Composites of aligned (K,Na,Li)NbO₃ (KNLN) piezoceramic particles in a PDMS polymer matrix are presented as promising materials for flexible sensors and energy harvesters. Their ease of processing is matched with a relatively high damage tolerance and piezoelectric performance at low dielectric loss. To maximize piezoelectric performance, the effective poling conditions for dielectrophoretically structured KNLN-PDMS composites are studied, and compared with random composites. Effective poling is identified at 7.5 kV/mm, for 6 min at 150°C in structured composites. The structured composites demonstrate improved piezoelectric performance, with respect to random composites, while retaining the low stiffness of the PDMS polymer matrix.

A highly sensitive, lead-free, and flexible piezoelectric touch sensor is reported based on composite films of alkaline niobate K_0.485Na_0.485Li_0.03NbO₃ (KNLN) powders aligned in a polydimethylsiloxane (PDMS) matrix. KNLN powder is fabricated by solid-state sintering and consists of microcubes. The particles are dispersed in uncured PDMS and oriented by application of an oscillating dielectrophoretic alignment field. The dielectric constant of the composite film is almost independent of the microstructure, while upon alignment the piezoelectric charge coefficient increases more than tenfold up to 17 pC N^-1. A quantitative analysis shows that the origin is a reduction of the interparticle distance to under 1.0 μm in the aligned bicontinuous KNLN chains. The temperature stable piezoelectric voltage coefficient exhibits a maximum value of 220 mV m N^-1, at a volume fraction of only 10%. This state-of-the-art value outperforms bulk piezoelectric ceramics and composites with randomly dispersed particles, and is comparable to the values reported for the piezoelectric polymers polyvinylidenefluoride and its random copolymer with trifluoroethylene. Optimized composite films are incorporated in flexible piezoelectric touch sensors. The high sensitivity is analyzed and discussed. As the fabrication technology is straightforward and easy to implement, applications are foreseen in flexible electronics such as wireless sensor networks and biodiagnostics.

In this study, the influence of Li substitution on the piezoelectric performance of lead-free K_0.5Na_0.5NbO₃ (KNN)-epoxy composites is explored. KNN piezoceramic particles modified with 0-12 mol% of Li are prepared via a double calcination technique, resulting in a perovskite particulate which transitions from an orthorhombic to tetragonal crystal structure between 6 and 9 mol% of Li, and contains a minor nonperovskite second phase from 6 mol%. A cuboid particle morphology is evident in all cases, though tetragonal KNN-based particles have formed with serrated edges and fractures. The particles are dispersed at 10 vol% in an epoxy matrix to develop both random and dielectrophoretically structured (K,Na,Li)NbO₃-epoxy composites. The dielectric constant of the composites appears almost independent of Li content, while the piezoelectric charge constant of structured composites peaks before the polymorphic phase transition, at 3 mol% of Li. The peak in performance can be attributed to the increased primary particle size of the composition in combination with its single phase orthorhombic crystal structure. The enhancement of the energy harvesting figure of merit, derived from substituting 3 mol% of Li in the KNN particulate, makes these composites an interesting choice for flexible energy generators.