Non-Contact Spatially Resolved Monitoring of Ceramic Drying Process

The drying phase in ceramic production has a large influence on the quality of the end product. The industry standard for monitoring this process is gravimetry, but the lack of spatial moisture information from this approach leads to limited quality control. Furthermore, the process can be inefficient and invasive, relying on previous data to determine optimal drying time and requiring an operator to handle samples during testing. This paper demonstrates that TeraHertz time-domain spectroscopy (THz-TDS) is an improved inline moisture sensor in ceramic production. The spatially resolved transmission and reflection properties of two ceramic samples were investigated with a commercial THz-TDS during their drying process and cross-correlated with gravimetry. A water loss of 0.033 – 0.034 g per g of sample weight was mapped onto a 1300 – 1700% increase of the peak-to-peak (p2p) amplitude of the THz transmission signal, and in turn, into a 0.05 - 0.07 (5 – 7%) decrease of refractive index, as well as a 0.53 - 0.60 (53 – 60%) reduction in absorption coefficient, demonstrating the sensitivity of the THz-TDS approach to water content. A simple effective medium theory (EMT) model for transmission measurements was used to confirm the potential of THz-TDS for quantitative spatial information of moisture content. The cross-correlation of reflection and transmission THz-TDS data with the gravimetry method also highlighted the potential of the former technique to discern between free and bound water, opening avenues for fundamental research on the topic.