Novel lead-free 2–1–2 composite: predicted high piezoelectric sensitivity and significant hydrostatic response

Abstract

A parallel-connected composite with 2–1–2 connectivity and two lead-free single-crystal components was first proposed to highlight its high piezoelectric sensitivity and to describe its large hydrostatic parameters. The first type of the composite layers represents a ferroelectric domain-engineered [001]-poled [Li, (K, Na)](Nb, Ta)O3 single crystal. In the second type of the composite layers, piezoelectric Li2B4O7 single-crystal rods in the form of an elliptic cylinder are regularly aligned in a large polymer matrix. The effective electromechanical properties of the composite were found in three stages, using the effective field and matrix methods that are applicable to piezoelectric media. A new orientation effect was studied that was linked with rotations of single-crystal rod bases in the polymer medium that forms the layers of the second type. Since these rotations are in a Li2B4O7 single crystal with a unique elastic and piezoelectric anisotropy, the layers of the second type exhibit a noticeable influence on the piezoelectric performance and hydrostatic response of the composite. A variation of the aspect ratio of the Li2B4O7 rod base becomes an additional important factor that can improve the hydrostatic parameters of the composite. Some effective parameters of related 2–1–2 composites were compared for a case of different polymer components therein. Three diagrams were first built for the high-performance 2–1–2 [Li, (K, Na)](Nb, Ta)O3 / Li2B4O7 / polyethylene composite to show ranges of rotation angles and volume fractions, which correspond to the longitudinal piezoelectric voltage coefficient g33* > 
500 mV m N-1, hydrostatic piezoelectric coefficient gh* > 200 mV m N-1 and hydrostatic figure of merit dh*gh* > 10-11 Pa-1. These effective parameters are important in piezoelectric sensor, hydroacoustic and other applications of novel lead-free composites.

Two orientation effects and large anisotropy of parameters in piezo-active 2–1–2 composites based on [0 1 1]-poled crystals

“…Hereafter, the aspect ratio of each rod base is η c = 100 (in notations at the rotation angle γ = 0 • ). As follows from recent studies [23], layer II at η c = 100 exhibits a large anisotropy of elastic properties even at small volume fractions of the KNNLT ceramic m c from 0.03 to 0.10. Taking the macroscopic symmetry of layers I and II into account, we note that the orientation effects are to be analysed when changing the rotation angles 0…”

“…The domain-engineered [0 1 1]-poled SC layer (layer I in our notations, inset 1 in figure 1) and ceramic/polymer layer (layer II in our notations, inset 2 in figure 1) are arranged periodically along the coordinate axis OX 1 . Here, we develop model concepts [23] and consider two orientation effects that are associated with the ferroelectric components in the adjacent layers. The spontaneous polarisation vector P (1) s of the SC in layer I is parallel to the OX 3 axis.…”

“…The effective properties are found in the long-wave approximation [13]. Such an approximation is valid under the condition [13,23] that a wavelength of an external acoustic field acting on a composite is much greater than the width of each layer of the composite (see figure 1).…”

“…Using elements of the ∥C * ∥ matrix from equation ( 6), we analyse the piezoelectric coefficients d * 3j and g * 3j of the 2-1-2 composite, where j = 1, 2 and 3. These properties are important [13,23] to describe piezoelectric activity and sensitivity. Knowing d * 3j and g * 3j allows us to study the behaviour of the traditional energy-harvesting (or squared) FOMs of the composite:…”

“…A novel piezo-active 2-1-2 composite represents an example of a hybrid structure [23] with elements of the 2-2 and 1-3 connectivity patterns that can influence the piezoelectric performance and anisotropy of a series of parameters. Recently, the parameters of related 2-1-2 composites with two ferroelectric components, namely, a domain-engineered SC poled along either the [0 0 1] or [0 1 1], and ceramic, were analysed [24], and the advantages of the composite containing [0 1 1]-poled SC over the similar composite containing [0 0 1]-poled SC with the same chemical composition were highlighted.…”