University of Tsukuba

About: University of Tsukuba is a education organization based out in Tsukuba, Ibaraki, Japan. It is known for research contribution in the topics: Population & Gene. The organization has 36352 authors who have published 79483 publications receiving 1934752 citations. The organization is also known as: Tsukuba daigaku & Tsukuba University.

Dynamics of wood transport in streams: a flume experiment

Abstract: The influence of woody debris on channel morphology and aquatic habitat has been recognized for many years. Unlike sediment, however, little is known about how wood moves through river systems. We examined some dynamics of wood transport in streams through a series of flume experiments and observed three distinct wood transport regimes: uncongested, congested and semi-congested. During uncongested transport, logs move without piece-to-piece interactions and generally occupy less than 10 per cent of the channel area. In congested transport, the logs move together as a single mass and occupy more than 33 per cent of the channel area. Semi-congested transport is intermediate between these two transport regimes. The type of transport regime was most sensitive to changes in a dimensionless input rate, defined as the ratio of log volume delivered to the channel per second (Qlog) to discharge (QW); this ratio varied between 0·015 for uncongested transport and 0·20 for congested transport. Depositional fabrics within stable log jams varied by transport type, with deposits derived from uncongested and semi-congested transport regimes having a higher proportion of pieces orientated normal to flow than those from congested transport. Because wood input rates are higher and channel dimensions decrease relative to piece size in low-order channels, we expect congested transport will be more common in low-order streams while uncongested transport will dominate higher-order streams. Single flotation models can be used to model the stability of individual pieces, especially in higher-order channels, but are insufficient for modelling the more complex intractions that occur in lower-order streams. © 1997 John Wiley & Sons, Ltd.

Correlation between the photoluminescence lifetime and defect density in bulk and epitaxial ZnO

Abstract: Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

Development of a Ru/C catalyst for glycerol hydrogenolysis in combination with an ion-exchange resin

Abstract: The combination of Ru/C and Amberlyst ion-exchange resin is effective for the dehydration and hydrogenation (denoted as hydrogenolysis) of glycerol to 1,2-propanediol under mild reaction conditions (393 K). A Ru/C catalyst prepared by using active carbon with a low surface area (∼250 m2/g) showed better performance than that prepared by using active carbon with a high surface area. In addition, treatment of Ru/C catalysts prepared from Ru(NO)(NO3)3 with Ar flowing at the appropriate temperature enhanced the performance compared to that of the commercially available Ru/C catalysts. This temperature treatment can be influenced by the decomposition of Ru precursor salt and aggregation of Ru metal particles. In addition, the degradation reaction as a side-reaction to C1 and C2 compounds of glycerol hydrogenolysis was more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. The combination of Ru/C with the Amberlyst resin enhanced the turnover frequency of 1,2-propanediol formation drastically, and this indicates that 1,2-propanediol can be formed mainly by dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-propanediol catalyzed by Ru/C.