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 (BaTiO 3 ) 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 BaTiO 3 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 BaTiO 3 NW arrays, it is shown that piezoelectric NWs based NEMS energy harvesters capable of harnessing this low frequency ambient vibrational energy can be conceived.

Evolution of the domain structure in bulk polycrystalline PZT during poling was studied using Piezoresponse Force Microscopy (PFM). For the study, two different experimental methods were employed. First, a trapezoidal PZT specimen was subjected to electric field so as to produce a wide variation of electric field intensity in the specimen. PFM images were then acquired from several different areas that have experienced different field strengths. Histograms of pixel intensity show a distinct difference in the pattern of piezoresponse signal between poled and unpoled areas. The presence of non-180° domain structure in the scanned area significantly affects the histogram pattern. At high levels of electric field the presence of mainly 180° domain structures leads to a bi-modal M-shaped histogram. To illustrate the evolution of the non-180° domain structure, in-plane poling was conducted with the electric field level increased in steps, and the domain evolution process was observed by PFM after each step. The resulting images demonstrate that non-180° domain structures gradually disappear from the specimen surface during the poling process. The PFM data can be exploited to study domain evolution in bulk ferroelectric materials via both qualitative observation and statistical analysis.

The strain hystereses of lead-free a (Bi 0.5 Na 0.5 )TiO 3 - b BaTiO 3 - c (Bi 0.5 K 0.5 )TiO 3 (abbrev. as BNBK 100 a /100 b /100 c ) ferroelectric compositions inside and outside the morphotropic phase boundary (MPB) are investigated. It is found that BNBK 85.4/2.6/12, a composition well within the MPB, possesses notable actuating properties such as an induced electrostrain of about 0.14% and an apparent d 33 of 295 pCN −1 . BNBK 85.4/2.6/12 is further doped with various amounts of manganese (Mn) to improve its sinterability and ferroelectric characteristics. Intricate hysteresis behaviors are observed upon Mn doping. The total induced electrostrain of BNBK 85.4/2.6/12 in the 33-directon decreases dramatically from 0.14 to 0.05% when 0.2 mol% of Mn is introduced into the composition. It then recovers sharply as the Mn doping amount is increased progressively to 0.5 and then to 1.0 mol%. However, when the doping amount is further increased above 1.0 mol%, a significant decrease in electrostrain is observed again. The hysteresis data indicate that an electrostrain above 0.1% can be maintained when the Mn doping amount is in between 0.5–1.5 mol%. Once outside this doping range, the induced electrostrain is considerably smaller. By examining the evolution of crystalline phase composition with Mn doping, the mole content of rhombohedral phase is shown to be a critical factor in deciding the straining behaviors of the Mn-doped BNBK 85.4/2.6/12 ceramics. To each increasing step in Mn doping level, there is a marked similarity in the evolutions of the induced electrostrain and rhombohedral phase content.