Reinforced Ferromagnetic Properties in CNC@CoFe₂O₄/P(VDF-TrFE) Nanofiber Composites for Magnetic Energy Harvester

After the fourth industrial revolution, the worldwide commercialization of electronic devices has been proceeding. According to the rising demand for electronic devices, demands for energy sources have also been increasing. To satisfy it, energy harvesters which can scavenge energy from ambient circumstances have been developed. Despite the diverse ways to convert energy obtained from the environment to a practical energy source, there are several limits to its application into daily life. The energy harvesters working under contact mode, in particular, would be exposed by performance degradation due to the continued contact or restricted in specific areas where the physical contact is not permitted. In this respect, the issues mentioned above could be solved by introducing the magneto-mechano-electric (MME) materials. The MME effect is defined as the electrical polarization in the ferroelectric piezoelectric (FP) phase induced by strain originated from physically adjacent ferromagnetic magnetostrictive (FM) phase under external magnetic field, and it is basically based on the non-contact mode. When it comes to polymer-based MME composites, representatively, it has been drawn attention from many research groups because of its advantages including generated voltage which is sufficient enough to operate the low voltage device, high cost-efficiency, and versatility of applicable structure due to the high flexibility. The reported method in general to achieve high output voltage from this type of MME composites so far was the surface or composition modification for FM phase material which is responsible for stress causing the polarization of the FP phase. Many researchers have tried to treat the FM phase materials by adding a coating agent to control the morphological character or regulating the chemical composition via post-treatment. In consequence, the overall process was getting longer due to the additional treatment, moreover, the ferromagnetism related to MME properties in direct, fundamentally, was diminished ascribed to the lower actual weight fraction of FM phase compared to the pure state.<br/>In this research, the new way to reinforce the ferromagnetism, which is in proportional to MME properties, in magnetic filler was suggested. To be specific, the cellulose nanocrystal(CNC) was incorporated into the reaction for the synthesis of the CNC@CoFe₂O₄(CFO) nano hybrid filler. The prepared fillers exhibit higher overall ferromagnetic properties including maximal saturation magnetization (<i>M</i>_s) of 71.8 emu/g and remnant magnetization (<i>M</i>_r) of 32.9 emu/g increased by 22.5% and 24.2%, respectively, in comparison with conventionally synthesized pure CFO nanoparticles even without any further treatment. To interpret the origin of the results, the morphological, surface chemical structural, crystallographic, and ferromagnetic analysis were performed. Through the investigation, it was found that the reinforced magnetic properties were largely due to the expanded crystallite size attributed to the nucleation site provided by introduction of CNC. Thereafter, to investigate the influence from reinforced ferromagnetism of hybrid fillers to the MME properties in composites, the CNC@CFO/P(VDF-TrFE) nanofiber composites was designed. It was observed that the ferromagnetism in composites with hybrid filler was enhanced, manifesting <i>M</i>_s of 4.17 emu/g and <i>M</i>_r of 2.03 emu/g increased by 36.8% and 51.5%, respectively, compared to composites prepared with equal contents of CFO without CNC. Finally, the MME composites were subjected under cyclic magnetic driven voltage measurement based on the bending motion. As a result, it was confirmed that the output voltage generated from the MME composites with CNC was 219.7 mV which is as higher as 72% than the composites prepared without CNC. On the basis of these results, it was concluded that the prepared MME composites with CNC are of the potential for non-contact energy harvesting application.