SnO2-mixed and Sn-doped TiO2 nanoparticles were synthesized via a hydrothermal process. SnO2-mixed TiO2 nanoparticles prepared in a neutral condition consisted of anatase TiO2 nanoparticles(diamond shape, ~25 nm) and cassiterite SnO2 nanoparticles(spherical shape, ~10 nm). On the other hand, Sn-doped TiO2 nanoparticles obtained under a high acidic condition showed a crystalline phase corresponding to rutile TiO2. As the Sn content increased, the particle shape changed from rod-like(d~40 nm, 1~200 nm) to spherical(18 nm) with a decrease in the particle size. The peak shift in the XRD results and a change of the c-axis lattice parameter with the Sn content demonstrate that the TiO2 in the rutile phase was doped with Sn. The photocatalytic activity of the SnO2-mixed TiO2 nanoparticles dramatically increased and then decreased when the SnO2 content exceeded 4%. The increased photocatalytic activity is mainly attributed to the improved charge separation of the TiO2 nanoparticles with the SnO2. In the case of Sn-doped TiO2 nanoparticles, the photocatalytic activity increased slightly with the Sn content due most likely to the larger energy bandgap caused by Sn-doping and the decrease in the particle size. The SnO2-mixed TiO2 nanoparticles generally exhibited higher photocatalytic activity than the Sn-doped TiO2 nanoparticles. This was caused by the phase difference of TiO2.

Zirconium diboride (ZrB2) and mixed diboride of (Zr0.7Ta0.3)B2 containing 30 vol.% silicon carbide (SiC) composites were prepared by hot-pressing at 1800˚C. XRD analysis identified the high crystalline metal diboride-SiC composites at 1800˚C. The TaB2 addition to ZrB2-SiC showed a slight peak shift to a higher angle of 2-theta of ZrB2, which confirmed the presence of a homogeneous solid solution. Elastic modulus, hardness and fracture toughness were slightly increased by addition of TaB2. A volatility diagram was calculated to understand the oxidation behavior. Oxidation behavior was investigated at 1500˚C under ambient and low oxygen partial pressure (pO2~10-8 Pa). In an ambient environment, the TaB2 addition to the ZrB2-SiC improved the oxidation resistance over entire range of evaluated temperatures by formation of a less porous oxide layer beneath the surface SiO2. Exposure of metal boride-SiC at low pO2 resulted in active oxidation of SiC due to the high vapor pressure of SiO (g), and, as a result, it produced a porous surface layer. The depth variations of the oxidized layer were measured by SEM. In the ZrB2-SiC composite, the thickness of the reaction layer linearly increased as a function of time and showed active oxidation kinetics. The TaB2 addition to the ZrB2-SiC composite showed improved oxidation resistance with slight deviation from the linearity in depth variation.

2-dimensional silica-silica Continuous Fiber-reinforced Ceramic.matrix Composites (CFCCs) were fabricated by a sol-gel infilitration method that has a changing processing condition, such as the repetitions of infilitration. In order to investigate the relationship between the processing condition and the mechanical properties of composites, the mechanical properties of specimens were measured by means of a 4-point flexural strength test while the evidence of strength degradation were microstructurally characterized. There seemed to be a minimum density value that existed at which the delamination between the fabrics would not occur. In the case that the density of silica CFCCs exceeded 1.55 g/cm3, the flexural strength also exceeded approximately 18 MPa at least. By applying the Minimum Solid Area (MSA) analysis of the porous structure, the correlation between the relative density and the mechanical properties of composites will be discussed.