Non-destructive Characterization of Soot in Exhaust Filters Using Millimeter-wave Imaging

It is necessary to monitor the distribution of the soot that accumulates in the soot removal filter of an internal combustion engine to design the highly efficient filtering structure and optimize the timing of the filter renewal. However, there is no non-destructive inspection technique at present. In this study, we describe a non-destructive inspection system that can monitor the distribution of soot that accumulates in the filter using combination of millimeter-to-terahertz wave imaging and computed tomography. The transmittance of the filter shows a strong dependence on the angle of incidence because of its two-dimensional periodical shape. We identified a frequency at which photonic band gaps did not occur, and measured the three-dimensional tomographic image of the filter in a non-destructive manner.