Development of “lead-free” piezoceramics based on potassium sodium niobate (KNN) from mechanochemically activated precursors

With exceptional electromechanical properties and mild processing conditions, highly dense PZT (lead, zirconate, titanate), with a general formula Pb(ZrxTi1-x)O3, is the most commonly used compound in the manufacturing of piezoceramics-based devices. However, the toxic nature of lead oxide, which is the main component in PZT systems (~60 wt.%), has raised concern about its environmental impact during all stages of its “life-cycle”. Among the many lead-free based candidates, KNN (potassium, sodium, niobate), with a general formula of KxNa1-xNbO3, has received great interest in the recent past, due to its promising electromechanical properties coupled with a high Curie temperature (217–304 °C). However, their extensive penetration into the commercialized devices is still hampered by their scarce reproducibility, poor sinterability and as yet unclarified impact on human health. These processes-related problems can be overcome by increasing the reactivity of the powders by mechanical processing (High Energy Ball Milling) or introducing specific dopants. In the present PhD thesis work, this technique has been extensively explored on the preparation of promising “lead free” systems as the potassium sodium niobate (KNN) and KNN-based materials. The structure, microstructure, electrical properties and cytotoxic effect of the as prepared “lead free” system has been studied. Particular attention was given to the correlation between microstructure and electrical properties. The main sintering technique employed are the common Air Sintering (AS) and the Spark Plasma Sintering (SPS); in particular, the latter one has been tested on a promising composition as the KNN modified with bismuth ferrite (BF). In parallel with the study of the materials, this thesis work has been dedicated to the development of an innovative device to perform in-situ poling/temperature XRD experiments, suitable for common laboratory diffractometers.