Biomedical Engineering Reference
In-Depth Information
inoperative and dipoles become out of phase. When sev-
eral polarization processes such as ionic, orientation, space
charge, etc., occur in a material, it follows that the dielectric
loss decreases with increasing frequency.
•
Temperature:
When the power of the source increases, the
heat produced from the power of the source is more dissi-
pated in the surrounding medium. h us, the dielectric loss
increases with increasing temperature.
•
Humidity and Moisture:
h e temperature coei cient for
loss factor may have either positive or negative sign, depend-
ing upon moisture content. An increase in loss angle is also a
worse result of humidity.
13.4.1.4 Dielectric Measurements
h e dielectric constant and dissipation factor/dielectric loss have been
measured on polycrystalline ceramic samples using Hewlett Packard
4192A impedance analyzer and Hioki 3532-50 LCR HiTester as a func-
tion of frequency (50 Hz-1 MHz) and temperature (300 K-673 K) in air
atmosphere. A high temperature sample holder with a Chromel-Alumel
thermocouple was used for all the dielectric measurements.
13.4.1.5
Results and Discussion
13.4.1.5.1 Frequency Variation of Dielectric Constant and Dissipation
Factor for Bi
1-x
La
x
FeO
3
(x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5)
Figure 13.17 shows the variations of dielectric constant as a function of fre-
quency for all the pure and La-substituted BiFeO3 samples. From the i gure
it is clear that with the increase of frequency, the dielectric constants of all
the pure and substituted compounds decrease and show similar behavior
as the pure compound. h e variation of dielectric constant with frequency
at room temperature indicates that the dielectric constant decreases rap-
idly at lower frequencies while it remains almost constant at higher fre-
quencies. It is believed that defects such as oxygen vacancies inherently
present in these materials, due to the volatilization of Bi-O species, create
space charges whose polarization can respond to the external electric i eld.
At low frequencies, these charges have enough time to move longer dis-
tances in the sample, creating a larger electronic polarization. h erefore,
a high value of dielectric constant is measured. At higher frequencies the
space charges no longer follow the i eld, leading to lower values of dielec-
tric constant. From the i gure it is also evident that the room temperature
dielectric constant increases with the increase of La content.
