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MATERIALS 49
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The densities were relatively similar, with only minor variations. 0.036 to 0.062 at 20 deg C (an increase of 72%) and from
The mean values ranged from 639 kg/cubic metre to 702 kg/ 0.048 to 0.081 at 90 deg C (an increase of 69%).
cubic metre, which can be explained by the heterogeneity of the An increase in the value of dielectric properties is observed
anatomical structure of oak wood within the lamella samples. with increasing moisture content in wood samples. The values
The dielectric constant (ε′) depended greatly on the relative of the ε′ at lower moisture contents (4.7%–5%) showed low
humidity of the air during testing. This change was observed variability, ranging from 2.20 to 2.66.

at both test temperatures, 20 deg C and 90 deg C. Increasing As the moisture content increases, the ε′ also increases
the RH during testing led to higher measured values of the ε′. (in the range of 6.7 percent–10 percent, it ranges from 2.63 to
At 20 deg C, the ε′ ranged from 2.29 at a RH of 10 3.12, while at moisture contents above 10 percent it exceeds
percent to 4.62 at 100 percent, while at 90 deg C it ranged 4.0, reaching up to 6.7 in the highest measured case).
from 2.43 at 10 percent to 4.69 at 100 percent. Increasing RH This represents an increase of more than 150 percent
also resulted in a higher value of loss tangent (tan δ), from in ε′ between the lowest and highest moisture contents. An
0.07 at RH of 10 percent to 0.16 at RH of 100 percent (20 increase in variability within the sample groups is also evident
deg C), and from 0.07 to 0.13 (90 deg C). as moisture content rises.
Increasing relative humidity from 10 percent to 100 percent This variability is much more pronounced at a measurement

resulted in an approximately twofold increase in the dielectric temperature of 90 deg C. For the tan δ, this variability is even
constant, corresponding to a rise of about 102 percent at 20 more pronounced and remains quite large at all moisture
deg C and 93 percent at 90 deg C. Over the same RH range, contents and both measurement temperatures.
the loss tangent increased by approximately 130 percent at The values of the tan δ also increase with higher moisture
20 deg C and 86 percent at 90 deg C. content in the samples, but this increase is less pronounced.
The variability between samples within the same groups
was, on average, greater at higher RH. As expected, higher

values of ε′ and tan δ were recorded at 90 deg C compared to
those at 20 deg C. The steamed samples did not particularly Markus Winkler-pxhere.com
stand out compared to the other samples and on average
measured similar values of ε′ and tan δ as samples measured
at RH of 65 percent and 75 percent.
The effects of sample density on dielectric properties were
pronounced under all measurement conditions. The results of
dielectric properties measured by sample density are recorded.
At 20 deg C, ε′ ranged from 2.41 for the least dense lamellas

(~575 kg m−3) to 3.87 for the densest (~750 kg m−3), and
from 2.92 to 4.35 at 90 deg C.
Accordingly, an increase in ε′ of approximately 60 percent
at 20 deg C and 49 percent at 90 deg C is observed within
the tested density range. This corresponds to an average
increase of roughly 0.005–0.006 in ε′ per 10 kg m−3 increase
in density. The tan δ followed a similar trend, ranging from