Regina Monteiro

The thermal decomposition and crystallization behaviour of a sol-gel precursor used for the preparation of c-axis oriented Al-doped ZnO thin films were investigated in the temperature range 20-600°C by TG-DTA, IR spectroscopy and XRD analysis. At low temperature, the formation of ZnO crystallites from the sol precursor, prepared by dissolving Zn(CH 3COO)2·2H2O and AlCl 3·6H2O in 2-methoxyethanol and monoethanol amine, takes place via zinc carbonate hydroxide (sclarite/hydrozincite) and occurs simultaneously with the decomposition of this intermediary compound, which occurs above 150°C. At 200°C, the crystalline structure is well defined in terms of ZnO hexagonal lattice parameters, although an important amount of residual organic compounds and water was not yet removed. Increasing the treatment temperature up to 300, 400 and 600°C leads to a gradual removal of the residual organic compounds and therefore to a small change of the ZnO crystalline structure in terms of lattice parameters and grain size.

The thermoreactivity of a zinc acetate non-alkoxide solution used for the preparation of ZnO-based thin films was investigated in the temperature range 20-600°C by TG-DTA, XRD and SEM data. We found that the formation in air of ZnO crystallites from the sol-gel precursor occurs above 150°C simultaneously with the decomposition of an intermediary compound, most probably carbonate hydroxide (sciante and/or hydrozincite). At 200°C, the crystalline structure is well defined in terms of ZnO hexagonal lattice parameters, although residual organic compounds and water were not yet fully removed and an amorphous phase coexists. A kinetic investigation on the thermal decomposition of sol-gel precursor from DTA data using Kissinger differential equation is also presented. Apparent activation energy values of about. 100 kJ mol-1 corresponding to the non-isothermal decomposition of solid precursors in the temperature range 170-250°C have been found.

The densification during sintering of borosilicate glass matrix composites with 25 vol. % alumina (Al2O3) particles was investigated. The powder compacts, isostatically pressed at 200 MPa, were sintered at a temperature within the range 800-1000° C and maintained at this temperature during various times. The sintering behaviour of the composites was investigated by density measurement and by axial and radial shrinkage measurements. The crystalline phases present in the sintered composites were identified by XRD and the microstructure was analyzed by SEM. For temperatures up to 900°C, the relative density of the composites increased continuously with sintering temperature and sintering time, while for higher temperatures, the density increased rapidly and then slowed down to achieve a nearly constant value after sintering the composites for 30 minutes. The composites exhibited isotropic shrinkage behaviour when sintered at 800°C and 850°C, but at higher temperatures slightly higher axial shrinkage than radial shrinkage was observed. The shrinkage behaviour and microstructural characteristics of the composites indicate that densification during sintering can be attributed to the viscous flow of the borosilicate glass.