Piezoelectric Sensitivity Research Articles

Flexible film-transducers based on polypropylene piezoelectrets: Fabrication, properties, and applications in wearable devices

In this work, piezoelectrets were prepared by the microstructure modification and corona charging of polypropylene (PP) foam sheets, and film-transducers based on such fabricated piezoelectrets were further developed. The electromechanical performances of the as-prepared transducers were investigated, including the complex dielectric spectra, frequency response, thermal stability, humidity sustainability, and fatigue resistance. The experimental results showed that the typical capacitance and impedance of the transducers were 43.7pF and 3.6MΩ at 1kHz, respectively. Piezoelectric sensitivity up to 600pC/g was achieved, but after annealing treatments at temperatures of 50, 70, and 90°C for 24h, only 54%, 30%, and 13% of the initial values were remained, respectively. The transducers had an appreciable sustainability in high-humid environments, and about 70% of the initial sensitivity was still retained in a humidity of 98% after 37days. In fatigue tests, the sensitivity decreased to 70% of the initial value after 1.5 million cycles of operation with a dynamic pressure of 2MPa that was applied perpendicularly onto the samples. The performance assessments demonstrated that the PP-piezoelectret film-transducers would have promising applications in wearable devices, which were also preliminarily exploited.

Enhancement in piezoelectric sensitivity via negative structural stiffness

Effective piezoelectric sensitivity of bimorph strip actuators was enhanced by negative structural stiffness. Negative stiffness was achieved in brass strips post-buckled in compression to an “S” shape; the brass strip was placed in series with the bimorph. The negative stiffness was tuned by adjusting the brass strip length. The effective piezoelectric sensitivity in units of displacement per volt input increased as the inverse of the overall stiffness of the series element. The maximum observed enhancement in effective piezoelectric sensitivity was at least a factor of 6 at 1 Hz in comparison with the value when no negative stiffness was used. This corresponds to a maximum effective sensitivity of about 36 µm/V in comparison with the baseline effective sensitivity of about 6.1 µm/V at 1 Hz. The value is several orders of magnitude (almost 40,000 times) higher than the typical value of sensitivity for the material comprising the individual bimorph strip actuator.

Sensitivity enhancement of piezoelectric force sensors by using multiple piezoelectric effects

The sensitivity enhancement of piezoelectric force sensors is investigated based on multiple piezoelectric effects in this paper. At first, theoretical analysis showed that multiple piezoelectric effects had an influence on the piezoelectric force sensors. Secondly, a practicable method was adopted to perform the experimental validation and quantify the influence of multiple piezoelectric effects. In the method, different capacitors whose capacitances became much larger than that of the piezoelectric quartz sensor were in parallel with the quartz sensor. With the method, the details of the sensitivity enhancement of piezoelectric sensors were obtained. Experimental results showed that the sensor sensitivity increased from 4.00 pC/N to 4.07 pC/N when the capacitance of parallel capacitors was increased from 500 to 1000 and then 5000 times of that of the sensor. The force sensor sensitivity was improved by 1.75%. However, the nonlinearity and repeatability error of the force sensor were markedly increased with increasing the capacitance of the parallel capacitor. The method demonstrated that the piezoelectric force sensor sensitivity could be enhanced to a certain extent in the practical engineering, but it also brought some negative effects. Finally, further analysis indicated that multiple piezoelectric effects influenced the sensitivity by changing the piezoelectric coefficient. The well-known piezoelectric coefficient d11 of the quartz could actually reach 2.35 pC/N.

Structural and Electrical Characterization of Solution-Processed Electrodes for Piezoelectric Polymer Film Sensors

Solution-processable graphene and carbon nanotube-based electrode materials were used here to provide electrodes on flexible piezoelectric polyvinylidenefluoride sensors. Piezoelectric sensitivity measurements, image-based analysis, adhesion tests, and sheet resistance measurements were applied to these printable sensors to rigorously analyze their performance and structure. The printable sensors showed electrical performance similar to metallized sensors, whereas the adhesion of the solution-processed materials to the substrate is not as high as that of the evaporated metal films. This also affects the measured sensor sensitivity values. The measurements based on optical images were found to be a promising method to capture detailed information about the electrode surface structure.

Implementation of an acoustic stall detection system using near-field DIY pressure sensors

In this paper, the use of DIY transducers is proposed to detect the pressure instabilities in a low-speed industrial axial fan. The authors aim is to detect rotating stall, a well-studied aerodynamic instability with a typical frequency that can be even lower than 10 Hz in low-speed industrial fans. Pressure transducers and piezoelectric sensors, such as microphones, in turbomachinery are used respectively in the near and far field as standard methods to perform time-resolved pressure measurements. Other classes of sensors, such as electret microphones, may be not suited for pressure measurements, especially in the ultrasound region because their cut-off frequency is about 20 Hz. In this study, the authors use a low-cost DIY technology, as alternative technology to stall detection, in comparison with a high precision piezoelectric sensor. The authors performed the pressure measurements using a dynamic transducer, a piezoresistive transducer, and a piezoelectric high sensitivity sensor that provides the measurement baseline. They implemented and set-up a measurement chain to identify the typical rotating stall pattern in low-speed axial fans. The results have been validated with respect to the state-of-the-art acoustic control techniques described in literature. The signals acquired using the two technologies are discussed using a combination of spectral and time-domain space reconstruction. The acoustic patterns obtained through the phase space reconstruction show that the DIY dynamic sensor is a good candidate solution for the rotating stall acoustic analysis.

Piezoelectric Sensitivity of a Layered Film of Chitosan and Cellulose Nanocrystals

Free-standing biodegradable films were fabricated by mixing cellulose nanocrystals and nanofibers with a water-based chitosan solution. The piezoelectric sensitivity of the films was measured to find out their suitability in sensing and actuation applications. Also their structure was evaluated with scanning electron microscopy. Our initial results show that a simple solvent casting method can be used to prepare films with varying piezoelectric properties. The composite films of chitosan and cellulose nanocrystals were noticed to have a phase-separated layered structure, resulting in an interesting side-dependent piezoelectric response. Moreover, the sensitivity values of plain chitosan were close to those of a non-degradable commercial sensor material polyvinylidene fluoride. The fabrication process has to be studied further to control and optimize the structure and piezoelectric properties of the films.

Quantitative measurements of electromechanical response with a combined optical beam and interferometric atomic force microscope

An ongoing challenge in atomic force microscope (AFM) experiments is the quantitative measurement of cantilever motion. The vast majority of AFMs use the optical beam deflection (OBD) method to infer the deflection of the cantilever. The OBD method is easy to implement, has impressive noise performance, and tends to be mechanically robust. However, it represents an indirect measurement of the cantilever displacement, since it is fundamentally an angular rather than a displacement measurement. Here, we demonstrate a metrological AFM that combines an OBD sensor with a laser Doppler vibrometer (LDV) to enable accurate measurements of the cantilever velocity and displacement. The OBD/LDV AFM allows a host of quantitative measurements to be performed, including in-situ measurements of cantilever oscillation modes in piezoresponse force microscopy. As an example application, we demonstrate how this instrument can be used for accurate quantification of piezoelectric sensitivity—a longstanding goal in the electromechanical community.

Temperature and Substrate Dependence of Piezoelectric Sensitivity for PVDF Films

The piezoelectric sensitivity, via both the direct and converse effects, for commercial polyvinylidene fluoride (PVDF) films is measured as a function of temperature and frequency, for two substrates, nylon and aluminum. The average effective sensitivity for the PVDF on nylon was 29 pm/V for both direct and converse effect, independent of frequency over 0.5 to 200 Hz. Direct effect sensitivity on aluminum substrate was about a factor of five greater. Analysis of effects of substrate's thermal and elastic constraint disclosed insufficient effect to account for the observed increase of sensitivity. Flexoelectric effects were considered as the cause. The direct and converse sensitivity increased at approximately 2% per degree Celsius over the frequency range 0.5 to 200 Hz.

Piezoelectric Lead Zirconium Titanate Composite Touch Sensors for Integration with Flexible OLED Technology

To enable the design of more intuitive product user interfaces, the prospects of matching piezoelectric touch sensors with flexible organic light emitting diode (OLED) technology are investigated. Low stiffness piezoelectric composite sensors, combining piezoelectric Pb(Zr,Ti)O3 powder with a polymer matrix, are prepared and compared to existing ceramic and polymer sensors. The piezoelectric sensitivity per stiffness of these piezoelectric composites can be tailored by adapting the polymer matrix to the stiffness required by the OLED, while retaining a relatively high sensitivity. Concurrently, the relatively low energy output of these flexible piezoelectric sensors can be increased through optimized sensor design.

Phase Transitions in Smectic Bent-Core Main-Chain Polymer Networks Detected by Dielectric and Ultrasonic Techniques

The dielectric and ultrasonic properties of a bent-core main-chain liquid-crystalline polymer network have been investigated. Dielectric measurements on this material showed two maxima in the real and imaginary components of the dielectric permittivity at 365 K and 450 K along with a temperature hysteresis showing the first order nature of the two phase transitions. Longitudinal ultrasonic attenuation and ultrasonic velocity measurements at 10 MHz frequency revealed some anomalies at the phase transition temperatures respect to the dielectric results. Above room temperature, a large ultrasonic attenuation was observed which can be related to the glass transition of this sample. Moreover, the piezoelectric sensitivity of this bent-core main-chain liquid-crystalline polymer network has also been studied.

High-performance 1–3-type lead-free piezo-composites with auxetic polyethylene matrices

High piezoelectric sensitivity and large hydrostatic parameters are discussed for two 1–3-type single crystal/auxetic polymer composites. The single crystal components selected are based on lead-free (K,Na)(Nb,Ta)O3 and (Li,K,Na)(Nb,Ta)O3 solid solutions, and the auxetic polymers are characterised by Poisson’s ratios –0.83≤ ν ≤–0.29. The composites examined exhibit advantageous properties over conventional lead-based piezo-active composites and ceramics such as high longitudinal piezoelectric coefficient g33⁎~(102–103) mV·m/N, squared figure of merit (Q33⁎)2~10–11 Pa−1, hydrostatic piezoelectric coefficient gh⁎~(102–103) mV·m/N and squared figure of merit (Qh⁎)2~(10–11–10−10) Pa−1. The composites also demonstrate an infinitely large anisotropy of piezoelectric coefficients d3j⁎.

Features of the Piezoelectric Effect in a Novel PZT-Type Ceramic/Clay Composite

Piezoelectric properties, their anisotropy and sound velocity are first studied in the composite based on ferroelectric ZTS-19 ceramic of the PZT type, and the second component of the composite is piezo-passive clay. The composite as a whole is characterised by a specific porosity and elements of various connectivity patterns. The important role of microgeometric factors in the formation of piezoelectric coefficients d*3j (j = 1 and 3) and their anisotropy ζd3j = d*33/d*31 is discussed in terms of connectivity patterns at the volume fraction of clay 0.01 ≤ mcl ≤ 0.35. The role of the clay component and clay-containing thin layers in the formation of the piezoelectric activity and sensitivity is interpreted within the framework of models of the two- and three-component composites based on poled ferroelectric ceramics. Results of modelling are compared to experimental data on the studied composite.

Effect of dielectrophoretic structuring on piezoelectric and pyroelectric properties of lead titanate-epoxy composites

Functional granular composites of lead titanate particles in an epoxy matrix prepared by dielectrophoresis show enhanced dielectric, piezoelectric and pyroelectric properties compared to 0–3 composites for different ceramic volume content from 10% to 50%. Two structuring parameters, the interparticle distance and the percentage of 1–3 connectivity are used based on the Bowen model and the mixed connectivity model respectively. The degree of structuring calculated according to both models correlate well with the increase in piezoelectric and pyroelectric sensitivities of the composites. Higher sensitivity of the electroactive properties are observed at higher ceramic volume fractions. The effect of electrical conductivity of the matrix on the pyroelectric responsivity of the composites has been demonstrated to be a key parameter in governing the pyroelectric properties of the composites.

Rare-Earth Calcium Oxyborate Piezoelectric Crystals ReCa4O(BO3)3: Growth and Piezoelectric Characterizations

Rare-earth calcium oxyborate crystals, ReCa4O(BO3)3 (ReCOB, Re = Er, Y, Gd, Sm, Nd, Pr, and La ), are potential piezoelectric materials for ultrahigh temperature sensor applications, due to their high electrical resistivity at elevated temperature, high piezoelectric sensitivity and temperature stability. In this paper, different techniques for ReCOB single-crystal growth are introduced, including the Bridgman and Czochralski pulling methods. Crystal orientations and the relationships between the crystallographic and physical axes of the monoclinic ReCOB crystals are discussed. The procedures for dielectric, elastic, electromechanical and piezoelectric property characterization, taking advantage of the impedance method, are presented. In addition, the maximum piezoelectric coefficients for different piezoelectric vibration modes are explored, and the optimized crystal cuts free of piezoelectric cross-talk are obtained by rotation calculations.

Low-frequency flexible piezoelectric nanogenerators based on ZnO nanorods grown on Cu wires

A self-powering nanosystem that harvests its operating energy from the environment is an attractive proposition for sensing and personal electronics. In this paper, we present a simple, low-cost approach to convert low-frequency mechanical energy into electric power using piezoelectric ZnO nanorods grown on common Cu wires. This energy conversion device has ultrahigh flexibility and piezoelectric sensitivity and can produce an output current of about 50 nA, which can last for almost 5 seconds. Furthermore, the I–V curves of the device under different strains are also examined, and the I–V characteristic is highly sensitive to strain. This energy-harvesting technology provides a simple and cost-effective platform to capture low-frequency mechanical energy, such as body movements, for practical applications.

Influence of the connectivity pattern, the nature of the piezoelectric material, and rod thickness on the properties of 50 vol % PZT/50 vol % NiCo0.02Cu0.02Mn0.1Fe1.8O4 − δ magnetoelectric composites

We have studied the influence of the connectivity pattern, the nature of the piezoelectric material, and rod thickness on the properties of magnetoelectric composites with the composition 50 vol % PZT/50 vol % NiCo0.02Cu0.02Mn0.1Fe1.8O4 − δ (where PZT stands for various commercially available lead zirconate titanate based piezoelectric materials). The results indicate that the magnetoelectric conversion efficiency of the rod composites with any connectivity pattern (1-1, 1-3, or 3-1) is 10–20% higher than that of laminate composites. The largest magnetoelectric conversion coefficient ΔE/ΔH is offered by the 1-1 connectivity composites. The ΔE/ΔH of the composites correlates with both their piezoelectric sensitivity g 33 and the piezoelectric sensitivity of the corresponding piezoelectric materials. ΔE/ΔH is shown to be inversely proportional to the thickness of the piezoelectric and ferrite rods. The highest ΔE/ΔH at a frequency of 1 kHz, 450 mV/(cm Oe), has been reached in the PZT-36 based composites with 1-1 connectivity and 0.3-mm-thick rods.

Microgeometry, piezoelectric sensitivity and anisotropy of properties in porous materials based on Pb(Zr, Ti)O3

Relations between the microgeometry, piezoelectric sensitivity and piezoelectric anisotropy are studied in porous composite materials based on Pb ( Zr , Ti ) O 3 ceramics. Systems of micrographs of cuts being parallel and perpendicular to the polarization direction are first shown for the porosity range v p = 0.05 - 0.57, and examples of microcracking at v p = 0.57 are considered. Experimental data on longitudinal and hydrostatic piezoelectric coefficients and squared figures of merit are represented as functions of v p , and nonmonotonic behavior of the piezoelectric anisotropy factor with increasing v p is analyzed in the context of changes in the microgeometry. Experimental results are compared to those derived from approximation formulae for the related porous ceramic.

Auxetic Materials and Related Systems

to mechanical metamaterials and related systems with particular emphasis on auxetic systems. Such materials show counter-intuitive, negative behaviours. For example, auxetic materials exhibit negative Poisson’s ratios which mean that these systems get fatter rather than thinner when stretched. Due to the superior properties exhibited by these systems, their study and characterisation has experienced a signifi cant increase, particularly in the last decade, something which may be perceived from the always increasing number of publications and patents in these areas. In fact, this issue is the seventh in a series of special issues published in physica status solidi (b) dedicated to auxetic materials and related systems (see [1–6]).

A sensitive piezoelectric composite lattice: Experiment

Lattice structures based on bimorph rib elements are fabricated and studied experimentally. The effective piezoelectric sensitivity d is observed to be much larger, by a factor of at least 10 000, in magnitude than that of material comprising the lattice ribs. Bending of the ribs in response to input voltage is responsible for the large sensitivity.

Piezoelectric composite lattices with high sensitivity

Lattice structures are presented with a piezoelectric sensitivity, d, much larger in magnitude than that of material comprising the lattice ribs. Large sensitivity is achieved by using piezoelectric bimorph elements as ribs; these bend in response to electric input, giving rise to a much larger displacement than for axial elements. Complex electrical connectivity is helpful, but not necessary; surface contact can be sufficient. Lattices are amenable to piezoelectric vibration damping. Piezoelectric sensitivity can be arbitrarily large and is unbounded.