Establish the dielectric properties of a number of wood species for a range of frequencies, temperatures and pressures

The present research offers an assessment of a new way of approaching the dielectric properties of wood. The concept takes into consideration a probable influential factor which has never been considered and refers to the elevated pressure which develops within wood during microwave processing. In addition, in order to understand and analyse the wood dielectric properties’ dependency on the internal temperature and pressure, a custom-made device and the appropriate measuring procedure, as well as a numerical model which calculates complex dielectric permittivity of an anisotropic wood structure is presented. Initially, the fundamentals of microwave–wood interaction including polarisation mechanism, factors affecting the dielectric response of wood, microwave related characteristics of wood and its microwave heating process are examined. The importance of measuring the wood dielectric properties is also posed. Subsequently, an insight into the main aspects of the techniques used in other industries to measure the materials dielectric properties is provided. The report ends with a comparative discussion of the measurement techniques applicability. A method for measuring the dielectric permittivity of wood is then presented together with a description of the software required to extract the dielectric parameters from the measured quantities. The experimental results of the dielectric parameters of the main Australian wood species are also presented. Besides, a numerical model built to describe the 3-D wood structure is put forward. This model was introduced into an efficient solver that calculates the effective dielectric constant of any three dimensional structure of dielectric materials. The analysis of the calculated results for the permittivity and the comparison to the measured values shows a very practical qualitative and quantitative agreement between all the measured and calculated values. Finally, the importance of the research, the key results and the recommendations for further work are emphasized.