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MATERIALS 45
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The main dielectric properties that govern the conversion of Rasbak
electrical energy into thermal energy while wood is within an
electromagnetic field are the dielectric constant (ε′) and the
loss tangent (tan δ).
These dielectric values depend strictly on the moisture
content, density, chemical composition of the wood, and the

anatomical direction in which the electromagnetic field is
applied. All these parameters determine the efficiency, speed,
and uniformity of wood heating within an electromagnetic field.
The value of the dielectric constant (ε′) indicates the capacity
of wood to store electrical energy in an alternating current
field, while the loss tangent (tan δ) indicates the proportion
of this energy converted into thermal energy. Both values are of the electric field leads to its decrease.
intricately related to the frequency of the electric field and vary However, these tests were conducted with limited variables.
with changes in temperature and moisture content of wood. These studies were carried out either at a single specific

Increasing the frequency of the field, the temperature of the frequency or within a limited range of moisture content in the
wood, and its water content leads to higher values due to samples. Furthermore, previous research rarely addressed the
increased interfacial polarisation and enhanced dipolar mobility combined influence of parameters such as density, moisture
of the water molecules inside wood. content, and temperature.
When an alternating electric field is applied to wood, the As it is evident in other types of wood that these parameters
polarisation mechanisms within the material lag behind the affect their behaviour when exposed to HF radiation, it is
applied signal, and some of the electrical energy is converted important to understand how oak wood behaves under HF

into thermal energy through frictional and relaxation processes. exposure.
Because humidity and temperature alter the relative The specific selection of these parameters can lead to
contributions of bound- and free-water polarisation, their the optimal heating of oak wood required for pre-bending,
control is crucial for predicting and optimising HF processing which has been shown to be most effective form of heating
efficiency. Previous studies have shown that, for wood with considering the success of bending of oak wood.
increasing moisture content, ε′ may rise from about four (dry) For the reasons stated above, the aim of this work is
to 100 at one MHz, while tan δ can increase by more than to characterise the dielectric properties of oak wood under
an order of magnitude. environmental conditions and material properties relevant to
Despite extensive research on softwoods, fewer studies high-frequency heating prior to bending.

have focused on hardwoods such as pedunculate oak (Quercus Specifically, we investigated the influence of temperature,
robur L.), whose high density and complex structure may air humidity, and density on the dielectric properties of oak
influence dielectric behaviour differently. Previous studies of wood across the frequency range 0.079–25.1 MHz.
the dielectric properties of oak wood have shown that an The working hypothesis is that, under conditions relevant for
increase in moisture content and the higher density of oak wood bending, density has a dominant influence on dielectric
wood compared to other types of wood lead to an increase behaviour, while temperature and relative humidity primarily
in the dielectric constant, while an increase in the frequency modulate the response to changes in moisture content.