## 2 Overview of Piezoelectric Materials Used in EH  
The mode of action of the various piezoelectric materials is similar, however, the properties differ depending on the material used. The commonly used lead titanate zirconate has very good piezoelectric properties; however, it is very brittle and susceptible to damage. It also contains lead, the use of which is not permitted in every field where piezoelectric materials are used. For this reason, further materials with piezoelectric properties are constantly being developed. For materials that are crystals, an important feature is the lack of point symmetry in their elementary cells. This enables the phenomenon of polarisation of the crystals, and thus charge accumulation on the electrodes, to occur [11]. Piezoelectric materials can be divided into many groups, depending on their composition, applications or conditions of use. An example of this categorisation is presented in the study of [12]. PZTs are an example of a polycrystalline material. They consist of multiple grains, each of which can be polarised in a different way before the initial treatment [7]. Quartz or Rochelle salt are lead-free, single-crystal materials [12]. The electrical charges within them are naturally arranged uniformly throughout the entire volume of the crystal excluding defects. Ceramic materials have certain advantages over single-crystal materials, such as lower production cost, the possibility of obtaining more complex shapes, and less susceptibility to errors during crystal formation [13]. There are also some associated limitations, such as poorer reproducibility of material properties compared to single crystals, loss of properties above the Curie temperature, the practical temperature limit being much lower, and a stronger dependence of properties on temperature. Polymeric materials, such as poly(vinylidene fluoride) (PVDF), have a repetitive structure consisting of monomers that are joined together by polymerisation. They are characterised by better strength properties than the others; however, they generally have much weaker piezoelectric properties [7]. In its structure, PVDF partly reflects the action of crystals. The interconnected polymer molecules form plates that are irregularly distributed in the material. The mechanism of the piezoelectric phenomenon is therefore similar to that observed in crystals, so it can be said that the material is a'semi-crystalline piezoelectric polymer'. However, its use requires prior preparation in the form of material polarisation and mechanical stretching [11]. Lead-free materials can also take the form of polycrystalline materials, such as (Bi\({}_{1/2}\)Na\({}_{1/2}\))TiO\({}_{3}\) (BNT) or (K\({}_{1/2}\)Bi\({}_{1/2}\))TiO\({}_{3}\) (KBT), whose properties are in many ways inferior to those of PZT [8]. In order to combine the positive properties of certain materials, composites are often produced in practice, often consisting of a crystalline piezoelectric material in a polymer matrix. This method is used, for example, in the manufacture of certain types of sensors and for ultrasonic applications. It is also possible to create layered materials such as PZT thin films surrounded by silicon layers, which have applications in electromechanical microcircuits [12]. One type of piezoelectric transducer is MFC (micro fibre composite) films. These have much better strength properties than standard ceramic or crystalline materials, while having better piezoelectric properties than polymeric materials. This composite consists of a fibre phase in the form of rectangular PZT fibres immersed in epoxy resin, sandwiched between two layers of electrodes and resin, and all encapsulated in a polyimide film [14]. Due to their form and structure, they are very flexible, and it is possible to stick them on many surfaces with complex shapes [14]. The superposition of PZT electrodes and fibres has also allowed for higher values of the piezoelectric constant d33, as well as an increased electromechanical coupling coefficient [15].