A flexible Polyimide based SAW delay line for corrosion detection

The focus of this paper is to explore a most suitable piezoelectric material for a surface acoustic wave (SAW) sensor to monitor corrosion for curved and complex surfaces at elevated temperatures. A comparison between piezocrystal Quartz and piezopolymers PVDF and Polyimide is made on the basis of simulation. It is found that Piezoelectric Polyimide is a promising material that fulfils our requirements. Finally a flexible Polyimide based surface acoustic wave (SAW) delay line is proposed to detect and monitor corrosion. It consists of a pair of Interdigital Transducers (IDTs) and an intermediate metal sensing element. Generation and detection of SAW on Polyimide piezoelectric substrate are obtained through input and output IDTs made of highly corrosive resistant material i.e. platinum (Pt). The SAW is propagated between the IDTs and underneath a sensing element made of iron (Fe). The device parameters like central frequency, SAW wavelength, number of finger pairs and others are designed using impulse response model. The theoretical phase shift as a function of sensing element´s thinness is calculated using mathematical equations and then compared to the phase shift obtained using coupling-of-mode (COM) Model. It is found that thinner the sensing element, the higher the phase shift, especially when thinness of sensing element is at nm scale. It is also noticed that at central frequency greater than 400 MHz, the phase shift is quite high and easily measureable. Since corrosion can be considered as thinning of a metallic element, the results suggest that it is feasible to develop a flexible Polyimide SAW based sensor for corrosion detection.