In-situ monitoring of moisture diffusion process for wood with terahertz time-domain spectroscopy

The moisture diffusion process in balsa wood was monitored using the terahertz time-domain spectroscopy (THz-TDS) technique. The reflective THz signals were measured during the drying and the absorption phase of the moisture in the balsa wood. Measurements were conducted along the direction of the wood grain (longitudinal) and perpendicular to the direction of the wood grain (tangential). From the results, the relationship between the pulse intensity of the THz signal and moisture content was quantitatively investigated. Further, an image of the moisture concentration gradient was constructed through the THz intensity analysis. From the study, it was possible to monitor the variation of diffusion coefficient depending on the moisture content gradient from the THz measurement. Moreover, using the measured concentration gradient, the moisture diffusion coefficient could be derived regardless of complicated boundary conditions of diffusion. Consequently, the degree of moisture diffusion can be successfully measured using the THz-TDS technique. This can then be utilized for the determination of the moisture diffusion coefficient in various industrial structures.