Dependence of dielectric and piezoelectric properties on film thickness for highly {100}-oriented lead magnesium niobate
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Fiber textured {100}-oriented lead magnesium niobate–lead titanate (PMN–PT) (70/30) films with thicknesses between 0.35 and 2.1 m were prepared using chemical solution processing. The degree of preferred orientation changed little with increasing thickness. However, the measured dielectric constant, remanent polarization, and piezoelectric coefficients (d31) increased with increasing film thickness. The effective d31 coefficients of highly {100}-oriented PMN–PT films on Pt-coated Si substrates were found to range from −16 to −96 pC/N. Ultraviolet illumination during poling resulted in abnormal aging behaviors and lower overall aging rates for the films. The initial nonlinear aging behavior was attributed to the presence of an internal space-charge field that developed from photoinduced charge carriers. As the space-charge field decays over time, the magnitude of d31 increased until 450–500 min after poling, at which time d31 remained either constant or declined slightly. Thus, the changes in d31 were limited to 1–2%/decade 500–600 min after poling.
I. INTRODUCTION
Lead magnesium niobate–lead titanate (PMN–PT) solid solutions are candidate materials for dielectric, electro-optic, and piezoelectric devices such as multilayer capacitors, optical waveguides, sensors, and actuators.1,2,3 Smolenskii and Agranovskaya4 first reported PMN. This material exhibits relaxor ferroelectric behavior. PMN forms a solid solution with PT with a morphotropic boundary near 35 mol% PT.5 Recently, several processing methods for PMN–PT thin films have been reported, including sputtering,6 chemical vapor deposition,7 pulsed laser deposition,8 screen printing,9 and chemical solution deposition methods.10 Several research groups have reported previously on chemical processing, compositional control, and the related dielectric properties for PMN–PT films. Francis and Payne11 investigated the effect of chemical additives and heat treatment on phase development and microstructure of PMN–PT films. In their study, they found that films derived from solutions prepared by addition of benzoic acid as a chemical additive show uniform and dense microstructures. In addition, rapid heat treatment between successive solution depositions was effective in minimizing pyrochlore formation in the films. Nagakari et al.12 studied the degree of Mg–O–Nb bonding in 2-methoxyethanol for different refluxing times. They found that films prepared from solutions with a long 268
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J. Mater. Res., Vol. 16, No. 1, Jan 2001 Downloaded: 18 Dec 2014
reflux time showed a higher dielectric constant than those prepared from solutions with a short reflux time. The higher dielectric constant was attributed to the large number of strong Mg–O–Nb bonds in the precursor solution. Udayakumar et al.13 reported that a very-high dielectric permittivity of approximately 2900 could be achieved in 0.5-m-thick PMN–PT (65/35) films. It is also known that the dielectric constant, pyroelectric coefficients, and remanent polarization can be affected by preferent
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