Wireless and Opto-Stimulated Flexible Implants: Artificial Retina Constructed by Ferroelectric BiFeO3-BaTiO3/P(VDF-TrFE) Composites.

The degeneration of retinal photoreceptors is one of the primary causes of blindness, and the implantation of retinal prostheses offers hope for vision restoration in individuals who are completely blind. Flexible bioelectronic devices present a promising avenue for the next generation of retinal prostheses owing to their soft mechanical properties and tissue friendliness. In this study, we developed flexible composite films of ferroelectric BiFeO3-BaTiO3 (BFO-BTO) particles synthesized by the hydrothermal method and ferroelectric poly(vinyldene difluoride-trifluoroethylene) (P(VDF-TrFE)) polymer and investigated their applications in artificial retinas. Owing to the coupling of the photothermal effect of BFO-BTO particles and the pyroelectric effect of the P(VDF-TrFE) polymer, the composite films demonstrate a strong photoelectric response (a maximum peak-to-peak photovoltage > 80 V under blue light of 100 mW/cm2) in a wide wavelength range of light (from visible to infrared) with the inherent flexibility and ease of preparation, making it an attractive candidate for artificial retinal applications. Experimental results showed that blind rats implanted with artificial retinas of the composites display light-responsive behavior, showcasing the effectiveness of vision restoration. This study demonstrates a novel approach for employing ferroelectric materials in vision restoration and offers insights into future artificial retina design.

Giant Energy Density via Mechanically Tailored Relaxor Ferroelectric Behavior of PZT Thick Film.

Relaxor ferroelectrics (RFEs) are being actively investigated for energy-storage applications due to their large electric-field-induced polarization with slim hysteresis and fast energy charging-discharging capability. Here, a novel nanograin engineering approach based upon high kinetic energy deposition is reported, for mechanically inducing the RFE behavior in a normal ferroelectric Pb(Zr0.52 Ti0.48 )O3 (PZT), which results in simultaneous enhancement in the dielectric breakdown strength (EDBS ) and polarization. Mechanically transformed relaxor thick films with 4 µm thickness exhibit an exceptional EDBS of 540 MV m-1 and reduced hysteresis with large unsaturated polarization (103.6 µC cm-2 ), resulting in a record high energy-storage density of 124.1 J cm-3 and a power density of 64.5 MW cm-3 . This fundamental advancement is correlated with the generalized nanostructure design that comprises nanocrystalline phases embedded within the amorphous matrix. Microstructure-tailored ferroelectric behavior overcomes the limitations imposed by traditional compositional design methods and provides a feasible pathway for realization of high-performance energy-storage materials.

Enhanced Energy Storage Performance of Polymer/Ceramic/Metal Composites by Increase of Thermal Conductivity and Coulomb-Blockade Effect.

Polymer-based dielectrics have attracted considerable attention for a wide range of applications as energy storage devices with high power. However, high loss from low thermal conductivity (K) and leaky current may limit their practical utilization greatly. To overcome these issues, two-dimensional hexagonal boron nitride (h-BN) modified with polydopamine (PDA) and metal palladium nanoparticles (h-BN@PDA@Pd NPs) are introduced into a poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) copolymer matrix. The PDA coating improves the compatibility between the ceramic h-BN filler and the polymer matrix. Contrary to the general idea, the metallic Pd NPs enhance the breakdown strength of the polymer nanocomposites through the Coulomb-blockade effect. The nanocomposite film filled with 6 vol % h-BN@PDA@Pd NPs exhibits significantly improved recoverable energy density (Urec) of 58.6 J cm-3, which is increasedby 496% compared to pure P(VDF-HFP) film, maintaining an efficiency of 65%, even under a high voltage of 500 MV m-1. The in-plane thermal conductivity of the nanocomposites was improved from 0.21 to 1.02 W m-1 K-1 with increasing ceramic h-BN content. This study suggests that a dielectric polymer with surface-engineered ceramic h-BN fillers through a Coulomb-blockade effect of metal Pd NPs might be a promising strategy for high energy storage devices.

Crystallization Kinetics in BaTiO3 Synthesis from Hydrate Precursors via Microwave-Assisted Heat Treatment.

The crystallization kinetics in BaTiO3 synthesis from hydrate precursors via microwave-assisted heating (MWH) were investigated. The structural and chemical features of powders synthesized via MWH and conventional heating (CH) were compared. The charged radicals generated under microwave irradiation were identified by chemical analysis and real-time charge flux measurements. Using Ba(OH)2∙H2O (BH1), Ba(OH)2 (BH0), and BaCO3 (BC) as the precursors for a Ba source, and TiO2∙4H2O (TH) for a Ti source, three different mixture samples, BH1TH (BH1 + TH), BH0TH (BH0 + TH), and BCTH (BC + TH), were heat-treated in the temperature range of 100-900 °C. BaTiO3 powders were synthesized at temperatures as low as 100 °C when sample BH1TH was subjected to MWH. Based on the growth exponent (n), the synthesis reactions were inferred to be diffusion-controlled processes (3 ≤ n ≤ 4) for MWH and interface-controlled processes (2 ≤ n ≤ 3) for CH. Current densities of approximately 0.073 and 0.022 mA/m2 were measured for samples BH1TH and BH0TH, respectively, indicating the generation of charged radicals by the interaction between the precursors and injected microwaves. The radicals were determined as OH- groups by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

A Comparison Study of Fatigue Behavior of Hard and Soft Piezoelectric Single Crystal Macro-Fiber Composites for Vibration Energy Harvesting.

Designing a piezoelectric energy harvester (PEH) with high power density and high fatigue resistance is essential for the successful replacement of the currently using batteries in structural health monitoring (SHM) systems. Among the various designs, the PEH comprising of a cantilever structure as a passive layer and piezoelectric single crystal-based fiber composites (SFC) as an active layer showed excellent performance due to its high electromechanical properties and dynamic flexibilities that are suitable for low frequency vibrations. In the present study, an effort was made to investigate the reliable performance of hard and soft SFC based PEHs. The base acceleration of both PEHs is held at 7 m/s2 and the frequency of excitation is tuned to their resonant frequency (fr) and then the output power (Prms) is monitored for 107 fatigue cycles. The effect of fatigue cycles on the output voltage, vibration displacement, dielectric, and ferroelectric properties of PEHs was analyzed. It was noticed that fatigue-induced performance degradation is more prominent in soft SFC-based PEH (SS-PEH) than in hard SFC-based PEH (HS-PEH). The HS-PEH showed a slight degradation in the output power due to a shift in fr, however, no degradation in the maximum power was noticed, in fact, dielectric and ferroelectric properties were improved even after 107 vibration cycles. In this context, the present study provides a pathway to consider the fatigue life of piezoelectric material for the designing of PEH to be used at resonant conditions for long-term operation.

Pyroelectric Energy Conversion and Its Applications-Flexible Energy Harvesters and Sensors.

Among the various forms of natural energies, heat is the most prevalent and least harvested energy. Scavenging and detecting stray thermal energy for conversion into electrical energy can provide a cost-effective and reliable energy source for modern electrical appliances and sensor applications. Along with this, flexible devices have attracted considerable attention in scientific and industrial communities as wearable and implantable harvesters in addition to traditional thermal sensor applications. This review mainly discusses thermal energy conversion through pyroelectric phenomena in various lead-free as well as lead-based ceramics and polymers for flexible pyroelectric energy harvesting and sensor applications. The corresponding thermodynamic heat cycles and figures of merit of the pyroelectric materials for energy harvesting and heat sensing applications are also briefly discussed. Moreover, this study provides guidance on designing pyroelectric materials for flexible pyroelectric and hybrid energy harvesting.

Boosting the Recoverable Energy Density of Lead-Free Ferroelectric Ceramic Thick Films through Artificially Induced Quasi-Relaxor Behavior.

Dielectric ceramic film capacitors, which store energy in the form of electric polarization, are promising for miniature pulsed power electronic device applications. For a superior energy storage performance of the capacitors, large recoverable energy density, along with high efficiency, high power density, fast charge/discharge rate, and good thermal/fatigue stability, is desired. Herein, we present highly dense lead-free 0.942[Na0.535K0.480NbO3]-0.058LiNbO3 (KNNLN) ferroelectric ceramic thick films (∼5 µm) demonstrating remarkable energy storage performance. The nanocrystalline KNNLN thick film fabricated by aerosol deposition (AD) process and annealed at 600 °C displayed a quasi-relaxor ferroelectric behavior, which is in contrast to the typical ferroelectric nature of the KNNLN ceramic in its bulk form. The AD film exhibited a large recoverable energy density of 23.4 J/cm3, with an efficiency of over 70% under the electric field of 1400 kV/cm. Besides, an ultrahigh power density of 38.8 MW/cm3 together with a fast discharge speed of 0.45 µs, good fatigue endurance (up to 106 cycles), and thermal stability in a wide temperature range of 20-160 °C was also observed. Using the AD process, we could make a highly dense microstructure of the film containing nano-sized grains, which gave rise to the quasi-relaxor ferroelectric characteristics and the remarkable energy storage properties.

Upshift of phase transition temperature in nanostructured PbTiO3 thick film for high temperature applications.

Thick polycrystalline pure PbTiO3 films with nano size grains were synthesized for the first time by aerosol deposition. Annealed 7 µm thick films exhibit well-saturated ferroelectric hysteresis loops with a remanent polarization and coercive field of 35 µC/cm(2) and 94 kV/cm, respectively. A large-signal effective d33,eff value of >60 pm/V is achieved at room temperature. The measured ferroelectric transition temperature (Tc) of the films ∼550 °C is >50 °C higher than the reported values (∼490 °C) for PbTiO3 ceramics. First-principles calculations combined with electron energy loss spectroscopy (EELS) and structural analysis indicate that the film is composed of nano size grains with slightly decreased tetragonality. There is no severe off-stoichiometry, but a high compressive in-plane residual stress was observed in the film along with a high transition temperature and piezoelectric response. The ferroelectric characteristics were sustained until 200 °C, providing significant advancement toward realizing high temperature piezoelectric materials.

Co and Fe doping effect on negative temperature coefficient characteristics of nano-grained NiMn2O4 thick films fabricated by aerosol-deposition.

Spinel structured highly dense NiMn2O4-based (NMO) negative temperature coefficient (NTC) thermistor thick films were fabricated by aerosol-deposition at room temperature. To enhance the thermistor B constant, which represents the temperature sensitivity of the NMO thermistor material, Co and Co-Fe doping was applied. In the case of single element doping of Co, 5 mol% doped NMO showed a high B constant of over 5000 K, while undoped NMO showed -4000 K. By doping Fe to the 5 mol% Co doped NMO, the B constant was more enhanced at over 5600 K. The aging effect on the NTC characteristics of Co doped and Fe-Co co-doped NMO thick film showed very stable resistivity-time characteristics because of the highly dense microstructure.

Reflectivity spectra and colors of porous anodic aluminum oxide containing silver nanoparticles by plasmonic absorption.

The reflectivity spectra and color of porous anodic aluminum oxide (AAO) nanostructures containing the assembly of silver (Ag) nanoparticles (NPs) with a diameter of -10 nm were investigated. The Ag NPs were assembled inside the pores of AAO with a diameter of -60 nm by dip-coating process during which Ag NPs adsorbed on the surface of AAO and driven inside the pores by capillary force upon the evaporation of solvent. The reflectivity spectra and associated colors of AAO with Ag NPs were determined by the plasmonic absorption of light by Ag NPs. Even with the monolayer coverage of Ag NPs in the pores of AAO, the reflectivity is significantly reduced specifically at -465 nm wavelength by a strong plasmonic absorption, resulting in its golden color. Aggregating Ag NPs by post-annealing at 300 and 400 degrees C changed the color to pink due to the red-shift of absorption. These results are indicative of potential color-engineering of NPs/AAO platform by wavelength-selective reduction of reflected light intensity and using it in direct optical read-out of change of surface and morphology conditions.

Effect of NiZnFeO4 content on PZT-pZN + NiZnFeO4 magnetoelectric composite.

We fabricated and characterized the magnetoelectric (ME) properties of 3-0 ME composite materials comprised of the high piezoelectric voltage coefficient material, 0.9Pb(Zr0.52Ti0.48)O3-0.1 Pb(Zn1/3Nb2/3)O3 + 0.005Mn (PZT-PZN), and the magnetostrictive material, Ni0.8Zn0.2Fe2O4 (NZF). As the ME effect is generated by the product coupling between the piezoelectric properties and the magnetostrictive properties, the NZF content should be optimized for a higher ME coefficient. The dielectric constant and spontaneous polarization (P) were decreased with increasing NZF content before the percolation of the NZF particulates. However, as the NZF content exceeded the percolation content, the dielectric loss was dramatically increased due to the low resistivity of NZF. While the piezoelectric constant was decreased with increasing NZF content, the maximum magnetization was linearly increased. When we combined the piezoelectric and magnetostrictive effects, the ME composite sintered at 1200 degrees C with 20% NZF showed a maximum dE/dH of 27 mV/cm x Oe at a magnetic bias of 1240 Oe.

Resolution enhancement in surface plasmon resonance sensor based on waveguide coupled mode by combining a bimetallic approach.

In this study, we present and demonstrate a new route to a great enhancement in resolution of surface plasmon resonance sensors. Basically, our approach combines a waveguide coupled plasmonic mode and a kind of Au/Ag bimetallic enhancement concept. Theoretical modeling was carried out by solving Fresnel equations for the multilayer stack of prism/Ag inner-metal layer/dielectric waveguide/Au outer-metal layer. The inner Ag layer couples incident light to a guided wave and makes more fields effectively concentrated on the outer Au surface. A substantial enhancement in resolution was experimentally verified for the model stack using a ZnS-SiO2 waveguide layer.