Graphene/polymer strain sensors for structural health monitoring

Supervisory Team:

Dr Alain Nogaret, Department of Physics
Dr Dan Pantos, Department of Chemistry
Dr Richard Ball (Department of Architecture and Civil Engineering, Bath)
Prof Tim Ibell (Department of Architecture and Civil Engineering, Bath
Dr John Orr (Department of Architecture and Civil Engineering, Bath)

Motivation:

Deterioration in infrastructure, combined with limitations in visual inspection, calls for new ways of monitoring the health of buildings and vessels. In order to reduce inspection costs and increase public safety, key structures, such as bridges and tunnels, incorporate hundreds of sensors that validate structural design, improve maintenance planning, anticipate seismic risk and eventually assist with emergency response efforts. Traditional strain gauges measure changes in the resonant frequency of a steel wire tensioned between two end blocks anchored in concrete. These sensors are expensive ($15k/sensor at the Bill Emmerson memorial bridge) and are too large (up to 1 metre) to be embedded within infrastructure materials where they are needed to detect the onset of cracks. Lessons from catastrophic bridge failures have underlined the limitations of current technology and are calling for dense arrays of inexpensive sensors which are small enough to be implanted in construction materials and transmit wirelessly to a centralized data collection system.(1)

Work programme:

This studentship will demonstrate graphite/polysiloxane strain gauges for structural health monitoring (SHM) which will provide a first line of defence against various vulnerabilities:

1. Initially, the student will synthesize composites of HOPG graphite nanoparticles in a polysiloxane matrix and optimize their electromechanical response for SHM sensing. This composite is a conductive elastomer which may be embedded in construction materials without compromising structural integrity. Compressive strain increases resistance linearly as it modifies the percolation network. We have the ability to engineer the sensitivity to strain by tuning the height and width of the tunnelling barrier of the polysiloxane matrix. We achieve this at the microscopic level by functionalizing HOPG nanoparticles with naphthalene diimide molecules synthesized in Pantoş’s group and by controlling the graphite filling fraction.(2)(4) The PhD student will mix, functionalize and mould composite micro-sensors on flexible poly-ethylene naphthalate (PEN) substrates using imprint lithographically.(2)(3) Strain will be applied by bending the PEN/composite bilayer to optimize the material response to lime and concrete. 3D arrays of sensors will then be fabricated in lime/concrete blocks and remotely interrogated via a Bluetooth wireless link. This setup will allow the student to measure the volume distribution of strain in-situ for the first time, in particular the onset of cracks during plastic deformation. The student will conduct these experiments on concrete and lime blocks in the laboratory of Richard Ball and Tim Ibell in civil engineering. This work builds on existing instrumentation and knowhow and will be completed within the first year.

2. In years 2 and 3, the student will synthesize single tunnelling junctions consisting of two graphene electrodes separated by an elastomeric tunnelling barrier. The graphene electrodes will be functionalised with naphthalene diimide molecules. The 4 aromatic cores form Van der Waals bonds to the graphene electrode whilst the conjugated chains attached to the imide sites dangle out of the graphene plane. We will use metal catalyst AgNO2 to bind the molecular endings of opposite graphene electrodes when these are contacted together in a mask aligner. The change in tunnelling resistance with strain will be calibrated and studied as in Part 1. Graphene/NDI/graphene trilayers will have faster response times than composites, fully reversible strain cycles and be capable of directional strain sensing. Such advanced sensors would constitute a breakthrough enabling the 3D tomographic reconstruction of cracks and defects. The fabrication work will be carried out within the nanofabrication facility at Bath. Naphthalene-diimide molecules have already been successfully synthesized in the group of Dr Pantos with a range of aromatic, siloxane attachments which are useful to control the tunnelling current between graphitic nanoparticles.(1)(2) Cracking in concrete structures is a particular type of structural defect which is of primary importance to the longevity of our infrastructure, given that internal reinforcement is exposed directly to the surrounding environment, which can be highly toxic and/or inaccessible to humans.

This scientific project will contribute to improve the safety of infrastructure, protecting lives and benefiting society as a whole.

Anticipated start date: 2 October 2017.

Note: Applications may close earlier than the advertised deadline if a suitable candidate is found; therefore, early application is strongly recommended.