(a) Photograph of obtained NaNbO3 nanowires after one time reaction. (b) Piezoelectric device scheme. Yellow, blue, and light blue layers correspond to the Au/Cr-coated Kapton film, NaNbO3–PDMS composite, and PS film, respectively. We show the photograph of a flexible NG device (inset). (c) Top-view optical microscope (left) and cross-section SEM (right) image of the device. For the top-view image, the upper Kapton film is removed. (d) Schematics of the piezoelectric power generation mechanism. Top: Alignment of dipoles after poling. Individual nanowire has ferroelectric (piezoelectric) domains with different electric dipoles. Each dipole (black arrows) has a component parallel to the electric field (green arrows). For each nanowire, we simply draw the same electric dipole component along the electric field direction. Bottom: Accumulation of free carriers in electrodes after compressive strain (see text for details).Scavenging energy from the environment at meaningful power densities remains an elusive and costly target for a range of small-scale applications, such as wireless sensors and autonomous information networks. In addition, prospects for scaling such technologies to provide real energy production from renewable sources could be a driver, but require much more stringent cost and scaled manufacturing targets. Environmentally renewable sources of energy that scale from miniaturized power sources to large scale energy production include solar, thermal, and vibrational, all of which have been enhanced by the incorporation of nano-enabled materials and nanomanufacturing processes. In many scenarios, an ideal system might include each type of energy conversion device implemented in a thin stack with energy storage elements. As this vision gains momentum, optimization of each type of conversion element is still required, both from a materials and scaled fabrication standpoint. In the case of vibration energy scavenging, research on nanowire (NW) piezoelectric power generation has focused on the use of zinc oxide (ZnO) or ferroelectric materials (lead zirconate (PbZrO3), lead titanate (PbTiO3), barium titanate (BaTiO3)). Piezoelectric nanogenerators fabricated from these materials have demonstrated modest power densities, yet still have challenges of scalable materials processes, along with the concern of toxic materials for scaled implementations.
Recently, Jung et. al. reported on the use of sodium niobate (NaNbO3) nanowires for lead-free, high output voltage piezoelectric power generation. The authors synthesized the NaNbO3 NWs using a facile hydrothermal at low temperature (150°C) which is capable of producing large quantities of high-quality, uniform NWs after one reaction step. The resulting NWs had diameters of 200nm with lengths of many tens of microns. The as-grown NWs filtered and washed in distilled water, then annealed at 600°C in an argon atmosphere. The annealed NaNbO3 NWs were thoroughly mixed with polydimethylsiloxane (PDMS) (1:100 volume ratio), then cast onto a chrome-gold (Cr/Au) coated polyimide substrate at a spin speed of 1000 rpm for 15 sec to form a NW-PDMS composite film with a thickness of ~100 µm as the active region of the piezoelectric nanogenerator. After curing, a similar Cr/Au coated polyimide layer was placed on top of the NW-PDMS composite film, thereby providing electrodes for the nanogenerator. The NaNbO3 NWs were poled by applying an electric field of ~80KV/cm at room temperature, which effectively sets the electric polarization of the piezoelectric NW crystals.
Comparison of generated power for NaNbO3 nanowire-based (solid circles) and nanocube-based (open cubes) NGs. For all compressive strain values, the output voltage (black symbols) and current (red symbols) of the nanowire-based NG are almost two times larger than those of the nanocube-based NG.Characterization of power generation by the NaNbO3 NW-PDMS composite was achieved using a linear motor stage to periodically apply and release a compressive force on the film. The nominal strain applied was 0.23% at a rate of 12.8%/sec with a frequency of 0.33 Hz. A peak output voltage of 3.2 V, and current of 72 nA was observed for these conditions, which translates to a volumetric power density of 0.6 mW/cm3 when considering the volume of the active layers only. Stability characterization further demonstrated the NW nanogenerator retained its performance even after 36,000 compressive strain cycles. Thus a potentially scalable path towards meaningful power densities for lead-free, environmentally friendly NW piezoelectric generators has been reported. Further enhancements may be achieved by increased loadings of the NWs in the active layer composite, along with improved alignment of the NW arrays in the composite in order to provide optimal polarization domains within the NW structures. Regardless, the reported approach and performance demonstration establishes a method by which a key energy harvesting material and device component may be realized for both small-scale ubiquitous power sources, as well as large scale, renewable energy sources.
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
Hoon Jung J, Lee M, Hong JI, Ding Y, Chen CY, Chou LJ, Wang ZL. 2011. Lead-Free NaNbO3 nanowires for a high output piezoelectric nanogenerator. ACS Nano. 5(12): 10041-10046. http://dx.doi.org/10.1021/nn2039033
Figure reprinted with permission from Hoon Jung J, Lee M, Hong JI, Ding Y, Chen CY, Chou LJ, Wang ZL. 2011. Lead-Free NaNbO3 nanowires for a high output piezoelectric nanogenerator. ACS Nano. 5(12): 10041-10046. http://dx.doi.org/10.1021/nn2039033. Copyright 2011 American Chemical Society.
