MagSonic: Hybrid Magnetic-Ultrasonic Wireless Interrogation of Millimeter-Scale Biomedical Implants With Magnetoelectric Transducer.

Wireless interrogation (power and data transfer) of biomedical implants, miniaturized to millimeter (mm) dimensions, is critical for their chronic operation. Achieving simultaneous wireless power and data transfer at deep sites reliably within safety limits for closed-loop sensing/actuation functions of mm-sized implants is challenging. To enable this operation, a hybrid magnetic-ultrasonic interrogation approach (called MagSonic) is realized through a single magnetoelectric (ME) transducer at the implant that can generate and receive both magnetic field and ultrasound. The fabricated mm-sized bar-shaped ME transducer (5.2×2×1.6 mm3) operates at acoustic wave resonance, functioning at sub-MHz frequencies. For the first time, we demonstrate wireless power reception through one modality (magnetic field or ultrasound) and simultaneous uplink data transmission using the other. At 40 mm depth, the MagSonic link could achieve 100 kbps uplink data rate (bit error rate ≤ 10-5) using 190 pJ/bit transmitted energy and 8 mW delivered power in tissue. The robustness of the MagSonic interrogation link against power carrier interference and misalignments is also demonstrated.

High-Performance Thermomagnetic Gd-Si-Ge Alloys.

Exploring low-grade waste heat energy harvesting is crucial to address increasing environmental concerns. Thermomagnetic materials are magnetic phase change materials that enable energy harvesting from low-temperature gradients. To achieve a high thermomagnetic conversion efficiency, there are three main material requirements: (i) magnetic phase transition near room temperature, (ii) substantial change in magnetization with temperature, and (iii) high thermal conductivity. Here, we demonstrate a high-performance Gd5Si2.4Ge1.6 thermomagnetic alloy that meets these three requirements. The magnetic phase transition temperature was successfully shifted to 306 K by introducing Ge doping in Gd5Si4, and a sharper and more symmetric magnetization behavior with saturation magnetization of Mmax = 70 emu/g at a 2 T magnetic field was achieved in the ferromagnetic state. The addition of SeS2, as a low-temperature sintering aid, to the Gd-Si-Ge alloy improved the material's density and thermal conductivity by ∼45 and ∼275%, respectively. Our results confirm that the (Gd5Si2.4Ge1.6)0.9(SeS2)0.1 alloy is a suitable composite material for low-grade waste heat recovery in thermomagnetic applications.

Defect-Engineering-Stabilized AgSbTe2 with High Thermoelectric Performance.

Thermoelectric (TE) generators enable the direct and reversible conversion between heat and electricity, providing applications in both refrigeration and power generation. In the last decade, several TE materials with relatively high figures of merit (zT) have been reported in the low- and high-temperature regimes. However, there is an urgent demand for high-performance TE materials working in the mid-temperature range (400-700 K). Herein, p-type AgSbTe2 materials stabilized with S and Se co-doping are demonstrated to exhibit an outstanding maximum figure of merit (zTmax ) of 2.3 at 673 K and an average figure of merit (zTave ) of 1.59 over the wide temperature range of 300-673 K. This exceptional performance arises from an enhanced carrier density resulting from a higher concentration of silver vacancies, a vastly improved Seebeck coefficient enabled by the flattening of the valence band maximum and the inhibited formation of n-type Ag2 Te, and ahighly improved stability beyond 673 K. The optimized material is used to fabricate a single-leg device with efficiencies up to 13.3% and a unicouple TE device reaching energy conversion efficiencies up to 12.3% at a temperature difference of 370 K. These results highlight an effective strategy to engineer high-performance TE material in the mid-temperature range.

Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices.

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d33 ; high voltage coefficient, g33 ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Qm ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature Tr-t ) for covering off-resonance (figure of merit (FOM), d33  × g33 ) and on-resonance (FOM, Qm  × k2 ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2 mol% MnO2 doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Qm  × k 31 2 $k_{31}^2$ (227-261) along with high d33  × g33 (28-35 × 10-12 m2 N-1 ), low tan δ (0.3-0.39%) and high Tr-t of 140-190 °C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO2 doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

A Comprehensive Study on Magnetoelectric Transducers for Wireless Power Transfer Using Low-Frequency Magnetic Fields.

Magnetoelectric (ME) transducers, comprising of layered magnetostrictive and piezoelectric materials, are more efficient than inductive coils in converting low-frequency magnetic fields into electric fields, particularly in applications that require miniaturized devices such as biomedical implants. Therefore, ME transducers are an attractive candidate for wireless power transfer (WPT) using low-frequency magnetic fields, which are less harmful to the human body and can penetrate easily through different lossy media. The literature lacks a comprehensive study on the ME transducer as a power receiver in a WPT link. This paper studies the impact of different ME design parameters on the WPT link performance. An accurate analytical model of the ME transducer, operating in the longitudinal-transverse mode, is presented, describing both temporal and spatial deformations. Nine ME transducers with different sizes (ME volume: 5-150 mm3) were fabricated with Galfenol and PZT-5A as magnetostrictive and piezoelectric layers, respectively. Through the modeling and measurement of these ME transducers, the effects of the ME transducer dimension, DC bias magnetic field, loading (RL), and operation frequency on the resonance frequency, quality factor, and received power (PL) of the ME transducer are determined. In measurements, a 150 mm3 ME transducer achieved > 10-fold higher PL for a wide RL range of 500 Ω to 1 MΩ at 95.5 kHz, compared to an optimized coil with comparable size and operation frequency.

Autonomous self-repair in piezoelectric molecular crystals.

Living tissue uses stress-accumulated electrical charge to close wounds. Self-repairing synthetic materials, which are typically soft and amorphous, usually require external stimuli, prolonged physical contact, and long healing times. We overcome many of these limitations in piezoelectric bipyrazole organic crystals, which recombine following mechanical fracture without any external direction, autonomously self-healing in milliseconds with crystallographic precision. Kelvin probe force microscopy, birefringence experiments, and atomic-resolution structural studies reveal that these noncentrosymmetric crystals, with a combination of hydrogen bonds and dispersive interactions, develop large stress-induced opposite electrical charges on fracture surfaces, prompting an electrostatically driven precise recombination of the pieces via diffusionless self-healing.

In situ-grown organo-lead bromide perovskite-induced electroactive γ-phase in aerogel PVDF films: an efficient photoactive material for piezoelectric energy harvesting and photodetector applications.

The unique combination of piezoelectric energy harvesters and light detectors progressively strengthens their application in the development of modern electronics. Here, for the first time, we fabricated a polyvinylidene fluoride (PVDF) and formamidinium lead bromide nanoparticle (FAPbBr3 NP)-based composite aerogel film (FAPbBr3/PVDF) for harvesting electrical energy and photodetector applications. The uniform distribution of FAPbBr3 NPs in FAPbBr3/PVDF was achieved via the in situ synthesis of FAPbBr3 NPs in the PVDF matrix, which led to the stabilization of the γ-phase. The freeze-drying process induced an interconnected porous architecture in the composite film, making it more sensitive to small mechanical stimuli. Owing to this unique fabrication technique, the constructed aerogel film-based nanogenerator (FPNG) exhibited an output voltage and current of ∼26.2 V and ∼2.1 µA, respectively, which were 5-fold higher than that of the nanogenerator with the pure PVDF film. Also, the sensitivity of FPNG upon the irradiation of light was demonstrated by the output voltage reduction of ∼38%, indicating its capability as a light sensing device. Furthermore, the prepared FAPbBr3/PVDF composite was found to be an efficient candidate for light detection applications. A simple planar photodetector was fabricated with the 8.0 wt% FAPbBr3 NP-loaded PVDF composite, which displayed very high responsivity (8 A/W) and response speed of 2.6 s. Thus, this exclusive combination of synthesis and fabrication for the preparation of electro-active films opens a new horizon in the piezoelectric community for effective energy harvesting and light detector applications.

Morphological interference of two different cobalt oxides derived from a hydrothermal protocol and a single two-dimensional metal organic framework precursor to stabilize the ß-phase of PVDF for flexible piezoelectric nanogenerators.

Here, we have fabricated a piezoelectric nanogenerator (PENG) composed of a Co-oxide (Co3O4) doped electro active PVDF based nanocomposite for efficient piezoelectric energy harvesting application where the Co3O4 inclusion favours nucleation and polar ß-phase stabilization in the nanocomposite. The morphological effect on the nucleation and ß-phase stabilisation of PVDF has been explored experimentally. The flake-like morphology of Co3O4 nanoparticles, synthesized by using a MOF, has a more effective surface area to nucleate and stabilise the ß-phase of PVDF than that of rod-like (hydrothermal) and spherical (commercial) nanoparticles. The PENG with PVDF and the 1.5 wt% MOF based Co3O4 (MPNG) shows an excellent open circuit voltage (∼37 V) and short circuit current (∼0.711 µA) upon human finger tapping. The maximum power density generated from the MPNG is ∼8.55 µW cm-2, which is well sufficient for the driving of portable electronic devices like LEDs, calculator wrist watches, humidity sensors etc. Also, from various easily accessible mechanical and biomechanical energy sources like heel pressing, walking, and machine vibration, the MPNG is capable of harvesting energy.

Triboelectric Nanogenerator Driven Self-Charging and Self-Healing Flexible Asymmetric Supercapacitor Power Cell for Direct Power Generation.

The expeditious growth of portable electronics has endorsed the researchers to develop self-powered devices that synchronically harvest and store energy. However, it is quite challenging to integrate two distinct phenomena in a single portable device. Here, we emphasize the fabrication of a triboelectric driven self-charging and self-healing asymmetric supercapacitor (SCSHASC) power cell composed of magnetic cobalt ferrite grown on a stainless steel (SS) fabric (CoFe2O4@SS) as positive and iron oxides decorated reduced graphene oxide grown on a SS fabric (Fe-RGO@SS) as negative electrodes separated by a KOH-soaked self-healing polymer hydrogel electrolyte membrane. The membrane contains Fe3+ cross-linked polyacrylic acid, whereas self-healing carboxylated polyurethane was utilized for encapsulation. SS fabric and poly(vinylidene fluoride- co-hexafluoropropylene) (PVDF-HFP)/SS-impregnated micropatterned PDMS composite film-strip were employed as positive and negative triboelectric friction layers, respectively. During mechanical deformation, the SCSHASC harvests electrical energy and subsequently stores it as electrochemical energy for sustainable power supply. The sandwich-type SCSHASC power cell (a supercapacitor unit sandwiched between two parallelly connected high-performance triboelectric nanogenerators) was charged up to ∼1.6 V within ∼31 min under periodic compression/stress ( F ≈ 17.6 N, f ≈ 3.80 Hz). Furthermore, the SCSHASC# (with two supercapacitor units in series) can instantly power-up several portable electronic appliances on periodic compression and release. Thus, the SCSHASC with unique design will be extremely beneficial for self-powered electronics.

Au quantum dots engineered room temperature crystallization and magnetic anisotropy in CoFe2O4 thin films.

For the first time, this work presents a novel room temperature time-effective concept to manipulate the crystallization kinetics and magnetic responses of thin films grown on amorphous substrates. Conventionally, metal-induced crystallization is adopted to minimize the crystallization temperature of the upper-layer thin film. However, due to the limited surface area of the continuous metal under-layer, the degree of crystallization is insufficient and post-annealing is required. To expose a large surface area of the metal under-layer, we propose a simple and novel approach of using an Au nanodots array instead of a continuous metallic under-layer to obtain crystallization of upper-layer thin films. Spinel cobalt ferrite (CFO) thin film as a 'model' was deposited on an Au nano-dots array to realize this methodology. Our findings revealed that the addition of quantum-sized Au nano-dots as a metal under-layer dramatically enhanced the crystallization of the cobalt ferrite upper layer at room temperature. The appearance of major X-ray diffraction peaks with high intensity and well-defined crystallized lattice planes observed via transmission electron microscopy confirmed the crystallization of the CFO thin film deposited at room temperature on 4 nm-sized Au nano-dots. This crystallized CFO thin film exhibits 18-fold higher coercivity (Hc = 4150 Oe) and 4-fold higher saturation magnetization (Ms = 262 emu cm-3) compared to CFO deposited without the Au under-layer. The development of this novel concept of room-temperature crystallization without the aid of additives and solvents represents a crucial breakthrough that is highly significant for exploring the green and energy-efficient synthesis of a variety of oxide and metal thin films.

An Approach To Fabricate PDMS Encapsulated All-Solid-State Advanced Asymmetric Supercapacitor Device with Vertically Aligned Hierarchical Zn-Fe-Co Ternary Oxide Nanowire and Nitrogen Doped Graphene Nanosheet for High Power Device Applications.

We highlight the design and fabrication of a polydimethylsiloxane (PDMS) encapsulated advanced all-solid-state asymmetric supercapacitor (ASC) device consisting of hierarchical mesoporous zinc-iron-cobalt ternary oxide (ZICO) nanowire coated nickel (Ni) foam (ZICO@Ni foam) as a promising positive electrode and nitrogen doped graphene coated Ni foam (N-G@Ni foam) as negative electrode in the presence of PVA-KOH gel electrolyte. Owing to outstanding electrochemical behavior and ultrahigh specific capacitance of ZICO (≈ 2587.4 F/g at 1 A/g) and N-G (550 F/g at 1 A/g) along with their mutual synergistic outputs, the assembled all-solid-state ASC device exhibits an outstanding energy density of ≈40.5 Wh/kg accompanied by a remarkable long-term cycle stability with ≈95% specific capacitance retention even after 5000 charge-discharge cycles. The exclusive hierarchical ZICO nanowires were synthesized by a facile two-step process comprising of a hydrothermal protocol followed by an annealing treatment on a quartz substrate. While Zn2+ gives the stability of the oxide system, Fe and Co ions provide better electronic conductivity and capacitive response under vigorous cyclic condition. The extraordinary performance of as-fabricated ASC device resembles its suitability for the construction of advanced energy storage devices in modern electronic industries.

Self-powered flexible Fe-doped RGO/PVDF nanocomposite: an excellent material for a piezoelectric energy harvester.

In this work, we report the superior piezoelectric energy harvester ability of a non-electrically poled Fe-doped reduced graphene oxide (Fe-RGO)/poly(vinylidene fluoride) (PVDF) nanocomposite film prepared through a simple solution casting technique that favors the nucleation and stabilization of ≈99% relative proportion of polar γ-phase. The piezoelectric energy harvester was made with non-electrically poled Fe-RGO/PVDF nanocomposite film that gives an open circuit output voltage and short circuit current up to 5.1 V and 0.254 µA by repetitive human finger imparting. The improvement of the output performance is influenced by the generation of the electroactive polar γ-phase in the PVDF, due to the electrostatic interactions among the -CH2-/-CF2- dipoles of PVDF and the delocalized π-electrons and remaining oxygen functionalities of Fe-doped RGO via ion-dipole and/or hydrogen bonding interactions. Fourier transform infrared spectroscopy (FT-IR) confirmed the nucleation of the polar γ-phase of PVDF by electrostatic interactions and Raman spectroscopy also supported the molecular interactions between the dipoles of PVDF and the Fe-doped RGO nanosheets. In addition, the nanocomposite shows a higher electrical energy density of ≈0.84 J cm(-3) at an electric field of 537 kV cm(-1), which indicates that it is appropriate for energy storage capabilities. Moreover, the surface of the prepared nanocomposite film is electrically conducting and shows an electrical conductivity of ≈3.30 × 10(-3) S cm(-1) at 2 wt% loading of Fe-RGO.