Kazuya Kurokawa

Low temperature oxidation of fully dense and porous MoSi2

Abstract The effects of pore and H 2 O vapor on accelerated oxidation and pesting were observed from fully dense and porous molybdenum disilicide (MoSi 2 ) at 500°C in air and air–H 2 O vapor atmospheres. Occurrence of simultaneous oxidation of Mo and Si, resulting in pesting, was hindered by using fully dense MoSi 2 . In addition, H 2 O vapor has no influence on the oxidation behavior of fully dense MoSi 2 . On the other hand, porous MoSi 2 showed the accelerated oxidation behavior after an induction period. In the induction period, simultaneous oxidation occurred preferentially at pores and gradually spreaded over the surface. After this period, the mass gain increased linearly with a high rate due to generation and development of cracks, resulting in pesting. In porous MoSi 2 , H 2 O vapor in air accelerated the occurrence of pesting. This is probably due to the formations of metastable hydrates, H 2 Si 3 O 7 and H 4 Si 8 O 18 , at pores.

Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H2O form a Cr2O3 scale in the early stage of oxidation. However, the Cr2O3 scale gradually degrades with time. In order to determine the effect of H2O on the deterioration of a Cr2O3 scale, the evaporation behavior of Cr2O3 in N2–O2–H2O atmospheres was investigated. The rate of mass loss in an N2–O2–H2O atmosphere was found to be one order of magnitude higher than the rates in N2–O2 and N2–H2O atmospheres, indicating that deterioration of the Cr2O3 scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr2O3 is probably based on the following reactions: 1/2Cr2O3(s)+3/4O2(g)+H2O(g)=CrO2(OH)2(g). However, it is also speculated that the reaction, Cr2O3(s)+2/3O2(g)=2CrO3(g), affects the evaporation of Cr2O3 at temperatures higher than 1323 K. The evaporation rate of Cr2O3 is roughly comparable to the growth rate of the Cr2O3 scale. Therefore, a Cr2O3 scale can be degraded by the evaporation of Cr2O3.

Classification of Oxidation Behavior of Disilicides

This study focuses on classification of structures of oxide scales formed on disilicides. The oxide scales formed on disilicides can be grouped into 3 types, (a) silica scale, (b) mixed oxide (silica+metal oxide) scale, and (c) double layered (silica/metal oxide) scale. Disilicide that forms an oxide scale of the type (a) generally show excellent oxidation resistance. As such disilicides, there are FeSi 2 , CoSi 2 , MoSi 2 , WSi 2 , etc. In FeSi 2 and CoSi 2 , a protective silica scale must be formed due to the selective oxidation of Si, whereas in MoSi 2 and WSi 2 evaporation of metal oxide plays an important role for the formation of a silica scale. Oxidation of TaSi 2 and NbSi 2 belonged to the type (b), and only CrSi 2 the type (c).

TDS Measurement of Hydrogen Released from Stainless Steel Oxidized in H2O-Containing Atmospheres

Hydrogen dissolved in the Cr2O3 scale formed on the stainless steel in the H2O-containing atmospheres is observed by TDS (thermal desorption spectroscopy) measurements. The amount of dissolved hydrogen in the Cr2O3 scale reaches a maximum about 0.32 mol% when the H2O concentration in the gas reaches 20%. It was found from GDS (glow discharge spectroscopy) measurements that hydrogen may exist at the oxide scale / substrate interface or in Cr2O3 scale bounded that interface. However, results from the Vickers hardness and the observation of scale morphology by SEM (scanning electron microscopy), hydrogen dissolved in the Cr2O3 scale would have little effect on a decrease in the mechanical property of the Cr2O3 scale. Therefore, hydrogen dissolved in the Cr2O3 scale may not be main factor of the deterioration of the Cr2O3 scale.

Observation of heavy elements produced during explosive cold fusion

This paper reports on many-body fusion reactions that may take place during cold fusion. Heavy elements are observed that might have been produced by such reactions during electrolysis of heavy water. Elements such as sodium, magnesium, aluminum, and zinc are observed inside grain-shaped defects in a palladium rod used in a cold fusion experiment.

Measurement of Young’s modulus of oxides at high temperature related to the oxidation study

Abstract α-Al2O3, Cr2O3, and α-Fe2O3 specimens were prepared by a sintering process. A 400 – 1000-Hz sine wave was applied to the specimen at 290 – 1273 K. The applied and respond waves were monitored by using force and acceleration sensors. The intensity ratio and phase shift between the applied and respond waves were analysed, and the anti-resonance frequency was obtained. Young’s moduli of α-Al2O3, Cr2O3, and α-Fe2O3 are estimated to be 386, 286, and 220 GPa at 298 K, respectively. The temperature dependence values of these oxides are estimated to be 54.3, 46.9, and 42.0 MPa K-1, respectively. The temperature dependence of Young’s modulus can be classified on the basis of the crystal structure of solids. The estimation of Young’s modulus at 1273K is possible with an error range of 10 – 30 GPa for a crystalline solid if the crystal structure of the solid is known. It is found that the temperature dependence of Young’s modulus depends on the density of the oxides, and an experiment in which well-characterized crystalline solids are used must be conducted to minimize the error range.

The influence of Si on the microstructure and sintering behavior of ultrafine WC

The microstructure of sintered nanoscale tungsten carbide powders with 1 wt % Si addition was found to be populated by an abnormally large number of elongated grains. Interrupted sintering experiments were conducted to clarify the origins of the excessive abnormal grain growth seen in the microstructure. It was observed that rapid coarsening occurred at high temperatures owing to the formation of a liquid phase. However, the grain shape evolution during this coarsening period was found to be a consequence of excessive stacking faults and micro twins on the basal planes probably generated by reaction of WC with Si. Analyses of the microstructures and the isothermal and non isothermal coarsening behaviors suggested that the platelet morphology evolved by defect-assisted nucleation and growth on faceted grains. Based on experimental evidence from samples interrupted at low temperatures and crystal growth theories, we discuss the possible mechanisms that eventually led to the rampant platelet-type morphology. Further, the influence of such rapid grain growth on the shrinkage rate during sintering is also discussed. In comparison with the cyclic coarsening-densification process of sintering in pure nanoscale WC, the addition of Si leads to only two distinct sintering stages: either densification dominated or coarsening dominated. Concurrent densification and coarsening cannot be sustained particularly in the presence of a liquid phase that significantly enhances coarsening.

Advanced tokamak concepts

AbstractA coin-shaped proton conductor made from metal oxides of strontium and cerium can be charged in a hot D2 gas atmosphere to produce excess heat. Anomalous heat evolution was observed from the proton conductors charged with alternating current at 5 to 45 V at temperatures ranging from 400 to 700°C. The anomalous heat produced temperature increases as much as 50°C. Excess heat was estimated as a few watts in most cases, totaling up to several kilojoules.

Oxidation Behavior of Mo-Si-B In Situ Composites

Isothermal oxidation behavior of Al added Mo-Si-B in-situ composites was investigated under Ar-20%O2 and air atmosphere over the temperature range of 1073–1673 K. The Al added Mo-Si-B composites ((Mo-8.7mol%Si-17.4mol%B)-1mol%Al) were prepared by arc-melting, and homogenized at 2073 K for 24 h in an Ar-flow atmosphere. The ternary Mo-Si-B in-situ composite exhibited a rapid mass loss at the initial oxidation stage and then the passive oxidation after the substrates were sealed with borosilicate glass in the temperature range of 1173–1473 K, whereas it exhibited a rapid mass gain around 1073 K. On the other hand, the Al addition significantly improved the oxidation resistance of Mo-Si-B in-situ composites at temperatures from 1073–1573 K. These excellent oxidation resistances are considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides.

High Temperature Corrosion Behavior of Si-Containing Alloys in the Gas Phase of Air-(Na2SO4+25.7mass%NaCl)

The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi2 alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na2SO4+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi2 alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na2SO4+25.7mass%NaCl) due to the formation of protective Al2O3/SiO2 and SiO2 scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi2-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.