[en] A new micromechanical model is proposed to analyse the piezoelectric properties of freeze-cast porous composite materials based on a ferroelectric lead zirconate titanate-type (PZT) ceramics. The important influence of the composite microgeometry and the porous ceramic matrix on the piezoelectric coefficients $d_{3j}^{\ast <!--\; ???\; -->}$ and $g_{3j}^{\ast <!--\; ???\; -->}$ and the piezoelectric anisotropy factor $d_{33}^{\ast <!--\; ???\; -->}/\left|d_{31}^{\ast <!--\; ???\; -->}\right|$ in the porosity range of m _p = 0.2–0.6 is evaluated and discussed. The resulting piezoelectric parameters of parallel-connected freeze-cast composites with highly aligned pore channels are then compared to those of PZT-based porous materials with randomly distributed porosity. Due to the relatively large piezoelectric coefficients $d_{33}^{\ast <!--\; ???\; -->}$ ∼ 10² pC N⁻¹, $g_{33}^{\ast <!--\; ???\; -->}$ ≈ 40–100 mVm N⁻¹, anisotropy factor $d_{33}^{\ast <!--\; ???\; -->}/\left|d_{31}^{\ast <!--\; ???\; -->}\right|$ ≈ 3–5 and the presence of aligned porous channels, the parallel-connected freeze-cast composite has advantages over conventional monolithic PZT-type ceramics (e.g. g ₃₃ = 24.2 mVm N⁻¹ and d ₃₃/∣d ₃₁∣ = 2.2 in the PZT-5 ceramic) and is suitable for piezoelectric transducer, sensor, acoustic, and energy-harvesting applications. (paper)

[en] We demonstrate that trimethylamine borane can exhibit desirable piezoelectric and pyroelectric properties. The material was shown to be able operate as a flexible film for both thermal sensing, thermal energy conversion and mechanical sensing with high open circuit voltages (>10 V). A piezoelectric coefficient of d${}_{33}$$\approx <!--\; ???\; -->$ 10-16 pC N${}^{-<!--\; ???\; -->1}$, and pyroelectric coefficient of p $\approx <!--\; ???\; -->$25.8 μC m${}^{-<!--\; ???\; -->2}$ K${}^{-<!--\; ???\; -->1}$ were achieved after poling, with high pyroelectric figure of merits for sensing and harvesting, along with a relative permittivity of ${\mathrm{\epsilon <!--\; ??\; -->}}_{33}^{\mathrm{\sigma <!--\; ??\; -->}}$$\approx <!--\; ???\; -->$ 6.3. (© 2020 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] This paper combines experimental and modelling studies to provide a detailed examination of the influence of porosity volume fraction and morphology on the polarisation-electric field response of ferroelectric materials. The broadening of the electric field distribution and a decrease in the electric field experienced by the ferroelectric ceramic medium due to the presence of low-permittivity pores is examined and its implications on the shape of the hysteresis loop, remnant polarisation and coercive field is discussed. The variation of coercive field with porosity level is seen to be complex and is attributed to two competing mechanisms where at high porosity levels the influence of the broadening of the electric field distribution dominates, while at low porosity levels an increase in the compliance of the matrix is more important. This new approach to understanding these materials enables the seemingly conflicting observations in the existing literature to be clarified and provides an effective approach to interpret the influence of pore fraction and morphology on the polarisation behaviour of ferroelectrics. Such information provides new insights in the interpretation of the physical properties of porous ferroelectric materials to inform future effort in the design of ferroelectric materials for piezoelectric sensor, actuator, energy harvesting, and transducer applications.

[en] Highlights: • Manufacture methods and the pore characteristics were discussed. • Properties in terms of mechanical, electrical and harvesting performance were described and compared. • Modelling approaches for microstructural design or predicting vibrational and frequency dependent response were examined. • Perspectives for future application on pyroelectric energy harvester were provided. -- Abstract: This paper provides an overview of ferroelectret materials for energy harvesting applications. These materials take the form of a cellular compliant polymer with polarised pores that provide a piezoelectric response to generate electrical energy as a result of an applied strain or surrounding vibration. The manufacturing processes used to create ferroelectret polymer structures for energy harvesting are discussed, along with the range of microstructural features and pore sizes that are formed. Their important mechanical, electrical and harvesting performance are then described and compared. Modelling approaches for microstructural design or for predicting the vibrational and frequency dependent response are examined. Finally, conclusions and future perspectives for ferroelectret materials for energy harvesting applications are provided.

[en] Highlights: • BaTiO₃ composites scaffolds were successfully fabricated via a single-step extrusion 3D printing system. • 3D scaffolds exhibited a seamless structure with high degree of fidelity to the CAD design. • The incorporation of BaTiO₃ improved the mechanical and electroactive properties of polymer-based scaffolds. • BaTiO₃ scaffolds promoted cells adhesion, proliferation, and a distinctive deposition of osteocalcin and collagen I. Piezoelectric ceramics, such as BaTiO₃, have gained considerable attention in bone tissue engineering applications thanks to their biocompatibility, ability to sustain a charged surface as well as improve bone cells' adhesion and proliferation. However, the poor processability and brittleness of these materials hinder the fabrication of three-dimensional scaffolds for load bearing tissue engineering applications. For the first time, this study focused on the fabrication and characterisation of BaTiO₃ composite scaffolds by using a multi-material 3D printing technology. Polycaprolactone (PCL) was selected and used as dispersion phase for its low melting point, easy processability and wide adoption in bone tissue engineering. The proposed single-step extrusion-based strategy enabled a faster and solvent-free process, where raw materials in powder forms were mechanically mixed and subsequently fed into the 3D printing system for further processing. PCL, PCL/hydroxyapatite and PCL/BaTiO₃ composite scaffolds were successfully produced with high level of consistency and an inner architecture made of seamlessly integrated layers. The inclusion of BaTiO₃ ceramic particles (10% wt.) significantly improved the mechanical performance of the scaffolds (54 ± 0.5 MPa) compared to PCL/hydroxyapatite scaffolds (40.4 ± 0.1 MPa); moreover, the presence of BaTiO₃ increased the dielectric permittivity over the entire frequency spectrum and tested temperatures. Human osteoblasts Saos-2 were seeded on scaffolds and cellular adhesion, proliferation, differentiation and deposition of bone-like extracellular matrix were evaluated. All tested scaffolds (PCL, PCL/hydroxyapatite and PCL/BaTiO₃) supported cell growth and viability, preserving the characteristic cellular osteoblastic phenotype morphology, with PCL/BaTiO₃ composite scaffolds exhibiting higher mineralisation (ALP activity) and deposited bone-like extracellular matrix (osteocalcin and collagen I). The single-step multi-material additive manufacturing technology used for the fabrication of electroactive PCL/BaTiO₃ composite scaffolds holds great promise for sustainability (reduced material waste and manufacturing costs) and it importantly suggests PCL/BaTiO₃ scaffolds as promising candidates for load bearing bone tissue engineering applications to solve unmet clinical needs.