Structural Health Monitoring
The verification of design assumptions and tracking of structural performance over time are critical for numerous civil, mechanical, environmental, naval, aerospace, and medical applications. Visual inspection is a common method used to detect structural damage, but can be expensive, subjective, and limited by accessibility. Discrete sensor measurement at highly localized points on a structure is a valuable and reliable procedure for structural health monitoring; however can be limited by the types of surfaces that these sensors can adhere to as well as the amount of surface area that can be assessed. There is great interest in advancing sensing technologies through cost reduction, improving sensor materials, and increasing the types and amounts of surfaces that can be successfully monitored.
Patterned Nano-engineered Thin Films on Flexible Substrates for Sensing Applications
Carbon nanotubes are an appealing material for sensing applications due to their exceptional physical properties and electrical characteristics, as well as their ability to increase film strength and elicit electrical responses to mechanical deformation when dispersed within polymers. The proposed technology is a carbon nanotube thin film for sensing applications developed using standard MEMS lithography and lift-off processes. Thin films were layered onto a glass slide and the fabricated material was subsequently detached from the slide to produce a free-standing thin film. Four layers of polyimide (approximately 12 microns thick) were spun coated onto the slide to produce a film with a thickness of approximately 48 microns. The polyimide was cured and a photoresist (PR) was patterned onto the substrate using optical lithography. During the optical lithography process the polyimide was dried and primed with the application of a hexamethyldisilazane (HDMS) layer. The PR layer was spun over the sample and the polyimide that was not covered with PR was soaked with poly-l-lysine to promote surface adhesion with the polyelectrolytes that comprised the nanocomposite film.
The nanocomposite film was created with the layer-by-layer (LbL) fabrication process which utilized oppositely charged polyelectrolyte solutions to attract thin layers of each solution to the substrate surface. The substrate was sequentially dipped in two solutions of opposite charge to build up a well-controlled and uniform thin film. The positively charged solution was 1.0 wt. % Poly(vinylalcohol) (PVA) and the negatively charged solution was 1.0 wt. % poly(sodium 4-styrenesulfonate) (PSS). Single wall carbon nanotubes (0.1 wt. %) were dispersed in the PSS solution. Single monolayers (PSS or PVA) were deposited and the process was repeated using the oppositely charged polyelectrolyte solution to yield a single thin film bi-layer. The bilinear process was repeated until a thin film with a thickness of 50 bilayers was fabricated.
The nanocomposite film was deposited over the polyimide and PR by the LbL directed assembly method. After this step was completed, an acetone bath and sonication was used to lift the carbon nanotube composite film from the PR covered areas. The specimens fabricated by the LbL assembly on glass slides were patterned as 5 parallel strips approximately 2 cm long with varying thicknesses of 10 µm, 250 µm, 500 µm, 1000 µm, and 1500 µm. The film was then annealed and the polyimide substrate was removed by etching a layer of glass with buffered hydrofluoric acid. Square pads where silver paste could be applied to create a wired electrical connection to a data acquisition system for film resistance measurements were added to the ends of the strips. Fabrication can be scaled up for the development of component-specific strain sensors over large areas. Larger-scale sensors were patterned in the center of a 10.2 cm glass wafer in 8 parallel strips approximately 3.5 cm long, 1.5 mm thick, and 4.5 mm apart. The fabricated sensors were defect free and able to effectively track the strain of steel-beam column structural specimens during cyclic load testing. The developed sensors display limited noise and comparable sensitivities as well as higher strain sensitivities for the more resistive films.
- Monitoring structures
- Research and Development
- Utilizes standard MEMS lithography and lift-off processes
- Patterning has two-dimensional resolutions of 1 μm or less
- Can measure complex structural responses
- Sensor can be applied to complex structural surfaces or on soft bodies
- Multiple sensors can be built on a single flexible substrate