Sol-Gel Synthesis of beta-Al2TiO5 Thin Films at low Temperature

beta-Al2TiO5 thin films were obtained by dip-coating silicon substrates in acid-catalyzed precursor sols of aluminum and titanium alkoxides with and without acetyl acetone as a chelating agent and subsequent firing at 700 °C. Nuclear magnetic resonance and mass spectra of the precursor sols demonstrated that synthesis without a chelating agent leads to the most extensive polymerization. X-ray photoelectron spectroscopy depth profiles showed that the composition of the films was homogeneous throughout their thicknesses. The crystallization of the films was studied as a function of the firing time and temperature. The films were amorphous up to 600 °C and crystalline beta-Al2TiO5 was formed at 700 °C. From 800 up to 1200 °C, titania (anatase and rutile) phases were also detected, without decomposition of the metastable beta-Al2TiO5 phase. The films fabricated from the sols without chelating agents showed a lower tendency to decompose into the thermodynamically stable phases rutile titania and R-Al2O3. The formation of beta-Al2TiO5 at low temperatures is attributed to a diffusion-limited crystallization process, which takes place in the conditions of high homogeneity at the molecular level reached in the sol-gel synthesis as shown by spectroscopic analysis.