A strain sensor based on nano carbon composite can be designed its piezoresistive characteristic by various factors such as type of the nano carbon isotope, content weight percent (wt%) of the nano carbon filler, the geometric sensor pattern, and so on. However, the piezoresistivity design for the sensor still mainly depends on trial and error approaches due to not only lack of fabrication information but also uncertainties of the nano composites mainly comes from fabrication process as well. Thus, from engineering view point, the piezoresistivity design strategy is urgently required for sensors applications.
In this study, a piezoresistive characteristics for sensor fabrication was studied based on piezoresistivity models in physics. Sensitivity characteristics of strain sensor were studied through the fabrication variables as described above. The sensors made of MWCNT (multi-walled carbon nanotube) / epoxy composites were prepared by varying wt% and nano filler types. After that, their piezoresistive sensing characteristics were experimentally studied. From this investigation, piezoresistive sensitivity was proportional to length and thickness of the sensors, and the sensitivity was inversely proportional to width of the sensors. As the sensor thickness increases, it could improve the piezoresistivity but it might deteriorate the actual deformation of the installed target material.
For applications of the piezoresistive sensors, following nano carbon composite sensors were developed and their piezoresistive characteristics were studied as well. Strain and torque sensors based on MWCNT/epoxy were fabricated and their sensing performances such as sensitivity, signal stability and linearity were proved as equivalent compared with conventional foil strain gauges. Pressure sensors based on MWCNT/epoxy were fabricated by using a bulk type and a 3D printed cantilever type. The cantilever type pressure sensor could measure up to 16,500kPa, which was about 200% higher pressure range than the bulk type. Novel impact paint sensors were developed MWCNT, exfoliated graphite nano-platelets (xGnP), and a hybrid type of the two nano-carbon fillers and they were sprayed onto a carbon fiber reinforced plastic (CFRP) panel for lab testing. In ball drop impact test, the MWCNT-xGnP-based hybrid sensor showed the best characteristics in impact energy sensing within the range 0.07-1.0J. The piezoresistive mechanism due to structural dimension variations of nano carbon isotopes for sensor design was also investigated by means of the piezoresistive model. From this study, the piezoresistivity of nano-carbon sensor was significantly dominated the electrical contact variation of the electrical fillers in a matrix.