Silicon oxycarbide glasses derived from polymeric networks with different molecular architecture prepared by hydrosilylation reaction

We have prepared two polysiloxane networks with different molecular structures and similar compositions as suitable precursors of silicon oxycarbide glasses (SiOC), in order to understand the influence of the molecular architecture of the polymeric networks on the structure of these glasses. The structural evolution from the polymeric precursor to glasses was followed by solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and FTIR spectroscopies, X-ray diffraction, thermogravimetric analysis, and density measurements up to 1000 °C. The high- temperature behavior of SiOC glasses, up to 1600 °C, was studied by X-ray diffraction, and ²⁹Si MAS NMR spectroscopy. The formation of carbosilane bridges in the polymeric networks during the hydrosilylation reaction contributed to the generation of a larger amount of carbidic sites in the final products. An increase in the pyrolysis temperature led to a distribution of silicon sites and crystallization profiles that depended on the molecular architecture of the polymeric precursors.     

Novel structured yttria-stabilized zirconia coatings fabricated by hybrid thermal plasma spraying

Yttria-stabilized ZrO₂ powders with initial sizes of 5–22 mm were chsosen as feedstock for hybrid thermal plasma deposition. At 100 kW RF input power, the microstructures of the deposited coatings varied from mostly sprayed splats to physical-vapor-deposited nanostructures when the powder feeding rate was reduced from 4 to 1 g/min. At a powder feeding rate of 2 g/min, a peculiar layered coating consisting of both structures was deposited at a rate over 50 mm/min, which is promising for the fabrication of next-generation novel thermal barrier coatings.     

Molecular dynamics study of deposition mechanism of cubic boron nitride

We have performed molecular dynamics simulations of bombardment of graphitic boron nitride (gBN) by energetic boron and nitrogen particles in order to examine the roles of ion bombardment in ion/plasma-assisted deposition of cubic boron nitride (cBN) thin films. We have found that the interaction of the energetic particles with gBN creates four-fold coordinated local structures (sp³-formation) inside gBN. We have also found that clusters of sp³-formations are created as a result of successive bombardment, some of which have cBN-like structures. On the basis of these results, we propose an atomic-scale model of cBN nucleation in which successive sp³-formation converts gBN into cBN.     

Thermoelectric properties of SiC thick films deposited by thermal plasma physical vapor deposition

SiC thick films of about 300 µm could be prepared with a deposition rate above 300 nm/s by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The thermoelectric properties were investigated as a function of composition and doping content. The nondoped films showed n-type conduction. Although the Seebeck coefficient reached as high as -480 µV/K, the power factor was only around 1.6 × 10^-4 Wm^-1 K^-2 at 973 K due to the relatively high electrical resistivity. In order to reduce the electrical resistivity and to deposit layers with n-type and p-type conduction, N₂, B and B₄C were selected as the dopants. Nitrogen-doped samples exhibit n-type characterization, B and B₄C-doped samples exhibit p-type characterization, and the electrical resistivity decreased from 10^-2–10^-3 to 10^-4–10^-5 Ωm after doping. The maximum power factor of the nitrogen-doped SiC and the thick films deposited with B₄C powder reached 1.0 × 10^-3 and 6.4 × 10^-4 Wm^-1 K^-2 at 973 K, respectively.

© 2003 Elsevier Science Ltd. All rights reserved.     

Heat Transfer Characteristics and Reynolds Stress Budget of Spatially Advancing Turbulent Flow in a Curved Channel

Direct numerical simulation was performed for the spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel equating the radius ratio to 0.92 or 0.8. The frictional Reynolds number was fixed at 150, whereas the Prandtl number was set at 0.71. According to the numerical result, the remarkable enhancement of heat transfer occurred on the outer wall, suggesting the organized vortex activated the heat transfer. The budgets of Reynolds stresses clarified that the onset and growth of the organized flow was assisted by the direct energy transfer from the mean flow.     

An Investigation of Compressible Turbulent Forced Convection by an Implicit Turbulence Model for Large Eddy Simulation

An investigation of compressible turbulent forced convection in a three-dimensional channel flow is studied numerically by an implicit turbulence model for large eddy simulation (LES). Because of a high temperature difference between two walls and turbulent flow, the compressibility and viscosity of fluid should be taken into consideration simultaneously. Methods of the Roe scheme, preconditioning, and dual time stepping coordinating an implicit turbulence model for LES are used for resolving the effect of the compressibility of fluid on a low speed flow field. The magnitudes of Re _τ based on the friction velocity changing from 180 to 940, with the high temperature difference of two walls of 500 k are conducted. The results of the mean velocity profiles and turbulent intensities are in good agreement with the benchmark DNS data obtained by spectral codes from a low Reynolds number (Re _τ  = 180) to a high Reynolds number (Re _τ  = 940). Besides, the larger the Re _τ is, with the exception of acquirement of larger average Nusselt number, the more drastic variation of local instantaneous Nusselt number is observed.     

Numerical and Experimental Study on Residual Stress in Gray Cast Iron Stress Lattice Shape Casting

The prediction of residual stress in a stress lattice shape casting (stress lattice) has been conducted and discussed by some researchers via the Finite Element Method (FEM). However, most of the previous studies used the first-order tetrahedral element, which has poor analysis accuracy in problems including bending. The use of the first-order tetrahedral element makes the verification of these studies uncertain because the bending deformation essentially occurs in the stress lattice casting. This study first shows that the thermal stress analysis for the stress lattice should use the element that can represent the bending deformation in principle for bending of the thin parts. Second, the simulated residual stress was compared with the measured value. The thermal stress analysis successfully predicted the residual stress of the stress lattice casting with and 11 pct difference. In addition to the prediction of the residual stress, it is important from the viewpoint of the productivity of castings to reveal the effect of the shake-out temperature on the residual stress. However, in the previous studies, conclusions concerning the effect of the shake-out temperature on the residual stress were not consistent (i.e., the one study said the higher shake-out temperature decreased the residual stress, and another study said a higher shake-out temperature increased the residual stress). Therefore, the current study first discusses the reason for the inconsistent conclusions in the previous studies. Second, stress lattice castings were cast and shaken out at various shake-out temperatures. Then, the current study validated the effect of the shake-out temperature on the residual stress. Consequently, the experimental results supported the conclusion of Kasch and Mikelonis that the shake-out at higher temperature contributed to the increase of the residual stress in the casting.     

Joint strength of Inconel 718 alloy and its improvement by post-weld heat treatment – joint performance and its controlling factors in friction welding of Inconel 718 alloy

The influences of welding parameters on tensile properties of friction-welded joints of Inconel 718 alloy (subjected to a post-weld heat treatment (PWHT) consisting of a solution treatment at 1253 K and double ageing treatments at 993 and 893 K) have been investigated to reveal the controlling factor of the joint performance. All joints obtained were fractured near the bond interface at smaller elongations and area reductions than the base metal on tensile tests, although most of them showed tensile strengths comparable with that of the base metal. The observations of fractured surface and its cross-sectional microstructure suggested that an interfacial fine grain zone including numerous fine Laves phase particles 30–100 nm in size was responsible for a low ductility fracture at shorter friction time and lower friction pressure. As the friction time and pressure were increased, the fine grain zone disappeared, and a reduction in hardness near the bond interface became significant, causing a rather ductile fracture near the bond interface. With an increase in friction time, coarse Laves phase particles, a few micrometres in size remaining near the bond interface increased and they acted as a crack nucleation site of ductile fracture. An increase in the solution treatment temperature during the post-weld heat treatment enhanced the dissolution of the coarse Laves phase in the low-hardness region, and enabled us to obtain joints that were free from unacceptable grain growth and fractured in the base metal at a solution treatment temperature of 1323 K.     

Plutonium breeding of light water cooled fast reactors

Light water cooled fast reactor with new fuel assemblies (FA) has been studied for high breeding of fissile plutonium. It achieves fissile plutonium surviving ratio (FPSR) of 1.342 (discharge/loading), 1.013 end and beginning of equilibrium cycle (EOEC/BOEC), and compound system doubling time (CSDT) of 95.9 years at the average coolant density of pressurized water reactor (PWR). It is further improved for reduced moderation boiling water reactor (BWR) (RMWR) coolant density. Fissile plutonium surviving ratio reaches 1.397 (discharge/loading), 1.030 (EOEC/BOEC) and CSDT is 37 years. The present study has shown the possibility of breeding at the PWR coolant density and meeting the growth rate of energy demand of advanced countries at the RMWR and Super FR coolant density for the first time. The new FA consist of closely packed fuel rods. The integrity of welding of fuel rods at the top and bottom ends is maintained as the conventional fuel rods. The coolant to fuel volume fraction is reduced to 0.085, one-sixth of that of RMWR. The volume fraction remains unchanged with the diameter of the fuel rod. The thermal hydraulic design of the cores remains for the future study.