Study of improved design and physical properties of 12CaO.7Al2O3 thin films
Abstract
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.
Authors
Feizi, ElnazCollections
- Theses [4223]