Module 5: Nonlinear Dielectrics
  Piezoelectric Ceramics
 
5.4.5 Common PIezoelectric Materials

5.4.5.1 Barium Titanate (BaTiO3)

This was the first piezoelectric material which was developed commercially for application in the generation and detection of acoustic and ultrasonic energy. We discussed its structure and transitions in the previous section (5.3.11). The transition temperature can be modified by chemical substitutions:
Ba substitution by Pb and Ca lowers the Tc of tetragonal to orthorhombic transition. This has been used to control the piezoelectric properties around 0°C, and is important for underwater detection and echo sounding.

Ti substitution by Zr or Sn increases the transition temperature for both the tetragonal–orthorhomic and orthorhomic–rhombohedral transitions and enhances piezoelectric properties. Ti substitution by 1-2 at % Co3+  leads to much reduced losses as high fields useful in ultrasonic applications. Care must be taken during processing to avoid reduction of Co3+ to Co2+ which occurs very easily.

5.4.5.2 Pb(Zr,Ti)O3 or PZT

PZT is one of the most used piezoelectric in a variety of applications due to its excellent properties and high enough transition temperatures. It has a perovskite structure with B sites randomly occupied by either of isovalent Ti and Zr  ions.

The phase diagram of PZT is shown below.

 

Figure 5.23 Phase diagram of PZT (Courtesy: © DoITPoMS, University of Cambridge, UK)

The typically used composition is about Zr:Ti::50:50  which gives excellent properties. The reason for this is that this is closed to morphotropic phase boundary and here rhombohedral and tetragonal structures co-exist. As we know that PS vector is along [001]-direction in tetragonal phase and <111>- direction in rhombohedral phase, it permit material to be poled easily as there are quite a few poling directions available making it a useful piezoelectric.

Donor doping in PZT such as La3+ on Pb site reduces the concentration of oxygen vacancies. This in turn reduces the concentration of defect pairs which otherwise impede the domain wall motion. This leads to noticeable increases in the permittivity, dielectric losses, elastic compliance and coupling coefficients, and reduction in the coercivity.