The continuing need for improved sensor performance and/or reduced power requirements of electronic components, for example for wireless sensor networks or intelligent components, has prompted renewed interest in the development of piezoelectric and pyroelectric sensors which can be coupled with harvesting technologies capable of capturing energy from ambient vibrations and/or heat. This presentation therefore provides an overview of piezoelectric materials for both sensing and harvesting. The particular advantages of exploiting porosity in these materials are emphasised, including how the pore structure and volume fraction can be tailored to optimise the dielectric and ferroelectric properties of these materials to enhance particular figures of merit; these include highly aligned pores formed by freeze casting. Examples of modeling and manufacture of porous materials sensors and energy harvesters are discussed, including SONAR applications that operate under hydrostatic conditions and devices that benefit from acoustic impedance matching with its environment. The potential of novel porous, composites, and sandwich structures are briefly described, and the range of potential benefits of using porosity in ferroelectrics and finally overviewed. This work is supported by UKRI Frontier Research Guarantee on “Processing of Smart Porous Electro-Ceramic Transducers - ProSPECT”, project No. EP/X023265/1.

Porous BaTiO3 ferroelectric ceramics with variable porosity level produced by sacrificial template method (i.e. with relative density between 74%-96%) are investigated by an experimental-numerical approach. The ceramics present anisotropy as derived from the deformation of sacrificial soft polymeric additives (PMMA spheres) during the pressing step. 3D micro X-ray computed tomography experimental data with specific numerical procedures have been firstly used to reconstruct the ceramic 3D microstructures and to extract sample geometrical information as: pore size distribution, pore anisotropy, phase interconnectivity and tortuosity. These 3D microstructures have been employed as input in FEM-based models, in order to determine the local electric field and potential distributions inside the ceramic body. The final aim is to evaluate the low field effective permittivity and high field ferroelectric switching P(E) responses as a function of porosity and applied field direction. The effective permittivity is estimated by computing the total electrostatic energy of the discretized system, while the polarisation-field response is calculated by using a Preisach model for describing the switching properties of the dense ceramic component under the specific local field of each element. Similarly, the tunability response is computed by using a Johnson approach to describe the nonlinear dielectric response of the dense ceramic component. The resulted dielectric and ferroelectric properties derived by using this procedure are discussed in comparison with the experimentally determined ones. The work demonstrates the usefulness of analyses and simulations of properties at different lenghtscales based on real 3D microstructures for completing the understanding of the complex relationship between composition – microstructure – local/macroscopic properties and pave the way of development of new tools for material design of porous or composite multi-materials.

Porous ferroelectric films are promising materials for various electronics applications, as a lower dielectric constant can lead to a higher figure of merit (e.g. in pyroelectric and piezoelectric devices). In contrast to bulk porous ferroelectric ceramics, there is limited information available on the electrical properties of porous films. The introduction of porosity alters the microstructure and, consequently, the electrical properties of the films. Here, we discuss the effect of porosity on the electrical properties of lead zirconate-titanate (PZT) prepared by chemical solution deposition.
The film's porosity is controlled by adding different amounts of polyvinylpyrrolidone (PVP) as a structure-directing agent. Depending on the PVP content, the film structure was changed from a columnar perovskite grain with porous inclusions (3-0 connectivity) to highly interconnected spongy-granular structures with uniaxial grains (3-3 connectivity).
We have found that current models for binary composites, which only account for the linear decrease in material volume in a porous film, do not fully encompass all the factors that govern film properties. The decrease in permittivity and polarization with an increase in porosity is faster than predicted by known models. This is due to texture destruction and the emergence of additional interfaces and defects, which cause depolarization fields and domain wall clamping. On the other hand, the relaxation of mechanical stresses in porous films leads to a decrease in non-switchable polarization, in contrast to the predicted monotonic growth with porosity for bulk materials.
The pore boundaries create new pathways for the flow of electric current, so the currents flowing along the pore boundaries exceed those flowing inside the grains. As a result, the leakage current in the highly porous films with 3-3 connectivity increases compared to the dense film. We suggest that understanding the mechanisms governing the electrical properties in porous ferroelectric films opens up new opportunities for their applications in electronics.
Acknowledgments
This work was supported by Russian Science Foundation, grant No 23-79-30016.

Since their discovery in 1959, lead-based relaxor ferroelectrics like Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) have been the focus of considerable interest for their extraordinary electromechanical characteristics. Nevertheless, the inclination towards lead-free alternatives has made it necessary to find substitutes for PMN-PT and similar Pb-based relaxors. The lead-free BiFeO3-BaTiO3 (BFO-BT) system, particularly with the morphotropic composition (approximately 67% BFO), emerges as notably promising, offering desirable piezoelectric capabilities (>110 pC/N) and a high Curie temperature (>400°C). The extant challenges with BFO-BT are mainly twofold: i) the non-uniform composition accentuated by complex core-shell structures and the emergence of secondary phases, and ii) low DC electrical resistivity mostly originating from the presence of oxygen vacancies and mixed Fe valence states. To tackle these challenges, it is essential to employ an effective synthesis procedure that promotes chemical homogeneity; moreover, it is necessary to engage in defect engineering for the BFO-BT system, calling for a detailed investigation of its point defects. This study sheds light on the synthesis and point defects in BFO-BT ceramics synthesized through Mechanochemical Activation (MA). MA is verified as a straightforward, efficacious approach for fabricating high-purity BFO-BT ceramics with high densities (>93% of the theoretical limit) while reducing core-shell structures and secondary phases. Ultimately, this research contributes to the understanding of enhancing ferroelectric material performance through strategic annealing and doping, shedding light on ways to improve functional properties on BFO-BT ceramics.