Dielectric Relaxation Based on Adsorbed Water in Wood Cell Wall under Non-Equilibrium State. Part 3. Desorption

Summary The temperature and frequency spectra of dielectric constant ε′ and dielectric loss factor ε″ of Sikkim spruce (Picea spinulosa Griff.) were measured to investigate the change in dielectric relaxation of water in wood during desorption. In order to control the rate of moisture content change, the measurements were carried out in three steps: from fiber saturation point to 80% RH, from 80% RH to 60% RH and from 60% RH to 20% RH, at 25°C. Two dielectric relaxation processes were observed in different temperature and frequency regions which changed their position and strength with the desorption process. Using the ε′ and ε″ spectra, two groups of Cole-Cole plots were obtained, on which basis two groups of dielectric parameters including the static dielectric constant εS, optic dielectric constant ε∞, relaxation strength (εS–ε∞), and the relaxation time distribution coefficient α were calculated. Both groups of parameters showed similar trends, that is, ε∞ remained nearly constant during the whole desorption process. εS and (εS–ε∞)changed little during the first step of desorption, decreased obviously during the second step and declined slightly during the third step. The α value obtained from the lower frequency data changed significantly during the second and third desorption steps, while there was little change in the other group of α values. These differences can be explained by different mechanisms of the relaxation processes in the lower and higher frequency regions.