Influence of nanoparticle seeding on the phase formation kinetics of sol-gel-derived Sr_0.7Bi_2.4Ta₂O₉ thin films

Sr_0.7Bi_2.4Ta₂O₉ (SBT) thin films were deposited on unseeded and SBT nanoparticle (approximately 60-80 nm) seeded Pt/Ti/SiO₂/Si substrates via sol-gel and spin-coating techniques. The SBT thin films were heated at 600 °C for l h to form the fluorite phase, and these fluorite films were further heated at 730-760 °C for fluorite-to-Aurivillius phase transformation. The volume fractions of Aurivillius phase forrnation obtained through quantitative x-ray diffraction analyses showed highly enhanced kinetics in seeded SBT thin films. Johnson-Mehl-Avrami isothermal kinetic analyses were performed for the characterization of Aurivillius phase formation in unseeded and seeded SBT thin films using the volume fraction values. The Avrami exponents were determined as approximately 1.4 and approximately 0.9 for unseeded and seeded SBT films, respectively, which reveals different nucleation modes. By using Arrhenius-type plots, the activation energy values for the phase transformation of unseeded and seeded SBT thin films were determined to be approximately 264 and approximately 168 kJ/mol, respectively. This gives a key reason for the enhanced kinetics in seeded films. Microstructural analyses on unseeded SBT thin films showed formation of randomly oriented needlelike crystals, while those on seeded ones showed formation of domains comprising directionally grown wormlike crystals. On the basis of the phase formation kinetics and microstructural development, a model representing different nucleation and crystal growth mechanisms for the unseeded and seeded SBT thin films was proposed.