Long-range Ferroelectric Phase Research Articles

Comprehensive optimization of piezoelectric coefficient and depolarization temperature in Mn-doped Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-BaTiO3 lead-free piezoceramics

Lead-free Bi0.5Na0.5TiO3 (BNT) piezoelectric ceramics have the advantages of large coercive fields and high Curie temperatures. But the improvement of piezoelectric coefficient (d33) is usually accompanied by a huge sacrifice of depolarization temperature (Td). In this work, a well-balanced performance of d33 and Td is achieved in MnO2-doped 0.79(Bi0.5Na0.5TiO3)-0.14(Bi0.5K0.5TiO3)-0.07BaTiO3 ternary ceramics. The incorporation of 0.25 mol% MnO2 enhances the d33 by more than 40%, while Td remains almost unchanged (i.e., d33=181 pC/N, Td=184 °C). X-ray diffraction (XRD) shows that an appropriate fraction of the small axis-ratio ferroelectric phase (pseudo-cubic, Pc) coexists with the long-range ferroelectric phase (tetragonal, T) under this MnO2 doping. Piezoelectric force microscopy (PFM) has revealed a special domain configuration, namely large striped and layered macro domains embedded with small nanodomains. This study provides a distinctive avenue to design BNT-based piezoelectric ceramics with high piezoelectric performance and temperature stability.

Electric field-induced two-step phase transformation and its contribution to the electromechanical strain in lead-free relaxor-based ceramics

Sodium bismuth titanate-based solid solutions are important lead-free piezoelectrics with potential applications. Large electromechanical strain is of particular interest, which can be realized via the structural change in the relaxor states under an electric field. In this work, a polycrystalline ceramic 0.85Na1/2Bi1/2TiO3–0.15BaZr0.2Ti0.8O3, in which a long-range ordered ferroelectric phase and a relaxor state coexist, has been investigated to unveil the origin of its electromechanical strain using various experimental techniques. An ergodic-relaxor to nonergodic-relaxor transition is first observed under a relatively weak electric field, and a more stable long-range ferroelectric phase is induced under a larger electric field. This two-step phase transformation is accompanied with the process of local polarization freezing as well as ferroelectric domain growth. The domain formation during the reversible phase transition is found to be the main contribution to the macroscopic strain. Our investigation provides an in-depth understanding of the origin of reversible electromechanical strain in the NBT-based relaxor-ferroelectric system.

Large strain and low hysteresis in (1-x)Bi0.5(Na0.75K0.25)0.5TiO3-xSrTiO3 lead-free piezoceramics

(1-x)Bi0.5(Na0.75K0.25)0.5TiO3-xSrTiO3 (BNKT-xST) incipient piezoelectric ceramics with x = 0.05-0.35 were fabricated using solid-state reaction technique. The microstructure and electrical properties are discussed as functions of ST content. The X-ray diffraction patterns confirm that all the compositions form a solid solution without any secondary phase. As the ST content increases, the grain growth is inhibited, and the ferroelectric phase transforms into the relaxor phase, which can be confirmed by the P-E and I-E loops. For the compositions of x = 0.05, 0.30 and 0.35, a large strain of 0.28%, 0.24% and 0.18% with a low hysteresis of 16.2%, 10.0% and 4.7% can be obtained. We propose that the large strain and low hysteresis behavior are beneficial from the disruption of long-range ferroelectric phase and the increase in relaxor phase, which should provide a feasible paradigm to promote the applications of precise micro-actuators.

Thermal-induced structural transition and depolarization behavior in (Bi0.5Na0.5)TiO3-BiAlO3 ceramics

The depolarization temperature Td determines the upper temperature limit for the application of piezoelectric materials. However, the origin of depolarization behavior for Bi-based materials still remains controversial and the mechanism is intricate for different (Bi0.5Na0.5)TiO3-based systems. In this work, the structure and depolarization behavior of (1-x)(Bi0.5Na0.5)TiO3-xBiAlO3 (BNT-BA, x = 0, 0.02, 0.04, 0.06, 0.07) ceramics were investigated using a combination of X-ray diffraction and electrical measurements. It was found that as temperature increased, the induced long-range ferroelectric phase irreversibly transformed to the relaxor phase as evidenced by the temperature-dependent ferroelectric and dielectric properties, which corresponded to a gradual structural change from the rhombohedral to the pseudocubic phase. Therefore, the thermal depolarization behavior of BNT-BA ceramics was proposed to be directly related to the rhombohedral–pseudocubic transition. Furthermore, Td (obtained from thermally stimulated depolarization currents curves) was higher than the induced ferroelectric-relaxor phase transition temperature TFR (measured from dielectric curves). The phenomenon was quite different from other reported BNT-based systems, which may suggest the formation of polar nanoregions (PNRs) within macrodomains prior to the detexturation of short-range ferroelectric domains with PNRs or nanodomains.

Lead-free (Ba,Ca)(Ti,Zr)O3 ceramics within the polymorphic phase region exhibiting large, fatigue-free piezoelectric strains

Lead-free xBaZrO3-(0.85-x)BaTiO3-0.15CaTiO3; x=0.00–0.20 (xBZ) ceramics were prepared successfully using the conventional solid-state reaction method. Unipolar electric-field-induced strains of the composition x=0.125 in the polymorphic phase region (PPR) composition show an extraordinarily high normalized piezoelectric coefficient (d33⁎) of 2244pm/V with relatively low hysteresis at a low electric field of 5kV/cm, which is higher than that of most reported lead-free ceramics. The PPR composition also exhibits excellent fatigue resistance to bipolar electric cycling with negligible loss of electric-field-induced strain after 106cycles. A two-phase mixture model featuring short-range ordering, which is dispersed in the long-range ferroelectric phase, is proposed to explain the outstanding piezoelectric properties. The reversible electric-field-induced phase transition between the two states is responsible for the large normalized piezoelectric coefficient and fatigue resistance. Under repeated electric cycling, the domains become more dynamic, and the change in domain configuration becomes easier due to decreased energy requirement upon polarization reversal. Furthermore, the ceramic shows single crystal-like behavior characterized by a nearly vertical slope in the polarization hysteresis data, which correlates to the electric field induced transformation from a multi-domain state to a single-domain state. This environmentally benign lead-free ceramic, with outstanding properties, has great potential use for highly responsive and reliable actuators.

Enhanced energy storage properties in La(Mg1/2Ti1/2)O3-modified BiFeO3-BaTiO3 lead-free relaxor ferroelectric ceramics within a wide temperature range

A new ternary lead-free (0.67-x)BiFeO3-0.33BaTiO3-xLa(Mg1/2Ti1/2)O3 ferroelectric ceramic exhibited an obvious evolution of dielectric relaxation behavior. A significantly enhanced energy-storage property was observed at room temperature, showing a good energy-storage density of 1.66J/cm3 at 13kV/mm and a relatively high energy-storage efficiency of 82% at x=0.06. This was basically ascribed to the formation of a slim polarization-electric field hysteresis loop, in which a high saturated polarization Pmax and a rather small remnant polarization Pr were simultaneously obtained. Particularly, its energy storage properties were found to depend weakly on frequency (0.2Hz–100Hz), and also to exhibit a good stability against temperature (25°C–180°C). The achievement of these characteristics was attributed to both a rapid response of the electric field induced reversible ergodic relaxor to long-range ferroelectric phase transition and a typical diffuse phase transformation process in the dielectric maxima.

Origin of the large strain response in tenary SrTi0.8Zr0.2O3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free piezoceramics

The perovskite oxides (1 − x)Bi0.5(Na0.9K0.1)0.5TiO3–xSrTi0.8Zr0.2O3 (SZT1000x, x = 0, 0.2, 0.4, 0.6, 0.8, and 1 %) were prepared via the conventional solid-state reaction method. The room temperature ferroelectric P–E loops coordinate with polarization current density J–E curves illustrated the changes of ferroelectric domains and polar nanoregions under different driving fields exhaustively. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44 % (S max/E max = 744 pm/V) at an applied field of 50 kV/cm was obtained at the composition of 0.6 mol% SZT. Temperature-dependent hysteresis measurements reveal the primary origin of the large strain is due to the presence of a nonpolar phase at a zero field. Upon the application of an electric field, the nonpolar phase that can easily transform into a long-range ferroelectric phase, and then brings the system back to its unpoled state once the applied electric field is removed. Notably, the electric field required to deliver large strains is reduced to 40 kV/cm while the S max/E max reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.

Structural, dielectric, ferroelectric and strain properties in CaZrO3-modified Bi(Mg0.5Ti0.5)O3–PbTiO3 solid solutions

The structural, dielectric, ferroelectric, strain properties of (0.89−x)Bi(Mg0.5Ti0.5)O3–xPbTiO3–0.11CaZrO3 ceramics (x=0.45–0.55) were investigated. A morphotropic phase boundary between rhombohedral relaxor and tetragonal normal ferroelectric phases was identified in the composition range of 0.49<x<0.51. Moreover, a phase transition from ergodic to nonergodic relaxor states was noticed in the composition range of 0.45<x<0.51 with increasing the PbTiO3 content, during which a giant electric field induced strain (∼0.32%, d33*∼640pm/V) was observed in the proximity of x=0.48. Such a giant strain is believed to originate from the field induced reversible phase transformation between ergodic relaxor and the long-range ferroelectric phase and this was further confirmed by the measurements of the temperature-dependent ferroelectric polarization/strain loops.

A micro-Brillouin scattering study on relaxor ferroelectric Pb[(Zn 1/3Nb 2/3) 1− xTi x]O 3 single crystals with x = 0.07

The longitudinal acoustic (LA) mode of Pb[(Zn 1/3Nb 2/3) 0.93Ti 0.07]O 3 (PZN-0.07PT) single crystals grown by Bridgman method was investigated over a wide temperature range by using a micro-Brillouin scattering technique. The ferroelectric phase transition of PZN-0.07PT at ∼166 °C was accompanied by a minimum of the Brillouin frequency shift and a sharp maximum of hypersonic damping. These acoustic properties exhibited a thermal hysteresis between cooling and heating processes indicating the first-order nature of the phase transition. The substantial softening of the frequency shift and the increasing damping are typical in ferroelectric relaxors where nano-sized polar clusters appear in the paraelectric phase and interact with acoustic waves via electrostrictive coupling. However, in the present case, acoustic damping consists of a small, sharp peak at the transition temperature ( T c) superposed on the broad increasing damping. The broad damping, which becomes substantially large already in the prototype cubic phase, is due to the fluctuation interactions, while the additional sudden peak at T c may be attributed to the growth and merging of polar nanoregions approaching percolation limit that drives the system into a long-range ferroelectric phase at T c.

An optically uniaxial antiferroelectric smectic phase in asymmetrical bent-core compounds containing a 3-aminophenol central unit

Two new series of asymmetrical bent-core compounds possessing 3-aminophenol and its N-methyl derivative were synthesized. In the obtained compounds, the asymmetry comes from the different linkages between the central unit and its arms and in two cases also from different lateral substituents. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, switching current and electro-optical measurements, X-ray analysis, and second-harmonic generation. All materials possessing 3-aminophenol as a central unit showed liquid crystalline properties, and two of them exhibited the orthogonal smectic phase (SmA family phase) and very stable columnar phases, while the N-methyl analogues exhibited no mesophase. Because of the optically uniaxial nature, double switching current peaks, and SHG activity associated with a distinct threshold electric field, this orthogonal smectic phase is assigned to a novel type, which has a short-range antiferroelectric order with a long-range randomized polar plane (SmAPAR). This is very unique in the sense that distinct threshold behavior emerges at the field-induced phase transition from the short-range antiferroelectric phase to a long-range ferroelectric phase.

Axial Pressure Influence on Dielectric and Ferroelectric Properties of Na 0.5 Bi 0.5 TiO 3 Ceramic

It was shown that axial pressure strongly changes the dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 ceramic. This includes: (i) the shift of rhombohedral-tetragonal phase transition to higher temperature, suppressing the dielectric dispersion, and reduction of thermal hysteresis of the electric permittivity which can be evidence of transition nature changes to second order one, and (ii) the decrease of polarization in the long-range ferroelectric phase and the shift of phase decay temperature to higher temperature. Also the thermodynamic parameters of the rhombohedral-tetragonal phase transition were estimated.

Dielectric properties of relaxor ferroelectric (PbMg1/3Nb2/3O3)1−x(PbTiO3)x single crystals

Successive phase transitions have been observed in relaxor ferroelectric single crystals (PbMg1/3Nb2/3)1−x(PbTiO3)x (PMN-xPT) for x=0.24. 0.31 and 0.33, by using dielectric measurements and polarization-electric field (P-E) hysteresis loops. Phase coexistence that is characterized by broad dielectric maximum and gradual-evolution of remanent polarization, has been observed in these crystals. The field-cooled-zero-field-heated dielectric permittivity shows that the dc bias field induces a long-range ferroelectric phase transition.

Field-cooled-zero-field-heated and zero-field-heated dielectric behaviors of (Pbmg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Temperature and frequency dependent dielectric measurements in the relaxor ferroelectric single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33%PT) are reported for measured fields along the [001] direction. A sharp phase transition (cuḃic paraelectric phase↔tetragonal ferroelectric phase) was observed near 420 K and 425 K for the zero-field-heated and field-cooled-zero-field-heated runs, respectively. The field-cooled-zero-field-heated dielectric permittivities show that the dc bias field induces a long-range ferroelectric phase transition at 357 K. In addition, the dielectric data ε″c(f,T) taken from the field-cooled-zero-field-heated process, imply the phase coexistence of a short-range ordering (which is attributed to the local structural fluctuations between rhombohedral and tetragonal symmetries) and a long-range ordering due to macro-domain pinning in the region of -330-380 K.