Piezoelectric materials are capable of converting mechanical energy into electrical energy and vice versa. These materials have been at the forefront of technological innovations ever since their emergence in the late 19th century. Piezoelectrics have found their niche in communication, automobile, aerospace, and several other industries relying on electromechanical systems. The excellent piezoelectric properties of materials like lead zirconium titanate (PZT) have made them a popular candidate in the production of actuators, sensors, and transducers. However, PZT was classified as a hazardous material in 2003 by the European Union due to the toxic nature of its lead oxide precursor. Efforts have been made by the scientific community to shift to more environmentally friendly, lead-free, piezoelectric systems. Bismuth, sodium, and potassium have garnered much attention as substitutes for lead due to their non-toxic nature. Piezoceramics such as barium titanate (BT), bismuth ferrite (BFO), potassium sodium niobate (KNN), and potassium sodium lithium niobate (KNLN) have become increasingly popular.

Bismuth ferrite’s (BFOs) high piezoelectric Curie temperature (825 °C) coupled with its lead-free nature makes it one of the more interesting PZT alternatives. However, the synthesis of phase pure BFO is difficult due to the presence of parasitic iron and bismuth-rich secondary phases. The electrical conductivity of BFO, which is considerably higher than that of PZT, is also an issue. The presence of a high electrical leakage current is generally attributed to the defects and oxygen vacancies in bismuth ferrite. Through doping, it is possible to control the density of these free moving charges and oxygen vacancies. In this thesis, we use the method of solid-state reaction synthesis with subsequent sintering for the preparation of doped BFO samples. This work aims to develop highly polarizable doped BFO systems with low leakage characteristics which could potentially be utilized for high-temperature sensing applications.

An enhancement in polarization was observed by doping BFO with trace amounts of cobalt. The incorporation of titanium into BFO led to the reduction of conductivity and caused an improvement in the leakage characteristics. A co-doped system of cobalt and titanium was further explored to combine the positive characteristics of both individual dopants. The result was a highly polarizable system (0.25-0.25 at% Co-Ti) with low conductivity and excellent leakage characteristics. A solid solution system of BFO with strontium titanate (STO) was also explored. The 30 at% STO system showed extremely large values of piezoelectric charge constant. The thesis was concluded by performing some high-temperature measurements on the 0.25-0.25 at% Co-Ti and 30 at% STO systems.

Piezoelectric materials have the ability to convert mechanical to electrical energy (direct effect) and vice versa. They are readily used in the aerospace, automobile, telecommunication industry etc. as both sensors and actuators. For this work the focus is on the sensor application, which utilizes the direct piezoelectric effect. With the rapidly growing technological demands, sensors should be flexible enough to adapt to different applications while also having adequate sensing capabilities. Currently, lead- and lead-based piezoelectrics are used in the industry due to their excellent piezoelectric properties. However, due to their toxic nature, research has been ongoing into more lead-free systems which are capable of replicating the performance of these lead-based systems. In this work, we aim to improve the sensing capabilities of lead-free piezoelectric composites. To further improve their performance, reducing the dielectric constant (ε) of the composite is the main strategy of this work. The reduction is achieved by fabricating a porous composite structure. The reduction in permittivity leads to an increase in the piezoelectric voltage constant (g), which defines the sensitivity of the piezoelectric composite.

The main focus of this work is to optimize a polymer and polymer foaming technique to obtain a high level of porosity, while also retaining adequate mechanical properties. The next step is to achieve a high poling efficiency for the composite in order to obtain good piezoelectric properties. For the polymer system, polyvinyl alcohol (PVA) is selected as the matrix due to its excellent film forming ability as well as its relatively high dielectric properties (compared to polymers). The direct foaming technique is used for this work, due to its simplicity and its reproducibility. For the lead-free ceramic system, Barium Titanate (BaTiO3) and Sodium Potassium Niobate doped with Lithium (KNLN3) is selected as they have good piezoelectric properties, and have been used in piezoelectric composites extensively. As a porous piezoelectric composite is used in this work, the contact poling is replaced by the corona poling method to prevent localized dielectric breakdowns and non-uniform poling.

With the direct foaming technique, foams with porosites in the range of 90-95 % are obtained, resulting in a drastic reduction in the permittivity of the composite. Such a high porosity level also results in a much softer composite. The optimization of the corona poling process is done by selecting the adequate poling temperature and the grid voltage, which is found to be 110 °C and 6 kV respectively. The effective piezoelectric charge coefficient is measured using Al plates as electrodes, to prevent the soft composites from compressing locally. The foam composites exhibit remarkably high g33 values exceeding the 1000 mV.m/N mark, almost double the best sensor used in the industry currently (PVDF). This is attributed to the high poling efficiency and the reduced dielectric permittivity of the composite. This opens up the vast number of possibilities for future systems based on porous structures to be used as sensors which can showcase good piezoelectric properties as well as being more flexible/conformable.

In this thesis, we focus on flexible haptic feedback applications which rely on the sensation of touch to display a feedback to the user that establishes a system to user interaction. Piezocomposites made of piezoceramic and polymer phases can be used here due to their high flexibility and piezoelectric properties that enable them to deform under an electric field. A lead-free piezoceramic: KNLN-3 has been chosen as the ceramic phase due to lead-based ceramics having issues with toxicity and dielectric mismatch in composites. A flexible polymer: CFE was chosen as the polymer phase due to its high dielectric constant that decreases the dielectric mismatch to the maximum extent. The first objective of this thesis is to optimize the scale-up in production of KNLN-3 when employed as a random 0-3 composite. Thus, the production of KNLN-3 was analyzed with respect to the microstructure, purity, surface topology, poling, lithium content, precursors used and correlated with its performance in a composite. Based on this set of data, an optimized powder was obtained. The second objective of thesis was to perform mechanical tests on a composite made with this powder to obtain mechanical properties that quantify its effectiveness as a flexible haptic feedback device. From these tests, the optimized powder was seen to be the most ideal when it was employed as an R50 (0-3 composite with 50 % filler) composite with no glass fibres used during production.