Energy Harvesting Figure Of Merit 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.

Two orientation effects and large anisotropy of parameters in piezo-active 2–1–2 composites based on [0 1 1]-poled crystals

The results of a comparative study on piezo-active 2–1–2 composites with two ferroelectric components are discussed. The composite structure combines elements of 2–2 (laminar) and 1–3 (fibrous) connectivity patterns. The first component in each composite is domain-engineered [0 1 1]-poled single crystal with macroscopic mm2 symmetry and high piezoelectric activity. The second component is a poled ferroelectric ceramic that is represented by parallel rods in the shape of an elliptic cylinder with a large ratio of semi-axes at its base. The first orientation effect is appreciable due to rotations of the main crystallographic axes X and Y around Z || OX 3 by an angle α in each crystal layer. Rotation of the ceramic rod bases by an angle γ in a polymer medium leads to the second orientation effect in the 2–1–2 composite. The two orientation effects contribute to a large anisotropy of electromechanical coupling factors k3j∗ , energy-harvesting figures of merit (FOM) (Q3j∗)2 and modified FOM F3j∗σ for a stress-driven harvester. The large level of (Q32∗)2 , (Q33∗)2 , F32∗σ and F33∗σ (these parameters are of the order of 10−11–10−10 Pa−1) indicates that the studied composites are suitable for piezoelectric sensors, transducers and energy-harvesting systems. New m—α diagrams put forward in the present study show regions wherein the large anisotropy of the effective parameters ( |k33∗ / k3f∗|⩾5 , (Q33∗/Q3f∗ )2 ⩾10 and F33∗σ/F3f∗σ⩾10 , f = 1 and 2) is achieved when changing the volume fraction m of the single crystal and the rotation angle α. As a result, the leading role of the first orientation effect is emphasised.

Temperature-Dependent Ferroelectric Properties and Aging Behavior of Freeze-Cast Bismuth Ferrite-Barium Titanate Ceramics.

Lead-free BiFeO3-BaTiO3 (BF-BT) piezoceramics have sparked considerable interest in recent years due to their high piezoelectric performance and high Curie temperature. In this paper, we show how the addition of highly aligned porosity (between 40 and 60 vol %) improves the piezoelectric performance, sensing, and energy harvesting figures of merit in freeze-cast 0.70BiFeO3-0.30BaTiO3 piezoceramics compared to conventionally processed, nominally dense samples of the same composition. The dense and porous BF-BT ceramics had similar longitudinal piezoelectric coefficients (d33) immediately after poling, yet the dense samples were observed to age faster than those of porous ceramics. After 24 h, for example, the porous samples had significantly higher d33 values ranging from 112 to 124 pC/N, compared to 85 pC/N for the dense samples. Porous samples exhibited 3 and 5 times higher longitudinal piezoelectric voltage coefficient g33 and energy harvesting figure of merit d33g33 than dense samples due to the unexpected increase in d33 and decrease in relative permittivity with porosity. Spontaneous polarization (Ps) and remnant polarization (Pr) decrease as the porosity content increased from 37 to 59 vol %, as expected due to the lower volume of active material; however, normalized polarization values with respect to porosity level showed a slight increase in the porous materials relative to the dense BF-BT. Furthermore, the porous ceramics showed improved temperature-dependent strain-field response compared to the dense. As a result, these porous materials show excellent potential for use in high temperature sensing and harvesting applications.

Analysis of energy conversion capability among various magnetostrictive materials for energy harvesting

This work addresses vibrational energy harvesting using magnetostrictive materials. In this field, materials with exceptional magneto-mechanical coupling properties (e.g. Galfenol, Terfenol-D) have attracted significant attention. Only a few magnetostrictive materials have been tested in devices, however, leaving the actual influence of these materials’ properties on the energy harvesting device open to question. This work compares an extensive range of ferromagnetic materials through analysis of their magnetic behavior under static stress. To enable fair comparison of the materials, a model was developed to interpolate their magnetic anhysteretic curves under fixed stress of σ = ±50 MPa. The energy harvesting process was then simulated using a theoretical Ericsson thermodynamic cycle, where the area represents the energy density. This approach estimates the ultimate energy density of the materials using a fair approach, without placing conditions on the applied magnetic field. The correlation between ultimate energy density and the magnetoelastic coefficient show that highly magnetostrictive materials achieve higher ultimate energy densities, as expected. In the low field range, it is however concluded that all materials exhibit energy densities of the same order of magnitude. Secondly, the magnetoelastic coefficient versus excitation field characteristics revealed an optimal bias magnetic field for each material. Finally, for realistic implementation, the paper considers a pre-stress in combination with a bias magnetic field and the small dynamic variations that result from currents induced in surrounding coils. A model was developed and revealed an optimum output energy density that was independent of the geometry and the coil. An energy harvesting figure of merit was then defined to enable a final comparison of the materials, encompassing both material characteristics and realistic applications. Under these working conditions and with all costs considered, some low-magnetostriction materials appeared able to compete with giant magnetostriction materials.

A comprehensive energy flow model for piezoelectric energy harvesters: Understanding the relationships between material properties and power output

Piezoelectric materials have significant potential for mechanical harvesting and have an important influence in determining the power output. While the energy harvesting figure of merit (FoMij) is frequently used to evaluate the performance of an energy harvesting material, inconsistencies between the FoMij and power output makes material design and selection complex. To address this challenge, this paper establishes a new comprehensive energy flow model to assess the influence of the mechanical, piezoelectric and dielectric properties of a material on the complete harvesting process. The model is experimentally verified via detailed experimental evaluation at a range of excitation conditions, with piezoelectric materials fabricated to yield contrasting properties. The comprehensive model provides powerful capabilities to enable (i) analysis of the influence of material modification on energy flow, (ii) precise quantification of the dependence of power output on material properties and (iii) prediction of the power output of a harvester, where the energy flow model significantly reduces the maximum error between predicted and measured output power by ∼70%. This work therefore provides a new holistic approach to fill the knowledge gap in understanding the relationships between material properties and harvesting performance to inform future research in material design, selection and modification.

Effect of bicontinuous minimal surface meso-scale geometry on piezoelectric performances of piezoelectric composites

This paper examines the effective piezoelectric properties of ceramic-based composite materials that contain bicontinuous minimal surface structures. Three structures are introduced bicontinuous Schwarz P, bicontinuous Gyroid, and bicontinuous I-WP and the material model is subjected to both compressive and tensile stresses. Using the finite element method, the relationship between the average piezoelectric coefficient and the volume fraction of piezoelectric ceramics is analyzed for different structures. The study reveals that the bicontinuous Gyroid structure exhibits the highest average piezoelectric coefficient of 501 pc/N under a compressive stress of 0.5 Gpa and a piezoelectric ceramic volume fraction of 50%. Additionally, the study compares the three bicontinuous minimal surface structures with 3–3 model piezoelectric composite materials (interconnected random distribution) to explore the piezoelectric ceramic transfer path and the impact of arrangement and combination on piezoelectric properties. The results show that bicontinuous minimal surface piezoelectric composite materials display superior piezoelectric properties, as demonstrated by their high piezoelectric charge coefficient, piezoelectric voltage coefficient, and energy harvesting figure of merit. Overall, this paper provides valuable insights into how the structure of piezoelectric ceramics can be altered to enhance the performance of piezoelectric composite materials.

3D-Printed Flexible PVDF-TrFE Composites with Aligned BCZT Nanowires and Interdigital Electrodes for Piezoelectric Nanogenerator Applications

Piezoelectric nanogenerators based on piezoelectric nanocomposites have attracted much interest in recent decades owing to their excellent piezoelectric properties and application in self-powered systems and wearable sensors. As a promising piezoelectric ceramic filler in composite-based generators, one-dimensional (1D) piezoelectric nanowires were filled into a polymer matrix to enhance its dielectric and piezoelectric properties. In this paper, flexible PVDF-TrFE composite films containing highly aligned Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) nanowires (NWs) have been manufactured via a direct-ink writing method. The effect of BCZT NW content on the dielectric, ferroelectric, and piezoelectric properties was investigated using multiphysics modeling and detailed experiments. An optimum composite composition was discovered, and the piezoelectric composite film with 15 wt % BCZT NWs possessed the highest energy harvesting figure of merit of 5.3 × 10–12 m2/N. Interdigital electrodes were combined with the composite to fabricate a patterned piezoelectric nanogenerator, where the piezoelectric nanogenerator can produce an open-circuit output voltage of 17 V, and the maximum output power density could reach 5.6 μW/cm2. This work provides opportunities for the optimization and fabrication of piezoelectric materials for energy-harvesting and sensing applications.

Enhanced energy harvesting performance in lead-free multi-layer piezoelectric composites with a highly aligned pore structure

The harvesting of mechanical energy from our living environment via piezoelectric energy harvesters to provide power for next generation wearable electronic devices and sensors has attracted significant interest in recent years. Among the range of available piezoelectric materials, porous piezoelectric ceramics exhibit potential for both sensing and energy harvesting applications due to their reduced relative permittivity and enhanced piezoelectric sensing and energy harvesting figures of merit. Despite these developments, the low output power density and the lack of optimized structural design continues to restrict their application. Here, to overcome these challenges, a lead-free multi-layer porous piezoelectric composite energy harvester with a highly aligned pore structure and three-dimensional intercalation electrodes is proposed, fabricated and characterized. The effect of material structure and multi-layer configuration of the porous piezoelectric ceramic on the dielectric properties, piezoelectric response and energy harvesting performance was investigated in detail. Since the relative permittivity is significantly reduced due to the introduction of aligned porosity within the multi-layer structure, the piezoelectric voltage coefficient, energy harvesting figure of merit and output power are greatly enhanced. The multi-layer porous piezoelectric composite energy harvester is shown to generate a maximum output current of 80 μA, with a peak power density of 209 μW cm−2, which is significantly higher than other porous piezoelectric materials reported to date. Moreover, the generated power can charge a 10 μF capacitor from 0 V to 4.0 V in 150 s. This work therefore provides a new strategy for the design and manufacture of porous piezoelectric materials for piezoelectric sensing and energy harvesting applications.

Porous pyroelectric ceramic with carbon nanotubes for high-performance thermal to electrical energy conversion

The recycling of low-grade thermal energy from our surroundings is an environmental-friendly approach to contribute to sustainability, which remains a grand challenge. Herein, a high-performance porous pyroelectric ceramic formed using carbon nanotubes (CNT) is designed and fabricated using a modified solid-state reaction technique. Localized characterization of PMN-PMS-PZT and PMN-PMS-PZT with 0.3 wt% CNT additions by piezoelectric force microscopy suggests that the presence of porosity and defects in grains can restrict the reversal of domains and weaken the local piezoresponse; that is due to the influence of porosity on the electric field, domain morphology, or screening effects induced by defects at the pore surface. More importantly, the porous ceramics showed enhanced figure of merits, including voltage responsibility and energy harvesting figure of merit, compared to the dense ceramic. The harvested energy increased by 208% when the 0.3 wt% of CNT was added to produce porosity, which has a potential application in thermal energy harvesting and sensing system.

Evaluation of the pore morphologies for piezoelectric energy harvesting application

Piezoelectric energy harvesting has attracted significant attention in recent years due to their high-power density and potential applications for self-powered sensor networks. In comparison to dense piezoelectric ceramics, porous piezoelectric ceramics exhibit superiority due to an enhancement of piezoelectric energy harvesting figure of merit. This paper provides a detailed examination of the effect of pore morphology on the piezoelectric energy harvesting performance of porous barium calcium zirconate titanate 0.5Ba(Zr 0.2 Ti 0.8 )O 3 -0.5(Ba 0.7 Ca 0.3 )TiO 3 (BCZT) ceramics. Three different pore morphologies of spherical, elliptical, and aligned lamellar pores were created via the burnt-out polymer spheres method and freeze casting. The relative permittivity decreased with increasing porosity volume fraction for all porous BCZT ceramics. Both experimental and simulation results demonstrate that porous BCZT ceramics with aligned lamellar pores exhibit a higher remanent polarization. The longitudinal d 33 piezoelectric charge coefficient decreased with increasing porosity volume fraction for the porous ceramics with three different pore morphologies; however, the rate of decrease in d 33 with porosity is slower for aligned lamellar pores, leading to the highest piezoelectric energy harvesting figure of merit. Moreover, the peak power density of porous BCZT ceramics with aligned lamellar pores is shown to reach up to 38 μW cm -2 when used as an energy harvester, which is significantly higher than that of porous BCZT ceramics with spherical or elliptical pores. This work is beneficial for the design and manufacture of porous ferroelectric materials in devices for piezoelectric energy harvesting applications.

Flexible pillar-base structured piezocomposite with aligned porosity for piezoelectric energy harvesting

Flexible piezoelectric energy harvesters have attracted significant attention in recent years due to their ability to convert strain energy associated with bending into electrical energy for low-power wearable electronic devices. Compared with 0–3 type piezoelectric composites, where the piezoelectric phase is isolated within the material, flexible piezoelectric energy harvesters based on a three-dimensional interconnected ceramic skeleton are of particular interest due to their high stress transfer capability and high piezoelectric coefficients. Herein, a three-dimensional interconnected porous barium calcium zirconate titanate (BCZT) pillar-based structure is created via freeze casting, where polydimethylsiloxane (PDMS) is impregnated into the aligned pore channels to form a flexible piezoelectric energy harvester. The effect of porosity on piezoelectric coefficients, ferroelectric properties and energy harvesting performance is investigated in detail. The piezoelectric coefficient and energy harvesting figure of merit are significantly enhanced due to the presence of an aligned pore structure which leads to a high transfer of applied stress into the piezoelectric phase. The highly aligned porosity leads to the creation of a piezoelectric composite with 60 vol% of polymer that exhibits a high output voltage of 30.2 V and current of 13.8 μA. Moreover, the peak power density can reach up to 96.2 μW cm −2 , which is significantly higher than that of nanoparticle-based piezoelectric composites. This work demonstrates a promising prospect of utilizing flexible piezoelectric composites for energy harvesting applications. A flexible piezoelectric energy harvester based on BCZT pillar-base composite with aligned porosity is achieved via a freeze casting technique. Enhanced piezoelectric properties and energy harvesting performance ( V = 30.2 V, I = 13.8 μA at a porosity of 60 vol%) are realized due to the high poling efficiency and improved stress transfer, which is capable of charging capacitors and illuminating commercial LEDs. • Flexible pillar-base structured piezocomposite with highly aligned porosity is developed via freeze casting. • The piezoelectric energy harvesting performance is significantly enhanced due to the presence of an aligned pore structure. • Piezoelectric composite with 60 vol% of polymer exhibits a maximum power density of 96.2 μWcm −2 . • It is demonstrated to have the ability to charge capacitors and illuminate commercial LED bulbs.

A simple scheme for calculating the energy harvesting figures of merit of porous ceramics

Abstract We propose a scheme based on recursively applying analytical formulae for effective properties to a class of porous ceramics for calculating their energy harvesting figures of merit. We approximate the structure of freeze-cast PZT parallel laminae joined by links (or bridges) by a model that can be broken down into two directions along which the structure resembles a laminate. The effective coefficients obtained in the first step of the recursion are then used as input on the second step which gives the final effective moduli. The comparison of those with calculations via Finite Element Method (FEM) on a non-recursive model shows good agreement. Finally, we calculate the piezoelectric and pyroelectric figures of merit and compare them with experimental results. The proposed scheme is a good alternative since it relies only on known simple analytical formulae and has a very low computational cost with respect to other methods that may be applied to such a geometry.

Comparison of K0.5Na0.5NbO3 and PbZr0.52Ti0.48O3 compliant-mechanism-design energy harvesters

Piezoelectric energy harvesting from ambient vibrations offers an environmentally friendly approach to powering distributed sensors for the Internet of Things. This paper gives a direct comparison of Pb(Zr,Ti)O3 (PZT)- and (K,Na)NbO3 (KNN)-based harvesters using a compliant mechanism harvester design for resonant frequencies of 20, 40, and 70 Hz. At 70 Hz, the measured power densities for PZT- and KNN-based devices are 1139 and 31 μW/mm3, respectively, for unimorph structures on nickel foils of 25 and 50 μm in thickness. The power density ratios scale proportionally to the material energy harvesting figures of merit. Energy harvesting with the compliant mechanism design is twice as efficient when compared to harvesting with a simple cantilever beam.

Effect of surfactants on the growth and characterization of triglycine sulfate crystals

This study investigates the effect on crystal growth and physical properties of triglycine sulfate (TGS) single crystals grown in the presence of surfactants, namely the anionic surfactants sodium dodecyl sulfate (SDS) and sodium 1-decanesulfonate (S1-DS). Pure TGS as well as SDS_TGS and S1-DS_TGS single crystals have been grown from aqueous solution, using a temperature lowering technique. The use of surfactants affects both crystal growth rate and shape. Concerning the latter feature, SDS_TGS crystals show larger (010) faces (b-face) and the S1-DS_TGS crystals show an elongation in [010] direction (b-direction). All samples are monoclinic and show the same Curie temperature (49 °C) as pure TGS. The UV–Vis transmittance of the crystals is found to be decreased due to the presence of surfactants during the growth process. At room temperature, the relative permittivity of the samples is decreased from 10.5 for pure TGS to 7.5 and 5.5 for SDS_TGS and S1-DS_TGS crystals, respectively. However, all crystals show the same pyroelectric coefficient of 168 μC/(m2·K) at room temperature. With that, the pyroelectric voltage figure of merit (FOM) (pc/ (ε.ε0)) is increased from 201 kV/(m·K)for pure TGS up to 245 kV/(m·K) for SDS_TGS, and up to 317 kV/(m·K)for S1-DS_TGS. The energy harvesting figure of merit(pc2/ε·ε0) is increased from 30.37 J/(m3.K2) for pure TGS to 42.52 and 58 J/(m3.K2) for SDS_TGS and S1-DS_TGS crystals, respectively.

Iridium oxide top electrodes for piezo- and pyroelectric performance enhancements in lead zirconate titanate thin-film devices

Iridium oxide films of various morphologies are produced by manipulating the reactive gas flow rate in a reactive sputtering process. This study explores the benefits of using planar and nanostructured IrOx top electrodes for Pb(Zr0.52Ti0.48)O3 (PZT) devices employed in piezoelectric and pyroelectric applications in comparison with conventional platinum electrodes. For the first time, the Young’s modulus of a planar IrOx thin film was directly measured to be 300 ± 15 GPa. The piezoelectric performance of devices with planar IrOx showed up to 245% more strain compared to those capped with a Pt electrode. Nanostructured, 2D IrOx platelet networks are observed for films deposited with high oxygen flow rates. The compatibility with various cleanroom processes, such as liftoff, ultrasonic cleaning, and deposition on etched surfaces, is tested for the IrOx nanostructured layers. These films are shown to have low optical reflectivity while retaining good PZT thin-film capacitor polarization and dielectric permittivity. An improved pyroelectric energy harvesting figure of merit by a factor of 2.5× was observed for the IrOx compared to Pt films on the same PZT material.

3D Hierarchical lattice ferroelectric metamaterials

Hierarchical cellular materials are ubiquitous in nature and lead to many extraordinary mechanical properties, such as ultralight, ultrastiff, and high toughness properties. Inspired by the biological materials, the purpose of this paper is to analyze three families, including cubic, octahedron, and hybrid, of 3D hierarchical lattice ferroelectric metamaterials and determine the relationship between architecture and effective thermo-electro-mechanical properties by proposing a multiscale asymptotic homogenization technique. The effect of hierarchical order, lattice topology and relative density on piezoelectric and pyroelectric figures of merit, measure for assessing the performance of ferroelectric metamaterials as sensors and energy harvesters, is explored. The 1st-order ferroelectric metamaterials remarkably improve the figures of merit compared to the fully-solid ferroelectrics; increasing hierarchical order can magnify these improvements. Hybrid hierarchical ferroelectric metamaterials show further improvement in ferroelectric properties, not achievable by fractal-like metamaterials. For example, compared to the 1st-order body centered cube (BCC) with a piezoelectric energy harvesting figure of merit (FOM33) of more than 50 times higher than the bulk ferroelectric materials, FOM33 of 2nd-order hybrid hierarchical octet truss/BCC can be 50.7% higher, this improvement is 43.8% and 43.2% for 2nd-order hierarchical fractal-like BCC and octet truss, respectively. Finally, scaling relationships for predicting the multiphysical behavior of ferroelectric metamaterials, covering the whole range of relative densities, are proposed. This study introduces bioinspired hierarchical ferroelectric metamaterials as a new class of lightweight multifunctional advanced materials with integrated mechanical, piezoelectric and pyroelectric properties for developing the next generation of hydrophones, pressure and temperature sensors, and energy harvesters.

A pyroelectric thin film of oriented triglycine sulfate nano-crystals for thermal energy harvesting

This study introduces a method for the fabrication of transparent and flexible pyroelectric thin films containing oriented triglycine sulfate (TGS) nanorods for thermal energy conversion or thermal sensing. For the fabrication of such thin films a highly nanoporous anodic aluminum oxide (AAO) template is mounted on top of a b-cut TGS seed crystal before filling the template’s nanopores with TGS solution. Crystal growth in the filled pores is done via the so-called temperature-lowering technique. Two principles are applied to grow oriented nanocrystals in the AAO template: first, the effect of different crystal growth rates in different directions of a nanopore’s confined space is exploited. Second, crystal growth at the bottom of the pores is initiated in a preferential direction by the TGS seed crystal. As confirmed by x-ray diffraction the crystal nanorods obtained are oriented in the desired spontaneous polarization vector of TGS (b-direction). Optical transparency of the filled AAO thin film is good when compared to a pure AAO film which indicates a reasonably high homogeneity and a reasonably low defect density of the TGS nanocrystals. Dielectric characterization at 1 kHz shows a relative permittivity similar to a bulk TGS crystal and a high resistivity of about 199 MΩ m. At its phase transition temperature (44 °C), the dielectric loss factor of the thin film is about 0.3. An average pyroelectric coefficient of 133 μC m−2 K−1 and a relative permittivity of 15.6 are calculated from measurement results at room temperature, taking the effect of the AAO thin film into account. With that, the pyroelectric voltage figure of merit (FOM) (pc/εrε0) and the energy harvesting FOM (pc2/εrε0) and are about 820 kV m−1 K−1 and 72 J m−3 K−2, respectively. In comparison to other approaches for growing thin films of oriented TGS crystals, the described method exhibits advantages concerning the growth of oriented nanocrystals, simplicity, fast growth, and an access to both sides of the nanocrystals.

Modified energy harvesting figures of merit for stress- and strain-driven piezoelectric systems

Piezoelectrics are an important class of materials for mechanical energy harvesting technologies. In this paper we evaluate the piezoelectric harvesting process and define the key material properties that should be considered for effective material design and selection. Porous piezoceramics have been shown previously to display improved harvesting properties compared to their dense counterparts due to the reduction in permittivity associated with the introduction of porosity. We further this concept by considering the effect of the increased mechanical compliance of porous piezoceramics on the energy conversion efficiency and output electrical power. Finite element modelling is used to investigate the effect of porosity on relevant energy harvesting figures of merit. The increase in compliance due to porosity is shown to increase both the amount of mechanical energy transmitted into the system under stress-driven conditions, and the stress-driven figure of merit, FoM33X, despite a reduction in the electromechanical coupling coefficient. We show the importance of understanding whether a piezoelectric energy harvester is stress- or strain-driven, and demonstrate how porosity can be used to tailor the electrical and mechanical properties of piezoceramic harvesters. Finally, we derive two new figures of merit based on the consideration of each stage in the piezoelectric harvesting process and whether the system is stress- (FijX), or strain-driven (Fijx).

Architected cellular piezoelectric metamaterials: Thermo-electro-mechanical properties

Advances in additive manufacturing have recently made possible the manufacturing of smart materials with arbitrary microarchitectures, which leads to developing lightweight smart metamaterials with unprecedented multifunctional properties. In this paper, asymptotic homogenization (AH) method is developed for predicting the effective thermo-electro-mechanical properties of architected cellular piezoelectric metamaterials. The effect of pore microarchitecture (relative density and cell topology) and polarization direction on elastic, dielectric, piezoelectric, pyroelectric and thermal properties of periodic cellular piezoelectric metamaterials is explored. The pore topology is determined by Fourier series expansion. Alternative pore microarchitectures are considered by tailoring shape parameters, scaling factor, and rotation angle of the constitutive pore. Smart cellular metamaterials made of both single-phase (BaTiO3) and bi-phase (BaTiO3-expoy) piezoelectric materials are considered. Apart from effective thermo-electro-mechanical properties, a series of figures of merit for the cellular piezoelectric metamaterials, i.e. piezoelectric coupling constant, acoustic impedance, piezoelectric charge coefficient, hydrostatic figure of merit, current responsivity, voltage responsivity and pyroelectric energy harvesting figures of merit, are presented and the reason for difference between the figures of merit of different types of piezoelectric metamaterials is discussed. The figures of merit shed lights on the effect of microarchitecture on optimizing the multifunctional performance of smart cellular metamaterials for applications as structurally efficient multifunctional energy harvesters. It is found that the piezoelectric and pyroelectric figures of merit of cellular piezoelectric metamaterials can be significantly improved compared to the commonly used honeycomb cellular materials and composite materials with solid circular inclusion if an appropriate microarchitecture is selected for the pore. For example, piezoelectric charge coefficient (dh) for a transversely polarized single-phase cellular piezoelectric metamaterial with a solid volume fraction of 0.4 can be 350% higher than the corresponding figures of merit of honeycomb piezoelectric material; voltage responsivity of transversely polarized bi-phase cellular piezoelectric metamaterials with an inclusion volume fraction of 0.3 can be also 249% higher than the corresponding value of composite materials with a solid circular inclusion.

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.