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JPS5025160

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DESCRIPTION JPS5025160
■ Electret App. No. 48-288 ■ Japanese Patent Application No. 44-101746 [Phase] Application
No. 42 (1967) Dec. 5 Priority claim [Phase] May 15 1967 0 Mark United States No. 06384630
Japanese Patent No. 42-77704 Division 0 Inventor Breston Buy Murphy Uniston Massachusetts
Bartridge Hill Road 42 出 願 Applicant Thermo Fist Electron Engineering Corp. Massachusetts
Ortsam First Avenue 85 [Fa] Attorney Attorney Motohiro Kurauchi
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing the parts of the
apparatus for carrying out one step in the process of manufacturing the electret according to the
present invention, and FIG. 2 is a sectional view of a converter using the electret according to the
present invention FIG. 3 is a fragmentary enlarged plan view showing a portion of an electret
backplate that forms a portion of the apparatus of FIG. 2;
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel electret. In
order to obtain satisfactory linearity of the response in an electrostatic transducer such as a
condenser microphone or electrostatic loudspeaker, it is necessary to provide the transducer
with a relatively high DC bias. It has long been known in principle that the required bias can be
obtained by using an electret with a metallized surface as the active element of the transducer.
Electrets are dielectric materials that have received a strong electrostatic field sufficient to
sustain residual internal [111111] polarization after electrostatic field removal. As a result, an
electrostatic charge is obtained in the material which acts to supply the required DC bias to the
electrostatic transducer. In fact, in the manufacture and use of electrets, a number of problems
have been encountered which impede its widespread adoption. The main problem was that
electrets made from known materials and methods prior to the present invention lacked good
polarization stability. Both temperature and humidity extremes cause severe and rapid loss of
polarization. However, under controlled conditions, the internal charge of conventional electrets
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diminishes at a rate which is too great to be acceptable for general purposes. Another problem
encountered arises from the need to place the dielectric material of the electret between the
conductive metal parts of the electrodes of the transducer. The physical thickness of the electret
inherently limits the transducer capacity. The shape of the electret at the beginning, such as that
consisting of a charged low disk, was inherently limited in this regard. Stray capacitance, such as
the capacitance between the leads of the associated electronic circuit, is such as to impede
satisfactory performance. It has been proposed to solve both problems by producing electrets
from thin films such as gly ethylene terephthalate which are commercially available as Mylar
(trade name) thin films. The converter made of Mylar thin film indeed exhibits a drive capacity
high enough that stray capacitances in the related circuits do not pose a problem, so that the
converter can be used in fairly low impedance transistor circuits. However, the apparent stability
of the transducer has been found to be at least partially due to the transducer's malfunction.
More specifically, the inventor made and tested an electrostatic transducer using an electret
made of mylar thin film. The thin film is metallized on one side to form one electrode which acts
as the diaphragm of the converter [111111 1 EndPage: 1 converter] and the other side faces the
metal backplate consisting of the other electrode of the converter.
The electret diaphragm is necessarily mounted in close proximity to the backplate in order to
utilize the Mylar film to obtain high drive capacity. In transducers of this type of conventional
construction, it has been found that when the electret is highly charged there is a marked
tendency for the diaphragm to stick to the backplate. The transducer is initially severely limited
in response by this attachment. If the adhesion is the only effect involved, then the transducer
will show a gradual increase in response as the electret loses polarization until the problem of
adhesion disappears and the diaphragm is fully oscillatory. I will. Subsequently, as the electret
declines, there will be a continuous drop in response. However, other influences may hide this
process and give the apparent stability of the response. First, the diaphragm consisting of the
electret, which is initially under tension, tends to relax with time. Thus, the initial response of the
transducer is improved by tightening the diaphragm if the electret is not charged too strongly. As
the diaphragm gradually relaxes, the response of the transducer tends to decrease, but as the
adhesion of the diaphragm to the back plate gradually decreases, the response of the transducer
gradually increases, and both of them cancel each other out. The response appears to be stable.
Furthermore, the decrease in transducer sensitivity due to charge loss over time tends to be
compensated as well. That is, as the tendency to adhere decreases due to the reduction of
electrostatic force and thus the transducer sensitivity gradually increases, the sensitivity becomes
constant even though the decrease in charge reduces the sensitivity. In case of coincidence, these
compensatory factors may result in stable performance for 12 to 18 months, but the sensitivity
will then be intense. The object of the present invention is to increase the charge stability of the
electret. The electrets of the present invention are made of a polymer, in particular a
thermoplastic polymer dielectric material of poly / X40 carbon. The inventors of the present
application have surprisingly found [111111] thin films in comparison with similar thin films
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such as Mylar et al. Preferably, the electret comprises a coating of members of the class
consisting of polymers and copolymers of styrene, halocarbon and vinylidene chloride. The
inventors of the present application have discovered that this class of materials is surprisingly
superior to other materials in their ability to capture and maintain surface electrostatic charge.
According to the invention, the dielectric material from which the electret can be made is
selected according to two property criteria. The first is the ability to capture and maintain
internal static charge. Dielectric materials suitable for internal polarization have a polar
molecular structure, low mobility for charge transport and diffusion, high concentration of traps
for ions and electrons, and high glass or crystal dislocation temperature . A second desired
reference characteristic is the ability to capture and maintain surface charge. For that purpose
the material should have high resistance, adequate charge selectivity and low vapor absorption.
Of the broad family of hand-held dielectrics, it has been found that commercially hand-held
polyhalocarbon sheets and films are surprisingly well suited for the production of electrets. This
material exhibits the desired properties which are unexpectedly required for internal and surface
side polarization. This material can be further improved by surface treatment to provide
improved surface polarization characteristics. The surface properties of polystyrene and its
derivatives, polyhalocarbons and polyvinylidene chloride films are particularly good. When these
materials are coated on or selected at the surface of the material selected as the electret body,
materials with excellent internal and surface properties can be produced. FIG. 1 schematically
shows the process of producing an electret according to the present invention. A thin film or
sheet type polyhalocarbon dielectric material 1 is contained in a suitable furnace 3 as shown
schematically. The substance 1 is shown in FIG. 1 (provided with a metal coating on its lower
surface, but this metal coating is in direct mechanical and electrical contact with the first
electrode 5. A second electrode 7 in the form of a grid is provided on the dielectric material 1 at
a distance. A conventional DC high voltage, shown schematically at 9, is connected between the 5
and 7 electrodes. The DC power source 9 can generate a DC electric field of about 10 kilovolts
strength per senna meter in the range between the electrodes 5 and 7 in which the substance 1
is located. Although it can heat also by the method of (1), it is preferable to provide the induction
heating coil 11 preferably. The coil 11 is disposed in any conventional manner surrounding the
dielectric material and the electrodes 5 and 7. The coil can be energized with a conventional AC
power supply of radio frequency, shown schematically at 12. The electric field generated by the
coil 11 should be sufficient not only to heat the dielectric material 1 to the desired temperature
in the furnace, but also to cause significant ionization in the gas such as air.
This ionized gas serves to provide a plasma electrode whose impedance is greatly reduced
compared to the impedance of the non-ionized gas, and to connect the electrode 7 to the top
surface of the dielectric material 1. This arrangement produces a more uniform surface charge
on the dielectric material than in the case of a continuous electrode in direct contact with the
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dielectric surface and avoids the inherent problems resulting from the local breakdown of the
dielectric in the latter arrangement I found that. If means other than an induction field are used
as a heating source, the AC field required for ionization can be provided by providing a separate
screen guard on top of the first electrode 5 in class 3. This AC power source is connected
between the screen grid and the electrodes 5. Alternatively, an AC power supply could be
connected between the electrodes 5 and 7 using conventional means to isolate the AC and DC
power supplies. Although the AC field was supposed to have all four effects or reduce the effect
of the DC field, it was found that a higher degree of polarization can be obtained by using a
superimposed electric field. After the above-mentioned internal polarization, substance 1 is given
a surface charge. The dielectric surface opposite to the metal coating may be rubbed by exposing
the material to an ionizing gas while applying a very high bias, or with other materials that
exhibit an opposite charge selectivity selected by known methods. Can be charged to a selected
polarity. A net charge of io-3 to 1 o-7 coulomb / square senna defined as surface charge minus
internal polarization was generated in this way. Next, specific examples will be described.
EXAMPLE 1 In the configuration of FIG. 1, Aelar (trade name of polychlorotrifluoroethylene)
having a thickness of about 0.00127 cm (o, ooos gauge) provided with a metal coating on the
lower surface thereof is used as dielectric substance 1; This metal coating is brought into direct
mechanical [111111] contact with the first electrode 5 in the form of a metal plate, and a second
in the form of a grid in which 0.01 cm diameter wires are arranged at intervals of 0.12 cm.
Electrode 7 was placed at a distance of 1 cm from the upper surface of substance 1. A normal
high voltage direct current source 9 was connected between the electrodes 5 and 7, and a 12
kilovolt, 455 kilohertz alternating current source 12 was connected to the induction coil 11. The
furnace 3 was filled with argon at a pressure of about 1 mff 1 Hf, and an alternating current was
supplied to the induction coil 11 to heat the dielectric material 1 to 100 ° C. for polarization.
This alternating current simultaneously ionizes the argon. When the temperature reached 100 °
C., a direct current of +50 volts was supplied to the second electrode 7, and this combination of
direct current and alternating current was maintained in a 100 ° C. furnace for 10 minutes.
The temperature was then reduced to room temperature (30 ° C.) for 1 hour while maintaining
alternating current and direct current. At this stage, the conductive material 1 contains internal
polarization (different charge) and a slight excess of surface charge (same charge). Next, the DC
voltage was increased to +300 volts to increase the surface charge, and the AC and DC voltages
were maintained for 15 minutes, at which point charging was complete. The material 1 was
removed from the furnace and the charge was measured to be an equivalent voltage of +300
volts (positive polarity is preferentially received by this material) and a uniform, stable net of 6
× 10 −8 clones / cm 2. The charge density is shown. EXAMPLE 2 In the configuration of FIG. 1,
using Ac1ar (trade name of polychlorotrifluoroethylene) having a thickness of about 0.00127 cm
(0,0005 gauge) provided with a metal coating on the lower surface thereof as dielectric
substance 1; This metal coating is brought into direct mechanical and electrical contact with the
first electrode 5 in the form of a metal plate, and a second electrode 7 in the form of a grid in
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which wires of 0.01 cm diameter are arranged at intervals of 0.12 cm. A normal high voltage
direct current source 9 was connected between the electrodes 5 and 7 while maintaining a
distance of 1 cm from the upper surface of the substance 1. A second grid of the form in which a
wire of 0.005 cm in diameter is arranged at an interval of 1 cm between centers spaced apart by
20 cm on one side from the second electrode 7, and this second grid is argon Directly connected
to a 10 kilovolt, 60 hertz alternating current source to ionize the gas. This method does not use
an induction coil. The furnace 3 is heated by conventional resistance heating means. This method
allows independent control of temperature and ionization [111111] EndPage: 3. The furnace 3 is
filled with argon at atmospheric pressure, the dielectric material 1 is heated to 100 ° C. with a
resistance heater, and 10 kilovolts of alternating current is supplied simultaneously to the
ionization grid and +50 volts of direct current to the second electrode 7 The polarization was
performed by Maintain furnace 3 at 100 ° C. for 15 minutes, then reduce heating and reduce
temperature from 100 ° C. to room temperature (30 ° C.) between> 60 minutes while
maintaining AC and DC voltage The Complete internal polarization when temperature reaches 30
° C. and increase DC voltage to +300 volts to increase surface charge and maintain both DC and
AC voltages for 15 minutes, at this point The charging was completed. The material 1 was
removed from the furnace and the charge was measured to be an equivalent voltage of +300
volts (positive polarity is preferentially received by this material) and a uniform, stable net charge
of 6 × 10 ′ clones / cm 2. It showed the density.
Observation on the charge stability of the electret produced in this way was found to be stable
over several tens of months. Thus, the use of the electret according to the invention in an
electrostatic converter such as a condenser microphone or an electrostatic loudspeaker
significantly improves the performance of the electrostatic transformer. 2 and 3 show an
example of an electrostatic transducer using an electret according to the present invention. The
device of FIG. 2 is symmetrical about axis A, with some exceptions apparent from FIGS. 2 and 3,
the structure of which can be understood from a single view. The transducer comprises a
substrate 13 such as plastic with an upstanding annular flange 15. The flange 15 comprises a
shelf 17 supporting a metal back plate 19. Backplate 19 is preferably drilled through a series of
holes such as 21 by chemical grinding. The holes 21 preferably have a diameter of about 0.0127
centimeters (about 5 mils) and a center spacing of 0.0381 centimeters (15 mils), although
various other dimensions and spacings may be employed. The holes provide an air passage in the
backplate and serve to make the gap between the backplate and the substrate 13 act as an
acoustic compression chamber. The upper surface of the back plate 19 is made of a fiber metal
wire 23 such as electric nickel, and preferably a net having a spacing of about 40 to 100 mils
and a gap of about 0.1016 to [111111] is formed. Is fixed to the back plate 19 by welding, for
example. Alternatively, the lines 23 can be integrally formed with the backplate. Another suitable
alternative to the line 23 is a series of posts integrally formed with the backplate. These posts
have a height of about 0.0003175 to about 0.00127 cm (% to% mil), a diameter of about
0.00508 to about 0.0127 cm (2 to 5 mil), and a spacing of about 0.0508 to about 0. It is 1524
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cm (20 to ω mil). On the backplate 19 is an electret diaphragm, generally indicated at 25, having
an exposed dielectric surface 27 and a metalized top surface 29. The diaphragm 25 may be
slightly separated from or in contact with the line 23. In any case, the line 23 limits the minimum
gap between the diaphragm and the back plate, and the diaphragm is the back plate. It prevents
electrostatic adhesion. The diaphragm is held in place by the annular rim of the protective metal
screen 33 which serves to clamp it against the flange 15 and the outer metal cap 35. The cap 35
can be fixed to the substrate 13 in any suitable conventional manner, for example by cutting off
parts or by gluing or the like.
Electrical connections to the converter of FIG. 2 can be made in a variety of conventional ways.
As shown, one lead 37 can be connected to the metal cap 35 and can be in contact with the metal
surface 29 of the diaphragm 35 via the cap 35 and the screen 33. The second conductor 39 is
directly connected to the back plate 19 and can be removed via a suitable passage comprising
the substrate and the conventional insulation arrangement (not shown) in the cap as shown.
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