Electret

Abstract

This electret (1) is obtained by subjecting a composite oxide, which contains two or more metal elements, to a polarization treatment. The present invention is a crystalline or amorphous oxide which contains trivalent metal elements A and B of two different kinds, and which has a basic composition represented by composition formula A3B5O12, and a band gap energy of 3 eV or more. The metal element A includes at least one element that is selected from among trivalent rare earth elements, and the metal element B includes at least one element that is selected from among trivalent typical elements.

Description

electret CROSS-REFERENCE TO RELATED APPLICATIONS

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

[0002] The present disclosure relates to electrets.

[0003] Electrets are electrically charged materials that generate an electrostatic field around them and have traditionally been used in applications such as electret condenser microphones and dust-collecting filters. Recently, they have been expected to be applied to vibration power generation, a type of energy harvesting technology. For example, there is a desire to commercialize small vibration power generation elements using electrets as integrated circuit-embedded elements for electrostatic vibration power generators that use environmental vibrations as a power source.

[0004] Organic polymer materials such as fluororesins are commonly used as materials for electrets. While organic polymer materials offer excellent control over the degree of freedom in shape and film thickness when forming thin films, concerns remain about the thermal stability of the surface potential and the deterioration of performance over time. Therefore, inorganic compound materials with excellent thermal stability have been explored. As an example of an inorganic compound material, an electret using a bulk sintered body of hydroxyapatite has been proposed. It is believed that the high surface potential is due to defects in hydroxide ions caused by sintering and dehydration treatments.

[0005] Furthermore, Patent Document 1 proposes an electret using a composite oxide of a specific composition having an ABO3-type perovskite structure containing two different metal elements A and B. In the composite oxide that serves as the electret, oxygen vacancies are introduced by substituting a portion of at least one of the metal elements A and B with a dopant element having a lower valence, which has the advantage that the amount of oxygen vacancies can be controlled by the amount of substitution.

[0006] Japanese Patent Application Laid-Open No. 2021-97213

[0007] It has been thought that electrets using complex oxides with a perovskite structure can suppress a decrease in surface potential after electretization by performing polarization treatment at a high temperature appropriate to the usage environment. However, it has been found that although electrets exhibit a stable surface potential in temperature environments up to about 100°C, the surface potential of the electret significantly decreases at temperatures above 100°C (e.g., 200°C or higher). On the other hand, when applied to devices such as vibration power generation elements, the manufacturing process includes a heat treatment step, and the devices are expected to be used in high-temperature environments, so there is a demand for electrets with higher thermal stability.

[0008] An object of the present disclosure is to provide an electret that can maintain a high surface potential even in a high-temperature environment and has excellent thermal stability.

[0009] One aspect of the present disclosure is an electret obtained by subjecting a composite oxide containing two or more metal elements to a polarization treatment, wherein the composite oxide contains two different trivalent metal elements A and B and has a composition formula of ABO 12 The electret is a crystalline or amorphous oxide having a basic composition represented by the formula: and having a band gap energy of 3 eV or more.

[0010] The electret of this embodiment has the composition formula A3B5O 12 It was found that the material is composed of an oxide having a basic composition where A:B:O is 3:5:12, and that it is possible to maintain the surface potential generated by polarization and suppress a decrease in the value of the surface potential, even in a high-temperature environment of, for example, 200° C. The reason for this is not entirely clear, but since a similar effect is not observed in oxides with an ABO3-type perovskite composition containing the same metal elements, it is presumed that the higher proportion of metal elements in the B site compared to the A site somehow contributes to thermal stability.

[0011] As described above, according to the above-described embodiment, it is possible to provide an electret that can maintain a high surface potential even in a high-temperature environment and has excellent thermal stability.

[0012] The above and other objects, features, and advantages of the present disclosure will become clearer from the following detailed description taken in conjunction with the accompanying drawings, in which Fig. 1 is a schematic diagram illustrating an example of the general configuration of an electret and a polarization method in embodiment 1, Fig. 2 is a schematic diagram illustrating the general configuration of an electret in embodiment 2, Fig. 3 is a diagram illustrating the relationship between the heat treatment of an electret and the surface potential in examples and comparative examples, Fig. 4 is a diagram illustrating the relationship between the Mg substitution amount and the surface potential in examples, Fig. 5 is a diagram illustrating the relationship between the heat treatment of an electret and the surface potential in examples and comparative examples, Fig. 6 is a diagram illustrating the relationship between the morphology of an electret and the surface potential in examples, and Fig. 7 is a diagram illustrating the relationship between the Mg substitution amount and the crystal structure of an electret in examples.

[0013] (Embodiment 1) An embodiment 1 relating to an electret will be described with reference to Fig. 1. As shown in Fig. 1, an electret 1 of this embodiment is formed by subjecting a composite oxide containing two or more metal elements to a polarization treatment. The composite oxide is configured as a pellet-shaped polycrystalline body 2 here. The composite oxide contains two different trivalent metal elements A and B, and has a composition formula ABO. 12 The oxide is a crystalline or amorphous oxide having a basic composition represented by the formula: and has a band gap energy of 3 eV or more, so that it can be polarized to develop a high surface potential.

[0014] The electret 1 is a charged substance that holds a positive or negative charge on its surface and generates an electrostatic field around it, and can exhibit electret performance by being subjected to a polarization treatment. As shown in the figure, the polarization treatment can be performed by, for example, sandwiching the polycrystalline body 2 between a pair of insulating sheets 31 having Au films 32 on their outer surfaces, connecting the Au films 32 to a DC power source 100, and applying a predetermined high voltage at a high temperature.

[0015] The treatment method for producing the electret is not limited to the method using the polarization treatment device shown in the figure, and a treatment method using corona discharge, for example, can also be adopted. The electret 1 obtained in this way can be used as an integrated circuit-embedded power generation element in various devices that convert mechanical energy into electrical energy and vice versa, for example, in small electrostatic vibration power generation devices that use ambient vibration as a power source.

[0016] The composition formula of the composite oxide is A3B5O 12 In the above formula, the metal element A includes at least one element selected from trivalent rare earth elements, and the metal element B includes at least one element selected from trivalent typical elements. Specifically, the 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 the metal element A. Furthermore, the metal element B can be at least one element selected from Al and Ga, and Al and Ga can also be used in combination.

[0017] As will be described in detail later, the composition formula A3B5O 12 In the above, at least one of the metal elements A and B may be substituted with a dopant element D having a lower valence than the metal elements A and B. Specifically, one or both of the trivalent metal elements A and B may be partially substituted with, for example, a divalent dopant element D.

[0018] Here, the composite oxide has the composition formula A3B5O 12 It is sufficient that the composite oxide has a basic composition expressed by the formula: where the ratio A:B:O of the basic constituent metal elements A, B, and O is approximately 3:5:12 for the entire composite oxide. For example, even when the A site or the B site is substituted with a dopant element D, it is sufficient that the content ratio of the metal elements occupying the A site or the B site is A:B:O = 3:5:12 or a relationship close to this. In this case, the oxide does not need to have a uniform composition as a whole, and it may have a composition that is partially different from the basic composition.

[0019] In general, the composition formula of the composite oxide is ABO, taking into consideration the amount of oxygen deficiency δ that occurs when sintering the powder raw material. 12－δ However, for simplicity, the oxygen vacancy amount δ is omitted here. When the trivalent metal elements A and B are substituted with a dopant element D having a lower valence, oxygen vacancies occur according to the amount of substitution. In this case, too, the basic composition before substitution is expressed by the composition formula A3B5O 12 shall represent the following.

[0020] The form of the electret 1 is not necessarily limited, and may be a crystalline oxide or an amorphous oxide. Here, for example, a powder raw material can be formed into a desired pellet shape and sintered to form the polycrystalline body 2. The shape of the polycrystalline body 2 can be selected arbitrarily, such as a specific rectangular plate shape or a disk shape, without any particular restrictions. Furthermore, a single crystal of a desired shape can also be used as the crystalline oxide. When an amorphous oxide is used, it can also be configured as an amorphous thin film.

[0021] When the composite oxide is a crystalline oxide, the composition formula is ABO 12 For example, when the composite oxide is polycrystalline body 2, it is not necessary for all of the constituent particles to be made of garnet-type oxide crystals, and they may have partially different compositions, but it is thought that an increase in the amount of garnet-type oxide crystals will further improve the electret performance.

[0022] Composition formula A3B5O 12 In the above, when at least one of the metal elements A and B is substituted, the substitution amount X by the dopant element D is preferably 60 atm% or less when the total of the metal elements A and B is 100 atm%. It has been found that when the substitution amount X by the dopant element D is large, for example, a crystal structure other than a garnet type is observed in a part of the oxide crystals constituting the polycrystalline body 2. Therefore, from the viewpoint of mainly comprising a garnet type crystal, it is preferable to appropriately set the substitution amount X by the dopant element D within a range in which the substitution ratio of the metal elements A and B to the whole does not exceed 60 atm%.

[0023] By introducing the dopant element D, which has a lower valence than the metal elements A and B, oxygen defects are introduced into the crystal structure according to the substitution amount X. It is thought that the larger the substitution amount X, the easier it is to introduce oxygen defects, which contributes to the development of the surface potential. On the other hand, when the substitution amount X is large, the crystal grains constituting the polycrystalline body 2 are more likely to have a structure of the composition formula A3B5O 12 It is presumed that this makes it difficult to maintain the garnet-type crystal structure represented by the formula (1), and that this makes it more likely that the crystals will partially have a spinel-type (AB2O4) or perovskite-type (ABO3) composition.

[0024] Composition formula A3B5O 12 In the formula (I), the metal elements A and B can be partially substituted with a first dopant element D1 and a second dopant element D2, respectively. When the metal element A is partially substituted with the first dopant element D1 having a lower valence than the metal element A, the substitution amount X1 of the metal element A with the first dopant element D1 is preferably in the range of 0 atm % < X1 ≦ 60 atm %.

[0025] The first dopant element can be at least one element selected from alkaline earth metal elements. Examples of alkaline earth metal elements include Mg, Ca, Sr, Ba, and Zn. The substitution amount X1 can be appropriately set depending on the desired electret performance so that the total substitution amount X of the metal elements A and B, taking into account substitution with the second dopant element D2, is within the above range. The substitution amount X1 is preferably in the range of 0 atm% < X1 ≦ 40 atm%, more preferably in the range of 0 atm% < X1 ≦ 20 atm%.

[0026] Even when a portion of the metal element B is substituted with a second dopant element D2 having a lower valence than the metal element B, the substitution amount X2 of the metal element B with the second dopant element D2 is preferably in the range of 0 atm % < X2 ≦ 60 atm %. The second dopant element may be at least one element selected from alkaline earth metal elements and Zn. Examples of alkaline earth metal elements include Mg, Ca, Sr, Ba, and Zn.

[0027] The substitution amount X2 of the second dopant element D2 can also be appropriately set in consideration of the substitution by the first dopant element D1 so that the total substitution amount X of the metal elements A and B falls within the above range. In this case as well, the substitution amount X2 is preferably set in the range of 0 atm%<X2≦40 atm%, more preferably in the range of 0 atm%<X2≦20 atm%.

[0028] In the polycrystalline body 2 having such a basic composition, by appropriately selecting the metal elements A and B and the first and second dopant elements D1 and D2, a complex oxide material with a relatively large band gap energy of 3 eV or more is obtained. This increases the breakdown voltage, making it possible to apply a high voltage during polarization treatment and achieve a desired high surface potential. Furthermore, by changing the types of the first and second dopant elements D1 and D2 relative to the metal elements A and B and adjusting the substitution amount X, the basic composition of the polycrystalline body 2 and the amount of oxygen vacancies introduced can be controlled, resulting in an electret 1 with the desired electret performance.

[0029] (Embodiment 2) An embodiment 2 relating to an electret will be described with reference to Fig. 2. Note that, among the symbols used in the second and subsequent embodiments, the same symbols as those used in the previous embodiments represent the same components as those in the previous embodiments unless otherwise specified.

[0030] As shown in FIG. 2, the electret 1 of this embodiment is made of a non-crystalline thin film, and specifically, can be configured as an amorphous film 20 formed on the surface of a substrate 10. The amorphous film 20 has the composition formula ABO, similar to the polycrystalline body 2 in the first embodiment. 12 The composite oxide has a basic composition represented by the formula: and has a band gap energy of 3 eV or more, and is made into an electret by being subjected to a polarization treatment.

[0031] In this embodiment, too, it is sufficient that the ratio A:B:O of the basic constituent metal elements A, B, and O is approximately 3:5:12 for the entire composite oxide. In addition, in the basic composition of the amorphous film 20, the metal elements A and B can be selected in the same manner as in embodiment 1, and a configuration can be adopted in which a portion of the metal elements A and B are substituted with the first and second dopant elements D1 and D2, respectively. In this case, the substitution amounts X1, X2, etc. can also be selected in the same manner as in embodiment 1.

[0032] The method for forming the amorphous film 20 is not necessarily limited, but for example, sputtering can be used. In this case, a polycrystalline composite oxide prepared to the desired composition is prepared in advance, and sputtering is performed in an inert gas using this as a target. This allows for a thin film of 0.1 mm (100 μm) or less, and preferably allows for the formation of an amorphous film 20 having a desired thickness in the range of 0.01 μm to several tens of μm.

[0033] The amorphous film 20 thus formed has the same basic composition as the target polycrystalline body. The film formation method is not limited to sputtering, and any method can be used. Other methods include physical vapor deposition, chemical vapor deposition, sol-gel methods, and deposition methods, and an appropriate method can be selected taking into consideration the desired film quality and film thickness.

[0034] The composition of the substrate 10 is not particularly limited, and a conductive substrate made of, for example, (Nb, Sr)TiO3, Si, or the like can be used. Alternatively, a conductive substrate made of a conductive material such as metal, or an insulating substrate made of a glass material, or the like, can also be used. The shape of the substrate 10 can be any external shape, such as a rectangular plate or a disk.

[0035] In this way, by converting the amorphous film 20 formed in a thin film on the substrate 10 into an electret, it is possible to obtain a high-performance electret 1 having a desired thin film shape according to the application, etc., and excellent thermal stability.

[0036] (Examples 1 to 3) Electret 1 having the configuration shown in Figure 1 was produced by the following method, and the electret performance was evaluated. Electret 1 was a material having the composition formula A3B5O 12 A composite oxide in which the metal element A is Y and the metal element B is Al was used to prepare a pellet-shaped polycrystalline body 2, which was then subjected to polarization treatment to form an electret. 12 (hereinafter referred to as YAO) was used as Example 1, and dopant-substituted YAO in which a portion of the Y at the A site was substituted with 1 atm % Mg and 5 atm % Sr as a dopant element D was used as Examples 2 and 3 (Mg 1%: YAO, Sr 5%: YAO).

[0037] <Preparation of Composite Oxide Powder> Powder reagents (raw material powders: YO, Al, MgO, SrO) shown in Table 1 were prepared as starting materials and weighed. For each of Examples 1 to 3, the raw material powders were weighed to the desired composition and placed in a glass tube. 30 g of grinding balls (ZrO; φ2 mm) and 30 ml of ethanol were then added and ground and mixed using a ball mill at 300 rpm for 24 hours. The solution, excluding the grinding balls, was then transferred to a Petri dish and dried for 24 hours. The dried powder was then placed in an agate mortar and crushed, then placed in an alumina crucible and calcined. The calcination conditions were as follows: First, the temperature was increased to 1400°C at a rate of 2°C / min, held at 1400°C for 8 hours, and then cooled to room temperature at a rate of 2°C / min to obtain a calcined powder.

[0038]

[0039] <Preparation of Sintered Pellets (Polycrystalline Body 2)> The calcined powder sample was placed in an agate mortar and pulverized, then further classified (<100 μm) to obtain a molding powder. Approximately 0.55 g of the molding powder was placed in a φ13 mm pellet molder and pressed at 250 MPa for 3 minutes to form a disk-shaped pellet. The resulting molded pellet was then placed in an alumina crucible and sintered. The firing conditions were as follows: First, the temperature was increased to 1700°C at a rate of 2.5°C / min and held at 1700°C for 2 hours. Then, the temperature was decreased to room temperature at a rate of 2.5°C / min. The sintered pellet thus obtained had an outer diameter of approximately φ11 mm and a thickness of approximately 1.3 mm.

[0040] <Polarization Treatment> The polycrystalline bodies 2 of Examples 1 to 3 were each subjected to polarization treatment using the method shown in FIG. 1 . The polycrystalline bodies 2 were sandwiched between a pair of insulating sheets 31 processed to a diameter of 11 mm and brought into contact with a pair of electrodes (not shown) connected to a DC power supply 100 of the polarization treatment device. The insulating sheets 31 had an Au film 32 formed on the side that contacted the electrodes. The polarization tool with the polycrystalline bodies 2 placed between the pair of electrodes was placed in a box furnace and left until the temperature inside the furnace stabilized at 200°C. Next, while the temperature remained stable at 200°C, a DC electric field of 8.0 kV / mm was applied between the pair of electrodes, performing polarization treatment for 3 minutes. After the predetermined treatment time had elapsed, the polycrystalline bodies 2 were allowed to cool to below 40°C while the DC electric field was still applied.

[0041] <Surface Potential Measurement> The surface potential of each of the electrets 1 of Examples 1 to 3 obtained in this manner was measured immediately after the polarization treatment. For the measurement, a surface potential meter (MODEL 341-B: manufactured by Trek Japan Co., Ltd.) was used to measure the surface potential in a non-contact manner. Following the measurement immediately after the polarization treatment, heating was performed using a drying oven, and the surface potential was measured after heating at 100°C for 30 minutes and then at 200°C for 30 minutes, and the influence of the thermal environment was evaluated. The results are shown in Figure 3, immediately after polarization, after heating at 100°C for 30 minutes, and after heating at 200°C for 30 minutes.

[0042] (Comparative Example 1) For comparison, a composite oxide (YAlO3) having a perovskite-type composition containing the same metal elements as in Example 1 was used as a conventional electret material, and a polycrystalline body 2 was prepared using raw material powder weighed to obtain a predetermined composition in the same manner, and then subjected to polarization treatment to form an electret. The surface potential of the electret 1 of Comparative Example 1 obtained in this manner was measured in the same manner as in Example 1 immediately after the polarization treatment (hereinafter referred to as "immediately after polarization"), after heating at 100°C for 30 minutes (hereinafter referred to as "after heating at 100°C"), and after heating at 200°C for 30 minutes (hereinafter referred to as "after heating at 200°C"). The results are shown in FIG. 3 in comparison with Examples 1 to 3.

[0043] As shown in Figure 3, the surface potential (absolute value) of the electrets 1 of Examples 1 to 3 immediately after polarization is in the range of 3.5 kV to over 4.5 kV, which is higher than the value of the electret 1 of Comparative Example 1, which is over 3 kV. In addition, the tendency after heating differs depending on the presence or absence of the dopant element D, and the surface potential (absolute value) of the electret 1 of Example 1 (unsubstituted: YAO) is the highest value of over 4.5 kV immediately after polarization. After heat treatment, the surface potential (absolute value) decreases slightly, but remains stable at around 4 kV, and after heating at 200 ° C., it is actually higher than after heating at 100 ° C.

[0044] The surface potential (absolute value) of the electret 1 of Example 2 (1% Mg:YAO) and the electret 1 of Example 3 (5% Sr:YAO) immediately after polarization was in the range of about 3.5 kV to 4 kV, but after heating at 100 ° C, it improved to about 6 kV to 7 kV, a value nearly twice as high. Furthermore, after heating at 200 ° C, although it decreased compared to after heating at 100 ° C, it maintained a value of about 4 kV to 5 kV, which was equal to or higher than immediately after polarization. In contrast, the surface potential (absolute value) of the electret 1 of Comparative Example 1 was slightly higher after heating at 100 ° C, but after heating at 200 ° C, it decreased significantly to about 1.5 kV, halving.

[0045] These results show that the use of a composite oxide with a garnet-type composition improves the overall surface potential compared to a composite oxide with a perovskite-type composition, and is particularly effective in maintaining the surface potential after heating at 200° C. Furthermore, the dopant element substitution exhibits a significant effect in improving the surface potential after heating at 100° C., which is presumably due to the contribution of oxygen defects introduced into the garnet-type composition.

[0046] The measured values ​​(absolute values) of the surface potential for the electrets 1 of Examples 1 to 3 and Comparative Example 1 are shown in Table 2 together with the band gap energy of the composite oxides of the basic compositions. As shown in Table 2, the composite oxides (YAO, dopant-substituted YAO) of the garnet-type compositions of Examples 1 to 3 have a band gap energy of 4.49 eV, and the composite oxide (YAlO) of the perovskite-type composition of Comparative Example 1 has a band gap energy of 5.54 eV.

[0047]

[0048] (Test Example 1) For the electret 1 (Mg 1%: YAO) of Example 2, the substitution amount X of Mg, which is the dopant element D, was changed in the range of 10 atm% to 80 atm%, as shown in Table 3, to obtain test samples 1 to 7. Sample 1 had the same substitution amount X as the electret 1 of Example 2, and the raw material powder of the electret 1 of Samples 2 to 7 was weighed so that the Mg substitution amount was 10 atm%, 20 atm%, 30 atm%, 40 atm%, 60 atm%, and 80 atm%, respectively. Otherwise, a polycrystalline body 2 was produced in the same manner as in Example 1, and subjected to polarization treatment to form an electret.

[0049]

[0050] As shown in Table 4, a plurality of electrets 1 (a, b) of each of Samples 1 to 7 were prepared, and the surface potential was measured immediately after polarization in the same manner as in Example 1. In addition, the surface potential was measured after heating at 100°C for 30 minutes and then at 200°C for 30 minutes. The measurement results for Samples 1 (1a, 1b) to 7 (7a, 7b) are shown in Table 4, and the surface potentials (absolute values) after heating at 100°C and 200°C are compared in Figure 4. The results for Sample 1a are the same as the results for Example 2 shown in Table 2.

[0051]

[0052] As shown in Table 4 and Figure 4, the surface potential (absolute value) of the electrets 1 of samples 2a, 2b to 6a, and 6b after heating at 100 ° C for 30 minutes is in the range of about 3 kV to 5 kV, which is lower than the value of samples 1a and 1b, which exceeds 6 kV. However, all of them show values ​​that are almost equal to or higher than those immediately after polarization. Samples 7a and 7b also show values ​​that exceed 1.5 kV immediately after polarization, and relatively high values ​​of about 1 kV even after heating at 100 ° C for 30 minutes.

[0053] Furthermore, the surface potential (absolute value) after heating at 200°C for 30 minutes is maintained at a high value in the range of approximately 3 kV to more than 4 kV for the electrets 1 of samples 2a, 2b to 3a, and 3b. In contrast, the electrets 1 of samples 4a, 4b to 6a, and 6b are in the range of approximately 1 kV to 2 kV, and the electrets 1 of samples 7a and 7b are approximately 0.3 kV, and there is a tendency for the surface potential (absolute value) to decrease as the Mg substitution amount increases.

[0054] From these results, when using a composite oxide substituted with a dopant element D, it is preferable to select the substitution amount X of the dopant element D so as to obtain a desired surface potential in the temperature environment of use, taking into consideration that the surface potential (absolute value) after heat treatment changes depending on the substitution amount X of the dopant element D. For example, by setting the substitution amount X in the 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, and it becomes possible to achieve a high surface potential (absolute value) of about 1 kV or more when the substitution amount X is 40 atm% or less, and about 3 kV or more when the substitution amount X is 20 atm% or less.

[0055] (Example 4) For the electret 1 having the configuration shown in FIG. 1, the composition formula A3B5O 12 The electret performance was evaluated using a composite oxide in which the metal elements A and B in the above were changed. The composite oxide was an oxide (hereinafter referred to as GAGO) in which the metal element A was Gd and the metal element B was Al and Ga. A part of the Gd in the A site was substituted with 1 atm% Sr as the dopant element D to form a dopant-substituted GAGO (Sr 1%:GAGO).

[0056] The electret 1 of Example 4 was produced by weighing out the powder reagents (raw material powders: GaO, AlO, GaO, SrO) as starting materials to obtain the desired composition as shown in Table 5, and preparing a pellet-shaped polycrystalline body 2 in the same manner as in Example 1, followed by polarization treatment to obtain an electret. The surface potential of the obtained electret 1 was measured immediately after polarization, after heating at 100°C for 30 minutes, and after heating at 200°C for 30 minutes in the same manner as in Example 1, and the results are shown in Figure 5. For comparison, the results of Comparative Example 1 are also shown in Figure 5.

[0057]

[0058] 5, the electret 1 (1% Sr:GAGO) of Example 4 exhibits a high surface potential (absolute value) of more than 3.5 kV immediately after polarization, and after heating at 100° C. for 30 minutes, it further improves to more than 5 kV. After heating at 200° C. for 30 minutes, although it falls below 3 kV, it shows a value about twice as high as that of the electret 1 of the perovskite composition of Comparative Example 1.

[0059] In this way, even when 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, an electret 1 exhibiting equivalent electret performance can be obtained. From these results, it is presumed that it is important to use a composite oxide having a garnet-type composition, and that this contributes to the development of a surface potential in the electret 1 and the maintenance of the surface potential in a high-temperature environment.

[0060] For the electret 1 of Example 4, the measured values ​​(absolute values) of the surface potential are shown in Table 2 together with the band gap energy of the complex oxide of the basic composition. As shown in Table 2, the complex oxide (GAGO) of the garnet-type composition of Example 4 has a band gap energy (estimated value) of 3.2 eV. Therefore, it can be seen that a complex oxide having a garnet-type composition and a band gap energy of 3 eV or more exhibits a desired high surface potential and has excellent thermal stability.

[0061] (Examples 5 to 7) The electret performance was evaluated for electrets 1 having a garnet-type composition equivalent to that of electret 1 (Mg 1%: YAO) in Example 2 but with different configurations. First, as Example 5, electret 1 having the configuration shown in FIG. 2 was produced by the following method. Electret 1 was produced by forming an amorphous film 20 on a substrate 10 and performing polarization treatment by corona discharge. As the substrate 10, a conductive substrate (0.6 mm) made of SrTiO3 containing 0.5 mass % Nb was prepared.

[0062] <Film formation> A polycrystalline material having a garnet-type composition [(Y 0.99 Mg 0.01 )3Al5O 12 A film containing Y, Mg, Al, and O was formed by sputtering under conditions of 360° C. in an Ar atmosphere using the target. The formed film was amorphous and contained Y, Al, and O in a ratio of approximately 3:5:12, and had a film thickness of 1 μm.

[0063] <Polarization Treatment> Next, the substrate 10 having the amorphous film 20 formed on its surface was placed on the surface of a grounded heater plate, and polarization treatment was performed by corona discharge with the back side of the substrate 10 in contact with the heater plate. The heating temperature by the heater plate and the conditions for the corona discharge treatment were as follows. Corona discharge was performed by placing a discharge needle facing the surface of the amorphous film 20 to be used as a sample. Heating temperature: 200°C Discharge voltage: -5.5 kV Discharge time: 3 minutes Distance between discharge needle and sample: 10 mm

[0064] In addition, as the electret 1 of Example 6, a single crystal having a garnet-type composition [(Y 0.99 Mg 0.01 )3Al5O 12 ] was prepared and subjected to the same polarization treatment to form an electret. Furthermore, a polycrystalline body 2 prepared in the same manner as in Example 2 was subjected to the same polarization treatment to form an electret, which was used as Example 7. The surface potential of each of the thus obtained electrets 1 of Examples 5 to 7 was measured in the same manner as in Example 1, and the results are shown in FIG.

[0065] As shown in Figure 6, the electrets 1 of Examples 6 and 7, which used crystalline oxides, had surface potentials immediately after polarization of -1832 V (single crystal) and -1824 V (polycrystalline), respectively, which were almost equivalent high values. Also, the electret 1 of Example 5, which used a non-crystalline oxide, exhibited a surface potential of -103.5 V (amorphous) immediately after polarization. From these results, it can be seen that by having a garnet-type composition, electretization is possible regardless of the form of the composite oxide, and a practical electret 1 can be obtained.

[0066] In addition, for the electret 1 (Mg 1%: YAO) of Example 2, in order to investigate the influence of the Mg substitution amount on the crystal structure, structural analysis (XRD) was performed using an X-ray diffractometer, and the results are shown in Figure 7. In Figure 7, the XRD analysis results for the electret 1 (sample 1) of Example 2 and the samples used in Test Example 1, with Mg substitution amounts of 20 atm% (sample 3), 40 atm% (sample 5), and 60 atm% (sample 6) are compared and shown.

[0067] As shown in Fig. 7, the results of structural analysis (XRD) using an X-ray diffractometer were compared for the samples used in Test Example 1, with Mg substitution amounts of 1 atm% (Sample 1), 20 atm% (Sample 3), 40 atm% (Sample 5), and 60 atm% (Sample 6). As a result, Samples 1 and 3, with Mg substitution amounts of 20 atm% or less, were found to have a substantially garnet-type composition (YAlO 12 ), and only a slight peak based on the perovskite-type composition (YAlO3) is observed, whereas as the Mg substitution amount increases, a tendency for different compositions to increase is observed. For example, in samples 5 and 6, where the Mg substitution amount is 40 atm% or more, peaks based on the spinel-type composition (MgAl2O4) begin to appear, and in sample 6, where the Mg substitution amount is 60 atm%, the peak based on the perovskite-type or spinel-type composition becomes larger.

[0068] 4, when the Mg substitution amount exceeded 40 atm%, the surface potential (absolute value) after heating tended to decrease, which is presumably due to the influence of an increase in crystals other than garnet-type crystals. Furthermore, when the Mg substitution amount was 20 atm% or less, the surface potential remained stable even after heat treatment at 200°C, which indicates that the thermal stability was improved by increasing the amount of crystals with a garnet-type composition. It has also been confirmed that a similar tendency occurs when the dopant element is Sr.

[0069] In this way, by using a crystalline or amorphous oxide that is a composite oxide having a garnet-type composition and has a band gap energy of 3 eV or more, it is possible to improve the thermal stability of the electret 1. Therefore, when producing the electret 1, it is possible to make a highly practical electret 1 by appropriately selecting a form such as a single crystal, a polycrystalline body, or an amorphous film depending on, for example, the use environment, the required shape or performance, etc.

[0070] Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments or structures. The present disclosure also encompasses various modifications and modifications within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and spirit of the present disclosure.

[0071] The present disclosure is not limited to the above-described embodiments, and can be applied to various embodiments within the scope of the present disclosure. The features of the present disclosure are as follows: [1] An electret (1) obtained by subjecting a composite oxide containing two or more metal elements to a polarization treatment, wherein the composite oxide contains two different trivalent metal elements A and B and has a composition formula ABO 12An electret that is a crystalline or amorphous oxide having a basic composition represented by the formula (I) and has a band gap energy of 3 eV or more. [2] The electret according to [1], wherein the metal element A includes at least one element selected from trivalent rare earth elements, and the metal element B includes at least one element selected from trivalent main group elements. [3] The electret 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. [4] The electret according to any one of [1] to [3], wherein at least one of the metal elements A and B is partially substituted with a dopant element D having a lower valence than the metal elements A and B. [5] The electret according to [4], wherein the substitution amount X of the metal elements A and B with the dopant element is 60 atm% or less when the total of the metal elements A and B is 100 atm%. [6] The electret according to [4] or [5], wherein a portion of the metal element A is substituted with the dopant element selected from alkaline earth metal elements, and the substitution amount 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 [6], wherein the substitution amount X1 of the metal element A by the dopant element is in the range of 0 atm% < X1 ≦ 40 atm%. [8] The electret according to any one of [4] to [7], wherein a portion of the metal element B is substituted with the dopant element selected from alkaline earth metal elements and Zn, and the substitution amount 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 [8], wherein the substitution amount X2 of the metal element B by the dopant element is in the range of 0 atm% < X2 ≦ 40 atm%.

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

Claims

1. An electret (1) obtained by polarizing a complex oxide containing two or more metal elements, wherein the complex oxide contains two different trivalent metal elements A and B and has a composition formula of ABO. 12 The electret is a crystalline or amorphous oxide having a basic composition represented by the formula: and having a band gap energy of 3 eV or more.

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

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

4. The electret according to claim 1 or 2, wherein at least one of the metal elements A and B is partially substituted with a dopant element D having a lower valence than the metal elements A and B.

5. The electret according to claim 4, wherein the amount X of substitution of said metal elements A and B by said dopant element is 60 atm % or less when the sum of said metal elements A and B is 100 atm %.

6. The electret according to claim 5, wherein a portion of the metal element A is substituted with the dopant element selected from alkaline earth metal elements, and the amount X1 of substitution 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 substitution amount X1 of the metal element A by the dopant element 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 substituted with the dopant element selected from an alkaline earth metal element and Zn, and the amount X2 of substitution 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 X2 of the metal element B substituted with the dopant element 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, a polycrystal, or an amorphous film.

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