Scanning acoustic microscopy for GDL water distribution characterization

The objective of this research was to identify and apply possible acoustic imaging techniques for water transport studies in PEM fuel cell diffusion mediums. To accomplish this, a scanning acoustic microscope (SAM) was designed and built along with dedicated control and analysis software to implement all investigative tests and scans. The results of the study were in two parts, acoustic response characterization of the fibrous material and implementation of the developed imaging technique for water saturation distribution characterization.Having used a focused 30MHz centered wide band transducer with focal point FWHM beam diameter of 90mum the target material ensured a ka number approximately equal to one given the average fiber diameter around 8mum. The result was a diffractive acoustic response subject to phase sensitive interference. Traditional ultrasonic techniques for material characterization or imaging were inapplicable so a signal correlation ratio method was developed which relies on the variations in acoustic responses between two scans. Results from different GDL samples at varying levels of saturation were compared to characterize their individual spatial distribution patterns.