Porous Ferroelectrics Research Articles

Porous Structure Enhances the Longitudinal Piezoelectric Coefficient and Electromechanical Coupling Coefficient of Lead-Free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3.

The introduction of porosity into ferroelectric ceramics can decrease the effective permittivity, thereby enhancing the open circuit voltage and electrical energy generated by the direct piezoelectric effect. However, the decrease in the longitudinal piezoelectric coefficient (d33) with increasing porosity levels currently limiting the range of pore fractions that can be employed. By introducing aligned lamellar pores into (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3, this paper demonstrates an unusual 22-41% enhancement in the d33 compared to its dense counterpart. This unique combination of high d33 and a low permittivity leads to a significantly improved voltage coefficient (g33), energy harvesting figure of merit (FoM33) and electromechanical coupling coefficient ( ). The underlying mechanism for the improved properties is demonstrated to be a synergy between the low defect concentration and high internal polarizing field within the porous lamellar structure. This work provides insights into the design of porous ferroelectrics for applications related to sensors, energy harvesters, and actuators.

Flexible ferroelectric wearable devices for medical applications.

SummaryWearable electronics are becoming increasingly important for medical applications as they have revolutionized the way physiological parameters are monitored. Ferroelectric materials show spontaneous polarization below the Curie temperature, which changes with electric field, temperature, and mechanical deformation. Therefore, they have been widely used in sensor and actuator applications. In addition, these materials can be used for conversion of human-body energy into electricity for powering wearable electronics. In this paper, we review the recent advances in flexible ferroelectric materials for wearable human energy harvesting and sensing. To meet the performance requirements for medical applications, the most suitable materials and manufacturing techniques are reviewed. The approaches used to enhance performance and achieve long-term sustainability and multi-functionality by integrating other active sensing mechanisms (e.g. triboelectric and piezoresistive effects) are discussed. Data processing and transmission as well as the contribution of wearable piezoelectric devices in early disease detection and monitoring vital signs are reviewed.

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 (${d}_{33}=630\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$) and strain responses (${S}_{\mathrm{bip}}=0.25%$ and ${S}_{\mathrm{neg}}=0.13%$, respectively), comparable to the dense material (${d}_{33}=648\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$, ${S}_{\mathrm{bip}}=0.31%$, and ${S}_{\mathrm{neg}}=0.16%$), 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 (${\ensuremath{\varepsilon}}_{33}^{\ensuremath{\sigma}}=200$ compared to ${\ensuremath{\varepsilon}}_{33}^{\ensuremath{\sigma}}=4100$ for the dense PZT at 1 kHz), yielding the highest piezoelectric voltage coefficient (${g}_{33}=216\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.28em}{0ex}}\mathrm{Vm}/\mathrm{N}$) and energy-harvesting figure of merit (${d}_{33}{g}_{33}=73\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{2}/\mathrm{N}$). 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.

Freeze cast porous barium titanate for enhanced piezoelectric energy harvesting

Energy harvesting is an important developing technology for a new generation of self-powered sensor networks. This paper demonstrates the significant improvement in the piezoelectric energy harvesting performance of barium titanate by forming highly aligned porosity using freeze casting. Firstly, a finite element model demonstrating the effect of pore morphology and angle with respect to poling field on the poling behaviour of porous ferroelectrics was developed. A second model was then developed to understand the influence of microstructure-property relationships on the poling behaviour of porous freeze cast ferroelectric materials and their resultant piezoelectric and energy harvesting properties. To compare with model predictions, porous barium titanate was fabricated using freeze casting to form highly aligned microstructures with excellent longitudinal piezoelectric strain coefficients, d33. The freeze cast barium titanate with 45 vol.% porosity had a d33 = 134.5 pC N−1 compared to d33 = 144.5 pC N−1 for dense barium titanate. The d33 coefficients of the freeze cast materials were also higher than materials with uniformly distributed spherical porosity due to improved poling of the aligned microstructures, as predicted by the models. Both model and experimental data indicated that introducing porosity provides a large reduction in the permittivity () of barium titanate, which leads to a substantial increase in energy harvesting figure of merit, , with a maximum of 3.79 pm2 N−1 for barium titanate with 45 vol.% porosity, compared to only 1.40 pm2 N−1 for dense barium titanate. Dense and porous barium titanate materials were then used to harvest energy from a mechanical excitation by rectification and storage of the piezoelectric charge on a capacitor. The porous barium titanate charged the capacitor to a voltage of 234 mV compared to 96 mV for the dense material, indicating a 2.4-fold increase that was similar to that predicted by the energy harvesting figures of merit.

Effect of the nanopore on ferroelectric domain structures and switching properties

Porous ferroelectrics have some special properties and features for specific promising applications. The nanopores structure have been observed in experiments may have great influence on the ferroelectric domain structures and the switching properties of ferroelectrics. In this study, we proposed a phase field model to predict the domain structures of porous ferroelectrics and their evolution under a switching electric field. The effects of the pores size/shape on domain structure, switching properties, and dielectric/piezoelectric properties are investigated and analyzed. The simulated hysteresis loop is in good agreement with experimental reports. It is shown that the porosity can strongly affect the ferroelectric domain size, and furthermore, will influence the hysteresis loop. Therefore controlling the porosity of ferroelectrics may provide a possible way to adjust the properties of porous ferroelectric materials.

Manufacture and characterization of porous ferroelectrics for piezoelectric energy harvesting applications

ABSTRACTPorous ferroelectric materials have been evaluated for their piezoelectric energy harvesting capabilities. Macro-porous barium titanate (BaTiO3) ceramics were fabricated with a range of porosities using the burned out polymer spheres process. The pore fraction was tailored by mixing a pore forming agent with BaTiO3 powder in varying amounts by weight before cold-pressing and pressureless sintering. Introducing porosity into the ferroelectric significantly increased the energy harvesting figure of merit, with a maximum of 2.85pm2/N obtained at ∼40% relative density compared with ∼1.0 pm2/N for the dense material. The results demonstrate that introducing porosity into a piezoelectric potentially provides an effective route to improving the vibration energy harvesting capability of these materials.

Understanding the peculiarities of the piezoelectric effect in macro-porous BaTiO3

This work demonstrates the potential of porous BaTiO3 for piezoelectric sensor and energy-harvesting applications by manufacture of materials, detailed characterisation and application of new models. Ferroelectric macro-porous BaTiO3 ceramics for piezoelectric applications are manufactured for a range of relative densities, α = 0.30–0.95, using the burned out polymer spheres method. The piezoelectric activity and relevant parameters for specific applications are interpreted by developing two models: a model of a 3–0 composite and a ‘composite in composite’ model. The appropriate ranges of relative density for the application of these models to accurately predict piezoelectric properties are examined. The two models are extended to take into account the effect of 90° domain-wall mobility within ceramic grains on the piezoelectric coefficients . It is shown that porous ferroelectrics provide a novel route to form materials with large piezoelectric anisotropy at 0.20 ≤ α ≤ 0.45 and achieve a high squared figure of merit . The modelling approach allows a detailed analysis of the relationships between the properties of the monolithic and porous materials for the design of porous structures with optimum properties.

Porous ferroelectrics for energy harvesting applications

This paper provides an overview of energy harvesting using ferroelectric materials, with a particular focus on the energy harvesting capabilities of porous ferroelectric ceramics for both piezo- and pyroelectric harvesting. The benefits of introducing porosity into ferro- electrics such as lead zirconate titanate (PZT) has been known for over 30 years, but the potential advantages for energy harvesting from both ambient vibrations and temperature fluctuations have not been studied in depth. The article briefly discusses piezoelectric and pyro- electric energy harvesting, before evaluating the potential benefits of porous materials for increasing energy harvesting figures of merits and electromechanical/electrothermal coupling factors. Established processing routes are evaluated in terms of the final porous structure and the resulting effects on the electrical, thermal and mechanical properties.

Microstructural modelling of the polarization and properties of porous ferroelectrics

Micromechanical models of porous ferroelectric ceramics have often assumed that thematerial is fully polarized in a particular direction and/or consists of a single isolated pore.In this work the polarization state in three-dimensional porous polycrystalline ferroelectricnetworks has been modelled to eradicate the oversimplification of these idealized unit cells.This work reveals that microstructural network models more closely represent a porousferroelectric microstructure since they are able to take into account the complexpolarization distribution in the material due to the presence of high and low permittivityregions. The modelling approach enables the prediction of the distribution ofpoled and unpoled material within the structure. The hydrostatic figures of meritand permittivity were determined for a variety of porous lead zirconate titanatemicrostructures and found to be in good agreement with experimental data. The decreasein piezoelectric activity with porosity was observed to be associated with thecomplex polarization state within the material. Model results were shown to bemuch improved when compared to a model assuming a fully polarized model.

Observation of Hierarchical Porous BaTiO3 Derived from Hard Template

AbstractThe morphology control at the mesoscale, via the design of size, shape, surface, interface, porosity and patterning of the meso-components, has gained increasing attention in the recent past and is expected to endue materials with novel functions. Most reports on mesoporous materials are on amorphous silica or simple oxides with amorphous or polycrystalline framework. It is still quite challenging to achieve functional multi-metal-oxides possessing highly porous structure. On the other hand, the synthesis and characterization of nano-sized ferroics has recently become important since it was predicted a dependence, for instance of the ferroelectric response, on the size and morphology. Moreover, the combination of different materials at the nanoscale creating multifunctional nanocomposites, where new properties resulting from scale, interface and defect phenomena are expected, is a new field that requires exploitation. Crystal size, surface area, surface curvature, and charges on the surface singnificantly influence the physical properties of ferroics at the nano-scale. Within our studies of the porous ferroelectrics, we have prepared BaTiO3 crystals with nanoporosity inside with a sol-precipitation process involving polymer or surfactant micelles. In this work, we present the microscopic characterization of porous BaTiO3 derived from hard matrix.

Design and processing of porous materials for electronic applications

The use of porosity, either unintentionally or intentionally, in the fabrication of materials for electronic and optoelectronic applications is introduced. Unintentional uses include the fabrication of ceramic magnets, where high electrical resistivities are required to reduce eddy currents at high frequency, and the powder technology, often used, inevitably results in porosity. The generation of light from porous silicon created a huge impact in the early 1990s, followed by extensive work on the mechanism responsible, and has now been followed by a more balanced evaluation of its potential. Porous ferroelectrics have shown significant advantages over dense materials for positive temperature coefficient of resistance applications, and for sensors such as hydrophones, and these will be discussed. Low dielectric constant materials are required for the next generation of silicon integrated circuits, where a reduction compared with silicon dioxide is required, and here porosity is a convenient strategy. Finally, the use of deliberately engineered porous nanostructures, with dimensions in the range of the wavelength of light, are discussed for applications in optical processing.