Microwave complex permittivity and anisotropy of conifer wood chips vs moisture content: experiments and modeling

• Published in: Journal of wood science 2022-12, Vol.68 (1), p.22-14, Article 22
• Main Authors: Rönnow, D., Ottosson, P., Andersson, D.
• Format: Article
• Language: English
• Subjects: Anisotropy; Biomedical and Life Sciences; Characterization and Evaluation of Materials; Complex permittivity; Effective medium modeling; Life Sciences; Materials Science; Moisture content; Moisture effects; Original Article; Permittivity; Radio transmission; Saturation; Soft wood; Softwoods; Ultra wideband radar; Wood; Wood chips; Wood Science & Technology
• ISSN: 1435-0211; 1611-4663; 1611-4663
• DOI: 10.1186/s10086-022-02026-5

The complex microwave permittivity—including anisotropy- of wood chips of softwood has been measured for different moisture contents in the band 0.75 to 2.5 GHz using an ultra-wide band radio transmission technique. The real and imaginary parts increase monotonically with moisture content. The wood chips are oriented by gravity, which gives anisotropic permittivity. The anisotropy ratio of the real part increases from 1.1 to 1.6 with moisture content from 0 to 120%. The anisotropy ratio of the imaginary part is around 2.5 at all moisture contents. Effective medium models were used to model the permittivity. The Bruggeman, and two versions of the Maxwell Garnett model gave good results at low moisture content (below the fiber saturation point). Above the fiber saturation point only the Bruggeman model gave results in agreement with experiments. The difference in model performance suggests that the free water does not follow the wood chips geometry.