WO2015055336A1

WO2015055336A1 - Electret structure

Info

Publication number: WO2015055336A1
Application number: PCT/EP2014/067623
Authority: WO
Inventors: David Bartylla; Nicolas Maier; Erhard Hirth
Original assignee: Robert Bosch Gmbh
Priority date: 2013-10-17
Filing date: 2014-08-19
Publication date: 2015-04-23
Also published as: DE102013221140A1; EP3058760A1; CN105659627A

Abstract

According to the invention, an electret structure (1) is proposed which comprises an electrically conductive substrate (10), a first electret layer (30) and a porous dielectric layer (20), which is arranged between the substrate (10) and the first electret layer (30). The electrically conductive substrate (10) in this case preferably forms an electrode of the electret structure (1) according to the invention. The invention is based on the concept of using the porous dielectric layer (20) to separate the charged electret layer (30) from the electrode (10). As a result, the migration of electrical charges (50) out of the electret layer (30) towards the rear electrode (10) is impeded. Likewise, the migration of oppositely charged charge carriers (55) out of the rear electrode (10) into the electret layer (30) is impeded. The charge stability of an electret structure (10) according to the invention in comparison with conventional addons without a porous dielectric layer (20) between the electret layer (30) and the rear electrode (10) is thus markedly increased.

Description

description

title

Electret

State of the art

Electrets are materials that can store electrical charges over a long period of time. Electrets (eg in the form of polymer films, but also inorganic electrets such as Si0 ₂ ) can be electrically charged by a variety of methods (corona processes, electron beams, contact charging, ...). When charging charge carriers are injected into portions of the electret or charge separation in the electret caused, whereby the electret becomes the carrier of an electric field. Charged electrets can be used in a variety of electronic components,

For example, in microphones, speakers or air filters. Electrets are contacted and charged with electrodes on at least one side to form an electret structure. A comprehensive account of previous knowledge can be found, for example, in G.M. Sessler (Ed.), Electrets, Vol. 1, 3rd Edition, Laplacian Press, Morgan Hili, CA, 1999 or in G.M. Sessler, Electrets: recent developments, Journal of Electrostatics, Vol. 51-52, pp. 137-145, 2001.

A challenge for technically deployable electret structures is the stability of charge separation in the electret for long periods of time, ideally several years or decades for application-relevant ones

Ambient conditions (e.g., elevated temperatures of, for example, up to 85 ° C), especially at high temperatures

Charge carriers can travel through the electret. As a result, charge carriers can be lost over time if charges migrate from the electret into a contacting electrode and compensate there. Numerous methods for improving the charge stability of charged electrets have been described in the literature, for example pretreatment with plasma, see Chen, Q., Investigation of Corona Charge Stability Mechanisms in Polytetrafluoroethylene (PTFE) Teflon Films After Plasma Treatment, 2003, Journal of Electrostatics 59 (FIG. 2003) 3-13, with chemicals see Haridoss, S., Perlman, M.

M., Chemical Modification of Near-Surface Charge Trapping in Polymers, 1984, J. Appl. Phys. 55 (5), 1 March 1984 or at elevated temperatures, see van Turnhout, J., The Use of Polymers for Electrets, 1975, Journal of Electrostatics, 1, pp. 147-163.

Since the last decade, porous electret films that internally store the charges on pore surfaces have been intensively researched as piezoelectric material ("Piezoelectrics"), as described, for example, in M. Paajanen, J. Lekkala, K. Kirjavainen, ElectroMechanical Film (EM Fi). As soon as two electrodes have been applied to the material, such a material can already be used as a mechanical-electrical converter and, due to its flexibility, allows different applications, whereby a surface of the charged porous electret (piezoelectret) oscillates. active. Particularly interesting for applications as a porous piezoelectric are electret materials, which are excellent

Provide charge storage, such as Teflon FEP, PTFE and PFA, and therefore can be used even at higher temperatures for a long time. When porous electret foils (piezoelectrons) are charged, there are positive and negative charge carriers on opposite sides of the pore in each pore. As a result, there is the disadvantage that a discharge of the electret takes place over longer periods, since charge carriers only have to travel over relatively short distances in order to meet and recombine with an oppositely charged charge carrier. For an application of electrets in components that have a consistent

Performance over many years even with harsh ones

Environmental conditions (for example, for applications in the

Automotive), electrets with very high charge stability are required. By choosing suitable materials and pretreatment methods electrets can be achieved which have good charge stability at room temperature and elevated Temperatures and which can potentially be used for the above applications. However, there are only a few options for choosing electrets that are suitable for use under such harsh conditions

Environmental conditions come into question.

New methods for producing electrets with very high charge stability and a further improvement in the charge stability over the prior art are therefore desirable and open up new fields of application for electrets.

DE 44 21 859 C2 shows a filter for cleaning gases with an electret and the electret at least partially surrounding layers of a porous dielectric. The porous dielectric is made of a foam, in particular of an open-cell or open-cell foam. The electret is as a thin layer or plate of a polymer, such. As polytetrafluoroethylene, formed and is polarized by thermal treatment.

DE 2 232 264 shows an electret structure, which consists of two thin dielectric films, between which an ionized air layer is introduced. This electret structure can be used as an ultrasonic transducer.

Disclosure of the invention

The invention describes a novel structure of an electret structure and a method for producing such an electret structure, in which the disadvantages of the prior art are avoided. The migration of charge carriers after the electrical charge is through the

Structure according to the invention difficult and thus ensures a high charge stability.

According to the invention, an electret structure is proposed comprising an electrically conductive support, a first electret layer and a porous dielectric layer interposed between the support and the electret layer is arranged. The electrically conductive carrier preferably forms the electrode of the electret structure according to the invention.

The invention is based on the idea of using the porous dielectric layer to separate the charged first electret layer from the back electrode.

This will make the migration of electrical charges out of the first

Electret layer to the electrode obstructed because the charges on average have to travel a longer distance to the electrode. Likewise, any migration of oppositely charged charge carriers from the electrode into the first electret layer is hindered. The charge stability of an electret structure according to the invention over conventional structures without porous,

Dielectric layer between the electret and electrode is thereby significantly increased.

Compared to known from the prior art porous electrets (so-called. Piezoelektreten), where positive and negative charges are present in each pores of the electret, the structure of the invention offers the advantage of a stronger spatial separation of positive and negative charges, since the porous layer itself is charged. This significantly reduces the risk of loss of charge due to recombination of charge carriers within the charged electret layer.

Electroacoustic transducers which use an electret structure according to the invention are preferably constructed in such a way that the electret structure itself does not oscillate, but instead builds up an electric field, so that the generation or the detection of sound waves follows the known principle of the art

electrostatic converter takes place.

According to the invention, a porous dielectric is applied to an electrically conductive carrier which forms the electrode. For example, porous polymers such as porous Teflon (eg PTFE, PFA, FEP, AF etc.), but also other materials with dielectric properties, for example porous ceramics, can be used for this purpose. The porous, dielectric layer thus formed may preferably be metallized on its side facing the carrier or the electrode. As a result, a good electrical connection is advantageously achieved and / or a

Potential barrier generated for the transfer of charge carriers. In particular, if the porous, dielectric layer comprises porous Teflon, it is advantageous to use aluminum for the metallization of the porous, dielectric layer, which has both the electrical connection of the porous,

improved dielectric layer to the carrier, as well as the passage of electrical charges from the back electrode into the Teflon material difficult.

According to the invention, an electret material is applied to the porous dielectric layer, for example by laminating a film of teflon (e.g., PTFE, PFA, FEP, AF, etc.) forming the first electret layer in which

Charge carriers are introduced by charging the film. Alternatively, the outer material (e.g., Teflon) may be dropped as a wet solution and then dried.

In order to prevent ingress of atmospheric moisture into the pores of the porous, dielectric layer, it is preferably provided that the entire electret structure is sealed off to the side. This can be realized, for example, in that the upper electret layer at the edge of the structure extends to the support and is joined to it (e.g., glued or laminated). Alternative seals are possible, e.g. by applying an adhesive, the side of the electret structure or a ring around the electret structure.

In a preferred embodiment of the invention, it is provided to introduce a further, second electret layer between the back electrode and the porous dielectric. The metallization of the porous, dielectric layer is omitted in this case. Instead, the side of the second electret layer facing the carrier or the return electrode can be metallized. This optional structure can be advantageously chosen if the metallization of the porous dielectric and / or the connection of the porous dielectric to the carrier cause problems due to the material properties. During the lifetime of the electret structure according to the invention, the charge carriers migrate slowly from the surface (or from the volume) of the upper electret layer in the direction of the electrode. In structures known from the prior art with electrets which are applied directly to electrodes, the charge carriers become the electrode after some time

reach and compensated by removal or recombination.

By contrast, in the case of the electret structure constructed according to the invention, the charge carriers from the electret, when they have reached the underside of the charged electret layer, reach the porous dielectric. At

Where electret and dielectric can touch each other

Carrier carried out and take place a further migration of charge carriers. However, due to the porosity of the porous, dielectric layer, the electret will in most cases be adjacent to a pore, that is to say an air-filled cavity. A trespassing of charge carriers from the

Electret material in the air of the pore does not take place, whereby the charges are immobilized at the border between electret and air (pore in the dielectric). The so immobilized charge carriers remain for long periods of time, for several years, in moderate environmental conditions

(Room temperature and low humidity) even several decades, stably bound in the electret structure according to the invention, whereby the

inventive structure has a very high charge stability.

The inventive method for producing such an electret structure accordingly comprises the following steps: a) Provision of an electrically conductive carrier, in particular one

metallic carrier, which is designed as an electrode,

b) applying a porous dielectric layer to the carrier,

c) applying a, in particular nonporous, electret material,

For example, a Teflon film on the porous dielectric layer, d) electrically charging the nonporous electret, whereby the first

Electret layer is formed. The sequence of method steps b) and c) is not defined in the context of the invention. It is also possible according to the invention to first apply the porous dielectric layer to the electret material (step c)), for example by lamination, and then to apply the composite of the porous dielectric layer and the electret material to the carrier (step b).

The electrical charging of the film in step d) can be carried out, for example, by a corona process, electron beams or contact charging.

Optionally, the electrically charged sandwich structure before use in a component in a further process step f) are additionally pre-aged, for example by aging at a temperature at which a high mobility of the charge carriers is given in the electret. This temperature depends on the materials chosen. Mobile charge carriers migrate rapidly under these conditions until they reach positions within the electret where they have high energy stability (so-called traps), or until they reach the electret-air interface at the transition from the electret to the porous dielectric where they immobilize be as described above. By this pre-aging, a system is achieved in which even before use in a component, the charge carriers in a very stable and immobile

Configuration available. Accordingly, a possible change in the charge characteristics of the electret structure at the early stage of use is prevented or minimized.

The electret structure according to the invention is suitable, for example, for use in a sound transducer for environmental sensor technology (eg environment detection in FIG

Driver assistance systems based on ultrasound, in robotics or the

Ultrasound surveillance). Other possible applications include electret applications known in the art, such as microphones, speakers, or air filters, particularly where such systems require very constant electrical charge of the electret over long periods of time, even under harsh environmental conditions.

Brief description of the drawings Figure 1 illustrates an electr et structure according to a first embodiment of the invention. Figure 2 illustrates an electret structure according to a second embodiment of the

Invention.

Embodiments of the Invention FIG. 1 shows schematically a section through an electret structure 1 according to a first exemplary embodiment of the invention. The electret structure 1 comprises a metallic carrier 10, which forms the electrode of the electret structure 1. An electrically charged Teflon film forms the first electret layer 30.

Between the carrier 10 and the electret layer 30 is a layer 20 of a porous, dielectric material, e.g. a ceramic, a polymer or porous Teflon, so that the sandwich structure according to the invention results. The porous, dielectric layer 20 thus comprises a solid dielectric 22 with, for example, differently sized pores 24. Between the porous, dielectric layer 20 and the carrier 10 is in this

Ausführungsbeilspiel provided a second electret layer 40, at its the

Carrier facing surface 42 is provided with a metal coating 45.

The first electret layer has charge carriers 50, in this example with negative electrical charge. Compensating charges 55 with

opposite polarity can be found in the carrier 10. By this

Charge separation of the known electret effect is achieved and the electret structure 1 can be used for example in sound transducers.

Only at the points where the first electret layer 30 and the

Dielectric 22 touch can occur a transfer of charge carriers and take place a further migration of charge carriers. However, at most points along the interface 60 between the first electret layer 30 and the porous dielectric layer 20, due to the porosity of the layer 20, the electret 30 will abut a pore 24, ie an air-filled cavity. A transgression of the charge carriers 50 into the air of the pore 24 takes place not happening. Thus, the charge carriers 50 are immobilized in the dielectric 22 at the boundary between the electret 30 and the air of the pore 24 and remain stably bound in the electret layer 30 over long periods of time. In order to prevent the ingress of atmospheric moisture, water or dirt into the pores 24 of the porous, dielectric layer 20, the electret layer 30 extends laterally as far as the carrier 10 and is glued or laminated there. Thereby, a circumferential seal 35 is realized around the electret structure 1, which improves the life under severe environmental conditions.

FIG. 2 schematically shows a section through an electret structure 1 according to a second exemplary embodiment of the invention. The operation is essentially the same as that in connection with FIG. 1

described embodiment.

In contrast to FIG. 1, in this exemplary embodiment no second electret layer is provided between the porous, dielectric layer 20 and the carrier 10. Instead, the surface 25 of the porous, dielectric layer 20 facing the carrier 10 has a metallization layer 45, which serves directly for connecting the porous, dielectric layer 20 to the carrier 10.

For sealing the porous dielectric layer 20 is in

Embodiment of Figure 2, a circumferential ring 36 of a

Adhesive material is provided which prevents the penetration of moisture and dirt into the porous dielectric layer 20.

In summary, high charge stability is achieved by the electret structure 1 according to the invention, since charge carriers are effectively immobilized at interfaces between the electret layer and pores of the porous, dielectric layer 20 and recombination with oppositely polarized charge carriers is prevented. By further intermediate layers, such as an electret layer 40 and / or a metallization layer 45, the connection of the porous, dielectric layer 20 to the carrier 10, which preferably forms the back electrode of the electret structure 1, for each selected material can be additionally optimized.

Claims

claims

1. Electret structure (1) comprising

an electrically conductive support (10)

a first electret layer (30)

a porous dielectric layer (20) disposed between the support (10) and the electret layer (30).

2. electret structure (1) according to claim 1, characterized in that the electrically conductive carrier (10) is formed to form an electrode of the electret structure (1).

3. electret structure (1) according to any one of claims 1 or 2, characterized

in that the porous, dielectric layer (20) comprises at least one of the following materials: a porous polymer, in particular porous Teflon, a porous ceramic.

4. electret structure (1) according to one of claims 1 to 3, characterized

in that the first electret layer (30) is formed as a polymer film, in particular as a Teflon film, in which electrical charge carriers (50) are incorporated.

5. electret structure (1) according to one of claims 1 to 3, characterized

in that the first electret layer (30) is formed by coating with a polymer solution in which electrical charge carriers (50) are introduced.

6. electret structure (1) according to one of claims 1 to 5, characterized

in that the electret structure (1) has a sealing structure (35) which seals the side surfaces of the porous, dielectric layer (20) against dirt and / or moisture.

7. electret structure (1) according to one of claims 1 to 6, characterized in that the porous, dielectric layer (20) on its surface facing the electrically conductive carrier (25) a

Metal coating (45), in particular an aluminum coating.

8. electret structure (1) according to one of claims 1 to 7, characterized

characterized in that the electret structure (1) has a second electret layer (40) which is arranged between the porous, dielectric layer (20) and the electrically conductive carrier (10), the second electret layer (40), in particular at its The electrically conductive carrier facing surface (42) has a metal coating (45).

9. A process for producing an electret structure (1) according to any one of claims 1 to 8, comprising the following steps:

a) providing an electrically conductive carrier (10), in particular a metallic carrier,

b) applying a porous dielectric layer (20) on the

Support layer,

c) applying an electret material, in particular a teflon film, to the porous dielectric layer (20),

d) electrically charging the film, making the film the first one

Electret layer (30) forms.

10. The method according to claim 9, characterized in that further a Voralterungsschritt f) is provided, in which the electret structure (1) after execution of step d) is stored at a predetermined temperature, for a predetermined time.

11. Sound transducer arrangement with an electret structure (1) according to one of claims 1 to 8.