Synthesis of perovskite electrooptic thin films by polymer modified sol-gel processing
Date of Issue2009
School of Materials Science and Engineering
Advanced Materials Research Centre
In the present work, Pb0.91La0.09(Zr0.65,Ti0.35)0.9775O3 (PLZT(9/65/35)) and (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT, x=0.23-0.43) perovskite electrooptic (EO) thin films have been synthesized by PVP modified sol-gel processing with lead acetate and lead nitrate as lead precursor, respectively. The effects of PVP content and molecular weight on the gel particles towards crystallization and microstructure evolution of the films have been systematically investigated. The mechanisms controlling the perovskite crystallization and microstructure development were discussed. The electrical, optical and electrooptic properties of the as-prepared films have also been studied. Our results have shown that adding PVP can significantly increase the critical thickness of a single PLZT layer and promote the crystallization of perovskite phase. However, rosette-like structures are always observed in our PVP-modified PLZT films. Although the occurrence of the rosette-like structures has been a typical morphological problem in Pb-containing ferroelectric films, its formation mechanism has not been well understood from sol-gel chemistry yet. Our results indicate that the formation of the rosette-like structures is closely related to a premature nucleation of PbO grains induced by the presence of PVP. The coarse PbO grains, compared to the fine PLZT gel particles, can lead to inhomogeneous distribution of Pb in the resultant films. Pb-rich domains accelerate the growth of perovskite crystals and as a result, a porous rosette-like structure is formed. Despite the perovskite-promotion effect of PVPs, the presence of rosette-like structure significantly deteriorates the electrical and optical properties of the PLZT films due to its porous nature. Nevertheless, the effective methods for removing rosette-like structures have been found and the resultant films exhibit very promising electrical, optical and EO properties. With PVP and lead nitrate as precursors, the forming of pyrochlore-free and non-rosette PMN-PT films was also studied. Perovskite phase can directly crystallize from amorphous PMN-PT thin films at the temperature ≤ 430ºC and bypass the metastable pyrochlore phase. Kinetic studies have shown that the perovskite crystallization is a diffusion controlled reaction with activation energy of 167.7kJ/mol, which indicates that the process could follow a columbite-like route. This is further supported by the findings on the formation of special core-shell like gel particles with PVP-Pb2+ as shell layer and Mg-O-Nb-(O-Ti) clusters as core in the PVP modified sol-gel solutions. As the cores with Mg-O-Nb-(O-Ti) tend to form columbite-like phases (i.e. MgNb2O6) during annealing, external Pb ions could diffuse into the cores, resulting in the direct crystallization of perovskite phase. Our method by developing such unique core-shell like gel-particles could enlighten the synthesis of the other materials in which, pyrochlore phase is difficult to be removed, e.g. PZN (Pb(Zn1/3Nb2/3)O3), Pb(Sc1/2Nb1/2)O3 and PNN (Pb(Nb1/3Nb2/3)O3). The developed 0.67PMN-0.33PT films exhibit very interesting dielectric behavior, characterized by a diffusive ε–T response and independence of Tmax on the measured frequency range. The refractive index no and ne of the films are 2.515 and 2.494 respectively, and the quadratic EO coefficient is 1.02x10-17 (m/v)2.
DRNTU::Engineering::Materials::Photonics and optoelectronics materials