Browsing by Subject "Surface morphology"
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Item Open Access Fine-tuning the dispersion and the mobility of BaO domains on NO x storage materials via TiO2 anchoring sites(American Chemical Society, 2010) Andonova, S. M.; Şentürk, G. S.; Ozensoy, E.In an attempt to control the surface dispersion and the mobility of BaO domains on NOx storage materials, TiO2/TiOx anchoring sites were introduced on/inside the conventional γ-Al 2O3 support matrix. BaO/TiO2/Al 2O3 ternary oxide materials were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies. The synthesized NOx storage materials were studied via XRD, Raman spectroscopy, BET surface area analysis, TPD, XPS, SEM, EDX-mapping, and in situ FTIR spectroscopy of adsorbed NO2. NOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology and the surface structure of the TiO2/TiOx domains. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors, which was found to originate mostly from the increased fraction of accessible TiO 2/TiOx sites on the surface. These TiO2/ TiOx sites function as strong anchoring sites for surface BaO domains and can be tailored to enhance surface dispersion of BaO. TPD experiments suggested the presence of at least two different types of NOx species adsorbed on the TiO2/TiOx sites, with distinctively different thermal stabilities. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+/N2O 3 on alumina ≪ nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. © 2010 American Chemical Society.Item Open Access The superconducting transition width and illumination wavelength dependence of the response of MgO substrate YBCO transition edge bolometers(Elsevier BV * North-Holland, 2007) Oktem, B.; Bozbey, A.; Avci, I.; Tepe, M.; Abukay, D.; Fardmanesh, M.Dependence of the phase and magnitude of the response of MgO substrate YBa2Cu3O7-δ (YBCO) transition edge bolometers to the near infrared radiation on the superconducting transition width is presented in this work. The bolometers were made of YBCO thin films of 200 nm thickness that were grown on single crystal MgO (1 0 0) substrates by DC inverted cylindrical magnetron sputtering. We have measured the responses of both large and small area devices with respect to the bias temperature and radiation modulation frequency. We have observed that the superconducting transition width has major effects on the response of the bolometers such as; on a dip of the phase of the response versus modulation frequency curve around 1 Hz, the rate of decrease of the magnitude of the response, and dependence of the phase of the response on temperature at mid-range modulation frequency. We have investigated a correlation between the superconducting transition width and the YBCO film surface morphology of the devices. In addition, the illumination wavelength dependence of the optical response of both wide and narrow transition width devices has been investigated. Here we present the analysis and the possible mechanisms that can affect the response of the bolometers at the superconducting transition region.Item Open Access Surface evolution of 4H-SiC(0001) during in-situ surface preparation and its influence on graphene properties(Trans Tech Publications, Switzerland, 2013) Ul Hassan J.; Meyer, A.; Çakmakyapan, Semih; Kazar, Özgür; Flege J.I.; Falta J.; Özbay, Ekmel; Janzén, E.The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions. © (2013) Trans Tech Publications, Switzerland.