Electret

Abstract

An electret (1) is obtained by charging an inorganic dielectric (2). The inorganic dielectric (2) is composed of a metal compound containing at least one of Al and Mg, and the total molar fraction of Al and Mg among all metal elements contained in the metal compound exceeds 50%. The metal compound contains a metal oxide or a metal salt and can be an oxide or salt containing Al, an oxide or salt containing Mg, or a composite oxide containing Al and Mg.

Description

electret CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Patent Application No. 2023-119797, 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] For example, organic polymer materials containing fluorine are commonly used as materials for electrets, and they have the advantage of excellent controllability of shape and film thickness when forming thin films. However, concerns about the thermal stability of the surface potential and the deterioration of performance over time have led to the study of inorganic compound materials with excellent thermal stability, and for example, electrets using bulk sintered bodies of hydroxyapatite have been proposed. It is believed that hydroxyapatite exhibits a high surface potential after polarization treatment due to defects in hydroxide ions caused by sintering and dehydration treatment.

[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] Electrets using hydroxyapatite generate crystal defects during the high-temperature dehydration process, making it difficult to adjust the amount of dehydration or control the number of defects. Furthermore, the sintering process required makes it difficult to form a thin film on a substrate. Therefore, when researchers considered applying complex oxides with a perovskite structure to a typical integrated circuit manufacturing process, they found that the surface potential of the electret decreases over time and at high temperatures. Because the creation of devices such as vibration-generating elements involves a heat treatment process, there is a need for electrets with higher thermal stability and that can suppress the decrease in surface potential over time.

[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 and temporal stability.

[0009] One aspect of the present disclosure is an electret obtained by charging an inorganic dielectric (2), wherein the inorganic dielectric is made of a metal compound containing at least one of Al and Mg, and the total molar fraction of Al and Mg in all metal elements contained in the metal compound exceeds 50%.

[0010] The electret having the above configuration uses a metal compound in which the total molar fraction of Al and Mg exceeds 50% as the inorganic dielectric to be charged, and it has been found 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 the same effect is not seen, for example, even in an oxide containing Al, if the molar fraction is 50% or less, it is presumed that the high proportion of Al and Mg in the metal elements contributes to improved 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 and temporal stability.

[0012] The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which Fig. 1 is a schematic diagram showing the general configuration of an electret in embodiment 1, Fig. 2 is a schematic diagram showing the general configuration of an electret in embodiment 2, Fig. 3 is a diagram showing the relationship between the thickness and surface potential of an inorganic dielectric in examples, Fig. 4 is a diagram showing the relationship between the metal compound constituting the inorganic dielectric and the band gap energy and the relative dielectric constant in examples and comparative examples, Fig. 5 is a diagram showing the relationship between the heat treatment of an electret and the surface potential in examples and comparative examples, Fig. 6 is a diagram showing the relationship between the elapsed time after electretization and the surface potential in examples and comparative examples, Fig. 7 is a diagram showing the relationship between the elapsed time after electretization and the surface potential in examples and comparative examples, Fig. 8 is a diagram showing the relationship between the elapsed time after electretization and the surface potential in comparative examples, and Fig. 9 is a diagram showing the relationship between the heat treatment of an electret and the surface potential in examples.

[0013] (Embodiment 1) Embodiment 1 of the electret will be described with reference to Fig. 1. As shown in Fig. 1, the electret 1 of this embodiment is obtained by charging an inorganic dielectric 2 made of a metal compound. The metal compound that becomes the inorganic dielectric 2 is a compound containing at least one of Al and Mg as a metal element, and the total molar fraction of Al and Mg in the total metal elements exceeds 50%. In other words, the metal compound has a composition in which the molar fraction of metal elements other than Al and Mg is 50% or less.

[0014] When the total molar fraction of the metal elements contained in the metal compound is 100%, the total molar fraction of Al and Mg is greater than 50%, which makes it possible to stably maintain a high surface potential generated by polarization. Specifically, an electret 1 is obtained that is less likely to experience performance degradation over long periods of use, even in high-temperature environments exceeding 100°C. The form of the inorganic dielectric 2 is not particularly limited, and here it is formed as a thin film on the substrate 10.

[0015] The electret 1 is a charged substance that holds a positive or negative charge on its surface and provides an electrostatic field to the surrounding area, and a surface potential can be generated by subjecting the inorganic dielectric 2 to a charging treatment described later. Such an electret 1 can be applied to various devices that convert mechanical energy into electrical energy and vice versa, and can be used, for example, as an integrated circuit-embedded power generating element in an electrostatic vibration power generator that uses environmental vibration as a driving source to obtain an output of about microwatts to milliwatts.

[0016] Specifically, the metal compound may be a metal oxide or a metal salt. The metal oxide or metal salt may contain metal elements other than Al and Mg, as long as the compound composition is selected so that the main metal element is Al, Mg, or both, and the total molar fraction is more than 50%.

[0017] Preferably, from the viewpoint of stably maintaining a high surface potential thermally and over time, the metal compound has a total molar fraction of Al and Mg of 60% or more. More preferably, the metal compound can be selected so that the total molar fraction of Al and Mg is 65% or more, and even more preferably, the metal elements contained in the metal compound are almost entirely Al and Mg. Here, "almost entirely Al and Mg" means, for example, that the total molar fraction is 90% or more, preferably 99% or more, and unavoidable impurities based on the starting materials, etc., may be contained in an amount of, for example, about 1%.

[0018] Such a metal compound can be selected from an oxide or salt containing Al, an oxide or salt containing Mg, or a composite oxide containing one or both of Al and Mg. The salt containing Al or the salt containing Mg is not particularly limited and can be appropriately selected from, for example, nitrides, hydroxides, chlorides, carbonates, sulfates, nitrates, etc.

[0019] Specifically, examples of oxides containing Al include Al2O3 (alumina), and examples of salts containing Al include aluminum salts such as AlN, Al(OH)3, and Al2(SO4)3. Examples of oxides containing Mg include MgO (magnesia), and examples of salts containing Mg include magnesium salts such as MgCO3, MgSO4, Mg(NO3)2, Mg(OH)2, and MgCl2. When these metal oxides or salts are used, the molar fraction of Al or Mg in all metal elements is 100%.

[0020] Examples of composite oxides containing one or both of Al and Mg include MgAl2O4 (spinel), or oxides having a spinel-type basic composition represented by the following composition formula 1 (hereinafter referred to as spinel-type oxides), Composition formula 1: X1Al2O4 (wherein X1 is a divalent metal element), or oxides having a garnet-type basic composition represented by the following composition formula (2) (hereinafter referred to as garnet-type oxides), etc. Composition formula 2: X23Al5O 12 (wherein X2 is a trivalent metal element)

[0021] In composition formula 1, the divalent metal element X1 can be at least one element selected from alkaline earth metal elements such as Mg and Sr, or transition metal elements such as Fe, Zn, and Mn. Specific examples of spinel-type oxides having such a composition include MgAlO (spinel), SrAlO, FeAlO, ZnAlO, and MnAlO. In such cases, the total molar fraction of Al and Mg in all metal elements is 66.7% to 100%.

[0022] In composition formula 2, the trivalent metal element X2 can be, for example, Y or at least one element selected from rare earth elements selected from lanthanides such as La, Sm, and Gd. Specific examples of garnet-type oxides having such a composition include YAlO 12 In this case, the molar fraction of Al in all the metal elements is 62.5%.

[0023] Here, the spinel-type oxide or garnet-type oxide does not necessarily have to have a spinel-type crystal structure or a garnet-type crystal structure as long as the ratio of the constituent elements X1 or X2 to Al and O is approximately 1:2:4 or 3:5:12 overall. In such cases, the oxide does not necessarily have to have a uniform composition overall, and may have a composition partially different from the basic composition.

[0024] In addition, in the garnet-type oxide, a portion of the trivalent metal element X2 may be substituted with a dopant element. Examples of the dopant element include alkaline earth metal elements such as Mg and Sr. When Mg is used as the dopant element, the total molar fraction of Al and Mg increases depending on the amount of substitution. When a dopant element is introduced, for example, it is desirable that the substitution ratio of the dopant element substituting X2 be 20 atm% or less.

[0025] The electret 1 is obtained by subjecting the inorganic dielectric 2 made of such a metal compound to a charging treatment. As a charging method, a method using corona discharge or the like to apply a voltage between a ground electrode and a counter electrode connected to the inorganic dielectric 2 can be adopted. In addition to the method using corona discharge, charging can also be performed by a thermal electret method in which a high voltage is applied at high temperature.

[0026] The metal compound may be either a crystalline compound or an amorphous compound, and the shape and form of the inorganic dielectric 2 can be appropriately selected depending on the application and required properties. For example, as shown in FIG. 1, it can be formed as an amorphous film on a substrate 10, and a thin film of 0.1 mm (100 μm) or less can be achieved. A film of the inorganic dielectric 2 having a desired thickness, preferably in the range of about 0.01 μm to several tens of μm, can be easily formed by, for example, a sputtering method.

[0027] When a sputtering method or the like is used as a film formation method, a polycrystalline body of a desired metal compound is used as a target, and a high voltage is applied in an inert gas to form an amorphous film of the same composition. The film formation method is not limited to the sputtering method, and any method can be used. Other methods include physical vapor deposition, chemical vapor deposition, sol-gel methods, and deposition methods, and can be selected appropriately taking into account the desired film quality and film thickness.

[0028] The substrate 10 is not particularly limited, and may be, for example, a conductive substrate made of (Nb,Sr)TiO3, Si, or the like. Alternatively, a conductive substrate made of a conductive material such as metal, or an insulating substrate made of a glass material, or the like, may be used. The substrate 10 and the inorganic dielectric 2 may have any external shape (for example, a rectangular plate or a disk).

[0029] The form of the inorganic dielectric 2 is not limited to the amorphous film described above, but may be a crystalline film such as a polycrystalline film. Alternatively, a plate-shaped single crystal or polycrystalline body may be used as the inorganic dielectric 2. An example thereof is shown below.

[0030] (Embodiment 2) An embodiment 2 relating to an electret will be described with reference to Fig. 2 and Fig. 3. 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.

[0031] In this embodiment, the substrate 10 of the first embodiment is not used, and instead, as shown in Fig. 2, a single crystal 11 having a predetermined thin plate shape is used as the inorganic dielectric 2. The single crystal 11 may be any of the various metal compounds described above, and for example, an alumina single crystal having a sapphire (α-Al2O3) structure may be used. The single crystal 11 is not limited to a single crystal 11, and may also be a polycrystalline body.

[0032] As shown in the upper part of Figure 2, the plate-shaped single crystal 11 can be used alone, but as shown in the lower part, a plurality of plate-shaped single crystals 11 can be stacked together to form a laminate to form a laminated inorganic dielectric 2. In this case, a predetermined number of plate-shaped single crystals 11 can be made into electrets and then stacked to form an integrated body. By making the electret 1 a laminated body, the surface potential of the integrated inorganic dielectric 2 becomes approximately equal to the sum of the surface potentials of the single crystals 11, and a high surface potential corresponding to the number of layers can be obtained.

[0033] As will be described in detail later (see Example 2), it has been found in Fig. 3 that the surface potential of the inorganic dielectric 2 does not change significantly depending on its thickness. Therefore, when a laminated inorganic dielectric 2 is used, the plate thickness of each single crystal 11 and the number of laminated sheets can be appropriately combined to achieve the desired performance depending on the application, etc. Preferably, by setting the plate thickness of each single crystal 11 to 100 µm or less, for example, about several tens of µm, the electret performance can be improved while suppressing an increase in the size of the inorganic dielectric 2.

[0034] For example, if ten 50 μm single crystals 11 are used to form a laminate, the total thickness of the inorganic dielectric 2 will be 0.5 mm (500 μm), making it possible to obtain a surface potential up to ten times higher than when single single crystals 11 of the same thickness are used. Note that, in such a laminate, it has been confirmed that adjacent single crystals 11 are closely attached to each other and integrated, and an electret 1 that can efficiently exhibit the desired electret performance with a simple configuration can be obtained.

[0035] As shown in Fig. 4, in these embodiments 1 and 2, the metal compound used as the inorganic dielectric 2 preferably has a band gap energy and a relative dielectric constant within a predetermined range. Here, a metal compound having a band gap energy of 2.9 eV or more is exemplified, and by using a metal compound having a relatively large band gap energy, the breakdown voltage of the inorganic dielectric 2 increases. Therefore, it becomes possible to apply a high voltage during the charging process and develop a desired high surface potential (for example, 1000 V or more).

[0036] FIG. 4 shows the relationship between various metal oxides including the metal compounds used in the inorganic dielectric 2 described above, and the band gap energy and the relative dielectric constant. Among the metal oxides, the example samples (MgO, Al2O3, MgAl2O4, Y3Al5O4) shown in the figure with a circle are 12 ) all have a band gap energy of 4 eV or more and a relative dielectric constant in the range of 9 to 25. Furthermore, in evaluation tests in the examples described in detail below, it has been confirmed that the following 1) to 3) are satisfied (see Examples 3 to 6). 1) The absolute value of the surface potential is 1000 V or more. 2) The decrease in surface potential due to heating at 200°C after electretization is 10% or less. 3) The change in surface potential over time is small.

[0037] Figure 4 also shows the band gap energy and dielectric constant of comparative samples, including metal oxides with a total molar fraction of Al and Mg of 50% or less (LiAlO2, LaSrAlO4, NdCaAlO4, YAlO3) and metal oxides that do not contain Al or Mg (LiGaO2, ZnO, ZrO2, TiO2). All of these comparative samples have band gap energies of 2.9 eV or more and dielectric constants ranging from 2 to over 100. The dielectric constant is the ratio of the dielectric constant (ε) of each sample to the dielectric constant of a vacuum (ε0), expressed by the following formula: Dielectric constant = Dielectric constant of material (ε) / Dielectric constant of vacuum (ε0)

[0038] It has been confirmed that the metal oxides of these comparative samples do not satisfy at least one of the above conditions 1) to 3). In particular, comparative samples (ZnO, ZrO) that do not contain Al or Mg exhibit band gap energy and dielectric constant values ​​relatively close to those of the example samples, but they hardly exhibit a surface potential. Furthermore, when the dielectric constant is significantly outside the range equivalent to that of the example samples, they do not exhibit a sufficient surface potential, regardless of the presence or absence of Al or Mg (LiAlO, LiGaO, TiO). Furthermore, when Al or Mg is contained but the dielectric constant is within the range equivalent to that of the example samples, although an initial surface potential is obtained, a decrease in the surface potential due to heating is observed, or stability over time is not achieved (see Figures 5 to 9). Details of these will be described later.

[0039] These results show that, in order to achieve a desired surface potential, a metal compound having a band gap energy of 2.9 eV or more and containing Al or Mg is advantageous. Specifically, a metal compound having a total molar fraction of Al or Mg of more than 50% can achieve a surface potential that is stable over time and temperature, and preferably has a band gap energy of 4 eV or more and a relative dielectric constant in the range of 9 to 25.

[0040] In this way, by using an inorganic dielectric 2 whose relative dielectric constant and band gap energy are within a predetermined range, it is possible to obtain an electret 1 that has a desired high surface potential, stability of the surface potential at high temperatures, and stability of the surface potential over time.

[0041] Example 1 An electret 1 having the configuration shown in Fig. 1 was produced by the following method, and its electret performance was evaluated. The electret 1 was prepared by forming a film made of a composite oxide with a spinel composition containing Mg, Al, and O as the inorganic dielectric 2 on a substrate 10, and then subjecting the film to a charging treatment. As the substrate 10, a conductive substrate (0.5 mm) made of SrTiO3 containing 0.5 mass % Nb was prepared.

[0042] <Film formation> A film containing Mg, Al, and O was formed on the surface of the substrate 10 by sputtering using a previously prepared spinel (MgAlO) polycrystal as a target in an Ar atmosphere at 360°C. The formed film was an amorphous film containing Mg, Al, and O in a ratio of approximately 1:2:4, and had a thickness of 0.3 μm. Subsequently, a heat treatment was performed in an air atmosphere at 600°C for 10 hours to form a thin film that would become the inorganic dielectric 2.

[0043] <Conversion to electret> Next, the substrate 10 on whose surface a thin film that would become the inorganic dielectric 2 was formed was placed on the surface of a grounded heater plate, and a charging treatment by corona discharge was carried out 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 charging treatment by corona discharge were as follows. Corona discharge was carried out by placing a discharge needle facing the surface of the inorganic dielectric 2 that would become the sample. Heating temperature: 200°C Discharge voltage: -5.5 kV Discharge time: 3 minutes Distance between discharge needle and sample: 10 mm

[0044] <Surface Potential Measurement> The surface potential of the thus obtained electret 1 (Example 1) was measured. For the measurement, a known surface potential meter (MODEL 341-B: manufactured by Trek Japan Co., Ltd.) was used to measure the surface potential in a non-contact manner. At this time, the surface potential was measured immediately after electretization by charging treatment, after heating at 100°C for 30 minutes, and after further heating at 200°C for 30 minutes, and the influence of the thermal environment and the passage of time was evaluated.

[0045] As a result, ions charged by corona discharge collide with the exposed surface of the inorganic dielectric 2 formed on the substrate 10, and become negatively charged, thereby becoming an electret. The obtained electret 1 had a surface potential (absolute value) of about 200 V immediately after the charging process, and even after being heated at 100°C and then further heated at 200°C, it maintained a surface potential (absolute value) of over 100 V. In this way, it was confirmed that high thermal stability can be obtained by using an amorphous film with a spinel type composition.

[0046] (Example 2) An electret 1 having the configuration shown in Figure 2 was produced by using a metal oxide containing Al as the inorganic dielectric 2 and performing a charging process. The metal oxide containing Al was an alumina single crystal with a sapphire (α-Al2O3) structure, and 10 mm square substrates with various thicknesses were prepared as Samples 1 to 5 for evaluation. For each of Samples 1 to 5, a plurality of samples with the following thicknesses were prepared. Sample 1: 50 μm Sample 2: 100 μm Sample 3: 200 μm Sample 4: 300 μm Sample 5: 625 μm

[0047] Each of these samples 1 to 5 was placed on a grounded heater plate in the same manner as in Example 1 and subjected to a charging treatment by corona discharge under the following conditions: Heating temperature: 200°C Discharge voltage: -5.5 kV Discharge time: 3 minutes Distance between discharge needle and sample: 10 mm

[0048] The surface potential of the electrets 1 of Example 2 was measured in the same manner as in Example 1. The measurements were carried out immediately after the electretization, after heating at 100°C for 30 minutes, and after heating at 200°C for 30 minutes, and the respective results are shown in Figure 3.

[0049] 3, the electrets 1 of Samples 1 to 5 using alumina single crystals all exhibit a high surface potential (absolute value) of over 3000 V immediately after electretization, with little variation. After heating at 100°C and 200°C, a decrease in surface potential was observed overall, with somewhat larger variations, but a high surface potential (absolute value) of over 2000 V to over 3000 V was maintained, demonstrating excellent thermal stability.

[0050] Furthermore, since there is almost no difference in surface potential depending on the thickness of Samples 1 to 5, it is inferred that the charge due to corona discharge is concentrated in the surface layer of the electret 1 (for example, within 50 μm). Therefore, for example, as shown in the upper part of Figure 2, when an alumina single crystal is used alone, by making it into a thin plate with a thickness of about 50 μm or more, approximately the same characteristics can be obtained regardless of the thickness. Furthermore, as shown in the lower part of Figure 2, when alumina single crystals are stacked to form the electret 1, single crystals of any thickness can be combined to achieve the desired characteristics or stack thickness. As an example, by stacking 10 alumina single crystals of Samples 1 to 5 to form the electret 1, the stack thickness will be in the range of about 0.5 mm to several mm, making it possible to achieve a high surface potential (absolute value) of about 20 kV to 30 kV, which is difficult to achieve with a single sintered body of the same thickness.

[0051] (Examples 3 to 5) Metal oxides of various compositions containing at least one of Al and Mg were used as the inorganic dielectric 2, and electrets 1 were produced by performing a charging process. First, a sample of an alumina single crystal having a sapphire (α-Al2O3) structure similar to that of Example 2 was prepared as a substrate having a size of 10 mm square and a thickness of 0.5 mm (500 μm), and a charging process by corona discharge was performed (Example 3). The charging process was performed under the following conditions, similar to Example 1, by placing the sample on a heater plate and heating it. Heating temperature: 200°C Discharge voltage: -5.5 kV Discharge time: 3 minutes Distance between discharge needle and sample: 10 mm

[0052] Furthermore, for the inorganic dielectric 2 using a magnesia single crystal (Example 4) and a spinel single crystal (Example 5), substrate-shaped samples similar to those in Example 3 were prepared and subjected to the same charging treatment to produce electrets 1. For these electrets 1 of Examples 3 to 5, the surface potential was measured in the same manner as in Example 1. The measurements were performed immediately after electretization, after heating at 100°C for 30 minutes, and then after heating at 200°C for 30 minutes, and the respective results are shown in Figure 5 as immediately after electretization, after heating at 100°C (30 minutes), and after heating at 200°C (30 minutes).

[0053] 5, the electret 1 using the alumina single crystal of Example 3 exhibits a surface potential (absolute value) close to 1500 V both immediately after electretization and after heating at 100° C., and after heating at 200° C., it becomes even higher at over 1500 V. Furthermore, the electret 1 using the magnesia single crystal of Example 4 exhibits a surface potential (absolute value) of around 1800 V, and the electret 1 using the spinel single crystal of Example 5 exhibits a surface potential (absolute value) of around 2000 V, both higher than that of Example 3, and the decrease due to heating is small or even high.

[0054] 5, it was confirmed that the electrets 1 of Examples 3 to 5 all had a surface potential (absolute value) of 1000 V or more, and that the decrease in the surface potential (absolute value) after heating at 200° C. was within 10%. Thus, it can be seen that by using a single crystal containing at least one of Al and Mg, the surface potential after electretization is greatly improved, and the effect of suppressing the decrease in surface potential due to a high-temperature environment is enhanced, resulting in a significant improvement in thermal stability.

[0055] Similar samples were also prepared using the metal oxides of Examples 3 to 5, and the changes in surface potential over time were evaluated. The results are shown in Figure 6. In Figure 6, the elapsed time (unit: hr) on the horizontal axis excludes the 30-minute heating times at 100°C and 200°C after the electretization described above, and the surface potential at the time when these heating times had elapsed is taken as the surface potential at elapsed time 0. Thereafter, the relationship between elapsed time and surface potential was examined up to 2000 hours elapsed.

[0056] 6, the electrets 1 using the metal oxides of Examples 3 to 5 all showed a tendency for the surface potential (absolute value) to decrease over time, but the decrease was not significant and the potential was almost constant by 1000 hours. Furthermore, there was almost no change thereafter, and a high surface potential (absolute value) of 1000 V to over 1500 V was maintained even after 2000 hours.

[0057] (Comparative Example 1) For comparison, a metal oxide containing 50% (molar fraction) or more of metal elements other than Al and Mg was used as the inorganic dielectric 2, and the electret 1 was produced by performing the same charging treatment as in Example 1. As the inorganic dielectric 2 of Comparative Example 1, a NdCaAlO4 single crystal containing Nd, Ca, and Al in equal proportions was used, and a substrate-shaped sample similar to that of Example 3 was prepared. In this case, the molar fraction of Al was about 33.3% (i.e., the total molar fraction of Al and Mg was <50%).

[0058] The surface potential of the electret 1 of Comparative Example 1 was measured in the same manner as in Example 1, and the results are shown in Figure 5. As shown in Figure 5, the surface potential (absolute value) of the electret 1 of Comparative Example 1 immediately after electretization was below 1000 V, which was lower than that of the electrets 1 of Examples 3 to 5. Furthermore, a decrease in the surface potential (absolute value) was observed upon heating at 100°C, and after heating at 200°C, the surface potential (absolute value) was significantly reduced to about 200 V.

[0059] From the results of FIGS. 5 and 6 , it can be seen that by using a metal oxide in which the total molar fraction of Al or Mg exceeds 50% as the inorganic dielectric 2, the electret performance is improved, and an electret 1 can be obtained which exhibits a high surface potential (absolute value) of 1000 V or more and has excellent thermal and temporal stability.

[0060] (Example 6, Comparative Example 2) As the inorganic dielectric 2, a single crystal having a garnet-type composition containing Al and Y (YAlO 12 ) was used to prepare a substrate-shaped sample similar to that of Example 3, and subjected to a charging treatment to produce electret 1 (Example 6). For comparison, electret 1 was produced in the same manner using a single crystal (YAlO3) having a perovskite-type composition containing Al and Y (Comparative Example 2). In Example 6, the proportion (molar fraction) of Al in the total metal elements was 62.5%, and in Comparative Example 2 it was 50%.

[0061] For the electrets 1 of Example 6 and Comparative Example 2, multiple test samples were prepared and tested for stability over time, with the results shown in Figure 7. In this case, similar to Example 1, heating was performed at 100 ° C for 30 minutes and at 200 ° C, and the surface potential of the sample after the heat treatment was determined as the surface potential at the start of the test (elapsed time: 0), and the change over the elapsed time of 4000 to 5000 hours was examined. It has been confirmed that the electret 1 of Example 6 also has a surface potential (absolute value) of 1000 V or more, and that the decrease in the surface potential (absolute value) after heating at 200 ° C is within 10%.

[0062] As shown in Figure 7, the electret 1 of Example 6 had a sample surface potential (absolute value) at the start of the test that varied from about 1000 V to 1800 V, but no significant decrease over time was observed or was relatively small. Therefore, even after 4000 hours had elapsed, the sample showed a high surface potential of about 1000 V to 1500 V. In contrast, the electret 1 of Comparative Example 2 had a sample surface potential (absolute value) at the start of the test that was in roughly the same range, but it rapidly decreased after the start of the test, and at around 2000 hours after the test, it was around 200 V or less, and had almost converged.

[0063] From these results, it can be seen that even when there is no significant difference in the surface potential (absolute value) after electretization, as in Example 6 and Comparative Example 2, in which the constituent elements of the metal oxide that becomes the inorganic dielectric 2 are the same, Comparative Example 2, in which the molar fraction of Al in the metal elements is 50% or less, exhibits a significant decrease in stability over time.

[0064] (Comparative Example 3) For comparison, a LiAlO2 single crystal containing equal proportions of Al and Li was used as the inorganic dielectric 2, and a substrate-like sample similar to that of Example 3 was prepared and subjected to a charging treatment to produce an electret 1. The surface potential of this electret 1 of Comparative Example 3 was measured from immediately after electretization until 15 minutes later, and the results are shown in Figure 8.

[0065] 4 shows the relationship between the band gap energy and the relative dielectric constant for the metal oxides constituting the inorganic dielectric 2 of Examples 3 to 6 and Comparative Examples 1 to 3, and oxides having similar compositions. As described above, the metal oxides of Examples 3 to 6 all have a band gap energy of 4 eV or more and a relative dielectric constant in the range of 9 to 25, whereas the metal oxide of Comparative Example 3 has a relative dielectric constant outside this range.

[0066] As shown in FIG. 8, the surface potential (absolute value) of the electret 1 of Comparative Example 3 immediately after electretization was low at 9.5 V, and after 10 minutes, it dropped to 1 V. As shown in FIG. 4, the electret 1 of Comparative Example 3 had a higher band gap energy than the LiGaO2 single crystal having a similar composition, and was equivalent to Examples 3 to 6. Generally, elements with a small atomic number have a larger band gap energy, so it is believed that the inclusion of Al (or Mg) resulted in a larger band gap energy than metal oxides of similar composition. However, since the molar fraction of Al was 50% or less and the relative dielectric constant was lower than the range equivalent to Examples 3 to 6, it is presumed that the surface potential was not sufficiently developed and stability over time was also reduced.

[0067] (Example 7) Alumina polycrystalline body (α-Al2O3 sintered body) was used as the inorganic dielectric 2, and a substrate-like sample similar to that of Example 3 was prepared and subjected to a charging treatment to produce an electret 1. The surface potential of the electret 1 of Example 7 was measured in the same manner as in Example 1, and the results are shown in Figure 9 together with the results of Example 3.

[0068] 9, the surface potential (absolute value) of the electret 1 of Example 7 immediately after electretization is high, at about 1200 V. The surface potential (absolute value) does not change significantly even after heating at 100° C., but after heating at 200° C., it drops to about 600 V. These results show that when an alumina polycrystalline body is used, it is possible to maintain a relatively high surface potential (absolute value), although it is less stable at high temperatures of 200° C. or higher than the alumina single crystal of Example 3.

[0069] In this way, by using a metal compound containing at least one of Al and Mg as the inorganic dielectric 2, it is possible to greatly improve the thermal and temporal stability of the electret 1. Furthermore, when producing the electret 1, it is possible to appropriately select a form such as a single crystal, a polycrystalline body, or an amorphous film, and by using these forms appropriately depending on, for example, the use environment, the required shape or performance, etc., it is possible to obtain an electret 1 with high practicality.

[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 without departing from the spirit of the present disclosure. The features of the present disclosure are as follows: [1] An electret (1) obtained by charging an inorganic dielectric (2), wherein the inorganic dielectric is composed of a metal compound containing at least one of Al and Mg, and the total molar fraction of Al and Mg in the total metal elements contained in the metal compound exceeds 50%. [2] The electret according to [1], wherein the metal compound is a metal oxide or a metal salt. [3] The electret according to [1] or [2], wherein the metal compound is an oxide or salt containing Al, an oxide or salt containing Mg, or a composite oxide containing one or both of Al and Mg. [4] The electret according to [3], wherein the composite oxide is an oxide having a spinel-type basic composition represented by the following composition formula 1, or an oxide having a garnet-type basic composition represented by the following composition formula 2: Composition formula 1: X1Al2O4 (wherein X1 is a divalent metal element) Composition formula 2: X23Al5O 12(wherein X2 is a trivalent metal element) [5] The electret according to [4], wherein in the composition formula 1, X1 is at least one element selected from alkaline earth metal elements or transition metal elements. [6] The electret according to [4] or [5], wherein in the composition formula 2, X2 is at least one element selected from rare earth elements. [7] The electret according to any one of [1] to [6], wherein the total molar fraction of Al and Mg in the total metal elements contained in the metal compound exceeds 60%. [8] The electret according to any one of [1] to [7], wherein the metal compound has a relative dielectric constant in the range of 9 or more to 25 or less. [9] The electret according to any one of [1] to [8], wherein the metal compound has a band gap energy of 4 eV or more.

[10] The electret according to any one of [1] to [9], wherein the inorganic dielectric is a single crystal, a stack of single crystals, a polycrystalline body, or an amorphous film.

Claims

1. An electret (1) obtained by charging an inorganic dielectric (2), wherein the inorganic dielectric is made of a metal compound containing at least one of Al and Mg, and the total molar fraction of Al and Mg in the total metal elements contained in the metal compound exceeds 50%.

2. The electret according to claim 1, wherein the metal compound is a metal oxide or a metal salt.

3. The electret according to claim 2, wherein the metal compound is an oxide or salt containing Al, an oxide or salt containing Mg, or a composite oxide containing one or both of Al and Mg.

4. The electret according to claim 3, wherein the composite oxide is an oxide having a basic composition represented by the following composition formula 1, or an oxide having a basic composition represented by the following composition formula 2. Composition formula 1: X1Al2O4 (wherein X1 is a divalent metal element) Composition formula 2: X23Al5O 12 (wherein X2 is a trivalent metal element) 5. The electret according to claim 4, wherein in said composition formula 1, X1 is at least one element selected from alkaline earth metal elements or transition metal elements.

6. The electret according to claim 4, wherein in said composition formula 2, X2 is at least one element selected from rare earth elements.

7. The electret according to claim 1 or 2, wherein the total molar fraction of Al and Mg in the total metal elements contained in the metal compound exceeds 60%.

8. The electret according to claim 1 or 2, wherein the metal compound has a relative dielectric constant in the range of 9 or more and 25 or less.

9. The electret according to claim 1 or 2, wherein the metal compound has a band gap energy of 4 eV or more.

10. The electret according to claim 1 or 2, wherein the inorganic dielectric is a single crystal, a laminate of single crystals, a polycrystalline body, or an amorphous film.

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