Stress–Strain Hysteresis Analysis of PVDF-Based Films Containing Nanofibers/Piezoelectric Particles for Vibration Energy Harvesting

Specific vibration energy harvesting applications require high deformability to harvest mechanical energy efficiently. Polymer-piezoelectric particle composites have been attractive as they inherently possess high flexibility. However, this flexibility can be further enhanced by increasing porosity. In this work, three types of flexible composite structures, dense sheet, porous film, and fiber mat, which were composed of polyvinylidene fluoride (PVDF) matrix and (Ba,Ca)-(Zr,TI)-O₃ (BCZT) ceramic particles, were produced. To evaluate the mechanical properties of these composites, the focus was on measurements of the dynamic stress-strain hysteresis loop. By analyzing the hysteresis loop, it was found that the storage modulus <i>G</i>' reduced by 77% for the porous thick film compared to the dense sheet, and the mechanical loss (tan ∂) was 0.51 for both the sheet and the thick film at the tensile strain rate of 2.0 mm/s (=2.5 Hz). Mechanical energy dissipated per cycle at 2 mm/s was 67.6 kJ/m³ for the sheet, 15.8 kJ/m³ for the thick film, and 0.127 kJ/m³ for the fiber mat. This study highlights the understanding that microstructure control, including pores and fibers, modulates dynamic stress patterns.