U.S. patent number 3,924,324 [Application Number 05/485,112] was granted by the patent office on 1975-12-09 for method of making electret.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Yoichi Kodera.
United States Patent |
3,924,324 |
Kodera |
December 9, 1975 |
Method of making electret
Abstract
A method of making an electret by annealing a corona charged
dielectric film on a metal plate is disclosed.
Inventors: |
Kodera; Yoichi (Yokohama,
JA) |
Assignee: |
Sony Corporation (Tokyo,
JA)
|
Family
ID: |
13593326 |
Appl.
No.: |
05/485,112 |
Filed: |
July 2, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Jul 5, 1973 [JA] |
|
|
48-76027 |
|
Current U.S.
Class: |
29/592.1;
361/229; 381/191; 307/400; 361/233 |
Current CPC
Class: |
H01G
7/02 (20130101); H01G 7/023 (20130101); Y10T
29/49002 (20150115) |
Current International
Class: |
H01G
7/00 (20060101); H01G 7/02 (20060101); H01S
004/00 () |
Field of
Search: |
;29/592,592E ;317/262A
;307/88ET ;179/111E,1.41B ;310/8.2,8.8,9.1,9.2,9.7 ;340/173PP
;427/39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lanham; C. W.
Assistant Examiner: Walkowski; Joseph A.
Attorney, Agent or Firm: Eslinger; Lewis H. Sinderbrand;
Alvin
Claims
I claim as my invention:
1. A method of making a mono-charge electret, comprising the steps
of charging a dielectric film which is mounted on a metal electrode
by subjecting said film to a corona discharge; and then, after said
corona discharge is completed and while said film is still mounted
on said electrode, heating said charged dielectric film.
2. A method of making a mono-charge electret as claimed in claim 1
in which said step of heating is carried out at a temperature
higher than the glass transition temperature of said dielectric
film.
3. A method of making a mono-charge electret as claimed in claim 1
in which said dielectric film is made of fluoride.
4. A method of making a mono-charge electret as claimed in claim 1
in which dielectret is charged by a negative corona discharge.
5. A method of making a mono-charge electret, comprising the steps
of charging a dielectric film which is mounted on a metal electrode
by subjecting said film to a corona discharge to thereby charge one
surface of said film with the polarity of said corona discharge;
peeling off said charged dielectric film from said electrode;
placing said dielectric film on a metal backing with the other
surface of said film in contact with said metal backing; and, while
said film is on said metal backing, heating said dielectric
film.
6. A method of making a mono-charge electret as claimed in claim 5,
in which said step of heating is carried out at a temperature
higher than the glass transition temperature of said dielectric
film.
7. A method of making a mono-charge electret as claimed in claim 5
in which said dielectric film is made of fluoride.
8. A method of making a mono-charge electret as claimed in claim 5
in which said dielectret is charged by a negative corona discharge.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a method of making an electret,
and more particularly is directed to a method of making an electret
with a single electric charge or a monocharge electret.
2. Description of the Prior Art
Recently, a thin film electret is used as an oscillating plate or
vibrating diaphragm of a microphone, speaker or the like. As the
material for the thin film electret, by way of example, a high
polymer film such as "Teflon FEP" (Trade name) consisting of a
copolymer of tetrafluoro ethylene-hexafluoro propyrene or "Teflon
TEF" (Trade name) is used.
Various methods have been known in the art to produce an electric
charge on a dielectric material. In general, a dielectric material
is gripped between two electrodes and subjected to heating process
to make it as an electret by a heterocharge, which is known as a
so-called thermo-electret method. In this case, when an electret,
which is charged with positive and negative charges on its both
surfaces, is used as an electrode layer is attached to one surface
of the electret and the electric charge produced on the other
surface thereof is utilized.
If an electret produced by the above prior art method is employed
in an ear-speaker, two of such electrets are used in such a manner
that they are mounted on fixed electrodes or back electrodes with
apertures to face the charges of the electrets of the same polarity
or sign, and an electric conductive vibrating diaphragm is located
between the two electrets at their central portions from a spacial
point of view to vibrate the vibrating diaphragm with an input
signal applied to an input terminal.
However, such a method can not be free from the following defect.
That is to say, when an dielectric film adhered to the back
electrode is subjected to an electret making process, it is
difficult to make an electret with a homogeneous electric charge
due to the distortion of the back electrode caused by heating
carried out in the electret-treatment, a time lapse change of the
adhesive between the dielectric film and the back electrode, and a
plurality of apertures bored through the dielectric film formed by
a corona discharge. Further, when a film which is already made as
an electret is attached to the back electrode, it is trouble to
aligne the apertures of the film with those of the back electrode
and there are a charge decay and so on due to a stain of the
film.
Another method is also known in the art in which an electron beam
with high energy is radiated to an dielectric film from a Van de
Graaff generator. With such a method, upon an initial irradiation
of the electron beam, a positive electric charge is produced on the
surface of the dielectric film facing the electron beam
irradiation, while a negative electric charge is produced on the
other surface thereof. After a predetermined time period has
lapsed, whole the film is charged with a negative electric charge
entirely. This method, however, requires a much great size of an
apparatus which is not preferred from a practical point of view,
and the dielectric film is apt to be broken due to the irradiation
of the electron beam, which causes the deterioration of a produced
electret in characteristics.
SUMMARY OF THE INVENTION
According to this invention, there is proposed a method of making
an electret, in which a high polymer film with no electrode is
placed on a metal table and is charged with a negative corona, and
then the high polymer film is subjected to an annealing treatment
to be an electret with an electric charge of a single sign or a
monocharge electret.
Further, after a corona discharge the film is peeled off from the
metal table once, and the film is mounted on the table with its
surface, which is charged with a positive charge, being faced to
the surface of the table, and then the film is subjected to the
annealing treatment to carry out a charging process with high
efficiency for a short time period.
With this invention, it is possible that a high polymer film is
charged at its both surfaces with a negative charge by a simple
construction of a device with the utilization of a corona
discharge.
In a practically used charging device, a negative corona discharge
is applied to a non-polar high polymer film disposed on a metal
table, and thereafter the film is annealed to obtain a mono-charge
electret film.
In this invention, a stylus electrode is disposed apart from the
surface of the film by 15mm and supplied with a voltage of -4 to
-8KV (kilovolts). By the exposure of about 30 sec (seconds), an
electret with an initial surface charge density of 10.sup.-7 to
10.sup.-8 C/cm.sup.2 (coulombs per square centimeter) is
obtained.
Thereafter, the electret is, at it is, annealed at a temperature of
about 150.degree.C to have a negative surface charge density of
-10.sup.-9 C/cm.sup.2, and then peeled off from the metal table to
be a monocharge electret film with a net charge as maintained.
Further, if the film peeled off from the metal table is again
mounted on the metal table with its positive charged surface in
contact therewith and is then subjected to an annealing treatment,
a monocharge electret film can be obtained for a short time
period.
If after the corona discharge an annealing treatment is applied in
the above method, the fact that a charged surface by the corona
discharge is transferred from the discharging side to that of the
metal table is shown by measuring a thermoelectric current
spectrum. As a result, an electrified or charged film is obtained
which is homogeneous and stable as compared with a film naturally
charged by a static electricity.
Other objects, features and advantages of this invention will
become obvious from the following description taken in conjunction
with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1A to 1E, inclusive, are diagrams showing a process of one of
the methods according to this invention;
FIGS. 2A to 2C, inclusive, are diagrams showing a process of
another of the methods of this invention;
FIGS. 3 and 4 are respectively schematic theoretical diagrams
showing head-phones used for the explanation of the invention;
and
FIG. 5 is a graph showing the surface charge density-time lapse
characteristics used for the explanation of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the prior art, when an electret is used in a head-phone, as
shown in FIG. 3, a pair of opposed electrets 1 and 2 are attached
to a pair of fixed electrodes or back electrodes 4 and 5 which have
bored there through apertures 3, respectively, and an electrically
conductive vibrating diaphragm 6 is disposed between the electrets
1 and 2 to vibrate in response to an input signal applied to an
input terminal t. With such a prior art construction, the two
electrets 1 and 2 are used in a single transducer, and upon the
manufacture thereof two dielectric films to be made as the
electrets 1 and 2 are attached to the back electrodes 4 and 5,
respectively, which is thereafter subjected to an electret
treatment. However, there may be a difficulty in such an electret
treatment due to the type of material used in the back electrodes.
That is, there are problems of the distortion of the back
electrodes and the deterioration of the adhesive used to attach the
dielectric films to the back electrodes which may be caused by the
heating during the electret treatment. Further, in the case of an
electret treatment wherein the dielectric film is charged on its
surface by a corona discharge, if there are many apertures in the
dielectric film, there is the unfortunate possibility that a
homogeneous charge production is not obtained on the surface of the
dielectric film.
In order to avoid such a defect, there has been proposed a method
with which a dielectric film is charged firstly to be an electret
and the electret is attached to the back electrode. However, this
method requires complicated procedures in that a plurality of
apertures are bored through the electret film and the electret film
is adhesively attached to the back electrode with the apertures of
the electret film and those of the back electrodes being aligned
with each other. In this process, there is a fear that the electret
film is blured and the surface charge thereof is decreased.
In order to eliminate such a defect, it may be considered that a
vibrating diaphragm is made as an electret. However, in such a case
the vibrating diaphragm must have a monocharge that is, an electric
charge of a single polarity so as to be operative as a vibrating
diaphragm. By way of example, if such a vibrating diaphragm is used
in a head-phone, as shown in FIG. 4, the vibrating diaphragm 7,
which is made to be an electret to have, for example, only a
negative charge, is centrally disposed and the back electrodes 4
and 5, each of which has formed therethrough a plurality of
apertures 3, are disposed at the both sides of the diaphragm 7 in
opposed relation with each other. Thus, the head-phone becomes
simple in construction.
Hereinbelow, the method of making an electret according to this
invention, which has a negative charge on its both surfaces and is
stable, will be described in conjunction with the drawing.
As shown in FIG. 1A, a dielectric film or a high polymer film 10 to
be made as an electret is mounted on a metal table or plate 11 and
a stylus electrode 12 is positioned above the high polymer film 10
apart from its upper surface 10A by a predetermined distance d. A
predetermined high voltage is applied across the stylus electrode
12 and the metal plate 11 from a high voltage source 13 with the
metal plate 11 being connected to a positive electrode while the
stylus electrode 12 being connected to a negative electrode to
carry out a corona discharge between the stylus electrode 12 and
the high polymer film 10 and to charge the high polymer film 10
with negative charge on its upper surface 10A.
Thereafter, as shown in FIG. 1B, the high polymer film 10 is peeled
off from the metal plate 11. At this time, a discharge occurs
between a peeled off surface 10B of the polymer film 10 and an
upper surface 11A of the metal plate 11, and hence the surface 10B
of the high polymer film 10 is charged with a positive charge.
Thus, as shown in FIG. 1C, the high polymer film 10 is charged with
negative and positive charges on its surfaces 10A and 10B,
respectively.
Thereafter, the high polymer film 10 is again placed on another
metal plate 14, with the surface 10B having the positive charge
being in contact with the plate, and is heated in a furnace 15 for
a constant time period and then cooled. In this case, it may be
preferred that the metal plate 14 is grounded, as shown in FIG. 1D.
In the figure, reference numeral 16 indicates a heater. In this
process, the heating temperature is selected higher than the glass
transition temperature of the high polymer film 10, for example,
more than 80.degree.C for the case that the high polymer film 10 is
made of Teflon FEP. If the heating temperature is too high, there
may be a fear that the film is distorted or molten. Accordingly, it
is necessary that the heating temperature is selected lower than
the melting point of the film.
Thus, an electret film 17 which is charged with the negative charge
on its both surfaces 10A and 10B is produced as shown in FIG.
1E.
That is, it is ascertained by measuring the electric charge on the
surfaces 10A and 10B of the high polymer film 10 that the surface
10B, which is charged with the positive charge before heating, is
charged with the negative charge by suitably selecting the time
interval of the heating, while the amount of the negative electric
charge stored in the surface 10A before the heating is decreased
somewhat, but the surface 10A has still the negative charge, that
is, both the surfaces 10A and 10B carry the negative charge. The
amount of initial charge by the corona discharge is much greater
than can be attained by the thermo electret method, and the charge
level is sufficient for practical use after being heat-treated.
In general, if the heating temperature is low, the heating time
interval required is long, so that it is most convenient and
practicable that the heating temperature is selected about
120.degree. to 160.degree.C for the Teflon FEP, with no thermal
distortion of the high polymer film as a result.
An electric field E established by a single sign of an electric
charge layer is expressed by ##EQU1## where .sigma. represents the
charge density and .epsilon..sub.0 the dielectric constant in
vacuum. Since the critical discharge of a uniform electric field in
atmosphere is 30KV/cm at normal temperature and pressure, the
maximum charge density of such a charged body becomes to 5.3
.times. 10.sup.-.sup.9 C/cm.sup.2, which is smaller than that where
the high polymer film is charged with positive and negative charges
on its both surfaces or where the film is provided with a back
electrode. In other words, a small charge density can provide the
necessary electric field.
FIGS. 2A to 2C show another method of the invention. With the
second method, the high polymer film 10 to be made as an electret
is mounted on the metal plate 11 and the stylus electrode 12 is
located above the film 10 apart from its upper surface 10A by a
predetermined distance in opposed relation. A predetermined high
voltage is applied across the stylus electrode 12 and the metal
plate 11 from the high voltage source 13 with the metal plate 11
being positive and the stylus electrode 12 being negative to
produce a negative electric charge on the upper surface 10A of the
film 10 by the corona discharge (refer to FIG. 2A).
The film 10 and the metal plate 11 are located in the heating
furnace 15 without the film 10 first being peeled off from the
metal plate 11 as before; and the film 10 is heated (annealed)
therein for a constant time interval. Thereafter the film 10 is
cooled. In this case, the heating treatment is carried out longer
than that in the case of the first method (FIGS. 1A-1D).
As a result of measuring the charge on both the surfaces 10A and
10B of the high polymer film 10 after cooling, it is ascertained
that, by suitably selecting the heating time interval, an electret
18 (FIG. 2C), which is charged with the negative charge on its both
surfaces 10A and 10B or carries a single sign of an electric charge
(as desired), can be obtained.
Examples of the present invention will be provided.
EXAMPLE I
A "Teflon FEP 100" (a copolymer of
tetrafluoroethylenehexafluoropropyren) film with an area of 1.5
.times. 1.5 cm.sup.2 and a thickness of 50.mu.m (micro-meter) is
mounted on the metal plate 11 and the stylus electrode 12 is
located above the film apart therefrom by 15mm, as shown in FIG.
1A. A voltage of -6KV is applied to the tip end of the stylus
electrode 12 to carry out the corona discharge for a time interval
of 30 seconds. After the corona discharge is finished, the film is
peeled off from the metal plate 11 and placed on the electrode of
an electric charge measuring apparatus. After measuring the charge
amount of the film, it is -8 .times. 10.sup.-.sup.8 C/cm.sup.2 at
its portion opposing the stylus electrode 12. After turning over
the film and measuring the charge amount on the surface opposite to
the discharged surface, it is +7.8 .times. 10.sup.-.sup.8
C/cm.sup.2. After the measurement, the film is mounted on the metal
plate with its positive charged surface facing the plate, and
heated at 155.degree.C for a time interval of ten minutes. When the
charge amount of the film after heating is measured, it is -1.9
.times. 10.sup.-.sup.9 C/cm.sup.2 on the surface of the film which
was charged with the negative charge initially and -2.3 .times.
10.sup.-.sup.9 C/cm.sup.2 on the surface of the film which was
charged with the positive charge initially, that is, both the
surfaces of the film are charged negatively.
EXAMPLE II
A Teflon FEP film with a thickness of 12.7 .mu.m is attached to a
metal ring with an outer diameter of 32mm and an inner diameter of
28mm and the film is mounted on the metal plate 11 with the film
being directly contacted with the metal plate 11. The stylus
electrode 12 is disposed above the film apart therefrom by 15mm and
a voltage of -7KV is applied to the stylus electrode 12 to carry
out the corona discharge for a time interval of 30 seconds. After
the corona discharge is finished, the film is peeled off from the
metal plate 11. Then, the film is placed on the second metal plate
14 with its corona-discharged surface facing upward and heated at
160.degree.C for a time interval of 20 minutes. It is ascertained
by the aforedescribed measurement that the charged amount after
heating is -4.7 .times. 10.sup.-.sup.9 C/cm.sup.2 on the
corona-discharged surface and -4.0 .times. 10.sup.-.sup.9
C/cm.sup.2 on the surface that was in contact with the metal plate,
that is, both the surfaces are charged negatively. The change in
the amount of the charge of the film with respect to the time lapse
is shown in FIG. 5 in which the ordinate represents the surface
charge density Q in multiple of 10.sup.-.sup.9 C/cm and the
abscissa the elapsed time T in day, when the film is left at a room
temperature.
In the graph of FIG. 5, a curve I shows the charge on the
corona-discharged surface and a curve II shows the charge on the
opposite surface. As may be obvious from the characteristic of the
graph, it is noted that the charges on both the surfaces of the
film are negative and settled after 10 days and the settled
negative charge is stable without being changed with further time
elapse.
EXAMPLE III
A Teflon FEP film with an area of 1.5 .times. 1.5 cm.sup.2 and a
thickness of 50 .mu.m is mounted on the metal plate 11 and the
stylus electrode 12 is located above the film apart therefrom by
15mm. A voltage of -6KV is applied to the tip end of the stylus
electrode 12 to carry out the corona discharge for a time period of
30 seconds. After the corona discharge is finished, the film is
heated at 150.degree.C for a time interval of 1.5 hours without the
film being peelef off from the metal plate 11. It is ascertained
that the charge amount of the film after heating is -1.3 .times.
10.sup.-.sup.9 C/cm.sup.2 on its corona-discharged surface and -2.4
.times. 10.sup.-.sup.9 C/cm.sup.2 on the surface which was in
contact with the metal plate, both surfaces thus negatively
charged.
ANOTHER EXAMPLE (a)
A Teflon FEP film with an area of 1.5 .times. 1.5 cm.sup.2 and a
thickness of 50.mu.m is placed on the metal plate 11 and the stylus
electrode 12 is located above the film apart therefrom by 15mm. A
voltage of -6KV is applied to the tip end of the stylus electrode
12 for a time interval of 30 seconds to carry out the corona
discharge. As a result of this, an electric charge of -5.5 .times.
10.sup.-.sup.8 C/cm.sup.2 is produced on the corona-discharged
surface of the film. Then, the film is peeled off from the metal
plate 11 and again placed on the metal plate. At this time, the
peeled off surface of the film is charged with the positive charge.
Thereafter, a metal mesh is disposed between the tip end of the
stylus electrode and the metal plate and a voltage of about -100V
is applied to the metal mesh relative to the metal plate, while a
voltage of -6KV is applied to the tip end of the stylus electrode
to carry out the corona discharge. In this case, the amount of the
charge is -5.5 .times. 10.sup.-.sup.9 to -7 .times. 10.sup.-.sup.9
C/cm.sup.2 on the corona-discharged surface but + 1 .times.
10.sup.-.sup.9 to 0 to - 1.4 .times. 10.sup.-.sup.9 C/cm.sup. 2 on
the surface in contact with the metal plate. Thus, there is the
case that both the surfaces are charged with the negative charge
but its reproducibility is deteriorated.
FURTHER EXAMPLE (b)
A separate specimen (Teflon FEP film), which is charged by the
similar corona discharge to that of the Example I, has the charge
of -8.6 .times. 10.sup.-.sup.8 C/cm.sup.2 on its corona-discharged
surface and that of +8.3 .times. 10.sup.-.sup.8 C/cm.sup.2 on its
surface in contact with the metal plate. Next, the film is placed
on the metal plate 14 with its negatively charged surface in
contact therewith and heated under the similar condition of the
Example I or at 155.degree.C for 10 minutes. When measuring the
charge on both the surfaces of the film, the surface which is
charged with the negative charge initially is charged at -5.7
.times. 10.sup.-.sup.9 C/cm.sup.2 and the surface which is charged
with the positive charge initially is charged at +2.1 .times.
10.sup..sup.-9 C/cm.sup.2, that is, the sign of the charges is not
changed.
THE OTHER EXAMPLE (c)
A separate specimen (Teflon FEP film), which is charged by the
similar corona discharge to that of the Example I, is peeled off
from the metal plate. Then, the film which is charged with the
negative and positive charges on its both surfaces, respectively,
is heated without being in contact with the metal plate. After
heating, the measurement of the charges shows that the positively
charged surface remained positive and the other surface remained
negative. That is, the film with a single sign of charge can not be
obtained.
If a Teflon FEP film with a thickness of 50.mu.m is placed on a
metal plate and a negative charge is produced on the surface of the
film of -5.5 .times. 10.sup.-.sup.8 C/cm.sup.2, this charge is
below its critical stability. However, since the stable charge is
on the order of -5 .times. 10.sup.-.sup.9 C/cm.sup.2 when the film
is peeled off from the metal, a discharge occurs on the peeled off
surface upon the peeling off to cause the production of the
positive charge. Accordingly, if the film on the metal plate is
charged at lower than about -5 .times. 10.sup.-.sup.9 C/cm.sup.2
initially, theoretically there appears no discharge on the peeled
off surface when the film is peeled off from the metal plate. Thus,
there must be obtained an electret with a monocharge or a single
sign of electric charge. However, from a practical point of view,
its reproducibility is low, and the reproducibility and the amount
of a charge in the case of heating treatment are less than desired
as compared with the former case. Further, in order to shorten the
time interval of heating treatment, it is better that after the
corona discharge the film is peeled off from the metal plate
once.
In the above example, a Teflon FEP film is used as a high polymer
film, but even if a Teflon TFE film, a Tefzel film and the like are
used as the high polymer film, an electret film with a single sign
of an electric charge can be also obtained.
As described above, with this invention, by taking the step of
charging the surface of a high polymer film with the corona
discharge and the step of heating the high polymer film after the
corona discharge, an electret having a monocharge, that is, a
single sign of an electric charge such as a negative electric
charge, can be easily obtained. Further, an apparatus for
performing the method of this invention requires no complete
re-design and thus can be the apparatus used for making the prior
art electret.
It may be obvious that many variations and changes could be
effected by those skilled in the art without departing from the
spirit and scope of the novel concepts of this invention.
* * * * *