Design and mechanical characterization of lead-free piezoelectric ceramics
Résumé
In order to fulfil European Union directives that want to banish lead contained in piezoelectric devices such as lead zirconate titanate (PZT) based ceramics, 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 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. K0.5Na0.5NbO3 (KNN) and K0.5Na0.5Nb0.85Ta0.15O3 (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 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 ranging from 1 to 3.5 μm. Evaluation of microhardness highlights densification and grain size as driving forces to improve it. The substitution of Nb by Ta shows a composite microstructure that decreases the global grain size and enhances the microhardness by 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.