Torsten Granzow

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.

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...

Influence of electric fields on the depolarization temperature of Mn-doped (1-x)Bi1/2Na1/2TiO3-xBaTiO3

The transition between induced long-range order and relaxor-like behavior upon heating is investigated in lead-free (1-x)Bi1/2Na1/2(Ti0.995Mn0.005)O3-xBa(Ti0.995Mn0.005)O3 piezoceramics with x = 0.03, 0.06, and 0.09 (BNT-100xBT:Mn). Temperature-dependent permittivity ɛ′(T) and thermally stimulated depolarization currents (TSDC) of poled samples were measured under identical heating conditions to clarify the depolarization mechanism. In both methods, the influence of electric bias fields on the transition temperature was investigated. Fields applied in the poling direction shift the transition to higher temperatures, with corresponding results in ɛ′(T) and TSDC measurements. While the response of transition temperature to external fields displays a similar trend in all investigated compositions, the shape of TSDC is clearly connected with the composition and, hence, the crystal symmetry of the sample. Furthermore, the comparison of ɛ′(T) and TSDC data reveals a systematic shift between transition temperatu...

Effect of tetragonal distortion on ferroelectric domain switching: A case study on La-doped BiFeO3–PbTiO3 ceramics

The ferroelectric and piezoelectric properties of (1−x)BiFeO3–xPbTiO3 (BF-PT) ceramics were investigated as a function of tetragonal distortion. The latter was adjusted by employing La-doping (0–30 at %) while keeping the material near the morphotropic phase boundary by varying x between 0.35 and 0.46. This allows changing the c/a ratio of tetragonal BF-PT in the range from 1.10–1.01 and consequently alters the level of compatibility stresses. It was found that the c/a ratio has a significant influence on domain switching as inferred from electric field induced polarization, strain hysteresis, and Rayleigh measurements. Specifically, a threshold c/a ratio of about 1.045 was identified below which the electric field induced domain mobility increases sharply.

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...

Effect of Nb-donor and Fe-acceptor dopants in (Bi1/2Na1/2)TiO3-BaTiO3-(K0.5Na0.5)NbO3 lead-free piezoceramics

The role of Fe as an acceptor and Nb as a donor in [0.94−x](Bi1/2Na1/2)TiO3–0.06BaTiO3−x (K0.5Na0.5)NbO3 (100xKNN) (x=0.02 and 0.03) lead-free piezoceramics was investigated. X-ray diffraction analyses show that all the profiles are best-fitted with a cubic symmetry where Fe doping tends to induce a lattice expansion, while Nb doping does the opposite. The strain and polarization characteristics are enhanced and suppressed by the acceptor and donor dopants, respectively. The improvement in the electrical properties with acceptor doping is accompanied by the stabilization of a ferroelectric order. Electron paramagnetic resonance spectroscopic analysis suggests that the stabilization of the ferroelectric order by the Fe dopant originates from the formation of (FeTi′–VO••)• defect dipoles.

Electric-field–temperature phase diagram of the ferroelectric relaxor system (1 − x)Bi1/2Na1/2TiO3 − xBaTiO3 doped with manganese

The electric-field–temperature phase diagram for the lead-free relaxor material (1 − x)(Bi1/2Na1/2)TiO3 − xBaTiO3 (x = 0.03, 0.06, and 0.09) doped with 0.5 mol% Mn (BNT-100xBT:Mn) was established. Transition lines between ergodic or nonergodic relaxor states and the field-induced ferroelectric state were determined at constant temperatures with electric-field-dependent measurements of the polarization as well as of the piezoelectric coefficient and permittivity. Near the depolarization temperature T d, the switching between two ferroelectric poling directions occurs in two steps via an intermediate relaxor state. This effect is closely related to the pinching of the ferroelectric hysteresis loop.

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.