Property characterization of PMN-PT thin films prepared at low annealing temperature.
Date Issued
2011
Date
2011
Author(s)
Chao, Chia-Hsien
Abstract
Lead magnesium niobate-lead titanate ((1-x)PMN-xPT) thin films are an important class of ferroelectric materials and high temperature annealing is typically required to produce the perovskite phase in these thin films. Existing studies have suggested that in order to avoid the formation of second phases (e.g., pyrochlore), annealing at more than 700C is commonly needed. The high annealing temperature hinders the integration of ferroelectric/piezoelectric (1-x)PMN-xPT thin films into the production of engineering components. The main purpose of this study is therefore to prepare (1-x)PMN-xPT thin films of various PMN:PT ratios at low annealing temperatures. This is achieved with the aid of a polymer, polyvinylpyrrolidone (PVP).
The (1-x)PMN-xPT thin films were prepared by the sol-gel method. PVP was added to the PMN-PT sol before the sol mixture was spin-coated onto the indium tin oxide (ITO)/glass substrates. A lead lanthanum titanate (Pb0.86La0.14TiO3) seeding layer was deposited onto the ITO/glass substrate first in order to promote the crystallization of PMN-PT. Three different thin film compositions were prepared in this study: 0.9PMN-0.1PT, 0.7PMN-0.3PT and 0.55PMN-0.45PT. The crystalline, microstructure, dielectric and ferroelectric properties of the prepared thin films were then characterized.
The characterization data indicate that with the aid of PVP, the ferroelectric perovskite phase can be successively produced in the 0.9PMN-0.1PT, 0.7PMN-0.3PT and 0.55PMN-0.45PT thin films with a low annealing temperature of 450 C.This is due to the exothermic reaction/decomposition of PVP during annealing, providing the necessary heat for forming the perovskite phase. However, the addition of PVP renders the thin films to exhibit a porous microstructure. The 0.9PMN-0.1PT thin film in particular exhibits a highly porous microstructure characterized by loose lattices of fused small grains. This is believed to be caused by the evaporation of a large quantity of Mg during annealing, causing the increase in interfacial surface energy. This would in turn promote the fast nucleation of small PMN-PT grains at the expense of densification and grain growth. The density of the 0.9PMN-0.1PT thin film can be improved by lengthening the annealing time or by increasing the amount of Mg (i.e., higher than what is required by the stoichiometric ratio) in the original sol mixture.
By lengthening the annealing time, the density and ferroelectric properties of the prepared (1-x)PMN-xPT thin films can be improved. The relative permittivities of the films show a broad peak as a function of temperature; this indicates the relaxor nature of the films. However, the frequency dispersion for the temperature of maximum permittivity (Tmax) is not obvious. This is believed to be caused by the microstructural porosity, weakening the frequency dependence of dielectric behaviors.
Subjects
PMN-PT
Thin film
Relaxor ferroelectrics
Perovskite
Pyrochlore
ITO
PVP
Microstructure
Type
thesis
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