**1. Introduction**

The ongoing technological developments lead undoubtedly to the quest for anisotropic ceramic material which presents a specific property or a combination of them. To this end, it

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

is necessary to control the growth parameter of nanostructures controlling the morphology of the particles.

Research on unidimensional nanostructures (1D) arouses multidisciplinary interest owing to their properties which allow the use of these nanostructures in self-assembly devices applica‐ ble in optical, electronic, photonic and biological systems [1,2]. Currently, researchers have been trying to develop efficient techniques for obtaining 1D ordered particles in nanoscale, controlling the size and shape of the particles since this is the key to improving the use of existing materials while allowing room for new multifunctional devices.

Ferroelectric and piezoelectric materials are much influenced by structural control, because some materials are found to exhibit piezoelectricity only when they are grown with 1D morphology. Another interesting property of 1D material that is worth mentioning is the photoluminescence (PL). Photoluminescence spectroscopy provides us with important information about electronic and optical structures of the materials such as the intermediate states between the valence band (VB) and the conductive band (CB) known as the band gap [3]. When a material combines the photoluminescence and piezoelectric properties, it can be applicable in piezophotonic devices [4].

Lead zirconate titanate (PZT)-based materials present excellent optical and electronic proper‐ ties; oddly enough though, despite their good properties, materials like PZT are prone to cause environmental damage which may eventually cause the need for their substitution for leadfree materials.

Alkali niobates, such as sodium niobate (NaNbO3), are promising materials for substituting materials like PZT. NaNbO3 is a lead-free piezoelectric perovskite semiconductor with different phase transitions, which has attracted increasing attention among researchers owing to its capacity to form the basis of the class of environmentally friendly materials [5,6].

The efficiency of these materials is said to be directly related to their crystalline and morpho‐ logical structures. One possibility of obtaining particles with crystalline and morphological well-defined characteristics lies in hydrothermal synthesis, which is a variation of solvother‐ mal synthesis.

The heating in hydrothermal synthesis can be carried out by microwave-assisted technique and when this happens the synthesis is named microwave hydrothermal synthesis. This process, in particular, is truly a low-temperature method for the preparation of monophasic materials of different sizes and shapes. This method is found to save energy and is environ‐ mentally friendly in that the reactions take place in closed system conditions. Unlike the conventional heating which requires a long time of reaction, microwave-assisted heating is a greener approach toward the synthesis of materials in a shorter time (several minutes to hours) with lower power consumption as a result of the direct and uniform heating of the contents [7].

Although some ceramics are known to have high ferroelectric and piezoelectric properties, their poor mechanical properties and other properties including the mismatch of the acoustic impedance with water and human tissue are found to restrict their application. On the other hand, ferroelectric polymers possess excellent mechanical properties such as flexibility and deformation, but their piezoelectric activity is low. To circumvent these problems, composite materials made with ferroelectric ceramic and polymer have been investigated as an alterna‐ tive material which combines the electric properties of ceramic and mechanical properties of polymer [8].

is necessary to control the growth parameter of nanostructures controlling the morphology of

Research on unidimensional nanostructures (1D) arouses multidisciplinary interest owing to their properties which allow the use of these nanostructures in self-assembly devices applica‐ ble in optical, electronic, photonic and biological systems [1,2]. Currently, researchers have been trying to develop efficient techniques for obtaining 1D ordered particles in nanoscale, controlling the size and shape of the particles since this is the key to improving the use of

Ferroelectric and piezoelectric materials are much influenced by structural control, because some materials are found to exhibit piezoelectricity only when they are grown with 1D morphology. Another interesting property of 1D material that is worth mentioning is the photoluminescence (PL). Photoluminescence spectroscopy provides us with important information about electronic and optical structures of the materials such as the intermediate states between the valence band (VB) and the conductive band (CB) known as the band gap [3]. When a material combines the photoluminescence and piezoelectric properties, it can be

Lead zirconate titanate (PZT)-based materials present excellent optical and electronic proper‐ ties; oddly enough though, despite their good properties, materials like PZT are prone to cause environmental damage which may eventually cause the need for their substitution for lead-

Alkali niobates, such as sodium niobate (NaNbO3), are promising materials for substituting materials like PZT. NaNbO3 is a lead-free piezoelectric perovskite semiconductor with different phase transitions, which has attracted increasing attention among researchers owing to its capacity to form the basis of the class of environmentally friendly materials [5,6].

The efficiency of these materials is said to be directly related to their crystalline and morpho‐ logical structures. One possibility of obtaining particles with crystalline and morphological well-defined characteristics lies in hydrothermal synthesis, which is a variation of solvother‐

The heating in hydrothermal synthesis can be carried out by microwave-assisted technique and when this happens the synthesis is named microwave hydrothermal synthesis. This process, in particular, is truly a low-temperature method for the preparation of monophasic materials of different sizes and shapes. This method is found to save energy and is environ‐ mentally friendly in that the reactions take place in closed system conditions. Unlike the conventional heating which requires a long time of reaction, microwave-assisted heating is a greener approach toward the synthesis of materials in a shorter time (several minutes to hours) with lower power consumption as a result of the direct and uniform heating of the contents [7].

Although some ceramics are known to have high ferroelectric and piezoelectric properties, their poor mechanical properties and other properties including the mismatch of the acoustic impedance with water and human tissue are found to restrict their application. On the other hand, ferroelectric polymers possess excellent mechanical properties such as flexibility and

existing materials while allowing room for new multifunctional devices.

applicable in piezophotonic devices [4].

60 Ferroelectric Materials – Synthesis and Characterization

the particles.

free materials.

mal synthesis.

A core parameter to be considered when discussing about composites lies in the connectivity patterns, which establish the arrangement of the phases comprising the composites. The first concept of connectivity was developed by Skinner et al [9] and Newnham et al [10], where the manner in which the individual phases are self-connected are described. There are 10 types of arrangements in which the two components, the matrix phase and the dispersed phase, can be connected forming the composite, ranging from unconnected 0-3 pattern to a 3-3 pattern in which both phases are three-dimensionally self-connected. The first number in the notation represents the dimension of connectivity for the piezoelectric active phase, while the second number refers to the electromechanically inactive polymer phase [11,12]. The composites with 0-3 connectivity possess the merits of being highly flexible and having a relatively high piezoelectric coefficient, though it is very difficult to obtain 0-3 composites with a high ceramic content. The high ceramic content provides a mixed connectivity in 0-3 composites due to the percolation of the particles, besides that the high concentration of big ceramic particles is found to reduce the flexibility of the composite [8]. The 3-3 composites when compared with the 0-3 composites are found to present a relatively higher piezoelectric coefficient [12] though they are quite more rigid and difficult to manufacture than the latter (0-3). Studies conducted on lead-free composites using KNN particles (sodium potassium niobate) and poly(vinylidene‐ fluoride) (PVDF) as polymer concluded that the piezoelectric activity tends to increase given an increase in the ceramic content. However, a decrease in the polymeric phase of the com‐ posite weakens the mechanical properties. The piezoelectric properties of the KNN/PVDF composite were compatible with the PZT-based materials, implying that it is a good option to be used in place of PZT-based materials [13].

A wide array of studies on piezoelectric composites can be found in the literature. Dargahi et al [14] developed a composite to be used in the medical area as a sensor in minimally invasive surgeries without damage to the body. Fuzari Jr et al [15] studied a promising material to use as an acoustic emission sensor. A NaNbO3/PVDF composite used in energy harvest was obtained by Mendoza et al. and Srinivas [16,17], where they compared the NaNbO3/PVDF and lead-based ceramic/PVDF composite in order to demonstrate the feasibility of replacing leadcontaining materials in high-energy-density dielectric capacitors.

Despite the range of advantages attributable to composite materials, the poling process still poses a challenge owing to the fact that the effective electric field for polarizing the ceramic particles is much lower compared to the applied electric field [15,18,19]. The poling efficiency of ceramic particles dispersed in a polymer can be enhanced inserting a third phase into the composite. This phase creates an electrical flux path between the ceramics particles, and to promote this effect a conducting polymer, polyaniline (PAni), for instance, can be included in the composite to control the electrical conductivity [15].

In light of that, the main purpose of this study is to discuss the preparation and characterization of NaNbO3 ceramic particles with different morphology obtained by microwave-assisted hydrothermal synthesis. The influence exerted by the synthesis parameter in obtaining the particles with the best characteristics as well as the use of these particles in the fabrication of composite films with 0-3 connectivity, using PVDF as polymer matrix, are all presented and discussed.
