Phase transformations and microstructure evolution in sol-gel derived yttrium-aluminum garnet films

Abstract

Diphasic yttrium-aluminum garnet (Y₃Al₅O₁₂, YAG) sols were made by hydrolysis of aluminum and yttrium isopropoxides. The sols were gelled across TEM grids to make films. The films were heat-treated up to 1550 °C for as long as 300 h. Heat-treatments of bulk gel were also done. Microstructure and phase evolution were observed by TEM. Some observations were done in situ in a TEM hot-stage. YAG fraction and grain size, matrix grain size, nuclei/area, and film thickness were measured. Bulk samples were characterized by x-ray, DTA, and TGA. Yttrium-aluminum monoclinic (YAM) and transition alumina appeared at 800 °C. YAG nucleated between 800 °C and 950 °C. Nucleation was weakly correlated with the transient presence of YAlO₃ garnet, and was eventually site-saturated at 0.3/μm³. The change in grain growth rate of the YAM and transition alumina matrix correlated with the change in the growth rate of YAG. Between 850 °C and 1000 °C YAG growth had t^1/2 dependence and 280 kJ/mole activation energy. Below 850 °C nucleation was continuous, and growth had t^0.85 dependence. Above 1000 °C YAG growth had t^1/4 dependence, and the matrix grains coarsened with t^1/4 dependence. Thicker films reacted faster because the nuclei/area and the growth rate after nucleation scaled with thickness. YAG growth was accompanied by formation of 20–100 nm subgrains. In the late stages of matrix grain coarsening there was also some reaction to YAG by a different process. Nucleation and growth kinetics are compared with other systems. Possible mechanisms are discussed.