Zinc oxide nanowires for dynamic strain sensing

A dynamic strain sensor using piezoelectric zinc oxide nanowires was demonstrated for potential application in structural health monitoring. Simulations and reviews of literature determined that strain of the nanowires by uniaxial compression yields the largest piezoelectric potential and that the piezoelectric coefficient of zinc oxide nanowires is enhanced due to nanoscale size effects. The fabrication of zinc oxide nanowires on various substrates was investigated in order to determine the ideal materials and seed layer deposition methods to yield high quality vertically-aligned nanowires. Nanowires were grown on indium tin oxide-coated glass slides. The tips of the nanowires were electrically connected using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) conductive polymer, which formed a Schottky barrier with zinc oxide allowing for the separation of charge across the nanowire-electrode junction. The piezoelectric coefficients of several fabricated devices were measured by applying pressure to the top of the nanowires and measuring the charge. Variations in performance between the different sensors were observed due to differences in the fabrication of each sensor. The highest coefficient measured was 11.5 pC/N, which is 16% higher than the bulk value for zinc oxide. The charge and voltage sensitivity to quasistatic pressure loading of the best performing sensor was calculated to be 1.32 pC/kPa and 16.7 mV/kPa. The response to clamped pressure stimulation from 1-90 kHz was evaluated using a piezoelectric stack actuator coupled with the zinc oxide nanowire sensor. The sensor showed excellent linearity to different amplitude vibrations at 1 kHz, and reasonably constant magnitude of charge output over the 1-90 kHz range for a constant vibration amplitude. The resonant frequency of the sensor and the response to free vibration could not be measured due to limitations in the available measuring equipment. The fabrication process for the nanowire sensor was found to be simple but inconsistent and could be improved by using repeatable processes such as photolithography for precisely defining electrode and seed layer geometries. The as-fabricated nanowire sensor shows promise as a dynamic strain sensor for structural health monitoring applications or pressure sensing but requires further characterization and optimization through modeling in order to compete with commercial sensors.