Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
Browse
16 results
Search Results
Publication Open Access Enhanced sinterability, thermal conductivity and dielectric constant of glass-ceramics with PVA and BN additions(Multidisciplinary Digital Publishing Institute (MDPI), 2022) Akkasoğlu, Ufuk; Çiçek, Buğra; N/A; Department of Chemistry; Arıbuğa, Dilara; Balcı, Özge; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 295531With the rapid development of the microelectronics industry, many efforts have been made to improve glass-ceramics' sinterability, thermal conductivity, and dielectric properties, which are essential components of electronic materials. In this study, low-alkali borosilicate glass-ceramics with PVA addition and glass-BN composites were prepared and successfully sintered at 770 degrees C. The phase composition, density, microstructure, thermal conductivity, and dielectric constant were investigated. It was shown that PVA addition contributes to the densification process of glass-ceramics (~88% relative density, with closed/open pores in the microstructure) and improves the thermal conductivity of glass material from 1.489 to 2.453 W/K.m. On the other hand, increasing BN addition improves microstructures by decreasing porosities and thus increasing relative densities. A glass-12 wt. % BN composite sample exhibited almost full densification after sintering and presented apparent and open pores of 2.6 and 0.08%, respectively. A high thermal conductivity value of 3.955 W/K.m and a low dielectric constant of 3.00 (at 5 MHz) were observed in this material. Overall, the resulting glass-ceramic samples showed dielectric constants in the range of 2.40-4.43, providing a potential candidate for various electronic applications.Publication Open Access Improved superconducting properties in the Mg11B2 low activation superconductor prepared by low-temperature sintering(Nature Publishing Group (NPG), 2016) F. Cheng, Y.; Liu, Z. Ma; Hossain, M. S.; Department of Chemistry; Somer, Mehmet Suat; Faculty Member; Department of Chemistry; College of Sciences; 178882Mg11B2 has a great application prospect in the superconducting coils for fusion reactor as the “low activation superconductors”. The un-doped Mg11B2 and Cu-doped Mg11B2 bulks using 11B as a boron precursor were fabricated by low-temperature sintering in present work. It was found that the prepared Mg11B2 low activation superconductors exhibit better Jc performance than all of other Mg11B2 samples reported in previous studies. As for Cu doped Mg11B2, minor Cu addition can obviously improve the Mg11B2 grain crystallization and reduce the amount of MgO impurity. Hence, improved grain connectivity and higher Jc at low fields is obtained in Cu doped Mg11B2 samples. For un-doped samples, refined grains and more MgO impurity with proper size brought about more flux pinning centers, resulting in better Jc performance at high fields.Publication Open Access Optimal reduced dimensional representation of classical molecular dynamics(American Institute of Physics (AIP) Publishing, 2003) Dey, Bijoy K.; Rabitz, H.; Department of Mathematics; Aşkar, Attila; Faculty Member; Department of Mathematics; College of Sciences; 178822An optimal reduced space method for capturing the low-frequency motion in classical molecular dynamics calculations is presented. This technique provides a systematic means for carrying out reduced-dimensional calculations in an effective set of reduced coordinates. The method prescribes an optimal reduced subspace linear transformation for the low frequency motion. The method is illustrated with a dynamics calculation for a model potential, where the original six-dimensional space is reduced to two (three) dimensions, depending on the desired frequency cutoff value.Publication Open Access Meso-2,3-dimercaptosuccinic acid: from heavy metal chelation to CdS quantum dots(Royal Society of Chemistry (RSC), 2012) Özen, Can; N/A; Department of Chemistry; Sevinç, Esra; Ertan, Fatoş Sibel; Ulusoy, Gülen; Acar, Havva Funda Yağcı; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 178902DMSA (meso-2,3-dimercaptosuccinic acid) a prescription drug and a heavy-metal chelating agent, is shown to act both as a sulfur source and a capping agent in the aqueous synthesis of CdS quantum dots under mild conditions. Release of sulfur from DMSA depends on the solution pH and the reaction temperature. Combination of 70 C and pH 7.5 was determined as the best reaction conditions for a well-controlled reaction. Changing the SH/Cd ratio from 2.5 to 7 provides QDs emitting from blue to orange with 6–9% quantum yield with respect to Rhodamine 2B. Viability tests performed with HeLa and MCF-7 cell lines indicate a very low cytotoxicity. Mild reaction conditions and biocompatibility makes these particles valuable candidates for bio applications.Publication Open Access 3D printing of elastomeric bioinspired complex adhesive microstructures(Wiley, 2021) Dayan, Cem Balda; Chun, Sungwoo; Krishna Subbaiah, Nagaraj; Drotlef, Dirk Michael; Akolpoğlu, Mükrime Birgül; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Bioinspired elastomeric structural adhesives can provide reversible and controllable adhesion on dry/wet and synthetic/biological surfaces for a broad range of commercial applications. Shape complexity and performance of the existing structural adhesives are limited by the used specific fabrication technique, such as molding. To overcome these limitations by proposing complex 3D microstructured adhesive designs, a 3D elastomeric microstructure fabrication approach is implemented using two-photon-polymerization-based 3D printing. A custom aliphatic urethane-acrylate-based elastomer is used as the 3D printing material. Two designs are demonstrated with two combined biological inspirations to show the advanced capabilities enabled by the proposed fabrication approach and custom elastomer. The first design focuses on springtail- and gecko-inspired hybrid microfiber adhesive, which has the multifunctionalities of side-surface liquid super-repellency, top-surface liquid super-repellency, and strong reversible adhesion features in a single fiber array. The second design primarily centers on octopus- and gecko-inspired hybrid adhesive, which exhibits the benefits of both octopus- and gecko-inspired microstructured adhesives for strong reversible adhesion on both wet and dry surfaces, such as skin. This fabrication approach could be used to produce many other 3D complex elastomeric structural adhesives for future real-world applications.Publication Open Access Determination of the surface electronic structure of Fe3O4(111) by soft X-ray spectroscopy(Elsevier, 2015) Ogasawara, Hirohito; Nilsson, Anders; Department of Chemistry; Kaya, Sarp; Faculty Member; Department of Chemistry; College of Sciences; 116541The determination of surface terminations in transition metal oxides is not trivial because many structural configurations could be possible. They exhibit various terminations depending on the oxidation states of metal cations exposed to the surface. Fe3O4 is one example in which octahedrally and tetrahedrally coordinated Fe2+ and Fe3+ cations coexists with oxygen anions. For the identification of the surface termination of Fe3O4(1 1 1) grown on Pt(1 1 1) we have employed surface sensitive synchrotron based X-ray photoelectron and absorption spectroscopy. It has been shown that the topmost surface is octahedrally coordinated Fe3+ rich. (C) 2014 Elsevier B.V. All rights reserved.Publication Open Access Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer(American Institute of Physics (AIP) Publishing, 2015) Dell'Angela, M.; Anniyev, T.; Beye, M.; Coffee, R.; Fohlisch, A.; Gladh, J.; Kaya, S.; Katayama, T.; Krupin, O.; Nilsson, A.; Nordlund, D.; Schlotter, W. F.; Sellberg, J. A.; Sorgenfrei, F.; Turner, J. .J.; Ostrom, H.; Ogasawara, H.; Wolf, M.; Wurth, W.; Department of Chemistry; Kaya, Sarp; Faculty Member; Department of Chemistry; College of Sciences; 116541Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.Publication Open Access A high-performance metal-free hydrogen-evolution reaction electrocatalyst from bacterium derived carbon(Royal Society of Chemistry (RSC), 2015) Wei, Li; Karahan, Hüseyin Enis; Goh, Kunli; Jiang, Wenchao; Yu, Dingshan; Jiang, Rongrong; Chen, Yuan; Department of Chemistry; Birer, Özgür; Researcher; Department of Chemistry; College of SciencesWe report a sustainable approach to obtain carbon materials with nitrogen and phosphorus dual functionalities from a common bacterium strain (S. aureus) as a highly efficient hydrogen-evolution reaction (HER) catalyst. With mesoporous structure introduced by ZnCl2 salt and cathodic activation, it demonstrates an onset overpotential as low as 76 mV, a Tafel slope of 58.4 mV dec(-1) and a large normalized exchange current density of 1.72 x 10(-2) mA cm(-2), which are comparable to those of hitherto best metal-free and well-fabricated metallic HER catalystsPublication 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 Direct observation of the dealloying process of a platinum–yttrium nanoparticle fuel cell cathode and its oxygenated species during the oxygen reduction reaction(Royal Society of Chemistry (RSC), 2015) Malacrida, Paolo; Casalongue, Hernan G. Sanchez; Masini, Federico; Hernandez-Fernandez, Patricia; Deiana, Davide; Ogasawara, Hirohito; Stephens, Ifan E. L.; Nilsson, Anders; Chorkendorff, Ib; Department of Chemistry; Kaya, Sarp; Faculty Member; Department of Chemistry; College of Sciences; 116541Size-selected 9 nm PtxY nanoparticles have recently shown an outstanding catalytic activity for the oxygen reduction reaction, representing a promising cathode catalyst for proton exchange membrane fuel cells (PEMFCs). Studying their electrochemical dealloying is a fundamental step towards the understanding of both their activity and stability. Herein, size-selected 9 nm PtxY nanoparticles have been deposited on the cathode side of a PEMFC specifically designed for in situ ambient pressure X-ray photoelectron spectroscopy (APXPS). The dealloying mechanism was followed in situ for the first time. It proceeds through the progressive oxidation of alloyed Y atoms, soon leading to the accumulation of Y3+ cations at the cathode. Acid leaching with sulfuric acid is capable of accelerating the dealloying process and removing these Y3+ cations which might cause long term degradation of the membrane. The use of APXPS under near operating conditions allowed observing the population of oxygenated surface species as a function of the electrochemical potential. Similar to the case of pure Pt nanoparticles, non-hydrated hydroxide plays a key role in the ORR catalytic process.