**12. Porous ceramics and piezoelectric materials**

The piezoelectric materials contain crystalline structures that do not overlap the positive and negative charge centers, leading to bipolar moments. When subjected to mechanical vibrations or movement, the mechanical strain of this material is applied and leads to distortion of the electrode, creating electrical charge [24, 25]. The electrical energy can be harvested by storing it in capacitors or rechargeable batteries. The piezoelectric material has been widely recognized with unique electromechanical coupling properties for its potential benefits in a large number of sensors and application engines. Moreover, it has also been shown that the microstructure of a piezoelectric material can be modified by adding a second phase (as in piezoelectric composites) or by introducing porosity (as in piezoelectric foams), in order to improve the piezoelectric materials properties for specific applications [26]. For example, by introducing porosity into piezoelectric material, the signal-to-noise ratio, sensitivity properties and impedance matching can be improved, thus the piezoelectric material can be made more suitable for the applications of hydrophone.

## **Author details**

Uday M. Basheer Al-Naib

Address all correspondence to: ummb2008@gmail.com

Faculty of Mechanical Engineering, UTM Centre for Low Carbon Transport in Cooperation with Imperial College London, Institute for Vehicle Systems and Engineering, University of Technology Malaysia, Johor, Malaysia
