Electret

Abstract

The electret (1) is formed by polarizing a composite oxide containing two or more metal elements. A crystalline or amorphous oxide containing two different trivalent metal elements A and B and having a basic composition represented by the compositional formula A3B5O12, the crystalline or amorphous oxide having a band gap energy of 3 eV or more. The metal element A contains at least one element selected from trivalent rare earth elements, and the metal element B contains at least one element selected from typical trivalent elements.

Description

Cross-reference of related applications

[0001] This application is based on Japanese Patent Application No. 2023-119798, filed on July 24, 2023, the contents of which are incorporated herein by reference. Technical Field

[0002] This disclosure relates to electrets. Background Technology

[0003] Electrets are charged materials that provide an electrostatic field to their surroundings. They have long been used in applications such as electret condenser microphones and dust filters. In recent years, applications in vibration power generation, as an energy harvesting technology, have been anticipated. For example, the practical application of small vibration power generation components using electrets is expected as integrated circuit-assembled components used in electrostatic vibration generators driven by environmental vibrations.

[0004] As constituent materials for electrets, organic polymers such as fluorinated resins are commonly used. Organic polymers offer excellent freedom of shape and control over film thickness during film formation. However, concerns remain regarding the thermal stability of the surface potential and the degradation of performance over time. Therefore, research is underway on inorganic compound materials with superior thermal stability. For example, electrets using bulk sintered hydroxyapatite have been proposed, which are believed to exhibit high surface potentials due to the defects in hydroxide ions caused by sintering and dehydration processes.

[0005] Furthermore, Patent Document 1 proposes an electret using a composite oxide with a specific composition containing two different metal elements A and B, exhibiting an ABO3-type perovskite structure. The composite oxide serving as an electret has the following advantages: for at least one of the metal elements A and B, a portion is replaced by a dopant element with a lower valence number, thereby introducing oxygen defects, and the amount of oxygen defects can be controlled by the amount of replacement.

[0006] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2021-97213 Summary of the Invention

[0007] It is believed that electrets using composite oxides with perovskite structures suppress the decrease in surface potential after electretization by polarization treatment at high temperatures corresponding to the operating environment. However, although a stable surface potential is observed at temperatures up to around 100°C, it has been determined that the surface potential of the electret decreases significantly at temperatures exceeding 100°C (e.g., above 200°C). On the other hand, in applications such as vibration-generating devices, where heat treatment processes are included in the manufacturing process and the devices are used in high-temperature environments, there is a need for electrets with higher thermal stability.

[0008] The purpose of this disclosure is to provide an electret that can maintain a high surface potential and has excellent thermal stability even at high temperatures.

[0009] One aspect of this disclosure is an electret, which is an electret formed by polarizing a composite oxide containing two or more metal elements, wherein the composite oxide contains two different trivalent metal elements A and B and has the compositional formula A3B5O. 12 It represents a crystalline or amorphous oxide with a basic composition and a band gap energy of 3 eV or higher.

[0010] The electret in this method consists of a component with the compositional formula A3B5O. 12 The oxide composition, with an A:B:O ratio of 3:5:12, demonstrates that, for example, even at high temperatures of 200°C, the surface potential manifested through polarization can be maintained, suppressing the decrease in surface potential value. The reason for this is not yet clear, but the same effect is not observed in oxides composed of ABO3-type perovskites containing the same metal elements. Therefore, it is speculated that a higher proportion of metal elements at the B sites compared to the A sites contributes to thermal stability.

[0011] As described above, an electret that maintains a high surface potential and exhibits excellent thermal stability even under high-temperature conditions can be provided. Attached Figure Description

[0012] The foregoing and other objects, features, and advantages of this disclosure will become more apparent from the accompanying drawings and from the detailed description that follows. In these drawings, [ Figure 1 ] Figure 1 This is a schematic diagram illustrating an example of the general structure and polarization method of the electret in Embodiment 1. [ Figure 2 ] Figure 2 This is a schematic diagram showing the general structure of the electret in Embodiment 2. [ Figure 3 ] Figure 3 This is a graph showing the relationship between heat treatment and surface potential of the electret in the embodiments and comparative examples. [ Figure 4 ] Figure 4 This is a graph showing the relationship between the Mg replacement amount and the surface potential in the examples. [ Figure 5 ] Figure 5 This is a graph showing the relationship between heat treatment and surface potential of the electret in the embodiments and comparative examples. [ Figure 6 ] Figure 6 This is a graph showing the relationship between the morphology of the electret and the surface potential in the embodiments. [ Figure 7 ] Figure 7 This is a graph showing the relationship between the Mg substitution amount and the crystal structure of the electret in the examples. Detailed Implementation

[0013] (Implementation Method 1) Reference Figure 1 Implementation method 1 of the electret will be described.

[0014] like Figure 1 As shown, the electret 1 in this method is formed by polarizing a composite oxide containing two or more metal elements. The composite oxide is here configured as a granular polycrystalline material 2. The composite oxide contains two different trivalent metal elements A and B, and has the compositional formula A3B5O. 12 The basic composition is a crystalline or amorphous oxide, and the band gap energy is above 3 eV, which is then polarized to exhibit a high surface potential.

[0015] Electret 1 is a charged material that maintains a positive or negative charge on its surface and provides an electrostatic field to the surroundings. By performing polarization treatment, it can exhibit electret properties. As shown in the figure, polarization treatment can be performed, for example, by sandwiching a polycrystalline material 2 between a pair of insulating sheets 31 having an Au film 32 on its outer surface, connecting the Au film 32 to a DC power supply 100, and applying a specified high voltage at high temperature.

[0016] The processing method for electret formation is not limited to the polarization processing device shown in the figure; for example, a processing method utilizing corona discharge can also be used. The electret 1 obtained in this way can be used as an integrated circuit assembly type power generation element in various devices that convert mechanical energy and electrical energy into each other, such as small electrostatic vibration power generation devices that use environmental vibration as a power source.

[0017] The composite oxide has the composition formula A3B5O 12In this composition, metal element A contains at least one element selected from trivalent rare earth elements, and metal element B contains at least one element selected from trivalent typical elements. Specifically, metal element A can be selected from at least one element selected from La, Y, Sm, and Gd. Two or more rare earth elements can also be used in combination as metal element A. Furthermore, metal element B can be selected from at least one element selected from Al and Ga, or a combination of Al and Ga can be used.

[0018] Furthermore, details are as follows, in the composition formula A3B5O 12 Alternatively, it can be a composition in which at least one of metal elements A and B is replaced by a dopant element D with a lower valence than metal elements A and B. Specifically, for one or both of the trivalent metal elements A and B, a portion of them can be replaced, for example, by a divalent dopant element D.

[0019] Here, the composite oxide only needs to have the compositional formula A3B5O 12 The basic composition is sufficient. The ratio of the basic constituent metal elements A, B, and O, A:B:O, in the overall composite oxide, only needs to be approximately 3:5:12. For example, even if sites A or B are replaced by dopant element D, as long as the ratio of the metal elements occupying sites A or B is A:B:O = 3:5:12 or close to it, it is acceptable. Furthermore, in this case, it is not necessary to be an oxide with a uniform overall composition; it can also have a composition that differs locally from the basic composition.

[0020] It should be noted that, typically, for example, considering the oxygen deficiency δ generated during the sintering of powder raw materials, the composition of the composite oxide is A3B5O 12-δ This indicates that, for simplicity, the notation for the oxygen defect amount δ is omitted here. Furthermore, when trivalent metal elements A and B are replaced by a lower-valence dopant element D, oxygen defects corresponding to the amount of replacement are generated. In this case, the basic composition before replacement is also represented as A3B5O. 12 .

[0021] The morphology of electret 1 is not limited; it can be either a crystalline oxide or an amorphous oxide. For example, powdered raw materials can be shaped into desired granules and sintered to produce polycrystalline 2. The shape of polycrystalline 2 can be arbitrarily selected, such as a specifically limited rectangular plate or disk shape. Alternatively, a single crystal of a desired shape can be used as a crystalline oxide. When using an amorphous oxide, it can also be formed into an amorphous thin film.

[0022] When the composite oxide is a crystalline oxide, it is preferable to use an oxide with the composition A3B5O.12 The crystal structure represented is predominantly garnet-type. For example, in the case of a polycrystalline composite oxide 2, it is not necessary for all the constituent particles to be composed of garnet-type oxide crystals; they can be composed of some different elements. However, it is believed that by increasing the number of garnet-type oxide crystals, the electret performance is further improved.

[0023] In the composition formula A3B5O 12 In this process, when at least one of metal elements A and B is substituted, and the total amount of metal elements A and B is 100 atm%, the substitution amount X of dopant element D is preferably 60 atm% or less. It is understood that if the substitution amount X of dopant element D increases, for example, a crystal structure other than garnet type will be observed in a portion of the oxide crystal constituting polycrystalline 2. Therefore, from the viewpoint of predominantly garnet-type crystals, it is preferable to appropriately set the substitution amount X of dopant element D within a range that does not exceed 60 atm% relative to the overall substitution ratio of metal elements A and B.

[0024] By introducing a dopant element D with a lower valence than metallic elements A and B, oxygen defects corresponding to the substitution amount X are introduced into the crystal structure. It is believed that the higher the substitution amount X, the easier it is to introduce oxygen defects, which contributes to the manifestation of surface potential. On the other hand, it is speculated that if the substitution amount X increases, the grains constituting polycrystalline 2 will find it difficult to maintain the composition of the A3B5O group. 12 The garnet-type crystal structure is easily transformed into crystals with localized spinel-type (AB2O4) and perovskite-type (ABO3) compositions.

[0025] Composition formula A3B5O 12 In this process, metal elements A and B can be partially replaced by a first dopant element D1 and a second dopant element D2, respectively. When a portion of metal element A is replaced by a first dopant element D1 with a lower valence than metal element A, the replacement amount X1 of metal element A by the first dopant element D1 is preferably set to the range of 0 atm% < X1 ≤ 60 atm%.

[0026] The first dopant element can be at least one element selected from alkaline earth metals. Examples of alkaline earth metals include Mg, Ca, Sr, Ba, and Zn. The substitution amount X1 can be appropriately set according to the desired electret performance, taking into account the total substitution amount X of metal elements A and B based on the substitution of the second dopant element D2, within the range described above. The substitution amount X1 is preferably in the range of 0 atm% < X1 ≤ 40 atm%, and more preferably in the range of 0 atm% < X1 ≤ 20 atm%.

[0027] Even when a portion of metallic element B is replaced by a second dopant element D2 with a lower valence than metallic element B, the amount of replacement X2 by the second dopant element D2 with metallic element B is preferably set to the range of 0 atm% < X2 ≤ 60 atm%. The second dopant element can be at least one element selected from alkaline earth metals and Zn. Examples of alkaline earth metals include Mg, Ca, Sr, Ba, and Zn.

[0028] Regarding the substitution amount X2 of the second dopant element D2, it can also be appropriately set considering the substitution of the first dopant element D1, such that the total substitution amount X of metal elements A and B is within the above-mentioned range. In this case, the substitution amount X2 is preferably in the range of 0 atm% < X2 ≤ 40 atm%, and more preferably in the range of 0 atm% < X2 ≤ 20 atm%.

[0029] In a polycrystal 2 with such a basic composition, by appropriately selecting metal elements A and B, and first and second dopant elements D1 and D2, a composite oxide material with a relatively large band gap energy of 3 eV or higher is obtained. This increases the insulation breakdown voltage, thus allowing the application of a high voltage during polarization treatment to exhibit the desired high surface potential. Furthermore, by changing the types of the first and second dopant elements D1 and D2 relative to metal elements A and B, and adjusting the substitution amount X, the basic composition of the polycrystal 2 and the amount of introduced oxygen defects can be controlled, thereby forming an electret 1 with the desired electret properties.

[0030] (Implementation Method 2) Reference Figure 2 Implementation method 2 of the electret will be described.

[0031] It should be noted that, in the symbols used in Embodiment 2 and thereafter, symbols that are the same as those used in the previously implemented embodiments, unless otherwise specified, represent structural elements that are the same as those in the previously implemented embodiments.

[0032] like Figure 2 As shown, the electret 1 in this embodiment is formed of an amorphous thin film, specifically, it can be constituted as an amorphous film 20 formed on the surface of the substrate 10. The amorphous film 20, like the polycrystalline film 2 in Embodiment 1, is composed of a film having the formula A3B5O. 12 The basic composition is represented by a composite oxide with a band gap energy of 3 eV or higher, which is electretized by polarization treatment.

[0033] In this method, the ratio of metal elements A, B, and O, which are the basic constituent elements, to the composite oxide is approximately 3:5:12. Furthermore, in the basic composition of the amorphous film 20, metal elements A and B can be selected in the same way as in Embodiment 1, or a portion of metal elements A and B can be replaced by first and second dopant elements D1 and D2, respectively. The replacement amounts X1, X2, etc., in this case can also be selected in the same way as in Embodiment 1.

[0034] The method for forming the amorphous film 20 is not necessarily limited; for example, sputtering can be used. In this case, a polycrystalline composite oxide with the desired composition is prepared in advance and used as a target for sputtering in an inert gas. As a result, thin films with a thickness of less than 0.1 mm (100 μm) can be achieved, and preferably, an amorphous film 20 with a desired thickness can be formed in the range of about 0.01 μm to tens of μm.

[0035] The amorphous film 20 formed in this way becomes a film with the same basic composition as the target polycrystalline material. The film formation method is not limited to sputtering; any method can be used. Other methods include physical vapor deposition, chemical vapor deposition, sol-gel deposition, and welding, etc., and the appropriate method can be selected based on the desired film quality and thickness.

[0036] The composition of the substrate 10 is not particularly limited; for example, a conductive substrate formed of (Nb,Sr)TiO3, Si, etc., can be used. Alternatively, a conductive substrate utilizing a conductive material such as a metal, or an insulating substrate utilizing a glass material, can also be used. The shape of the substrate 10 can be any shape, such as a rectangular plate or a disk.

[0037] In this way, by electretting the amorphous film 20 formed as a thin film on the substrate 10, a high-performance electret 1 with a desired thin film shape corresponding to the application and excellent thermal stability can be formed.

[0038] Example (Examples 1-3) Make using the following methods Figure 1 The electret 1 with the shown configuration is used to evaluate its performance. Electret 1 uses a component with the formula A3B5O. 12 A composite oxide with metal element A designated as Y and metal element B designated as Al was used to fabricate a granular polycrystalline material 2, which underwent polarization treatment to achieve electret formation. At this point, the undoped Y3Al5O3... 12(Hereinafter referred to as YAO) As Example 1, YAO was replaced by a dopant obtained by replacing a portion of Y at site A with 1 atm% Mg and 5 atm% Sr, which are dopants D, as Examples 2 and 3 (Mg 1%: YAO, Sr 5%: YAO).

[0039] <Preparation of Composite Oxide Powder> As starting materials, the powdered reagents shown in Table 1 (raw material powders: Y₂O₃, Al₂O₃, MgO, SrO) were prepared and weighed separately. For each of Examples 1-3, the raw material powders weighed in accordance with the specified composition were placed in a glass tube, and 30g of grinding balls (ZrO₂; φ2mm) and 30ml of ethanol were further added. The mixture was then ground and mixed using a ball mill at 300rpm for 24 hours. Then, the solution after removing the grinding balls was transferred to a petri dish and dried for 24 hours. The dried powder was then ground in an agate mortar and then placed in an alumina crucible for pre-calcination. The pre-calcination conditions were as follows: First, the temperature was raised to 1400℃ at a heating rate of 2℃ / min, held at 1400℃ for 8 hours, and then lowered to room temperature at a cooling rate of 2℃ / min to obtain the pre-calcined powder.

[0040] <Preparation of sintered particles (polycrystalline 2)> The pre-fired powder sample was pulverized in an agate mortar and further graded (<100μm) to prepare molding powder. Approximately 0.55g of the molding powder was placed in a φ13mm particle shaper and pressurized at 250MPa for 3 minutes to form disc-shaped particles. The resulting shaped particles were then sintered in an alumina crucible under the following conditions: First, the temperature was increased to 1700℃ at a heating rate of 2.5℃ / min and held at 1700℃ for 2 hours. Then, the temperature was decreased to room temperature at a cooling rate of 2.5℃ / min. The resulting sintered particles had a diameter of approximately φ11mm and a thickness of approximately 1.3mm.

[0041] <Polarization Treatment> For the polycrystalline 2 of Examples 1-3, respectively, using... Figure 1The polarization treatment is performed using the method shown. A polycrystalline silicon 2 is sandwiched between a pair of insulating sheets 31, each φ11 mm in diameter, and brought into contact with a pair of electrode portions (not shown) of the polarization treatment apparatus connected to a DC power supply 100. The insulating sheets 31 are those with an Au film 32 formed on the contact surface with the electrode portions. The polarization apparatus, with the polycrystalline silicon 2 positioned between the pair of electrode portions, is placed in a box furnace and left until the furnace temperature stabilizes at 200°C. Then, while the temperature is stabilized at 200°C, a polarization treatment is performed for 3 minutes by applying a DC electric field of 8.0 kV / mm between the pair of electrode portions. After a predetermined treatment time, the temperature is cooled to below 40°C while the DC electric field is continuously applied.

[0042] <Surface Potential Measurement> For the electrets 1 obtained in Examples 1-3, the surface potential after initial polarization treatment was measured. A surface potentiometer (MODEL341-B: manufactured by TREK JAPAN Co., Ltd.) was used to measure the surface potential in a non-contact manner. Following the initial polarization treatment, the surface potential was measured in a drying oven after heating at 100°C for 30 minutes, followed by further heating at 200°C for 30 minutes, to evaluate the influence of the thermal environment. The results are presented as follows: after initial polarization, after heating at 100°C for 30 minutes, and after heating at 200°C for 30 minutes. Figure 3 .

[0043] (Comparative Example 1) For comparison, as a conventional electret material, a perovskite-type composite oxide (YAlO3) containing the same metallic elements as in Example 1 was used. Similarly, polycrystalline material 2 was prepared using raw material powder weighed in a manner that resulted in a specified composition, and then polarized to electret. For the electret 1 of Comparative Example 1 obtained in this way, the surface potential was measured using the same method as in Example 1, after immediate polarization treatment (hereinafter appropriately referred to as "after immediate polarization"), after heating at 100°C for 30 minutes (hereinafter appropriately referred to as "after heating at 100°C"), and after heating at 200°C for 30 minutes (hereinafter appropriately referred to as "after heating at 200°C"). The results were compared with those of Examples 1-3 and are shown below. Figure 3 .

[0044] like Figure 3As shown, the surface potential (absolute value) of electret 1 in Examples 1-3 after initial polarization is in the range of 3.5kV to 4.5kV, showing a higher value than that of electret 1 in Comparative Example 1, which exceeds 3kV. Furthermore, depending on the presence or absence of dopant element D, the surface potential (absolute value) of electret 1 (unsubstituted: YAO) in Example 1 reaches a maximum value exceeding 4.5kV after initial polarization. After heat treatment, the surface potential (absolute value) decreases slightly but stabilizes at around 4kV; however, heating to 200°C results in a higher value than heating to 100°C.

[0045] The surface potential (absolute value) of electret 1 (Mg1%: YAO) in Example 2 and electret 1 (Sr5%: YAO) after initial polarization was in the range of approximately 3.5 kV to 4 kV. In contrast, after heating at 100°C, it increased to approximately 6 kV to 7 kV, becoming nearly twice as high. Furthermore, after heating at 200°C, it decreased compared to the value after heating at 100°C, but maintained a value of approximately 4 kV to 5 kV, the same as or higher than the value after initial polarization. In contrast, the surface potential (absolute value) of electret 1 in Comparative Example 1 was slightly higher after heating at 100°C, but decreased significantly after heating at 200°C, halving to approximately 1.5 kV.

[0046] These results show that, compared with perovskite-type composite oxides, the overall surface potential is improved by using garnet-type composite oxides, especially in maintaining the surface potential after heating to 200°C. Furthermore, a significant effect was observed in the improvement of the surface potential after heating to 100°C with dopant-substituted materials, suggesting that the introduction of oxygen vacancies in the garnet-type composition plays a role.

[0047] In addition, for electret 1 of Examples 1-3 and Comparative Example 1, the measured values ​​(absolute values) of surface potential and the band gap energy of the composite oxide that forms the basic composition are shown in Table 2. As shown in Table 2, the band gap energy of the garnet-type composite oxide (YAO, dopant-substituted YAO) of Examples 1-3 is 4.49 eV, and the band gap energy of the perovskite-type composite oxide (YAlO3) of Comparative Example 1 is 5.54 eV.

[0048] (Experimental Example 1) For electret 1 (Mg 1%: YAO) of Example 2, as shown in Table 3, the substitution amount X of Mg as dopant element D was varied within the range of 10 atm% to 80 atm%, and samples 1 to 7 were used for testing. Sample 1 was set with the same substitution amount X as electret 1 of Example 2, and the raw material powder of electret 1 of samples 2 to 7 was weighed with Mg substitution amounts of 10 atm%, 20 atm%, 30 atm%, 40 atm%, 60 atm%, and 80 atm%, respectively. Otherwise, polycrystalline 2 was prepared in the same manner as in Example 1 and electretized by polarization treatment.

[0049] As shown in Table 4, multiple electrets 1 of these samples 1-7 were prepared (a, b), and their surface potentials after immediate polarization were measured using the same method as in Example 1. Furthermore, the surface potentials were measured after heating at 100°C for 30 minutes and then at 200°C for 30 minutes. The measurement results for these samples 1 (1a, 1b)-7 (7a, 7b) are shown in Table 4, and the absolute values ​​of the surface potentials after heating at 100°C and 200°C are compared with... Figure 4 The results are shown in comparison. Furthermore, the results for sample 1a are the same as those for Example 2 shown in Table 2.

[0050] As shown in Table 4, Figure 4 As shown, the surface potential (absolute value) of electret 1 in samples 2a, 2b to 6a, and 6b after heating at 100°C for 30 minutes is in the range of approximately 3kV to 5kV, which is lower than that of samples 1a and 1b, which exceed 6kV, but all show values ​​roughly the same as or higher than those after immediate polarization. For samples 7a and 7b, the surface potential exceeds 1.5kV after immediate polarization, and even after heating at 100°C for 30 minutes, it shows a relatively high value of approximately 1kV.

[0051] Furthermore, regarding electret 1 of samples 2a, 2b~3a, and 3b, the surface potential (absolute value) remained high after heating at 200°C for 30 minutes, exceeding approximately 3kV~4kV. In contrast, the electret 1 of samples 4a, 4b~6a, and 6b was in the range of approximately 1kV~2kV, and the electret 1 of samples 7a and 7b was approximately 0.3kV. This indicates a tendency for the surface potential (absolute value) to decrease with increasing Mg substitution.

[0052] Based on these results, when using composite oxides substituted with doped element D, considering that the surface potential (absolute value) after heat treatment varies depending on the substitution amount X of doped element D, it is preferable to select the substitution amount X of doped element D in a manner that yields the desired surface potential at the operating temperature. For example, by setting the substitution amount X to a range of 60 atm% or less, a high surface potential (absolute value) of about 1 kV or more can be obtained even at 200 °C, a high surface potential (absolute value) of about 1 kV or more can be exhibited when the substitution amount X is 40 atm% or less, and a high surface potential (absolute value) of about 3 kV or more can be exhibited when the substitution amount X is 20 atm% or less.

[0053] (Example 4) For those with Figure 1 The electret 1 shown uses the composition formula A3B5O 12 The performance of electrets is evaluated by using composite oxides with altered metal elements A and B. The composite oxide is an oxide (hereinafter referred to as GAGO) in which metal element A is set as Gd and metal element B is set as Al or Ga, and a portion of the Gd at site A is replaced by a dopant of 1 atm% Sr as the dopant element D to replace GAGO (Sr1%:GAGO).

[0054] In the electret 1 of Example 4, as shown in Table 5, powdered reagents (raw material powders: Ga2O3, Al2O3, Ga2O3, SrO) as starting materials were weighed to have the desired composition, and granular polycrystalline 2 was prepared in the same manner as in Example 1, and polarized to form an electret. Figure 5 The results are shown, obtained by measuring the surface potential of electret 1 after polarization, heating at 100°C for 30 minutes, and heating at 200°C for 30 minutes, in the same manner as in Example 1. Additionally, for comparison, in... Figure 5 The results of Comparative Example 1 are shown in the figure.

[0055] like Figure 5 As shown, in electret 1 (Sr1%: GAGO) of Example 4, the surface potential (absolute value) immediately after polarization exhibits a high value exceeding 3.5 kV, and after heating at 100°C for 30 minutes, the surface potential further increases to over 5 kV. Furthermore, after heating at 200°C for 30 minutes, although the voltage is below 3 kV, it shows approximately twice the high value compared to electret 1 with the perovskite composition of Comparative Example 1.

[0056] In this way, even if the metal element A occupying the A site is changed and multiple elements (Al, Ga) are used as the metal element B occupying the B site, electret 1 with the same electret properties can be obtained. Based on these results, it is speculated that the use of composite oxides with garnet-type composition is important and contributes to the surface potential performance of electret 1 and the maintenance of surface potential at high temperature environments.

[0057] For electret 1 of Example 4, the measured surface potential (absolute value) and the band gap energy of the composite oxide with the basic composition are shown in Table 2. As shown in Table 2, the band gap energy (estimated value) of the garnet-type composite oxide (GAGO) of Example 4 is 3.2 eV. Therefore, it can be seen that if a composite oxide has a garnet-type composition and a band gap energy of 3 eV or higher, it exhibits the desired high surface potential and excellent thermal stability.

[0058] (Examples 5-7) For electret 1 with the same garnet-type composition as electret 1 (Mg1%: YAO) in Example 2 but different morphologies, the electret performance was evaluated. First, as Example 5, electret 1 with the following method was manufactured. Figure 2 The electret 1 is shown in the diagram. The electret 1 is obtained by forming an amorphous film 20 on a substrate 10 and performing a polarization treatment based on corona discharge. The substrate 10 is a conductive substrate (0.6 mm) made of SrTiO3 containing 0.5% by mass of Nb.

[0059] <Membrane Formation> On the surface of substrate 10, a pre-prepared polycrystalline material composed of garnet type [(Y] is placed... 0.99 Mg 0.01 )3Al5O 12 Using [a target], a film containing Y, Mg, Al, and O was formed by sputtering under an Ar atmosphere at 360°C. The film was amorphous, containing Y, Al, and O in a ratio of approximately 3:5:12, and had a thickness of 1 μm.

[0060] <Polarization Treatment> Next, a substrate 10 with an amorphous film 20 formed on its surface is placed on the surface of a grounded heating plate. With the back side of the substrate 10 in contact with the heating plate, a polarization treatment based on corona discharge is performed. The heating temperature of the heating plate and the corona discharge treatment conditions are as follows. The corona discharge is performed by placing a discharge needle opposite to the surface of the amorphous film 20, which is the sample.

[0061] Heating temperature: 200℃ Discharge voltage: -5.5kV Discharge time: 3 minutes Distance between the discharge needle and the sample: 10mm Additionally, as electret 1 in Example 6, a single crystal composed of garnet type with a diameter of 10 mm and a thickness of 0.5 mm was prepared [(Y 0.99 Mg 0.01 )3Al5O 12 The same polarization treatment was performed to electretize the polycrystal 2. Furthermore, the same polarization treatment was performed on the polycrystal 2 prepared in the same manner as in Example 2, and the electretized polycrystal was designated as Example 7. For the electrets 1 obtained in Examples 5-7, the surface potential was measured in the same manner as in Example 1, and the results are shown below. Figure 6 .

[0062] like Figure 6 As shown, the surface potentials of electret 1 in Examples 6 and 7, which used crystalline oxides, after initial polarization were -1832V (single crystal) and -1824V (polycrystalline), respectively, yielding approximately equally high values. Furthermore, for electret 1 in Example 5, which used amorphous oxides, a surface potential of -103.5V (amorphous) was also observed after initial polarization. These results indicate that by having a garnet-type composition, electretization can be performed regardless of the morphology of the composite oxide, resulting in a practical electret 1.

[0063] It should be noted that, for electret 1 (Mg 1%: YAO) in Example 2, structural analysis (XRD) was performed using an X-ray diffraction apparatus to investigate the effect of Mg substitution on the crystal structure. The results are shown in... Figure 7 .exist Figure 7 In addition to electret 1 (sample 1) in Example 2, samples with Mg replacement amounts of 20 atm% (sample 3), 40 atm% (sample 5), and 60 atm% (sample 6) used in Test Example 1 were compared and the XRD-based analysis results are shown.

[0064] like Figure 7 As shown, the results of structural analysis (XRD) using an X-ray diffraction apparatus were compared for samples with Mg substitution amounts of 1 atm% (sample 1), 20 atm% (sample 3), 40 atm% (sample 5), and 60 atm% (sample 6) used in Experimental Example 1. The results showed that samples 1 and 3, with Mg substitution amounts below 20 atm%, exhibited a roughly garnet-type composition (Y3Al5O3). 12A slight peak based on the perovskite type composition (YAlO3) was observed. In contrast, as the Mg substitution amount increased, a tendency for different compositions to increase was observed. For example, in samples 5 and 6 with a Mg substitution amount of 40 atm% or more, a peak based on the spinel type composition (MgAl2O4) was observed. In sample 6 with a Mg substitution amount of 60 atm%, the peaks based on the perovskite type or spinel type composition became larger.

[0065] In the above Figure 4 The results showed that when the Mg substitution amount exceeded 40 atm%, a tendency for the surface potential (absolute value) to decrease after heating was observed, suggesting that the increase in crystals other than garnet type had an impact. Furthermore, it was found that when the Mg substitution amount was below 20 atm%, a stable surface potential was maintained after heating at 200 °C, indicating that more garnet-type crystals were formed, thus improving thermal stability. Additionally, the same tendency was confirmed when the doping element was Sr.

[0066] Thus, by using crystalline or amorphous oxides that are composite oxides with a garnet-type composition and a band gap energy of 3 eV or higher, the thermal stability of electret 1 can be improved. Therefore, when manufacturing electret 1, by appropriately selecting morphologies such as single crystals, polycrystalline films, or amorphous films according to the usage environment, required shape, or performance, highly practical electret 1 can be formed.

[0067] This disclosure has been described according to embodiments, but it should be understood that this disclosure is not limited to these embodiments or constructions. This disclosure also includes various modifications and equivalent variations. In addition, various combinations, methods, and other combinations or methods that include only one element or more of them also fall within the scope and spirit of this disclosure.

[0068] This disclosure is not limited to the above-described embodiments, and can be applied to various embodiments without departing from its spirit.

[0069] The features of this disclosure are as follows.

[0070] [1] An electret, which is an electret formed by polarizing a composite oxide containing two or more metal elements (1), The composite oxide contains two different trivalent metal elements A and B, and has the compositional formula A3B5O. 12 It represents a crystalline or amorphous oxide with a basic composition and a band gap energy of 3 eV or higher.

[0071] [2] According to the electret described in [1], wherein the metal element A contains at least one element selected from trivalent rare earth elements, and the metal element B contains at least one element selected from trivalent typical elements.

[0072] [3] According to [1] or [2], wherein the metal element A is at least one element selected from La, Y, Sm and Gd, and the metal element B is at least one element selected from Al and Ga.

[0073] [4] An electret according to any one of [1] to [3], wherein at least one of the metal elements A and B is replaced by a dopant element D with a lower valence than the metal elements A and B.

[0074] [5] According to the electret described in [4], when the total of the metal elements A and B is set to 100 atm%, the amount of substitution X of the dopant element for the metal elements A and B is 60 atm% or less.

[0075] [6] According to the electret described in [4] or [5], a portion of the metal element A is replaced by the dopant element selected from alkaline earth metal elements, wherein the amount of replacement X1 of the metal element A by the dopant element is in the range of 0 atm% < X1 ≤ 60 atm%.

[0076] [7] According to the electret described in [6], wherein the amount of substitution X1 of the dopant element for the metal element A is in the range of 0 atm% < X1 ≤ 40 atm%.

[0077] [8] An electret according to any one of [4] to [7], wherein a portion of the metal element B is replaced by the dopant element selected from alkaline earth metal elements and Zn, and the amount of replacement X2 of the metal element B by the dopant element is in the range of 0 atm% < X2 ≤ 60 atm%.

[0078] [9] According to the electret described in [8], wherein the amount of substitution X2 of the dopant element for the metal element B is in the range of 0 atm% < X2 ≤ 40 atm%.

[0079]

[10] An electret according to any one of [1] to [9], wherein the composite oxide is in the form of a single crystal, a polycrystalline or an amorphous film.

Claims

1. An electret, which is an electret formed by polarizing a composite oxide containing two or more metal elements (1), The composite oxide contains two different trivalent metal elements A and B, and has the composition formula A3B5O. 12 It represents a crystalline or amorphous oxide with a basic composition and a band gap energy of 3 eV or higher.

2. The electret according to claim 1, wherein, The metal element A contains at least one element selected from trivalent rare earth elements, and the metal element B contains at least one element selected from trivalent typical elements.

3. The electret according to claim 1 or 2, wherein, The metal element A is at least one element selected from La, Y, Sm and Gd, and the metal element B is at least one element selected from Al and Ga.

4. The electret according to claim 1 or 2, wherein, In at least one of the metal elements A and B, a portion of it is replaced by a dopant element D with a lower valence than the metal elements A and B.

5. The electret according to claim 4, wherein, When the total of the metal elements A and B is set to 100 atm%, the substitution amount X of the dopant element for the metal elements A and B is less than 60 atm.

6. The electret according to claim 5, wherein, A portion of the metal element A is replaced by the dopant element selected from alkaline earth metal elements, and the amount of replacement X1 of the metal element A by the dopant element is in the range of 0 atm% < X1 ≤ 60 atm%.

7. The electret according to claim 6, wherein, The amount of substitution X1 by the dopant element for the metal element A is in the range of 0 atm% < X1 ≤ 40 atm%.

8. The electret according to claim 5, wherein, A portion of the metal element B is replaced by a dopant element selected from alkaline earth metal elements and Zn, wherein the amount of replacement X2 of the metal element B by the dopant element is in the range of 0 atm% < X2 ≤ 60 atm%.

9. The electret according to claim 8, wherein, The amount of substitution X2 of the dopant element for the metal element B is in the range of 0 atm% < X2 ≤ 40 atm%.

10. The electret according to claim 1 or 2, wherein, The composite oxide is in the form of a single crystal, polycrystalline, or amorphous film.

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