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Description 1, title of the invention
Electret device
3. Detailed Description of the Invention The present invention relates to an electret apparatus
and, more particularly, to an induction (N) type that can be used to illustrate the equalization of
the surface charge density of the electret and the like. As is well known, electrets have the
property that a charged electric charge lasts semipermanently, and are widely used in various
fields as electrostatic transducers and the like. FIG. 1 shows the principle configuration of a
conventional condenser microphone using such an electret, wherein an electret material 12 such
as a polymer film is adhered to a back electrode J1 made of a conductive metal substrate to form
an electret. The selected electret structure is used as a fixed electrode. Then, a diaphragm 16
made of metal thin film or the like stretched over the holding ring 15 is disposed as a movable
electrode on the fixed electrode formed of the electret structure U via a sweep J4 of an insulating
material or the like. Is configured. Thus, when the diaphragm 16 of such an electrostatic unit is
displaced by the external sound pressure, the capacitance C between the two electrodes is
changed so that K changes, but Q = CV and so on. In this case, the voltage V between the two
electrodes will be changed if the electret structure 1 (provided by 3) is constant. Therefore, a
capacitor microphone is realized which converts the sound pressure into an electric signal in an
electrostatic manner by converting the change voltage V into an impedance by converting the
impedance V through the field effect transistor FET or the like and extracting it as an EndPage: 1
impedance. Be done. 1g2 iri same, <a book showing nine cases applied to a condenser head unit,
in principle, by conducting the operation opposite to the case of the condenser microphone
described above. Can be converted to sound pressure. In this case, the diaphragm 16 as a
movable pole stretched in the holding ring 15 'is placed in the center, and the spacers J4. A pair
of electret structures via J4, 1! By setting 7 as a fixed pole 25 and arranging them in a pair, a socalled push-nor-type no-condenser headphone unit is realized. In addition, the signal source Em
and the high-resistance 8-fold high-resistance resistor are connected to each electrode of the
headphone unit. An electrical signal is provided via R ,. By the way, the electrostatic transducer
using the electret structure as the fixed pole as described above is different from the usual one
and there is no need for a DC power source for polarization or polarization, so this configuration
is as simple as possible. It has the advantage of being able to be miniaturized and thus of course
be able to be less expensive.
However, the electret structure which is most important has a serious drawback that it tends to
adversely affect various properties as a trans-) easer in that the variation of the surface charge
density, that is, the unevenness of the charge distribution is large. The FIG. 3 shows an example
of measurement of the surface charge distribution of the conventional electret structure, in which
the electret material 12 is deposited on the back electrode JJ made of an aluminum plate having
an outer diameter of 6 ° φ and electretized. There is, however, as you can see from the figure, it
can be known that the variation is very large within the range of several 1 oov. In the same
figure, the flat portion JJb at the center excluding the wavy undulations JJa in the periphery is a
non-electretized portion provided for manufacturing reasons etc., but the surface charge density
of the non-electretized portion Jjb is that of the periphery. There is also a problem that the
variation will be much larger and extremely lower, and the effective area given to the
electrostatic action will be reduced accordingly, so in this case various elements including
conversion efficiency as an electrostatic type transducer. The j conversion characteristic is
further degraded. In addition, since the conventional electret structure is exposed so that the
surface portion of the electret material 12 is exposed to air directly, it has poor moisture
resistance and has a problem in the stability over time, and the life is shortened. It had a serious
drawback of Therefore, the present invention has been made in view of the above-described
points, and in particular, it is possible to illustrate the uniformity of the electret surface charge
density well as the induced (N) form and to greatly improve the stability. It is an object of the
present invention to provide an electret device. The following is a detailed description of an
embodiment of the present invention with reference to FIG. That is, as shown in FIG. 4, electret
materials 221 and 222 made of polymer films such as PP, TFE, and FEP are used as back
electrodes 211.2 J2 fC each made of a conductive metal substrate, one of which is at the same
potential as earth. A conductive metal material or an organic material in which a pair of electret
elements 231.232 formed by depositing and electretizing are facing each other with the electret
material side, and the area shape is the same or less between them. Is stacked to sandwich the
dielectric 24 in a state of being insulated from the ground, and a charge of a potential
substantially equal to the algebraic sum of the single potentials of the electret elements 231 °
232 is displayed on the surface of the other back electrode 212. It is the parent of electret device
25 configured to be ejected, where dielectric 24 is usually In the following, the term
insulator alone refers to one that includes the conductive metal material and the organic
material (which will be described later) as described above.
Here, the organic material is a thermoplastic and thermosetting resin EndPage: 2 material, a
composite material combining these with another material, and a dielectric material such as
glass, and the volume resistance is 10 120-countries or more. The so-called insulating materials
of Then, when the charges of the electret device 25 and the pair of electret elements 231 ° 232
configured as described above respectively have a negative sign θ and a positive sign, charges of
the negative sign θ are induced on the surface of the back electrode 212 of one element 232 As
a result, the dielectric (polarization) effect is provided. In this case, laminating the pair of electret
elements inward with the dielectric 24 interposed therebetween does not merely cause the
dielectric action as described above k, and the surface as the electret device U as described later
Uniform density of charge density! It is important that they are well designed and that their
stability is secured. In the above, when the surface potential of the pair of electret elements
231.232 alone is CBIJCEx) and L7, and the surface potential induced on the back electrode 212
of one of the elements is [E,], E, = (α1- 1cI) (αgag 寓) In this case, (, α1), (α5) Fi is a book also
called an induction coefficient, so α1 = α3-α = 0.77 to approximately 1.0 It was confirmed that
they were included. However, when the dielectric 24 is present, it is approximately in the range
of α = 0.6 to 0.99. Therefore, considering the equation as α ÷ 1 and simplifying it, the above
equation is E, = El-E. となるからE、=E、→E、=OEl)El→E、
=■Er<Ex→Il=。 As described above, K can be regarded as inducing a charge having a
positive or negative sign potential substantially equal to the algebraic sum of each element single
potential. In this case, an electret element alone generally charged with a positive sign is inferior
in time-wise stability to the single tread element singly charged with a negative sign θ, and these
are combined and laminated. It is advantageous to be able to stabilize by means of K and to
obtain a surface charge of 4E, which is obtained as follows: = 1 El-θ E 寞 = 2 K (where El-gs = E).
FIG. 5 shows a specific example in the case of obtaining the electret device 25 realized as
described above. First, the contacts of the switch SW connected to the dielectric 24 and the back
electrode 212 of the upper electret element 232 are sequentially In the closed state, the back
electrode 212 and the dielectric 24 are brought into the same potential tb zero charge state as
the earth, and then the switch SW is opened to make the earth and the insulation state.
Then, the upper electret element 232 and the dielectric 24 are previously held on the upper
electret element 231 to which the back electrode 211 has the same potential as the ground while
keeping the zero lightning load state by using, for example, insulating tweezers. A pair of
elements 231.2: II! If the dielectric 24 is laminated to form an interlayer, as described above, the
dielectric action on the surface of the back electrode of the upper element occurs. In this case,
when the dielectric 24 is a conductive metal material, charges are instantaneously induced, but
when it is an insulating material, it takes a certain time for the WJ implant charge to reach a
constant voltage ( For example, it requires 30 minutes of chords). Also, assuming that the switch
SW is closed again from such an induced state, the electric charge disappears as if the dielectric
24 is a metal so that the potential becomes zero immediately, but in the case of a P edge material
(in the case of a fluorine resin system). In the case of metal, it has a time constant with respect to
the movement of charge so as to gradually decrease and disappear. Since FIG. 6 shows a case of
using a glass whose area shape is smaller than that of the electret element as the dielectric 24 as
another specific example, the induction coefficient α = 0. It was ascertained that approximately
83 EndPage: 3 degrees (the dielectric charge of the glass dielectric is approximately 5 sov for the
electret element single charge of 700 V) and that it is induced almost equivalent to the case of
metal. When it is necessary to adjust the surface potential, since the decrease time of the glass
dielectric 24 & when the switch SW is closed again from the dielectric state is relatively faster
than the insulating material such as ABS resin and slower than the metal material. It is
advantageous to FIG. 7 shows the time-dependent change (leakage characteristic) of the charge
induced on the surface of one back electrode 212 as described above, that is, one index of
stability, in which (a) is the dielectric 24. This is the case of a lightning-conducting gold-plated
material (but the same applies in the case of a fluorine resin system), and almost unchanged from
the initial state (good stability is obtained). Also, (b) is a case where the dielectric 24 is fii, and in
the case of a thermal insulation material such as an ABS resin, and after about 5 minutes from
the initial state, the flakes are collected and good stability is obtained thereafter. However, the
stability is such that the electret device 25 has a pair of electret elements' 2JJ. Because the
surfaces of each element 231 ° 232 are protected by sandwiching the dielectrics 24 with layers
232 facing each other and laminating the layers, the electret surface may be exposed to air
directly unlike in the conventional case. It is promoted by being made not to be exposed.
That is, each time the surface of each work and lefttlet element 231.232 is protected, it is not
only rich in moisture resistance, it is advantageous in terms of temporal stability as well, and as a
result, a button as long as possible to achieve long life extension It is because stability can be
ensured. As shown in FIGS. 8 (a) and 8 (b), if undulations are provided on the surfaces of the pair
of electret elements 231.232, respectively, the dielectric 24 interposed therebetween is directly
attached. As the area of contact is reduced, the stability can be further enhanced from the
viewpoint of handling. FIG. 9 shows an example of measurement of the surface charge
distribution of the electret apparatus U as described above, and electret materials: 221 h, 222 *
'fr on a back electrode 211 mm 212 a made of an aluminum plate having an outer diameter of
60 絽 φ. 1. A pair of electret elements that have been electretized. ? 1ha 232 m mutually inward,
the area shape is the same root between them. It is laminated so as to sandwich the dielectric
24b of a degree, and the distribution state of the charges which are induced on the surface of
one back electrode 2121. That is, as can be seen from this figure, it can be known that the
surface charge density is uniform and hardly dispersed in all parts unlike the conventional case.
It should be noted that the decrease slightly in the vicinity of both ends in the figure is due to the
accuracy of the measuring instrument and is not essential. The flat portion 22c at the center
excluding the undulations fB 22b in the periphery in FIG. 9 is a non-electretized portion provided
for manufacturing reasons as in the conventional case, but the dielectric corresponding to the
non-electretized portion Even in the central portion of the surface of the body 24b, no lower drop
is observed, and uniform charge density equivalent to that of the periphery can be obtained. That
is, unlike the conventional case, the effective area given to the electrostatic action is not
substantially reduced a little if there is a non-electretized portion, and in this respect, conversion
as an electrostatic transformer is possible. It is possible to significantly improve various
conversion characteristics including efficiency. That is, the electret device 25 in which the
dielectric 24 is sandwiched and stacked between the pair of electret elements 231.232 that are
mutually inward facing as described above merely induces a predetermined charge on the
surface of one back electrode 212. Not only does it exhibit the phenomenon, it has the very
excellent advantage of improving the stability provided by the protective effect of the electret
element 231.232 and the possible equalization of the surface charge density provided by the
surface charge uniforming effect. As soon as it becomes clear.
In the above, the dielectric 24 is not necessarily required to be used, and even if the dielectric 24
is not present, the charge density exposed on the surface of the back electrode of one of the
elements is a distribution unevenness in each element alone. Kana EndPage: It will be improved.
By the way, although the electret device as described above has various application aspects as an
electrostatic transducer, there is a pick-up cartristo and a speaker besides the above-described
microphone and headphones especially for an audio device. When applied to these acoustic
devices, there are the following advantages. That is, first, low noise and high efficiency by the
increase of effective capacity, low distortion by uniform voltage distribution, and high reliability
by the improvement of stability. It is a point of improvement of retention, simplification of
accuracy by the simplification of structure, and reduction of cost. In particular, in the case of a
microphone, the noise can be further reduced by increasing the capacitance to lower the input
impedance to the impedance converting field effect transistor and lowering the noise level.
Further, in the case of headphones and speakers, it is possible to achieve so-called
transformerless operation that can eliminate the need for a signal supply transformer by
increasing the efficiency. The following fields can be considered as applications other than
acoustic equipment. For example, it can be applied to switches and sensors as sensors in a
second-to-one basis as well as measuring instruments (including radiation dose needles) as
sensors in principle and substantially the same as the electrostatic transducers described above.
It is also applicable to digital devices. The present invention is also applicable to various
electrostatic devices (electrostatic printing machine, vacuum generator, electrostatic dust
removing device, etc.), and also applicable to medical devices and high impedance high voltage
power supplies. [7] Then, it is needless to say that other applications as well as the abovementioned modifications of the embodiment and the specific example are possible without
departing from the scope of the present invention. Therefore, as described above, according to
the present invention, an electret apparatus capable of achieving a good uniformity of the
electret surface charge density, particularly as the induced (N) form, and greatly improving the
stability. Can be provided.
4, Brief description of the drawing 1 m! FIG. 2 is an explanatory view of the principle
configuration of a conventional condenser microphone and condenser headphone unit, FIG. M 3
is a measurement drawing showing surface charge density of the conventional electret structure
used in FIG. 1 and FIG. 4 is an explanatory view showing one embodiment of the electret
apparatus according to the present invention, FIG. 5 LV, FIG. 6 is a view showing a concrete
example of FIG. 4 and FIG. 7 is FIG. FIG. 8 is a diagram showing another specific example of FIG.
4, and FIG. 9 is a diagram showing surface charge density of the electret device of FIG. 21. 212 · ·
· back electrode, 221.222 · · · · electret material% 231. 232 · · · electret 1 element, bi · · · ·
dielectric, U · · · · · · electret device. Applicant agent Patent attorney Takehiko Suzue 11th I 2nd I
16 third! l□EndPage: 5