Piezoelectric nanoparticles offer a chance for advancement toward self-powered systems through energy harvesting. The use of a system or device that performs a function without the requirement for an external power source, such as a battery or any other type of source, is made possible by the newly emerging technology known as self-powered systems. For instance, this technology can capture energy from nearby sources and transform it into electric energy utilizing the piezoelectric effect. The creation of some self-powered systems based on nano-devices would be facilitated by the development of a nanogenerator (NG) to transform environmental energy into electric energy. Zinc oxide (ZnO) has been employed in a wide range of applications, including optoelectronics, chemical sensors, piezoelectric transducers, and varistors, due to its many functions. Consequently, ZnO-based piezoelectric composites would be a different option for flexible and small implantable sensors that may immediately detect and monitor mechanical stimulation or pressure changes occurring in the patient's organs.
Top1. Introduction To Piezoelectric Effect
The discovery that compressing and stretching quartz can produce macroscopic polarization and, subsequently, electric charges was made in 1880 by Pierre and Jacques Curie (Curie & Curie, 1880). The term “piezoelectricity,” which means “electricity resulting from the press,” was used to describe this phenomenon (Taylor, 1985). The Greek word “piezo” means “to press or squeeze.” The first official use of piezoelectric material wasn't made until 1921 when quartz crystal was used in a transducer to find submarines via echolocation. Since the development of artificial piezoelectric materials, the emergence of piezoelectricity has opened up new avenues for research in the field of crystal physics and has quickly generated an industry market. The study and use of piezoelectric materials in modern engineering were hastened in particular by the discovery of ferroelectricity (Valasek, 1921). In 1969, it was discovered that polyvinylidene fluoride polymer had a high piezoelectric effect and met the criteria for mechanical flexibility in piezoelectric materials (Murayama et al., 1976). Piezoelectric materials, such as single crystal, ceramic, and polymer, have several uses in the fields of medical, telecommunications, information, and the military up to this point.
The generation of an electric charge in response to a mechanical stimulus is known as the direct piezoelectric effect. As demonstrated in Figures 1 (a) to (c), if compressive stress is applied, the result is a positive electric voltage. If tension is applied, the result is a negative electric voltage. As roughly depicted in Figure 1 (d) to (f), the inversed piezoelectric effect is the generation of mechanical deformation in the material in response to electric stimulation. When a voltage is applied to a piezoelectric material, cocompressive deformation results, but a reversed voltage results in tension.
Figure 1. Direct piezoelectric effect: (a) at zero stress, (b) at applied compression, and (c) at applied tension. Converse piezoelectric effect: (d) at zero voltage, (e) at applied voltage, and (f) at applied reversed voltage.
Indeed, the straightforward molecular model shown in Figure 2 can be used to illustrate the fundamentals of piezoelectricity. Centrosymmetric and non-centrosymmetric materials can both be categorized as crystals. A centrosymmetric crystal cannot produce electric polarization in response to external stimulation, as shown in Figure 2(a). Positive and negative charge centers coincide in the initial condition, absent any external force, creating an electrical neutral in the basic unit cell.
Figure 2. (a) A centrosymmetric molecular and (b) a non-centrosymmetric molecular model subjected to an external force (Arnau & Soares, 2008)
Due to the presence of the center of symmetry, the polarization dipole pi always cancels one another after the application of an external force, resulting in a null total dipole moment. As a result, there is no internal net polarization (P), proving that centrosymmetric crystals are nonpolar and non-piezoelectric. The unperturbed non-centrosymmetric unit cell, on the other hand, starts out in an electrically neutral condition, just like a centrosymmetric crystal (Figure 2(b)). Because a unit cell lacks a center of symmetry, when external stress is applied, the internal structure is deformed, causing the separation of the positive and negative centers to produce a dipole moment. Inside the substance, even nearby half-dipoles carrying the opposing charge indications cancel one another out. However, this substance polarized and surface net charges that produce an electrical field elaborate the genesis of piezoelectricity (Arnau & Soares, 2008).