Study of improved design and physical properties of 12CaO.7Al2O3 thin films
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Calcium aluminate compound, 12CaO.7Al2O3, was prepared via an improved sol-gel technique in the form of thin film on magnesium oxide (MgO) single crystal substrate as well as powder. The microstructures of the films were observed before and after crystallization, and the effect of solution processing parameters, including the molar fractions of the ingredients, on the continuity of the films and the formation of surface defects was studied. An optimized sol-gel process using a new solution recipe was developed based on the microstructural observations. Homogeneous thin films of 12CaO.7Al2O3 with high critical thickness (~ 5 − 6 μm)were produced using this optimized technique. The chemical composition of the films was determined using energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Raman and Fourier transform infrared (FTIR) spectral analyses were employed in order to investigate the effect of heat treatment temperature on the crystallization of 12CaO.7Al2O3 film on magnesium oxide substrate. The results of the phase analysis show that a single-phase film of 12CaO.7Al2O3 is formed at a temperature of 1300 oC. A crystallized structure with well-defined grain boundaries is obtained after 2 hr of heat treatment at this temperature under normal air atmosphere. The phase formation of 12CaO.7Al2O3 in powder form was investigated via room-temperature and high-temperature X-ray diffraction (XRD) and crystallization of 12CaO.7Al2O3 and CaO.Al2O3 powders started taking place simultaneously at a temperature of ~ 900 oC. A comparison between the FTIR results of the films with XRD results of the powder proved the crystallization of 12CaO.7Al2O3 thin film to start at a higher temperature compared to the powder. Furthermore, a single-phase 12CaO.7Al2O3 tends to form in thin film on MgO substrate, whereas the formation of 12CaO.7Al2O3 is accompanied by the formation of secondary phases of CaO.Al2O3 and 3CaO.Al2O3. The optical absorption properties of the 12CaO.7Al2O3 films were investigated at different temperatures from room temperature to 300 oC and the experimental data were analysed in Tauc and Urbach regions. The optical band gap decreased from 4.088 eV at 25 oC to 4.051 eV at 300 oC, while Urbach energy increased from 0.178 eV at 25 oC to 0.257 eV at 300 oC. The relationship between the optical band gap and the Urbach energy at different temperatures showed an almost linear relationship from which the theoretical values of 4.156 and 0.065 eV were evaluated for the band gap energy and Urbach energy of a 12CaO.7Al2O3 crystal with zero structural disorder at 0 K.
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