The present work aims to design, produce, and test piezoelectric sensors based on optimized Ba0.85Ca0.15Ti0.90Zr0.10O3 (BCTZ) porous ceramics with controlled microstructures, in complex experimental set-ups for energy harvesting applications for a new generation of self-powered sensors devices. This study demonstrates a new concept based on the use of controlled porosity in ferroelectric ceramics as a tool for enhancing the piezoelectric figures of merit (FOMs), by decreasing permittivity values while preserving high piezoelectric constants. The BCTZ ceramic structures with various levels of microporosity (3% to 31% porosity) have been produced using poly(methyl-methacrylate) (PMMA) microspheres as sacrificial templates and were analyzed from the point of view of their dielectric, ferroelectric properties and their piezoelectric sensing performances. An enhanced piezoelectric response was found in the BCTZ ceramic with intermediate porosity around 18 vol.%, with the highest value of piezoelectric response of 470 pC/N and figure of merit of 7,3 pm2/N. The optimum piezoelectric properties at the intermediate porosity level are related to the microstructural changes (pore shape and connectivity) and possible field-induced structural modifications. The theoretical simulations concerning the influence of pore geometry on electrical properties and the experimental piezoelectric energy harvesting results have shown the possibility of using Pb-free porous ferroelectric materials in devices for energy harvesting applications.

Zirconia (ZrO2) ferroelectric (FE) thin films show promising potential for diverse applications in energy storage, non-volatile memories, and neuromorphic computing devices. Their compatibility with complementary metal-oxide-semiconductor (CMOS) technology, high dielectric constant and natural abundance makes them particularly attractive for large-scale use. However, despite these advantages, ZrO2 thin films have been understudied when compared to HfO2 and Hf0.5Zr0.5O2 (HZO), resulting in a limited understanding of the optimization and stabilization of FE properties of ZrO2. In these materials, the FE behavior arises from the presence of the metastable orthorhombic (o-phase, space group Pca21), but the most stable polymorph is the non-FE monoclinic phase (m-phase, space group P21/c). Thus, many efforts have been aimed at understanding how to stabilize the FE phase. Among various factors, doping has shown great potential for the tuning of properties such as polarization magnitude, reducing wake up effects, endurance, coercive field and switching times. Because of this, it has been extensively studied both theoretically and experimentally for HfO2 and HZO, but not as much for ZrO2. Besides doping, oxygen vacancy (V¬O) engineering is important as the V¬O concentration influences relative phase stability. Moreover, reducing the V¬O concentration is seen as one of the best ways for achieving wake-up free o-phase thin films. Knowledge of the effects of doping and defect engineering on the polarization and relative phase stability of ZrO2 is still widely missing. In this study, through density functional theory (DFT) calculations, we systematically investigate the effects of B, Si, Mg, Sc, Ca, Ce, Ta, Hf, Y, Sr, La and Ba doping on the m, o- and tetragonal (t)-phases of ZrO2 at 3.125 cat%. We obtained insight into structural distortions caused by the presence of the dopants and its effect on spontaneous polarization, relative phase stability, and defect formation energies for the m-, o- and t-phases of ZrO2. We highlighted key candidates with great potential for practical applications for their clear advantages in tuning these properties. Our findings add to and further solidify the current state of the art theoretical insight of the material and its potential for applications.

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