ELECTRICITY

Abstract

An electret (1) has an electret layer (2). The electret layer is formed by subjecting a composite film, in which inorganic dielectric particles (21) are dispersed and held in a base film (22), to a polarization treatment. The inorganic dielectric particles consist mainly of an inorganic dielectric material with a band gap energy of 4 eV or more.

Description

[0001] The present disclosure relates to an electret.

[0002] As an energy harvesting technology that converts energy present in the environment into electrical energy, a practical application of a vibration energy generating element or the like, using an electret, has been investigated. An organic polymer material, such as fluorinated resin, is generally used as a component of the electret. For example, a chain-like fluorinated resin or a polymer with a fluoroaliphatic ring structure in a main chain is used.

[0003] While the organic polymer material offers advantages such as shape freedom and excellent controllability of layer or film thickness during thin-film formation, concerns exist regarding the thermal stability of a surface potential and potential performance degradation over time in high-temperature environments, due to its organic nature. In contrast, JP 5 563 746 B2 proposes an electret containing fine metal oxide particles with a relative permittivity of 2.0 to 4.0 × 10⁻⁶. 3 to be mixed to an organic polymer with a polar functional group at the end of a main or side chain with a volume fraction of 0.02 volume percent or more and less than 10 volume percent to obtain improved thermal stability.

[0004] On the other hand, the formation of an electret using an inorganic composite material with excellent stability at high temperatures has been investigated. For example, JP 6 465 377 B2 proposes an electret material using a sintered body with a crystal structure of hexagonal hydroxyapatite and a hydroxide ion content lower than that of stoichiometric hydroxyapatite. This sintered body is obtained by sintering and dehydrating a molded body of hydroxyapatite powder at a temperature above 1250 °C and below 1500 °C, assuming that a high surface potential is generated after polarization treatment due to hydroxide ion defects.

[0005] By incorporating an energy generation element or the like, using an inorganic electret, into an integrated circuit or the like, formed on a substrate, an energy generation device can be miniaturized and used in a high-temperature environment, and is expected to be suitable for various applications. However, since the electret material in JP 6 465 377 B2 is a sintered bulk material made from a raw powder, it is difficult to deposit it onto the substrate of a device. Alternatively, it is possible to form a thin film using a film-forming device, but in this case, film formation not only takes some time, but the range of controllable film thickness is also limited.Since the electret material in JP 5 563 746 B2 consists mainly of an organic polymer, the controllability of the film thickness is improved, but the thermal stability is insufficient, and there is concern that the performance will deteriorate over time when used in a high-temperature environment.

[0006] EP 1 144 075 B1 relates to polymeric electret materials and in particular polymeric electret filter materials, wherein the electret material comprises: a porous material with an electrostatic charge comprising a composite material, wherein the composite material comprises: (i) a polymer matrix with a first thermoplastic polymer being a nonpolar polymer and a second thermoplastic polymer, wherein the second thermoplastic polymer is a telomer with polar functional end groups, and (ii) 0.01 to 50 wt.% of a ferroelectric material dispersed therein.

[0007] From DE 698 05 865 T2, a device and a method for producing platinum layers with controlled preferred orientation using nitrogen are also known.

[0008] The purpose of the present disclosure is to provide an electret that is thermally stable, has easily controllable film thickness, and is suitable for use in a high-temperature environment.

[0009] The problem is solved by the subject matter of the main claim. Advantageous further developments are specified in the dependent claims.

[0010] According to the invention, the composite film is used for the electret layer, and the inorganic dielectric particles, which acquire electret properties through polarization treatment, are dispersed in the base film. This allows for easy control of the film thickness of the electret layer formed over the substrate and improves the degree of freedom in shaping. Furthermore, since the inorganic dielectric particles consist of inorganic dielectric material with a high bandgap energy of 4 eV or more, the dielectric breakdown voltage can be increased, and a high surface potential can be achieved by applying a high voltage during the polarization treatment. Therefore, it is possible to obtain an electret that is thermally stable and less likely to experience a decrease in surface potential due to high-temperature environments or long-term use.

[0011] As described above, it is possible to provide electrets that are thermally stable, have easily controllable film thickness, and are suitable for use in a high-temperature environment.

[0012] Further tasks and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. The drawings show: Fig. 1 a schematic view to illustrate an electret layer as a schematic configuration example and a basic structure of an electret according to a first embodiment; Fig. 2 a schematic view to illustrate another schematic configuration example of an electret according to the first embodiment; Fig. 3 a schematic view to illustrate a further schematic configuration example of an electret and to illustrate a further example of a base film forming an electret layer; and Fig. 4 A schematic view to illustrate another schematic configuration example of an electret and to illustrate another example of inorganic dielectric particles forming an electret layer. (First embodiment)

[0013] Below is an electret according to a first embodiment with reference to the Fig. 1 to 3 described. As in Fig. As shown in Figure 1, the electret 1 of the present embodiment has an electret layer 2 as its base structure. The electret layer 2 is formed by subjecting a composite film, in which inorganic dielectric particles 21 are dispersed and held in a base film 22, to a polarization treatment, wherein the inorganic dielectric particles 21 are particles consisting mainly of an inorganic dielectric material with a band gap energy of 4 eV or more.

[0014] As in Fig. As shown in Figure 2, the electret 1 of the present embodiment can have a configuration comprising a substrate 10 and an electret layer 2 formed over a surface of the substrate 10. As shown in Fig. As shown in Figure 1 as the basic structure, the electret layer 2 is formed by subjecting a composite film in which inorganic dielectric particles 21 are dispersed and held in a base film 22 to a polarization treatment, wherein the inorganic dielectric particles 21 are particles consisting mainly of an inorganic dielectric material with a band gap energy of 4 eV or more.

[0015] Here, the term "particles consisting mainly of an inorganic dielectric material" means that the particles can consist solely of the inorganic dielectric material, the particles can contain impurities attributable to the raw material of the inorganic dielectric material, or some other components can be added to the particles during a process for producing the inorganic dielectric material. Furthermore, the term "particles consisting mainly of an inorganic dielectric material" indicates that the particles may consist only of the inorganic dielectric material. Fig. 1 Electret 1 shown (i.e., the electret layer 2) is, for example, in a state in which the electret layer 2 in the electret 1 of Fig. 2 was detached from substrate 10.

[0016] The electret 1 is a charged substance that generates an electrostatic field in its surroundings, and the surface potential is expressed by the electret layer 2, in which the composite film containing the inorganic dielectric particles 21 is polarized. The inorganic dielectric particles 21 are held in the base film 22 and are uniformly dispersed, so that the electret layer 2, exhibiting stable properties, can be formed.

[0017] Since the electret layer 2 uses the inorganic dielectric material with a relatively large bandgap energy of 4 eV or more for the inorganic dielectric particles 21, and the breakdown voltage becomes large, the electret layer 2 can generate a desired high surface potential by being subjected to a high voltage during the polarization treatment. Furthermore, since the electret layer 2 consists of the composite film in which the inorganic dielectric particles 21 are dispersed in the base film 22 suitable for film formation, film formation and control of the film thickness according to the shape of the substrate 10 are simple, and the electret 1 can exhibit excellent shape flexibility and thermal stability.

[0018] The electret 1 is used, for example, as an energy generation element built into an integrated circuit in various devices that mutually convert mechanical energy and electrical energy, e.g., in a small electrostatic vibration energy generation device that uses environmental vibrations as an energy source.

[0019] The electret 1 has an arbitrary external shape corresponding to the shape of the substrate 10 (such as a rectangular flat plate shape or a disc shape), and the electret layer 2 is laminated and formed over a surface of the substrate 10. Here, the vertical direction in the drawings is defined as the thickness direction X of the substrate 10, the surface over which the electret layer 2 is laminated is referred to as the top surface, and the surface on the opposite side is referred to as the bottom surface.

[0020] The material of which the inorganic dielectric particles 21 are composed is not specifically limited, as long as it is an inorganic dielectric material with a band gap energy of 4 eV or more. Preferably, such an inorganic dielectric material is a mixed or composite oxide with a perovskite structure of the ABO3 type, containing two different metal elements A and B. In the present embodiment, the electret layer 2, which uses a mixed oxide with a perovskite structure, is mainly described below.

[0021] The mixed oxide with the perovskite structure is a mixed oxide with a perovskite-like crystal structure, described by the molecular formula ABO3, and typically exhibits a cubic unit cell. The metal element A is located in the center of a cubic crystal, the metal element B is located at each vertex of the cubic crystal, and an oxygen atom O is coordinated with respect to each of the metal elements A and B in a regular octahedron. In the perovskite structure, the mixed oxide often has a non-stoichiometric composition due to the lack of oxygen atoms. In this case, the mixed oxide can be described by the molecular formula ABO3. x(x < 3) and lattice defects occur because the amount of oxygen is less than the stoichiometric ratio. Preferably, a configuration is used in which the amount of oxygen is less than the stoichiometric ratio, which contributes to improving the surface potential.

[0022] In the electret layer 2, the inorganic dielectric particles 21 used as the inorganic dielectric material consist of mixed oxide particles with a perovskite structure, which is described by the molecular formula ABO3 as a basic composition. The combination of the metal elements A and B in the molecular formula ABO3 is not specifically restricted, but preferably a rare-earth aluminate (RAO3) is used in which a trivalent rare-earth element R (metal element A) and a trivalent Al (metal element B) are combined.

[0023] As a specific example, in the ABO3-type perovskite structure, the A-site (metal element A) can be occupied by a rare-earth element R from the group La, Y, Pr, Sm, and Nd, and the B-site (metal element B) can be occupied by Al. The rare-earth aluminate in which these components are combined exhibits a large band gap energy of 4 eV or more and a relatively low relative permittivity (e.g., 100 or less), thus enabling a high surface potential.

[0024] Furthermore, rare earth aluminate can be produced using a relatively inexpensive material, which is advantageous in terms of manufacturing costs.

[0025] The mixed oxide forming the inorganic dielectric particles 21 can have a composition in which at least one of the A-site (metal element A) and the B-site (metal element B) in the perovskite structure is replaced by a dopant that differs from metal elements A and B. If a metal element with a lower valence than metal elements A and B is used as the dopant, oxygen defects are likely to be introduced into the crystal structure.

[0026] If the A-site is, for example, the trivalent rare-earth element R described above, the dopant element can be a divalent alkaline earth metal (including Mg) occupying part of the A-site. Examples of such divalent alkaline earth metals are Ca and Sr.

[0027] Alternatively, if the B site is the trivalent aluminum described above, the dopant element can be one or more chosen from the divalent alkaline earth metals (including magnesium) and zinc, occupying part of the B site. Naturally, part of both the A and B sites can be replaced by the dopant elements.

[0028] Oxygen defects can be easily introduced by using a composition in which some of the metal elements A and B, which form the mixed oxide, are replaced in this way. The presence of defects in the inorganic dielectric material is considered important for obtaining a high surface potential in the electret layer 2. By using the material with the perovskite structure for the inorganic dielectric particles 21, the number of defects can be easily controlled by element substitution.

[0029] The substitution ratio of the dopant element replacing metal A can be suitably adjusted in the range of, for example, 0.5 atomic percent to 20 atomic percent. Similarly, the substitution ratio of the dopant element replacing metal B can be suitably adjusted in the range of, for example, 0.5 atomic percent to 20 atomic percent, and a desired high surface potential can be achieved according to the substitution ratio. In this way, defects can be created by introducing the dopant element, and by controlling the substitution ratios of metals A and B, the number of defects can be controlled, and stable surface potential properties can be achieved.

[0030] The electret layer 2 consists of a composite film formed on the substrate 10 using a material obtained by preparing a mixed oxide into particles to form inorganic dielectric particles 21, and mixing the inorganic dielectric particles 21 with a base material to obtain the base film 22. The base material can be an organic or inorganic material that can be uniformly mixed with the inorganic dielectric particles 21 and formed into a film of a predetermined thickness, exhibiting excellent heat resistance and dielectric strength.

[0031] Preferably, the base film 22 can have heat resistance and dielectric strength required according to the polarization treatment conditions described below. It is desirable that the base film 22 be made of a material that is stable at the temperature during the polarization treatment, has a melting point or thermal decomposition temperature (e.g., 200 °C or higher) that is higher than the temperature during the polarization treatment, and has an electric field strength at dielectric breakdown (e.g., 4 kV / mm or more; hereinafter referred to as dielectric breakdown field strength) that is higher than the electric field strength during the polarization treatment.

[0032] Such a base material can be, for example, an organic material such as polyimide, silicone resin, polyamide-imide, allyl resin, and fluorocarbon resin, or an inorganic material such as sodium silicate. Furthermore, any liquid material capable of forming a film can be used, such as glassy SOG (spin-on glass), which contains silicon and silicon resin.

[0033] The material of the substrate 10 is not specifically limited. In the present embodiment, for example, a conductive Si substrate is used. Conductive substrates made of conductive oxides and metals, such as aluminum, iron, and copper, as well as insulating substrates made of quartz glass, soda glass, sapphire, PP (polypropylene), PET (polyethylene terephthalate), and polycarbonate can also be used as the substrate 10.

[0034] The composite film for forming the electret layer 2 can be formed over the substrate 10 by any method, such as printing, dispensing, casting, or spin coating. For example, in the case of the printing method, a liquid material containing the inorganic dielectric particles 21 and the base material is applied to the substrate 10 by printing to achieve a desired film thickness, and is then dried and heat-cured to form a composite film of the desired thickness.

[0035] By forming the electret layer 2 in a film structure, in which the inorganic dielectric particles 21 are dispersed in the base material film 22 in this manner, it is easier to form the electret layer 2, which produces a high surface potential on the substrate 10, without using a film-forming device or the like, which requires high temperature and high vacuum conditions. The electret layer 2 can have any desired film thickness, and a composite film (e.g., 10 µm or more) that is thicker than the film thickness formed with a normal film-forming device can be easily formed.

[0036] In the composite film forming the electret layer 2, the volume ratio of the inorganic dielectric particles 21 can be adjusted to achieve a desired surface potential and film thickness. Preferably, the volume ratio of the inorganic dielectric particles 21 is set in the range of 30 volume percent or more and 75 volume percent or less. Increasing the volume ratio of the inorganic dielectric particles 21 results in a higher surface potential and also improves the strength of the composite film. Furthermore, using spherical particles as the inorganic dielectric particles 21 makes it easier to control the film thickness.In this case, since a densely packed structure is formed at 75 volume percent, a greater surface potential can be achieved by setting this as the upper limit, and the strength of the composite film can be ensured without reducing the absorption capacity of the inorganic dielectric particles 21.

[0037] The electret 1 is obtained by subjecting the composite film to polarization treatment in a state where the composite film is formed above the upper surface of the substrate 10. Therefore, it is desirable to have a conductive layer 3 on the lower surface of the substrate 10, opposite the electret layer 2 in the thickness direction X. The conductive layer 3 consists of a conductive film formed using a conductive metal such as Ti, Au, and Pt, and may have a structure in which several conductive films are laminated. By using the conductive layer 3 as an electrode during the polarization treatment, the composite film can be polarized to form the electret layer 2.

[0038] At the in Fig. In the electret 1 shown in Figure 2, the conductive layer 3 is formed with a two-layer structure on the lower surface, or underside, of the conductive silicon substrate 10. A first conductive layer 31 in contact with the lower surface of the substrate 10 consists of a metal such as titanium with good adhesion, and a second conductive layer 32 in contact with a lower surface of the first conductive layer 31 is a noble metal with good conductivity (such as platinum, gold, and the like). A silicon oxide film 11 is located on the upper surface of the substrate 10, and the electret layer 2 is arranged in contact with an upper surface of the silicon oxide film 11.

[0039] The polarization treatment method is not specifically limited and is carried out, for example, using a corona discharge or the like to apply a voltage between the conductive layer 3, serving as the ground electrode, and a counter electrode. Regarding the polarization treatment conditions, it is desirable to apply a voltage at a temperature of 100 °C or higher, resulting in an electric field strength of 1 kV / mm or higher, preferably 4 kV / mm or higher. To achieve efficient energy generation, for example, as a device for vibration energy generation, a surface potential of 400 V or more is required. For example, in the case of an electret layer 2 with a film thickness of 100 µm, a desired surface potential can be achieved by a polarization treatment with an electric field strength of 4 kV / mm or more.

[0040] Since the surface potential of the electret layer 2 after polarization treatment is proportional to a voltage applied to the composite film formed on the substrate 10, it is necessary to apply a voltage capable of achieving the surface potential required for the application. Alternatively, the film thickness can be increased according to the required voltage so that no dielectric breakdown occurs.

[0041] Since the electret layer 2 formed over the substrate 10 is mainly composed of the inorganic material, the electret 1 produced in this way has a higher durability in use in a high temperature environment and a higher surface potential compared to an electret made of an organic material, while suppressing performance degradation over time. (Example 1)

[0042] The Electret 1 with the in Fig. The configuration shown in point 2 was manufactured using the following procedure. <Lösungsvorbereitung>

[0043] To form the composite film that would constitute electret layer 2, lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and calcium carbonate (CaCO₃) powders were first mixed as raw materials for the inorganic dielectric material until the molar ratio of La, Al, and Ca was 99:100:1, and thoroughly mixed to obtain a mixture. The mixture was then heated and sintered for 2 hours at 1600 °C to produce a sintered powder of mixed oxide based on lanthanum-aluminate La₂O₃. 0,9 Approx 0,01 AlO x (hereinafter referred to as LAO-based sintered powder). The resulting sintered powder was sufficiently pulverized to achieve an average particle size of approximately 50 µm in order to obtain the inorganic dielectric particles 21.

[0044] Next, an organic mixture of N-methyl-2-pyrrolidone and polyamic acid was prepared as the raw material for the base material that would form the base film 22, such that the polyamic acid concentration was approximately 6%. The sintered LAO-based powder, which would form the inorganic dielectric particles 21, was added to the organic mixture at a concentration of 30% by mass and mixed to produce a powder mixture for forming the composite film. <filmbildung>

[0045] The upper surface of the conductive silicon substrate 10 was thermally oxidized to form the silicon oxide film 11 with a film thickness of 50 nm. Subsequently, the conductive layer 3 was formed on the lower surface of the substrate 10 by a sputtering process. The conductive layer 3 was formed by forming a 30 nm titanium film, which is the first conductive layer 31, and a 200 nm gold film, which is the second conductive layer 32, in that order from the side in contact with the substrate 10, using appropriate layer-forming metal materials as targets.

[0046] The powder mixture, prepared as described above, was then applied to the upper surface of the Si oxide film 11 on the substrate 10 using a pressure application process to achieve a thickness of approximately 160 µm. If the sintered LAO-based powder settled to the bottom after application, an upper organic mixture without the sintered LAO-based powder was removed, if necessary. The volume ratio of the sintered LAO-based powder after the formation of the electret layer 2 was 70% by volume.

[0047] Subsequently, the substrate applied with the powder mixture solution was dried for 10 to 15 minutes at 110 °C in an air atmosphere. The polyamic acid was then converted to polyimide by a 1-hour heat treatment at 280 °C. As a result, an organic-inorganic composite film is formed in which the inorganic dielectric particles 21, consisting of sintered LAO-based powder with a perovskite structure, are surrounded and held by the polyimide base material (thermal decomposition temperature ≥ 500 °C) and are uniformly dispersed within the base material film 22. The temperature rise rate during the heat treatment was 3 °C / min. <polarisationsbehandlung>

[0048] The organic-inorganic composite film, intended to form the electret layer 2, was formed over the substrate 10 as described above and subjected to a polarization treatment to obtain the electret 1. For the polarization treatment, a corona discharge was used. The conductive layer 3, in contact with the lower surface of the substrate 10, was grounded as the ground electrode. The corona discharge electrode was positioned on the upper surface of the electret layer 2, and a negative voltage was applied to induce the corona discharge. The conditions for the corona discharge were as follows: the voltage was applied, and the corona discharge continued even when the temperature was reduced. Discharge voltage: -6 kV Temperature: 200 °C Processing time: 1 hour

[0049] Consequently, the organic-inorganic composite film formed on substrate 10 was polarized and negatively charged on its upper surface, thus yielding the electret layer 2, which exhibits electret properties. A high surface potential can be achieved according to the polarization treatment conditions, and by performing the polarization treatment at a temperature (e.g., 200 °C) higher than room temperature, fluctuations in the surface potential can be suppressed even in applications where the ambient operating temperature is high. This makes it possible to achieve stable electret properties.The temperature and other conditions of the polarization treatment can be appropriately modified depending on the melting point of the material from which the organic-inorganic composite film is made, the properties required in the assumed application environment, and the like.

[0050] In electret 1 of Example 1, lanthanum aluminate (LaAlO3), which is a typical composition of the inorganic dielectric material based on LAO from which the inorganic dielectric particles 21 are composed, has a band gap energy of 5.6 eV, and the surface potential of a polycrystal with a thickness of 1 mm was 4000 V (electric field strength during polarization of 1 kV / mm or more). (La 0,99 Approx 0,01 AlO x ), in which some of the Al is replaced by Ca, a dopant, also exhibited almost the same band gap energy, and the band gap energy of (La 1-y Approx y AlO 3-δ ), in which the substitution ratio y of Ca was varied in the range of 0.5 atomic% to 20 atomic%, was also almost the same. The surface potentials of (La 1-y Approx y AlO 3-δ ), which were formed into polycrystals with a thickness of 1 mm, were 1000 V to 3500 V (electric field strength during polarization of 1 kV / mm or more).

[0051] For other rare-earth aluminates, the band gap energies in typical compositions are shown below. Even when some of the metallic elements of these rare-earth aluminates are replaced, they exhibit almost the same band gap energy. A surface potential of YAlO3, when formed into a polycrystal with a thickness of 1 mm, was 1000 V (electric field strength during polarization of 1 kV / mm or more). YAIO3: 7.9 eV PrAIO3:4.7 eV SmAIO3:4.6 eV NdAIO3:4.4 eV

[0052] In contrast, BaTiO3 (band gap energy: 3.5 eV) with a perovskite structure exhibited a surface potential of 4 V (electric field strength during polarization of 1 kV / mm or more) when formed into a polycrystal with a thickness of 1 mm. Comparing these surface potentials confirmed that the electret property of the electret 1, which contains the inorganic dielectric particles 21 in the electret layer 2, can be improved by using a mixed oxide with a band gap energy of 4 eV or more, such as rare-earth aluminate.

[0053] Here, in Example 1, the sintered LAO-based powder from which the inorganic dielectric particles 21 are composed was pulverized to have an average particle size of about 50 µm, but the average particle size can be larger or smaller than 50 µm. The average particle size can be appropriately determined within a range where uniform dispersion is possible, depending on the thickness of the composite film and the like. Although polyimide was used as the organic material for the base film 22 in Example 1, the present disclosure is not limited to this, and other organic or inorganic materials can be used for the base film 22.

[0054] For example, in electret layer 2, as in Fig. Figure 3 shows that the base film 22 for dispersing and retaining the inorganic dielectric particles 21 consists of sodium silicate, which is an inorganic material. In this case, a powder mixture solution can be used to form a composite film by adding a sintered powder made of La 0,99 Approx 0,01 AlO x They are prepared by mixing an inorganic solution with sodium silicate as the base material in a predetermined ratio. Other configurations and manufacturing processes can be the same.

[0055] Since, as described above, the electret layer 2 is formed from the inorganic composite film using sodium silicate, an inorganic material, the electret 1 can exhibit higher thermal stability.

[0056] Although conductive silicon was used for substrate 10, the present disclosure is not limited to this. Substrate 10 can be a conductive substrate using a conductive oxide or a metal such as aluminum, iron, and copper, or an insulating substrate using quartz glass, soda glass, sapphire, PP (polypropylene), PET (polyethylene terephthalate), polycarbonate, or the like. However, it is necessary to apply an electric field to the composite film, which is to form the electret layer 2, at the time of polarization, and the conductive layer 3 is provided on one surface of the composite film in the thickness direction X to ground one surface.In the present embodiment, the conductive layer 3 is provided on the lower surface of the substrate 10 and an electric field is applied indirectly, but the conductive layer 3 can be provided in such a way that it is in direct contact with a surface of the composite film and is grounded.

[0057] In this way, the electret 1, as in Fig. Figure 2 shows a configuration in which the electret layer 2 is arranged above the substrate 10. Furthermore, the electret 1 can be, as in the configuration of Fig. Figure 1 shows a state in which the electret layer 2 is detached from the substrate 10. In this case, after the composite film, which is to form the electret layer 2, has been formed over the substrate 10, it can be detached from the substrate 10 and subjected to a polarization treatment, e.g., by placing the composite film between a pair of metal plates, which are to be electrodes, and directly applying a voltage from a DC power supply.

[0058] Alternatively, the polarization treatment can be carried out in a state where the composite film, which is to form the electret layer 2, has formed over the substrate 10, after which the electret layer 2 can be detached from the substrate 10 to obtain the electret 1. The electret 1 is obtained by detaching the electret layer 2 from the substrate 10 in this way and can be used by placing the electret 1 at a desired position. (Second embodiment)

[0059] Below is an electret according to a second embodiment with reference to Fig. 4 described, the second embodiment mainly serving to illustrate the present invention embodied in the first embodiment. As described in Fig. As shown in Figure 4, the basic structure of the electret 1 in the present embodiment is the same as in the first embodiment and comprises a substrate 10 and an electret layer 2 formed over a surface of the substrate 10. In the present embodiment, an inorganic dielectric material forming inorganic dielectric particles 21 in the electret layer 2 differs from that in the first embodiment. The differences are discussed in detail below. Incidentally, in the second and subsequent embodiments, the same reference numerals used as in the embodiment already described represent the same components as in the embodiment already described, unless otherwise specified.

[0060] In the present embodiment, the electret layer 2 also has a structure in which the inorganic dielectric particles 21, which consist mainly of an inorganic dielectric material with a band gap energy of 4 eV or more, are dispersed and held in a base film 22. In the first embodiment, the inorganic dielectric material is the mixed oxide with the perovskite structure of the ABO3 type, but in the present embodiment, an inorganic compound with an apatite structure containing phosphate ions and hydroxide ions is used.

[0061] Apatite is a general term for compounds that have a molecular formula M 10 (ZO4)6(X)2, which can be described, typically have a crystal structure in which the unit cell is classified in the hexagonal system and is the space group P63 / m, and often have a non-stoichiometric composition. The inorganic compound with an apatite structure containing phosphate ions and hydroxide ions is a compound described by a molecular formula M 10 (PO4)6(OH)2 is described, and the metal element M includes divalent alkaline earth metal elements such as Ca.

[0062] Preferably, hydroxylapatite (HA) is used as the inorganic compound with the apatite structure, containing phosphate ions and hydroxide ions. Hydroxylapatite has a molecular formula (Ca 10 (PO4)6(OH)2) when the stoichiometry is satisfied, and has a crystal structure in which the unit cell is classified into the hexagonal system and the space group is P63 / m.

[0063] Preferably, the inorganic compound used as the inorganic dielectric material has a hydroxide ion content in the apatite structure that is lower than the stoichiometric ratio. Hydroxyapatite exhibits a hexagonal hydroxyapatite crystal structure when a raw material powder containing, for example, hydroxide and phosphate ions is sintered at a temperature above 1250 °C and below 1500 °C. During this process, the hydroxide ion content becomes lower than the stoichiometric ratio. This is due to the dehydration of the hydroxyl group upon heating of the hydroxyapatite, resulting in the formation of oxyhydroxyapatite (OHA) and the appearance of lattice defects.

[0064] In the present embodiment, the electret layer 2 uses an inorganic compound with an apatite structure, such as hydroxyapatite, as the inorganic dielectric material for the inorganic dielectric particles 21. The inorganic compound is prepared into particles to form the inorganic dielectric particles 21, and a composite film in which the inorganic dielectric particles 21 are dispersed in the base film 22 is formed as the electret layer 2 over the substrate 10 to form the electret 1. The substrate 10 material, the base material to form the base film 22, and the method for forming the composite film to form the electret layer 2 can be the same as in the first embodiment. (Example 2)

[0065] The Electret 1 with the in Fig. The configuration shown in section 4 was manufactured using the following procedure. <Lösungsvorbereitung>

[0066] Initially, to form the composite film that would constitute the electret layer 2, hydroxylapatite powder (HA powder) with a particle size of 50 to 60 µm was used as raw material for an inorganic dielectric material. This material was sintered and dehydrated by heating at 1400 °C for 2 hours to obtain a sintered powder including oxyhydroxylapatite (OHA) (hereinafter referred to as sintered HA powder including OHA). Since the powder size is increased by sintering, the powder is sufficiently pulverized such that the particle size after pulverization is approximately 20% smaller than before sintering. The resulting sintered powder was then used as the inorganic dielectric particles 21.

[0067] Subsequently, an organic mixture of N-methyl-2-pyrrolidone and polyamic acid was prepared as raw material for the base material to form the base film 22, resulting in a polyamic acid concentration of approximately 6%. The sintered HA powder, including OHA, was added to the organic mixture at a concentration of 30% by mass and blended to produce a powder mixture for forming the composite film. <filmbildung>

[0068] The upper surface of the conductive silicon substrate 10 was thermally oxidized to form the silicon oxide film 11 with a film thickness of 50 nm. Subsequently, the conductive layer 3 was formed on the lower surface of the substrate 10 by a sputtering process. The conductive layer 3 was formed by forming a 30 nm titanium film, which is the first conductive layer 31, and a 200 nm gold film, which is the second conductive layer 32, in that order from the side in contact with the substrate 10, using appropriate layer-forming metal materials as targets.

[0069] The powder mixture solution, prepared as described above, was then applied to the upper surface of the Si oxide film 11 on the substrate 10 using a printing process to achieve a thickness of approximately 160 µm. If the sintered HA powder with OHA settled to the bottom after application, the organic mixture solution in the upper layer, excluding the sintered HA powder with OHA, was removed if necessary. The volume ratio of the sintered HA powder with OHA after formation of the electret layer 2 was 70% by volume.

[0070] Subsequently, the substrate applied with the powder mixture solution was dried for 10 to 15 minutes at 110 °C in an air atmosphere. The polyamic acid was then converted to polyimide by a 1-hour heat treatment at 280 °C. As a result, an organic-inorganic composite film was formed in which the inorganic dielectric particles 21 from the sintered HA powder, including OHA, are surrounded and held by the polyimide base material (thermal decomposition temperature ≥ 500 °C) and are uniformly dispersed within the base film 22. The temperature rise rate during the heat treatment was 3 °C / min. <polarisationsbehandlung>

[0071] The organic-inorganic composite film, intended to form the electret layer 2, was formed over the substrate 10 as described above and subjected to a polarization treatment to obtain the electret 1. For the polarization treatment, a corona discharge was used. The conductive layer 3, which is in contact with the lower surface of the substrate 10, was grounded as the ground electrode. A corona discharge electrode was placed on the upper surface of the electret layer 2, and a voltage was applied between the electrodes to induce the corona discharge. The conditions for the corona discharge were as follows: the voltage was applied, and the corona discharge continued even when the temperature was reduced. Discharge voltage: -6 kV Temperature: 200 °C Processing time: 1 hour

[0072] In the electret 1 of Example 2, the HA-based inorganic dielectric material (HA including OHA) forming the inorganic dielectric particles 21 had a band gap energy of 7 eV or more, which was greater than a band gap energy of 4 eV. Furthermore, the surface potential of the polarized electret layer 2 (thickness 160 µm) was 1.3 kV, and it was confirmed that a high surface potential was also achieved when the composite film in which the inorganic dielectric particles 21 were dispersed in the base film 22 was formed.

[0073] In this way, as in the first embodiment, the electret layer 2 exhibiting electret properties can also be obtained by polarizing the organic-inorganic composite film formed over the substrate 10 and negatively charging the upper surface. In this case as well, a high surface potential can be achieved depending on the polarization treatment conditions, and a stable electret property can be realized.< / polarisationsbehandlung> < / filmbildung> < / polarisationsbehandlung> < / filmbildung>

Claims

[1] Electret (1) with an electret layer (2), wherein - the electret layer (2) is formed by subjecting a composite film in which inorganic dielectric particles (21) are dispersed and held in a base film (22) to a polarization treatment; - the inorganic dielectric particles consist mainly of an inorganic dielectric material with a band gap energy of 4 eV or more - the inorganic dielectric material is a mixed oxide with a perovskite structure of the ABO3 type, containing two different metal elements A and B; - in the mixed oxide, an A-site of the perovskite structure is occupied by a rare earth element R, chosen from the group consisting of La, Y, Pr, Sm and Nd, and a B-site of the perovskite structure is occupied by Al; - in the mixed oxide at least one of the metal elements A and B is partially replaced by a doping element consisting of another metal element; - the doping element that replaces the metal element A is an alkaline earth metal element; and - the doping element that replaces the metal element B is at least one element chosen from the group consisting of alkaline earth metal elements and Zn. [2] Electret according to claim 1, further comprising a substrate (10) wherein the electret layer is formed over a surface of the substrate. [3] Electret according to claim 1 or 2, wherein - a substitution ratio of the dopant element replacing the metal element A, ranging from 0.5 atomic% to 20 atomic%; and - a substitution ratio of the dopant element replacing the metal element B, ranging from 0.5 atom% to 20 atom%. [4] Electret according to any one of claims 1 to 3, wherein the base film has a dielectric breakdown field strength which is higher than an electric field strength during the polarization treatment, and is made of a material which is stable at a temperature during the polarization treatment. [5] Electret according to any one of claims 1 to 4, wherein the content of inorganic dielectric particles in the composite film is 30 percent by volume or more and 75 percent by volume or less.

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