostatic cutoffs in free energy simulations: toward consistency between simulations with different cutoffs(American Institute of Physics (AIP) Publishing, 1998) McCammon, J. Andrew; Department of Physics; Reşat, Haluk; Faculty Member; Department of Physics; College of SciencesThe use of electrostatic cutoffs in calculations of free energy differences by molecular simulations introduces errors. Even though both solute-solvent and solvent-solvent cutoffs are known to create discrepancies, past efforts have mostly been directed toward correcting for the solute-solvent cutoffs. In this work, an approach based on the generalized reaction field formalism is developed to correct for the solvent-solvent cutoff errors as well. It is shown using a series of simulations that when the cutoff lengths are significantly smaller than the half unit cell size, and the solute-solvent cutoff is not much larger than the solvent-solvent cutoff, the new algorithm is able to yield better agreement among simulations employing different truncation lengths.Publication Open Access Correcting for solvent-solvent electrostatic cutoffs considerably improves the ion-pair potential of mean force(American Institute of Physics (AIP) Publishing, 1999) Department of Physics; Reşat, Haluk; Faculty Member; Department of Physics; College of SciencesA recently developed algorithm based on the continuum treatment of the solvent molecules beyond the electrostatic cutoff sphere is applied to the potential of mean force results between sodium and chloride ions to study the effects of the solute-solvent and solvent-solvent cutoff errors. The results show that although the solute-solvent correction improves the thermodynamic results slightly, physically realistic results are obtained only when the solvent-solvent correction is applied. This further supports past findings that proper treatment of solvent-solvent interactions is as important as that of the solute interactions, and should not be ignored.