Shunkichi Ueno

Comparison of Water Vapor Corrosion Behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBC's

The corrosion behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBCs were examined at 1500oC in water vapor environment. These oxides were coated on silicon nitride specimens by oxidation-bonded reaction sintering technique. Among Ln2Si2O7 system, though the thermal expansion coefficient of Yb2Si2O7 phase is closer to silicon nitride than that of Lu2Si2O7 phase, the corrosion resistance of silicon nitride with Lu2Si2O7 EBC was higher than that of Yb2Si2O7 EBC sample. In these EBC materials, boundary silica phase was easily corroded by water vapor. Among ASiO4 system, though the corrosion rate of HfSiO4 bulk was larger than that of ZrSiO4 and TiSiO4 phases, the oxidation of the silicon nitride substrate for HfSiO4 coated sample was smaller than that of other two. Many cracks were in ASiO4 EBC layer during the corrosion test. The introduced crack length in HfSiO4 EBC layer is shorter than in ZrSiO4 EBC layer due to the thermal expansion mismatch between EBC materials and the silicon nitride substrate.

Stress analysis in shear/compression tests

Abstract Stress analysis by means of the finite element method was done on glass-fibre-reinforced plastics subjected to combined shear and compressive stresses. Two types of experimental set-up were analysed, that is, parallel and series methods, where the specimens were compressed by tilted jigs that allow the application of combined stresses on the specimens. The Tsai-Hill criterion was employed to judge the failures under the combined stresses. Different failure envelopes were obtained for the two set-ups. In the parallel system, the shear strength increased once with compressive stress and then decreased. In the series system, the shear strength decreased montonically with compressive stress. The difference was brought about by different stress distributions due to different constraint conditions.

Development of EBC for Silicon Nitride

Various coating methods of EBC layer for silicon nitride were discussed. High density EBC layer was successfully coated by different techniques such as sputtering, sol-gel and reaction sintering methods. Water vapor corrosion and recession mechanisms of Lu2Si2O7 which is a potential material for EBC were discussed. The problems in the development of EBC revealed by corrosion tests were summarized. The most important problem addressed here was the corrosion of silica at grain boundary. Due to corrosion of silica at the boundary, formation of porous surface is inevitable, then the silicon nitride substrate gets easily oxidized and/or corroded by water vapor. To resolve this issue, we propose a new EBC material without boundary silica and the corrosion mechanism of this improved EBC material is discussed.

Exposure Test Results of Lu2Si2O7 in Combustion Gas Flow at High Temperature and High Speed

In order to understand recession behavior and the amount of recession of Lu2 Si2 O7 in the combustion gas flow, sintered Lu2 Si2 O7 specimens were manufactured by hot pressing and exposed under various combustion gas flow conditions (T = 1300–1500 °C, P = 0.3 MPa, V = 150 m/s, PH2O = 27–69 kPa, t = 10h). After the exposure tests, etch pits, which are assumed to form due to volatilization of SiO2 in the grain boundary phase, were observed at the surface of specimen. The amount of Lu2 SiO5 phase at the surface of specimen increased with the increase of gas temperature or water vapor partial pressure. A corresponding decrease in the amount of Lu2 Si2 O7 phase was observed. Furthermore, by using the average weight loss rate for exposure times of ten hours, the influence of gas temperature and water vapor partial pressure on weight loss rate was examined, and the amount of recession under gas turbine conditions was calculated.Copyright

Thermally stable high-strength porous alumina

A two-step heating schedule involving pulse electric current sintering, a kind of pressure-assisted vacuum sintering, and a subsequent postsintering in air was used to fabricate sintered porous alumina compacts. During pressure-assisted vacuum sintering, a dense microstructure of the A1 2 0 3 -C system was obtained and in the second stage (i.e., during postsintering in air at different temperatures ranging from 800 to 1300 °C for more than 10 h) carbon particles present in the A1 2 O 3 -C system burned out to form a highly porous A1 2 O 3 compact. In this work, the porosity (30%) was successfully controlled and did not change with the postsintering temperature. The intriguing aspect of this study is that porous alumina compacts are fabricated with high strength and remain stable against the postsintering temperature and extended soaking time. This behavior merits the material fabricated here as a potential porous compact, mechanically withstanding for high-temperature applications.

High Temperature Hydro Corrosion Resistance of Silica Based Oxide Ceramics

Water vapor corrosion behavior of Ln2 Si2 O7 (Ln = Nd, Er, Lu), mullite, CaYb4 Si3 O13 and Al2 O3 were investigated at 1500°C. In Ln2 Si2 O7 phases, Ln = Nd and Er samples were completely dissolved in water vapor environment. CaYb4 Si3 O13 phase underwent decomposition during the corrosion test. Lu2 Si2 O7 and mullite showed excellent water vapor corrosion protection. In the case of mullite, Al2 O3 rich phase was formed on the surface and the corrosion progression was successfully protected. In the case of Lu2 Si2 O7 phase, phase transition occurred and the grain boundaries of surface layer were slightly corroded by the corrosion test.© 2003 ASME

Physical properties and high resolution electron microscope study of barium neodymium manganese tetra oxide [BaNdMnO4]

The condition for the formation of single phase of BaNdMnO4 was studied. ca ratio decreases with oxygen nonstoichiometry because of liberation from the Jahn-Teller effect of trivalent Mn ion. The electrical conductivity shows a semiconducting behavior with the activation energy of 0.2 eV. BaNdMnO4 has paramagnetism and high spin state of Mn+3. The electron diffraction patterns of BaNdMnO4 reveal the existence of superstructure corresponding to 100 and 010 spots in this body centered tetragonal phase, from which a rotation of oxygen octahedra about c-axis is deduced. The high resolution electron microscopy (TEM) shows that thin layered BaNd2Mn2O7 phase is also included in the matrix of BaNdMnO4 phase as a kind of defect structure.

Thermo-mechanical stability of porous alumina: effect of sintering parameters

Abstract Recently, we proposed a two-step heating schedule involving pulse electric current sintering (PECS), a kind of pressure assisted vacuum sintering, and subsequent post-heat treatment in air to fabricate porous alumina support, using commercially available alumina and carbon powders [J. Mater. Res. 18 (2003) 751]. During pressure assisted vacuum sintering, Al2O3–C system of low porosity was obtained and in second stage, i.e. during post-heat treatment in air, carbon particles present in the Al2O3–C system burnt out to form highly porous Al2O3 support. Following our previous brief study, the effects of sintering parameters such as temperature, pressure, and heating rate on the properties of the porous alumina were investigated. The porosity varied between 28 and 38% depending on the sintering parameters. As desired, the pore size distribution did not change with post-heat treatment temperature and hence the mechanical properties as well. It was concurred from this present study that the sintering parameters of PECS greatly influenced pore characteristics and other properties of porous compacts. We admit that the initial composition ratio of powder mixture (Al2O3:C) also plays important role on properties such as porosity, pore size, etc. which is beyond the scope of this present study.

Crystal structure of a layered perovskite, barium europium manganese oxide [BaEu2Mn2O7]

The crystal structure of BaEu2Mn2O7 with a layered perovskite was determined for the powder X-ray diffraction data by Rietveld analysis. The unit cell is orthorhombic with a a0 = 0.5503(1), b0 = 0.5481(3), c0 = 2.030(9) nm amd Z = 4 with the space group Fmmm. The oxygen octahedron in this structure was elongated along c axis and distorted along [110]t direction of fundamental body centered tetragonal cell due to Jahn-Teller effect.

Comparison of Recession Behavior between Lu2Si2O7 and Lu2SiO5 by High Speed Steam Jet at High Temperatures

The recession behavior of Lu2SiO5 and Lu2Si2O7 under high speed steam jet with a velocity of 50 m/s in temperature range between 1300o and 1500oC was compared. Material recession and porous microstructure were observed on the bulk surface for all samples after steam jet tests at the elevated temperatures. The porous structure for Lu2Si2O7 bulk samples become refine with increasing test temperature. On the other hand, the porosity of the porous structure for Lu2SiO5 bulk samples reduced with increasing test temperatures. Phase change was observed for the test of Lu2Si2O7 sample. Lu2SiO5 phase was formed on the Lu2Si2O7 bulk surface in the 1400oC test and Lu2Si2O7 phase was formed again on the surface of the bulk in the test at 1500oC. No phase change was observed in the tests for Lu2SiO5 between 1300o and 1500oC.