Developments in Ceramic Materials Research

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R. Ramesh, H. Kara, Ron Stevens and C. R. Bowen
create the pores. The spherical ATR results in rounded pores (Figure 1) whereas irregular
PEO results in more irregular pore morphology (Figure 2).
2.3.1. Dielectric constant
Figure 3 shows the variation in dielectric constant of 3-3 piezocomposites prepared using
different techniques. Dielectric constant decreases with decreasing ceramic content. There is
no significant difference in properties between PZT-Air and PZT-Polymer composites. This is
due to the comparable permittivity values of air and polymer phases. It is also interesting to
note that at ~ 60% PZT ceramic volume, where the 3-0 connectivity starts appearing, the
permittivity deviates into two separate lines. This is due to the extensive cracks, revealed by
cross-sectional SEM examinations, lying parallel to the surface. These cracks, which occur
after green compaction due to relaxation of the elastic ATR spheres, prevent connectivity of
the ceramic between the electroded surfaces, hence reducing the permittivity considerably.
However, it should also be noted that pore anisotropy can also be a reason for this deviation
between ATR and PEO samples since higher permittivity is obtained when specimen with
pores align along the thickness direction compared to those normal to the thickness direction
[32].
Figure 3. Variation of dielectric constant of 3-3 composites with ceramic volume fractions.
2.3.2. Piezoelectric Coefficients
Piezoelectric coefficients depend strongly on the amount of piezoceramic component
present in a piezocomposite, since this is the dominant contributor to the net piezoelectric
property of the composite. The polymer component, being a passive material, contributes
mainly to the mechanical properties and indirectly influences the piezoelectric properties of
the composites as well.