The setup for power generation experiment is shown schematically
in Fig. 5, which consists of the electret plate, the
counter electrode plate, an alignment stage, and an electromagnetic
shaker (APS-113, APS Dynamics Inc.). The electret
plate and the counter electrode plate are respectively fixed to
the shaker and the alignment stage. The gap is measured with
a laser displacement meter (LC-2430, Keyence Inc.). The
counter electrode plate is moved sinusoidally in the in-plane
direction by the shaker. In order to measure the output voltage
for various external loads, a simple measuring circuit with a
voltmeter and two resistors in series is employed.
For obtaining large capacitance and thus large output power,
it is necessary to reduce the gap between the electret and counter
electrode plates. However, electric breakdown of the air gap
between the plates should occur, since the surface potential of
the electret is higher than -600 V, which is close to the breakdown
voltage of air for a 50 μm gap. In the present study, SF6
gas, which has 2.5 times larger breakdown voltage than air, is
introduced into the gap.
Table 2 shows the capacitance between the electret and the
counter electrode plates for the gap g = 100 μm. Without the
guard electrode, the capacitance measured for both 100% and
0% overlap are significantly larger than their theoretical values
of 17.7 pF and 0 pF. On the other hand, with the guard
electrode, the parasitic capacitance is significantly decreased,
and the capacitance for 0% overlap drops to 2.3 pF.
The distance between the top and bottom glass substrate
and the oscillation amplitude are respectively set to 40 μm and