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Publication Metadata only An experimental study on heat transfer performance of iron oxide based ferrofluids(ASME, 2012) Kaya, Alihan; Kurtoglu, Evrim; Kosar, Ali; Department of Chemistry; Acar, Havva Funda Yağcı; Faculty Member; Department of Chemistry; College of Sciences; 178902Nanofluids are colloidal compounds, where the solid phase material is composed of nano sized particles, and the liquid phase can potentially be any fluid but aqueous media are common. As a common nanofluid type, ferrofluids are formed by holding solid nanoparticles in suspension by weak intermolecular forces and may be produced from materials with different magnetic properties. Magnetite is one of the materials used for its natural ferromagnetic properties. Heat transfer performance of ferrofluids is one of the crucial properties among many others that should be analyzed and considered for their wide range of applications. For this purpose, experiments were conducted in order to characterize heat transfer properties of ironoxide based ferrofluids flowing through a microchannel. Promising results were obtained from this study, which are suggesting the use of ferrofluids for heat transfer applications can be advantageous.Publication Metadata only A coarse graining approach in molecular simulations: fuzzy potentials(Istanbul Technical University, 2003) Department of Chemistry; Department of Chemistry; Yurtsever, İsmail Ersin; Eşsiz, Şebnem; Faculty Member; Undergraduated Student; Department of Chemistry; College of Sciences; College of Sciences; 7129; 191615A new representation for interaction potential functions is presented. Unlike the orthodox approaches, the potential function is not a fixed function in terms of internuclear coordinates but a probabilistic one which contains information over a wide range of angular degrees of freedom. It is shown that such approaches can provide practical solutions for bulk systems of high density.Publication Metadata only Polyurethane synthesis revisited: effect of solvent and reaction conditions on prepolymer formation and polymer properties(American Chemical Society (ACS), 2014) N/A; Department of Chemistry; Department of Chemistry; Yıldırım, Armen; Yılgör, Emel; Yılgör, İskender; Master Student; Researcher; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; College of Sciences; College of Sciences; N/A; 40527; 24181N/APublication Metadata only Influence of competitive hydrogen bonding between hard and soft segments on the properties of siloxane and polyether-based segmented copolymers.(Amer Chemical Soc, 1999) Department of Chemistry; Department of Chemistry; N/A; Yılgör, İskender; Burgaz, Engin; Metin, Burak; Yurtsever, İsmail Ersin; Yılgör, Emel; Faculty Member; Undergraduate Student; Undergraduate Student; Faculty Member; Researcher; Department of Chemistry; College of Sciences; College of Sciences; N/A; 24181; 17956; N/A; N/A; 40527N/APublication Metadata only High average-power diode-pumped femtosecond Cr3+: LiCAF laser(Association for Computational Linguistics, 2008) Kärtner, Franz X.; Fujimoto, James G.; Demirbaş, Ümit; Benedick, Andrew; Siddiqui, Aleem; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 2385167-fs pulses with an average power of 300 mW and pulse repetition rate of 120 MHz were obtained from a diode-pumped Cr3+: PLiCAF laser. A semiconductor saturable absorber mirror enabled stable and self-starting mode-locked operation.Publication Metadata only Magnetic nanoparticle based nanofluid actuation with dynamic magnetic fields(ASME, 2011) Bilgin, Alp; Kurtoglu, Evrim; Erk, Hadi Cagdas; Sesen, Muhsincan; Kosar, Ali; Department of Chemistry; Acar, Havva Funda Yağcı; Faculty Member; Department of Chemistry; College of Sciences; 178902Magnetic nanoparticle suspensions and their manipulation are becoming an alternative research line and have very important applications in the field of microfluidics such as microscale flow control in microfluidic circuits, actuation of fluids in microscale, and drug delivery mechanisms. In microscale, it is possible and beneficial to use magnetic fields as actuators of such nanofluids, where these fluids could move along a gradient of magnetic field so that a micropump without any moving parts could be generated with this technique. Thus, magnetically actuated nanofluids could have the potential to be used as an alternative micro pumping system. Actuation of ferrofluid plugs with a changing magnetic field has been extensively studied in the literature. However; the flow properties of ferrofluids are sparsely investigated when the ferrofluid itself is forced to continuously flow inside a channel. As an extension of previous studies, this study aims to investigate flows of magnetic nanoparticle based nanofluids by a generated magnetic field and to compare the efficiency of the resulting system. Lauric Acid coated Super Paramagnetic Iron Oxide (SPIO-LA) was used as the ferrofluid sample in the experiments to realise actuation. Significant flow rates up to 61.8μL/s at nominal maximum magnetic field strengths of 300mT were achieved in the experiments. Results suggest that nanofluids with magnetic nanoparticles merit more research efforts in micro pumping. Thus, magnetic actuation could be a significant alternative for more common techniques such as electromechanical, electrokinetic, and piezoelectric actuation. Copyright © 2011 by ASME.Publication Metadata only Synthesis, properties, and applications of polycaprolactone-polydimethylsiloxane-polycaprolactone triblock copolymers(Amer Chemical Soc, 2015) N/A; Department of Chemistry; Department of Chemistry; Söz, Çağla Koşak; Yılgör, Emel; Yılgör, İskender; PhD Student; Researcher; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 24181Poly(ε-caprolactone)–polydimethylsiloxane–poly(ε-caprolactone) (PCL-PDMS-PCL) triblock copolymers with block lengths in 1,000 – 32,000 g/mol range were synthesized by the ring-opening polymerization of ε-caprolactone using aminopropyl terminated PDMS oligomers as initiators. Reactions were carried out in bulk or solution at 125±5 ºC using tin octoate catalyst. Products obtained in high yields were characterized by FTIR, GPC, DSC, AFM, SEM, XRD, OM and water contact angle measurements. Effect of the copolymer composition and the molecular weight of the PDMS and PCL blocks on; (i) microphase separation and copolymer morphology, (ii) crystallization of PCL segments, and (iii) surface properties of the copolymers were investigated. Regardless of the block lengths, all copolymers displayed microphase separated morphologies. The extent of microphase separation, resultant morphology and sizes of the microphases were strongly dependent on copolymer composition and block lengths of PCL and PDMS. Crystalline PCL microphase was observed in all copolymers, which increased as a function of PCL content and molecular weight. All copolymers displayed hydrophobic surfaces as determined by static water contact angle measurements. Hydrophobicity improved with an increase in the PDMS block length. Copolymer morphologies were also obtained by computational studies at the molecular and mesoscopic levels via density functional theory (DFT) and dissipative particle dynamics (DPD) methods respectively. Computational results obtained were in very good agreement with the microphase morphologies determined by AFM studies. PCL-PDMS-PCL copolymers can be used as reactive oligomers or surface or bulk modifying additives for polymeric systems. We evaluated them as a processing aid in the melt extrusion of polyolefins, where they improved the extruder output substantially. They were also used as surface modifying additives for electrospun polyacrylonitrile fibers and silica modified epoxy networks, which led to the formation of superhydrophobic surfaces.Publication Metadata only Acousto-optic mode-locking of a Cr2+: ZnSe laser(Optica Publishing Group, 2000) Carrig, Timothy J.; Wagner, Gregory J.; Jeong, Jay Y.; Pollock, Clifford R.; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Acousto-optic mode-locking of a Cr2+:ZnSe laser that produces 4.4 psec duration, transform-limited, Gaussian shaped pulses is described. The laser outputs 82 mW of output power at an 81 MHz pulse repetition frequency.Publication Metadata only Development of highly stable and luminescent aqueous CdS quantum dots with the poly(acrylic acid)/mercaptoacetic acid binary coating system(Amer Scientific Publishers, 2009) Lieberwirth, I.; Department of Chemistry; N/A; Department of Chemical and Biological Engineering; Acar, Havva Funda Yağcı; Çelebi, Serdar; Serttunalı, Nazlı İpek; Faculty Member; Master Student; Undergraduate Student; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; Graduate School of Sciences and Engineering; College of Engineering; 178902; N/A; N/AHighly stable and luminescent CdS quantum dots (QD) were prepared in aqueous solutions via in situ capping of the crystals with the poly(acrylic acid) (PAA) and mercaptoacetic acid (MAA) binary mixtures. The effect of reaction temperature and coating composition on the particle size, colloidal stability and luminescence were investigated and discussed in detail. CdS QDs coated with either PAA or MAA were also prepared and compared in terms of properties. CdS-MAA QDs were highly luminescent but increasing reaction temperature caused an increase in the crystal size and a significant decrease in the quantum yield (QY). Although less luminescent and bigger than CdS-MAA, CdS-PAA QDs maintained the room temperature size and QY at higher reaction temperatures. CdS-MAA QDs lacked long term colloidal stability whereas CdS-PAA QDs showed excellent stability over a year. Use of PAA/MAA mixture as a coating for CdS nanoparticles during the synthesis provided excellent stability, high QY and ability to tune the size and the color of the emission. Combination of all of these properties can be achieved only with the mixed coating. CdS coated with PAA/MAA at 40/60 ratio displayed the highest QY (50% of Rhodamine B) among the other compositions.Publication Metadata only Luminescent Nd3+ doped thermoplastic silicone-urea copolymers(Amer Chemical Soc, 2006) Department of Chemistry; N/A; Department of Physics; Department of Physics; Department of Chemistry; Yılgör, İskender; Demirbaş, Ümit; Sennaroğlu, Alphan; Kurt, Adnan; Yılgör, Emel; Faculty Member; Master Student; Faculty Member; Teaching Faculty; Researcher; Department of Physics; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; 24181; 20328; 23851; 194455; N/AA simple method for the preparation of rare earth ion-doped polymers, which display luminescence, is reported. For this purpose silicone–urea copolymers were doped with Nd(NO3)3·6H2O. Various structural and physicochemical properties of the resultant materials were investigated. FTIR studies indicated strong interaction of Nd3+ ions with urea groups, but no interaction with siloxane backbone, which is expected. Absorption measurements in the visible and near infrared region were performed and the radiative decay rates and branching ratios for the meta-stable 4F3/2 level were determined by using Judd–Ofelt theory. The samples were also excited at 800nm and emission spectra were observed in the near infrared at 905, 1059, and 1331nm. In Nd3+ doped silicone–urea systems the highest emission cross section at 1059nm was determined to be 60.7×10−21cm2. Spectroscopic parameters determined in this study suggest that Nd3+ doped silicone–urea copolymers are promising candidates for the development of fiber lasers or amplifiers near 1.06 and 1.3μm.