Piezoelectric Composites

April 10, 2017

Piezoelectric materials generate electric current when placed under stress and will change size or experience mechanical deformation upon being subjected to an electric current.

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An important subject of current research is the applicability of piezoelectric composites for use in
acoustic transducers for medical ultrasonic imaging.

Piezoelectric materials are materials which generate electric current when placed under stress. The converse effect can also be observed, in which a piezoelectric material will change size or experience mechanical deformation upon being subjected to an electric current. Piezoelectricity is related to the formation of electric dipole moments in crystalline solids. The dipoles in a solid are usually randomly oriented, but can be aligned through poling (a process in which a strong applied electric field is combined with an elevated temperature). In piezoelectric materials, an applied mechanical stress changes the polarization (either strength or direction) of the material, which then varies the electric charge of the material.

In piezoelectric composites, polymer material is used to modify desired features of a piezoceramic material. Piezoceramic rods are embedded in a polymer matrix to form a composite material by the dice and fill technique. In this manufacturing technique, a piezoceramic plate is diced through 80% of its thickness with a very small pitch (less than 0.1 mm generally). The notches are then filled with a polymer matrix, and the uncut side of the plate is ground off. This method allows for the manufacture of much larger piezocomposite plates, used to produce more than one smaller transducer.

Injection molding is another manufacturing process that can be used to make piezoelectric ceramics, which can then be used to produce piezoelectric composites. One advantage of this technique is the ability to produce complex shapes rapidly and at a low cost. The liquid ceramic is injected under pressure into a cavity the shape of the finished product. The ceramic is then heat treated and sintered to produce a shape that requires minimal finishing. However, the ceramic must then be poled to produce a functional piezoceramic.

Unlike conventional piezoceramics, piezocomposites can be engineered to meet specific needs by altering several material parameters. One of these engineered parameters involves increasing the ceramic volume to alter the value of the dielectric constant. Additionally, the type of polymer used as the matrix material can modify the acoustical or mechanical performance of the piezocomposite.

An important subject of current research is the applicability of piezoelectric composites for use in acoustic transducers for medical ultrasonic imaging. This application is focused mainly on the 1-3 PZT (lead zirconate titanate) -rod/polymer composite type.

Currently, PZT is the most common piezoelectric material used to manufacture piezocomposites. However, in the future, it is very possible that new varieties of piezocomposites will be developed for much more varied applications than are currently available.

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