GB2040642A

GB2040642A - Transducer

Info

Publication number: GB2040642A
Application number: GB7903224A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-01-30
Filing date: 1979-01-30
Publication date: 1980-08-28
Also published as: JPS55103780A; AU5484080A; ES8100591A1; ZA797036B; ES488162A0

Abstract

Electro-acoustic or electro- mechanical transducers are known in which the diaphragm is a foil of an electrically active plastics material, such as a foil of an electret material or of a piezo-electric material. With such transducers, electrodes (2) are needed on one at least of the faces of the foil (1), and to overcome difficulties due to the making of contact to such electrodes (2), at least one of the electrodes (2) is a layer of an elastomer loaded with particles of an electrically conductive material such as carbon. <IMAGE>

Description

SPECIFICATION Transducer This invention relates to electro-acoustical or electro-mechanical transducers of the type in which the diaphragm is of an electrically active plastics material. Examples of such materials are piezo-electrically active plastics foils and foils which carry electret charges.

Such a diaphragm requires for its operation thai one or both of its surfaces carry electrodes to which electrical contact can be made.

Conventionally such electrodes consist of a thin metal film applied by vacuum coating or electroplating methods. The connections to the electrodes are usually pressure contacts, since a soldering or welding operation would damage the plastics foil. In addition, most metals that would be otherwise suitable for use as electrodes suffer from one or both of the following problems: (a) The thin film may be damaged by corrosive atmospheres containing, for example, sulphur dioxide or salt.

(b) The metal surface may have an oxide layer which causes poor contact at the low current produced by the transducer with the light contact pressure which can be applied.

Some of the precious metals do not suffer from the above problems, but they are costly. Further, they are often difficult to apply to the plastics films, as it is usually necessary to coat the film with another metal first to overcome adhesion problems.

An object of the present invention is to provide an electro-acoustic or electro-mechanical transducer in which the above problems are minimized or even overcome.

According to the invention there is provided an electro-acoustic or electro-mechanical transducer which includes a diaphragm of an electrically active plastics material having electrically conductive electrodes on one or both of its two faces, wherein each said electrode is of an elastomer material having particles of an electrically conductive material dispersed in it.

According to the invention there is also provided an electro-acoustic transducer, which includes a diaphragm of a plastics material which is an electret or is piezo-electric, said diaphragm having electrically conductive electrodes on one or both of its two faces, wherein each said electrode is of an elastomer material having carbon particles dispersed in it.

Such an arrangement provides an inexpensive simply-applied electrode which is free of corrosion and to which a simple, reliable, pressure contact can be made. Such an electrode is a preferred arrangement consists of a layer of an elastomer material containing carbon particles to make it conductive. These particles do not corrode, and the elastic matrix does not unduly increase the stiffness of the transducer element. As the layer is an elastomer, it will not crack during operation, nor when the diaphragm is made into a domed or cylindrical form, as required by some forms of piezo-electric transducer.

For the elastomer it is preferred to use synthetic materials, as these do not deteriorate over a period of time, as happens with a natural latex.

Fluorine-containing polymers are especially suitable for this purpose as they are highly stable and are not damaged by high temperature during use, or during the polarization of the foil.

Embodiments of the invention will now be described with reference to the drawings in which Fig. 1 is a cross-section of a first embodiment of the invention.

Fig. 2 is a second embodiment of the invention, also shown in cross-section.

Fig. 3 is a third embodiment of the invention, also shown in cross-section.

We refer first to Fig. 1. Here a transducer diaphragm 1 of plastics foil is shown, which is coated on each face with electrodes 2 of a conductive carbon-loaded elastomer. The diaphragm is assumed to be circular, being gripped at the rim, and having electrode connections (not shown) made to the electrode 2.

These electrodes may be applied by brushing, spraying or printing on of the elastomer-carbon mixture which has been dissolved in a suitable solvent. When the solvent evaporates a thin layer of carbon-loaded elastomer is left on the surface of the foil 1.

Note that the foil 1 may be piezoelectrically active material or an electret material.

It is sometimes convenient, e.g. when the diaphragm material has to be piezoelectrically active, to polarize the diaphragm using metal electrodes. In this case the elastomer may be applied over the metal electrodes to protect them and also to provide a reliable means of connection. An example of this is shown in Fig. 2 where metal electrodes 3 on the plastics foil 1 are coated with a carbon-loaded elastomer 2.

Fig. 3 shows how electrical contact may be made to an electrode 2 on a foil 1 by clamping it to a metal ring 4 by a frame 5. The ring 4 connected to one transducer terminal may be coated with precious metal or may itself have a coating of carbon-loaded polymer 3 on its contacting surface. Another possibility is to make the clamping material itself out of a conductive plastics material.

Claims

1. An electro-acoustic or electro-mechanical transducer which includes a diaphragm of an electrically active plastics material having electrically conductive electrodes on one or both of its two faces, wherein each said electrode is of an elastomer material having particles of an electrically conductive material dispersed in it.

2. A transducer as claimed in claim 1, and in which the particles are of carbon.

3. An electro-acoustic transducer, which includes a diaphragm of a plastics material which is an electret or is piezo-electric, said diaphragm having electrically conductive electrodes on one or both of its two faces, wherein each said electrode is of an elastomer material having carbon particles dispersed in it.

4. A transducer as claimed in claim 'i, 2 or 3, in which the foil is provided, on one or both of its faces, with a thin metallic layer between the face of the foil and its elastomer layer.

5. A transducer as claimed in claim 1 ,2,3 or 4, and in which the foil and its elastomer electrode on one of its faces is retained in a frame by a metallic clamping ring.

6. An electro-acoustic or electro-mechanical transducer substantially as described with reference to Fig. 1, Fig. 2, and Fig. 3 of the accompanying drawings.

7. An electro-acoustic transducer, which includes a diaphragm having at least one face in contact whith the atmosphere, which diaphragm is a foil of a plastics material which is an electret material or a piezo-electric material, and an electrically-conductive electrode on one or both of the faces of the foil, wherein the or each said electrode is a layer of an elastomer material having particles of an electrically conductive material dispersed in it, and wherein the or each said electrode has a thickness which is of the same order as the thickness of the foil.

8. A transducer as claimed in claim 7, and in which the electrically conductive material is carbon.

9. A transducer as claimed in claim 6 or 7 and in which the foil is provided with a thin metallic layer between the or each face of the foil and the corresponding electrode.