Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/80—Constructional details
  • H10N30/85—Piezoelectric or electrostrictive active materials
  • H10N30/857—Macromolecular compositions
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/01—Manufacture or treatment
  • H10N30/09—Forming piezoelectric or electrostrictive materials
  • H10N30/098—Forming organic materials

Definitions

• the present invention relates to a capacitive transducer of the kind having a polymer film arranged between two electrodes in the form of electrically conductive layers arranged on opposing surfaces of the polymer film.
• the invention further relates to a method for manufacturing such a capacitive transducer.
• Capacitive transducers have previously been provided, which comprise a thin polymer film where a first electrode, in the form of a first electrically conductive layer, is arranged on a first surface of the polymer film, and a second electrode, in the form of a second electrically conductive layer, is arranged on a second, opposite, surface of the polymer film. Thereby the electrodes form a capacitor with the polymer film arranged therein. If a potential difference is applied between the electrodes, the electrodes are attracted to each other, and the polymer film is compressed in a direction perpendicular to the electrodes, and elongated in a direction parallel to the electrodes. If the transducer is designed in a careful manner, this results in a mechanical stroke from the transducer, i.e. the electrical energy supplied to the electrodes is converted into mechanical work, i.e. the transducer acts as an actuator.
• the electrodes are mechanically attracted to each other, and if a potential difference is then applied between the electrodes, then by mechanically decreasing the distance between the electrodes, i.e. by compressing the polymer film in a direction perpendicular to the electrodes, and elongating the polymer film in a direction parallel to the electrodes, it is possible to convert mechanical energy into electrical energy. If the transducer is designed in a careful manner, this results in mechanical work being converted into electrical energy, i.e. the transducer acts as an electrical energy generator. Similarly, if the electrodes are pushed towards each other or pulled away from each other, the capacitance of the capacitor is changed, due to the altered distance between the electrodes. Such a mechanism can be used for sensing purpose, where any dimensional changes in the transducer can be monitored by reading resulting capacitance changes.
• suitable manufacturing techniques including spin coating, spray coating, casting and/or roll-to-roll processes, in order to be able to mass produce the transducers in an easy and cost effective manner.
• the materials which have previously been selected for the polymer film have a relatively low viscosity, in the region of 1,000-100,000 mPa-s, prior to curing the polymer film, in order to allow easy handling of the polymer material in the processes described above.
• the performance of the transducer depends on the strength of the electrical field applied across the polymer film of the transducer in the sense that the higher the electrical field strength achieved, the larger the elongation of the transducer, in the case that the transducer acts as an actuator, or the more electrical energy is gained, in the case that the transducer acts as a generator.
• the electrical or dielectric breakdown strength is referred to as the electrical or dielectric breakdown strength. This is a material property.
• the applied voltage should be kept sufficiently low to prevent that the electrical field strength applied to the capacitive transducer exceeds the electrical breakdown strength of the polymer film. Since the applied electrical field strength determines the elongation of the transducer, the maximum possible elongation, and thereby the maximum mechanical work which can be delivered by the transducer, acting as an actuator, is limited by the electrical breakdown strength of the polymer film. Similarly, the maximum possible electrical energy gained for a transducer, acting as a generator, is limited by the electrical breakdown strength of the polymer film. Thus, in some instances it may not be possible to fully utilise the maximum potential of the transducer.
• the invention provides a capacitive transducer comprising:
• the polymer film is at least partly made from a material having a molecular weight which is at least 21,000 g/mol.
• the first electrically conductive layer forms a first electrode
• the second electrically conductive layer forms a second electrode.
• the polymer film is arranged between the electrodes, and thereby a capacitor is formed.
• the polymer film is at least partly made from a material having a molecular weight which is at least 21,000 g/mol.
• a material having a molecular weight normally have a relatively high viscosity, typically higher than 100,000 mPa-s, and they are therefore normally considered inappropriate for use in manufacturing processes including gravure, slot-die, spin coating, spray coating, casting or similar techniques.
• most solvents require ATEX certified equipment. This is disadvantageous because it complicates the manufacturing process.
• SEBS belongs to the thermoplastic elastomers being tri-block rubbers and crosslinked due to phase separation of polystyrene end groups into styrene rich domains linked by the ethylene butadiene chain. This phase separation leads to physical cross-linking of the elastomer.
• Thermoplastic elastomers are also sometimes referred to as thermoplastic rubbers and are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) which consist of materials with both thermoplastic and elastomeric properties. While most elastomers are thermosets, thermoplastics are in contrast relatively easy to use in manufacturing, for example, by injection molding. Thermoplastic elastomers show
• thermoset elastomers and thermoplastic elastomers are different from thermoset elastomers.
• crosslinking is a critical structural factor which contributes to impart high elastic properties.
• the crosslink in thermoset polymers is a covalent bond created during the vulcanization process being chemically cross-linked as opposed to the physically cross-linked elastomer, like SEBS, where the crosslink is a weaker dipole or hydrogen bond or takes place in one of the phases of the material.
• thermoplastic elastomers like SEBS however only exhibit dielectric breakdown strength in the same ranges as electrical breakdown strength of the polymer materials which is traditionally used for capacitive transducers such as polymer materials having a molecular weight of approximately 16,000 g/mol, and belongs to a group of silicone elastomers which is sometimes referred to as room temperature vulcanizing (RTV-2) elastomers having a breakdown in the range of 40-90 V/pm. They are rarely seen above 100 V/pm.
• RTV-2 room temperature vulcanizing
• the inventors of the present invention have surprisingly found that chemically crosslinked (and/or thermoset) polymer materials having a molecular weight which is at least 21,000 g/mol exhibit very high electrical breakdown strengths, such as above 100 V/pm. Therefore, when the polymer film of a capacitive transducer is made at least partly of such a material, it is possible to apply a larger electrical field to the transducer without risking electrical breakdown. Accordingly, the transducer is capable of delivering more mechanical work in a single stroke, or more electrical energy in a cycle, without risking damage to the transducer. In some applications this improved performance of the transducer is so advantageous that it is more important to obtain the improved performance than to maintain an easy manufacturing process.
• the electrically conductive layers may preferably be made from a metal or an electrically conductive alloy, e.g. from a metal selected from a group consisting of silver, gold, copper, aluminium and nickel. Alternatively other suitable metals or electrically conductive alloys may be chosen.
• the polymer film may be at least partly made from a material having a degree of
• polymerization which is at least 300, such as at least 500, such as between 300 and 1,000, such as between 500 and 1,000, or at least 1,000, such as at least 5,000, such as between 5,000 and 10,000.
• a large degree of polymerization of a polymer material implicates large molecules.
• a material having a high molecular weight may therefore also have a large degree of polymerization.
• the polymer film may be at least partly made from a material having a Si0 2 filler content of at least 10% by weight, such as at least 25% by weight, such as at least 30% by weight, such as between 10% by weight and 35% by weight, such as between 25% by weight and 35% by weight, or between 30% by weight and 45% by weight.
• the polymer film is expected to exhibit high tear strength.
• the polymer film may be at least partly made from a material having a pyrogenic silica filler content of at least 10% by weight, such as at least 25% by weight, such as at least 30% by weight, such as between 10% by weight and 35% by weight, such as between 25% by weight and 35% by weight, or between 30% by weight and 45% by weight.
• the polymer film may be at least partly made from a material comprising an additive which increases the relative permittivity or the dielectric constant of the polymer film.
• the polymer film which exhibits high electrical breakdown strength, further has a high relative permittivity or dielectric constant. This allows the mechanical work or the electrical energy provided by the transducer to be increased, thereby improving the overall performance of the transducer.
• the additive may, e.g., be an organic or inorganic filler, conductive or non conductive, micro- particles or nano-particles, surface treated or not treated, prepared by grafting or another suitable method.
• the additive may be carbon, graphite, polyaniline-based, titanium oxide, barium titanate, nanoclays, etc. It is preferred that these additives be coated with appropriate surface treatment to increase compatibility with the polymer film having high molecular weight, thus allowing for achieving very high relative permittivity while still having high electric breakdown strength.
• the amount of additive in the polymer film may be at least 0.1% by weight, such as between 0.1% by weight and 30% by weight, such as between 0.2% by weight and 10% by weight.
• the polymer film may be made from an elastomer.
• An elastomer is a polymer with the property of viscoelasticity, generally having low Young's or elastic modulus and high yield strain. Thus, an elastomer film is highly stretchable, thereby allowing elongation of the capacitive transducer in response to an applied electrical field.
• the elastomer may be a silicone elastomer, such as an addition curing silicone elastomer, a platinum catalysed silicone elastomer, or polydimethylsiloxane (PDMS), such as silicone elastomer made of linear polysiloxanes, polysiloxanes with at least vinyl terminations, polysiloxanes with lateral and/or end vinyl groups, etc.
• a silicone elastomer such as an addition curing silicone elastomer, a platinum catalysed silicone elastomer, or polydimethylsiloxane (PDMS), such as silicone elastomer made of linear polysiloxanes, polysiloxanes with at least vinyl terminations, polysiloxanes with lateral and/or end vinyl groups, etc.
• PDMS polydimethylsiloxane
• the polymer film may be at least partly made from a material having a molecular weight which is at least 35,000 g/mol, such as between 35,000 g/mol and 350,000 g/mol, such as between 35,000 g/mol and 72,000 g/mol.
• a group of silicone elastomers known as liquid silicone rubber (LSR) have molecular weights within the interval 35,000 g/mol to 72,000 g/mol, degrees of polymerization within the interval 500 to 1000, and Si0 2 filler contents within the interval 25% by weight to 35% by weight.
• LSR liquid silicone rubber
• Such elastomer materials have previously been used in medico applications, such as medical tubing and wound healing items.
• LSR elastomer materials have previously been considered inappropriate for use in manufacturing processes such as roll-to-roll coating, spin coating, spray coating or casting, due to their high viscosity, being within the interval 100,000 mPa-s to 8,000,000 mPa-s.
• the inventors of the present invention have, however, surprisingly found that the breakdown strength of LSR elastomer materials is very high, typically above 100 V/pm, and even as high as approximately 160 V/pm for some materials. This makes the LSR elastomers very suitable for use in a capacitive transducer, despite the disadvantages of their high viscosity. It is further noted that in typical manufacturer technical data sheets, stated values of electrical breakdown strength for elastomers are almost equal independent of molecular weight of the elastomer.
• the polymer film may be at least partly made from a material having a molecular weight which is at least 350,000 g/mol, such as between 350,000 g/mol and 720,000 g/mol.
• a group of solid silicone elastomers known as high temperature vulcanizing (HTV) elastomers have molecular weights within the interval 350,000 g/mol to 720,000 g/mol, degrees of polymerization within the interval 5,000 to 10,000, and Si0 2 filler contents within the interval 30% by weight to 45% by weight, or even higher.
• HV high voltage
• these materials have even higher viscosities, being within the interval 150,000,000 mPa-s to 250,000,000 mPa-s, than the LSR elastomers, and are not flowable at all, and therefore they have previously been considered even less appropriate for use in manufacturing processes such as roll-to-roll coating, spin coating, spray coating or casting, than the LSR elastomers.
• the inventors of the present invention have, however, surprisingly found that the breakdown strength of HTV elastomer materials is very high, typically above 100 V/pm, and even as high as approximately 160 V/pm for some materials. This makes the HTV elastomers very suitable for use in a capacitive transducer, despite the disadvantages of their very high viscosity.
• the first surface of the polymer film and/or the second surface of the polymer film may comprise a surface pattern of raised and depressed surface portions, and the first electrically conductive layer and/or the second electrically conductive layer may be deposited onto the surface pattern of the first and/or second surface, the first electrically conductive layer and/or second electrically conductive layer thereby having a corrugated shape.
• the electrically conductive layers with a corrugated shape allows elongation of the capacitive transducer, without having to stretch the electrode(s) formed by the electrically conductive layer(s). Instead, the corrugated shape of the electrically conductive layer(s) is simply substantially evened out while the polymer film stretches. Thus, the electrode(s) is/are compliant. Thereby the mechanical work obtained in a single stroke or electrical energy gained per cycle of the capacitive transducer is significantly increased as compared to transducers having flat electrodes.
• polymer films made at least partly from a material having a molecular weight which is at least 21,000 g/mol in capacitive transducers with corrugated electrodes.
• such transducers are capable of delivering very large mechanical work in a single stroke or high electrical energy gain per cycle due to the compliant electrode(s).
• the polymer film is made from a material having a high electrical breakdown strength, in order to avoid electrical breakdown when the transducer is operated in such a manner that its full potential is used.
• the inventors of the present invention have surprisingly found that polymer materials having a high molecular weight exhibit a high electrical breakdown strength, and therefore they are particularly
• the raised and depressed surface portions of the first and/or second surface may have a shape and/or size which vary periodically along at least one direction of the respective surface.
• the corrugated electrically conductive layer is compliant along a first direction, but relatively stiff along a second, substantially
• the variations of the raised and depressed surface portions may be relatively macroscopic and easily detected by the naked eye of a human being, and they may be the result of a deliberate act by the manufacturer.
• the periodic variations may include marks or imprints caused by one or more joints formed on a roller used for manufacturing the film.
• the periodic variations may occur on a substantially microscopic scale.
• the periodic variations may be of the order of magnitude of manufacturing tolerances of the tool, such as a roller, used during manufacture of the film. Even if it is intended and attempted to provide a perfect roller, having a perfect pattern, there will in practice always be small variations in the pattern defined by the roller due to manufacturing tolerances. Regardless of how small such variations are, they will cause periodical variations to occur on a film being produced by repeatedly using the roller.
• the film may have two kinds of periodic variations, a first being the imprinted surface pattern of structures such as corrugations being shaped perpendicular to the film, this could be called a sub- pattern of variations, and further due to the repeated imprinting of the same roller or a negative plate for imprinting, a super-pattern may arise of repeated sub-patterns.
• the surface pattern may comprise waves forming troughs and crests extending in essentially one common direction, each wave defining a height being a shortest distance between a crest and neighbouring troughs.
• the crests and troughs resemble standing waves with essentially parallel wave fronts.
• the waves are not necessarily sinusoidal, but could have any suitable shape as long as crests and troughs are defined.
• a crest (or a trough) will define substantially linear contour- lines, i.e. lines along a portion of the corrugation with equal height relative to the polymer film in general.
• This at least substantially linear line will be at least substantially parallel to similar contour lines formed by other crest and troughs, and the directions of the at least substantially linear lines define the common direction.
• the common direction defined in this manner has the consequence that anisotropy occurs, and an electrically conductive layer arranged on the corrugated surface is compliant in a direction perpendicular to the common direction.
• An average height of the waves may be between 1/3 and 20 pm, such as between 1 pm and 15 pm, such as between 2 pm and 10 pm, such as between 4 pm and 8 pm.
• the first electrically conductive layer and/or the second electrically conductive layer may have a thickness in the range of 0.01-0.2 pm, such as in the range of 0.01-0.1 pm, such as in the range of 0.1-0.2 pm, such as in the range of 0.02 pm to 0.09 pm, such as in the range of 0.05 pm to 0.07 pm.
• the electrically conductive layer is preferably applied to the film in a very thin layer. This facilitates good performance and facilitates that the electrically conductive layer can follow the corrugated pattern of the surface of the film upon deflection.
• the polymer film may be a structure comprising at least two layers of polymer material, wherein at least a first of the layers of polymer material is made from a material having a molecular weight which is at least 21,000 g/mol, and at least a second of the layers of polymer material is made from a material having a molecular weight which is between 7,000 g/mol and 21,000 g/mol.
• the first of the layers of polymer material has a high molecular weight, and thereby a high electrical breakdown strength and a high viscosity.
• the second of the layers of polymer material has a low molecular weight, corresponding to the polymer materials which are normally used for capacitive transducers, and thereby a lower electrical breakdown strength and a lower viscosity.
• the first layer ensures that the polymer film has a required high electrical breakdown strength, while the second layer is easier to handle during manufacture, due to the lower viscosity.
• the second layer may advantageously form a part of the polymer film which requires careful handling during manufacture, such as a part having an electrically conductive layer arranged thereon, and/or a part being provided with a surface pattern, such as a corrugated surface pattern.
• the first layer may form a part of the polymer film which does not require such careful handling during manufacture.
• the polymer film may be a structure comprising at least three layers of polymer material, wherein at least a first of the layers of polymer material is made from a material having a molecular weight which is at least 21,000 g/mol, at least a second of the layers of polymer material is made from a material having a molecular weight which is between 7,000 g/mol and 21,000 g/mol, and at least a third of the layers of polymer material is made from a material having a molecular weight which is between 7,000 g/mol and 21,000 g/mol, and wherein the first layer is arranged between the second layer and the third layer.
• a layer of polymer material which has a high molecular weight, and thereby a high electrical breakdown strength and a high viscosity is arranged between two layer of polymer material which have a low molecular weight, and thereby a lower electrical breakdown strength and a low viscosity.
• the first surface of the polymer film and the second surface of the polymer film may both be made from the low viscosity polymer material, and they are therefore easy to handle during manufacture.
• any surface patterns, such as corrugations, as well as deposition of electrically conductive layers, including any required surface treatment can be performed in a polymer material which is well known for the purpose and suitable for the appropriate manufacturing processes.
• the second layer arranged between the first layer and the third layer forms a barrier layer which provides the desired high electrical breakdown strength to the structure.
• the second of the layers of polymer material may preferably be one of the kinds of material described above, such as an LS elastomer or a HTV elastomer.
• at least the second of the layers of polymer material may be made from a material having a degree of polymerization which is at least 300, such as at least 500, such as between 500 and 1,000.
• the invention provides a method for manufacturing a capacitive transducer, the method comprising the steps of:
• step of providing a polymer film comprises providing a polymer film which is at least partly made from a material having a molecular weight which is at least 21,000 g/mol.
• the transducer which is manufactured by means of the method according to the second aspect of the invention may preferably be a transducer according to the first aspect of the invention. Therefore the remarks set forth above are equally applicable here.
• the polymer film is at least partly made from a material having a molecular weight which is at least 21,000 g/mol, the polymer film exhibits a high electrical breakdown strength, and thereby it is possible to increase the obtained performance of the transducer, as described above.
• the step of providing a polymer film may comprise adding a solvent to a polymer material.
• polymer materials having high molecular weights often have a high viscosity. Therefore, in order to handle the polymer film, e.g.
• the step of providing a polymer film may comprise the steps of: providing a polymer material,
• the polymer film is shaped after the curing agent has been added to the polymer material, but before the polymer mixture cures. This allows handling of the polymer material. Once the polymer film has cured, the shape of the polymer film is maintained.
• the curing agent may be of a kind which causes the polymer film to cure when a specific time interval has lapsed after mixing the polymer material and the curing agent. In this case the curing step simply takes place after the specific time interval has lapsed.
• the curing agent may be of a kind which requires activation, such as heat activation, before curing of the polymer film takes place.
• the step of allowing the polymer film to cure comprises the step of activating the curing agent, e.g. heating the polymer film to a relevant curing temperature in the case that the curing agent requires heat activation.
• the method may further comprise the steps of: - adding a solvent to the polymer material, and
• solvent is added to the polymer material as well as to the curing agent.
• the mixture of polymer material and solvent is then mixed with the mixture of curing agent and solvent, thereby obtaining the polymer mixture. Since adding solvent decreases the viscosity of the polymer material and of the curing agent, respectively, it becomes easier to properly mix the polymer material and the curing agent. Thereby it is easier to obtain a uniform polymer mixture.
• solvent may only be added to the polymer material or only to the curing agent. This may be relevant if only one of these materials has a high viscosity.
• the polymer material and the curing agent may be mixed first, and solvent may be added to the polymer mixture before the polymer film is formed.
• the polymer film may advantageously be made from an elastomer, such as a silicone elastomer.
• a silicone elastomer typically comprises a silicone base, a filler, a catalyst, e.g. platinum, tin or another suitable catalyst, and a cross-linker.
• the silicone elastomer may comprise other ingredients, such as pigments, stabilizers, silicone oil, etc.
• Some silicone elastomers are of a two-component type, i.e. a component A is mixed with a component B, and the mixture is allowed to cure when the elastomer film has been formed.
• component A may be a silicone base
• component B may be a curing agent.
• the filler may be added to component A, to component B, or to both.
• the cross-linker may be added to component A, while the catalyst is added to component B.
• the cross-linker may be added to component B, while the catalyst is added to component A.
• the cross-linker and the catalyst must not both be added to component A or to component B, since this may lead to partial curing of said component.
• component A may be or act as a silicone base
• component B may be or act as a curing agent, or vice versa.
• the step of providing a polymer film may comprise providing the first surface of the polymer film and/or the second surface of the polymer film with a surface pattern of raised and depressed surface portions. As described above, a corrugated surface is thereby obtained, onto which the layer of electrically conductive material is deposited. Thereby a compliant electrode is obtained, which allows a long elongation of the transducer in response to an electrical or mechanical excitation of the transducer.
• the surface pattern may, e.g., be provided by means of a shape defining element, e.g. in the form of a roller or a mould, which is used for imprinting the surface pattern onto a surface of the polymer film.
• the step of providing a polymer film may comprise the steps of: providing a first layer of polymer material,
• the resulting polymer film is a multi-layered structure.
• the multi-layered structure is gradually built up by applying a new layer onto an already cured layer, and then allowing the newly applied layer to cure.
• the step of providing a polymer film may further comprise the steps of:
• an additional layer of polymer material is added to the multi- layered structure.
• the procedure could be repeated as many times as required in order to obtain a desired multi-layered structure.
• a fourth, fifth, sixth, etc., layer may be added to the structure in a sequential manner, allowing each layer to cure before the next layer is applied.
• the material of the first layer of polymer material may differ from the material of the second layer of polymer material.
• the material of one layer may have a low molecular weight and thereby a low electrical breakdown strength, but a low viscosity, thereby allowing easy handling of this layer during manufacture.
• the other layer may have a high molecular weight and thereby a high electrical breakdown strength, but a high viscosity, thereby making handling of this layer during manufacture more difficult.
• a polymer film in the form of a multi-layered structure may be obtained by shaping and curing the individual layers, and subsequently attaching the layers to each other, e.g. by laminating the layers together.
• the first and second layers of electrically conductive material may be deposited onto the relevant surfaces before the layers are attached to each other.
• two substantially identical layers of polymer material may be formed from any suitable polymer material exhibiting desired viscosity properties, but without considering the properties regarding electrical breakdown strength of the polymer material.
• a layer of electrically conductive material is deposited onto a surface of each of the two polymer layers.
• the surfaces onto which the electrically conductive material is deposited may be provided with a surface pattern prior to depositing the layers of electrically conductive material, as described above.
• An additional layer of polymer material may then be formed from a polymer material which exhibits desired properties with respect to electrical breakdown strength, i.e. a polymer material having a high molecular weight, but without considering the viscosity properties of the polymer material.
• the additional layer of polymer material is not provided with surface patterns, and no layer of electrically conductive material is deposited on a surface of the additional layer.
• the three layers are attached to each other in such a manner that the additional layer is arranged between the two layers carrying the layers of electrically conductive material, and in such a manner that the layers of electrically conductive material form outer surfaces of the resulting structure.
• Such a laminated structure is sometimes referred to as a back-to-back structure.
• the first and second layers of electrically conductive material may be deposited onto first and second surfaces of a multi-layered structure after the layers have been attached to each other.
• the layers may be laminated together in such a manner that one or more of the electrically conductive layers are arranged inside the laminated structure.
• two layers, each being provided with an electrically conductive layer may be laminated together in such a manner that the electrically conductive layer of one layer faces a surface of the other layer which is not provided with an electrically conductive layer.
• Such a laminated structure is sometimes referred to as a front-to-back structure.
• Figs, la and lb show a capacitive transducer according to an embodiment of the invention being exposed to zero electrical potential difference and being exposed to a high electrical potential difference, respectively,
• Fig. 2 illustrates a mixing process forming part of a method according to an embodiment of the invention
• Fig. 3 illustrates an alternative mixing process forming part of a method according to an alternative embodiment of the invention
• Figs. 4-6 are flow diagrams illustrating method steps of methods according to various embodiments of the invention
• Figs. 7a and 7b are graphs illustrating performance of a prior art capacitive transducer
• Figs. 8a and 8b are graphs illustrating performance of a capacitive transducer according to an embodiment of the invention.
• Fig. 9 is a cross sectional view of a capacitive transducer according to an embodiment of the invention.
• Fig. 10 is a cross sectional view of a capacitive transducer according to an alternative embodiment of the invention.
• Figs, la and lb show a capacitive transducer 1 according to an embodiment of the invention.
• the capacitive transducer 1 comprises a polymer film 2 having a first surface and a second surface.
• the surfaces are both provided with a surface pattern of raised and depressed surface portions.
• the raised and depressed surface portions form waves of crests and troughs, extending in one common direction.
• An electrically conductive layer 3 is deposited onto each of the surfaces of the polymer film 2, the electrically conductive material being deposited so that the electrically conductive layers 3 are formed according to the pattern of raised and depressed surface portions. Thereby a designed corrugated profile of the electrically conductive layers 3 is obtained.
• the electrically conductive layers 3 form electrodes of the transducer 1, and they are electrically connected to a power source.
• the electrically conductive layers 3 are exposed to zero electric potential difference
• Fig. lb the electrically conductive layers 3 are exposed to a high electric potential difference.
• the electrically conductive layers 3 are attracted to each other. Thereby the thickness of the polymer film 2 arranged between the electrically conductive layers 3 is reduced, and the polymer film 2 is stretched, i.e. elongated, along a direction which is perpendicular to the common direction of the waves defined by the surface pattern.
• Fig. 2 illustrates a mixing process forming part of a method according to an embodiment of the invention.
• a component A in the form of a polymer material, is mixed with a solvent to form a mixture, Mix A. Thereby the viscosity of the polymer material is decreased.
• a component B in the form of a curing agent, is mixed with a solvent to form a mixture, Mix B. Thereby the viscosity of the curing agent is also decreased. Finally, the two mixtures, Mix A and Mix B, are mixed to form a final mixture, Final Mix. The decreased viscosity of the polymer material and of the curing agent makes it easier to obtain a uniform final mixture.
• the final mixture may subsequently be used for forming a polymer film in a manner which will be described in further detail below. Since the two mixtures, Mix A and Mix B, both contain solvent, the final mixture, Final Mix, also contains solvent. Thereby the viscosity of the final mixture is lower than would be the case if solvent had not been added, and easy handling of the final mixture is ensured.
• Fig. 3 illustrates a mixing process forming part of a method according to an alternative embodiment of the invention.
• a component A in the form of a polymer material, is mixed with a component B, in the form of a curing agent, to form a mixture, Mix A+B.
• the mixture, Mix A+B is mixed with a solvent to form a final mixture, Final Mix.
• the viscosity of the final mixture is reduced, due to the added solvent, thereby ensuring easy handling of the final mixture.
• the final mixture may subsequently be used for forming a polymer film in a manner which will be described in further detail below.
• Fig. 4 is a flow diagram illustrating method steps of a method according to an embodiment of the invention.
• a polymer mixture is provided.
• the polymer mixture may, e.g., be provided in a manner described above with reference to Fig. 2 or Fig 3.
• the polymer mixture is coated onto a shape defining element in the form of a mould or carrier web, using a suitable coating technique, such as gravure coating, slot coating, spin coating, or other similar techniques.
• the carrier web may advantageously be provided with a surface pattern, which is imprinted onto the polymer mixture as the polymer mixture is coated onto the carrier web.
• the polymer mixture is allowed to cure, and as a result, an elastomer film, coated onto the carrier web, is obtained.
• the elastomer film is then de-laminated from the carrier web.
• the carrier web is returned, and polymer mixture is once again coated onto the carrier web.
• the carrier web may be or form part of a roller. This process results in a long web of elastomer film.
• the elastomer film is also provided with a surface pattern, which is an imprint of the surface pattern of the carrier web.
• the de- laminated elastomer film is rolled up, and a roll of elastomer film is thereby obtained.
• the elastomer film is supplied to a vacuum deposition chamber, where a metal layer is deposited onto a surface of the elastomer film. Thereby an electrode is formed on the surface of the elastomer film.
• the surface is provided with a surface pattern as described above, the deposited metal layer follows the surface pattern, and the resulting electrode is compliant.
• Fig. 5 is a flow diagram illustrating method steps of a method according to an embodiment of the invention.
• Rolls of elastomer film are provided.
• One or more of the rolls of elastomer film may be provided with a metal electrode deposited onto a surface of the elastomer film.
• the elastomer film may be manufactured in the manner described above with reference to Fig. 4.
• one or more of the elastomer films may be of a kind which is not provided with a metal electrode.
• Two or more layers of elastomer film are laminated together, using a suitable lamination technique, thereby obtaining rolls of laminated film.
• the lamination process may be performed in such a manner that surfaces being provided with a metal electrode form outer surfaces of the laminate.
• the resulting laminated structure forms a capacitive transducer comprising metal electrodes arranged with a multi-layered elastomer film there between.
• Such a structure is sometimes referred to as a back-to-back laminate.
• the lamination process may be performed in such a manner that one or more metal electrodes is/are arranged inside the laminated structure.
• This may, e.g., be obtained by arranging a metal electrode of one layer in such a manner that it faces a surface of another layer which is not provided with a metal electrode.
• Such a structure is sometimes referred to as a front-to-back laminate.
• the layers of elastomer film may be made from the same polymer material. However, the layers may alternatively be made from different polymer materials exhibiting different properties. For instance, some layers may be made from a polymer material with a low viscosity in order to allow easy handling of these layers, while other layers may be made from a polymer material with a high molecular weight, and thereby a high electrical breakdown strength, in order to ensure that the laminated film has a high electrical breakdown strength.
• the laminated film may comprise two layers being provided with a metal electrode arranged on a surface being provided with a surface pattern. These two layers may advantageously be made from a polymer material with low viscosity, thereby allowing easy handling, in particular with respect to providing the surface pattern.
• the laminated film may comprise an additional layer arranged between the two layers described above.
• the additional layer may be made from a polymer material with a high molecular weight, and thereby a high electrical breakdown strength.
• the metal layers form outer surfaces of the laminated structure. Thereby a capacitive transducer with a high electrical breakdown strength is provided.
• Fig. 6 is a flow diagram illustrating method steps of a method according to an alternative embodiment of the invention. Similarly to the process described above with reference to Fig. 4, a polymer mixture is coated onto a shape defining element in the form of a mould or carrier web and allowed to cure, thereby obtaining an elastomer film attached to a carrier web.
• polymer mixture is coated onto the elastomer film attached to the carrier web.
• the polymer mixture may be the same kind of polymer mixture which was used for forming the first elastomer film.
• another kind of polymer mixture may be used, e.g. a polymer mixture with properties which differ from the properties of the first polymer mixture.
• an additional polymer layer is added to the elastomer film.
• the additional layer is allowed to cure, and thereby an elastomer film comprising two layers is formed.
• the process may be repeated until a multi-layered structure comprising a desired number of layers has been obtained.
• Such multi-layer structures can then be de-laminated from the carrier web, thereby obtaining rolls of multi-layer elastomer film.
• Figs. 7a and 7b illustrate electrical breakdown strength of a polymer material which has previously been used for capacitive transducers.
• Fig. 7a shows number of breakdown events as a function of applied electrical field strength
• Fig. 7b shows cumulative probability of failure, due to electrical breakdown, as a function of applied electrical field strength.
• the tested polymer material has a molecular weight of approximately 16,000 g/mol, and belongs to a group of silicone elastomers which is sometimes referred to as room temperature vulcanizing (RTV-2) elastomers. These elastomers have relatively low viscosities, and are therefore easy to handle during manufacture. For this reason it has previously been preferred to use these elastomers for capacitive transducers where a thin layer of elastomer is arranged between electrically conductive layers.
• RTV-2 room temperature vulcanizing
• Figs. 8a and 8b show graphs which are similar to the graphs of Figs. 7a and 7b.
• the material being tested has a molecular weight of approximately 35,000 g/mol, i.e. significantly higher than the material which was tested in Figs. 7a and 7b.
• the polymer material which was tested in Figs. 8a and 8b belongs to a group of silicone elastomers which is sometimes referred to as liquid silicone rubbers (LSR). These materials have previously been used for medical purposes, such as for medical tubing or wound healing applications.
• LSR liquid silicone rubbers
• a typical electrical breakdown strength for elastomer films made from the LSR elastomer is significantly higher than a typical electrical breakdown strength for elastomer films made from the RTV-2 elastomer, even though technical data sheets from manufacturers state similar electrical breakdown strength to that of RTV-2 elastomers. More particularly, it can be seen that electrical breakdown typically occurs for the RTV-2 elastomer film, illustrated in Figs, 7a and 7b, at approximately 65 V/pm, while electrical breakdown typically occurs for the LSR elastomer film, illustrated in Figs. 8a and 8b, at approximately 135 V/pm.
• Fig. 9 is a cross sectional view of a capacitive transducer 1 according to an embodiment of the invention.
• the capacitive transducer 1 of Fig. 9 comprises an elastomer film 2 comprising a first surface and a second surface, each surface being provided with a surface pattern of raised and depressed surface portions.
• An electrically conductive layer 3 is deposited onto each of the surfaces of the elastomer film 2.
• the elastomer film 2 is a multi-layer structure comprising three layers.
• a first layer 2a is arranged in a centre part of the elastomer film 2, while a second layer 2b and a third layer 2c are arranged adjacent to the first layer 2a, and at opposing sides of the first layer 2a.
• the second layer 2b and the third layer 2c comprise the surfaces being provided with the surface pattern and having the electrically conductive layers 3 deposited thereon.
• the first layer 2a is made from a polymer material which has a molecular weight which is at least 21,000 g/mol.
• the material may, e.g., be a liquid silicone rubber (LSR).
• LSR liquid silicone rubber
• the second layer 2b and the third layer 2c are both made from a material which differs from the material of the first layer 2a.
• the second layer 2b and the third layer 2c may be made from the same material, or they may be made from different materials, in which case the three layers 2a, 2b, 2c of the elastomer film 2 is made from three different materials.
• the material(s) of the second layer 2b and the third layer 2c may advantageously be selected to exhibit low viscosity prior to curing, thereby making it easy to handle the second layer 2b and the third layer 2c during manufacture of the capacitive transducer 1.
• the first layer 2a of the elastomer film 2 acts as a barrier layer, providing the desired high electrical breakdown strength to the elastomer film 2.
• the second layer 2b and the third layer 2c which form the part of the elastomer film 2 which is difficult to manufacture, are easy to handle, due to the low viscosity of the material used for these layers 2b, 2c.
• the capacitive transducer 1 of Fig. 9 is a back- to-back multi-layer structure.
• the layers 2a, 2b, 2c of the elastomer film 2 may be manufactured separately and subsequently laminated together.
• the elastomer film 2 may be manufactured by sequentially adding layers to the structure and allowing each layer to cure before the next layer is added.
• Fig. 10 is a cross sectional view of a capacitive transducer 1 according to an alternative embodiment of the invention. Similarly to the embodiment shown in Fig. 9, the capacitive transducer 1 of Fig.
• a 10 comprises an elastomer film 2 arranged between two electrically conductive layers 3.
• a first layer 2a is made from a polymer material having a molecular weight of at least 21,000 g/mol, thereby exhibiting high electrical breakdown strength.
• a second layer 2b is made from a different material, e.g. having low viscosity prior to curing. Similarly to the situation described above with reference to Fig. 9, the first layer 2a forms a barrier layer, providing the desired high electrical breakdown strength to the elastomer film 2.
• the capacitive transducer 1 of Fig. 10 is a front-to-back multi-layer structure. Therefore one of the electrically conductive layers 3 is arranged between the first layer 2a and a third layer 2c of elastomer material.
• the third layer 2c may, e.g., be made from the same material as the second layer 2b, similarly to the situation described above with reference to Fig. 9.

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