Biomedical Engineering Reference
In-Depth Information
the discs obtained were ~ 0.4-0.5 mm. In order to make samples with 0% humidity, the
discs were annealed at 100ºC in oven for 24 hr. Discs were also placed inside sealed
containers having MgNO
3
and NaCl salt for at least 2 weeks to achieve a relative hu-
midity of 51% and 75%, respectively. The samples were quickly transferred (less than
10 min) from the sealed containers to the sample holder in the DRS machine so as to
minimize absorption of atmospheric moisture.
Dielectric relaxation spectra were collected isothermally using a Novocontrol
GmBh Concept 40 broadband dielectric spectrometer in the frequency range 0.1-10
6
Hz. Temperatures were controlled within 0.2°C. The diameter of the top electrode
was 15 mm while the diameter of the bottom electrode was 30 mm. In order to better
resolve the spectra due to high values for dielectric loss caused by high conduction
loss, the dielectric loss was calculated using the expression (1) (Wubbenhorst and
Turnhout, 2002).
ε''der = -(π/2)(∂ε'(ω)/ ∂ln ω)
(1)
The relaxation positions were determined using WINFIT software.
dielectric Constant as a Function of Water Content
The variation of ε' with respect to the frequency for various temperatures for starch
stored at 0% humidity is as shown in Figure 1a and 1b. From Figure 1a and 1b, it
is observed that the dielectric constant reaches a maximum of ~10 at 100°C. From
Figure 1a, it is observed that a relaxation (a broad peak) exists at temperature range
-100-0°C. Another relaxation is observed at temperatures close to 100°C.
Figure 1a
Figure 1b
The variation of ε' with respect to the frequency for various temperatures for starch
stored at 51% humidity is as shown in
Figure 2a
and 2b. A relaxation is observed at
temperatures -100-0°C. Another relaxation is observed at higher temperatures. The
dielectric constant at low frequencies is observed to increase rapidly as temperature
increases. The value of the dielectric constant is large at low frequencies due to elec-
trode polarization.
Search WWH ::
Custom Search