Piezo Composites Research Articles

Enhancing Manufacturability of SU-8 Piezoelectric Composite Films for Microsystem Applications.

Piezoelectric thin films are extensively used as sensing or actuating layers in various micro-electromechanical systems (MEMS) applications. However, most piezoelectrics are stiff ceramics, and current polymer piezoelectrics are not compatible with microfabrication due to their low Curie Temperature. Recent polymer-composite piezoelectrics have gained interest but can be difficult to pattern. Photodefinable piezoelectric films could resolve these challenges by reducing the manufacturability steps by eliminating the etching process. But they typically have poor resolution and thickness properties. This study explores methods of enhancing the manufacturability of piezoelectric composite films by optimizing the process parameters and synthesis of SU-8 piezo-composite materials. Piezoelectric ceramic powders (barium titanate (BTO) and lead zirconate titanate (PZT)) were integrated into SU-8, a negative epoxy-based photoresist, to produce high-resolution composites in a non-cleanroom environment. I-line (365 nm) light was used to enhance resolution compared to broadband lithography. Two variations of SU-8 were prepared by thinning down SU-8 3050 and SU-8 3005. Different weight percentages of the piezoelectric powders were investigated: 5, 10, 15 and 20 wt.% along with varied photolithography processing parameters. The composites' transmittance properties were characterized using UV-Vis spectroscopy and the films' crystallinity was determined using X-ray diffraction (XRD). The 0-3 SU-8/piezo composites demonstrated resolutions < 2 μm while maintaining bulk piezoelectric coefficients d33 > 5 pm V-1. The films were developed with thicknesses >10 μm. Stacked layers were achieved and demonstrated significantly higher d33 properties.

Improved corona discharge-based modified poling method for 0–3 PZT/PEGDA piezoelectric composites

Among piezo composites, 0–3 type ceramic/polymer-based composites have balanced properties and flexible manufacturing processes, allowing them to tackle challenges in wearable electronics and sensing technology. However, due to a misfit between the dielectric coefficients of the ceramic particles and polymer matrix, poling 0–3 composites is difficult. Therefore, in this study, we develop a novel poling method that combines the merits of normal corona poling and oil-based poling methods to activate a lead zirconium titanate (PZT)/Polyethylene glycol diacrylate (PEGDA) composite. The influence of various parameters (voltage, temperature, poling duration, and time of ageing) on the piezoelectric and dielectric properties of the PZT/polymer composite were evaluated. For the 0–3 PZT/PEGDA composites (67 wt%), the optimal poling parameters were found to be 60 min poling duration, 25 °C, and 24 kV. The d33 (32 pC/N) obtained by the proposed poling method is about 3 times that obtained by the normal corona poling method (11 pC/N). This modified poling method was shown to be more effective than the conventional method for a range of filler contents (50 wt%, 67 wt%, and 75 wt%). It was also found that the d33 value of the PZT/PEGDA composite increased over time and was attributed to the increase of the composite dielectric constant after poling. Finally, sensing elements were assembled from the poled samples and evaluated, which shows the potential for detecting tiny signals in sensing and soft robotics.

Generalized homogenization model of piezoelectric materials for ultrasonic transducer applications

Although piezocomposite (PC) materials have increasingly attracted researchers, there is still a need to properly and easily derive their properties. We develop a generalized homogenization model (GHM) that accounts for Smith and Cha approaches to evaluate the equivalent characteristics of piezocomposites. This method could be applied to all connectivities patterns, but restricted herein to 2-2 and 1-3 piezocomposites for comparison with Smith (1-3) and Cha (2-2) analytical results. In the proposed GHM is a parameter θ, is changed for various connectivities. The 1-3 and 2-2 PZT-7A/Araldite D (PCs) data are used and equivalent characteristics of these Pcs are determined as function of volume fraction of PZT-7A piezoelectric. Results show that the electromechanical coefficients are well fitted by Voigt and Reuss models. Results obtained for some parameters show that the proposed GHM is consistent with the analytical existing models used for the 1-3 and 2-2 connectivities and is in line with measured values from Chan and Unsworth (1989). Based on the GHM 2-2 configuration results of piezocomposite materials, the electroacoustic responses of transducers having some of these properties are simulated using the KLM model. A performance trade-off was chosen, resulting in an improved thickness coupling coefficient and a lowered acoustical impedance, and a similar approach as that on a pure PZT-7A.

An analytical approach to evaluate effective coefficients of 1–3 piezoelectric composites

In this paper, a micromechanics method is developed to evaluate effective coefficients of piezoelectric fiber-reinforced composites. An exact solution is derived for effective elastic, piezoelectric and dielectric coefficients of such piezocomposites subjected to the applied load in the direction transverse to the fiber orientation. Simultaneously, based on finite element method, a numerical study is performed on a representative volume element of such piezo composite containing fiber in square packing arrangement. The finite element method provides a numerical solution to evaluate effective elastic, piezoelectric and dielectric coefficients for discrete volume fraction of fiber, the range being 0.1–0.6 for this study. The results are interpolated to understand the overall behavior of such piezocomposites. The results obtained from the micromechanics method and the finite element method are compared with the results obtained from other models based on strength of materials method given in the literature. It is observed that the method developed in this study provides better results for effective coefficients susceptible to fiber packing arrangements.

Ultrahigh current density and fatigue stability in flexible energy harvester by designing delivery paths

Flexible piezoelectric energy harvesters that efficiently convert mechanical energy into electrical energy have been extensively studied due to their great application potential in low-power wearable electronics and self-powered sensors. However, the low current output and poor thermal fatigue resistance severely restrict their practical applications. Here, we propose a new strategy to simultaneously improve the current density and thermal conductivity of the flexible piezocomposites (PCs) by designing delivery paths. High-quality (Ba0·85Ca0.15) (Ti0·90Zr0.10)O3/copper nanorods/polydimethylsiloxane (BCZT/Cu NRs/PDMS) PCs with a novel co-chained structure prepared using a well-suited technique of dielectrophoresis were reported for the first time. The BCZT particles and Cu NRs aligned in same chains throughout PDMS matrix results in an efficient delivery path both for induced charge transfer and heat dissipation, thus leading to ultrahigh current density (4.7 μA/cm2) and thermal conductivity (0.31 W/(m · K)). Further, a faster charging speed and a dramatically fatigue stability were realized in the co-chained PCs. Our work is expected to provide a potential solution for simultaneously enhancing current density and heat dissipation for a variety of composites in the field of flexible energy harvesting.

An analytical model for shape morphing through combined bending and twisting in piezo composites

Bilayer piezo macro fiber composites (MFCs) with specific fiber orientations are experimentally known to undergo bending as well as twisting, under the action of an externally applied electric field. An analytical model is developed for this shape-morphing phenomenon using continuum mechanics and energy minimization considering material anisotropy. A detailed parametric study is carried out using a coupled electro-mechanical finite element model and the results are used to validate the analytical model. The analytical model quantitatively predicts the variation of curvatures with the system parameters (bilayer geometry and electric field magnitude) with good accuracy. The tendency of a bilayer to undergo pure bending, pure twisting or a helical deformation is predicted using stress-free strain mismatch between anisotropic layers. Both bending and twisting deformations are found to be strongly dependent on the fiber orientation, vary linearly with the applied electric field magnitude, independent of length and decrease with an increase in width or thickness of MFC.

Influence of polymer insertion on the dielectric, piezoelectric and acoustic properties of 1-0-3 polyurethane/cement-based piezo composite

Cement/polyurethane based piezo composite was fabricated with 1-0–3 connectivity. Microstructural analysis shows that polyurethane (PU) acts as a binder, reducing the porosity and improving the matrix adhesion to the piezoelectric phase. Dielectric characterisation shows that PU strongly reduces both the interfacial polarisation process and the ionic mobility in the matrix, thus decreasing the dielectric constant and increasing the values of voltage constant (g33) and electromechanical coupling coefficient (kt). The composite was surface mounted on a porous concrete bar and evaluated as an acoustic emission sensor (AE). The results show that this material is a highly sensitive sensor for AE detection in the frequency range 50–220 kHz.

Performance of vertically reinforced 1-3 piezo composites for active damping of smart sandwich beams

This paper deals with the performance of vertically reinforced 1-3 piezoelectric composite material as a constraining layer for active constrained layer damping of sandwich beams. A finite element model, considering both in-plane and out-of-plane actuation of the constraining layer of the active constrained layer damping, has been developed for analyzing the active damping of sandwich beams integrated with the patches of such active constrained layer damping treatment. The analysis revealed that vertically reinforced 1-3 piezo composites, which are popularly used as sensor materials, can be used as distributed actuators of smart sandwich beam.

Microstructuring by a Combination of Micro Impact Extrusion and Shear Displacement Forming

The Collaborative Research Centre SFB/TR 39 PT-PIESA is developing mass production technologies and process chains for the fabrication of aluminium piezo composites, which can be used as raw material for "smart sheet metal" [1]. Microstructuring by forming is a challenging task concerning material flow, tool and process design [2]. In this study, a hybrid forming process combined of micro impact extrusion and shear displacement is presented and discussed. The formed microstructure, depicted in figure 1, consists of ten parallel primary cavities with cross sections of 0.3×0.3 mm² and four larger secondary cavities which are surrounding the primary cavities. High demands are made concerning precision and reproducibility of the cavities' geometry according to the function of the cavities, which is to serve as collets for sensitive piezo rods. The microstructure has to be formed with one stroke of the stamp. Micro backward impact extrusion is chosen for structuring the primary cavities since it allows accurate forming without aligning die plate and stamp due to a flat die plate. Shear displacement forming, which is the selected process for the secondary cavities, requires a structured and aligned die plate but the forming forces are significantly lower than forming the same geometry with an extrusion process which in turn increases the accuracy. The investigations are focused on the characterization of samples formed with the hybrid process in comparison to structures which are formed solely by impact extrusion. Geometric parameters, material flow and process parameters were evaluated to assess the hybrid process. First experiments show promising results, whereas higher degrees of deformation could be reached at lower forming forces. Exemplary, sections for both processes are depicted in figure 2.