In order to clarify the mechanism of rapid sintering including the densification and interparticle binding of ODS alloys during pulsed electric-current sintering (PECS) process, the micro-distribution of temperature in a sintered particle was analyzed by computer simulation based on the Joule's heating model combined with the heat conduction model. The temperature at the neck between sintered particles during PECS was compared with that during direct electric-current sintering (DECS) and hot-pressed sintering (HP). The temperature at the necked region was higher than the average temperature in a particle during PECS and DECS due to the concentration of electric-current at the neck. The temperature difference between the average temperature in a particle and the neck temperature was increased and attained the maximum of 235 K and 132 K for PECS and DECS of MA956 alloy and 225 K and 140 K for PECS and DECS of MA754 alloy, respectively, and then it was decreased with the lapse of heating time. The rise in temperature at the necked region was increased in the order of HP < DECS < PECS even at the same sintering condition. The rapid densification and interparticle binding in PECS of ODS alloys were dominantly attributed to the promotion of plastic-flow in a particle and the decomposition of oxide film at prior particle boundary by the local heat generation at the necked region.
Static grain growth behavior in 1 mol% of GeO2, TiO2, MgO or BaO-doped ZrO2-3 mol%Y2O3 (3Y-TZP) was examined at 1400°C with a special interest in dopant effect on superplastic flow stress in fine-grained 3Y-TZP. The static grain growth can be described as normal grain growth in single-phase ceramics, and the growth constant K is in the order of 10% flow stress for each material. The value of K in cation-doped TZP correlates closely to dopant cation ionic radius. Assuming that the activation energy for diffusivity of constituent ions can be given as a linear function of strain caused by the ionic size difference, the dependence of the growth constant and the flow stress on the ionic radius can be described as a function of the dopant cation ionic radius. The calculated activation energy for the diffusivity is in good agreement with the experimental data in cation-doped TZP. The small dopant effect on the flow stress can be given by the activation energy difference due to the dopant cation ionic size.
Tensile deformation was carried out for a mechanically milled Al-1.1 mol%Mg-1.2 mol%Cu alloy at temperatures of 523-823 K and a true strain rate of 1×100 s-1. The largest uniform elongation prior to local necking occurred at an intermediate temperature (748 K). This temperature dependence of the uniform elongation was analyzed from a strain hardening viewpoint in which the balance of the stored dislocations and the thermal recovery of them is responsible for maintaining a high mobile dislocation density that is exclusively temperature-dependent to sustain a large uniform elongation. It was found that thermal activation process of liberating unlocked immobile dislocations from solute atmosphere is responsible for the temperature dependence of thermal recovery of stored dislocations. The same process applies for the deformation parameters such as the re-mobilization probability of immobile dislocations and the mobile dislocation density that were previously obtained from the analysis of stress-strain behavior on a basis of dislocation dynamics. An activation energy of approximately 32 kJ/mol is equivalent to the vacancy migration energy in aluminum minus the binding energy between a vacancy and a solute atom of magnesium or copper.
In order to evaluate the high-temperature durability of the plasma sprayed thermal barrier coating (TBC) systems in connection with their coating characteristics such as the coating microstructures and interfacial natures depending on the coating processing, high-temperature oxidation test was conducted at 1000 and 1100°C under both the isothermal and thermal cycle conditions for several kinds of TBC systems. Specimens with different coating features were prepared systematically by controlling different coating parameters such as the sorts of the ceramic top-coat powders and the post-spraying heat-treatment conditions. High-temperature oxidation behavior was found to depend strongly on both the sorts of top-coat powder and the heat-treatment conditions after spraying such as a combination of treatment temperature and atmosphere. Then, the top-coat structure with large numbers of micropores and microcracks introduced by hollow spherical powder was proved to have the superior top-coat spalling resistance even in the thermal cycle conditions as compared to those with only a few microdefect by angular powder, because of the thermal stress relaxation effect associated with the preexisting microdefects. It was also clarified that the segmented top-coat made by short spray distance using the angular powder was much effective in suppressing the top-coat spalling. Furthermore, the appropriate reheat-treatment in argon atmosphere delayed the development of the thermally grown oxides (TGO) layer which is formed at top-coat/bond-coat interface. On the contrary, the combination of heat-treatment at higher temperature and in air atmosphere was revealed to promote the development of the heterogeneous TGO layer with many voids and to cause the premature top-coat spalling for TBC system with the dense top-coat by angular powder. From the microstructural analyses of TGO layer using TEM-EDS, it was confirmed that Y-Al-mixed oxide particles existed selectively at Al2O3 grain boundary in TGO layer and Y content in these oxide particles tended to increase as the testing temperature was raised. Affecting factors for high-temperature oxidation property of TBC systems was discussed in connection with the coating parameters for developing the high performance TBC system.
In order to avoid chlorine evolution on the anode in hot seawater electrolysis, the oxygen evolution anodes were tailored. γ-MnO2-type Mn1−x −yMoxFeyO2+x −0.5y anodes consisting of Mn4+, Mo6+ and Fe3+ were prepared by anodic deposition. The anodes with the 100% oxygen evolution efficiency in the electrolysis of 0.5 kmol m-3 NaCl at pH 12, 353 K and a current density of 1000 Am-2 were prepared in solutions consisting of 0.003 kmol m-3 Na2MoO4, 0.1 kmol m-3 Fe(NH4)(SO4)2 and 0.2 kmol m-3 or higher concentrations of MnSO4 at pH 0.33-0.5, 363 K and a current density of 600 Am-2. In the outside of this range, generally because of lower deposition efficiency, the electrode surface was not completely covered with anodically deposited oxides with a consequent lower oxygen evolution efficiency.
Tailoring of active cobalt alloy cathodes for hydrogen evolution in a hot concentrated sodium hydroxide solution was attempted by electrodeposition. Enhancement of cathodic activity of cobalt for electrolytic hydrogen evolution has been carried out by the formation of Co-Fe and Co-Fe-C alloys containing different content of iron. The carbon addition to Co-Fe alloys was made to enhance the electrolytic hydrogen evolution activity and to prevent open circuit corrosion in 8 kmol m-3 NaOH at 363 K. The best condition for electrodeposition of Co-Fe alloy was pH 4 and the iron sulfate concentration of more than 5 kg m-3. Under this condition the alloy was obtained with nanocrystalline bcc phase as a dominant and with the highest hydrogen evolution activity. The carbon addition slowed down preferential iron dissolution by open circuit corrosion in the hot alkaline solution but was not effective in complete prevention of the open circuit corrosion and in enhancing the hydrogen evolution.
Recently, organic polymer dielectric films attract attention as promising dielectric films that reduce environmental burdens and inhibit RC-delays. However, its mechanical properties have not been investigated. Therefore, we evaluated its reliability by measuring the interfacial energy release rates during the delamination under various conditions. In this study, SiLK (trademark of the Dow Chemical Company) is selected as dielectric material. We prepared two types of specimens. One consists of spin-coated SiLK layer sandwiched between Si substrates using adhesion bond. The other is made by the following process; solution of SiLK is sandwiched between Si substrates, and it is cured by heating. Four point bending tests and double cantilever beam tests were performed. In the meantime, delamination paths were identified using SEM and ESCA. In the case of heat curing specimens, delamination propagated along interface between Si/SiLK, so we were able to estimate the interfacial energy release rates. By the results of tests under different mode mixties, we could evaluate the criterion for fracture using elipse approximation.
TiAl was implanted with nitrogen and its effect on cyclic oxidation behavior of TiAl has been investigated in purified oxygen at 1200 K and in static air at 1100 K. The nitrogen implantation was carried out at room temperature, 973 K or 1173 K. The acceleration voltage was 50 kV and the ion dose was 1020 m-2 or 1021 m-2. For the specimen implanted with nitrogen at room temperature with a dose of 1021 m-2, the nitrogen distributed into the depth of about 200 nm with a maximum concentration at about 120 nm, and the concentration changed gradually in the penetration depth. The implantation of nitrogen resulted in the formation of Ti2AlN. The amount of the nitride was larger for the implantation carried out at room temperature than that at 973 K or 1173 K because of the nitrogen diffusion into the matrix during high temperature implantation. The nitrogen implantation deteriorates the oxidation behavior of TiAl in oxygen due to enhanced scale spallation, while the cyclic oxidation behavior of the nitrogen-implanted specimen in air was comparable to that of TiAl without implantation. From the specimen oxidized in oxygen for a very short period, small amount of Ti2N was detected along with TiO2 by X-ray diffraction, indicating that the nitride formed by implantation is not stable during oxidation. The scale formed on the specimen oxidized in oxygen had thick outer layer of TiO2 and stratified inner layer with micro pores. This scale structure seems to be responsible for the lower scale strength. The present results suggest that nitride layer formed by implantation is not continuous thus ineffective for improvement of the oxidation resistance of TiAl.
Si3N4 ceramics with added Fe3O4 and Mo are often used for sliding parts because of the highly lipophilic properties and self-lubricity of these additives. However, flaws are generated by differences in thermal expansion between these additional components and the matrix during sintering. Therefore, materials and processes that improve these mechanical properties are needed. In this study, such ceramics were hot-pressed to enhance their strength, lipophilic properties and abrasion resistance and compared with atmosphere-sintered Si3N4. Micro-lipophilic properties were also considered. The bending strength, relative density, hardness and fracture toughness were all improved by hot-pressing Si3N4 ceramics containing Fe3O4 and Mo. Bending strength was increased by about 430 MPa, and density was increased by over 4 percent. Macro and micro-lipophilic properties were enhanced by the presence of Fe3O4, MoO3 and Si2N2O on the material's surface. Abrasion resistance was enhanced by a falling of grains and the presence of self-lubricating MoO3 on the material's surface.
n-type Bi2Te2.85Se0.15 sintered materials were prepared by hot pressing alone and combination of pressureless sintering and hot pressing, and the effect of pressureless sintering on the microstructures and thermoelectric properties of the sintered materials were studied. Samples hot pressing after pressureless sintering showed greater power factors in comparison with samples prepared by hot pressing alone and pressureless sintering after hot pressing. This is considered to be attributed to increased mobility caused by increased in grain size and anisotropic texture due to pressureless sintering. Thermal conductivity also showed a little higher value due to an increase in both carrier and lattice thermal conductivities. These thermoelectric properties did not exhibit a dependency on the pressureless sintering period. Consequently, the samples prepared by hot pressing after pressureless sintering showed the highest figure of merit of Z =2.89×10-3K-1 for a pressureless sintering period of 7.2 ks.
Three-stage blazed plate fin core model of ultra fine off-set fin, -(thickness × height × pitch × off-set pitch = 0.2 mm × 1.2 mm × 1.6 mm × 5 mm) blazed by Ni blazing material of the recuperator for 600 MW High Temperature Gas Cooled Reactor Gas Turbine (GTHTR300) system was fabricated and tested on its high temperature mechanical properties. The following results were derived. (1) High temperature static, fatigue and creep mode of fracture mainly occurred at ultra fine off-set fin made of SUS304. (2) Tested model showed almost the same high temperature strength, fatigue and creep behaviors of SUS304 as a main structural material at elevated temperatures up to 873 K, (3) Recuperator designed for the GTHTR300 could be testified on the database of SUS304 base material and (4) The recuperator with the same fin plate structure could be practically applied to the GTHTR300.