Stochastic model of dispersive multi-step polarization switching in ferroelectrics due to spatial electric field distribution

A stochastic model for polarization switching in tetragonal ferroelectric ceramics is introduced, which includes sequential 90{\deg}- and parallel 180{\deg}-switching processes and accounts for the dispersion of characteristic switching times due to a nonuniform spatial distribution of the applied field. It presents merging of the recent multistep stochastic mechanism (MSM) with the earlier nucleation limited switching (NLS) and inhomogeneous field mechanism (IFM) models. The new model provides a much better description of simultaneous polarization and strain responses over a wide time window and a deeper insight into the microscopic switching mechanisms, as is exemplarily shown by comparison with measurements on lead zirconate titanate.Comment: 11 pages, 3 figure

Orienting anisometric pores in ferroelectrics:Piezoelectric property engineering through local electric field distributions

Ferroelectrics are a technologically important class of materials that are used in actuators, sensors, transducers, and memory devices. Introducing porosity into these materials offers a method of tuning functional properties for certain applications, such as piezo- and pyroelectric sensors and energy harvesters. However, the effect of porosity on the polarization switching behavior of ferroelectrics, which is the fundamental physical process determining their functional properties, remains poorly understood. In part, this is due to the complex effects of porous structure on the local electric field distributions within these materials. To this end, freeze-cast porous lead zirconate titanate (PZT) ceramics were fabricated with highly oriented, anisometric pores and an overall porosity of 34 vol.%. Samples were sectioned at different angles relative to the freezing direction, and the effect of pore angle on the switching behavior was tracked by measuring simultaneously the temporal polarization and strain responses of the materials to high-voltage pulses. Finite-element modeling was used to assess the effect of the pore structure on the local electric field distributions within the material, providing insight into the experimental observations. It is shown that increasing the pore angle relative to the applied electric field direction decreases the local electric field, resulting in a reduced domain-wall dynamic and a broadening of the distribution of switching times. Excellent longitudinal piezoelectric (d33 = 630 pm/V) and strain responses (Sbip = 0.25% and Sneg = 0.13%, respectively), comparable to the dense material (d33 = 648 pm/V, Sbip = 0.31%, and Sneg = 0.16%), were found in the PZT with anisometric pores aligned with the poling axis. Orienting the pores perpendicular to the poling axis resulted in the largest reductions in the effective permittivity (εσ33= 200 compared to εσ33= 4100 for the dense PZT at 1 kHz), yielding the highest piezoelectric voltage coefficient (g33 = 216×10−3 Vm/N) and energy-harvesting figure of merit (d33g33 = 73×10−12 m2/N). These results demonstrate that a wide range of application-specific properties can be achieved by careful control of the porous microstructure. This work provides an understanding of the interplay between the local electric field distribution and polarization reversal in porous ferroelectrics, which is an important step towards further improving the properties of this promising class of materials for sensing, energy harvesting, and low-force actuators

Electromechanical properties of Ce-doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 lead-free piezoceramics

Abstract Lead-free piezoceramics based on the (Ba, Ca)(Zr, Ti)O3 (BCZT) system exhibit excellent electromechanical properties for low-temperature actuation applications, but suffer from relatively high processing temperatures. Here we demonstrate an approach for the reduction of the sintering temperature and simultaneous increase of the electromechanical strain response of (Ba, Ca)(Zr, Ti)O3 piezoceramics by aliovalent doping with Ce. The samples were prepared by solid state synthesis and their crystallographic structure, dielectric, ferroelectric, and electromechanical properties were investigated. The highest d*33 value of 1189 pm/V was obtained for the sample with 0.05 mol% Ce, substituted on the A-site of the perovskite lattice. The results indicate a large potential of these materials for off-resonance piezoelectric actuators

Interplay of conventional with inverse electrocaloric response in (Pb,Nb) (Zr,Sn,Ti)O3 antiferroelectric materials

The electrocaloric effect in ferroics is considered a powerful solid-state cooling technology. Its potential is enhanced by correlation to the inverse electrocaloric effect and leads into mechanisms of decreasing or increasing dipolar entropy under applied electric field. Nevertheless, the mechanism underlying the increase of the dipolar entropy with applied electric field remains unclear and controversial. This study investigates the electrocaloric response of the antiferroelectric Pb0.99Nb0.02[(Zr0.58Sn0.43)0.92 Ti0.08]0.98O3 in which the critical electric field is low enough to induce the ferroelectric phase over a broad temperature range. Utilizing temperature- and electric-field-dependent dielectric measurements, direct electrocaloric measurements, and in situ transmission electron microscopy, a crossover from conventional to inverse electrocaloric response is demonstrated. The origin of the inverse electrocaloric effect is rationalized by investigating the field-induced phase transition between antiferroelectric and ferroelectric phases. The disappearance of the latent heat at field-induced transition coincides with the crossover of the electrocaloric effect and demonstrates that the overall electrocaloric response is an interplay of different entropy contributions. This opens new opportunities for highly efficient, environmentally friendly cooling devices based on ferroic materials

An ideal amplitude window against electric fatigue in BaTiO3-based lead-free piezoelectric materials

Electric fatigue has been a vexing issue for Pb(Zr,Ti)O3 ceramics, the material-of-choice for piezoelectric technologies, where higher field amplitudes always lead to a more severe property degradation. Thus, piezoelectric devices must be driven under low electric fields to ensure performance reliability, which results in a low efficiency. In the past decade, the intensive worldwide research on lead-free compositions has identified a few ceramics with piezoelectric properties comparable to those of lead-containing ones. However, their resistance to electric fatigue has not been well studied. In this work, we report an abnormal amplitude dependence of electric fatigue in lead-free piezoelectrics: A BaTiO3-based ceramic suffers fatigue degradation when the field amplitude is low, but exhibits an amplitude window at higher fields with essentially no fatigue. Furthermore, electric-field in-situ transmission electron microscopy (TEM) experiments up to 105 cycles are conducted to clearly reveal that the degradation at low fields is due to the unique single-domain state. We, therefore, have identified an ideal amplitude window with performance at full potential and, at the same time, extremely high reliability for a lead-free piezoelectric ceramic that is promising to replace Pb(Zr,Ti)O3

Dynamic scaling properties of multistep polarization response in ferroelectrics

Ferroelectrics are multifunctional smart materials finding applications in sensor technology, micromechanical actuation, digital information storage etc. Their most fundamental property is the ability of polarization switching under applied electric field. In particular, understanding of switching kinetics is essential for digital information storage. In this regard, scaling properties of the temporal polarization response are well-known for 180{\deg}-switching processes in ferroelectrics characterized by a unique field-dependent local switching time. Unexpectedly, these properties were now observed in multiaxial polycrystalline ferroelectrics, exhibiting a number of parallel and sequential non-180{\deg}-switching processes with distinct switching times. This behaviour can be explained by a combination of the multistep stochastic mechanism and the inhomogeneous field mechanism models of polarization reversal. Scaling properties are predicted for polycrystalline ferroelectrics of tetragonal, rhombohedral and orthorhombic symmetries and exemplarily demonstrated by measurements of polarization kinetics in (K,Na)NbO3-based ferroelectric ceramic over a timescale of 7 orders of magnitude. Dynamic scaling properties allow insight into the microscopic switching mechanisms, on the one hand, and into statistical material characteristics, on the other hand, providing thereby the description of temporal polarization with high accuracy. The gained deeper insight into the mechanisms of multistep polarization switching is crucial for future ultrafast and multilevel digital information storage.Comment: 22 pages, 3 figure

Multi-step stochastic mechanism of polarization reversal in rhombohedral ferroelectrics

A stochastic model for the field-driven polarization reversal in rhombohedral ferroelectrics is developed, providing a description of their temporal electromechanical response. Application of the model to simultaneous measurements of polarization and strain kinetics in a rhombohedral Pb(Zr,Ti)O3 ceramic over a wide time window allows identification of preferable switching paths, fractions of individual switching processes, and their activation fields. Complementary, the phenomenological Landau-Ginzburg-Devenshire theory is used to analyze the impact of external field and stress on switching barriers showing that residual mechanical stress may promote the fast switching.Comment: 33 pages, 9 figure

Requirements for the transfer of lead-free piezoceramics into application

The recent review for the Restriction of Hazardous Substances Directive (RoHS) by the expert committee, appointed by the European Union, stated that the replacement of PZT “… may be scientifically and technologically practical to a certain degree …”, although replacement “… is scientifically and technically still impractical in the majority of applications.” Thus, two decades of sustained research and development may be approaching fruition, at first limited to a minority of applications. Therefore, it is of paramount importance to assess the viability of lead-free piezoceramics over a broad range of application-relevant properties. These are identified and discussed in turn: 1. Cost, 2. Reproducibility, 3. Mechanical and Thermal Properties, 4. Electrical Conductivity, and 5. Lifetime. It is suggested that the worldwide efforts into the development of lead-free piezoceramics now require a broader perspective to bring the work to the next stage of development by supporting implementation into real devices. Guidelines about pertinent research requirements into a wide range of secondary properties, measurement techniques, and salient literature are provided

Synthesis procedure and properties of NiFe2O4 – BaTiO3 composites

NiFe2O4 (NF) powder was prepared by auto combustion method starting from nickel and iron nitrates. After the process of self-ignition, fine precursor powder was thermally treated at 1000 oC for 1h forming the nickel ferrite powder [1]. XRD analysis proved the formation of well crystallized nickel-ferrite cubic spinel structure. Particle size distribution measurements showed the existence of agglomerates. SEM micrographs presented nanometer particles from 100 to 500 nm. DBET calculated from specific surface area was ~ 700 nm and factor of agglomeration of obtained NF powder was ~ 27 %. Cubic barium titanate (BT) powder was prepared by soft chemical method (modified Pechini process). Spherical particles of around 75 nm were obtained in the BT powder [2]. Composites (NF-BT) with the general formula x NiFe2O4 – (1-x) BaTiO3 (x = 0.2, 0.3, 0.5) powders were prepared by mixing previously obtained powders of nickel ferrite and barium titanate in planetary ball mill for 24h. As a milling medium were used tungsten carbide balls and iso-propanol. Powder was pressed and sintered at 1170 oC for 4 h and from X-ray measurements the presence of NF and BT phases was detected. No secondary phases were found. Magnetic measurements of composite materials were carried out and presented in Table 1. Saturation magnetization moment of composite materials decrease with barium titanate amount and the fields at which saturation occur increase with BT content. The coercivity HC (Oe) increases with barium titanate concentration in obtained multiferroic material

Domain morphology of newly designed lead‐free antiferroelectric NaNbO₃‐SrSnO₃ ceramics

Reversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO₃‐modified NaNbO₃ lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark‐field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron‐diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron‐diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃‐modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well‐defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology