Lead-free piezoelectrics: Current status and perspectives

In the last few years, there has been tremendous effort to develop lead-free ferroelectric ceramics with high piezoelectric properties by various doping and alloying routes. Several material systems have been explored, however, no prominent alternative to the versatile lead zirconate titanate (PZT) system has been found yet. Despite the achieved improvement in piezoelectric properties, there are problems in the synthesis, processing and poling of the sintered ceramics. Various processing techniques including microwave, hydrothermal, sol–gel, Pechini and spark plasma sintering have been used to overcome the drawbacks related to synthesis issues. In this paper, an attempt is made to review recent developments on lead-free piezo materials emphasizing their preparation, structure–property relations, and consequent physical properties. In this context, both compositional and structural engineering approaches to achieve acceptable piezoelectric properties in lead-free materials are discussed. Piezoelectric properties of the most promising lead-free compositions/families including titanates, alkaline niobates and bismuth perovskites and their solid solutions, along with non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. A brief coverage of the recent developments in the area of piezoelectric energy harvesting is also encompassed.