Takashi Akatsu

A role of charge-transfer complex with iodine in the modification of coal tar pitch

Abstract Iodine introduced into coal tar pitch (CTP) can drastically alter its reology and carbonization behavior. Here, the mechanism of interaction between iodine and CTP has been investigated by using various spectroscopic methods such as 127I-NMR, EPR and FT–IR. It is shown that some iodine molecules infiltrated into the CTP and form charge transfer complexes with the relative large aromatic components of the CTP. Hyperfine sublevel correlation spectroscopy (HYSCORE) revealed the molecular size of cation radicals, which contain more than ten benzene rings, and the location of iodine anion that is incorporated at ca. 0.3 nm from the aromatic cation radicals. The elemental H/C ratio decreased and the viscosity of CTP increased with the density of the charge-transfer complexes, which was also increased by the iodine treatment. These results strongly suggest that dehydrogenative polymerization of CTP occurs during the iodine treatment. The cation radicals in the charge-transfer complexes accelerate the dehydrogenative polymerization and result in a high carbon yield.

Divorced eutectic and interface characteristics in a solidified YAG-spinel composite with spinel-rich composition

Solidified microstructures of YAG (Y3Al5O12)-spinel (MgAl2O4) composite with spinel-rich composition (YAG : spinel = 1 : 30 molar ratio) were investigated. Intergranular YAG divorced eutectic exhibiting fine-grained structures was observed in the solidified composite. Formation of the divorced eutectic was attributed to the metastable growth of primary phase spinel during solidification, which was verified by the presence of in situ spinel precipitates. TEM-SAD technique and crystal structure analysis reveal that a set of specific crystallographic orientation relationships are present between {640} plane of YAG and {111} plane of spinel crystals. The presence of low-energy YAG/spinel interfaces is believed to be responsible for the transgranular fracture characteristics exhibited for the solidified composite.

THE MICROSTRUCTURE AND CREEP DEFORMATION OF HOT-PRESSED SI3N4 WITH DIFFERENT AMOUNTS OF SINTERING ADDITIVES

Compressive creep deformation of hot-pressed silicon nitride with different amounts of grain boundary glassy phase was investigated at 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity. The apparent activation energy of silicon nitride with a larger amount of glassy phase was measured to be about 700 kJ/mole, and that with a smaller amount of glassy phase was found to be 400 kJ/mole. In addition, the microstructural observation found that no cavity appeared and grain boundary glass was recrystallized during creep test. Thus, the rate-limiting steps in solution/precipitation creep mechanism change from the solution-reprecipitation of Si 3 N 4 grains to the diffusion through the grain boundary with increasing the amount of glassy phase.

Anisotropy of creep deformation rate in hot-pressed Si3N4 with preferred orientation of the elongated grains

Compressive creep deformation of hot-pressed silicon nitride with two different preorientations of grain was investigated at temperatures in the range of 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity of the apparent activation energy of creep rate was found to be about 500 kJ mol-1. It means the creep deformation is due to diffusion controlled solution/precipitation. Creep rate of specimen with creep loading direction in parallel to the hot-pressing axis was determined to be higher than that in perpendicular to the hot-pressing axis. In addition, microstructural observation revealed that no cavity appeared and grain boundary glass was recrystallized during creep. X-ray diffraction (XRD) analysis confirms that needle-like Si3N4 grains were reoriented during creep test. These results indicate that the anisotropy of creep rate results from the disparity in the rate of solution–reprecipitation of grains rather than that in diffusion through the grain boundary, which is dependent on the preferred orientation of the needle-like grains.

Linear strain hardening in elastoplastic indentation contact

Finite-element analyses for elastoplastic cone indentations were conducted in which the effect of linear strain hardening on indentation behavior was intensively examined in relation to the influences of the frictional coefficient (μ) at the indenter/material contact interface and of the inclined face angle (β) of the cone indenter. A novel procedure of “graphical superposition” was proposed to determine the representative yield stress Y R . It was confirmed that the concept of Y R applied to elastic-perfectlyplastic solids is sufficient enough for describing the indentation behavior of strainhardening elastoplastic solids. The representative plastic strain of e R (plastic) ≈ 0.22 tan β, at which Y R is prescribed, is applicable to the strain-hardening elastoplastic solids, affording a quantitative relationship of Y R = Y + e; R (plastic) × E P in terms of the strain-hardening modulus E P . The true hardness H as a measure for plasticity is estimated from the Meyer hardness H M and then successfully related to the yield stress Y as H = C (β,μ) × Y for elastic-perfectly-plastic solids and as H = C (β,μ) × Y R for strain-hardening solids, by the use of a β- and μ-dependent constraint factor C (β,μ) ranging from 2.6 to 3.2.

Phase compositions and microstructural characteristics of solidified Al2O3-rich spinel solid solution/YAG composite

Abstract The phase compositions and microstructures of an Al 2 O 3 -rich spinel solid solution (MA ss )–YAG composite were investigated by conventional solidification. A binary eutectic system of YAG–MA ss exhibiting changed eutectic temperature was proven to exist in the YAG–spinel–alumina ternary phase region. A YAG–MA ss eutectic-like composite was obtained by controlled cooling process. It was found that metastable phase, YAlO 3 , was formed during rapid solidification of the YAG–MA ss composite from an annealing temperature of 1795°C. When reheating the as-solidified YAlO 3 –MA ss composite at 1500°C, a YAG–MA ss composite with fine-grained YAG phase was obtained.

Multifunctional porous titanium oxide coating with apatite forming ability and photocatalytic activity on a titanium substrate formed by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was used to make a multifunctional porous titanium oxide (TiO2) coating on a titanium substrate. The key finding of this study is that a highly crystalline TiO2 coating can be made by performing the PEO in an ammonium acetate (CH3COONH4) solution; the PEO coating was formed by alternating between rapid heating by spark discharges and quenching in the solution. The high crystallinity of the TiO2 led to the surface having multiple functions, including apatite forming ability and photocatalytic activity. Hydroxyapatite formed on the PEO coating when it was soaked in simulated body fluid. The good apatite forming ability can be attributed to the high density of hydroxyl groups on the anatase and rutile phases in the coating. The degradation of methylene blue under ultraviolet radiation indicated that the coating had high photocatalytic activity.

The rheological behavior during carbonization of iodine-treated coal tar pitch

Abstract Changes of rheological properties, especially thermal fusibility, during carbonization processes of iodine-treated coal tar pitches have been investigated by TG–Mass, viscometer and proton nuclear magnetic resonance thermal analysis. It was found that iodine could exist in two distinct forms in coal tar pitch (CTP). Introduced iodine molecules, about 30 wt% against the CTP, formed charge-transfer complexes with the CTP molecules. These charge-transfer complexes decompose at temperature above 200°C to form HI and free radicals, which promote cross-linkage and reduce the resolidification temperature of CTP by two hundred degrees. Additional iodine (at levels of greater than 30 wt%) could be eliminated by ethanol and be weakly bound to the CTP. Over extended periods of time at 100°C, latter form of iodine increases the softening temperature and viscosity of the CTP, possibly by acting as an oxidant. The combination of the two effects explains how iodine is so effective as a stabilizer of CTP.

Effect of characteristics of materials on fracture behavior and modeling using graphite-related materials with a high-velocity steel sphere

In order to investigate the impact fracture behavior of graphite-related materials, high-velocity steel spheres accelerated by a two-stage light gas gun were impacted onto three kinds of polycrystalline graphites and a 2D-C/C composite. The polycrystalline graphites have different maximum particle sizes and static dissipation energies. The in situ observation of impact fracture was performed by taking photographs with high-speed framing cameras of the impacted (front) surfaces of those specimens. High pressure caused by shock wave produced a gush of fine fragments in a fluidlike manner immediately after the impact. The apparent volume of fragments was dependent on the maximum particle size, but independent of the thickness of the specimen. The crack patterns of the impacted surface were closely related to the static dissipation energy. The fracture behavior of graphite having the lowest dissipation energy was classified into two cases: with and without fracturing on the rear surface. In the latter case, the time of crack appearance was a function of impact velocity. However, in the former case, both impact velocity and specimen thickness influenced the time of crack appearance.

Evolution of microtexture in furan resin-derived carbon with heat-treatment

Abstract Microtextural evolution in furan resin-derived carbon was investigated. The evolution was mainly studied from the viewpoint of the change in microtexture of surface and internal pores with heat-treatment and was characterized by observation with a scanning electron microscope, measurement of the BET surface area, X-ray diffraction and Raman spectroscopy. Microporous layers, about 60 μm thick, were formed on the surface during carbonization at 1000 °C when furan resin was polymerized in ambient air. The layer partially changed into graphite with a small surface area after heat-treatment at 2600 °C, the inside remaining amorphous. The microporous layer was easily rearranged to form a graphite layer, compared to the inside. The carbons that existed in the internal closed pore during heat-treatment at 1000 °C changed into turbostratic carbon by heat-treatment at 2600 °C. The texture resulted from mass transport through gas phase during the heat-treatment.