Enver Koray Akdoğan

Nonlinear dielectric response of (Ba _0.60 Sr _0.40 )TiO ₃ epitaxial films on and -oriented NdGaO ₃ substrates were investigated as a function of film thickness. The second, fourth and sixth order permittivities were determined at 10 GHz and at room temperature from which the so-called critical field for maximum tunability was computed using a thermodynamic formalism recently developed by the authors along the principal residual misfit strain directions. It is shown that the critical field is anisotropic in the plane of the film and its magnitudes are governed by the film thickness.

The effects of isotropic and anisotropic epitaxy on the nonlinear dielectric response in (Ba_0.6,Sr_0.4TiO₃) thin films were investigated experimentally. Under isotropic epitaxy, it is shown that the misfit is detrimental to the non-linear response of the films with decreasing film thickness. Nonlinear dielectric properties of films with in-plane compression exhibit more pronounced strain dependence, while the same does not set-in in film under tension down to 200 nm in thickness. In the case of anisotropic epitaxy, tunability exhibits directional dependence in the plane of the film and possesses orthorhombic symmetry in accordance to the Curie principle. The nonlinear dielectric response was found to be greater along the compressed directions.

MicroPen/spl trade/ direct-write technique has been used to fabricate polycrystalline paraelectric BST 60/40 11-140 /spl mu/m thick films on alumina substrates. The dielectric properties of the thick films were studied as a function of film thickness, microstructure, and the state of residual stresses. Sin/sup 2//spl Psi/ X-ray analyses indicated a compressive state of stress in the films which decreased from /spl sim/40 MPa at 36 /spl mu/m to /spl sim/15 MPa at 140 /spl mu/m. The cubic/spl rarr/tetragonal transition temperature was found to be -10/spl deg/C. No thickness dependence of the transition temperature was observed. Field induced polarization curves also indicate no apparent nonlinear behavior up to 4 kV/mm.

This work investigates the effects of anisotropic epitaxial strains on the microwave frequency dielectric response of BST (60/40) thin films. Dielectric properties such as permittivity and tunability along the <-110>, <-111> and crystallographic directions correlate well with the variation of elastic strain energy as a function of film thickness. We demonstrate unequivocally that the maximum permittivity and tunability is obtained along a crystallographic direction where the strain is a minimum.

Properties of nanosize PbTiO(3) are discussed from the standpoint of the effects of crystallite size on electrostrictive behavior. Experimental results based on crystallite size-dependent primary and secondary order parameters (spontaneous polarization and strain) show that the critical crystallite size (CCS) for the stabilization of the cubic paraelectric phase at 298 K is around 15 nm. The CCS inversely scales with the cubic-tetragonal transition temperature (T(tr)). Electrostrictive coefficients exhibit an order of magnitude increase at 28 nm, which is attributed to the anharmonicity of the perovskite lattice. First order calculations indicate that the increase in the electrostrictive coefficients offsets the decrease in spontaneous polarization and results in an increase in piezoelectric moduli suggesting that piezoelectric activity could, in principle, be observed in the nanosize regime.

Structure-property relations in meaoscopic BaTiO₃ and PbTiO₃ are reviewed from the stand-point of effects of crystallite size on intrinsic behavior using the Landau-Ginzburg-Devonshire formalism. Experimental results based on x-ray diffrdctometry and quantitative calorimetry indicate that the critical crystallite sizes (CCS) for the stabilization of the cubic paraelectric phase in BaTiO₃ and PhTiO₃ are in the vicinity of 67 and 15 nm, respectively. Thc CCS scales inversely with the cubic-tetragonal transition temperature (Ttr). The depression in Ttr in PbTiO₃ is -15 (C with decreasing crystallite size (CS). A decrease in Ttr for BaTiO₃ was not observed. Both systems preserve their first order phase transformation characteristics with reduced CS. Electrostrictive coefficients were determined as a function of CS and were found to exhibit a pronounced increase with decreasing CS. The increase in the elcctrobtrictive coefficients offsets the decrease in spontaneous polarization and results in an increase in piezoelectric moduli suggesting that strong piezoelectric activity could indeed be observed in the mesoscopic size range