background: Wood is increasingly used in more demanding, load bearing applications such as high-rise buildings, and a number of new engineered wood products have been developed fairly recently such as cross-laminated timber. In addition, hardwood species such as beech or ash are becoming more important for such products. In light of these new applications and with the goal of a more efficient and advanced use of wood, a better insight into structure-mechanics relationships and into wood-water interaction are needed.
Key parameters are structure and orientation of the cellulose microfibrils in the wood cell walls and the distribution and diffusion of water in wood. Terahertz (THz) technology provides the unique possibility to analyze and image these key parameters and properties of wood with the big advantage of being a non-hazardous technique allowing for immediate sample manipulation during the experiments. This makes the THz technology in particular suitable for in situ diffusion or mechanical experiments, applying various climatic conditions and imaging over longer times. Furthermore, due to the small size of sources and detectors, it is well-suited for the development of mobile instruments for on-site investigations and has the potential of becoming a frequently used key technology in wood science.
Project description: We have identified three key aspects in THz technology, which can have a significant scientific impact on the understanding and characterization of wood properties:
(i) THz radiation is very sensitive to humidity and water. Due to the different spectral properties of water compared to those of the cell wall polymers it allows for a spatially and temporally resolved measurement of water content and diffusion in wood.
(ii) As the anisotropic optical properties of wood and cellulose correlate strongly with the cellulose microfibril orientation, THz radiation is an excellent medium for probing the spatial structure of wood relevant to mechanical properties.
(iii) Internal stress in a material is also a cause for anisotropic optical properties. We presume that we can correlate changes in optical anisotropy at THz wavelengths with stress changes in wood.
Previous studies, including own work, dealt with these topics; however, they are mainly based on single spot measurements. Our goal is to build and use a spectroscopic THz imaging setup in a humidity controlled environment with the possibility to integrate additional devices such as simultaneous weighting or in-situ tensile tests. This will allow performing advanced research by in-situ imaging of wood structure and diffusion processes.