Kyle G. Webber

High temperature stress-induced "double loop-like" phase transitions in Bi-based perovskites

Polycrystalline 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 samples were tested under uniaxial mechanical compression at various temperatures in the vicinity of the polar tetragonal to nonpolar tetragonal phase boundary. They are shown to display double loop-like stress-strain behavior, marked by a closed ferroelastic hysteresis loop. Thus, it forms a mechanical analog to the polarization-electric field hysteresis behavior of barium titanate above the Curie temperature. As temperature is increased there is an apparent loss of macroscopically observable ferroelasticity, despite the persistence of tetragonality. Macroscopic experimental results are discussed in conjunction with temperature-dependent and stress-dependent high-energy x-ray diffraction data. This reveals a phase transition below the Curie temperature, marked by a discontinuous change in lattice parameters and octahedral tilting during compressive mechanical loading.

Mechanical constitutive behavior and exceptional blocking force of lead-free BZT-xBCT piezoceramics

The free strain during unipolar electrical activation and the blocking stress are important figures of merit for actuator applications. The lead-free (1 − x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) system has been shown to display exceptional unipolar strain at room temperature, making it very attractive as an electroactive material for large displacement, low frequency actuation systems. In this work, the temperature- and composition-dependent blocking stress is measured with the proportional loading method. It was found that BZT-xBCT outperformed Pb(Zr1− x Ti x )O3 and Bi1/2 Na 1/2TiO3–based ceramics for electric fields up to 2 kV/mm.

Electric-field-temperature phase diagram of Mn-doped Bi0.5(Na0.9K0.1)0.5TiO3 ceramics

An electric field–temperature (E-T) phase diagram for a lead-free 0.5 mol. % Mn-doped Bi(Na0.1K0.9)TiO3 ceramics was investigated. The x-ray diffraction, dielectric and polarization measurements revealed relaxor behavior and were used to characterize the stability regions of the non-ergodic relaxor, ergodic relaxor and electric field induced ferroelectric states. As indicated by the polarization–current density profiles, transformation between two electric fields, induced ferroelectric states with opposite polarization direction arise via a two-step process through an intermediate relaxor state. Interplay between the ferroelectric state conversion and intermediate relaxor state is governed by the dynamics of polarization relaxation. The presented E-T phase diagram revealed the effects of the applied electric field and temperature on stability regions. This is of special interest since the Bi0.5(Na0.1K0.9)0.5TiO3 ceramics were proposed as a potential piezoceramic material.

Domain switching energies: Mechanical versus electrical loading in La-doped bismuth ferrite–lead titanate

The mechanical stress-induced domain switching and energy dissipation in morphotropic phase boundary (1−x)(Bi1 − yLay)FeO3–xPbTiO3 during uniaxial compressive loading have been investigated at three different temperatures. The strain obtained was found to decrease with increasing lanthanum content, although a sharp increase in strain was observed for compositions doped with 7.5 and 10 at. % La. Increased domain switching was found in compositions with decreased tetragonality. This is discussed in terms of the competing influences of the amount of domain switching and the spontaneous strain on the macroscopic behavior under external fields. Comparison of the mechanically and electrically dissipated energy showed significant differences, discussed in terms of the different microscopic interactions of electric field and stress.

High temperature blocking force measurements of soft lead zirconate titanate

Piezoelectric actuators outperform other technological solutions in the area of high-speed, high-force and high-accuracy displacement, but are only able to generate strains of about 0.2%. The load capability is generally quantified in terms of a blocking force, which is the force sustained under electric field at zero displacement. Stress–strain curves in a temperature regime from room temperature up to 150 °C on electrically loaded soft lead zirconate titanate (PZT) are generated to determine the blocking stress. The ensuing nonlinear behaviour is discussed in terms of ferroelectric and ferroelastic switching and contrasted to idealized linear constitutive behaviour as often assumed by manufacturers. The blocking stress is shown to increase with temperature due to an additional stiffening effect as a function of electric field. The actual mechanical work done is found to be larger than in the idealized case where linear constitutive behaviour is assumed.

Application of a Classical Lamination Theory Model to the Design of Piezoelectric Composite Unimorph Actuators

This work presents the design, development, and characterization of unimorphtype laminated piezoelectric actuators. The actuators consist of a piezoelectric lead zirconate titanate (PZT) layer sandwiched between unidirectional Kevlar 49 and epoxy composite layers. Differential thermal expansion during processing places the ceramic plate in a state of residual compression and results in a curved actuator. Modified classical lamination theory (MCLT) (modified to include piezoelectricity) was used to design the actuators. Three layups were fabricated and characterized: [90/PZT/90/0], [902/PZT/90/02], and [903/PZT/90/03]. Results were compared to a commercially available unimorph-type actuator made from layers of metal, adhesive, and piezoelectric material. The classical lamination theory predictions were in good agreement with the measured response of the PZT composite actuators and provide a useful design tool for these actuators.

Phase transformation induced by electric field and mechanical stress in Mn-doped (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 ceramics

Electric-field- and stress-induced phase transformations were investigated in polycrystalline 0.5 mol. % Mn-doped (1−x)(Bi1/2Na1/2)TiO3-x(Bi1/2K1/2)TiO3 (x = 0.1, 0.2). To characterize the effect of electric field and stress on the stability of the ferroelectric and relaxor states, polarization- and current density-electric field curves, as well as the stress-strain response as a function of temperature were characterized. Analogous to the observed electrical behavior, the macroscopic mechanical constitutive behavior showed a closed hysteresis at elevated temperatures, indicating a reversible stress-induced relaxor-to-ferroelectric transformation. The electrical and mechanical measurements were used to construct electric field–temperature and stress-temperature phase diagrams, which show similar characteristics. These data show that a mechanical compressive stress, similarly to an electric field, can induce long-range ferroelectric order in a relaxor ferroelectric.

Tailoring of unipolar strain in lead-free piezoelectrics using the ceramic/ceramic composite approach

The electric-field-induced strain response mechanism in a polycrystalline ceramic/ceramic composite of relaxor and ferroelectric materials has been studied using in situ high-energy x-ray diffraction. The addition of ferroelectric phase material in the relaxor matrix has produced a system where a small volume fraction behaves independently of the bulk under an applied electric field. Inter- and intra-grain models of the strain mechanism in the composite material consistent with the diffraction data have been proposed. The results show that such ceramic/ceramic composite microstructure has the potential for tailoring properties of future piezoelectric materials over a wider range than is possible in uniform compositions.

Critical mechanical and electrical transition behavior of BaTiO3: The observation of mechanical double loop behavior

Strain response of polycrystalline barium titanate (BaTiO3) was investigated under high unipolar electric field (0 to 4 kV/mm) and compressive stress (0 to 400 MPa) in the temperature range from 25 to 160 °C. In the vicinity of the Curie point (TC), nonlinear and hysteretic strain-electric field and strain-stress constitutive behaviors were observed, persisting above TC where they correspond to the well-known electric field induced double loop polarization behavior. Analogous to the electrical double loops, the mechanical (strain-stress) hysteretic behavior above TC is caused by a stress induced phase transition from the paraelectric/paraelastic to ferroelectric/ferroelastic phase; the electro-elastic (strain-electric field) hysteresis is similarly caused by an electric field induced phase transition. The stress and electric field at which transitions occur increase linearly with increasing temperature, exhibiting critical behavior typical for the first order phase transitions. The temperature limit for the induced phase transition extends up to 12 °C over TC. Results are discussed in relation to the Landau-Ginzburg-Devonshire free energy expansion.

Influence of uniaxial stress on the ferroelectric-to-paraelectric phase change in barium titanate

The dielectric behavior near the ferroelectric-to-paraelectric phase transition of 〈001〉C-oriented single crystal and polycrystalline barium titanate (BaTiO3) was investigated as a function of uniaxial compressive stress in the temperature range from 25 to 200 °C. An increase in the Curie point (TC) and decrease in the Curie-Weiss temperature (θ) were observed with increasing stress for both single crystal and polycrystalline samples, resulting in an increase in the first order nature of the phase transition as measured by the temperature difference (TC – θ). With increasing applied stress levels, the permittivity versus temperature curves of polycrystalline samples were found to broaden and flatten near the Curie point, which was not observed for the single crystals. The experimental results were analyzed using a classical 2-4-6 Landau polynomial. The observed increase in the first order nature of the ferroelectric-to-paraelectric phase transition with uniaxial stress was explained by a linear dependence...