
where l is the length of the plate. Omitting the
intermediate calculations given in [4, 5], we give
the expressions for fr and fa, (6).
(6)
Thus, resonant and antiresonant frequencies can
be approximated, which is important for calculating
power plant specifications.
If an alternating electric voltage is applied to a
segmented dielectric, the polarization does not
follow the electric field, which leads to dielectric
losses. When a mechanical load is applied, the
deformation is established with a delay. That is,
these processes correspond to a phase shift. All
materials are subject to relaxation processes to a
greater or lesser degree. Relaxation processes in
ferroelectrics appear due to mechanical and
dielectric losses. The peculiarities of
ferropiezoelectrics of ferroelectrics operating in the
dynamic mode is the presence of both types of
losses. At low frequencies, the angles of dielectric
and mechanical losses in it make a total loss angle δ,
defined through Кс [3-7], see equation (7).
Therefore, the polarization is considered as a
complex number: =-
, where is
relative dielectric constant;
- imaginary part of the
complex number, loss coefficient (
= tgδ·; tgδ -
loss characteristic: δ - phase shift angle.
The frequency characteristics,
and δ
depending on the normalized frequency ω/ω0 are as
shown in figure 6.
Now it is possible to explain ECG operation
principle in more detail. Under mechanical load, as a
result of the clamping of ferroelectric, there is a
sharp decrease in , Figure 6, which leads to
electrical capacity reduction and an increase in Кс,
that is, to a sharp increase in the efficiency of
conversion of mechanical energy into electrical
energy. In a certain frequency range between the
resonance and the antiresonance, where the
deformation will increase sharply to a greater extent
than is due to mechanical load, a sudden absorption
of mechanical energy occurs. This leads to a sharp
increase in the degree of polarization.
Reducing the electrical capacitance of the
ferroelectric ECG leads to an increase in the
electrical voltage UO, see equation (8), and,
therefore, an increase in the electrical power in the
load EU. Phase transitions in ferroelectrics occur
within certain temperature ranges. In this connection,
it should be noted that in ferroelectrics some
piezoelectric moduli, characterizing the change in
the degree of polarization under mechanical loading,
reach very large values during phase transitions,
theoretically passing into infinity. Thus, the effect of
mechanical load in a certain frequency range and the
effect of thermal energy in a certain temperature
range are a sort of a catalyst of chemical reactions in
solid solutions of ferroelectrics and mainly increase
the specific power and specific energy of the ECG.
Figure 6. To the analysis of ECG resonance
parameters.
The absolute dielectric constants,
of the
clamped element in this case are, of course, less
than
of the free one and are linked by equation
of electromechanical coupling:
2
11
2
Кс
s
dT
a
T
a
E
T
a
S
a
(7),
where Кс is electromechanical coupling coefficient,
is elastic compliance while the electric field
strength is Е=0,
is absolute dielectric constants
where the mechanical stress is T=0 and d is
piezoelectric module [3-5].
In the general case, the transformation function
(ECG) is of the form: expression (8).
From equation (7) it is obvious that the value of
Кс has a significant effect on the ratio of dielectric
constant of clamped and free ferroelectrics, i. e
change in the dielectric constant under the action of
mechanical load. For example, in case of Кс = 0.5
(an averaged value), this ratio will be 0.75. Which,
in its turn, is highly important (especially since in
modern ferropiezoelectric ceramics Кс = 0.6...0.7
for output electricvoltage (output power) of the
power plant, see equation (8), as dielectric constant
and electrical capacity are directly proportional.
International Journal of Chemical Engineering and Materials
DOI: 10.37394/232031.2023.2.13