Author: Zhou, Pengyu
Title: Additive manufacturing-driven thin film ultrasound sensors : from sensing ink development to applications in ultrasonics-based structural health monitoring
Advisors: Su, Zhongqing (ME)
Zhou, Li-min (ME)
Degree: Ph.D.
Year: 2021
Subject: Detectors
Thin film devices
Structural health monitoring 
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xxx, 212 pages : color illustrations
Language: English
Abstract: Structural health monitoring (SHM), a bionic paradigm inspired by the manner of information perception and decision-making of human beings, has shown appealing promise in safeguarding engineering assets. Amidst diverse SHM approaches, the acousto-ultrasonic wave-driven SHM, which leverages numerous merits of acousto­ultrasonic waves, strikes a balance among resolution, detectability, practicality, and cost, well corroborating the concept of in situ SHM. Central to the realization of in situ acousto-ultrasonic wave-driven SHM is the acquisition of acousto-ultrasonic wave signals. Nevertheless, for most of the prevailing sensors that are developed for entertaining such a demand, a challenge remains: how to compromise "sensing effectiveness" with "sensing cost"?
In this PhD study, a series of thin film ultrasound sensors are developed by virtue of a direct-write additive manufacturing (AM) approach –inkjet-printing. The sensing inks and printed sensors are morphologically tuned at nano scales, driving the sensors to be highly sensitive to acousto-ultrasonic waves in a broad band regime, from static strain to high-frequency ultrasound of frequencies up to 1.6 MHz. Being ultra-thin and lightweight, the sensors feature a homogenous, consolidated nanostructure, with which transient change of the tunneling resistance among adjacent electrical-conductive nanoplatelets in the polymeric matrix can be triggered, when the sensors are loaded with dynamic strains induced by acousto-ultrasonic waves. It is the triggered quantum tunneling effect that endows the sensors with capability to respond to dynamic acousto-ultrasonic signals of high frequencies with excellent fidelity and accuracy.
Based on the mechanism study, a nanocomposite-based sensing ink, formulated with carbon black (CB) nanoparticles and polyvinyl pyrrolidone (PVP), is developed. The sensing ink is rigorously designed and morphologically optimized to be stable, printable and wettable. By directly depositing the sensing ink on flexible polyimide (PI) substrates, ultralight, flexible, nanocomposite thin film ultrasound sensors are produced via drop-on-demand inkjet printing. With the quantum tunneling effect triggered among CB nanoparticles, the printed CB/PVP film sensors have proven capability of in situ, precisely responding to dynamic strains in a broad range from quasi-static strain, through medium-frequency vibration, to strain induced by acousto­ultrasonic waves up to 500 kHz. Notably, the sensitivity of the sensors can be tuned by adjusting the degree of sensor conductivity via controlling the printed passes, endowing the sensors with capacity of resonating to strains of a particular frequency, authenticating that inkjet-printed thin film ultrasound sensors can be tailor-made to accommodate specific signal acquisition demands.
To further enhance the sensitivity and expanding responsive range of the sensors, morphologically optimized NGP/poly (amicacid) (PAA) hybrid-based nanocomposite ink is synthesized, with which nanographene platelets (NGP)/PI sensors are fabricated. The ink is produced with high-shear liquid phase exfoliation (LPE) from inexpensive bulk graphite, manifesting good printability and graphene concentration as high as 13.1 mg mL-1. Featuring an ultra-thin thickness (~ 1 μm only), the inkjet-printed NGP/PI film sensors are demonstrated to possess excellent thermal stability and high adhesive strength reaching the American Society for Testing and Materials (ASTM) 5B level. The uniform and consolidated NGP/PI nanostructure in the sensors enables the formation of π-π interactions between NGPs and PI polymer matrix, and consequently the quantum tunneling effect is triggered among NGPs when acousto­ultrasonic waves traverse the sensors. This sensing mechanism facilitates the NGP/PI sensors with comparable performance as prevailing commercial ultrasound sensors such as piezoelectric sensors. The film sensors demonstrate a gauge factor as high as 739, when sensing ultrasound at 175 kHz, and a ultrabroad responsive spectrum up to 1.6 MHz. This is first ever that an inkjet-printed thin film ultrasound sensor responds to dynamic strains in such a broad band and acousto-ultrasonic waves of such a high frequency.
To examine the effects of aggressive environmental exposures to the inkjet-printed thin film ultrasound sensors, the sensing performance of the sensors in acquiring broadband acousto-ultrasonic wave signals is scrutinized in an extensive regime of temperature variation from –60 to 150 °C, which spans the thermal extremes undergone by most aircraft and spacecraft. Under high-intensity thermal cycles from –60 to 150 °C, the sensors exhibit stability and accuracy in responding to signals in abroad band as well. Compared against conventional ultrasound sensors such as piezoelectric wafers, inkjet-printed film sensors avoid the influence of increased dielectric permittivity during the measurement of high-frequency signals at elevated temperatures.
With proven sensitivity, sensing accuracy and stability, the inkjet-printed thin film ultrasound sensors are further developed into an all-printed nanocomposite sensor array (APNSA), in lieu of conventional ultrasonic phased array which is of a low degree of integrity with composites, to ameliorate ultrasonic imaging of composites. Individual sensing elements of APNSA are inkjet printed by directly writing sensing inks on Kapton film substrates. Compared with a conventional ultrasonic phased array, APNSA can be fully integrated with the inspected composites. In conjunction with the use of the additively manufactured APNSA, ultrasonic imaging of composites can be implemented, spotlighting a nature of full integration of APNSA with composites for in situ SHM and anomaly detection, yet without degrading the original integrity of the composites.
In conclusion, starting from mechanism study, through design to fabrication of sensing inks, new breeds of thin film ultrasound sensors are developed via inkjet printing. Successful application paradigms of the thin film ultrasound sensors have accentuated the alluring potentials of the new sensors in fulfilling real-world in situ acousto­-ultrasonic wave-driven SHM.
Rights: All rights reserved
Access: open access

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