High-Temperature Fracture Mechanism of Low-Ca-Doped Silicon Nitride

High-purity Si₃N₄ (with 2.5 wt% glassy SiO₂) doped with 0 to 450 at.ppm of Ca was prepared as a model system to investigate the effects of grain-boundary segregants on fracture phenomenology at 1400°C. Subcritical crack-growth (SCG) resistance as well as creep resistance was degraded significantly by the presence of a small amount of Ca. The internal friction of the doped materials exhibited the superposition of a grain-boundary relaxation peak and a high-temperature background, and the apparent viscosity of the grain-boundary film was determined from the peak. Based on these experimental data, the fracture mechanism at 1400°C was divided into three regions: "brittle," SCG, and creep failure as a function of both external strain rate and Ca concentration, C _Ca. From the investigation of the C _Ca dependence of the critical strain rate for the transition from "brittle" to SCG fractures, the SCG phenomenon is suggested to be triggered by small-scale, grain-boundary sliding. The C _Ca dependence of "steady-state" creep rate was far from the theoretical dependence of diffusional creep via a solution-precipitation mechanism. The discrepancy was interpreted to be due to the presence of an impurity-insensitive creep component. This component may correspond to the lowest limit of the tensile creep rate in Si₃N₄ polycrystalline materials containing intergranular glassy-SiO₂ film.     

Improved polyhydroxybutyrate (PHB) production in transgenic tobacco by enhancing translation efficiency of bacterial PHB biosynthetic genes

Polyhydroxybutyrate [P(3HB)] was produced in the transgenic tobacco harboring the genes encoding acetoacetyl-CoA reductase (PhaB) and polyhydroxyalkanoate synthase (PhaC) from Ralstonia eutropha (Cupriavidus necator) with optimized codon usage for expression in tobacco. P(3HB) contents in the transformants (0.2mg/g dry cell weight in average) harboring the codon-optimized phaB gene was twofold higher than the control transformants harboring the wild-type phaB gene. The immunodetection revealed an increased production of PhaB in leaves, indicating that the enhanced expression of PhaB was effective to increase P(3HB) production in tobacco. In contrast, codon-optimization of the phaC gene exhibited no apparent effect on P(3HB) production. This result suggests that the efficiency of PhaB-catalyzed reaction contributed to the flux toward P(3HB) biosynthesis in tobacco leaves.     

Measurements of keV-neutron capture cross sections and capture gamma-ray spectra of 105Pd

The neutron capture cross sections and capture gamma-ray spectra of ¹⁰⁵Pd were measured in the region from 15 to 100 keV and at 585 keV. A neutron time-of-flight method was utilised with an anti-Compton NaI(Tl) spectrometer and a 1.5-ns pulsed neutron source by the ⁷Li(p,n)⁷Be reaction. The capture yields were obtained by applying a pulse-height weighting technique to the observed net capture gamma-ray pulse-height spectra. The capture cross sections of ¹⁰⁵Pd were derived with errors less than 5%, using the standard capture cross sections of ¹⁹⁷Au. The evaluated capture cross sections of JENDL-4.0 and ENDF/B-VII.1 were compared with the present results. The evaluations of JENDL-4.0 and ENDF/B-VII.1 were larger than the present results by 3%–15% in the region from 15 to 100 keV and at 585 keV. The capture gamma-ray spectra of ¹⁰⁵Pd were also derived by unfolding the observed net capture gamma-ray pulse-height spectra. The multiplicities of capture gamma rays of ¹⁰⁵Pd were obtained from the capture gamma-ray spectra.     

Formation of Continuous Pore Structures in Si–C–O Fibers by Adjusting the Melt Spinning Condition of a Polycarbosilane–Polysiloxane Polymer Blend

A polymer blend containing polycarbosilane (PCS) and 15 mass% of polyhydromethylsiloxane (H-oil) was prepared and the properties of the polymer melt were investigated in order to clarify the mechanisms of continuous pore formation. Melt viscosity decreased as the H-oil content increased. Moreover, the saturation of the plasticizer effect of H-oil on the viscosity and apparent turbidity of the melting polymer suggested that the compatibility of H-oil to PCS in melting was limited at 15 mass%. Gas chromatography data after heating showed that an amount of evolved hydrogen at 573 K was increased by 15 mass% of H-oil addition. The fibers formed by the melt-spun of the polymer blend at 578 K mainly possessed a single pore at the center of the fibers' cross section. On the other hand, the fibers melt-spun at 543 K usually included multiple pores, and the fibers melt-spun at 538 K included a number of tiny pores. It is proposed that the evolved hydrogen can be dissolved in the polymer melt, and the desaturation process of the dissolved gas during the fiber spinning with a sudden temperature reduction likely determined the size and location of pores in the fibers.     

Integrated nuclear data utilisation system for innovative reactors

A five-year research and development project on an integrated nuclear data utilisation system was initiated in 2002, for developing innovative nuclear energy systems such as accelerator-driven systems. The integrated nuclear data utilisation system will be constructed as a modular code system, which consists of two sub-systems: the nuclear data search and plotting sub-system, and the nuclear data processing and utilisation sub-system. The system will be operated with a graphical user interface in order to enable easy utilisation through the Internet by both nuclear design engineers and nuclear data evaluators. This paper presents an overview of the integrated nuclear data utilisation system, describes the development of a prototype system to examine the operability of the user interface and discusses specifications of the two sub-systems.     

Analysis of Smoothing Effect of Weather in Distribution System on PV Generation

This paper describes the smoothing effect and fluctuations of PV generation for the purpose of voltage management in a distribution system. The purpose of the analysis is to show quantitatively the fluctuations and the smoothing effect of PV generation by weather, using actual generation data from 25 distributed PVs in the distribution area. The analysis showed the impact of weather on PV output fluctuation based on the indexes of coherence and maximum fluctuation. In particular, it showed the relationship between the area size and the PV output fluctuations by frequency. It was found that the two indexes differed with the weather, and fine and cloudy days seemed to be the most difficult situation for voltage management in the distribution system.     

High-Temperature Strength of Fluorine-Doped Silicon Nitride

High-purity Si₃N₄ (with 2.5 wt% glassy SiO₂) doped with F was prepared by immersion of the starting powder into dilute HF and hot isostatic pressing without sintering additives, using a glass encapsulation method. Oxygen content and cation impurity content were almost the same for the F-doped and undoped materials. However, X-ray fluorescence analysis revealed the order of 100 ppm of F in the doped material, and a considerable amount of F was detected from the amorphous SiO₂ phase at grain-boundary triple points by analytical transmission electron microscopy. High-resolution electron microscopy found that an amorphous intergranular film was omnipresent in both of the materials, with an equilibrium thickness of 10 ± 1 å. Subcritical crack-growth resistance and creep resistance at 1400°C were degraded significantly by the presence of F. Internal friction of doped materials exhibited a dear grain-boundary relaxation peak, which suggested that F was present in the intergranular film at the two-grain junctions; this decreased the grain-boundary viscosity considerably. The film thickness of the doped material showed no apparent chemical effects and was explained by taking into account competing repulsive forces acting normal to the film.