Emil Aulbach

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.

Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics

The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3−0.06BaTiO3−0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rodel, Appl. Phys. Lett. 91, 112906 (2007) was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93(Bi0.5...

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.

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...

Experimental determination of sintering stresses and sintering viscosities

A loading dilatometer assisted by two high-resolution lasers was applied for the accurate measurement of radial and axial strains during uniaxial load assisted sintering. An improved hot forging technique was employed for the first time for the experimental determination of sintering stress and sintering viscosity by hot forging samples under various loads which were pre-sintered to different densities. The technique of discontinuous hot forging was discussed in detail and the effect of the developing anisotropy and different grain growth were analysed. The sintering stress and uniaxial viscosity were both obtained as functions of density ranging from 65 to 96% and compared with theoretical models. Alumina powder with a grain size of about 150 nm was used in this study.

Effect of uniaxial stress on ferroelectric behavior of (Bi1/2Na1/2)TiO3-based lead-free piezoelectric ceramics

Prior studies have shown that a field-induced ferroelectricity in ceramics with general chemical formula (1−x−y)(Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 and a very low remanent strain can produce very large piezoelectric strains. Here we show that both the longitudinal and transverse strains gradually change with applied electric fields even during the transition from the nonferroelectric to the ferroelectric state, in contrast to known Pb-containing antiferroelectrics. Hence, the volume change and, in turn, the phase transition can be affected using uniaxial compressive stresses, and the effect on ferroelectricity can thus be assessed. It is found that the 0.94(Bi1/2Na1/2)TiO3–0.05BaTiO3–0.01(K0.5Na0.5)NbO3 ceramic (largely ferroelectric), with a rhombohedral R3c symmetry, displays large ferroelectric domains, significant ferroelastic deformation, and large remanent electrical polarizations even at a 250 MPa compressive stress. In comparison, the 0.91(Bi1/2Na1/2)TiO3–0.07BaTiO3–0.02(K0.5Na0.5)NbO3 ceram...

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.

Viscous Poisson's coefficient determined by discontinuous hot forging

A high-resolution laser-assisted loading dilatometer was applied for the precise measurement of radial and axial strain rates under different uniaxial loads, using alumina as a model material. As a continuous application of an external load can lead to an anisotropic microstructure, a discontinuous hot forging technique was utilized to determine the viscous Poissons coefficient. In these studies, samples were presintered to different densities and only then were hot-forging tests performed. The result provides an isotropic viscous Poissons coefficient, which increases smoothly between 0.21 and 0.42 within the accessible density range. Combined with the uniaxial viscosity measured before using the same technique, the hydrostatic sintering stress, bulk viscosity, and shear viscosity as a function of density are now available for solid-state sintering. A comparison of the experimentally obtained results with several theoretical models is included.

Laser-assisted high-resolution loading dilatometer and applications

A new hot forging apparatus has been sucessfully developed and applied for the experimental determination of continuum mechanical sintering parameters and theoretical modeling of thin-film sintering. A precise automatic loading system and high-resolution laser measurement system enable accurate measurement of radial and axial strains of cylindrical specimens under constant or intermittent uniaxial loads of 0-5000 N during high-temperature densification. Preliminary experimental results are demonstrated and the key features of technological significance in the design introduced in detail.