Fundamentals of polymer precursor method for synthesizing silicon carbide based ceramic fibers

Inorganic fibers become major materials in modern industry. In particular, the carbon fiber technology shows remarkable development. Nowadays, carbon fibers with extreme strength and flexibility play an important role in airplane industry as carbon fiber reinforced plastic matrix composites (CFRP). Success of carbon fiber gives a clue that even the materials with no fusibility or with high melting points can be shaped to thin fiber form by using polymers as starting precursors. In a decade of 1970s, Yajima evidenced that thin continuous SiC based fibers can be obtained from a kind of organosilicon polymers. This polymer, PCS, was synthesized by thermal condensation reaction of polydimethylsilane (PDMS). PDMS is intrinsically infusible and not soluble to any kinds of organic solvents because of the symmetric backbone structure. During the thermal condensation, chain scissions and recombination process introduce Si-CH2-Si bridges and Si-H side groups in the polymer structure, which is called Kumada rearrangement process. PCS, thus obtained, is stable at a room temperature in an oxidation or humid atmosphere. The introduced Si-H side groups play an important role in curing process, which is necessary to make spun fiber infusible during the successive pyrolysis process. It should be paid attention that chemical composition of the PCS is not equal to the original PDMS. C/Si and H/Si molar ratios of PCS (1.87 and 4.55, respectively) is apparently lower than those of PDMS. It means that backbone of PCS possesses ladder-like or locally cross-linked structure. Relatively high softening point of PCS also suggests the existence of such rigid structure. Most of “polycarbosilane”, synthesized by synthetic chemistry is viscous liquid at room temperature. It is unfortunate that thermal condensation reaction of PDMS is mainly accompanied by evolution of gaseous silanes with monomethyl or dimethyl chemical groups, while evolution of methane and hydrogen is minor. Because of a large amount of such “silanes” removal, PCS yield from PDMS is known to be limited in the range of 60-65 mass%. Melt spinnability is an essential issue in applying the polymer precursors for ceramic fiber technology. “Spinability” is however, rather a pragmatic term and the assessment is not easy. Evaluation of PCS melt viscosity suggested that the viscosity range of 5-10 Pa·s was appropriate for the fiber spinning. From the temperature dependence, the apparent activation energy of 180 kJ mol-1 was estimated for PCS, and addition of the plasticizer seemed to decrease the activation energy. By adjusting the plasticizer combination, thinner precursor fiber or the fiber with unique - controlled morphology was synthesized The C/Si ratio of PCS exceeds 1.0, and thermal oxidation curing, adopted in early days, introduces a considerable amount of oxygen in the system. Thus, the classic SiC based fibers were generally amorphous with chemical compositions of Si-C-O or Si-C-O-M (M: metal). Development of ceramic-matrix composites (CMC), however, began to require maintenance of the fiber performance even at high temperatures beyond 1600 C. Near stoichiometric and more crystalline SiC fibers were developed in recent years in order to respond such market demands. One is Hi-Nicalon-Type S (NGS Advanced Fibers Co., Ltd), which uses electron beam curing and hydrogen decarboinization for adjusting the fiber chemical composition. Another is Tyranno SA (UBE industries, Ltd.), which becomes stoichiometric SiC via carbothermal reduction process. Existence of Al is believed to be essential for homogeneous crystallization. The technology of these advanced fiber production, however, contains more complex factors than the cases of classic ceramic fibers because the process contains the step of the large volume-weight shrinkage influenced by surrounding an atmosphere. Deeper information about local composition change, bond rearrangements and nano-micro crystallization process will become necessary for precise control of the whole process

Experimental study of cavitating flow influenced by heat transfer from heated hydrofoil

This study experimentally investigated the influence of heat transfer from a heated hydrofoil on cavitating flow to understand the evaporation phenomenon under high-heat-flux and high-speed conditions. A temperature difference was generated between the hydrofoil and mainstream by installing an aluminum nitride heater in a NACA0015 hydrofoil fabricated from copper. A cavitation experiment was performed in a high-temperature water cavitation tunnel at the Institute of Fluid Science, Tohoku University. The effect of heating on cavitating flow was evaluated by changing the mainstream velocity and pressure, namely the cavitation number, at a fixed heater power of 860 W. Results showed that the heat transfer from the hydrofoil affected cavitating flow in terms of the cavity length, cavity aspect, and periodicity. The effect on the cavity length became stronger at a lower velocity owing to a higher hydrofoil temperature. The variation in periodicity implied that the heating effect reduced the unsteadiness of cavitation. A modified cavitation number was proposed by considering the heat transfer from the heated wall. A thermal correction term was derived by considering that the fluid temperature close to the heated hydrofoil was affected by the turbulent convective heat transfer between the mainstream and hydrofoil. The corrected cavitation number can be considered as a parameter that describes the cavity length in the isothermal and heated cases in a unified expression.Comment: Accepted Manuscript in International Journal of Multiphase Flo

Caffeic acid inhibits the formation of 1-hydroxyethyl radical in the reaction mixture of rat liver microsomes with ethanol partly through its metal chelating activity

Effect of caffeic acid on the formation of 1-hydroxyethyl radicals via the microsomal ethanol-oxidizing system pathway was examined. The electron spin resonance spin trapping showed that 1-hydroxyethyl radicals form in the control reaction mixture which contained 0.17 M ethanol, 1 mg protein/ml rat river microsomes, 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone, 5 mM nicotinamide adenine dinucleotide phosphate and 30 mM phosphate buffer (pH 7.4). When the electron spin resonance spectra of the control reaction mixtures with caffeic acid were measured, caffeic acid inhibited the formation of 1-hydroxyethyl radicals in a concentration dependent manner. Gallic acid, dopamine, l-dopa, chlorogenic acid and catechin also inhibited the formation of 1-hydroxyethyl radicals. Above results indicated that the catechol moiety is essential to the inhibitory effect. Caffeic acid seems to chelate of iron ion at the catechol moiety. Indeed, the inhibitory effect by caffeic acid was greatly diminished in the presence of desferrioxamine, a potent iron chelator which removes iron ion in the Fe (III)-caffeic acid complex. Since Fe (III)-desferrioxamine complex is active for the 1-hydroxyethyl radicals formation, caffeic acid inhibits the formation of 1-hydroxyethyl radicals in the reaction mixture partly through its metal chelating activity

Rb-Sr, Sm-Nd ages of the Phalaborwa Carbonatite Complex, South Africa

We analyzed Rb-Sr and Sm-Nd isotopic compositions of whole-rock and minerals from syenite, biotite gneiss xenolith, dolerite dyke, phoscorite, and carbonatite from the Phalaborwa Carbonatite Complex located in northeastern Transvaal, South Africa. Syenite does not give significant Rb-Sr and Sm-Nd whole-rock and mineral isochron ages. Dolerite gives an Rb-Sr whole-rock and mineral isochron age of 2062±74Ma. This age overlaps with the timing of the magmatism of the Phalaborwa Carbonatite Complex. Biotite gneiss and phoscorite do not also give significant Rb-Sr and Sm-Nd isochrons. The Rb-Sr whole-rock and mineral isochron of phoscorite, however, gives an age of 2013±93Ma. The age is clearly the cooling age of this complex. Carbonatites are divided into two groups, having low and high initial Sr isotopic ratios. This coordinates with the result of S.C. Eriksson (Carbonatites, ed. by K. Bell, 1989). In addition, these groups indicate different initial Nd isotopic ratios. These suggest that carbonatite and related rocks were formed by mixing of two source magmas having different Sr and Nd isotopic compositions

Physiological variation among Tricholoma matsutake isolates generated from basidiospores obtained from one basidioma

ArticleMycoscience. 60(2): 102-109. (2019)journal articl

A case of sarcoidosis accompanying squamous cell carcinoma in the mandibular gingiva

A 51-year-oldmanwith a history of gingival cancer two years previously was referred to our hospital for further examination of chest abnormal shadow. Bilateral hilar and mediastinal lymphadenopathy, diffuse small nodular opacities and pleural nodules were observed in chest high resolution CT. Serum angiotensin converting enzyme and lysozyme were elevated. Transbronchial lung biopsy specimens demonstrated non-caseous granuloma. CD4-positive lymphocytes were increased in broncho-alveolar lavage (CD4/CD8 ratio 5.47). The patient was diagnosed as having sarcoidosis. Radiological findings were improved and serum angiotensin converting enzyme level was decreased to within the normal range by corticosteroid therapy (prednisolone 30 mg/day). This is the first report of sarcoidosis accompanying the gingival cancer

Ligation of MHC Class II Induces PKC-Dependent Clathrin-Mediated Endocytosis of MHC Class II

In addition to antigen presentation to CD4(+)T cells, aggregation of cell surface major histocompatibility complex class II (MHC-II) molecules induces signal transduction in antigen presenting cells that regulate cellular functions. We previously reported that crosslinking of MHC-II induced the endocytosis of MHC-II, which was associated with decreased surface expression levels in murine dendritic cells (DCs) and resulted in impaired activation of CD4(+)T cells. However, the downstream signal that induces MHC-II endocytosis remains to be elucidated. In this study, we found that the crosslinking of MHC-II induced intracellular Ca(2+)mobilization, which was necessary for crosslinking-induced MHC-II endocytosis. We also found that these events were suppressed by inhibitors of Syk and phospholipase C (PLC). Treatments with a phorbol ester promoted MHC-II endocytosis, whereas inhibitors of protein kinase C (PKC) suppressed crosslinking-induced endocytosis of MHC-II. These results suggest that PKC could be involved in this process. Furthermore, crosslinking-induced MHC-II endocytosis was suppressed by inhibitors of clathrin-dependent endocytosis. Our results indicate that the crosslinking of MHC-II could stimulate Ca(2+)mobilization and induce the clathrin-dependent endocytosis of MHC-II in murine DCs

Sibling spore isolates of Tricholoma matsutake vary significantly in their ectomycorrhizal colonization abilities on pine hosts in vitro and form multiple intimate associations in single ectomycorrhizal roots

ArticleFungal Ecology. 43: 100874. (2020)journal articl