Preliminary Research on the Flexible Spiral Sensing Element Based on α-Fe2O3-Doped MWCNTs Coated With PTFE to Measure the Gap and Gas Concentration Between Narrow Curved Surfaces Simultaneously

To realize the simultaneous measurement of gap and gas concentration between narrow curved surfaces, in this article, a flexible spiral sensing element based on $\alpha $ -Fe2O3-doped multiwalled carbon nanotubes (MWCNTs) (mass ratio is 100) coated with polytetrafluoroethylene (PTFE) is prepared by spraying and spin coating technology using polyimide (PI) film as the substrate. The spiral structure has eddy current effect under ac excitation, which is sensitive to the change in the gap between the sensing element and metal target. Meanwhile, the $\alpha $ -Fe2O3-doped MWCNTs can respond to gas. The experimental results indicate that the gas response values of impedance amplitude, impedance angle, and resistance are 0.509%, 0.216%, and 1.376% at ethanol vapor concentration of 5000 ppm when the curvature radius is 50 mm with a gap of 0.5 mm, respectively. The absolute/relative sensitivities of impedance amplitude and impedance angle to the gap are 175 $711.54~\pm ~1424.85~\Omega \cdot \text { mm}^{-1}$ /11.347% $\pm ~0.095{\%}\cdot \text { mm}^{-1}$ and $2.614^{\circ }~\pm ~0.015^{\circ }\cdot \text { mm}^{-1}$ /5.307% $\pm ~0.033{\%}\cdot \text { mm}^{-1}$ in the gap range of 0–0.5 mm, respectively. In addition, according to impedance complex plane diagram of the sensing element, a signal processing method based on R–X is proposed, and the ability of flexible spiral sensing element to simultaneously measure the gap and gas concentration between narrow curved surfaces is preliminarily verified.