239+240Pu concentrations of sediments and seawater in the coastal sea of Korea

Summary The ^239+240Pu content of the marine sediments and seawater of the costal sea of Korea was measured. In marine sediments the ^239+240Pu concentrations were in the range of 0.11-1.91 Bq/kg dry weight and in the coastal sea of Korea the ratio of ^239+240Pu/¹³⁷Cs was 0.27. The correlations between ^239+240Pu and ¹³⁷Cs concentrations and the content of organic matter (C, O, H, N, S) as well as the grain size of marine sediment were investigated by regression analysis. The distribution coefficient of ^239+240Pu was 1.22^. 10⁵. The ^239+240Pu concentration in seawater increased with seawater depth. However, the ¹³⁷Cs concentration in seawater did not change considerably with depth.     

Variable energy gap of SiCN nanopowders

SiCN and SiC nanopowders were prepared by infrared laser pyrolysis of gaseous precursors starting from a SiH₄C₂H₂NH₃ gas mixture. The SiCN powders were composed of an amorphous phase ascribed to the ternary compound with a β-SiC crystalline phase, while the SiC powders consisted of β phase only. The chemical bonding and the local atomic order in the SiCN powders are much more complicated than those of pure SiCSi₃N₄ mixtures and are strongly dependent on the variation in the initial gas composition. The average grain size was estimated from X-raydiffraction (XRD) patterns and atomic force microscopy (AFM). The UV-VIS transmittance data were used for the bandgap determination in these new materials. Absorption studies suggested that the direct energy gap is more favourable in SiCN and SiC nanoparticles and is blueshifted with regard to the crystalline bulk (SiC). The change of type of band-to-band transition in SiC nanopowders, due to the size effects, and increasing of the energy gap value in SiCN powders with different local atomic arrangement, makes them attractive for blue-UV optoelectronic applications. PACS 61.46.+w; 78.70.Ck; 68.3.Ps; 78.40.-q     

Visualizations of the transitional flow patterns at the mid-span between shrouded co-rotating disks

Transitional air flow patterns at the mid-plane of gap between co-rotating disks in a stationary cylindrical enclosure are visualized under the acceleration condition of disks. The flow visualization is performed using olive oil particles with a laser sheet and CCD camera. On disk spinning-up, the transition flow patterns are clearly observed. The affect of magnitude of acceleration on the onset of transition and the relations of the number of vortex cells in fully turbulent regime of the flow in an outer region of the flow field to the rotating speed are also investigated. The centrifugal effect of particle is evaluated from the comparison with visualization results using hollow glass spheres.     

Nematic domains and resistivity in an itinerant metamagnet coupled to a lattice

The nature of the emergent phase near a putative quantum critical point in the bilayer ruthenate Sr3Ru2O7 has been a recent subject of intensive research. It has been suggested that this phase may possess electronic nematic order (ENO). In this work, we investigate the possibility of nematic domain formation in the emergent phase, using a phenomenological model of electrons with ENO and its coupling to lattice degrees of freedom. The resistivity due to the scattering off the domain walls is shown to closely follow the ENO parameter. Our results provide qualitative explanations for the dependence of the resistivity on external magnetic fields in Sr3Ru2O7.     

Simultaneous synthesis and consolidation of nanostuctured MgAl2O4 compound by high-frequency induction heated sintering

The rapid sintering of a nanostuctured MgAl₂O₄ compound in a short time was investigated with high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostuctured materials. A highly dense nanostructured MgAl₂O₄ compound was produced with simultaneous application of 80 MPa pressure and induced current with output of total power capacity (15 kW) within 2 min. The sintering behavior, grain size, and mechanical properties of the MgAl₂O₄ compound were investigated.     

Rapid synthesis and consolidation of nanostructured 2MgAl2O4–MgAl2Ti3O10 composite and its mechanical properties

Rapid sintering of nanostructured 2MgAl₂O₄–MgAl₂Ti₃O₁₀ composite in a pulsed current activated sintering process was investigated. The advantage of this process is that it enables very rapid densification to near theoretical density and prevents grain growth in nanostructured materials. Highly dense nanostructured 2MgAl₂O₄–MgAl₂Ti₃O₁₀ composite was produced by simultaneous application of 80 MPa pressure and a pulsed current of 2,800 A for 2 min. The sintering behavior, grain size, and mechanical properties of the 2MgAl₂O₄–MgAl₂Ti₃O₁₀ composite were investigated.     

Mass- and heat-transfer-enhanced catalyst system for Fischer-Tropsch synthesis in fixed-bed reactors

Fischer-Tropsch synthesis (FTS) was carried out using Al₂O₃-supported Co catalyst coated on metallic monolith. Considering the liberation of a large amont of heat from the highly exothermic FTS reaction, catalytic activity of Co catalyst coated on metallic monolith was tested and compared with that of pellet-type catalysts. The reaction was carried out in a conventional tubular fixed-bed reactor and simulated distillation (SIMDIS) analysis method was used to determine the liquid products distribution. Proper control of degree of reaction, as well as the reaction temperature gave rise to a shift of products selectivity toward higher hydrocarbons, especially C₁₃?C₁₈ diesel range hydrocarbons.     

Iterative algorithms for part grouping and loading in cellular reconfigurable manufacturing systems

A reconfigurable manufacturing system (RMS), one of state-of-the-art manufacturing system technologies, is the one designed at the outset for rapid changes in its hardware and software components in order to quickly adjust its production capacity and functionality in response to market or system changes. In this study, we consider a cellular RMS with multiple reconfigurable machining cells (RMCs), each of which has numerical control machines, a setup station, and an automatic material handling and storage system. Each machine within the RMC has an automatic tool changer and a tool magazine of a limited capacity. Two important operational problems, part grouping and loading, are considered in this study. Part grouping is the problem of allocating parts to RMCs, and loading is the problem of allocating operations and their cutting tools to machines within the RMC. An integer programming model is suggested to represent the two problems at the same time for the objective of balancing the workloads assigned to machines. Then, due to the complexity of the problem, we suggest two iterative algorithms in which the two problems are solved repeatedly until a solution is obtained. Computational experiments were done on various test instances and the results are reported.     

Acoustic streaming in lithotripsy fields: preliminary observation using a particle image velocimetry method

This study considers the acoustic streaming in water produced by a lithotripsy pulse. Particle image velocimetry (PIV) method was employed to visualize the acoustic streaming produced by an electromagnetic shock wave generator using video images of the light scattering particles suspended in water. Visualized streaming features including several local peaks and vortexes around or at the beam focus were easily seen with naked eyes over all settings of the lithotripter from 10 to 18 kV. Magnitudes of the peak streaming velocity measured vary in the range of 10-40 mm s(-1) with charging voltage settings. Since the streaming velocity was estimated on the basis of a series of the video images of particles averaged over 1/60s, the time resolution limited by the video frame rate which is 1-2 orders of magnitude larger than driving acoustic activities, measured velocities are expected to be underestimated and were shown a similar order of magnitude lower than those calculated from a simple theoretical consideration. Despite such an underestimation, it was shown that, as predicted by theory, the magnitude of the streaming velocity measured by the present PIV method was proportional to acoustic intensity. In particular it has almost a linear correlation with peak negative pressures (r=0.98683, p=0.0018).