Abstract:
Piezoelectric composite materials, consisting of a ferroelectric ceramic in an
electrically-inactive polymer matrix, have been shown to greatly outperform single
phase materials for certain applications. A new assembly technique, which electrically
controls the spatial distribution of the ceramic within the polymer, promises to enhance
the sensitivity of 0-3 type piezoelectric composites. The materials so-produced have a
quasi 1-3 structure and it is intended that they will exhibit some of the advantages of 1-
3 piezoelectric composites, whilst retaining the simplicity of 0-3 manufacturing.
The electric field structuring technique exploits the electrokinetic phenomenon of
dielectrophoresis, which is responsible for the electrorheological effect. When a
suspension of ceramic particles in an insulating fluid is exposed to a moderate AC
electric field, the particles polarize and as a result they exhibit a mutually attractive
force. Under suitable conditions the particles assemble into `pearl-chains', `fibrils' or
columns, oriented parallel to the applied field. If the fluid is a resin pre-polymer, this
can then be cured and the newly formed structures frozen into place to form a
composite material with anisotropic properties. The key process parameters are
explored and the implications of employing this method to produce technologically
useful materials are discussed.
It is demonstrated, for the first time, that dielectrophoresis can be used to induce
anisotropic dielectric and piezoelectric properties in 55%vol. fraction ceramic /
polymer composites. A model composite system of pure lead titanate in an epoxy resin
is considered in basic detail. A method of producing a lead zirconate titanate (PZT)
powder with a narrow particle size distribution, by flux growth, has been shown to be
effective. New concepts in multiphase composites are introduced, whereby chains are
formed within the confines of a second immiscible fluid or where particles of two
different materials are mixed in a suspension, each material having its own `polarization
signature'.
