Biomedical Engineering Reference
In-Depth Information
303K
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(a)
4
2
0
3
4
5
6
7
0.50
(b)
0.25
0.00
2
4
6
8
(
rad s
-1
)
log
Figure 14.9
Frequency dependence of
(a) and
(b) of present material at various
temperatures.
temperature range as a gradual decrease in (ω) and as a broad peak in
tan δ(ω). Relaxation phenomena in dielectric materials are associated with
a frequency-dependent orientational polarization [11]. At low frequency,
the permanent dipoles align themselves along the i eld and contribute fully
to the total polarization of the dielectric.
At higher frequency, the variation in the i eld is too rapid for the
dipoles to align themselves, so their contribution to the polarization and
hence to the dielectric permittivity can become negligible. h erefore
the dielectric permittivity (ω) decreases with increasing frequency.
It is evident from Figure 14.9(b) that the position of loss peak tanδ
max
(centered at the dispersion region of ) shit s to higher frequency with
increasing temperature and that a strong dispersion of tanδ exists in
present material. Dielectric relaxation behaviors are generally described
