Oxidation mechanisms of ZRB2-based ultra high temperature ceramic matrix composites

Ultra-high temperature ceramics (UHTCs) are expected as the materials for the nose cones and leading edges for hypersonic and re-entry vehicles. Zirconium diboride (ZrB2) and its composites are a widely studied class of UHTCs. The oxidation of monolithic ZrB2 forms ZrO2 and B2O3. B2O3 acts as a surface protective layer; however, it evaporates above 1200℃. SiC particles are considered effective additives because the SiO2 formed by the oxidation of SiC protects the unreacted region. Simultaneously, excessive pores are formed under the surface in the SiC particle-dispersed ZrB2 matrix (hereafter denoted ZS) composites in a wide temperature range by the preferential oxidation of SiC (active oxidation of SiC) because solid SiO2 is not formed; instead, gaseous SiO forms by active oxidation because of the low oxygen partial pressure relative to that of the surface. The pore-rich porous layer is denoted the “SiC-depleted layer”. The SiC-depleted layer leads to spallation and delamination of the oxidized regions on the surface because strength and stiffness of this layer are quite low. Thus, excessive pore formation in ZS composites should be prevented to improve the oxidation resistance. The objective of this study is to understand oxidation mechanisms of ZrB2-based composites and to propose the way to prevent the formation of SiC-depleted layer in ZS composites. In the present study, we fabricated monolithic ZrB2, ZS, and ZrB2-SiC-ZrC (ZSZ) ternary composites by spark plasma sintering (SPS) technique. In addition, carbon fiber-reinforced ZSZ matrix (C/ZSZ) composites was also fabricated by Si melt infiltration (MI) process. Oxidation resistance of monolithic ZrB2, ZS, ZSZ, and C/ZSZ have specially designed fast heating system in order to characterize oxidation resistance above 2000℃. Please click Additional Files below to see the full abstract

Regulation of the MDM2-P53 pathway and tumor growth by PICT1 via nucleolar RPL11

PICT1 (also known as GLTSCR2) is considered a tumor suppressor because it stabilizes phosphatase and tensin homolog (PTEN), but individuals with oligodendrogliomas lacking chromosome 19q13, where PICT1 is located, have better prognoses than other oligodendroglioma patients. To clarify the function of PICT1, we generated Pict1-deficient mice and embryonic stem (ES) cells. Pict1 is a nucleolar protein essential for embryogenesis and ES cell survival. Even without DNA damage, Pict1 loss led to p53-dependent arrest of cell cycle phase G1 and apoptosis. Pict1-deficient cells accumulated p53, owing to impaired Mdm2 function. Pict1 binds Rpl11, and Rpl11 is released from nucleoli in the absence of Pict1. In Pict1-deficient cells, increased binding of Rpl11 to Mdm2 blocks Mdm2-mediated ubiquitination of p53. In human cancer, individuals whose tumors express less PICT1 have better prognoses. When PICT1 is depleted in tumor cells with intact P53 signaling, the cells grow more slowly and accumulate P53. Thus, PICT1 is a potent regulator of the MDM2-P53 pathway and promotes tumor progression by retaining RPL11 in the nucleolu

C/Cフクゴウザイリョウ ノ チョウコウオンカ ニオケル キョウドジンセイ

Mechanical properties of carbon fiber reinforced carbon (C/C) composites, fabricated by the preformed yarn method, were studied at elevated temperature up to 2300℃ in vacuum. Various mechanical tests including tensile, flexural, interlamina shear and fracture toughness tests were carried out to evaluate temperature dependence of strength, Young\u27s modulus and fracture toughness for three kinds of laminate C/C composites with uni-directional, 0°/90° and O°/90°/±45° fiber orientations. The failure processes of C/C composites were examined mainly focused on the influence of "transverse-cracks", which bad introduced during processing

Preparation and Properties of Dental Composite Resin Cured under Near Infrared Irradiation

A composite resin (CR) with photo-polymerization is widely used for dental filling material. Current CR has a restriction on the photo-polymerization depth because of the scattering of irradiated blue visible (VIS) light. Rare earth doped Y2O3 particles (Y2O3:RE) are known to emit blue VIS light under near infrared (NIR) excitation by an upconversion process. The particles can act as both filler to reinforce and illuminator to cure the resin by emitting blue VIS light under the irradiation of the NIR light, which propagates more deeply due to its longer wavelength. In this study, CR with Y2O3:RE was prepared. The Young's modulus of CR with Y2O3 was comparable with that of commercial CR, however, the hardness was lower than that of commercial CR. CR with Y2O3:RE was successfully cured with NIR irradiation. Then, the feasibility of Y2O3:RE as the NIR polymerization initiator using its upconversion emission for CR was suggested