Piezoelectric Thin Films Research Articles

Adaptive finite element simulation and experimental verification for fretting wear of PVDF piezoelectric thin films

An adaptive finite element method linked with user-defined subroutine UMESHMOTION is developed for simulating the tangential fretting wear of PVDF piezoelectric thin films. The three-dimensional (3D) sphere-on-flat contact model is established for the fretting wear simulation by using commercial software ABAQUS, where the Si3N4 ceramic ball is used as the punch in contact with PVDF piezoelectric thin films. The variable friction coefficient obtained from experiments is taking into consideration. The calculation of wear is based on the Archard's wear model. The tangential fretting motion with different loads and variable friction coefficients are applied in the finite element simulation. The fretting wear test are carried out to verify the numerical results under the same conditions. The results indicate that the present adaptive finite element model has a certain accuracy comparable to the experiment results for the fretting wear of PVDF piezoelectric thin films.

Light Modulation in Silicon Photonics by PZT Actuated Acoustic Waves.

Tailoring the interaction between light and sound has opened new possibilities in photonic integrated circuits (PICs) that range from achieving quantum control of light to high-speed information processing. However, the actuation of sound waves in Si PICs usually requires integration of a piezoelectric thin film. Lead zirconate titanate (PZT) is a promising material due to its strong piezoelectric and electromechanical coupling coefficient. Unfortunately, the traditional methods to grow PZT on silicon are detrimental for photonic applications due to the presence of an optical lossy intermediate layer. In this work, we report integration of a high quality PZT thin film on a silicon-on-insulator (SOI) photonic chip using an optically transparent buffer layer. We demonstrate acousto-optic modulation in silicon waveguides with the PZT actuated acoustic waves. We fabricate interdigital transducers (IDTs) on the PZT film with a contact photolithography and electron-beam lithography to generate the acoustic waves in MHz and GHz ranges, respectively. We obtain a V π L ∼ 3.35 V·cm at 576 MHz from a 350 nm thick gold (Au) IDT with 20 finger-pairs. After taking the effect of mass-loading and grating reflection into account, we measured a V π L ∼ 3.60 V·cm at 2 GHz from a 100 nm thick aluminum (Al) IDT consisting of only four finger-pairs. Thus, without patterning the PZT film nor suspending the device, we obtained figures-of-merit comparable to state-of-the-art modulators based on SOI, making it a promising candidate for a broadband and efficient acousto-optic modulator for future integration.

On-line monitoring of steel ball stamping by mechatronics cold heading equipment based on PVDF polymer sensing material

Abstract In order to explore the influence of cold heading equipment based on polyvinylidene fluoride (PVDF) polymer sensing material on steel ball stamping, a new method was proposed to reflect the defects of cold heading forming of steel ball by load signal. PVDF piezoelectric film is used as the sensor design of force and support structure of steel ball cold heading electromechanical sensor model. PVDF piezoelectric thin film force sensor is used for the structural optimization and simulation. The structural parameters affecting the natural frequency are numerically analyzed by MATLAB software. The mapping relationship between the external load and the output load of the sensor is obtained by using ANSYS software, and the simulation curve of the natural frequency of the sensor is compared with the theoretical curve to verify the factors affecting the natural frequency. The results show that the nonlinear error of sensors refers to the measured curve and the maximum deviation between the fitting line and the percentage of full-scale output.

Sputter Deposition and Characterization of Sm-Doped Pb(Mg1/3, Nb2/3)O₃-PbTiO₃ Epitaxial Thin Film on Si Toward Giant-Piezoelectric Thin Film for MEMS Actuator Application.

To meet the growing demand for better piezoelectric thin films for microelectromechanical systems (MEMSs), we have developed an SM-doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 (Sm-PMN-PT) epitaxial thin film as a next-generation piezoelectric thin film to replace Pb(Zr, Ti)O3 (PZT). The inherent piezoelectricity | e31,f | achieved 20 C/m2, which is greater than those of intrinsic PZT thin films and the best Nb-doped PZT thin film. Besides, the simulation results show that the | e31,f | value of the single Sm-PMN-PT film could be around 26 C/m2. Meanwhile, the breakdown voltage of the as-deposited thin film was higher than 300 kV/cm. These results suggest the high potential of the Sm-PMN-PT epitaxial thin film for piezo-MEMS actuators with large displacement or force.

Resonant Magnetometer for Ultralow Magnetic Field Detection by Integrating Magnetoelastic Membrane on Film Bulk Acoustic Resonator.

Here, we report on a composite nanomechanical resonant magnetometer with magnetoelastic thin film integrated on the surface of a film bulk acoustic resonator (FBAR). By exploiting the delta-E effect of magnetoelastic thin film and resonance characteristic in piezoelectric thin film, we theoretically and experimentally demonstrate the capability to realize ultrahigh resonance frequency and excellent magnetic field sensitivity in such composite configuration, thereby greatly improving the limit of detection of weak magnetic field. The proposed FBAR-based resonant magnetometer achieves maximum magnetic sensitivity of 137 kHz/Oe in a proof-of-concept device without structural optimization, corresponding to a noise equivalent power as low as 7 nT/Hz1/2. Further study indicates that by optimizing the thicknesses of the magnetic sensitive layer and piezoelectric layer, an unprecedented sensitivity of 5 GHz/Oe with an exceptional limit of detection of weak magnetic field down to 190 [Formula: see text]/Hz1/2 could be potentially achieved. Our work provides a forward new and exciting route toward ultralow magnetic field detection in civilian and military applications.

Temperature-induced potential barriers in piezoelectric semiconductor films through pyroelectric and thermoelastic couplings and their effects on currents

We study temperature effects in piezoelectric semiconductor thin films through pyroelectric and thermoelastic as well as piezoelectric couplings. Two-dimensional equations based on the macroscopic theory of piezoelectric semiconductors for extensional deformations of the films are used. A p-type ZnO film with an in-plane c-axis is analyzed. It is shown through a linear and theoretical analysis that a local temperature change produces a pair of a potential barrier and a well in the film. A nonlinear numerical analysis is then performed when a voltage is also applied, showing that the potential barrier blocks the in-plane motion of the holes and thus affects the current density distribution. The effects of various physical and geometric parameters are examined. The results obtained provide the theoretical foundation for thermally manipulating currents in piezoelectric semiconductor films for piezotronic applications.

Development and Characterization of ZnO Piezoelectric Thin Film Sensors on GH4169 Superalloy Steel Substrate by Magnetron Sputtering.

At present, piezoelectric sensors are primarily applied in health monitoring areas. They may fall off owing to the adhesive’s durability, and even damage the monitored equipment. In this paper, a piezoelectric film sensor (PFS) based on a positive piezoelectric effect (PPE) is presented and a ZnO film is deposited on a GH4169 superalloy steel (GSS) substrate using magnetron sputtering. The microstructure and micrograph of ZnO piezoelectric thin films were analyzed by an X-ray diffractometer (XRD), energy dispersive spectrometer (EDS), scanning electron microscope (SEM), and atomic force microscope (AFM). The results showed that the surface morphology was dense and uniform and had a good c-axis-preferred orientation. According to the test results of five piezoelectric sensors, the average value of the longitudinal piezoelectric coefficient was 1.36 pC/N, and the average value of the static calibration sensitivity was 19.77 mV/N. We selected the sensor whose parameters are closest to the average value for the dynamic test experiment and we drew the output voltage response curve of the piezoelectric film sensor under different loads. The measurement error was 4.03% when repeating the experiment six times. The research achievements reveal the excellent performance of the piezoelectric film sensor directly deposited on a GH4169 superalloy steel substrate. This method can reduce measurement error caused by the adhesive and reduce the risk of falling off caused by the aging of the adhesive, which provides a basis for the research of smart bolts and guarantees a better application in structural health monitoring (SHM).

Resonating AlN-thin film MEMS mirror with digital control

Piezoelectrically actuated resonant micromirrors were designed to meet the light detection and ranging (LiDAR) system requirements. Key features were a 3-mm mirror aperture, a 40-deg field of view, and a 50-Hz refresh rate. The presented micromirror provides biaxial symmetrical beam steering with ±12.7 deg mechanical tilt angle, resulting in a 50-deg field of view with an adjustable Lissajous XY-scanning pattern for a forward-looking LiDAR system. The mirrors were fabricated using silicon on insulator wafers, and actuation was based on piezoelectric aluminium nitride thin film. The mirrors were vacuum packaged for high-quality factor resonator operation. The device design contained eight separate piezoelectric aluminium nitride elements arranged as differential pairs for each axis, where each actuator was equipped with a sensing element providing a mechanically coupled electrical feedback signal. The piezoelectric elements connected as actuators required only minimal power and were directly compatible with CMOS low-voltage logic, which eases integration to driving digital systems. The sense elements are used to monitor phase, amplitude, and frequency. A digital control system connected to each of these elements provides accurate frequency and phase control of independent orthogonal resonators, permitting control of the X and Y amplitudes and the refresh rate of the Lissajous pattern.

Thin films of the alpha-quartz Si_xGe_{1-x}O_2 solid solution

SiO_2 with the alpha-quartz structure is one of the most popular piezoelectrics. It is widely used in crystal oscillators, bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) devices, and so on. GeO_2 can also be crystallized into the alpha-quartz structure and it has better piezoelectric properties, with higher piezoelectric coefficient and electromechanical coupling coefficients, than SiO_2. Experiments on bulk crystals and theoretical studies have shown that these properties can be tuned by varying the Si/Ge ratio in the Si_xGe_{1-x}O_2 solid solution. However, to the best of our knowledge, thin films of Si_xGe_{1-x}O_2 quartz have never been reported. Here we present the successful crystallization of Si_xGe_{1-x}O_2 thin films in the alpha-quartz phase on quartz substrates (SiO_2) with x up to 0.75. Generally, the films grow semi-epitaxially, with the same orientation as the substrates. Interestingly, the Si_{0.75}Ge_{0.25}O_2 composition grows fully strained by the quartz substrates and this leads to the formation of circular quartz domains with an ordered Dauphiné twin structure. These studies represent a first step towards the optimization of piezoelectric quartz thin films for high frequency (> 5 GHz) applications.

Vibration-induced nanoscale friction modulation on piezoelectric materials

Mechanical vibration, as an alternative of application of solid/liquid lubricants, has been an effective means to modulate friction at the macroscale. Recently, atomic force microscopy (AFM) experiments and model simulations also suggest a similar vibration-induced friction reduction effect for nanoscale contact interfaces, although an additional external vibration source is typically needed to excite the system. Here, by introducing a piezoelectric thin film along the contact interface, we demonstrate that friction measured by a conductive AFM probe can be significantly reduced (more than 70%) when an alternating current (AC) voltage is applied. Such real-time friction modulation is achieved owing to the localized nanoscale vibration originating from the intrinsic inverse piezoelectric effect, and is applicable for various material combinations. Assisted by analysis with the Prandtl—Tomlinson (P—T) friction model, our experimental results suggest that there exists an approximately linear correlation between the vibrational amplitude and the relative factor for perturbation of sliding energy corrugation. This work offers a viable strategy for realizing active friction modulation for small-scale interfaces without the need of additional vibration source or global excitation that may adversely impact device functionalities.

Acousto-Pi: An Opto-Acoustofluidic System Using Surface Acoustic Waves Controlled With Open-Source Electronics for Integrated In-Field Diagnostics.

Surface acoustic wave (SAW) devices are increasingly applied in life sciences, biology, and point-of-care applications due to their combined acoustofluidic sensing and actuating properties. Despite the advances in this field, there remain significant gaps in interfacing hardware and control strategies to facilitate system integration with high performance and low cost. In this work, we present a versatile and digitally controlled acoustofluidic platform by demonstrating key functions for biological assays such as droplet transportation and mixing using a closed-loop feedback control with image recognition. Moreover, we integrate optical detection by demonstrating in situ fluorescence sensing capabilities with a standard camera and digital filters, bypassing the need for expensive and complex optical setups. The Acousto-Pi setup is based on open-source Raspberry Pi hardware and 3-D printed housing, and the SAW devices are fabricated with piezoelectric thin films on a metallic substrate. The platform enables the control of droplet position and speed for sample processing (mixing and dilution of samples), as well as the control of temperature based on acousto-heating, offering embedded processing capability. It can be operated remotely while recording the measurements in cloud databases toward integrated in-field diagnostic applications such as disease outbreak control, mass healthcare screening, and food safety.

Data-Over-Sound With PMUTs

Data-over-sound is an emerging technology for digital communication which uses frequencies at the upper bounds of human hearing, usually between 15 kHz to 25 kHz. We report a successful development of Piezoelectric Micromachined Ultrasound Transducers (PMUTs) for low-power data-over-sound applications. Piezoelectric thin films used in PMUTs can have high residual tensile stresses ranging from 300MPa to 1.5GPa. These stresses have the effect of increasing the resonant frequencies of the transducers, making it a challenge to fabricate low frequency devices. Using the optimum dimensions by estimating the net residual stress in the fabricated diaphragms, transducers suitable for a frequency range of 17 kHz to 21 kHz were fabricated, capable of generating as much as 83 dB of sound pressure level at a distance of 5 cm in continuous operation.

Application of Ni-doped ZnO nanorods in surface acoustic wave ultraviolet photodetectors

A surface acoustic wave ultraviolet photodetector was fabricated on a ZnO thin film with pure and Ni-doped ZnO nanorods deposited on a Si substrate. Piezoelectric ZnO thin films were grown on Si through radio-frequency magnetron sputtering, and ZnO nanorods were synthesized on ZnO thin films by using the hydrothermal method. The crystalline structure, surface morphology, and luminescent characteristics of ZnO films and nanorods were examined using X-ray diffraction and photoluminescence spectrometers and scanning electron microscope. The performance of the surface acoustic wave photodetector was evaluated using the variations in surface capacitance, insertion loss, and phase shift. ZnO nanorods became shorter and thicker with an increase in the concentration of Ni doping; however, the variations in surface capacitance of Ni-doped ZnO nanorods were greater than those of pure ZnO nanorods obtained under ultraviolet irradiation. Devices with Ni-doped ZnO nanorods exhibited larger shifts in insertion loss and phase than those with pure ZnO nanorods did.

Acoustic Biosensors and Microfluidic Devices in the Decennium: Principles and Applications.

Lab-on-a-chip (LOC) technology has gained primary attention in the past decade, where label-free biosensors and microfluidic actuation platforms are integrated to realize such LOC devices. Among the multitude of technologies that enables the successful integration of these two features, the piezoelectric acoustic wave method is best suited for handling biological samples due to biocompatibility, label-free and non-invasive properties. In this review paper, we present a study on the use of acoustic waves generated by piezoelectric materials in the area of label-free biosensors and microfluidic actuation towards the realization of LOC and POC devices. The categorization of acoustic wave technology into the bulk acoustic wave and surface acoustic wave has been considered with the inclusion of biological sample sensing and manipulation applications. This paper presents an approach with a comprehensive study on the fundamental operating principles of acoustic waves in biosensing and microfluidic actuation, acoustic wave modes suitable for sensing and actuation, piezoelectric materials used for acoustic wave generation, fabrication methods, and challenges in the use of acoustic wave modes in biosensing. Recent developments in the past decade, in various sensing potentialities of acoustic waves in a myriad of applications, including sensing of proteins, disease biomarkers, DNA, pathogenic microorganisms, acoustofluidic manipulation, and the sorting of biological samples such as cells, have been given primary focus. An insight into the future perspectives of real-time, label-free, and portable LOC devices utilizing acoustic waves is also presented. The developments in the field of thin-film piezoelectric materials, with the possibility of integrating sensing and actuation on a single platform utilizing the reversible property of smart piezoelectric materials, provide a step forward in the realization of monolithic integrated LOC and POC devices. Finally, the present paper highlights the key benefits and challenges in terms of commercialization, in the field of acoustic wave-based biosensors and actuation platforms.

Enhanced output performance of piezoelectric energy harvester based on hierarchical Bi3.15Nd0.85Ti3O12 microspheres/PVDF-HFP composite

The inclusion of inorganic piezoelectric fillers in the polymeric matrix of flexible ferroelectric polymer synergies the piezoelectricity for wearable electronics. Herein, hierarchical Bi3.15Nd0.85Ti3O12 microspheres (H-BNdT MPs) embedded in the polymeric matrix of polyvinylidene fluoride–hexafluoropropylene (PVDF-HFP) based piezoelectric thin films are fabricated by using a facile and cost-effective film-forming method, drop-casting. The composites possess improved dielectric and ferroelectric properties comparing with that of the pure PVDF-HFP film without any electrical poling treatment. More importantly, the highest outputs of an open-circuit voltage of 20 V and a short-circuit current of 1.9 μA under the repeated mechanical force of 10 N at the frequency of 2 Hz were successfully achieved. The maximum power of the optimal PEH can reach as high as 4.6 mW, corresponding to a power density of 938 μW/cm2, and the output electricity can light up 10 LEDs with a full-wave rectifier circuit. The enhanced output performance is attributed to the high local stress around the H-BNdT MPs arising from the hollow hierarchical architecture of the inorganic fillers. The lead-free flexible PEH is also proved to have the ability to harvest the biomechanical energy of human movements, such as finger pinching, finger tapping, and fist punching. In particular, a PEH shoe is made to harvest energy from walking and running. This PEH is confirmed to be a potential energy harvester and effective self-powered generator to drive wearable devices.

Chip-level orthometric surface acoustic wave device with AlN/metal/Si multilayer structure for sensing strain at high temperature

A new AlN film-based surface acoustic wave (SAW) device was explored for sensing strain at high temperature. AlN/metal/Si multilayer composite structure was proposed to construct the strain sensing chip, and the corresponding theoretical analysis on SAW propagation were performed by using finite element method (FEM). High-quality AlN piezoelectric thin-film was prepared by using magnetron sputtering on Si substrate, and Pt electrodes was then lithographically prepared to build the sensing chips with one-port resonator pattern operating at 607 MHz and 620 MHz. To compensate the temperature cross sensitivity and enhance the strain sensitivity, two sensing chips were positioned perpendicular on a ceramic package, then the differential frequency signal was extracted to evaluate the applied strain. A high-temperature strain testing platform was constructed to characterize the prepared strain sensor with orthometric structure, larger strain sensitivity of 0.84 ppm/ με at 550 °C in the range 0 – 500 με and excellent temperature stability of 0.624 ppm/°C at the temperature range of 20–600 °C were achieved successfully.

Fractal grid-induced turbulence strength characterization via piezoelectric thin-film flapping velocimetry

The centerline streamwise and cross-sectional (x/Dh = 0.425) turbulence characteristics of a 2D planar space-filling square-fractal-grid (SFG) composed of self-similar patterns superimposed at multiple length-scales is experimentally unveiled via piezoelectric thin-film flapping velocimetry (PTFV). The fluid–structure-interaction between a flexible piezoelectric thin-film and SFG-generated turbulent flow at ReDh = 4.1 × 104 is investigated by analysis of the thin-film’s mechanical response. Measurements of the thin-film-tip deflection δ and induced voltage V demonstrate increasing flow fluctuation strength in the turbulence generation region, followed by rapid decay further downstream of the SFG. Interestingly, SFG-induced turbulence enables the generation of maximum centerline thin-film’s response (Vrms, δrms) and millinewton turbulence-forcing (turbulence-induced excitation force acting on the thin-film) Frms which are respectively, 7× and 2× larger than the classical square-regular-grid of similar blockage ratio. The low frequency, large-scale energy-containing eddies at SFG’s central opening plays a critical role in driving the thin-film vibration. Most importantly, the SFG-generated turbulence at (y/T = 0.106, z/T = 0.125) away from the centerline allows equivalent mechanical characteristics of turbulence generation and decay, with peak of 1.9× nearer from grid. In short, PTFV provides a unique expression of the SFG-generated turbulence, of which, the equivalent turbulence length-scale and induced-forcing deduced could aid in deciphering the flow dynamics for effective turbulence management.

Ferroelectric and Ferroelastic Domain Related Formation and Influential Mechanisms of Vapor Deposited Piezoelectric Thin Films

Compared to the bulk piezoelectric materials counterpart, piezoelectric thin films (PTFs) possess advantages of smaller size, lower power consumption, better sensitivity, and have broad application in advanced micro-electro-mechanical system (MEMS) devices. However, the performance of MEMS transducers and actuators are largely limited by PTFs piezoelectric properties. In this review, we focus on understanding structure-property relationship of vapor deposited PTFs, with emphasis on the effect of strain energy and electrostatic energy in thin films, especially, energy relaxation induced misfit dislocation and ferroelectric (FS) and ferroelastic (FC) domain formation mechanisms. We then discuss the microstructure of these domains and their influential mechanisms on piezoelectric properties, as well as the domain engineering strategies (i.e., internal and external stimuli). This review will motivate further experimental, theoretical, and simulation studies on FS and FC domain engineering in PTFs.

Visualization of acoustic power flow in suspended thin-film lithium niobate phononic devices

We report direct visualization of gigahertz-frequency acoustic waves in lithium niobate phononic circuits. Primary propagation parameters, such as the power flow angle and propagation loss, are measured by transmission-mode microwave impedance microscopy. Using a fast Fourier transform, we can separately analyze forward and backward propagating waves and quantitatively evaluate the propagation loss. Our work provides insightful information on the propagation, diffraction, and attenuation in piezoelectric thin films, which is highly desirable for designing and optimizing phononic devices for microwave signal processing.

Molecular dynamics between amorphous and crystalline phases of e-beam irradiated piezoelectric PVDF thin films employing solid-state NMR spectroscopy

Piezoelectric bi-axially and uni-axially stretched PVDF films subjected to e-beam irradiation doses of 5 kGy, 500 kGy and 1 MGy and films not irradiated have been studied. It was found that the piezoelectric responses of both untreated and e-beam irradiated PVDF films under strain strongly depends on their initial manufacturing processing. The initial structural differences were mainly based on crystalline phase composition, notably the α to β polymorph ratio. 19F MAS Solid State NMR measurements performed with fast sample spinning (31.25 kHz) allow us to accurately determine the proportion of crystal and amorphous phases. Considering only the crystalline phases, it was concluded that the uni-axial ((β)-PVDF) film was almost composed of 100% of β-phase while the biaxially-oriented ((α-β)-PVDF) was composed of 81% β-phase and 19% of α-phase. It was also shown that the changes of local dynamic processes of PVDF chains in kHz timescale enable to reflect the degradation of both the crystalline and amorphous phases. Presence of small molecular-weight chemical compounds was evidenced by Carr–Purcell–Meiboom–Gill (CPMG) 1H MAS NMR experiment. The degradation process was in agreement with melting peak decrease registered by DSC. 19F/1H T1ρ relaxation times and analysis of cross-polarization (CP) buildup curves are discussed. Significant scatter of relaxation parameters (19F T1ρ and 1H T1ρ) suggested an important degradation of polymer structure. The 1H T1ρ values for PVDF samples irradiated at 1 MGy mainly imply chain scission mechanisms of degradation rather than cross-linking processes. SS NMR spectroscopy data supported assumption that e-beam irradiation brings flexibility to piezoelectric PVDF films which improved their piezoelectric voltage output generated by deformation up to doses of 500kGy. Above this dose range, degradations were very important and affected severely PVDF films mechanical properties.