Browsing by Author "Koga, K."

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Correlation between glaze-colors and structural properties of the HIZEN celadons produced in the Edo period of Japan, by means of X-ray diffraction (?)
(Cer?mica, 2011) Hidaka, M.; Ohashi, K.; Wijesundera, R.P.; Kumara, L.S.R.; Watanabe, M.; Koga, K.; Choi Jae-Young; Sung, N.E.; Park, Y.J.
HIZEN celadons produced at Arita and Imari areas in Japan from 1630's to 1790's (Edo period) have been investigated by means of X-ray fluorescence analysis, and X-ray diffraction and X-ray absorption spectra using synchrotron radiation. It is found that, in the HIZEN celadons, the color brightness of the celadon glazes depends on the structural property of the raw basic ceramics taken at Imaizumi (Arita) and Ohkwachi (Imari), where the former is mainly Quartz-SiO2, and the later is Sanidine ((K,Na)Si3O8). It is confirmed that CaCO3 of natural wood ash added artificially into the raw celadon ceramics makes a glassy glaze on the surface of the basic body of the HIZEN celadons. Transition-metal ions (Cr, Cu, Zn) of very small amount are detected in the celadon glazes, in addition to Fe and Mn of small amount. It is considered that Cu and Cr are related to the color brightness of green-brown and blue-green in the HIZEN celadon glazes, respectively.
Correlation between the Izumiyama porcelain ceramics and the red-overglaze enamels of the Kakiemon-style porcelains
(Ceramics International, 2008) Kajihara, S.; Hidaka, M.; Wijesundera, R.P.; Kumara, L.S.R.; Kobayashi, H.; Koga, M.; Tsuru, T.; Koga, K.; Shimomura, K.; Choi Jae-Young; Sung, N.E.; Park, Y.J.
The Kakiemon-style porcelains made from 17th century at Arita are famous Japanese porcelains, characterized mainly by their colored underglaze and overglaze and by their original design of coloring spatial patterns in the porcelain surface. Raw materials of the red-overglaze enamels have been investigated by means of X-ray diffraction and X-ray absorption spectra using synchrotron radiations. It is found that Izumiyama porcelain ceramics of yellow color can produce the Kakiemon red-overglaze enamels by thermal treatment and water-washing, where Izumiyama is a collecting place of the raw porcelain ceramic at Arita. The brightness of the red-overglaze enamels is related on the local structure around Fe ions and the electronic band states of Fe ions near a Fermi level in ?-Fe2O3, in addition to the spatial density of the ?-Fe2O3 fine particles. The structural and electronic properties are slightly affected by an electron-hybridization between Fe ions of ?-Fe2O3 and oxygen ions of the (SiO2?Al2O3) complexes in the red overglaze.
Effects of annealing on the properties and structure of electrodeposited semiconducting Cu?O thin films
(Physica Status Solidi (basic solid state physics), 2007) Wijesundera, R.P.; Hidaka, M.; Koga, K.; Sakai, M.; Siripala, W.; Choi Jae-Young; Sung, N.E.
The structures and the electronic states in electrodeposited semiconductor Cu?O thin films have been investigated for each annealing temperature (TA) by X-ray diffraction (XD) and X-ray absorption spectroscopy (XAS) near the Cu K edge using synchrotron radiation. The thin films prepared as grown and annealed at TA ? 175 �C, 200 �C ? TA ? 300 �C, TA = 400 �C are characterized mainly by the pure Cu2O-type structure, the pseudo-Cu2O-type having a superlattice structure, and two phases of Cu2O-type and CuO-type structures, respectively, while the film annealed at TA = 500 �C is single-phase CuO-type. The XAS spectra suggest that there is a structural phase transition occurring at about 400 �C, which induces a modulation of the local structure around Cu ions observed in the extended X-ray absorption fine structure (EXAFS) and the occupational electronic band states of Cu-4p localized just above the Fermi level, taken from X-ray absorption near edge structure (XANES). The open-circuit voltage suggests that the photosensitivity of the Cu?O thin films strongly depends on the annealing treatment and shows a crossover from an n-type to a p-type semiconductor. (? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth and characterisation of potentiostatically electrodeposited Cu2O and Cu thin films
(Thin Solid Films, 2006) Wijesundera, R.P.; Hidaka, M.; Koga, K.; Sakai, M.; Siripala, W.
Cuprous oxide and copper thin films were potentiostatically electrodeposited in an acetate bath. Voltammetric curves were used to investigate the growth parameters; deposition potential, pH and temperature of the bath. Deposition potential dependency on the structural, morphological, optical and electronic properties of the films were investigated by the X-ray diffraction measurements, scanning electron micrographs, absorption measurements and dark and light current?voltage characterisations. It was observed that single phase polycrystalline Cu2O can be deposited from 0 to ? 300 mV Vs saturated calomel electrode (SCE) and co-deposition of Cu and Cu2O starts at ? 400 mV Vs SCE. Further increase in deposition potential from ? 700 mV Vs SCE produces single phase Cu thin films. Single phase polycrystalline Cu2O thin films with cubic grains of 1?2 ?m can be possible within the very narrow potential domain around ? 200 mV Vs SCE. Enhanced photoresponse in a photoelectrochemical cell is produced by the Cu2O thin film prepared at ? 400 mV Vs SCE, where Cu is co-deposited with Cu2O with random distribution of Cu spheres on the Cu2O surface. This study reveals that a single deposition bath can be used to deposit both Cu and Cu2O separately and an admixture of Cu?Cu2O by controlling the deposition parameters.
Structural and Electronic Properties of Electrodeposited Heterojunction of CuO/Cu2O
(Ceramics-Silik�ty, 2010) Wijesundera, R.P.; Hidaka, M.; Koga, K.; Choi Jae-Young; Sung, N.E.
The structures and the electronic band states of the electrodeposited thin film CuO/Cu2O heterojunction have been studied by means of the X-ray diffractions (XRD) and the X-ray absorption spectra (XAS) with different grazing angles of the incident X-ray beam using the synchrotron radiation. The heterojunction of about 2 ?m consists of n-type Cu2O (~1 ?m) and p-type CuO (~1 ?m) thin films bi-layer. Scanning electron micrographs (SEMs) show the existence of two different polycrystalline grain layers and the XRD reveals that the different grain layers are high quality CuO-type and Cu2O-type structures respectively. Photoactive performances of the Ti/CuO/Cu2O/Au heterojunction are Voc of ~210 mV and Jsc of ~310 mA/cm2. It reveals that the Cu2O grains are grown from the surfaces of the CuO polycrystalline grains and make very good contact with the CuO grains. It is found that the XAS of CuO/Cu2O heterojunction are convoluted independently by X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structures (XANES) spectra of the Cu2O and CuO grains, depending on the grazing angles. Present study reveals that bottom of the conduction band (Cu-4pp) of the Cu2O in the CuO/Cu2O heterojunction reduces by 0.57 eV relative to the Ti/Cu2O ohmic contact.