| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 89 === Abstract
Two formula, TB and TBP, were chosen for the preparation of mullite thin films. Basically, a sol contained TEOS (tetraethylorthosilicate) and boehmite colloid was used in this study. The sols were kept in stoichiometric composition 3Al2O3•2SiO2, or contained a crack-limiting agent, polyvinylpyrrolidone (PVP). These gels were dried, then heat-treated up to 1300oC. The properties of these sols were characterized by rheometer, various thermal analysis techniques (DTA TGA & TMA), also by XRD, SEM and TEM.
The gelation results show that the TBP sol with PVP has a shorter gelation time than the TB sol. Most thermal behavior of TB and TBP were similar to each other, but slightly different in burn-out schedule of PVP and in shrinkage during heat treatment. PVP is effective for crack sealing of thin film coatings. The microstructures appear that the mullite grains in the bulky sintered TB and TBP samples show equiaxial with an average size around 2 mm.
Uniform and crack-free mullite films of TBP were spin-coated on silica substrates with the thickness less than 4 mm. Some abnormal mullite grains on the films appeared acicular morphology with a long axis oriented parallel to [001]mullite direction, which should be the case of plate-like grains. The details about the formation of platy mullite grains need further investigation. The major fraction of mullite grains were equiaxial with sizes ranged from 0.5 μm to a few micrometers. The films while treated at 1280oC for 1 hr still contained fine q- and d-Al2O3 particles. The final thickness of the TBP films was determined to be a function of angular velocity (w) and spinning viscosity (m) of spin coating, in an exponential indexes –3 and 3, respectively. Mullite thin film on porous Al2O3 substrate also showed plate-like grains. But unsupported mullite films prepared on plastic tape were not able to maintain its shape and turned into broken pieces.
The preparation of multi-layer mullite/Al2O3 composites was done in company with the study on the wetting and thermal mechanical (TMA) behavior of TB sol on various Al-containing substrates. In order to reduce the wetting angle, hydration in 100ºC deionic water and anodization of thin Al foil using acidic solution were tried. These treatments could form an oxide or hydroxide surfaces which show a better wetting than on pure Al foil. After laminated at least 30 layers of the coated Al2O3 foils, the composites were sintered and showed three stages of shrinkage and two stages of expansions. The mullite layers in the composites showed porous and better bonding with porous anodized foil. Due to thermal expansion mismatch, the layers were easily bending during oxidation of residual metal Al in the foil, and formed cracks in Al2O3 and mullite layers. The cracks also lowered the strength of the composites.
Key words: mullite, sol-gel, spin coating, thin film, ceramic composite.
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