Electronic Material Properties - und Geowissenschaften ...

Fig. 2 shows the time dependencies for the charging currents IC (a), surface potentials VS
(b) and the light intensities ∆Ilight (c) for a single ePTFE film at grid voltages ranging from
−500 to −1500 V. As soon as the applied electric field exceeds a threshold value of -700 V
the film starts to emit light. The light emission is accompanied by a charging current IC and
a buildup of surface potential VS. After 5 s the current IC, the surface potential VS and the
light emission ∆Ilight saturate to non-zero values. The constant current can be explained by
an equivalent circuit model consisting of the sample capacitor and a leakage resistor in
parallel. The leakage current (saturated current IC) is caused by charge transport initiated
by ongoing local breakdowns. Hence, the charge carriers generated in the material voids
must permanently vanish during the poling process, most likely due to ejection at the metal
electrode and neutralization of corona induced charges at the ePTFE top surface. As a
consequence, the surface charge potential does not change for grid voltages VG < −700 V
[4].
I C , [µA]
V S , [ V ]
∆I light , [a.u.]
0,00
-0,05
-0,10
-0,15
-0,20
-800
-600
-400
-200
0
2,0
1,5
1,0
0,5
0,0
a)
0 5 10 15 20
b) -700... -1500V
0 5 10 15 20
c)
0 5 10 15 20
t 1
-500V
-700V
-800V
-900V
-1000V
-1250V
-1500V
-500V
-1500V
-1250V
-1000V
-900V
-800V
-700V
-500V
time, [sec]
Figure 2. Time dependencies of charging currents (a),
surface potentials (b) and light intensities (c)
during negative poling of an individual ePTFE film
at different grid voltages as indicated.
I C , [µΑ]
V S , [ V ]
∆I light , [a.u.]
0,00
-0,05
-0,10
-0,15
-2000
-1000
0
3
2
1
0
a)
0 25 50 75
b)
0 25 50 75
t 1
c)
0 25 50 75
time, [sec]
Figure 3. Time dependencies of charging currents (a),
surface potentials (b) and light intensities (c)
during negative poling of three-layer sandwich at
grid voltage of -2500 V.
In the contrary to the results obtained from single ePFTE films, the light intensity ∆Ilight and
the charging current IC detected during the poling of the sandwich structure exhibit initially
maxima and decay with time to zero. This is depicted in Fig. 3. Time t1 indicates the onset
of light emission. Concomitantly, the saturated surface potential coincides for all applied
grid voltage with VG. Due to the blocking character of the FEP films, on can anticipate that
the positive and negative charges generated during breakdown are trapped at opposite
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