Conventional Piezoelectric Transducers Research Articles

Radiation Impedance of Rectangular CMUTs.

Recently, capacitive micromachined ultrasound transducers (CMUTs) with long rectangular membranes have demonstrated performance advantages over conventional piezoelectric transducers; however, modeling these CMUT geometries has been limited to computationally burdensome numerical methods. Improved fast modeling methods, such as equivalent circuit models, could help achieve designs with even better performance. The primary obstacle in developing such methods is the lack of tractable methods for computing the radiation impedance of clamped rectangular radiators. This paper presents a method that approximates the velocity profile using a polynomial shape model to rapidly and accurately estimate radiation impedance. The validity of the approximate velocity profile and corresponding radiation impedance calculation was assessed using finite element simulations for a variety of membrane aspect ratios and bias voltages. Our method was evaluated for rectangular radiators with width:length ratios from 1:1 up to 1:25. At all aspect ratios, the radiation resistance was closely modeled. However, when calculating the radiation reactance, our initial approach was only accurate for low aspect ratios. This motivated us to consider an alternative shape model for high aspect ratios, which was more accurate when compared with FEM. To facilitate the development of future rectangular CMUTs, we provide a MATLAB script that quickly calculates radiation impedance using both methods.

Monitoring of cementitious materials hydration using 2–2 cement-based piezoelectric transducers based on electromechanical impedance method

Monitoring cementitious materials hydration enhances construction efficiency by providing crucial information regarding the optimal timing for shoring removal and pre-stress transferring. In recent years, many scholars have utilized the lead zirconate titanate (PZT)-based electromechanical impedance method (EMI) to monitor the cementitious materials hydration process. However, conventional piezoelectric transducers are less compatible with cementitious materials. In addition, the currently commonly used statistical-based metrics lack a clear physical meaning. The 2–2 cement-based transducer (2−2), composed of multiple PZT elements, demonstrates excellent electromechanical performance and good compatibility with cementitious materials. In this study, the 2–2 was employed to monitor the cementitious materials hydration, and an electromechanical impedance model for describing the interaction between the 2–2 and the structure was presented. Taking into account both resonance frequency and peak value of the conductance signatures, an objective function was defined. Genetic algorithm (GA) was employed to minimize the disparity between theoretical and experimental results, thus extracting equivalent stiffness and damping. The empirical relationship between equivalent stiffness, damping, and compressive strength was established to predict the compressive strength. The experimental results show that for the same monitoring process, the normalized equivalent stiffness and damping extracted from different 2–2 sensors are in some agreement. It is more sensitive to hydration processes than statistical indicators. More importantly, the equivalent parameters have a clear physical meaning and can better portray the hydration process of the cementitious materials.

A Novel Nondestructive Testing Probe Using AlN-Based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs).

Ultrasonic nondestructive testing (NDT) usually utilizes conventional bulk piezoelectric transducers as transceivers. However, the complicated preparation and assembly process of bulk piezoelectric ceramics limits the development of NDT probes toward miniaturization and high frequency. In this paper, a 4.4 mm × 4.4 mm aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) array is designed, fabricated, characterized, and packaged for ultrasonic pulse-echo NDT of solids for the first time. The PMUT array is prepared based on the cavity silicon-on-insulator (CSOI) process and packaged using polyurethane (PU) material with acoustic properties similar to water. The fabricated PMUT array resonates at 2.183 MHz in air and at around 1.25 MHz after PU encapsulation. The bandwidth of the packaged PMUT receiver (244 kHz) is wider than that of a bulk piezoelectric transducer (179 kHz), which is good for axis resolution improvement. In this work, a hybrid ultrasonic NDT probe is designed using two packaged PMUT receivers and one 1.25 MHz bulk transmitter. The bulk transmitter radiates an ultrasonic wave into the sample, and the defect echo is received by two PMUT receivers. The 2D position of the defect could be figured out by time-of-flight (TOF) difference, and a 30 mm × 65 mm detection area is acquired. This work demonstrates the feasibility of applying AlN PMUTs to ultrasonic NDT of solids and paves the way toward a miniaturized NDT probe using AlN PMUT technology.

Research on low-frequency bender disk transducer driven by multiple relaxor ferroelectric single crystal disks

In this paper, we investigate the low-frequency bender disk transducer designed by the relaxor ferroelectric single crystal. First, we introduce the principle of the bender disk transducer, and use multiple single crystal disks as driving materials to excite the bending vibration mode of the metal plate, which reduces the operating frequency of the transducer. Then we establish the three-dimensional finite element model of the bender disk transducer driven by multiple single crystal disks, analyze the influence of the number and position distribution of single crystal disks on the transducer performance, and compare with that of the piezoelectric ceramic transducer with the same structure. Finally, we manufacture the proposed relaxor ferroelectric single crystal, piezoelectric ceramic transducers and the conventional piezoelectric ceramic bender disk transducer respectively, and carry out the experimental measurement in an anechoic water tank. The experimental results show that for the proposed single crystal transducer, its measured maximum transmitting voltage response (TVR) is 126.2 dB at 1.1 kHz, which is 13.4 dB higher than that of the proposed piezoelectric ceramic transducer, and its frequency corresponding to the maximum TVR is reduced by 700 Hz compared with the conventional piezoelectric ceramic bender disk transducer.

Laser-plasma sound sources in atmospheric air: A systematic experimental study

Laser-plasma sound sources are increasingly utilized in technological and scientific applications as attractive alternatives to conventional electromechanical or piezoelectric transducers. This work presents a systematic experimental study of laser-plasma sound sources generated in atmospheric air by strong, short and ultrashort laser pulses. Emphasis is given on the dependence of their acoustic emission on the characteristics of the optical radiation. In the experiments, nanosecond and femtosecond laser pulses with different energies and optical wavelengths are focused in atmospheric air using lenses of different focusing power. The produced acoustic pulses are captured via a system with high dynamic range and broad frequency response in the far field of the plasma source. The acoustic signals are analysed both in the time and frequency domains to extract their primary characteristics, particularly the total acoustic energy, peak pressure, spectral magnitude and spectral peak frequency. The results are systematically examined and compared with respect to the characteristics of the exciting laser pulses. The experimental observations are supported by theoretical calculations of the generated plasma density via rate equations that account for the main ionization and electron loss mechanisms. The presented analysis provides a frame for controlling the acoustic characteristics of laser-plasma sound sources, which is particularly important for applications in the fields of acoustic measurements, sound reproduction and biomedicine.

Time reversal of long coda waves: Experiments in the nonlinear regime.

A problem that arises when the time-reversal process is applied in a nonlinear regime is related to the generation of harmonics: conventional piezoelectric transducers cannot work properly at the frequency of the second harmonic. Then, the time-reversed wave cannot be completely emitted. Few works provide a solution to this issue. Thus, we study the alternative of performing a cross correlation of the wavefield. In a linear regime, this procedure is an accurate method for estimating real time-reversal properties. To study both procedures in the nonlinear regime in detail, we measure the wavefield of a wave that (1) traverses a multiple scattering medium, composed by a random set of parallel copper rods and (2) propagates inside a reverberant cavity, consisting of an aluminum case immersed in water. Cross correlation yields a virtually focused wavefield, where the focal width at the frequency of the first, second, and third harmonics can be measured. We compare these values with those obtained in a real time-reversal experiment. Results suggest that both time-reversal procedures are equivalent. In addition, we discuss the possibility of amplitude estimation at the focal spot and the limits of this work based on a theoretical model.

Development of a linear array electromagnetic acoustic transducer for shear horizontal guided wave inspection

Ultrasonic guided wave screenings have proven to be fast and reliable in detecting various types of defects in plate-like structures. Today, low frequency ultrasonic guided waves are routinely used to screen long sections of pipelines. For many years, ultrasonic guided waves at frequencies beyond the cutoff of the first high-order mode attracted interest in the research community due to the plurality of modes that can propagate. During operation at a frequency beyond the cutoff of the first high-order mode, the transduction mechanism becomes crucial for selectively exciting and detecting a single high-order mode or a group of high-order modes. Ultrasonic comb transducers allow to selectively excite and detect high-order ultrasonic guided waves at a desired wavelength. Linear array transducers are even more flexible, and allow virtually full control in the frequency wavenumber space. High-order shear horizontal (SH) modes have multiple potential applications including, for example, remote thickness gauging and crack monitoring. However, these modes are notoriously difficult to excite and detect using conventional piezoelectric transducers. Electromagnetic acoustic transducers (EMAT) make the excitation and detection of SH modes relatively simple. Periodic permanent magnet (PPM) EMAT can be used to selectively excite and detect high-order SH modes based on a desired wavelength. However, in some applications, it may be necessary to excite and detect high-order SH modes with more control in the frequency wavenumber space. In this paper, a novel EMAT linear array is proposed using a configuration similar to a PPM EMAT but with individual coils wrapped around the magnets. The performance of the EMAT linear array in transmission and reception is demonstrated in simulations and experimentally on a 9.53 mm steel plate. The results show a successful transmission and detection of SH0 to SH4 for a 300 to 800 kHz frequency range.

Fatigue state characterisation of steel pipes using ultrasonic shear waves.

The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well studied in the literature. Additionally, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study utilises these phenomena to develop a method able to characterise the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in Electromagnetic Acoustic Transducers (EMAT) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.

MECHANIZED ULTRASONIC INSPECTION OF TIEIN WELDS OF THE MAIN GAS PIPELINE WITH AN AUGUR-ART FLAW DETECTOR

To perform ultrasonic testing of golden joints of the main gas pipeline, a procedure for mechanized ultrasonic testing was developed. The procedure complied with the requirements of EN ISO 13588:2019 inspection level C and EN ISO 10863:2020 inspection level C. Testing for transverse defects was carried out using conventional piezoelectric transducers using the echo-mirror method and frontal SAFT processing. The use of the AUGUR-ART flaw detector, the SPIDER scanner and the ECHO-scan and AUGUR-Analysis software demonstrated stable detection of defect simulators in the reference block. All this made it possible to analyze the shortcomings of welding at training joints and successfully perform golden joints of the main gas pipeline.

Phase-shift digital holography using multilayer ceramic capacitor actuators

In this paper, a four-step phase-shift digital holography (DH) system using multilayer ceramic capacitors (MLCCs) as actuators is proposed. This system is capable of implementing DH with low costs and small-form factors. As a material for actuators, MLCCs are more economical and easier to construct than conventional piezoelectric transducers. By applying a voltage to a single MLCC and measuring the displacement and standard deviation based on the shape change observed on the top surface of the MLCC with laser Doppler velocimetry, the applicability of the MLCC in terms of suitable driving characteristics for use as an actuator in the DH system was confirmed. As the accurate phase step actuation required to employ the MLCC actuator for DH cannot be performed, a four-step hologram extraction method was employed using the intensity information of the hologram obtained by continuously driving the MLCC actuator. Consequently, the proposed MLCC actuator was applied to a typical Michelson interferometer setup to measure a 1.8 µm step sample, and the quantitative phase information was obtained with a standard deviation of less than 1.3 nm.

Beam-Membrane Coupled Piezoelectric Micromachined Ultrasonic Transducers with Enhanced Transmitting Sensitivity.

Piezoelectric micromachined ultrasonic transducers (PMUTs) are a promising alternative to conventional bulk piezoelectric ceramic-based ultrasonic transducers. However, the transmitting sensitivity of the reported PMUTs is far from satisfactory. In this paper, we report a beam-membrane coupled PMUT (BM-PMUT), which enhances the transmitting sensitivity via simultaneously increasing the acoustic emission areas and maintaining the comparable vibration amplitude. Experimental results show that the center and edge transmitting sensitivities of the BM-PMUT are 108.1 and 96 nm/V at 370 kHz, which are 109.9 and 49.6 nm/V at 677 kHz for the traditional PMUT (T-PMUT). Thus, the BM-PMUT realizes piston-like mode shapes and achieves around twofold improvement in the effective acoustic emission area relative to the traditional T-PMUT of the same size. Due to the larger acoustic emission areas and comparable vibration amplitudes, the normalized far-field sound pressure level of the BM-PMUT is 8.5 dB higher than that of the T-PMUT.

Demodulation of Fiber Bragg Grating Accelerometer Using In-Line Sagnac Interferometers

We proposed a fiber Bragg grating (FBG) accelerometer demodulation method based an in-line Sagnac interferometric structure. In this method, an electro-optical phase modulator (EOM) is used as a phase-generating carrier (PGC) modulation module. The principle of in-line Sagnac optical path interference based on EOM is theoretically analyzed by Jones matrix, and the modulation mechanism of EOM is described. The experimental results of an FBG accelerometer based on flexible hinge show that the system background noise is <inline-formula> <tex-math notation="LaTeX">$1 \times {10^{ - 5}}\,\,{{\mathrm {rad}}} \cdot {{\mathrm {Hz}}^{-1 /2}}$ </tex-math></inline-formula>. The EOM based modulation scheme solves the problem of distortion of demodulation response due to the unstable modulation performance of the conventional piezoelectric transducer, which also provides a fast response to the interferometric optical path changes. The sensitivity of the in-line Sagnac based demodulation scheme is doubled compared to the Sagnac ring structure. The proposed scheme provides an integrated phase modulator for demodulation system for FBG sensors.

Coupling Effects of Crosstalk and Parasitic Loss on Capacitive Micromachined Ultrasonic Transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have been widely utilized in applications of biomolecule monitoring, ultrasound medical diagnosis, and ultrasound therapy because of their advantages in high-level integration with integrated circuits, good impedance matching for acoustic transmission, and flexible element and array configuration compared with conventional piezoelectric ceramic transducers. Crosstalk effects on dynamics of a CMUT element, <i>i.e.</i> electrical, mechanical, and acoustic performance, would lead to unexpected spurious resonances. In this study, the mutual interaction from fluid and nearby cells in a CMUT element is newly investigated specially aiming to characterize their coupling effects with series and parallel parasitic effects based on a nonlinear Equivalent Circuit Model (ECM). The mutual interaction is evaluated with the electromechanical and acoustic characteristics by comparison between ECM and 3D Finite Element Analysis (FEA). Two representative CMUT element configurations, that are the rectangular with nine cells and the hexagonal with seven cells, are analyzed with different center-to-center distance. The radii of cells are set to be <inline-formula> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula>/8 with different edge-to-edge distances. The parasitic effects are modeled by a series parasitic factor and a parallel-capacitance parasitic ratio. The coupling influence of crosstalk and parasitic effects on the performance of a CMUT element is evaluated, such as the impedance, phase, conductance, velocity, and ultrasound pressure field. The results of ECM-FEA-based methods show good agreements, and the improved ECM can be used for co-parasitic extraction with front-end interface.

A backward-mode optical-resolution photoacoustic microscope for 3D imaging using a planar Fabry-Pérot sensor

Optical-resolution photoacoustic microscopy (OR-PAM) combines high spatial resolution and strong absorption-based contrast in tissue, which has enabled structural and spectroscopic imaging of endogenous chromophores, primarily hemoglobin. Conventional piezoelectric ultrasound transducers are typically placed far away from the photoacoustic source due to their opacity, which reduces acoustic sensitivity. Optical ultrasound sensors are an alternative as their transparency allows them to be positioned close to the sample with minimal source-detector distances. In this work, a backward-mode OR-PAM system based on a planar Fabry-Pérot ultrasound sensor and coaxially aligned excitation and interrogation beams was developed. Two 3D imaging modes, using raster-scanning for enhanced image quality and continuous-scanning for fast imaging, were implemented and tested on a leaf skeleton phantom. In fast imaging mode, a scan-rate of 100,000 A-lines/s was achieved. 3D images of a zebrafish embryo were acquired in vivo in raster-scanning mode. The transparency of the FP sensor in the visible and near-infrared wavelength region makes it suitable for combined functional and molecular imaging applications using OR-PAM and multi-photon fluorescence microscopy.

Investigation of biogenic materials and ferroelectrets for energy harvesting on vibrating aircraft structures

In this publication the application of novel piezoelectric materials for energy harvesting on vibrating aircraft structures is investigated. These materials have significant advantages over conventional piezoelectric transducer materials like piezoceramics. In particular, biogenic materials in the form of wood-based materials and ferroelectrets in the form of irradiation cross-linked polypropylene are the subject of the investigation. The material characterization in terms of mechanical and electromechanical properties is shown for both material types. For the wood materials a compression test is used as the material has load-bearing properties. The ferroelectrets provide high compliances and are therefore investigated in a tensile test for material characterisation as well as in a four-point flexural test regarding its behaviour when glued to a dynamically bending surface. Additionally an FE-model of the material model for ferroelectrets is presented, which is validated by experimental results. An estimation of the power output is given for different concepts with both kinds of materials.

Compact Laser Driver for Ultrasonic Arbitrary Position and Width Pulse Sequences Generation

Laser ultrasound offers several benefits in comparison to immersion or air-coupled ultrasound: it directly generates stress on the sample surface; it has a wide bandwidth and a small acoustic source can be produced. Conventionally, high-power pulsed lasers are required therefore equipment is bulky and expensive. Here we propose to use mid-power laser diodes, which are small and low cost. To solve the low signal amplitude problem with lower power lasers, the use of spread spectrum signal, arbitrary position, and width binary pulse sets is proposed. To date, a laser driver for this task has not been available. This article describes the development of such a compact driver, where the essential electronics occupy an area of $10\,\,\times20$ mm. The whole system with a fixture for kinematic mount is comparable in size to a conventional piezoelectric ultrasonic transducer. The driver can supply pulse sets of up to 40 A. Individual pulse duration can vary between 20 and 1000 ns (0.5–25-MHz ultrasound range) and the total pulse train duration is limited by the laser type used. Experiments show that up to 10- $\mu \text{s}$ long pulse sets can be used at 10-A current, without laser degradation. Current waveforms, beam profile, and optical response signals were measured for three rectified topologies. It was concluded that the GaN-based constant current switch topology has the best performance, but a power MOSFET in a source current feedback topology can also be used to generate pulses down to 20 ns. Photoacoustic response signals from chirp and phase shift keying modulation have been demonstrated.

Toward an ultra-high resolution phased-array system for 3D ultrasonic imaging of solids

Ultrasonic phased-array (PA) systems have been widely adopted in the field of nondestructive evaluation for material characterization and imaging of internal defects. Whereas many defects exhibit complex three-dimensional structures, most PA systems provide only two-dimensional images. In this Letter, we demonstrate the ability to create high-resolution 3D images of internal defects using a PA system based on a piezoelectric and laser ultrasonic system (PLUS). The PLUS combines a piezoelectric transmitter to insonify the structure to be inspected with a laser Doppler vibrometer to create a matrix array of receiver points without contact. The small size of the laser beam results in an ultra-multiple number of elements on the order of thousands, which is impossible to achieve with a conventional piezoelectric matrix array transducer. An emission from a piezoelectric transmitter compensates for the intrinsically low sensitivity of a laser Doppler vibrometer. After formulating the 3D imaging algorithm of the PLUS, we demonstrate that the PLUS with 4096 receiving points (i.e., 64 × 64 points) achieves high-resolution 3D imaging in a specimen with a flat bottom hole. We also visualize the complex structure of stress corrosion cracking. We believe that the 3D imaging capability of the PLUS may open up new avenues to the accurate evaluation of material strength, the identification of the types of defects, and the elucidation of the mechanisms of defect initiation.

Multi-Frequency Piezoelectric Micromachined Ultrasonic Transducers

In this paper multi-frequency piezoelectric MEMS ultrasonic transducers (pMUTs) are designed, characterized and tested for nondestructive evaluation (NDE) of solids. The transducers operate in flexural mode, and are tuned to three different frequencies namely 1 MHz, 1.5 MHz and 2 MHz. The microstructural layers consist of aluminum nitride (AlN) as an active sensing layer sandwiched between metal and doped silicon electrodes. pMUTs are designed with octagonal and circular membranes. The vibration of silicon membrane assists piezoelectric element to convert energies. The transducers are modeled numerically to obtain their dynamic characteristics. Piezoelectric Multi-User MEMS Processes (PiezoMUMPs) are utilized to manufacture pMUTs. The electromechanical characterization shows that the circular design has higher figure of merit as compared to the octagonal design due to more uniform stress distribution transferred between silicon and AlN layers. It is demonstrated that the piezeoelectric layer should be deposited up to the inflection point of diaphragm deformation. This avoids the signal cancellation due to opposite polarization. The performance of pMUTs as receiver is evaluated to detect the creep damage by implementing nonlinear ultrasonic testing (NLUT). NLUT is based on detecting higher harmonics in solids due to heterogeneity in materials. Significant amplification in the second harmonics is obtained due to highly narrowband and low damping characteristics of pMUTs that improves the resolution of NLUT to detect subwavelength damage. Higher harmonics can be detected using small footprint pMUT device, which is not possible with conventional piezoelectric transducers. This allows better spatial resolution of nonlinear measurement.

Response Bandwidth Design of Fabry-Perot Sensors for Partial Discharge Detection Based on Frequency Analysis

The insulation of power equipment can be effectively assessed by analyzing the acoustic signals originated from partial discharges (PD). Fabry-Perot (F-P) sensors are capable of detecting PD acoustic signals. Although the frequency bandwidth of an F-P sensor is mainly referred to conventional piezoelectric transducer (PZT) sensor, it is still doubtful to identify a suitable bandwidth for fiber sensors in detection of PD signals. To achieve a suitable bandwidth for an F-P sensor, the frequency distribution of PD acoustic emission is investigated, and an extrinsic F-P sensor is designed to detect acoustic signals generated from PD. F-P sensors with different intrinsic frequencies are fabricated as possible design standards of bandwidth for acoustic detection. PD acoustic signals are detected by these F-P sensors and PZT sensors in the experimental system, in which four typical electrode models are employed. The measured results of frequency performance are analyzed in linear and semilogarithmic coordinates. The results show that F-P sensors can effectively detect PD acoustic emissions in both wideband and narrowband modes. Moreover, F-P sensors achieve a higher sensitivity in the narrowband mode. We propose that intrinsic frequency of the F-P sensor should be designed in the frequency range of 100–170 kHz to obtain maximum sensitivity.

SH guided wave excitation by an apparent face-shear mode (d36) piezocomposite transducer: experiments and theory

The fundamental shear horizontal (SH0) wave in plate-like structures is of practical importance in structural health monitoring (SHM) due to its non-dispersive characteristics. However, compared with Lamb waves, SH0 wave is less used in practice since it is difficult to be excited by using conventional piezoelectric transducers. In this work, an apparent face-shear mode (d36) piezocomposite transducer was proposed to excite and receive SH0 wave. The piezocomposite transducer was made up of epoxies and several hollow PZT squares of different sizes. The face-shear d36 mode was synthetized by the thickness-shear deformation of each side of the hollow PZT squares. Both finite element simulations and experimental testings showed that the apparent d36 piezocomposite transducer was capable of exciting pure SH0 wave along four main directions (0°, 90°, 180° and 270°) from 70 to 150 kHz. Moreover, the piezocomposite transducer can filter Lamb waves and receive SH0 wave only over a wide frequency range from 70 to 370 kHz. Based on Huygens’ principle and the method of solving inclusion problem, an eigenstrain-based theoretical model was then proposed to describe the SH0 wave field generated by face-shear transducers. Explicit analytical expressions for the piezocomposite transducer were successfully obtained and their accuracies were verified by both experiments and finite element simulations. Furthermore, some general guidelines based on the analytical model were presented for optimization of the transducer’s dimensions. The proposed apparent d36 piezocomposite transducer may be a good candidate for exciting and receiving low frequency SH0 wave in SHM systems, since it is lightweight and shows better surface conformability than PZT wafers.