Ceramic-ceramic composite structures are a viable solution to improve the electromechanical response of lead-free ferroelectrics through tuning of the local electrical and mechanical fields. The origin of the enhanced properties, however, remains unclear, as many of the possible effects, such as polarization and strain coupling as well as interface diffusion, are interrelated and difficult to separate or directly investigate. In this study, we use a custom-built digital image correlation system to directly investigate the influence of strain coupling on 2-2 composites consisting of 0.90Na1/2Bi1/2TiO3-0.06BaTiO3-0.04K0.5Na0.5NbO3 (NBT-6BT-4KNN) and 0.94Na1/2Bi1/2TiO3-0.06BaTiO3 (NBT-6BT) by varying the mechanical interface contacts between end members. Specifically, two model cases were utilized to separate the relative contributions of the polarization and strain coupling mechanisms: (i) electrically connected and (ii) mechanically and electrically connected. The local strain gradient was characterized through the thickness of the composite across different layers as well as the interface, where the macroscopic large signal longitudinal and transverse ferroelectric response was determined. Experimental results reveal an enhancement of the large signal piezoelectric coefficient d_33^* by approximately 10 % from 390 pm/V to 440 pm/V due to strain coupling.
The small‐signal dielectric and piezoelectric coefficients of paper‐derived sodium potassium niobate, K0.5Na0.5NbO3 (KNN), were compared with those of conventionally prepared samples. Results show similar functional properties of paper‐derived KNN without significantly decreasing the small‐signal piezoelectric coefficient. The structure and microstructure analysis of conventional KNN and paper‐derived KNN did not reveal any significant difference in the crystal structure and grain size. However, the temperature‐dependent inter‐ferroelectric phase transition temperature estimated from the temperature‐dependent dielectric permittivity data revealed a decrease of approximately 18°C for the paper‐derived KNN and is possibly associated with the structural and microstructural defects. This work indicates that optimizing suspension chemistry and sintering conditions will be critical to enhance the functional response of paper‐derived KNN further. Moreover, paper‐derived ceramic processing, a novel and cost‐effective additive manufacturing technology, can be potentially used to fabricate other electroceramics with a wide range of porosities and sizes as well as complex geometries and multilayer structures.
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