•Co-doped (0–10 mol%) Ba0.8Sr0.2TiO3 films were prepared by sol-gel method.•Co dopant increases the dielectric constant and loss of the BST films.•Figure of merit of 4.5 was obtained in the BSTC film with 5 mol% Co dopant.Ba0.8Sr0.2TiO3 thin films with different concentrations of Co dopant (0–10 mol%) were deposited on Pt/Ti/SiO2/Si substrates by sol-gel method. The microstructure, surface morphology, dielectric and tunable properties of the films were investigated as a function of Co concentration. It was demonstrated that all the Co doped Ba0.8Sr0.2TiO3 (BSTC) thin films exhibited dense microstructure with uniform crystal grains and the grain size of the BSTC films decreased with the increase of Co content. Both dielectric constant and loss decreased with the increase of Co dopant. It was proposed that the depressed dielectric loss in BSTC film originated from the reduction of intrinsic electrons by Co acceptor doping. Although tunability of the BSTC films was also reduced with the addition of Co dopant, a largest figure of merit of 4.5 was obtained in the BSTC film with 5 mol% Co dopant owing to the comparable tunability and low dielectric loss of the films, which made it an alternative source for potential tunable applications.

•Adjustment on the pore characteristics of ice-templated TiO2/chitosan/rGO composite.•Well oriented lamellar pores obtained in TiO2/chitosan/rGO by freeze casting.•The novel architecture of lamellar pores of TiO2/chitosan/rGO resulted in excellent photocatalytic degradation of methyl orange.In this article ice templating is used to fabricate novel TiO2/chitosan/reduced graphene oxide (rGO) composites with a highly aligned macroporous structure for photocatalytic applications. The structure of the composites was readily tailored using the composite composition, for example the lamellar pore width decreased from 50–45 to 5–10 μm, while the lamellar thickness increased from 2–3 to 20–25 μm, with an increase of the TiO2 content from 45 to 77 vol%. Lamellar pore channels between the layers exhibited a more uniform distribution when the rGO content was 1.0 wt%. The increase in viscosity of the composites with high TiO2 contents led to the formation of smaller ice crystals and smaller lamellar pore sizes to enable the production of composite structures with improved mechanical strength. The TiO2/chitosan/rGO composites exhibited excellent photocatalytic degradation of methyl orange and the photocatalytic efficiency was optimized by control of the active material content and microstructure. The hybrid composites with 1.0 wt% rGO showed a degradation percentage of 97%, which makes these novel TiO2/chitosan/rGO freeze cast structures attractive materials as high performance and high strength substrates for photocatalytic degradation applications.

•An analytical solution on the electroelastic response of piezoelectric fiber composites (PFCs) is proposed.•The problem is converted to a singular integral equation with logarithmic kernel.•The mechanical performance of PFCs is affected by the permittivity, piezoelectric constant of materials and the structural parameters.•The results is confirmed by finite element method.As important smart materials, piezoelectric fiber composites (PFCs) have shown excellent performance in many areas. However, the electric field strength concentration at the edge of the interdigital electrode may lead to crack propagation and eventual actuating failure of PFCs. In this paper, a novel analytical solution on the electroelastic response of PFCs is proposed to characterize the mechanical performance and obtain optimal structure parameters. The problem is converted to a singular integral equation with logarithmic kernel. By solving the resulting equation, the distributions of electric potential, electric displacement, electric field strength, and strain of PFCs fiber are obtained. The finite element method (FEM) is employed to confirm the results. The results demonstrate that the electric displacement and strain of PFCs are dramatically affected by the permittivity properties and piezoelectric constant of materials. The PFCs made by PZT-5H have higher surface electric displacement than PZT-5A and PZT-4. For a ratio of W/L = 1/4, both the electric field and strain obtained the minimal value at the electrode edges, which is better for the mechanical performance of PFCs. Moreover, when the thickness of fibers decreases, the actuating performance of PFCs improves and the probability of fracture failure lessens.Download high-res image (251KB)Download full-size image

•A shear piezoelectric fiber composites (SPFC) with interdigitated electrodes was prepared.•A fixed SPFC showed a shear strain of 176.5 ppm, corresponding to an effective piezoelectric constant d15 of 403.9 pm/V.•A 13 × 40 mm Mylar membrane cantilever beam structure was actuated and exhibited a tip displacement of 0.343 mm.•The vibrational mode of SPFC was discussed based on electrical impedance results.As smart materials, piezoelectric ceramics played an important role in actuation, sensing, structural health monitoring and vibration damping systems. This letter reports a novel structure of shear piezoelectric fiber composites (SPFC), which was polarized in the thickness direction, and drived on the transverse direction with interdigitated electrodes. Under a driving condition of 270 V at 0.1 Hz, a fixed SPFC showed a shear strain of 176.5 ppm and a Mylar membrane cantilever beam structure exhibited a tip displacement of 0.343 mm, corresponding to an effective piezoelectric constant d15 of 400 pm/V. These results provide a novel idea for the SPFC applied in actuation and vibration control areas.

The perovskite structured lead-free system Na0.5Bi0.5TiO3 (NBT) whiskers were synthesized from whiskers of layered tunnel structured Na2Ti6O13 (NT), using a topochemical route. Both NT and NBT whiskers show high aspect ratios with an average length of 15 µm and diameter of 1 µm. By prolonging the reaction time from 2 h to 6 h at 900 °C, NT whiskers with monoclinic phase completely transformed to NBT whiskers with pseudocubic phase. Typical strip-like nanodomains are observed in a NBT whisker, which are parallel to each other. The piezoelectric response amplitude for a NBT whisker indicates a large electric field induced strain, corresponding to a Smax/Emax value of as high as 300 pm/V. This work provides an in-depth instruction to prepare pure NBT whiskers, and gives the detailed piezoelectricity of NBT whiskers to promote their applications in energy harvesting and micro-electromechanical systems.

Two-dimensional monocrystalline platelets of the perovskite structured lead–free system (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) were synthesized from platelets of layer-structured Na0.5Bi4.5Ti4O15 (NBIT), using a topochemical route. Both NBIT and NBBT platelets showed high aspect ratios with an average size of 10 μm and thickness of 0.6 μm, for the latter. A structural transformation from layer-structured NBIT to perovskite NBBT was identified in a transitional platelet, which contained phases of both NBIT and NBBT. The composition of NBBT lies at a morphotropic phase boundary (MPB), with the coexistence of monoclinic and tetragonal polymorphs confirmed by X-ray powder diffraction analysis. Transmission electron microscopy and piezoresponse force microscopy were used to investigate the domain structure, with stripe-like domains in the unpoled system transformed into lamellar domains after poling. The piezoresponse amplitude for a NBBT platelet indicated a large piezoresponse strain, corresponding to a field-induced strain Smax/Emax of around 800 pm V−1 at 5 kV mm−1, which is significantly higher than that in ceramic samples of NBBT at the same applied potential.

This paper demonstrates the significant benefits of exploiting highly aligned porosity in piezoelectric and pyroelectric materials for improved energy harvesting performance. Porous lead zirconate (PZT) ceramics with aligned pore channels and varying fractions of porosity were manufactured in a water-based suspension using freeze-casting. The aligned porous PZT ceramics were characterized in detail for both piezoelectric and pyroelectric properties and their energy harvesting performance figures of merit were assessed parallel and perpendicular to the freezing direction. As a result of the introduction of porosity into the ceramic microstructure, high piezoelectric and pyroelectric harvesting figures of merits were achieved for porous freeze-cast PZT compared to dense PZT due to the reduced permittivity and volume specific heat capacity. Experimental results were compared to parallel and series analytical models with good agreement and the PZT with porosity aligned parallel to the freezing direction exhibited the highest piezoelectric and pyroelectric harvesting response; this was a result of the enhanced interconnectivity of the ferroelectric material along the poling direction and reduced fraction of unpoled material that leads to a higher polarization. A complete thermal energy harvesting system, composed of a parallel-aligned PZT harvester element and an AC/DC converter, was successfully demonstrated by charging a storage capacitor. The maximum energy density generated by the 60 vol% porous parallel-connected PZT when subjected to thermal oscillations was 1653 μJ cm−3, which was 374% higher than that of the dense PZT with an energy density of 446 μJ cm−3. The results are beneficial for the design and manufacture of high performance porous pyroelectric and piezoelectric materials in devices for energy harvesting and sensor applications.

Ceramic/polymer nanocomposites are attractive for energy storage applications due to their ability to exploit the high permittivity of ceramic fillers and high breakdown strength of the polymer matrix. One challenge for the development of high performance nanocomposites based on ceramic particulates or fibers in a polymer matrix is that they often require a high volume fraction (>50%) to achieve a high permittivity, which is often at the expense of a reduction in dielectric strength and mechanical flexibility. In this paper we demonstrate by both experiment and finite element simulation that high aspect ratio nanofiber fillers offer an effective approach to achieve high energy density and dielectric strength. Lead-free ferroelectric Na0.5Bi0.5TiO3 (BNT) nanofibers with a high aspect ratio (>200) are synthesized by a hydrothermal method and dispersed in a poly(vinylidene difluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. The increased fraction of β-phase and the alignment of BNT nanofibers perpendicular to the direction of the applied electric field lead to an enhanced dielectric strength, compared to spherical BNT/P(VDF-FHP) nanoparticles and pure P(VDF-HFP), and experimental measurements are compared with numerical simulations. The results demonstrate that the nanofiber nanocomposites exhibited an ultra-high discharged energy density (12.7 J cm−3) and provide an innovative approach to produce high-energy storage density materials.

Tailoring the rheological properties of colloidal inks is critical for the direct ink writing of three-dimensional structures. The rheological properties of zirconia colloidal inks are extremely sensitive to the concentration of ammonium polyacrylate (NH4PAA). The colloidal ink containing 47 vol% ZrO2, 0.005 g ml−1 cellulose and 0.8 wt% NH4PAA dispersant showed a low viscosity of 3 Pa s at a characteristic shear rate of 50 s−1 and maintained an equilibrium elastic modulus of 8 × 104 Pa when the stress amplitude was up to 200 Pa. Three-dimensional structures with a feature size of 200 μm were successfully assembled with fine integrity.

In this study, lead-free sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) was synthesized using a hydrothermal method with processing temperatures of 100–180 °C and NaOH concentrations of 2–14 M. NBT spherical agglomerates of primary nanocubes, NBT nanowires and NBT microcubes were obtained and their morphologies exhibited a strong correlation with the synthesis conditions. NBT nanowires with a high aspect ratio were proven to be single-crystalline with a [110] growth direction by high-resolution TEM analysis. The in situ transformation process and dissolution–recrystallization mechanism were successfully used to explain the formation of different NBT morphologies. Domain structures and piezoelectric characteristics were systematically studied for NBT by piezoresponse force microscopy (PFM). Clear ferroelectric domain structures and obvious polarization switching behaviors were observed in all types of NBT. The NBT microcubes possessed a larger piezoresponse compared with the NBT spherical agglomerates and nanowires.

High relative permittivity and low dielectric loss were simultaneously achieved in the percolative nanocomposites with methoxypolyethylene glycol (mPEG) modified multi-walled carbon nanotubes (MWCNTs). The dense mPEG layer with a thickness of approximately 1.7 nm was continuously coated on the surface of MWCNTs. MWCNTs exhibited excellent dispersibility after being functionalized by mPEG (mPEG@MWCNTs), the mPEG@MWCNTs/ethanol suspension was still turbid even when the suspension was deposited for two months. A high permittivity of 69.7 and a low dielectric loss of 0.042 were simultaneously achieved in the poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) nanocomposite with 4.02 vol% mPEG@MWCNTs at 1 kHz. The improved dielectric properties in the nanocomposite is mainly ascribed to the following reasons: (i) the increased microcapacitors formed by MWCNTs and insulated dielectric composite; (ii) the enhanced interfacial polarization due to the homogeneous dispersion of mPEG@MWCNTs in the nanocomposites and tight adhesion between mPEG@MWCNTs and P(VDF-HFP) matrix.Multi-walled carbon nanotubes (MWCNTs) were facilely modified by an amphiphilic polymer, the composites with modified MWCNTs exhibit high permittivity and suppressed dielectric loss.

•The aligned porous structure of HA/BaTiO3 composite was similar to the lamellar Haversian system in bone tissue.•The piezoelectric d33 coefficient of HA/BaTiO3 with porosity of 50% was 5.0 pC/N, much higher than that of natural bone.•HA/BaTiO3 with porosity of 50% promoted proliferation, differentiation and adhesion of MG63 cells remarkably.Osteoblasts growing into bone substitute is an important step of bone regeneration. This study prepared porous hydroxyapatite (HA)/BaTiO3 piezoelectric composites with porosity of 40%, 50% and 60% by ice-templating method. Effects of HA/BaTiO3 composites with different porosities, with and without polarizing treatment on adhesion, proliferation and differentiation of osteoblasts were investigated in vitro. Results revealed that cell densities of the porous groups were significantly higher than those of the dense group (p < 0.05), so did the alkaline phosphate (ALP) and bone gla protein (BGP) activities. Porosity of 50% group exhibited higher ALP activity and BGP activity than those of the 40% and 60% groups. Scanning electron microscopy (SEM) observations revealed that osteoblasts adhered and stretched better on porous HA/BaTiO3 than on the dense one, especially HA/BaTiO3 with porosity of 50% and 60%. However, there was no significant difference in the cell morphology, cell densities, ALP and BGP activities between the polarized group and the non-polarized group (p > 0.05). The absence of mechanical loading on the polarized samples may account for this. The results indicated that hierarchically porous HA/BaTiO3 played a favorable part in osteoblasts proliferation, differentiation and adhesion process and is a promising bone substitute material.Aligned porous structure of HA/BaTiO3 piezoelectric composites prepared by ice-templating method was similar to the lamellar Haversian system in bone tissue. When co-cultured with human osteosarcoma cells (MG63), porous HA/BaTiO3 composites exhibited remarkable biological activity in promoting proliferation, differentiation and adhesion of MG63 cells.

Tailoring the rheology of suspensions is an essential and persistent issue form many applications, especially three-dimensional (3D) printing. Colloidal suspensions of ceramic powder (Al2O3) dispersed by a special thermosensitive dispersant (poly(acrylic acid)–poly(N-isopropylacrylamide), PAA–PNIPAM) were designed, which underwent a remarkable fluid-gel transition in response to thermal stimulus due to the phase transition of the graft chains (-PNIPAM). 3D periodic structures with a fine size of 100 μm were assembled by 3D printing.Keywords: 3D printing; dispersant; rheology; suspensions; thermosensitive

Energy storage materials are urgently demanded in modern electric power supply and renewable energy systems. The introduction of inorganic fillers to polymer matrix represents a promising avenue for the development of high energy density storage materials, which combines the high dielectric constant of inorganic fillers with supernal dielectric strength of polymer matrix. However, agglomeration and phase separation of inorganic fillers in the polymer matrix remain the key barriers to promoting the practical applications of the composites for energy storage. Here, we developed a low-cost and environmentally friendly route to modifying BaTiO3 (BT) nanoparticles by a kind of water-soluble hydantoin epoxy resin. The modified BT nanoparticles exhibited homogeneous dispersion in the ferroelectric polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix and strong interfacial adhesion with the polymer matrix. The dielectric constants of the nanocomposites increased significantly with the increase of the coated BT loading, while the dielectric loss of the nanocomposites was still as low as that of the pure P(VDF-HFP). The energy storage density of the nanocomposites was largely enhanced with the coated BT loading at the same electric field. The nanocomposite with 20 vol % BT exhibited an estimated maximum energy density of 8.13 J cm–3, which was much higher than that of pure P(VDF-HFP) and other dielectric polymers. The findings of this research could provide a feasible approach to produce high energy density materials for practical application in energy storage.Keywords: BaTiO3; energy storage; ferroelectric polymers; hydantoin epoxy resins; nanocomposites;

(K, Na)NbO3 (KNN) nanorods were fabricated by molten salt synthesis (MSS) from Nb2O5 and K2Nb4O11 precursors, respectively. The phase and composition of the KNN nanorods were characterized by X-ray diffraction and inductively coupled plasma spectrometry. The microstructure and crystal growth orientation of the KNN nanorods were investigated by scanning electron microscopy and transmission electron microscopy. All the products were proved to be single-crystalline. The KNN nanorods produced from Nb2O5 precursors were short and agglomerate with an aspect ratio of 6:1, while those produced from K2Nb4O11 precursors showed an aspect ratio of nearly 15:1, which could be utilized for bio-sensing and energy-harvesting microdevices. It was proposed that the shortness and agglomeration of the KNN nanorods produced from Nb2O5 precursors originated from the layered structures of the pristine precursors used in fabricating Nb2O5. The KNN nanorods produced from K2Nb4O11 showed a preferable stoichiometry with a K/Na ratio closer to 50:50 than those produced from Nb2O5, which was attributed to the ability of the K2Nb4O11 nanorods to maintain the K content during the reaction process. In addition, the piezoelectric properties of individual KNN nanorods produced from Nb2O5 and K2Nb4O11 precursors were confirmed by piezoresponse force microscopy, with the converse piezoelectric coefficient d*33 calculated to be 95 pm V−1 and 201 pm V−1, respectively. The results obtained in this work would provide a valuable guide for further improvement of the MSS process to synthesize KNN nanorods with a large aspect ratio and high performance.

In this study, a relaxor ferroelectric ceramic, 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (PMN–PT), was synthesized by a molten-salt growth method with lower remnant polarization and slimmer hysteresis loops than traditional ferroelectric ceramics. The PMN–PT particles remained homogeneously dispersed in the composite and adhered tightly to a poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix due to the modification of the particles with dopamine. The composites had a maximum dielectric constant of 65.1 and a low dielectric loss of less than 0.037 at 1 kHz. Due to the low remnant polarization of the relaxor ferroelectric ceramic of PMN–PT, the energy density of the composites significantly increased. The discharged energy density of the sample with 50 vol% PMN–PT was 4 times that of P(VDF-HFP) at 80 kV mm−1. It was demonstrated that the dopamine functionalized PMN–PT/P(VDF-HFP) composite was a potential dielectric material with potential future applications in energy storage.

A sandwich-structured composite consisted of a pure poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) central layer and two BaTiO3–P(VDF-HFP) neighboring layers was developed for high energy storage density. The sandwiched structure effectively enhanced dielectric properties and energy storage capacity of the composites. Breakdown strength and energy storage density of the composite with 35 vol% P(VDF-HFP) central layer reached 315 kV mm−1 and 5.22 J cm−3, which were enhanced by 68% and 125% compared to 187 kV mm−1 and 2.32 J cm−3 of the single layer composite, respectively. The dielectric constant of the composites with sandwich-structure was approximately 3 times higher than the pure P(VDF-HFP) and the dielectric loss was as low as 0.026 at 1 kHz. These results demonstrated that the sandwich-structured ceramics/polymer composite was an effective way to produce high energy density composites.

Niobium-doped barium strontium calcium titanate (BSCTN) ceramics were prepared using a conventional solid-state reaction method. The effects of Nb contents on crystal structure, microstructure, dielectric properties and ferroelectric relaxor behavior of the BSCTN ceramics were investigated. The BSCTN ceramics showed dense microstructures with uniform crystal grains with Nb doping. It was demonstrated that Nb5+ entered the B-site of the perovskite BSCTN ceramic and substituted for Ti4+, which caused the expansion and distortion of crystal lattice of the tetragonal BSCTN ceramic. Doping of Nb resulted in more diffused phase transition and lower Curie temperature of the BSCTN ceramics. The diffuseness degree indicator γ increased until the addition of Nb dopant exceeded 1.5 mol% where a maximum γ of 1.98 was achieved. Among the compositions assayed in this work, the BSCTN ceramics with Nb contents of approximately 1.0–1.5 mol% yielded promising relaxor properties that made them alternative sources for development of environmental friendly lead-free relaxor ferroelectric materials.

Gelcasting is a novel forming method in fabricating complex ceramic parts. A low-toxicity gelling system with excellent properties based on Hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was developed for gelcasting PZT ceramics. Effects of solid loading on the rheological properties, gelation behaviors and mechanical properties were investigated. The solid loading of PZT suspension which was suitable for the gelcasting reached as high as 55.0 vol%. The presence of PZT powder exhibited a catalytic effect of the polymerization reaction of the suspension calculated by the activation energy Ea. The mean flexural strength, the characteristic strength and Weibull modulus of the green bodies reached the maximum of 34.1 ± 3.3 MPa, 35.6 MPa and 11.1, respectively, when the solid loading was 55.0 vol%. After sintering at 1120 °C, excellent irregular shaped pillar accurately reproduced the form of the mold and the structural integrity maintained well over a large area.

•Systematical characterizations on pore characteristic of HA/BT ceramics•Well aligned lamellar pores obtained by the ice-templating process•Adjustment on the pore characteristics of ice-templated HA/BT ceramics•Higher piezoelectric coefficient in porous HA/BT than that of the natural bone•None cytotoxic effect and excellent in vitro biocompatibility in porous HA/BTIt was proposed that the piezoelectric effect played an important physiological role in bone growth, remodelling and fracture healing. An aligned porous piezoelectric composite scaffold was fabricated by freeze casting hydroxyapatite/barium titanate (HA/BT) suspensions. The highest compressive strength and lowest porosity of 14.5 MPa and 57.4% with the best parallelism of the pore channels were achieved in the HA10/BT90 composite. HA30/BT70 and HA10/BT90 composites exhibited piezoelectric coefficient d33 of 1.2 and 2.8 pC/N, respectively, both of which were higher than the piezoelectric coefficient of natural bone. Increase of the solid loading of the suspension and solidification velocity led to the improvement of piezoelectric coefficient d33. Meanwhile, double-templates resulted in the coexistence of lamellar pores and aligned macro-pores, exhibiting the ability to produce an oriented long-range ordered architecture. The manipulation flexibility of this method indicated the potential for customized needs in the application of bone substitute. An MTT assay indicated that the obtained scaffolds had no cytotoxic effects on L929 cells.

•An improved ITSD method was used to prepare HA microspheres with high porosity.•Adjust pore structures by solvent types, solid loadings and PVA contents•Relationship between porosities and drug loadings was discussed in detail.•Influence of pore structures on drug release kinetics was discussed in detail.Hydroxyapatite (HA) microspheres with high porosities were successfully obtained using an improved ice-templated spray drying (ITSD) technique for drug delivery applications. Pore structures and pore sizes of microspheres have great impact on drug loading and release kinetics. Therefore, solvent types, polyvinyl alcohol (PVA) contents and solid loadings of suspensions were adjusted to control the pore structures and pore sizes. Microspheres with interconnected pore networks and aligned pore structures were obtained using camphene-based and tert-butyl alcohol (TBA)-based suspensions, respectively. With the increase of PVA contents in suspensions, the growth of sintering neck became more obvious and the surface of HA particles became smoother. The inner pore structures of microspheres transformed from uniformly distributed cellular pores to three-dimensional interconnected pore networks, with the increase of solid loadings in suspensions. Gentamicin was successfully loaded into porous HA microspheres. The drug loading percentage increased from 40.59 to 49.82% with the increase of porosity of HA microspheres. The release percentage during the initial 18 h increased from 48.72 to 65.68% with the transformation of pore structures from independent cellular pores (main diameter ~ 3 μm) to three-dimensional interconnected pore networks (main diameter > 3 μm).

•A free strain of 1900 microstrain was achieved in PFCs.•Decreased excitation frequency significantly enhanced PFCs’ free strain performance.•PFCs’ actuation performances were enhanced with the increase of voltage amplitude and the decrease of dc bias voltage.The free strain performance of piezoelectric fiber composites (PFCs) and the PFCs’ capability of actuating a cantilever exhibited strong dependence on the excitation voltage, e.g., the voltage amplitude, dc bias voltage and frequency. The enhanced free strain performance was observed when a large voltage amplitude and low dc bias voltage were applied, owning to the increase of d33 piezoelectric coefficient. The PFCs showed better free strain performance under the quasi-static condition than dynamic conditions due to the frequency dependence of d33 piezoelectric coefficient. The influence of voltage amplitude and dc bias voltage on the actuation capability was similar to that on the free strain performance. However, for the customized cantilever structure, the first bending resonance existed definitely at around 25 Hz, which indicated that the mechanical effect was the key factor influencing the actuation performance at measured frequency range of 0.1–35 Hz.

Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.Highlights► Systematical characterizations on HA suspensions' rheological properties. ► Well aligned lamellae pores obtained by ice-templated process. ► Porosity can be adjusted in the range of 68–81 vol.%. ► Compressive strength can be manipulated in the range of 0.9–2.4 MPa. ► Rheological properties showed strong influence on the porous HA ceramic.

•Intensity factors are obtained for functionally graded piezoelectric material with surface electrodes.•Relationship between stress intensity factor and electric displacement intensity factor is considered.•Intensity factors can be same at different geometric configuration if nonhomogeneous parameter is properly selected.Functionally graded piezoelectric material (FGPM) has attracted great attention as sensors and actuators due to its improved reliability and performance. This paper aims at the study of fracture behavior relating to FGPM. In this paper, electroelastic field concentration near electrodes for a FGPM under anti-plane shear deformation is obtained. The Fourier transform technique is applied to reduce the boundary value problem to a pair of dual integral equations, and then to singular integral equation with Cauchy kernel. The resulting singular integral equations are solved by Lobatto-Chebyshev quadrature method. The field intensity factors such as electric displacement and stress intensity factors have been obtained numerically. The influences of nonhomogeneity and geometry parameters on the field intensity factors have been discussed. Theoretical analysis reveals a linear relationship between stress intensity factors and the electric displacement intensity factors. Moreover, the nonhomogeneous parameter should be properly selected to balance the intensity factors on the top and bottom surface electrodes.

•The effect of dielectric mismatch on effective electric field in piezoceramic fibers was explained by a model.•The dispersibility and adhesion of BaTiO3 nanoparticles in epoxy was improved by the dopamine modification.•The actuation performance increased firstly and then decreased with adding BaTiO3 nanoparticles.•The maximum free strain and displacement of cantilever beam were up to 1820 ppm and 19 mm, respectively.Piezoelectric fiber composites (PFCs) have attracted much interest owing to their flexibility and toughness compared with conventional monolithic piezoceramic wafers. The free strain values and actuation property of PFCs strongly depend on the active electric field applied in Pb(Zr1 − xTix)O3 (PZT) fibers. Reducing the dielectric constant mismatch between PZT fiber and the assembling epoxy resin would greatly increase the active electric field in PZT fiber. Therefore, BaTiO3 (BT) nanoparticles were introduced into the epoxy resin to enhance the dielectric constant. Homogeneous dispersion of BT nanoparticles and tight adhesion with the epoxy resin were achieved through a surface modification by dopamine. The maximum dielectric constant of dopamine modified BT/epoxy (BT@Dop/epoxy) nanocomposites was 10.38 with 12 wt% BT@Dop content at 1 kHz. The maximum free strain of PFCs reached 1820 ppm with 6 wt% BT@Dop content, while PFCs assembled by pure epoxy showed 790 ppm at the same processing condition. The tip displacement of cantilever beam actuated by PFCs reached the peak of 19 mm at the resonance frequency with 6 wt% BT@Dop, which was improved by 90% comparing to PFCs with pure epoxy.

Two-dimensional (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) platelets with a size of up to ca. 5 μm and thickness of 0.2–0.5 μm were introduced as fillers into a polymer matrix to prepare energy storage composites for the first time. The NBBT platelets were treated with an aqueous solution of H2O2 and coated with polyvinylpyrrolidone (PVP) before mixing with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)). The final composite was denoted as NBBT@PVP/P(VDF–HFP). Composites were prepared with NBBT@PVP loadings from 1 to 30 vol%. The relative permittivity of the composites increased significantly with increasing NBBT@PVP loading, while the breakdown strength decreased. To improve the breakdown strength of the composites, a sandwich-structure of multilayer films was developed, which used NBBT@PVP/P(VDF–HFP) composites with 1 vol% NBBT loadings as central hard layers and the composites with 30 vol% NBBT loadings as neighboring soft layers. The five-layered film, which contained three central hard layers and neighboring soft layers, showed excellent energy storage properties. The breakdown strength and the maximum energy storage density of the film reached 258 kV mm−1 and 14.95 J cm−3, respectively. The energy efficiency remained 0.9 at an electric field of 200 kV mm−1. The findings provide a new approach to produce energy storage materials with high performance.

Tailoring the rheological properties of colloidal inks is critical for the direct ink writing of three-dimensional structures. The rheological properties of zirconia colloidal inks are extremely sensitive to the concentration of ammonium polyacrylate (NH4PAA). The colloidal ink containing 47 vol% ZrO2, 0.005 g ml−1 cellulose and 0.8 wt% NH4PAA dispersant showed a low viscosity of 3 Pa s at a characteristic shear rate of 50 s−1 and maintained an equilibrium elastic modulus of 8 × 104 Pa when the stress amplitude was up to 200 Pa. Three-dimensional structures with a feature size of 200 μm were successfully assembled with fine integrity.

Ceramic/polymer nanocomposites are attractive for energy storage applications due to their ability to exploit the high permittivity of ceramic fillers and high breakdown strength of the polymer matrix. One challenge for the development of high performance nanocomposites based on ceramic particulates or fibers in a polymer matrix is that they often require a high volume fraction (>50%) to achieve a high permittivity, which is often at the expense of a reduction in dielectric strength and mechanical flexibility. In this paper we demonstrate by both experiment and finite element simulation that high aspect ratio nanofiber fillers offer an effective approach to achieve high energy density and dielectric strength. Lead-free ferroelectric Na0.5Bi0.5TiO3 (BNT) nanofibers with a high aspect ratio (>200) are synthesized by a hydrothermal method and dispersed in a poly(vinylidene difluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. The increased fraction of β-phase and the alignment of BNT nanofibers perpendicular to the direction of the applied electric field lead to an enhanced dielectric strength, compared to spherical BNT/P(VDF-FHP) nanoparticles and pure P(VDF-HFP), and experimental measurements are compared with numerical simulations. The results demonstrate that the nanofiber nanocomposites exhibited an ultra-high discharged energy density (12.7 J cm−3) and provide an innovative approach to produce high-energy storage density materials.

Two-dimensional monocrystalline platelets of the perovskite structured lead–free system (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) were synthesized from platelets of layer-structured Na0.5Bi4.5Ti4O15 (NBIT), using a topochemical route. Both NBIT and NBBT platelets showed high aspect ratios with an average size of 10 μm and thickness of 0.6 μm, for the latter. A structural transformation from layer-structured NBIT to perovskite NBBT was identified in a transitional platelet, which contained phases of both NBIT and NBBT. The composition of NBBT lies at a morphotropic phase boundary (MPB), with the coexistence of monoclinic and tetragonal polymorphs confirmed by X-ray powder diffraction analysis. Transmission electron microscopy and piezoresponse force microscopy were used to investigate the domain structure, with stripe-like domains in the unpoled system transformed into lamellar domains after poling. The piezoresponse amplitude for a NBBT platelet indicated a large piezoresponse strain, corresponding to a field-induced strain Smax/Emax of around 800 pm V−1 at 5 kV mm−1, which is significantly higher than that in ceramic samples of NBBT at the same applied potential.

This paper demonstrates the significant benefits of exploiting highly aligned porosity in piezoelectric and pyroelectric materials for improved energy harvesting performance. Porous lead zirconate (PZT) ceramics with aligned pore channels and varying fractions of porosity were manufactured in a water-based suspension using freeze-casting. The aligned porous PZT ceramics were characterized in detail for both piezoelectric and pyroelectric properties and their energy harvesting performance figures of merit were assessed parallel and perpendicular to the freezing direction. As a result of the introduction of porosity into the ceramic microstructure, high piezoelectric and pyroelectric harvesting figures of merits were achieved for porous freeze-cast PZT compared to dense PZT due to the reduced permittivity and volume specific heat capacity. Experimental results were compared to parallel and series analytical models with good agreement and the PZT with porosity aligned parallel to the freezing direction exhibited the highest piezoelectric and pyroelectric harvesting response; this was a result of the enhanced interconnectivity of the ferroelectric material along the poling direction and reduced fraction of unpoled material that leads to a higher polarization. A complete thermal energy harvesting system, composed of a parallel-aligned PZT harvester element and an AC/DC converter, was successfully demonstrated by charging a storage capacitor. The maximum energy density generated by the 60 vol% porous parallel-connected PZT when subjected to thermal oscillations was 1653 μJ cm−3, which was 374% higher than that of the dense PZT with an energy density of 446 μJ cm−3. The results are beneficial for the design and manufacture of high performance porous pyroelectric and piezoelectric materials in devices for energy harvesting and sensor applications.