Alain Brice Kounga

Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system

Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na0.5TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO(3)-xK(0.5)Na(0.5)NbO(3) ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O-3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics, 1� xyBi 0.5 Na 0.5 TiO 3 -xBaTiO 3 -yK 0.5 Na 0.5 NbO 3 0.05x 0.07 and 0.01y 0.03, have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as 0.45% at an electric field of 8k V/ mm. Polarization in this composition was not stable in that the piezoelectric coefficient d33 at zero electric field was only about 30 pm/ V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.

Morphotropic phase boundary in (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 lead-free piezoceramics

The electromechanical behavior of (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 (BNT-KNN) lead free piezoelectric ceramics is investigated for 0⩽x⩽0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x≈0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3. II. Temperature dependent properties

The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1−x−y)Bi0.5Na0.5TiO3–xBaTiO3–yK0.5Na0.5NbO3 (0.05⩽x⩽0.07, 0.01⩽y⩽0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150°C, while for 150<T<200°C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature Td is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, Td is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a...

High-temperature poling of ferroelectrics

The poling behavior of a lead-zirconate-titanate piezoelectric ceramic is investigated by measurements of the ferroelectric hysteresis, the longitudinal piezoelectric coefficient, and field-cooling poling experiments. At high temperatures, the decrease in the coercive field facilitates poling at lower electric fields, resulting in higher values of the longitudinal piezoelectric coefficient. However, there exists a threshold field of about 150 V/mm, below which fully poled samples cannot be obtained even when field cooling from temperatures above the transition. Further, a temperature regime below the Curie temperature is observed, where a polarization under field can be measured, but a remanent polarization is not stable. The results are discussed with respect to the phase transition behavior.

Electromechanical poling of piezoelectrics

One of the main obstacles in the development of high-performance piezoelectric materials for advanced devices is reaching sufficient levels of electrical poling by application of electric fields. To overcome this obstacle, we suggest an electromechanical poling method, which makes use of the ferroelastic properties of ferroelectric perovskite structures. It is shown that the application of mechanical stress perpendicular to the electrical poling direction drastically improves the ferro- and piezoelectric properties. The electric field required for poling is decreased by 75%. Electromechanical poling thus can pave the way for the next generation of high-performance piezoelectric materials.

Ferroelectric properties of lead zirconate titanate under radial load

Ferroelectric and ferroelastic properties are closely entwined in piezoelectric perovskites. Research on this topic has been mostly limited to collinear action of electric field and mechanical load. Here, the effect of a radial mechanical load and an axial electric field applied simultaneously to a cylindrical lead zirconate titanate sample is investigated. The dielectric constant after poling under load indicates that domain wall movement is facilitated, while the number of domain walls is reduced. Hysteresis measurements show that the remnant polarization and coercive field depend characteristically on the load, with a clear behavior change at the ferroelastic coercive stress.

Stress assisted electrical poling of ferroelectrics

Poling of piezoelectric ceramics is required to induce piezoelectric behaviour. A new method has been designed to apply a transverse mechanical stress while applying an axial electrical field. The effect of this combined loading method on polarization and piezoelectric coefficient is studied as a function of loading path, electric field and mechanical stress. Due to this treatment, the electrical poling field is strongly reduced. Also, the effect of a transverse mechanical stress on the coercive field during bipolar electrical cycling is considered. The results are discussed in terms of 180deg domain switching and non -180deg domain switching. This method has mainly been applied to PZT with selected results on other materials. The transverse radial mechanical stress on cylinders is applied by using the Poisson effect. This ensures a very homogeneous stress transfer to the piezoceramic.