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An efficient method of production of high charge density electrets
Journal of Electrostatics, 19 (1987) 205-207
205
Elsevier SciencePublishersB.V., Amsterdam-- Printed in The Netherlands Short Communication
AN E F F I C I E N T M E T H O D OF P R O D U C T I O N OF H I G H C H A R G E DENSITY ELECTRETS w. MEDYCKIand B. HILCZER Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17/19, PI-60179 Poznari (Poland)
(ReceivedNovember26, 1985;acceptedin revisedform September28, 1986) Several methods of polymer foil electret production have been described [ 1-8 ]. Here, a modification of the breakdown method by Sessler and West [ 8 ] is discussed which allows high charge densities to be obtained in this polymer foil. In their methods the charging of polymer foil occurs between two metal electrodes. Between them there is a polymer foil metallized on one side and a dieletric glass plate which stabilizes the breakdown process during charging. The essence of this method is the procedure of separating all parts of the system (electrodes, insert and polarized foil). Sessler and West have suggested short-circuiting the metal layer of the foil with the opposite electrode, after switching off the d.c. power supply, and before separation of the charged foil from the dielectric plate. This recovery process should prevent the high charge densities obtained during polarization from discharging when the foil is stripped from the dielectric plate. It is suggested here that this recovery procedure be modified in order to increase the acquired charge density of electrets. First the authors propose using the same non-metallized foil as the polarized one as the insert. This form of insert has two main properties: (i) it has a thickness comparable with (or the same as) the polarized foil; (ii) it has the same, very low conductivity. In Fig. 1 the polarization method proposed by the authors is shown. The essential difference from the Sessler and West set-up beside the insert is the double key, which connects (and disconnects) simultaneously both electrodes from the d.c. power supply and the ground. During the polarization process the capacity formed by both electrodes is charged. (There occurs, of course, the process of electric breakdown in all the air layers which exist in the system. ) After the assumed polarization time the key is turned off. As soon as possible, the electrode to the adjacent insert is removed, which causes a compensation of the air breakdown in the air gap. The low conductivity of the insert should prevent the electret charge formed in the foil from devastating. The removal of the insert from the electret causes only a weak air breakdown due to the absence of the opposite electrode. 0304-3886/87/$03.50
© 1987ElsevierSciencePublishersB.V.
206 metal e l e c t r o d e ~ insert fad - - E ' "
A)
charged f m [ - metal layer /
~~ " , • |
I~1
" I"
~'"-
=''j
J
B)
F///////////~
c)
~
D)
¥ I ."" ,.", ;'.'., .:. 7. '.|
1:
....
:-'
h'.:
t'_-' :._.,;_.'.-:.': :'~. X-1
'.l
Fig. 1. Schematic set-up for charging foil electrets using the breakdown method, (A) - polarization, (B) - d.c. power supply disconnection, (C) - metal electrode removal, (D) - recovery of electret.
In this experiment, we have applied the above method to obtain electrets from polyethylene terephthalate (PET) of varying thickness. As the insertwe have used P E T foilof 20 # m thickness. The surface charge densities were calculated from the measured equivalent voltage [9 ]. In Fig. 2 the surface charge densities obtained with the previously described method of charging P E T foils are shown. There is a relation between the surface charge density and thickness of polarized foil [8]. The charge density increases as the thickness of polymer foildecreases. Most likelya similar number of charge carriershas been injected into foilsduring the polarization process for a given polarization voltage. This E '~
~0
10 08
i
i
,
i
,
O •
n
•
e,
o o
D
02
•
w
u
,
06 O~
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n
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I
4
,
tHICKNESS
'
OF
FOIL
•
0
x
A
o
•
5
10
20
23
36
254
;
l
6
APPLIED
b
{ /urn} •
25/, I
1
VOLTAGE
[kV]
Fig. 2. Initialcharge density of negatively charged electrets versus applied voltage (polarizing time I min) ; Sessler-West data [8]: • -- with polarizing time of I min, • -- with polarizing time of 38 rain.
207
statement is based on the fact that the equivalent voltage of electrets is about 70% of the polarization voltage, which was applied to polarized PET foil. In the case of a glass insert, all the polarization voltage is applied to the foil ( resistivity of glass is negligible as compared to the resistivity of the PET foil ) and the efficiency of the Sessler and West method is about 30%. In the figure points obtained by Sessler and West are marked for comparison. These points present the dependence of surface charge densities versus the polarization time. It is obvious, that for the time of one minute smaller charge densities have been obtained than in our experiment for 25/lm PET foil. Sessler and West have obtained better results for considerably longer polarization times. Thus we can say that suitable changes in the Sessler and West method, namely using a special disconnecting key in the polarizating circuit, applying thin polymer foil as the insert and special treatment during the recovery of the electret from polarizing capacitor should allow the production of high charge density electrets with improved efficiency. References 1 2 3 4 5 6 7 8 9
G.M. Sessler and J,E. West, J. Acoust. Soc. Am., 34 (1962) 1787. G.M. Sessler and J.E. West, J. Electrochem. Soc., 115 (1968) 836. P.V. Murphy and F.W. Fraim, J. Audio. Eng. Soc., 16 (1968) 450. R.W. Tyler, J.H. Webb and W.C. York, J. Appl. Phys., 26 (1955) 61. H. Seiwatz and J.J. Brophy, 1965 Annual Report, Conference on Electrmal Insulation, Natl. Acad. Sci. Washington D.C., 1966. G.M. Sessler and J.E. West, Polym. Lett., 7 (1969) 367. G.M. Sessler and J.E. West, Appl. Phys. Lett., 17 {1970) 507. G.M. Sessler and J.E. West, J. Appl. Phys., 43 (1972) 922. C.W. Reedyk and M.M. Perlman, J. Electrochem. Soc., 115 (1968) 49.
205
Elsevier SciencePublishersB.V., Amsterdam-- Printed in The Netherlands Short Communication
AN E F F I C I E N T M E T H O D OF P R O D U C T I O N OF H I G H C H A R G E DENSITY ELECTRETS w. MEDYCKIand B. HILCZER Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17/19, PI-60179 Poznari (Poland)
(ReceivedNovember26, 1985;acceptedin revisedform September28, 1986) Several methods of polymer foil electret production have been described [ 1-8 ]. Here, a modification of the breakdown method by Sessler and West [ 8 ] is discussed which allows high charge densities to be obtained in this polymer foil. In their methods the charging of polymer foil occurs between two metal electrodes. Between them there is a polymer foil metallized on one side and a dieletric glass plate which stabilizes the breakdown process during charging. The essence of this method is the procedure of separating all parts of the system (electrodes, insert and polarized foil). Sessler and West have suggested short-circuiting the metal layer of the foil with the opposite electrode, after switching off the d.c. power supply, and before separation of the charged foil from the dielectric plate. This recovery process should prevent the high charge densities obtained during polarization from discharging when the foil is stripped from the dielectric plate. It is suggested here that this recovery procedure be modified in order to increase the acquired charge density of electrets. First the authors propose using the same non-metallized foil as the polarized one as the insert. This form of insert has two main properties: (i) it has a thickness comparable with (or the same as) the polarized foil; (ii) it has the same, very low conductivity. In Fig. 1 the polarization method proposed by the authors is shown. The essential difference from the Sessler and West set-up beside the insert is the double key, which connects (and disconnects) simultaneously both electrodes from the d.c. power supply and the ground. During the polarization process the capacity formed by both electrodes is charged. (There occurs, of course, the process of electric breakdown in all the air layers which exist in the system. ) After the assumed polarization time the key is turned off. As soon as possible, the electrode to the adjacent insert is removed, which causes a compensation of the air breakdown in the air gap. The low conductivity of the insert should prevent the electret charge formed in the foil from devastating. The removal of the insert from the electret causes only a weak air breakdown due to the absence of the opposite electrode. 0304-3886/87/$03.50
© 1987ElsevierSciencePublishersB.V.
206 metal e l e c t r o d e ~ insert fad - - E ' "
A)
charged f m [ - metal layer /
~~ " , • |
I~1
" I"
~'"-
=''j
J
B)
F///////////~
c)
~
D)
¥ I ."" ,.", ;'.'., .:. 7. '.|
1:
....
:-'
h'.:
t'_-' :._.,;_.'.-:.': :'~. X-1
'.l
Fig. 1. Schematic set-up for charging foil electrets using the breakdown method, (A) - polarization, (B) - d.c. power supply disconnection, (C) - metal electrode removal, (D) - recovery of electret.
In this experiment, we have applied the above method to obtain electrets from polyethylene terephthalate (PET) of varying thickness. As the insertwe have used P E T foilof 20 # m thickness. The surface charge densities were calculated from the measured equivalent voltage [9 ]. In Fig. 2 the surface charge densities obtained with the previously described method of charging P E T foils are shown. There is a relation between the surface charge density and thickness of polarized foil [8]. The charge density increases as the thickness of polymer foildecreases. Most likelya similar number of charge carriershas been injected into foilsduring the polarization process for a given polarization voltage. This E '~
~0
10 08
i
i
,
i
,
O •
n
•
e,
o o
D
02
•
w
u
,
06 O~
z u.l o
n
01
I
4
,
tHICKNESS
'
OF
FOIL
•
0
x
A
o
•
5
10
20
23
36
254
;
l
6
APPLIED
b
{ /urn} •
25/, I
1
VOLTAGE
[kV]
Fig. 2. Initialcharge density of negatively charged electrets versus applied voltage (polarizing time I min) ; Sessler-West data [8]: • -- with polarizing time of I min, • -- with polarizing time of 38 rain.
207
statement is based on the fact that the equivalent voltage of electrets is about 70% of the polarization voltage, which was applied to polarized PET foil. In the case of a glass insert, all the polarization voltage is applied to the foil ( resistivity of glass is negligible as compared to the resistivity of the PET foil ) and the efficiency of the Sessler and West method is about 30%. In the figure points obtained by Sessler and West are marked for comparison. These points present the dependence of surface charge densities versus the polarization time. It is obvious, that for the time of one minute smaller charge densities have been obtained than in our experiment for 25/lm PET foil. Sessler and West have obtained better results for considerably longer polarization times. Thus we can say that suitable changes in the Sessler and West method, namely using a special disconnecting key in the polarizating circuit, applying thin polymer foil as the insert and special treatment during the recovery of the electret from polarizing capacitor should allow the production of high charge density electrets with improved efficiency. References 1 2 3 4 5 6 7 8 9
G.M. Sessler and J,E. West, J. Acoust. Soc. Am., 34 (1962) 1787. G.M. Sessler and J.E. West, J. Electrochem. Soc., 115 (1968) 836. P.V. Murphy and F.W. Fraim, J. Audio. Eng. Soc., 16 (1968) 450. R.W. Tyler, J.H. Webb and W.C. York, J. Appl. Phys., 26 (1955) 61. H. Seiwatz and J.J. Brophy, 1965 Annual Report, Conference on Electrmal Insulation, Natl. Acad. Sci. Washington D.C., 1966. G.M. Sessler and J.E. West, Polym. Lett., 7 (1969) 367. G.M. Sessler and J.E. West, Appl. Phys. Lett., 17 {1970) 507. G.M. Sessler and J.E. West, J. Appl. Phys., 43 (1972) 922. C.W. Reedyk and M.M. Perlman, J. Electrochem. Soc., 115 (1968) 49.