Modelling and fabrication of porous sandwich layer barium titanate with improved piezoelectric energy harvesting figures of merit

Abstract

This paper demonstrates that porous ‘sandwich’ structures can provide an effective route for the design and optimisation of piezoelectric materials for energy harvesting applications, which is becoming an increasingly important technology for self-powered wireless networks and sensors. A numerical model is presented that accounts for the complex poling distribution throughout a layered ferroelectric and helps to develop a detailed understanding of the relationship between the geometry of the porous structure and the poling characteristics of porous ferroelectric materials, with good agreement with experimental data. Novel layered barium titanate ceramics were fabricated whereby dense outer layers surround a highly porous sandwich layer, and for specific layer geometries an unusual condition was achieved where the longitudinal piezoelectric strain coefficients (d33) increased as the thickness of the porous layer and total porosity level of the layered structure increased. The permittivity (ε33T) decreased with increasing thickness and increasing porosity level of the porous layer due to the presence of a low permittivity air phase. These two factors in combination led to an increase in the longitudinal energy harvesting figure of merit, d332/ε33T, for the layered structure, with a maximum of 3.74 pm2/N when the relative thickness of the porous layer was 0.52 and the porosity within this layer was ∼60 vol%. This harvesting performance of these novel structures is much larger than both dense barium titanate (1.40 pm2/N) and barium titanate with randomly distributed porosity at the same 60% volume fraction (2.75 pm2/N).