Key Engineering Materials Vols. 317-318

Abstract: The corrosion behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBC's 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.

Abstract: The phase and microstructure of four kinds of plasma sprayed TiO2 coatings (P25, ST, NK, KT) were characterized by XRD (X-ray Diffraction), FE-SEM (Field Emission Scanning Electron Microscope) and TEM ( Transmission Electron Microscopy). Their photocatalytic properties were discussed in relation to phase composition and microstructure. The FE-SEM observation results reveal that there are quite a number of nanoparticles on the surfaces of all four kinds of plasma sprayed TiO2 coatings, which provide photocatalytic reactive sites. Based on the observation results of FE-SEM and TEM, the microstructure schematic of plasma sprayed TiO2 coatings using nanoparticles as feedstock is suggested. The best photocatalytic property of P25 coating among the four plasma sprayed TiO2 coatings is attributed to the combination of its fine “cauliflower” structure and the highest anatase content.

Abstract: Anatase nanocrystals were formed on sol-gel derived SiO2-TiO2 coatings with water vapor or hot water treatments. The anatase nanocrystals were precipitated through hydrolysis of Si-O-Ti bonds, dissolution of SiO2 component, migration of hydrolyzed titania species, and nucleation and growth. The process temperatures can be lowered below 60oC, which should open new applications. It was found that the crystal habit of titania was changes from roundish anatase to sheet-like titania by applying vibration during the hot water treatment. The nanosheets were identified as hydrated titania, TiO2 . mH2O, with layerd structure. The coatings of hydrated titania showed high photocatalytic activities and excellent antifogging properties due to their unique morphology.

Abstract: TiO2 sol with steady suspension and high photocatalytic activity was prepared via a precipitation-peptization method. The XRD results indicate that the TiO2 sol exhibited an anatase structure. TiO2 films on woven fabrics (carbon, PET and Nylon), prepared via dip-coating with TiO2 sol, were characterized by XRD and SEM analysis. The dependence of the woven fabrics properties on the amount of TiO2 coated and photocatalytic activity were also studied. When the diameter of fibers is smalle, large quantity of TiO2 is coated on woven fabrics leading to an improvement in the photocatalytic activity. It is observed that the photocatalytic activity of TiO2 film coated on woven fabrics exceeds that of TiO2 film coated on common glass substrate with the same surface area.

Abstract: Production of reinforced surface ceramic material was investigated by radiation curing-oxidation method. The originality of this research is to use along with polycarbosilane (PCS) an admixture of another preceramic polymer, polyvinylsilane (PVS). PCS, PVS or their combination in cyclohexane solution were spin-coated onto Al2O3 plates or deposited onto porous substrate by dipping method. After radiation oxidation step, used to preserve the coat on the substrate, a heat treatment was applied for crosslinking of the polymer. Thickness and quality of silicon carbide (SiC) film were investigated after pyrolysis.

Abstract: Thermal degradation of indium tin oxide (ITO) thin film has been investigated. ITO thin film was fabricated on glass substrate using RF magnetron sputtering and was characterized. The resistivity of the film which was thermally degraded at high temperatures in air atmosphere was increased highly. Thermally-degraded specimen was analyzed using XPS and Hall measurement to reveal failure mechanism. Result showed that failure mechanism was the decrease in charge carrier concentration and mobility due to oxygen diffusion and chemisorption. Accelerated degradation test (ADT) was performed to predict the lifetime of ITO thin film. The lifetime under normal operating condition could be predicted via statistical analysis and modeling of data acquired from ADT of a short period.

Abstract: The local acceleration effects, which are peculiar phenomena during atomic scale deposition process, were investigated byMolecular Dynamics (MD) simulation. The values of local acceleration were distributed widely for various surface orientations. Deposited atoms were accelerated along the potential energy surface, and accelerated values were evidently dependent on the local configuration of the surface. In contrast, the local acceleration became negligibly small for clusters consisting of many atoms.

Abstract: Amorphous/pseudoamorphous GaN was prepared by pulsed-laser ablation at room temperature without any heat treatment. The structure and chemical composition of the specimens were systematically investigated. Laser ablation at low Ar pressure (<50Pa) led to deposition of smooth Ga-rich films, which is independent with laser energy. Under same pressures, as laser energy increased, the film stoichiometry changed from Ga-rich to near stoichiometric composition. Varying background Ar pressure strongly affected the product structure showing little effect on the chemical composition. Under higher pressure than 100 Pa, fine nanoparticles with a size of 5 nm rather than films were deposited on substrate due to the increased collision by plume confining. The optical band-gap of the deposited a-GaN is 2.8 eV for thin films and 3.9 eV for nanoparticles.

Abstract: The nitrogenated diamond-like carbon films (a-C:H:N) were deposited on Si-wafer by the rf-PECVD method with the addition of nitrogen to the mixture gas of methane and hydrogen. We were investigating the effect of the additive nitrogen gases and annealing in relationship between bonding structure and electrical properties of the deposited films. The electrical conductivity of films increased with the flow rate of nitrogen increasing up to 10 sccm. Also as annealing temperature was increased, the electrical conductivity of films increased. The structure analysis results show that an increase of the flow rate of nitrogen and annealing temperature favor the formation of sp2 bonding in the films. Therefore, we confirmed that the increase of the electrical conductivity is due to structure change by graphitization of the films.

Abstract: Lu2O3 and SiO2 were used as sintering additives and rod-like β-Si3N4 seeds were added to enhance β-Si3N4 grain unidirectional growth. Silicon nitride ceramics were prepared by tape casting and gas pressure sintering
at 1950oC in 10 atm nitrogen atmosphere for 6 h. Compare to the no-seeded Si3N4, the seeded and tape-cast Si3N4 ceramics have obvious anisotropic microstructure and anisotropic properties. When a stress applied along with the grain alignment direction, the bending strength of the seeded and tape-cast Si3N4 at 1500oC was 738 MPa, which was almost the same as its room temperature bending strength. However, the bending strength of the seeded and tape-cast at room temperature was 556 MPa (perpendicular direction); and their thermal conductivity were 67 W/m·K (perpendicular direction) and 83 W/m·K (parallel direction), respectively. The anisotropic properties of the seeded and tape-cast Si3N4 were attributable to the elongated Si3N4 grain alignment.