Anatomically informed mesoscale electrical impedance spectroscopy in southern pine and the electric field distribution for pin-type electric moisture metres

Abstract

Electrical impedance spectra of wood taken at macroscopic scales below the fibre saturation point have led to inferences that the mechanism of charge conduction involves a percolation phenomenon. The pathways responsible for charge conduction would necessarily be influenced by wood structure at a variety of sub-macroscopic scales – at a mesoscale – but these questions have not yet been addressed. The goal of this work is to explore if mesoscale anatomical features in wood affect impedance spectra. Small (0.5 mm diameter) needles were used as electrodes and were configured such that the line segment between the electrodes could be oriented radially, tangentially, longitudinally and in combinations of those directions in both earlywood and latewood, including comparisons of earlywood–latewood transitions. The spectra were fit to an equivalent circuit model with a constant phase element in parallel with a resistor and Warburg element that describes ionic conduction. Finite element simulations were run to examine the effect of the fringing electric field near the electrodes. The simulations revealed that the current density was concentrated at the electrodes, resulting in a lack of dependence on electrode spacing thus explaining why measurements taken with pin-type electric moisture metres are nearly independent of electrode geometry.