Chris M. Fancher

Effect of Crystallographic Texture on the Field-Induced-Phase Transformation Behavior of Bi 0.5Na 0.5TiO 3−7BaTiO 3−2K 0.5 Na 0.5NbO 3

The electric-field-induced phase transformation of {001}pc oriented Bi0.5Na 0.5TiO 3−7BaTiO 3−2K 0.5Na 0.5NbO 3 bulk ceramics with a 8.3 multiples of a random distribution fiber texture was investigated using in situ diffraction. Field-dependent diffraction suggests that the high strain is a result of an electric-field-induced pseudo-cubic to tetragonal phase transformation, where the induced tetragonal phase has a strong domain texture. The effect of electric field direction was investigated by applying the electric field parallel and perpendicular to the fiber texture axis. Applied field direction affected the lattice spacing of the induced phase, domain texture, and increased poling field to induce the phase transformation.

Domain evolution in lead-free thin film piezoelectric ceramics

Due to environmental and health concerns lead-free piezoelectric systems are currently being evaluated for use as replacements for lead-based ceramics. (1-x)Na0.5Bi0.5TiO3 – xBaTiO3 (NBT-xBT) is a promising alternative. In order to develop materials with improved performance, it is necessary to understand local structure effects on the piezoelectric response at the grain level. Using piezoresponse force microscopy (PFM), we have studied the domain evolution under locally applied electric fields. Using pulsed laser deposition thin films are deposited on Pt/Ti/SiO2/Si substrates at a temperature of 650 °C in a 150 mTorr O2 environment. In NBT-0.05BT films, domains are clearly visible after the application of a bias field and they relax after a period of time. After the films are exposed to ambient conditions for several weeks, they must be annealed before domains are again visible with PFM. Domain images after poling indicate that the polarization direction is rotating in the plane of the sample.

Crystal structure of Si-doped HfO2

Si-doped HfO2 was prepared by solid state synthesis of the starting oxides. Using Rietveld refinement of high resolution X-ray diffraction patterns, a substitutional limit of Si in HfO2 was determined as less than 9 at. %. A second phase was identified as Cristobalite (SiO2) rather than HfSiO4, the latter of which would be expected from existing SiO2-HfO2 phase diagrams. Crystallographic refinement with increased Si-dopant concentration in monoclinic HfO2 shows that c/b increases, while β decreases. The spontaneous strain, which characterizes the ferroelastic distortion of the unit cell, was calculated and shown to decrease with increasing Si substitution.

Crystal structure of Si-doped HfO[subscript 2]

Si-doped HfO2 was prepared by solid state synthesis of the starting oxides. Using Rietveld refinement of high resolution X-ray diffraction patterns, a substitutional limit of Si in HfO2 was determined as less than 9 at. %. A second phase was identified as Cristobalite (SiO2) rather than HfSiO4, the latter of which would be expected from existing SiO2-HfO2 phase diagrams. Crystallographic refinement with increased Si-dopant concentration in monoclinic HfO2 shows that c/b increases, while β decreases. The spontaneous strain, which characterizes the ferroelastic distortion of the unit cell, was calculated and shown to decrease with increasing Si substitution.

Crystal structure of Si-doped HfO{sub 2}

Si-doped HfO2 was prepared by solid state synthesis of the starting oxides. Using Rietveld refinement of high resolution X-ray diffraction patterns, a substitutional limit of Si in HfO2 was determined as less than 9 at. %. A second phase was identified as Cristobalite (SiO2) rather than HfSiO4, the latter of which would be expected from existing SiO2-HfO2 phase diagrams. Crystallographic refinement with increased Si-dopant concentration in monoclinic HfO2 shows that c/b increases, while β decreases. The spontaneous strain, which characterizes the ferroelastic distortion of the unit cell, was calculated and shown to decrease with increasing Si substitution.

Reconstruction of the field-induced transformation strain in {001} pseudocubic oriented Bi 0.5 Na 0.5 TiO 3 -7BaTiO 3 -2K 0.5 Na 0.5 TiO 3 bulk ceramics

The high strain response of {001} oriented Bi_0.5Na_0.5TiO₃-7BaTiO₃-2K_0.5Na_0.5NbO₃ is ascribed to a pseudocubic to tetragonal electric-field-induced phase transformation. Using in situ electric field dependent X-ray diffraction the macroscopic strain response can be reconstructed from the observed peak shift and peak bifurcation due to phase transformation. A volume averaging of the lattice strain of Bi_0.5Na_0.5TiO₃-7BaTiO₃-2K_0.5Na_0.5NbO₃ estimates a maximum macroscopic strain response of 0.65% compared to 0.48% from macroscopic measurements.

Ferroelastic Domains and Anisotropy in Lead Free Piezoelectrics

Our research investigates the correlations between domain texture and microstructural features, including crystallographic texture in bulk and thin film polycrystalline materials to understand the development of piezoelectric and other anisotropic properties in a number of rapidly evolving lead free piezoelectric materials. We investigate approaches to understanding polarization distributions by starting from polarization measurements. In addition, 2D and 3D microstructural simulations are carried out in all types of ferroelectrics to rationalize and then engineer their equilibrium and kinetic response. This paper discusses recent findings associated with bulk piezoelectricity, phase stability, and ferroelastic and ferroelectric domain motion for materials such as Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) and Bi0.5Na0.5TiO3 (BNT). Conventional and synchrotron-based x-ray diffraction, electron and optical microscopy and piezoelectric characterization techniques are employed to assess texture, both as a function of poling and temperature. The coupling between microstructure and the inherent directional biases fundamental to piezoelectric and ferroelectric performance enable consideration of orientation and anisotropy in systems with unique constraints.