Ultra-Long Barium Titanate Nanowire Arrays for Low Frequency Energy Harvesting Applications

Piezoelectric nanowires (NWs) have recently attracted immense interest due to their excellent electro-mechanical coupling behavior that can efficiently enable conversion of low-intensity mechanical vibrations for powering or augmenting batteries of biomedical devices and portable consumer electronics. Specifically, nano-electromechanical systems (NEMS) composed of piezoelectric NWs offer an exciting potential for energy harvesting applications due to their enhanced flexibility, light weight, and compact size. Compared to the bulk form, high aspect ratio NWs can exhibit higher deformation to produce an enhanced piezoelectric response at a lower stress level. NEMS made of conventional semiconducting vertically aligned, ZnO NW arrays have been investigated thoroughly for energy harvesting; however, ZnO has a lower piezoelectric coupling coefficient as compared to many ferroelectric ceramics which limits its piezoelectric performance. Amidst lead-free ferroelectric materials, environmentally-friendly barium titanate (BaTiO₃) possesses one of the highest piezoelectric strain coefficients and thus can enable greater energy transfer when used in vibrational energy harvesters. In this paper, a novel NEMS energy harvester is fabricated using ultra-long (∼40 μm long), vertically aligned BaTiO₃ NW arrays which has a low resonant frequency (below 200 Hz) and its AC power harvesting capacity from low amplitude base vibrations (0.25 g) is demonstrated. The design and fabrication of low resonant frequency vibrational energy harvesters has been challenging in the field of MEMS/NEMS since the high stiffness of the structures results in resonant frequency often greater than 1 kHz. However, ambient mechanical vibrations usually exist in the 1 Hz to 1 kHz range and thus highly complaint ultra-long, NW arrays are beneficial to enable efficient energy conversion. Through the use of this newly developed synthesis process for the growth of highly compliant, ultra-long BaTiO₃ NW arrays, it is shown that piezoelectric NWs based NEMS energy harvesters capable of harnessing this low frequency ambient vibrational energy can be conceived.