Development of a piezoelectric micromachined ultrasonic transducer optimised to operate in the pore solution of reinforced concrete structures

Abstract

Structural Health Monitoring (SHM) of Reinforced Concrete (RC) is important to ensure that necessary interventions on concrete structures are conducted in a timely manner. Structural integrity may be effected by various chemical substances such as chloride ions which ingress the concrete’s pore structure and corrode the rebar. Timely detection of such chemical substances and subsequent intervention can avoid potential structural deterioration with ensuing potentially disastrous consequences. The conduct of an effective Structural Health Monitoring regime on civil engineering structures such as bridge decks, can be challenging due to inherent difficulties required to access specific, inaccessible parts of the structure, such as the underside of a bridge deck. This points to the setup of a SHM system, through a microscale distributed sensor network as being an effective proposition. Such a system can be made up of Micro Electromechanical Systems (MEMS) devices. The sensory elements forming the distributed network would be embedded within the concrete structure during the construction phase.

To achieve a durable system which is also easy to install during the structure’s construction phase, communication between the sensory elements would need to be conducted through wireless means. This dissertation explored the possibility of using microscale ultrasonic transducers as a means of implementing the inter device wireless communication channel required to achieve a viable distributed sensor system. This work’s primary contribution to the body of knowledge was therefore the development of the devices required to build the ultrasonic transmission path required to form the wireless communication channel. It needs to be clear that while the author has conducted prior work focusing on the sensory part of the system and also published papers in fields such as, the use of galvanic methods for detecting chloride ion ingress, research on the sensory system itself does not form part of this dissertation.

Reviewed literature indicated that for microscale ultrasonic devices to operate within an RC structure, two particular components needed to be considered. Firstly, liquid coupling was needed to effectively couple the transducer to the concrete structure. Secondly the frequency of the PMUTs’ operation needed to be in the region of 100 kHz and below. The focus of this dissertation was therefore the development of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) optimised to operate inside a liquid coupling fluid at this particular frequency range. This was found to be an area in which very sparse background research had been conducted and therefore it must be said that the nature of most of the research conducted in this dissertation was novel. This makes this dissertation a valuable tool which can act as an important background to other researchers in fields involving PMUTs deployed in liquid coupling fluids. Applications that may potentially utilise such technology are not limited to civil engineering but also encompasses areas such as the biomedical and marine engineering fields.

This dissertation outlines the extensive analytical, Finite Element Modelling (FEM) and experimental work conducted to explore the dynamics of PMUT design and operation. This included studies conducted with various variables being modified such as, filling the PMUT cavity with gas or liquid, the utilisation of different excitation frequencies, and also the utilisation of coupling fluids having different densities such as isopropanol or glycerine. Furthermore this dissertation also presents the development of various novel PMUT designs which were found to provide enhancements in ultrasonic reception or transmission performance. Such enhancements were based on designs such as multi electrode patterns and modified diaphragm structures. The devices developed in this dissertation were based on the PiezoMUMPSTM Multi Project Wafer (MPW) design concept. Aluminium nitride was used as the piezoelectric material found at the core of the devices’ operational dynamics.