Dang Moon Wee

Effects of phosphorus on stress of multi-stacked polysilicon film and single crystalline silicon

In multi-stacked polysilicon films, the stress was examined in terms of dopant distribution and the polysilicon/polysilicon interface at which phosphorus and oxygen atoms were piled up. The phosphorus dopant introduced the compressive stress in the films. The thin oxidized layer formed at the interface was an important factor governing the stress gradient in the multi-stacked film. This interface effect could be minimized using symmetrical stacking of polysilicon films and resulted in a low stress gradient of -0.15 MPa µm-1 for a 5.3 µm thick polysilicon layer. In single crystalline silicon, the phosphorus dopant induced the tensile stress. The lattice dilation coefficient of 4.5 × 10-24 cm3 for phosphorus was measured using a high-resolution x-ray rocking curve. A stress model was introduced on the basis of the lattice dilation theory to calculate the stress and stress gradient induced by the dopant.

Nitrogen-doped TiAl alloys. Part I : Microstructure control

Effects of nitrogen addition on the microstructure control of two-phase TiAl intermetallic compounds were investigated. 1.0 at.% nitrogen addition leads to remarkable grain refinement in the fully lamellar microstructure but had little effect on the duplex microstructure. Only Ti3AlN precipitate was formed in 0.3 at.% nitrogen-doped alloy and both Ti2AlN and Ti3AlN precipitates were formed in 1.0 at.% nitrogen-doped alloy. Since the formation of the gamma phase is retarded by nitrogen addition, long-term heat treatment is needed for the duplex microstructure in the 1.0 at.% nitrogen-doped alloy. Though 0.3 at.% nitrogen-doped alloy had much lower elongation than the mother alloy, 1.0 at.% nitrogen addition had little effect on room temperature elongation and increased room temperature yield strength by two times in the fully lamellar microstructure.

Nitrogen-doped TiAl alloys Part II Plastic deformation behavior

Effects of the nitrogen addition on high temperature tensile properties and creep resistance of the fully lamellar and the duplex Ti-48.5Al-1.5Mo (at.%) alloy were investigated. High temperature yield strength of the nitrogen-doped alloys increased due to solute hardening and precipitation hardening of Ti3AlN. Nitrogen addition led to remarkable improvement of creep resistance in the duplex microstructure as well as in the fully lamellar microstructure. In particular, the primary creep deformation of the 1.0 at.% nitrogen-doped alloy with the duplex microstructure decreased definitely to a similar or superior level of the fully lamellar alloy. We consider that precipitate hardening of p-phase (Ti3AlN) and solute hardening of nitrogen atoms may be responsible for such remarkable creep resistance of the nitrogen-doped alloys.

A novel angular rate sensor employing flexural plate wave

We designed and fabricated the silicon micromachined flexural plate wave actuator based on piezoelectric PbZr_xTi_1-xO₃ (PZT) thin films for use in angular rate sensor. The working principles of the angular rate sensor utilizing the wave were investigated.

Catalytic Activity of Oxidation-reduction Pre-treated Ni3Al for Methane Steam Reforming

The catalytic activity of oxidation-reduction pre-treated Ni3Al powder for methane steam reforming was examined. The oxidation-reduction pre-treatment consisted of two steps: oxidation in air at various temperatures from 973 to 1373 K, and then followed by reduction in H2 at 873 K. It was found that the oxidation-reduction treatments significantly reduced the onset temperature of activity, i.e., improved the activity of Ni3Al powder at low temperatures. The characterization of Ni3Al surface showed that an outer surface layer of fine NiO particles were formed on the surface of Ni3Al after oxidation. These NiO particles were reduced to metallic Ni by the subsequent reduction treatment, resulting in the high activity for methane steam reforming. These results indicate that the Ni3Al can form highly active surface structure with oxidation-reduction treatment, having excellent heat resistance.

Evolution of Surface Morphology in Ni(γ)/Ni3Al(γ´) Two-Phase Foil during Electrochemical Etching

Evolution of surface morphology in Ni(γ)/Ni3Al(γ´) two-phase foil of binary Ni-18 at.%Al was examined during the electrochemically selective etching in the electrolyte of distilled water including 1 wt.% (NH4)2SO4 and 1 wt.% citric acid. In the early stage (0.5 h), only the γ matrix was etched and the outmost γ´ particles were protected by a preexisting surface product. As the γ matrix was etched more, the side surfaces of the outmost γ´ particles and the γ´ particles that were located inside were exposed in the electrolyte. They were dissolved, and had a high density of fine dimples. However, the dissolution rate of the γ´ particles was slower than that of the γ matrix and thus the selective etching was retained in this stage. Finally, at 5h, more γ´ particles were exposed and the flat and smooth surfaces of the outmost γ´ particles were completely eliminated by the dissolution on the side surfaces. From these observations plus the saturation of the current density observed in the electrochemical test, we concluded that the change in the surface morphology was finished at this stage. Thus, the surface became more rough and irregular, which resulted from the original two-phase microstructure and the fine dimple structure by transpassivation.

Relationship between Microstructure and Tensile Strength in the Directionally Solidified (23-27) at. % Al-Ni Alloys

Directional or single crystal technique was applied to enhance the ductility, and two phases of γ (Ni) phase or β (NiAl) phase in γ‘(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50µm/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the γ dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the γ‘ single phase in the eutectic composition of 24.5 at. % Al, and the β dendrite fibers in the γ‘ matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including γ dendrites with γ‘ matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the γ dendritic microstructure changed to the β dendrite in the γ‘ matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle β dendrites in the ductile γ’ matrix.

Equilibrium and metastable eutectics near the Ni3Al composition

Ni3Al has been considerable research area due to its high temperature behavior increasing strength with increasing temperature. A series of directional solidification studies showed that the eutectic occurred between g’/b and the metastable eutectic of g/b forms under slightly different conditions, however, it is not well established whether the eutectic is composed of g/g‘, g’/b, or g/b . In order to understand solidification behavior of the eutectic structure, directional solidification experiments have been carried out with solidification rate near the Ni3Al composition in this study. The effects of the solidification rate and composition on formation of the equilibrium and metastable eutectics have been discussed. The (g’+g) coupled phase was also shown to form with the eutectic at the solid/liquid interface.

Plane Strain Compression of Single Crystalline Ni3Al-Base Intermetallic Compounds

Anisotropic deformation behavior of single crystalline Ni3Al-base intermetallic compounds, including Ni3Al single-phase and Ni/Ni3Al two-phase alloys, was systematically studied by the plane strain compression tests. Plastic flow behavior of single phase Ni3Al is strongly dependent on the initial crystal orientation and the flow stress becomes higher with increasing the numbers of the operative slip planes. In the case of the Ni/Ni3Al two phase alloys, the flow behavior is found to be divided into two stages. Such flow behavior is considered to be closely related to the difference in the deformation behavior between Ni solid solution and Ni3Al precipitates.

Effects of Si and B Additions on the Thermal Stability of Lamellar Microstructure of TiAl Alloys

Thermal stability of TiAl-1.5Mo-Si and TiAl-1.5Mo-B alloys wa s investigated by partial melting in floating zone (FZ) furnace. The lamellar stability of TiAl-1.5Mo alloys, which proved to be thermally unstable, was improved dramatically by Si addition. L amellar stability can be improved by preventing α-phase formation from β-phase due to the effect of shifting phase diagram to the low Al side by Si addition. However, it was found that B additi on in TiAl-1.5Mo alloys had no affirmative effect on lamellar stability. Contrary to Si a ddition, phase diagram cannot be shifted by B addition. Therefore, α-phase formation from β-phase cannot be prevented by B addition. In the result of partial melting, it can be concluded that in order t o have enough lamellar stability as a seed alloy, the elements that can shift phase diagram to low Al side must be added to Ti-Al-Mo alloy. In addition, lamellar orientation control was successfully performed by directional solidification using the Ti-46Al-1.5Mo-1Si alloy as a seed alloy.