Physical and mechanical properties of PVDF/KNN composite produced via hot compression molding

Abstract One of the most important lead-free piezoelectric compounds that has been studied extensively belongs to alkali niobate family, the sodium-potassium niobium (KNN) compound. In this study, KNN/PVDF composite was fabricated through hot compression molding of the components and its mechanical, thermal and electrical properties were studied. For this purpose, KNN was first synthesized by solid-state reaction during two-step calcination and its structural evolution was studied using XRD. Composite samples were prepared at different KNN to PVDF weight ratios. The powder mixture was then formed into disks by hot compression molding. The microstructure of the composite samples was investigated by SEM. The prepared samples were perfectly dense with a density in the range of 97.44% to 99.11% of the theoretical density. The thermal properties of the prepared composites were evaluated by thermogravimetric analysis and it was observed that final weight loss due to material degradation was reduced with increasing KNN weight percent. In addition, samples with higher KNN contents showed increased Young's modulus and yield strength values in compression. In order to investigate the electrical properties, the dielectric constant, dielectric loss factor, piezoelectric charge and voltage coefficients and electric polarization were studied. The results showed that the dielectric constant increased with KNN content. The same trend was observed in the piezoelectric charge coefficient and the dielectric loss factor of the PVDF/KNN composites. At 80 wt% KNN, the composite showed a distinct ferroelectric behavior with a remanent polarization of 0.255 μC/cm2 and 20.5 kV/cm coercivity. According to the results of the present study, hot compression molding is an effective method for producing KNN/PVDF composites with improved mechanical and thermal properties compared to its constituents.