Hideki Kita

FIM observation of GP zones in an Al-4%Cu alloy

Abstract GP zones in an aged Al-4%Cu alloy have been examined by a field ion microscope. When the spacing of Cu atoms is large on the observing plane, as, for example, on {024}, {113} or {135} planes, which a GP[1] zone intersects, the images of individual Cu atoms constituting the zone are clearly resolved. A change in image contrast upon successive field evaporation has shown that Cu atoms forming a GP[1] zone are brightly imaged by preferentially ionizing image gas atoms when those Cu atoms protrude from an otherwise smooth plane. When they do not protrude, a dark contrast appears because of the larger electronegativity of Cu. All the GP[1] zones, observed and analyzed with respect to their structures, are identified to consist of a single Cu {200} plane. GP[2] zones with two Cu planes separated by three or four {200} matrix planes are observed.

As-fired strength of sintered silicon nitride ceramics

Abstract An increased machined, bulk strength is seen to lead to an improved as-fired strength. Silicon nitride with an as-fired strength as high as 750 MPa can be obtained, sufficient for most structural applications. Microstructural examinations reveal that the reduction of surface and subsurface pores (or equivalent, may be treated as cracks) both in size and occurrence frequency is the primary reason for the strength increase observed. A linear elastic fracture mechanics model correlates the as-fired and bulk strength properties, and predicts that subsurface flaws located in a reasonably deep site underneath the surface would dominate the flexural strength if their length exceeds 2.3–2.8 times of the length of surface defects. The subsurface flaws very close to the surface are the most damaging and should be avoided by process optimizations.

Enhancement of Seebeck coefficient for SrO(SrTiO3)2 by Sm substitution: Crystal symmetry restoration of distorted TiO6 octahedra

We found that Sm3+ substitution in SrO(SrTiO3)2 is effective in improving the Seebeck coefficient (S). The ∣S∣ value increases notably with temperature, benefiting from an enhancement of the density of states (DOS) effective mass md* from ∼3m0 (300K)to∼7.5m0 (1000K), due to an improvement of the local symmetry of TiO6 octahedra, enhancing the degeneracy in the Ti 3d orbitals, which form the conduction band (CB), and also to an accompanying lattice expansion, which gives rise to a higher DOS at the bottom of the CB and, consequently, a larger md*.

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.

Preparation of a transition metal containing polymethylsilsesquioxane hybrids and silicon oxycarbide ceramics: The fabrication of coating and self-supported films

The simple polymerization of methyltriethoxysilane (MTES) has been achieved by the co-hydrolysis with chemically modified tantalum or niobium alkoxides, at room temperature, within 0.5 h and under a slight catalyst. The addition of the transition metal alkoxides into MTES is found to accelerate the formation of siloxane bonds, which was confirmed by 29Si- nuclear magnetic resonance measurement. The siloxane condensation degree of the obtained sol was found to be approximately 80%. The self-supported crack-free hybrid films could be obtained by using this solution. Crack-free ceramic films of 90 mm diameter and 0.2 mm thickness were also obtained by the pyrolysis of the above-described hybrid films. The solutions could also be used as coating precursors, allowing smooth and homogeneous coating films without cracks or pores to be obtained by the dip coating of sol to the various substrates. The adhesion between the coating films and substrates showed large values even for organic substrates, which was evaluated by a cross cut tape test. Scratch hardness was also high.

Resolution of Cu Atoms in a GP[1] Zone in Al–Cu by FIM and Determination of the GP[1] Structure

By FIM the atomic arrangement of a GP[1] zone in an aged Al–Cu alloy is resolved into isolated spots on the {024} planes. These resolved spots are identified as the images produced by individual Cu atoms of the GP[1] zone protruding on the {024} surfaces. By observing the position and shape formed by the spots corresponding to the Cu atoms during the successive field evaporation, it is concluded that a GP[1] zone consists of a single layer of Cu atoms on the {002} planes.

Effect of Diluents on Post-Reaction Sintering of Silicon Nitride Ceramics

Post-reaction sintering as a technique for the fabrication of Si3N4 ceramics has received much attention as a cost-effective process due to the use of cheap Si powder as a raw material. In this method, the rapid exothermic nitridation of Si results in local melting of Si to cause its agglomeration, which is expected to be a flaw after densification. Therefore, control of the exothermic reaction is needed to improve the reliability of post-reaction sintered Si3N4 ceramics. In this study, Si3N4 ceramics were fabricated by post-reaction sintering with Si3N4 or SiO2 powders in order to control the exothermic reaction. As a result, the microstructure and bending strength of Si3N4 ceramics was changed by adding these additives. In particular, the addition of SiO2 resulted in the high strength of Si3N4 ceramics. Consequently, it was found that Si3N4 and SiO2 particles played the role of diluents, and SiO2 was effective in post-reaction sintering as an oxygen donor.

Effect of joining conditions on microstructure and flexural strength of long silicon nitride pipes fabricated by local heat-joining technique

Abstract The microstructure and flexural strength of long silicon nitride pipes joined by a local heat-joining technique at different mechanical pressures, temperatures, and holding times were examined. Silicon nitride pipes sintered with Y2O3 and Al2O3 additives were used as the parent material, and a powder slurry of a Si3N4–Y2O3–Al2O3–SiO2 mixture was brush-coated on the rough or uneven end faces of the pipes. The flexural strength gradually decreased with decreasing mechanical pressure from 5 to 1.4 MPa at 1600 °C for 1 h. A large number of small voids measuring less than 1 μm were present in the joint layers of the samples joined with mechanical pressures of 1.4 and 3 MPa at 1600 °C for 1 h, although there were very few voids formed at 5 MPa. When the joining temperature was increased to 1700 °C for 1 h, some voids were observed in both the joint layer and the parent silicon nitride, and the flexural strength decreased to 529 MPa. Furthermore, some voids were formed in the parent silicon nitride when the holding time was increased from 1 to 4 h at 1600 °C and 5 MPa, which indicated slight degradation of the strength from 677 to 644 MPa.

Review and Overview of Silicon Nitride and SiAlON, Including their Applications

Silicon nitride (Si 3 N 4 ) and SiAlON ceramics are promising materials for high-temperature and high mechanical stress applications because they offer good thermal shock resistance, high strength retention at elevated temperatures, and excellent erosion resistance as well as corrosion resistance. Recently, they have received much more attention in the field of electronics.

Thermal Conductivity, Strength, and Microstructure of Si-Ti-Al-O-N Porous Ceramics

As a low thermal conductive material for ceramic heat insulated engine, reaction-sintered composite was fabricated from Si and Al2TiO5 powders, and the property and microstructure were investigated. Thermal conductivity showed minimum at the compounding ratio of 35wt% of Al2TiO5, moreover, it was lowered to 2.6W/m. K by oxidizing at 800°. During sintering, Al2TiO5 decomposed and new phases were generated by reaction with N2.