GB2079055A - Electret device - Google Patents

Abstract

An electret device (16) comprises a plurality of electret elements (10) which are stacked in layers, at least the lowest electret element of the stack having a conductive electrode (12) and each electret element being provided with a volume of surface charge of one polarity. <IMAGE>

Description

SPECIFICATION Electret device This invention relates to an electret device.

An electret is a dielectric body which is provided with a volume of surface electric charge. The electret dielectric body retains the electric charge for very long time, often measured in tens of years. The electric charge, or the dielectric polarization, is achieved, for example in one process, by heating a body of dielectric material to a relatively high temperature while the material is exposed to an intense electric field for a substantial time and then cooling the material while maintaining its exposure to the electric field. Upon cooling, the material exhibits characteristics of a permanent charge distribution.

Electrets have many uses as transducers such as microphones, loudspeakers or record-disc pick-ups. Moreover, it is hoped to use electrets as memories, high potential sources, measuring apparatus and other devices in the near future.

Electrets are dielectrics which produce static electric fields having relatively long lifetimes.

Their long lifetimes are mainly attributable to the development of new materials which are more stable than the earlier employed waxes. Today, plastics materials such as polyethylene, polypropylene, polyethylene terephthalate are extensively used to form electrets.

Conventionally, electrets are used in the form of electret devices of which an electret body is secured on a conductive electrode. These electret devices have drawbacks in that the surface charge potential is relatively low.

According to the present invention, there is provided an electret device, comprising: a plurality of electret elements which are stacked in layers and of which at least the lowest electret element comprises a polarized dielectric body adjacent to another electret element and a conductive electrode secured to said dielectric body, each electret element being provided with a volume of surface charge with one polarity.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional view of an embodiment of an electret device according to the present invention; Figure 2 is a sectional view of another embodiment of an electret device according to the present invention; Figure 3 shows graphs of charge potential distributions on electret devices; Figure 4 shows graphs of charge retention properties of embodiments of electret devices according to the- present invention; and Figures 5a and 5b are sectional views of modified embodiments of electret devices according to the present invention.

Throughout the drawings, like reference numerals will be used to designate like or equivalent portions.

Fig. 1 shows a sectional view of an embodiment of an electret device 16 according to the present invention. A dielectric body 10 is formed as a flat plate and provided with a volume of electric charge with one polarity. A conductive layer or base plate 12 is secured on one flat surface of dielectric body 10.

A combination of such a polarized dielectric body 10 and conductive base plate 12 forms an electret element 14. A plurality of such electret elements 14~,, 142, 142 14 are stacked one on another the conductive base plate 12 of the second electret element (142) being attached to the dielectric body 10 of the electret element 1 4. and so on, the stacking of electret elements 14 forming the electret device 16.

Each combination of a dielectric body 10 and a conductive base plate 12 is similar to a conventional electret device.

Usually, the polarization of each dielectric body 10 is carried out by, for example, imposing an electrical field or applying charged particles like ions, before or after the conductive base plate 12 is combined with the dielectric body 10. The polarity of the dielectric body 10 may be decided by the polarity of the electrical field or the charged particles.

Almost all dielectric materials with the nature of an electret may be adopted for a dielectric body 10. High molecular weight compounds like polypropylene, fluorine containing polyolefins (such as polytetrafluoroethylene) or polyethylene are, however, especially suitable because the electric charge in those materials becomes relatively high in potential and is maintained for a long time.

The electret device 16 of Fig. 1 has a very high surface charge potential as a whole. The charge potential Es is given by equation (1) below: Es = [ [ (E1.K2 + E2) . K3 + E3 ] . K4 + ..j. K5 + E5 = k,. E1 C k2 .E2 + k3 E3 + ... + En (1) where E5 represents the charge potential of electret device 1 6 as a whole, E" E2, E3 .... and E represent the charge potentials of electret elements 1 4" 1 42, 143 .... and 14 respectively, K1, K2, K3 .. and Kn represent polarization constants, as they may be called, of electret elements 1 41, 142, 143 ... and 1 4n and k1, k2, k3 .. represent respective synthetic polarization constants by which electret elements 1 4" 1 43, 143 ... and 14 located in the lower part of electret device 16 dielectrically affect electret element 1 4n located at the top of electret device 1 6, directly or via one or more electret elements between them.

As is apparent from the equation (1), the charge potential E5 of electret device 1 6 is exceedingly higher than that of an individual electret element 14. Therefore, electret device 1 6 described above is very useful for many fields. For example, an electrostatic transducer like a microphone or a headphone using electret device 1 6 becomes highly sensitive and has other effective properties. Especially, a powerful electrostatic loudspeaker which was difficult to realise is easily obtained by using electret device 16.

Fig. 2 shows a sectional view of another embodiment of an electret device according to the present invention. Electret device 1 6 of Fig. 2 is different from electret device 1 6 of Fig. 1 by having a cover body 1 8 on top of electret element 1 4n.

The material of cover body 1 8 may be a conductive, semi-conductive or non-conductive material or a mixture of such a material with at least one other substance. In the case of nonconductive materials, for example glass or an organic compound with a volume resistivity of more than 1 0l253-cm is particularly suitable.

In the electret device 1 6 describe above, the cover body 1 8 is polarized dielectrically under the affect of the charged electret device 1 6. A positive polarity appears on the cover body 1 8 at its surface remote from electret element 1 4n, providing the polarity of the surface charge of its dielectric body 10 is positive, and vice versa. That is, a polarity the same as the polarity of the surface charges of dielectric bodies 1 0 appears on the outer surface of the cover body 1 8.

The charge potential Es is given by equation (2) below: E5c = [ [ [ (E, . K2 + E2) . K3 + E3#. K4 + ] Kn + En ] sKc = k1 . E, + k2 . E2 + k3.E3 +... T Kc Ea (2) where K5 represents a polarization constant of the material of the cover body 1 8, and the other symbols are identical with the corresponding symbols in equation (1), except that k1, k2, k3 .. refer to the cover body 1 8 not the electret element 1 4n The charge potential E55 of the electret device 1 6 of Fig. 2 is also exceedingly higher than that of an individual electret element 14, as with the electret device 1 6 of Fig. 1. Furthermore, the charge potential E55 is uniform at every part of the upper surface of the cover body 1 8 as explained below.

Fig. 3 shows graphs comparatively illustrating potential distribution characteristics on an electret device 1 6 and a conventional electret device, tha charge potential in volts (V) being plotted against location on the upper surface, starting at one end and finishing at the opposite end. Graphs A and B are potential distribution characteristics of electret devices 1 6 of Fig. 1 and Fig. 2 but the number of electret elements 14 is three, and graph C is a potential distribution characteristic of a conventional electret device similar in its size with those of the embodiments of the present invention.

As will be seen from graphs A and B, the charge potential or its average value of such an electret device 1 6 is nearly three times of that of the conventional electret device as shown by graph C. The charge potential dispersion of graph A, in its ratio to its average potential, is smaller than that of graph C. Moreover, the charge potential of graph B has little dispersion. As will be understood from the relatively flat characteristics of graph A and the very flat characteristics of graph B, such a device 1 6 has exceedingly uniform surface charge at every part of its upper surface. On the other hand, the conventional electret device described above does not have uniform surface charge as will be understood from graph C.

An electret device 1 6 therefore provides a very good usefulness. That is, for example, transducers using an electret device 1 6 may provide high-efficiency tranducing characteristics due to the high surface charge potential of electret device 16.

Furthermore, transducers using an electret device 16 may provide high-fidelity transducing characteristics because the electrostatic forces arising between the electret device 1 6 as one electrode of the transducing capacitor and an opposite electrode are uniform at every part of the transducing capacitor.

An electret device 1 6 is provided with long-lived charge or polarization retention properties because dielectric bodies 10 except the one located at the top of the electret device 1 6 without a cover body are protected from being exposed to the air, and the charge becomes hard to drain away. Moreover, the electret device 1 6 shown in Fig. 2 is provided with exceedingly long-lived charge or polarization properties because dielectric body 1 0 of electret element 1 4n located at the top of the electret device 1 6 is protected from being exposed to the air by the cover body 1 8, and the charge becomes hard to drain away.

Charge retention properties are shown in Fig. 4 in which charge potential in volts (V) is plotted against time in hours (H). Graph D represents the property of an electret device 1 6 shown in Fig. 2 whose cover body 1 8 is made from conductive material or metal. On the other hand, graph E represents the property of an electret device 1 6 shown in Fig. 2 whose cover body 1 8 is made from non-conductive material, for example ABS (acrylonitrile-butadiene-styrene) copolymer. The charge potential of graph D is maintained at almost the same as the value at the time of its fabrication, for a long time. The charge potential of graph E gradually decreases after the time of its fabrication, but the charge potential approaches a steady state about 24 hours afterwards. Then, both of the charge potentials remain at the steady states for a very long time.

Furthermore, an electret device 1 6 whose cover body 1 8 is made of a conductive material is easily capable of having its decreased surface charge increased by means of supplying high potential voltage between the lowest conductive base plate 12 and the cover body 1 8 because the dielectric bodies 1 0 are repolarized by the high potential voltage.

Figs. 5a and 5b shown sectional views of modifications of the electret devices 16 shown in Figs. 1 and 2 respectively. In these modifications, electret element 141 located at the bottom of the electret device 1 6 is provided by both a polarized dielectric body 10 and a conductive base plate 12, but the other electret elements 1 43, 143 ... 1 4n are provided by only polarized dielectric bodies 10.

Claims (6)

1. An electret device, comprising: a plurality of electret elements which are stacked in layers and of which at least the lowest electret element comprises a polarized dielectric body adjacent to another electret element and a conductive electrode secured to said dielectric body, each electret element being provided with a volume of surface charge with one polarity.

2. An electret device according to claim 1, wherein it further comprises a cover body attached on the electret element located at the top of the stack.

3. An electret device according to claim 2, wherein the said cover body comprises conductive material.

4. An electret device according to claim 2, wherein said cover body comprises nonconductive material.

5. An electret device according to claim 2, wherein said cover body comprises semiconductive material.

6. An electret device, substantially as herein described with reference to Fig. 1, or Fig. 2, or Fig. 5a, or Fig. 5b of the accompanying drawings.

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