DE102016204031A1 - Method for producing an electret arrangement - Google Patents

Abstract

The manufacturing process comprises the following process steps: a.) In the first process step, an amorphous fluoropolymer is dissolved in a solvent, whereby a fluoropolymer solution is formed. b.) This is followed by printing the fluoropolymer solution on a support plate. c.) Subsequently, a solvent contained in the printed fluoropolymer solution is removed. This creates a printed, amorphous Fluropolymer. d.) In the fourth process step, the electrostatic charging of the printed, amorphous fluoropolymer occurs, whereby an electret structure is produced. e.) In the last process step, a thermal aftertreatment of the electret arrangement is performed. In this case, the electret arrangement comprising the carrier plate and the electret structure printed on the carrier plate is heated for a defined time to a temperature above a later operating temperature of the electret arrangement.

Description

State of the art

The invention relates to a method for producing an electret arrangement, the Elekretanordnung as a product of the manufacturing process, and a capacitive transducer.

The document DE 10 2012 202 422 A1 describes a sound transducer arrangement that can be used in particular for sound-based environment detection. The arrangement comprises an electret material which forms a capacitance between a first and a second electrode structure. On the first electrode structure, the electret material is applied. On or above the electret material, the second electrode structure is arranged. The electrical material can be applied by a printing process.

The document US 2007/0230722 describes an electret condenser microphone in which an electret film of a perfluoropolymer is used. For example, the perfluoropolymer can be dissolved in a fluorine-based solvent and then coated with a thin film coating method. The spin-on is followed by drying to remove the solvent from the film.

The known electret arrangements, however, have in common that the polarization stability of the electret used is often not high enough to maintain its electrical charge even at high operating temperatures over a longer period of time. However, this is absolutely necessary in order to guarantee the failure-free use of electret arrangements in the environment sensor system of, for example, vehicles.

Disclosure of the invention

• a.) Im ersten Verfahrensschritt wird ein amorphes Fluorpolymer in einem Lösemittel aufgelöst, wodurch eine Fluorpolymer-Lösung entsteht.
• b.) Darauf folgend wird die Fluorpolymer-Lösung auf eine Trägerplatte aufgedruckt. Insbesondere ist dieser Druckschritt durch eine frei wählbare und durch die vewendete Druckfrom definierte Strukturierung gekennzeichnet.
• c.) Anschließend wird ein in der aufgedruckten Fluorpolymer-Lösung enthaltenes Lösemittel entfernt. Dadurch entsteht eine beliebig strukturierte, amorphe Fluropolymerbeschichtung.
• d.) Im vierten Verfahrensschritt kommt es zu der elektrostatischen Aufladung des aufgedruckten, amorphen Fluorpolymers, wodurch eine Elektretstruktur erzeugt wird.
• e.) Im letzten Verfahrensschritt wird eine thermische Nachbehandlung der Elektretanordnung durchgeführt. Hierbei wird die Elektretanordnung, welche die Trägerplatte und das auf die Trägerplatte aufgedruckte Elektret umfasst, über eine definierte Zeit auf eine Temperatur oberhalb einer späteren Einsatztemperatur der Elektretanordnung erwärmt.

In order to counteract this problem, a production method for an electret arrangement as well as the electret arrangement resulting directly from this production process are proposed according to the invention. The invention additionally comprises a capacitive sound transducer in which the electret arrangement according to the invention is used. The production method according to the invention comprises the following method steps:

• a.) In the first step, an amorphous fluoropolymer is dissolved in a solvent to form a fluoropolymer solution.
• b.) Subsequently, the fluoropolymer solution is printed on a carrier plate. In particular, this printing step is characterized by a freely selectable structuring defined by the used printfrom.
• c.) Subsequently, a solvent contained in the printed fluoropolymer solution is removed. This creates an arbitrarily structured, amorphous Fluropolymerbeschichtung.
• d.) In the fourth process step, the electrostatic charging of the printed, amorphous fluoropolymer occurs, whereby an electret structure is produced.
• e.) In the last method step, a thermal aftertreatment of the electret arrangement is carried out. In this case, the electret arrangement comprising the carrier plate and the electret printed on the carrier plate is heated for a defined time to a temperature above a later operating temperature of the electret arrangement.

The invention is based on the finding that amorphous fluoropolymers, which are used in this Hersterstellungsverfahren as Elekretmaterial, due to a strong entanglement of the polymer chains with each other so-called traps or referred to as traps possess. Since these trapping sites extend over the entire volume of the polymer layer, they are also referred to as volume traps. Charge carriers placed in these volume traps are much more firmly anchored than surface charges. Amorphous fluoropolymers, such as poly [4,5-difluoro-2,2-bis (trifluoromethyl) -1,3-dioxole-co-tetrefluoroethylene], are therefore used as the electret material in the process described, since they are in contrast to the Usually semi-crystalline fluoropolymers such as PTFE, FEP or PFA in a suitable solvent, such as perfluorotrialkylamine or perfluortributlyamine dissolve and thus liquefy. In liquid form, the polymers can be used as Elekretmaterial in a simple manner for coating surfaces by means of printing technology. The use of printing technology in turn offers the advantage that this results in a maximum design freedom for electret structures with a layer thickness of 10-25 μm. While commercial film-based electrets must first obtain their geometry by punching or cutting technique, a plurality of Elekretschichten can be prepared via a polymer solution in printing technology by appropriate arrangement in the plane in one process step. During the drying process, ie during the removal of the solvent contained in the printed fluoropolymer solution, the degree of crystallinity of the fluoropolymers should be kept as low as possible. For the reason that as many volume rapps arise within the polymer layer, if the amorphous portion within the polymer and thus the largest possible amount of irregularities within the polymer is retained to a high degree. The drying temperature and drying time are crucial here because over Temperature and time the crystal growth and thus the crystallinity can be influenced. During the subsequent polarization or electrostatic charging of the printed, amorphous fluoropolymer, not all charge carriers are trapped in volume traps, but also position themselves on the surface. These surface charges are much worse anchored in the electret and therefore have no great stability. A thermal aftertreatment of the electret structure after the electrostatic charge causes on the one hand the partial removal of surface charges and on the other hand the diffusion of near-surface charge carriers into deeper volume rape. As a result, the charge carriers are better anchored in the electret, which leads to a higher polarization stability of the electret structure. The rate at which surface charges are removed is exponentially dependent on the thermal activation energy supplied. The removal of less stable charge carriers can thus be adjusted via the parameters temperature and exposure time of the thermal aftertreatment. It is important that the temperature level is above the later operating temperature of the Elekretanordnung to ensure the stability of the remaining polarization in the application.

It is preferred to print the fluoropolymer solution by a screen printing process. This makes it possible to achieve comparable layer thicknesses, as found in commercial electret films. These correspond to about 10-15 μm layer thickness. The screen printing method allows the simple production of multiple arrangements of acoustic transducers, so-called transducer arrays, since in a manufacturing step, a plurality of electret layers in almost any number and size can be produced. With regard to the required layer thickness, the screen printing method is the most advantageous method, but other printing methods, such as gravure printing or inkjet printing, are also possible for printing the fluoropolymer solution.

In a preferred embodiment of the invention, the electrostatic charging of the printed, amorphous fluoropolymer is carried out by a corona treatment. As a result, the polymer is polarized and an electret structure is produced, whereby, for example, a permanent capacitor voltage for a capacitive sound transducer can be supplied.

The later maximum operating temperature, above which the electret arrangement is heated during the thermal after-treatment, preferably corresponds to a temperature range between 90 and 100 ° C. This corresponds approximately to the maximum operating temperature of a capacitor electrode of a capacitive sound transducer for environment sensors. This ensures that the polarization of the electret is also retained when using the electret structure in a capacitive sound transducer, as used for example in the environmental sensor system of a vehicle.

If the electret arrangement produced by the method according to the invention is intended to be used in a sound transducer, it is possible according to a preferred embodiment of the invention to print at least two separate regions of the support plate with the fluoropolymer solution when printing the fluoropolymer solution onto the support plate. These two areas can have many different shapes, and they can also differ in shape from one another. For example, both areas may be rectangular in shape, but in another embodiment may also be a shape rectangular and the other circular shaped. These at least two areas have a horizontal distance of less than half the airborne sound wavelength to the nearest neighbors. At a working frequency of 48 kHz, this would be a distance of 3.6 mm. The operating frequency refers to the frequency of the received and processed by the sound transducers sound signals. This arrangement, which is also referred to as array arrangement, offers later in the application of the electret structure, for example in transducers compared to individual transducers advanced possibilities of signal processing.

The removal of the solvent contained in the printed fluoropolymer solution according to step b) of the method according to the invention is preferably carried out by means of a heat treatment of the carrier plate and the fluoropolymer solution printed on the carrier plate. The heat treatment solidifies the polymer and produces an amorphous polymer structure. In addition, the warming leads to the formation of so-called beta transformations. Beta transformations can result in twisting of the polymer chains or movement of side chains. This creates additional volume traps for later polarization. Beta-conversion means on the one hand the movements of side groups of a polymer chain, on the other hand the movement of several atoms along the polymer chain. A simple rotation along the polymer chain can not take place in a polymer, since in this case a large chain section would have to be moved as a whole. This is not possible because of obstruction by neighboring molecules. However, several atoms can be moved together along the chain. If a deformation of the polymer chain takes place by movement of several atoms along the chain, a so-called crankshaft relaxation occurs, because thereby the polymer chain is deformed in the form of a crankshaft. Beta conversions always find below the glass transition temperature. (Conversions at glass transition temperature are called alpha conversion)

The heat treatment used to solidify the polymer preferably begins with the heating of the backing plate and the fluoropolymer solution printed on the backing plate at a constant first temperature of between 50 and 100 ° C for a period of between 5 and 30 minutes. As a result, a large part of the solvent can evaporate, but the polymer concentration within the solution remains as long as possible below the crystallization limit. Upon reaching this critical crystallization limit, the solution is supersaturated and crystal growth can begin. However, the crystal growth of the polymer should be kept as low as possible, since as many volume rape as possible are formed within the polymer layer, if the amorphous content within the polymer is high, leaving as much as possible irregularities within the polymer. So if finally the crystallization limit is reached by the evaporation of the solvent, the crystal growth must be stopped here as quickly as possible. Therefore, in a second step of the heat treatment, the temperature is then increased linearly over a period of between 1 and 5 minutes to a constant second temperature between 100 and 150 ° C. By increasing the temperature, a higher polymer content in the solution can again be achieved without the solution becoming supersaturated and thus crystal growth beginning. The polymer concentration within the solution is thus still kept below the crystallization limit. The carrier plate and the fluoropolymer solution printed on the carrier plate are then heated at the second temperature for a period of between 5 and 15 minutes. This allows another part of the solvent to evaporate and solidify the polymer. In order to complete the drying process as quickly as possible upon reaching the crystallization limit, thereby keeping the crystal formation as low as possible, the second temperature is then linearly increased over a period of between 1 and 5 minutes to a maximum temperature between 150 and 200 ° C. At this maximum temperature, the support plate and the fluoropolymer solution printed on the support plate are heated consistently for a period between 5 and 15 minutes, until finally the residual solvent is completely evaporated and the polymer is fully solidified.

The adhesion of the printed, amorphous fluoropolymer can preferably be increased by chemical side groups, in particular carboxyl groups (COOH) on the support plate. The side groups are part of the molecule and arise in the polymer synthesis. This offers the advantage that the subsequent electret structure can no longer be easily removed by, for example, mechanical actions.

According to a further aspect of the invention, an electret arrangement is provided as a direct product of the method according to the invention, which comprises a carrier plate and at least one electret structure printed on the carrier plate. The printed electret structure has significantly higher polarization stability when used at elevated temperature than is required in industrial and automotive applications as the allowable operating temperature (e.g., up to + 85 ° C) and compared to commercially available film-based electret layers. For example, fluorine-ethylene-propylene (FEP), one of the best known semicrystalline sheet-based electret materials in long-term deposition at 95 ° C, discharges by 12% over 300 days (measured by surface potential). By contrast, the printed electret structure produced according to the invention only discharges by 4% in the same period. The electret structure thus has more than twice the polarization stability for these parameters.

According to a further aspect of the invention, a capacitive sound transducer is provided which comprises a spacer and a vibratable membrane as the first capacitor electrode. The spacer may be annular, but depending on the electret structure, it may also have any shape adapted to the electret structure. In addition, the capacitive sound transducer comprises the electret structure according to the invention with the electret structure and an additional conductive layer as the second capacitor electrode between the electret structure and the support plate of the electret arrangement. The electret structure supplies the voltage of the capacitive transducer. The membrane of the capacitive transducer is disposed at a first end of the spacer open to the environment and the electret assembly is disposed at a second end of the spacer open towards the environment. In this case, the membrane is arranged at the first end of the spacer such that it terminates the electret structure as part of the electret arrangement from the surroundings. Between the two capacitor electrodes, a dielectric for generating an electric field can now be introduced. In this case, the electret arrangement can be part of the dielectric, since there is between the two capacitor electrodes. This capacitive transducer has the advantage that it is simple in construction and also requires no external power supply.

Furthermore, the carrier plate of the capacitive transducer preferably carries at least two electret structures printed on the carrier plate. These at least two electret structures are through at least one membrane sealed from the environment. With a plurality of sound transducers it is possible to capture the environment more accurately.

Brief description of the drawings

1 shows a method sequence according to a first embodiment of the invention for producing an electret arrangement.

2 FIG. 12 is a graph showing the thermal step of the heat treatment for removing the solvent contained in the printed fluoropolymer solution. FIG.

3 shows a first embodiment of a capacitive transducer according to the invention.

4 shows a first embodiment of a capacitive transducer in a multi-array according to the invention.

embodiments

1 shows an embodiment of a method according to the invention for producing an electret arrangement as a flow chart. It is in a process step 5 an amorphous fluoropolymer such as poly [4,5-difluoro-2,2-bis (trifluoromethyl) -1,3-dioxole-co-tetrefluoroethylene] dissolved in a solvent. As a suitable solvent, for example perfluorotrialkylamine or perfluorotributylamine can be used here. Subsequently, in a process step 10 the fluoropolymer solution on a carrier plate 150 printed. This printing process can be done for example by means of the screen printing process. Optionally, at least two areas of the carrier plate 150 be printed with the fluoropolymer solution. The regions may have any shape adapted to the application intended for the electret assembly, eg rectangular, square, round, oval or L-shaped.

By removing the solvent contained in the printed fluoropolymer solution, next in a process step 20 produces a printed, amorphous fluoropolymer.

To an electret structure 120 The printed, amorphous fluoropolymer is subsequently produced in one process step 50 polarized. The polarization can be done for example by means of the corona treatment.

In a final process step 60 Finally, there is a thermal aftertreatment of the electret assembly, wherein the electret assembly is heated over a defined time to a temperature above a later maximum operating temperature of the electret. This later maximum operating temperature of the electret arrangement can correspond, for example, to a temperature range between 90 and 100 ° C.

Optionally, the removal of the solvent contained in the printed fluoropolymer solution in step 20 by means of a heat treatment of the carrier plate 150 and on the carrier plate 150 printed fluoropolymer solution. This optional heat treatment is through the process steps 25 . 30 . 35 . 45 and 50 on the 1 shown. This is in process step 25 the carrier plate 150 and those on the backing plate 150 printed fluoropolymer solution at a constant first temperature between 50 and 100 ° C for a period of between 5 and 15 minutes heated. Subsequently, in process step 30 the first temperature over a period of between 1 and 5 minutes to a second temperature between 100 and 150 ° C are increased. Subsequently, in one process step 35 the carrier plate 150 and those on the backing plate 150 printed fluoropolymer solution at the constant second temperature for a period of between 5 and 15 minutes heated. Subsequently, in process step 40 the second temperature is heated to a maximum temperature between 150 and 200 ° C over a period of between 1 and 5 minutes. Last is in process step 45 the carrier plate 150 and those on the backing plate 150 printed fluoropolymer solution at the same maximum temperature for a period of between 5 and 15 minutes heated.

2 shows an example of a temperature profile 80 or the thermal steps of the heat treatment for removing the solvent contained in the printed fluoropolymer solution. It is on the X-axis 100 the time in minutes and on the y-axis 90 the oven temperature is plotted in ° C. First, the temperature is maintained at 50 ° C for 5 minutes and then increased linearly over a period of 2 minutes at 100 ° C. Once there, the temperature is kept constant at 100 ° C for 5 minutes, before being then again linearly increased over a period of 2 minutes to 150 ° C. This temperature is then kept constant for 5 minutes.

3 shows in a side view in section an embodiment of a capacitive transducer 135 according to the invention. The capacitive transducer 135 includes a spacer 140 , a vibratory membrane 140 , an electret structure 120 , a support plate 150 and a conductive layer 145 between electret structure 120 and support plate 150 , Here, the oscillatory, electrically conductive membrane is used 110 as the first capacitor electrode and is at a first end of the spacer open towards the environment 140 arranged. The conductive layer 145 between carrier plate 150 and electret structure 120 serves as a second capacitor electrode. The electret assembly, which is the carrier plate 150 and the on the support plate 150 includes printed, electrostatically charged electret structure, provides the capacitive transducer 135 with tension and is at the second end of the spacer 150 arranged. The electret structure 120 is thus completed by the environment. The spacer 150 may be annular as a coherent piece or composed of individual pieces. Between the membrane 110 as the first capacitor electrode and the conductive layer 145 as the second capacitor electrode is a dielectric 130 arranged. It insulates the first and second capacitor electrodes from each other. As a dielectric 130 can serve any medium whose charge carriers are generally not movable (for example, air). Also, the electret structure 120 serve as a dielectric, since it is located between the two capacitor electrodes.

4 shows in a side view in section an embodiment of a capacitive transducer 136 , In this case, the electret arrangement consists of a carrier plate 151 that have several electret structures 120 wearing. Between the carrier plate 151 and the multiple electret structures 120 is in each case a conductive layer 145 arranged as a second capacitor electrode. The electret assembly is at the second end of the plurality of spacers open toward the environment 140 arranged. The individual electret structures 120 are each made up of a spacer 140 surrounded by a ring. The common oscillatory membrane 111 as each first capacitor electrode is at the other end of the four spacers 140 arranged so that the electret structures 120 be completed from the environment. It would also be conceivable here that the electret structures 120 of individual membranes 111 be completed from the environment. Also could be more than two electret structures 120 annular of the at least one spacer 140 be surrounded.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102012202422 A1 [0002]
• US 2007/0230722 [0003]

Claims (11)

Method for producing an electroretin arrangement, comprising a carrier plate ( 150 . 151 ) and at least one on the carrier plate ( 150 . 151 ) printed electret structure ( 120 ), the method comprising the steps of: a. Dissolving an amorphous fluoropolymer in a solvent to produce a fluoropolymer solution, b. Printing the fluoropolymer solution onto a carrier plate ( 150 . 151 c. Removing the solvent contained in the printed fluoropolymer solution, d. Electrostatic charging of a printed, amorphous fluoropolymer to produce an electret structure ( 120 ), characterized in that in step e. the method, a thermal aftertreatment of the electret is performed, wherein the electret is heated over a defined time to a temperature above a later maximum operating temperature of the electret. A method according to claim 1, characterized in that the printing of the fluoropolymer solution in step b.) Takes place by a screen printing process. Method according to one of claims 1 or 2, characterized in that the electrostatic charge of the printed, amorphous fluoropolymer for producing an electret structure ( 120 ) in step d.) by a corona treatment. Method according to one of claims 1 to 3, characterized in that the subsequent maximum operating temperature of the electret arrangement corresponds to a temperature in a temperature range between 90 and 100 ° C. Method according to one of claims 1 to 4, characterized in that in step b.) Of the method, at least two regions of the carrier plate ( 150 . 151 ) are printed with the fluoropolymer solution, said at least two areas in the horizontal direction have a distance of less than half the airborne sound wavelength as a function of the operating frequency of the transducer from each other Method according to one of claims 1 to 5, characterized in that the removal of the solvent contained in the printed fluoropolymer solution in step c.) By means of a heat treatment of the support plate ( 150 . 151 ) and on the carrier plate ( 150 . 151 ) printed fluoropolymer solution. A method according to claim 6, characterized in that the heat treatment comprises the following thermally sequential steps: - heating the carrier plate ( 150 . 151 ) and on the carrier plate ( 150 . 151 ) printed fluoropolymer solution at a constant first temperature between 50 and 100 ° C for a period between 5 and 15 minutes, - Linear increase of the first temperature over a period of between 1 and 5 minutes to a second temperature between 100 and 150 ° C, - heating the carrier plate ( 150 . 151 ) and on the carrier plate ( 150 . 151 ) printed fluoropolymer solution at the constant second temperature for a period of between 5 and 15 minutes, - linear increase of the second temperature over a period of between 1 and 5 minutes to a maximum temperature between 150 ° C and 200 ° C and - heating of the support plate ( 150 . 151 ) and on the carrier plate ( 150 . 151 ) printed fluoropolymer solution at the constant maximum temperature for a period of between 5 and 15 minutes. Method according to one of claims 1 to 7, wherein the adhesion of the printed, amorphous fluoropolymer to the support plate ( 150 . 151 ) is increased by side chemical groups, in particular carboxyl groups. Electret arrangement comprising a carrier plate ( 150 . 151 ) and at least one on the carrier plate ( 150 . 151 ) printed electret structure ( 120 ), produced by a method according to one of claims 1 to 8, characterized in that the polarization stability of the electret structure ( 120 ) is at least twice as high as a foil-based electret measured at a temperature higher than 80 ° C over a period of at least 300 days. Capacitive transducer ( 135 . 136 ) comprising: - at least one spacer ( 140 ), - a vibratory membrane ( 110 . 111 ) as the first capacitor electrode, wherein the membrane ( 110 . 111 ) at a first end of the environment of the at least one spacer ( 140 ), - the Elekretanordnung according to claim 9, wherein a conductive layer ( 145 ) between the carrier plate ( 150 . 151 ) and the Elekretstruktur ( 120 ) the Elektetanordnung serves as a second capacitor electrode, - a dielectric ( 130 ), which is arranged between first and second capacitor electrode, wherein the electret arrangement at the second end of the spacer open towards the environment ( 140 ) is arranged so that the dielectric ( 130 ) the first and second capacitor electrodes are electrically isolated from each other, the membrane ( 110 . 111 ) so at the first end of the spacer ( 140 ) is arranged that they the electret structure ( 120 ) completes the environment. Capacitive transducer ( 135 . 136 ) according to claim 10, characterized in that the carrier plate ( 150 . 151 ) at least two on the carrier plate ( 150 . 151 ) printed electret structures ( 120 ), wherein the at least two electret structures ( 120 ) by at least one membrane ( 110 . 111 ) be completed by the environment.

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