A quantitative assessment for the orientation and distribution of carbon fibers in the bipolar plate of fuel cell using high frequency ultrasound

Conductive polymer composites fabricated by adding a certain amount of conductive fillers, such as carbon black or carbon nanotube, into a polymer substrate are commonly used as the material of the bipolar plate in fuel cells. The electrical conductivity of polymer composites is affected by the distribution and orientation of fillers and is therefore crucial to be assessed. This study is to develop methods and techniques for nondestructively measuring properties of carbon fibers in the conductive polymer composite using a high frequency ultrasound system. The experiments were carried out from samples, fabricated by an injection molding machine, composed of a 5x5 cm polycarbonate added with carbon fibers of 6 μm diameter and 3 mm length and those of weight percentage at 0, 0.1, 0.2, and 0.3%. The ultrasonic signals of conductive polymer composites associated with different filler concentrations were acquired from a 48 MHz high frequency ultrasound imaging system. The raster scanning images corresponding to depths at 0.15 and 0.3 mm beneath the surface of conductive polymer composites were reconstructed. The orientation of filler carbon fibers parallel or perpendicular to the surface of conductive polymer composites were calculated according to their respective percentages of pixel content in the image. Results showed that percentages of pixel contents for fillers of carbon fibers at the depth of 0.15 mm beneath the surface of sample for those 0.1, 0.2, and 0.3 wt% filler carbon fibers were calculated to be 2.50±1.13, 5.19±1.70, and 5.93±1.29, respectively, and those of 0.3mm were to be 2.91±1.46, 6.18±1.60, and 6.42±1.77, respectively. Moreover, those percentages of pixel contents for fillers of perpendicular orientation with respect to depth of 0.15mm were 1.49±0.89, 2.49±0.96, and 3.62±0.95, respectively; those of 0.3mm were 1.39±0.72, 2.70±0.62, and 3.97±0.94, respectively. This study demonstrated that current high frequency ultrasound image incorporated with the analysis method is feasible to be applied to quantitatively and rapidly assess the distributions of fillers in conductive polymer composites.