Piezoelectric materials can be broadly classified as crystalline, ceramic and polymeric piezoelectric materials. The most commonly produced piezoelectric ceramics are lead zirconate titanate, barium titanate and lead titanate. Gallium nitride and zinc oxide can also be regarded as a ceramic due to its relatively wide band gap, that can generate an instantaneous polarisation inside their lattice on application of a force. The semiconducting PM possesses unique advantage such as compatibility with the Integrated circuits and semiconductor devices. Further, inorganic ceramic PM have several advantages over single crystal, such as the ease of fabrication into a variety of shapes and sizes as single crystals requires cutting along the crystallographic directions, thus minimising the possibilities of cutting into different shapes. The next class of PM namely organic polymer such as PVDF, have low Young's modulus compared to the inorganic PM. Piezoelectric polymers possess higher piezoelectric stress constants, an important parameter in sensors, than ceramics, which show that they can be better sensors than ceramics. Moreover, piezoelectric polymeric sensors and actuators, due to their processing flexibility, can be readily manufactured into large areas, and cut into a variety of shapes. In addition polymers also exhibit high strength, high impact resistance, low dielectric constant, low elastic stiffness, and low density, thereby a high voltage sensitivity which is a desirable characteristic along with low acoustic and mechanical impedance useful for medical and underwater applications. Among the PM, PZT ceramics are popular as they have a high sensitivity, a high g33 value. They are however brittle. Furthermore, they show low Curie temperature, leading to constraints in terms of applications in harsh environmental conditions. However, promising is the integration of ceramic disks into industrial appliances moulded from plastic. This resulted in the development of PZT-polymer composites, and the feasible integration of functional PM composites on large scale, by simple thermal welding or by conforming processes. Several approaches towards lead-free ceramic PM have been reported, such as piezoelectric single crystals, and ferroelectric ceramics with a perovskite structure and bismuth layer-structured ferroelectrics, which have been extensively researched. Also, several ferroelectrics with perovskite-structure TiO3 , TiO3 , KNbO3 , have been investigated for their piezoelectric properties.
Key piezoelectric properties
Important piezoelectric properties are:
"d" Constant : the "d" coefficient is a measure of the strain induced by an applied voltage. High dij constants indicate larger displacements which are needed for motoring transducer devices. Similarly, d33 suggest that the deformation is in the 3 direction same direction of the induced potential whereas d31 is used when the force is applied perpendicular to the polarization axis. The d15 indicate that the applied mechanical stress is due to shear deformation.
The electromechanical coupling factor k, is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy. The first subscript to k denotes the direction along which the electrodes are applied; the second denotes the direction along which the mechanical energy is applied, or developed.
Quality Quality Factor: Among the most important high power properties of piezoelectric ceramics is the mechanical quality factor, Qm, which is the inverse of the mechanical loss tan ϕ.
