Design and mechanical characterization of lead-free piezoelectric ceramics

In order to fulfil European Union directives that want to banish lead contained in piezoelectric devices such as lead zirconate titanate (PZT) based ceramics (PZT) , the (K,Na)NbO3 system has been one of the most investigated and promising candidates. However it that needs to be further investigated in order to introduce it at an industrial scale . However , and there remains a lack of data on the mechanical behaviour to scale-up this material and to push forward its introduction in different devices. K 0.5 Na 0.5 NbO 3 (KNN) and K 0.5 Na 0.5 Nb 0.85 Ta 0.15 O 3 (KNN-Ta) powders are prepared by solid-state synthesis ball milling and shaped in series of pellets using conventional and Spark Plasma Sintering (SPS). KNN-based ceramics sintered by conventional routes do not easily reach a relative density up to higher than 90 % in opposition to SPS-sintered ceramics which can achieve 98 % relative density. Optimization of SPS parameters permits also to reduce grain growth and to obtain ceramic grain size of between ranging from 1 and to 3.5 μm. Evaluation of microhardness highlights densification and grain size as driving forces to improve it. The substitution of Nb by Ta show ed shows a composite microstructure that decreases the global grain size and enhances the microhardness by more than 28 % 30% compared to pure KNN ceramics, confirming the identified driving forces. This study confirms the benefit of SPS in achieving high quality dense and homogeneous KNN-based ceramics. It features the engineering of microstructure by managing sintering parameters and chemical modification, leading to a large range of relative density and grain size, that proves their key role in mechanical behaviour of such materials.