Piezoelectric Film Research Articles

Self‐Powered Rapid Response Flexible Pressure Sensor for Wearable Application

Flexible pressure sensors have attracted a lot of attention in fields such as medicine and healthcare due to their ease of making wearable devices. However, the development of flexible pressure sensors is facing the challenges of a complex manufacturing process and high cost. Herein, a zinc oxide (ZnO) piezoelectric film flexible pressure sensor with a 3 × 3 sensor array presented through an extremely simple and ultralow‐cost fabrication process is reported. The 3 × 3 sensor array in series and again, in parallel. The output voltage of the 3 × 3 sensor array is significantly higher compared to the original thin‐film piezoelectric sensors at the same film thickness. The ZnO flexible pressure sensor shows good linear sensitivity and high durability over 4000 cycles of loading in the test range of 0–14 N. The response and recovery times tend to decrease as the dynamic pressure increases in the experimental range. Furthermore, a high‐precision dynamic force calibration system through a comparison between the open‐loop and closed‐loop strategies used in the dynamic force calibration experiments that are performed is presented. Potential applications of the sensor are demonstrated, including elbow flexure and finger tapping. The sensor also shows the ease of massive fabrication.

Thin, flexible, and biocompatible medical ultrasound array transducer using a sol–gel composite spray technique

A thin, flexible, and biocompatible medical ultrasonic transducer was developed using a sol–gel composite spray technique to fabricate a single sheet of piezoelectric material. The careful selection of materials prioritized flexibility, with silicone rubber being chosen for its biocompatibility as the material to be in direct contact with the living body. A porous lead zirconate titanate film with a dielectric constant of 134 was generated, and comb-shaped electrodes were printed on the piezoelectric film to form a 64-channel linear array, which successfully suppressed crosstalk values between adjacent elements and at a distance of two elements to −41.5 and −45.8 dB, respectively. Experiments using string targets achieved a spatial resolution of 2 mm in the lateral direction and 0.68 mm in the axial direction. In an imaging experiment of the carotid artery, the transducer was successful in visualizing the vessel wall before and after the carotid artery vessel.

Enhanced Polarization Effect of Flexible Magnetoelectric PVDF-TrFE/Fe3O4 Smart Nanocomposites for Nonvolatile Memory Application

High power consumption of nonvolatile memory is a major challenge, as it reduces the memory efficiency of information storage devices. The magnetoelectric (ME) coupling in multiferroic nanocomposites, which can be utilized in magnetoelectric random access memory, is an effective approach to reduce power consumption in information storage. Here, a type of ME nanocomposite embedded with 0.5 wt % Fe3O4 is presented, exhibiting higher ME voltage coefficients for piezoelectric thin films. Specifically, the ME voltage coefficient of the P(VDF-TrFE)/Fe3O4 composite developed in this study is 8.97 mV/(cm·Oe), which is 17.5% higher compared to that of the pure P(VDF-TrFE) at a Hdc of 1000 Oe. Meanwhile, the enhanced ME effect of smart nanocomposites is characterized by the increase of diffraction peak intensity at a microscopic level. The nanocomposite films exhibit high ME voltage coefficients and information storage performance, providing a great potential for creating next-generation memory devices in the realm of artificial intelligence and wearable devices.

Single-crystalline AlN/sapphire and composite electrode based ultra-high temperature surface acoustic wave devices

Single crystalline AlN material is very attractive for the development of high temperature electronic devices owing to its high temperature resistance. However, at ultrahigh temperatures, AlN film still will be oxidized and metal electrodes used for the devices will aggregate and vaporize. The deterioration of the material and metal electrodes eventually fails the AlN-based devices. Here, we report a single crystalline AlN film-based ultrahigh temperature surface acoustic wave (SAW) device. By using a thin alumina (Al2O3) protection layer on surface, oxidation of the AlN piezoelectric film was effectively inhibited at high temperatures. A composite electrode based on multilayer structure of Pt-Rh and Al2O3 was designed and fabricated, with the Al2O3 layer as the diffusion barrier layers to suppressed inter-diffusion, agglomeration and vaporization of the metals. The results showed that the developed AlN SAW devices can withstand high temperatures, and work well at temperatures up to 1400 °C.

Ultrathin Epidermal P(VDF-TrFE) Piezoelectric Film for Wearable Electronics

Ultrathin epidermal piezoelectric sensors are essential in the next generation of wearable flexible electronics due to their conformal contact with the skin. However, the fabrication and peeling of ultrathin film still face great challenges, and there is an urgent need to develop a facile and effective process. Here, we propose a method of combing spin-coating and aqueous phase exfoliating to prepare the ultrathin poly(vinylidene fluoride-ran-trifluoroethylene) (P(VDF-TrFE)) piezoelectric film with a thickness of about 3 μm. And the thermal treatment is employed to improve its crystallinity and piezoelectricity, resulting in the piezoelectric coefficient of 11.67 pC N–1 and fine sensitivity of 4.56 mV kPa–1. The ultrathin device is successfully applied to monitor the signals of arterial pulse and interactive force, which is expected to provide references for medical diagnosis and robot design. Notably, this work might motivate design principles and methods of ultrathin epidermal devices and promote its applications in wearable flexible electronics.

Fluorescent N-CQDs uniformly implanted into sponge-like PVDF-HFP piezoelectric film by facile two-step process for multifunctional pressure sensor

To meet the diversified demands for the booming intelligent systems, a multifunctional pressure sensor with two basic capabilities of ultraviolet excited fluorescence and force-electricity response has been proposed, and named as fluorescent pressure sensor (FPS). Carbon quantum dots (CQDs) are being rapidly developed as an reasonable photoluminescence materials. However, the CQDs were utilized in the solid-state film along with serious aggregation-induced quenching (AIQ). Here, we developed a two-step method instead of the typical one-step method to overcome the AIQ of the CQDs in the representative piezoelectric polymer. The sponge-like poly (vinylidene) fluoride-hexafluoropropylene (PVDF-HFP) film with high porosity formed based on nonsolvent-induced phase separation was selected as the host matrix of the guest nitrogen-doped CQDs (N-CQDs). The impressive bright blue fluorescence with enhanced intensity derived from the N-CQDs is successfully implanted into the obtained N-CQDs/PVDF-HFP piezoelectric hybrid film, attributing to two key factors: (i) the sponge-like matrix supported uniform distribution of the N-CQDs and (ii) the hydrogen bonding between the N-CQDs and the matrix chains. Meanwhile, the hybrid film used in the FPS still keeps a sensitivity of 92.7 mV N−1, thanks to the β piezoelectric phase formation in the matrix. Since the hybrid film based FPS has multiple features, such as ultraviolet excited fluorescence, force-electricity response, self-powered capability and flexibility, it will show a great potential in the intelligent systems used as the multifunctional sensor and energy harvester.

Optimal Design Towards High Performance of Sandwich Flexible Piezoelectric Energy Harvesters

AbstractThe flexible piezoelectric energy harvester (FPEH), as an effective strategy for long-term power supply of implantable and wearable electronics, requires high areal output energy density, low mechanical stiffness, and high energy efficiency, simultaneously. The widely adopted sandwich FPEH, consisting of one relatively hard substrate sandwiched between two piezoelectric films, can provide a high areal output energy density, but also high mechanical stiffness and low energy efficiency due to its energy-wasting deformation of the hard substrate. Here, we propose a novel optimal soft-substrate sandwich FPEH with designs of sufficient length and optimized Young’s modulus of the substrate, which is much smaller than that of the piezoelectric film. A sandwich beam model considering both the bending and shearing of the soft substrate and the one-way coupling of the piezoelectric effect was adopted for the theoretical analysis and optimal design. The optimal soft-substrate sandwich FPEH exhibits greatly improved overall performance with a 33% increase in areal output energy density, a 51% reduction in mechanical stiffness, and a 177% increase in energy efficiency, simultaneously. Systematic theoretical analysis is performed to illustrate the mechanism and guide the optimal design. The novel optimal soft-substrate sandwich FPEH is then applied to harvesting energy from various living subjects. This optimal design can be extended to other types of mechanical energy harvesters with a similar laminated structure.

Microstructure Evolution with Rapid Thermal Annealing Time in (001)-Oriented Piezoelectric PZT Films Integrated on (111) Si.

In our recently published paper (Y.-Y. Wang et al., High performance LaNiO3-buffered, (001)-oriented PZT piezoelectric films integrated on (111) Si, Appl. Phys. Lett. 121, 182902, 2022), highly (001)-oriented PZT films with a large transverse piezoelectric coefficient e31,f prepared on (111) Si substrates were reported. This work is beneficial for the development of piezoelectric micro-electro-mechanical systems (Piezo-MEMS) because of (111) Si's isotropic mechanical properties and desirable etching characteristics. However, the underlying mechanism for the achievement of a high piezoelectric performance in these PZT films going through a rapid thermal annealing process has not been thoroughly analyzed. In this work, we present complete sets of data in microstructure (XRD, SEM and TEM) and electrical properties (ferroelectric, dielectric and piezoelectric) for these films with typical annealing times of 2, 5, 10 and 15 min. Through data analyses, we revealed competing effects in tuning the electrical properties of these PZT films, i.e., the removal of residual PbO and proliferation of nanopores with an increasing annealing time. The latter turned out to be the dominating factor for a deteriorated piezoelectric performance. Therefore, the PZT film with the shortest annealing time of 2 min showed the largest e31,f piezoelectric coefficient. Furthermore, the performance degradation occurred in the PZT film annealed for 10 min can be explained by a film morphology change, which involved not only the change in grain shape, but also the generation of a large amount of nanopores near its bottom interface.

Flexible inorganic piezoelectric functional films and their applications

Piezoelectric materials play an increasingly important role in energy harvesters, sensors, and actuators. Flexible and thin piezoelectric films have been demonstrated to provide advanced functionalities and improved performances. However, the research on flexible inorganic piezoelectric thin films has rarely been systematically summarized. Here, we summarize the recent advances in the flexible inorganic piezoelectric thin films, focusing on their structural designs, fabrication techniques, and applications in various practical scenarios. Specifically, different fabrication techniques suitable for diverse inorganic piezoelectric nanostructures are reviewed, including sol–gel, hydrothermal, electrospinning, and other techniques, and the integration process with flexible substrates is further discussed. Biomedical and industrial applications of the flexible piezoelectric thin films are emphasized. Finally, some existing challenges and future perspectives are discussed.

All Acoustical Excitation of Spin Waves in High Overtone Bulk Acoustic Resonator

The hybrid high overtone bulk acoustic wave resonators (HBARs) consisting of a piezoelectric film transducers and gallium gadolinium garnet substrates with yttrium iron garnet films (YIG-GGG-YIG) are used for experimental excitation and detection of acoustically driven spin waves (ADSWs). Two types of HBAR transducers made of Al-ZnO-Al films (differed through the electrodes’ geometry) were deposited onto YIG-GGG-YIG trilayers with different YIG film thicknesses and doping levels and served for excitation of multimode HBAR at gigahertz frequencies. ADSWs were detected by measuring the shifts of resonant HBAR modes in a tangential external magnetic field when the conditions for magnetoelastic resonance (MER) were satisfied. It was shown that the design of the transducer with a continuous bottom electrode provides all acoustical excitation of spin waves (pure ADSWs), suppressing the additional inductive magnetic dynamics excitation due to the electrodes’ geometry. The theoretical study of the HBAR spectrum in a magnetic field showed that the resonance harmonics in the MER region can either almost continuously transfer from one to another, or decay and form an evident magnetoelastic gap. In this case, the shift of resonant frequencies can achieve several intermodal distances. The results obtained are important for applications of HBAR-based devices in spintronics and magnonics.

Piezoelectric thin films for MEMS

First Page

ScAlN Film-Based Piezoelectric Micromechanical Ultrasonic Transducers with Dual-Ring Structure for Distance Sensing

Piezoelectric micromechanical ultrasonic transducers (pMUTs) are new types of distance sensors with great potential for applications in automotive, unmanned aerial vehicle, robotics, and smart homes. However, previously reported pMUTs are limited by a short sensing distance due to lower output sound pressure. In this work, a pMUT with a special dual-ring structure based on scandium-doped aluminum nitride (ScAlN) is proposed. The combination of a dual-ring structure with pinned boundary conditions and a high piezoelectric performance ScAlN film allows the pMUT to achieve a large dynamic displacement of 2.87 μm/V and a high electromechanical coupling coefficient (kt2) of 8.92%. The results of ranging experiments show that a single pMUT achieves a distance sensing of 6 m at a resonant frequency of 91 kHz, the farthest distance sensing registered to date. This pMUT provides surprisingly fertile ground for various distance sensing applications.

Hybrid Organic/Inorganic Piezoelectric Device for Energy Harvesting and Sensing Applications

Novel hybrid organic/inorganic flexible devices with composite films, consisting of Ba0.5Sr0.5TiO3 (BST), were prepared by inserting BST nanocoating under spray deposited Poly-vinylidene fluoride-based co-polymer PVDF-TrFE. The study validated that the crystalline structure of BST remains unaffected by the presence of polymer. The 3D atomic force microscopic image of the composite sample confirmed the improved surface roughness and contact conditions after spraying the polymer. As a result, the hybrid sample exhibited a higher polarization current with reduced impedance and parasitic inductance. The enhancement of the stability of the piezoelectric parameters at multiple bending was observed for the hybrid sample in comparison with the BST single film transducer. The drop of the root mean square (RMS) voltage was 70% after approximately 340,000 numbers of bending against less than 3% for the hybrid BST+PVDF-TrFE device. Due to the effect of the separate layers and summed net charges, the piezoelectric voltage of the hybrid device was competitive to the piezoelectric oxide films, despite the lower piezoelectric coefficient of the polymer. The proposed solution paves the path toward lead-free, wearable energy harvesting devices for low-power consuming electronic devices.

Flexible piezoelectric PVDF nanofiber film sensor by blow spinning

Flexible piezoelectric PVDF nanofiber film sensor by blow spinning

Bionic gradient flexible fish skin acts as a passive dynamic micro-roughness to drag reduction

Fish skin has a unique biological feature that hard fish scales are embedded in a flexible dermis layer and covered by a viscoelasticity mucus or compliant epidermis layer. Therefore, the entire fish skin system can act as a dynamic resilient energy-absorbing coating with micro-roughness to absorb external turbulent pulsations. The microscopic morphology, arrangement of fish scales, and mechanical characteristics of tuna skin inspired the extraction and fabrication of the bionic gradient flexible fish skin (BGFFS) model. First, BGFFS was fabricated by the three-dimensional (3D) printing method, spraying and casting process. Then, the 3D morphology of the bionic fish skin was characterized. The results indicated that the BGFFS exhibited good morphology of fish scales and spatial gradient mechanical properties like fish skin. Moreover, the experimental results in a circulating water tunnel showed that the BGFFS had excellent drag reduction performance in a turbulent flow. When the Reynolds (Re) number was 6.8 × 104, the maximum drag reduction ratio could reach 13.8 %. The drag reduction mechanism was analyzed by the computational fluid dynamics (CFD) method, and the energy-absorbing effect of the BGFFS was measured by the polyvinylidene fluoride (PVDF) piezoelectric film. In this experiment, the maximum voltage signal reached 0.448 V. The drag reduction mechanism of the BGFFS was that the rolling friction caused by vortexes replaced sliding friction. In the meantime, the BGFFS could absorb the micro disturbances in a turbulent flow. These findings can serve as a foundation for an in-depth analysis of the hydrodynamic performance of fish as well as a new inspiration for drag reduction and antifouling.

Development of high uniformity Al1-xScxN piezoelectric film stack dry etching process on 8-inch silicon wafers

This paper have investigated the basic properties of inductively coupled plasma reactive ion etching (ICP-RIE) of scandium doped aluminum nitride (Al1-xScxN) thin films on 8-inch silicon wafers, and presented a pretreatment scheme which can effectively improve the etching uniformity and wafer yield in the mass fabrication of Al1-xScxN-MEMS (micro-electromechanical systems) devices. Based on BCl3/Ar pretreatment and Cl2/Ar main etching scheme, an etching rate of 128 nm/min is obtained, with the low nonuniformity, and selectivity of 1.8 with respect to the loss of molybdenum (Mo) electrode is achieved, respectively. The relationship between etching parameters and etching uniformity is revealed in detail by the 5-step Al1-xScxN etching model. Meanwhile, to optimize the etching compatibility in the fabrication of Al1-xScxN-piezoelectric MEMS devices, a variety of etching masks (e.g. photoresist and silicon dioxide) are respectively used to obtain the consistent sidewall profile at different locations, with photoresist of 24° and silicon dioxide of 57°.

Deep Learning Enabled Early Predicting Cardiovascular Status Using Highly Sensitive Piezoelectric Sensor of Solution‐Processable Nylon‐11

AbstractCardiovascular diseases are found as one of the major cause of deaths globally, these can be reduced substantially if early‐stage detection and intervention is possible. Regular monitoring of the arterial pulse is one of the possible solutions, however, existing technologies have put limitations, due instability in continuous monitoring, lack of information in real‐time recording of cardiovascular parameters and bulky instruments. A highly sensitive flexible piezoelectric sensor of nylon‐11 fabricated is introduced from simple solution processable technique. Which consists of a highly sensitive, flexible, conformable piezoelectric film, owing to its high mechanosensitivity (≈225 mV N−1) in the subtle pressure range (0.001–1 kPa), and fast responsivity (≈4 ms), it is tested for assessing risk factors of cardiovascular diseases based on arterial pulse data. It is integrated with the internet of things (IoT) via system on a chip to facilitate remote healthcare monitoring. Deep learning algorithms is further interfaced with sensor for early detect and predict cardiovascular risks, showing an accuracy of >94% for predicting cardiovascular status. This piezoelectric sensor equipped with artificial intelligence and IoT has potential for monitoring the risk analysis of the cardiovascular diseases, daily activities, and facilitate to early predict the anomalous physiological changes in the body.

Numerical and experimental investigation of the energy harvesting performance of electromechanically coupled piezoelectric prestressed beams subjected to nonlinear vibrations

Τhe piezoelectric energy harvesting from nonlinear vibrating structures has greatly attracted the attention of scientific community over the last decades, as it seems to provide one of the more promising ways for high electromechanical energy conversion. Current study deals with the development of a robust and accurate numerical tool capable of modelling and designing piezoelectric structures undergoing severe nonlinear vibrations. Specifically, a coupled multi-field generalized nonlinear mechanics framework for piezoelectric beams subjected to initial stresses and large rotations, extended to include passive external electric circuits in order to predict the dynamic response of the structure and the power generated at the external resistive load. An experimental setup was also developed providing great correlations with the numerical results in case of a vibrating axially prestressed composite beam under the prebuckling regime. At the end, a computational investigation was performed studying the complex dynamic behavior of the piezoelectric beam, in the postbuckling regime. The model successfully captures the measured energy harvesting response of the beam including additional effects induced by the asymmetric configuration of the piezoelectric film.

High Step-Up Ratio DC-AC Converter Using Fourth-Order LCLC Resonant Circuit for Ultrasonic Fingerprint-Sensor Drivers

Ultrasonic fingerprint sensors are becoming more widely used in thick or flexible displays. In order to better identify fingerprint information, ultrasonic sensors need to generate more ultrasonic energy, which can be transmitted to the display surface through media with higher acoustic impedance. In this paper, a DC-AC converter with a high lift ratio was proposed to enhance the transmission energy of the ultrasonic fingerprint sensor, thus helping to improve the identification. The converter comprises a full-bridge inverter and two LC resonant circuits. The introduction of an additional LC resonant circuit into the traditional Class-D LC resonant converter effectively increases the boost ratio of the proposed converter from 5 to 22. When used as a part of the ultrasonic fingerprint sensing system, the proposed converter can amplify the 20-V low-voltage DC required to drive the piezoelectric organic film to 376 V high-voltage AC. The voltage of the wave received from this new driver is equal to 970 mV, which greatly exceeds the 376 mV achieved by using the Class-D converter alone. In this paper, the topology proposed by the ultrasonic fingerprint sensor converter driver was experimentally verified, which greatly improved the boost ratio and can be considered suitable for wider applications.

Influence of annealing ambience on structural and piezoelectric properties of single crystal LiTaO3 thin film

The crystal-ion-slicing technology is a promising technology for fabricating high-quality single crystal piezoelectric thin films. This work attempted to investigate the influence of annealing ambience on the properties of LiTaO3 thin films fabricated by crystal-ion-slicing technology. Here, two 4-inch 42° rotated Y-X propagation LiTaO3 thin films were fabricated, as well as the LiTaO3 thin films were annealed in vacuum and O2 environment, respectively. The results demonstrate that the LiTaO3 thin films annealed in O2 environment possesses more homogeneous and denser structure and fewer defects. The Zratio of the resonators fabricated on the LiTaO3 thin films annealed in O2 environment is approximately 85.3 dB and the Q-factor reaches 5107, indicating the excellent piezoelectric properties of thin films.