JP7318310B2 - dielectric film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a dielectric film, and more particularly to a dielectric film comprising a composite of polymer and inorganic filler.

A capacitor has a dielectric inserted between two electrodes, and its electrostatic capacity is proportional to the relative permittivity of the dielectric. Ceramics, plastics, insulating oil, mica, and the like are known as dielectrics used in capacitors. In particular, BaTiO ₃ has a high relative permittivity, so BaTiO ₃ is mainly used for the dielectric of small-sized and large-capacity capacitors.

BaTiO ₃ is tetragonal at room temperature (25°C), but has a Curie point (about 125°C) at which the crystal structure changes from tetragonal (ferroelectric) to cubic (paraelectric). Highest dielectric constant. Therefore, the capacitance of a capacitor using BaTiO ₃ varies greatly near the Curie point. However, in order to obtain a dense sintered body of BaTiO ₃ , a high sintering temperature of around 1300°C is required. Furthermore, BaTiO ₃ is poor in workability, so it is difficult to process it into arbitrary or complicated shapes.

On the other hand, plastic films made of polymers such as polypropylene are used as dielectrics in film capacitors. Since the plastic film is flexible, it can be easily wound into a roll. However, since polymers have a low dielectric constant, the number of turns must be increased in order to increase the capacitance of the capacitor. Therefore, there is a problem that the film capacitor is larger than the multilayer ceramic chip capacitor.

On the other hand, by combining a flexible polymer with an inorganic filler having a high dielectric constant, both flexibility and a high dielectric constant can be achieved. Moreover, when a film capacitor is produced using such a composite, the film capacitor can be made smaller than when only a polymer is used. However, composites of polymers and inorganic fillers have a problem of low dielectric breakdown strength.

In order to solve this problem, various proposals have been conventionally made.
For example, Patent Literature 1 discloses a dielectric film in which high-voltage-resistant resin layers made of polyvinylacetoacetal resin are formed on both sides of a composite dielectric layer containing 40% by volume of strontium titanate powder.
In the same document,
(a) When the inorganic component is dispersed in the thermosetting resin, the smoothness of the surface of the composite dielectric layer is reduced, and the electrodes formed on both main surfaces of the composite dielectric layer and the composite dielectric layer are formed. Since electric field concentration occurs in the resin portion between the inorganic component and the resin portion, the withstand voltage is lowered, and (b) high withstand voltage is provided on both sides of the composite dielectric layer that is a composite of the organic component and the inorganic component. It is described that the formation of a flexible resin layer improves the withstand voltage of the dielectric film as a whole.

Further, Patent Document 2 discloses a dielectric film in which amorphous metal oxide particles are dispersed in an organic resin.
In the same document,
(a) When crystalline ceramic particles are dispersed in an organic resin, a sharp change in dielectric constant occurs between the organic resin and the ceramic particles, resulting in a decrease in breakdown electric field strength;
(b) Dispersing amorphous metal oxide particles in an organic resin alleviates the local increase in electric field intensity at the interface and increases the breakdown electric field intensity.

A film capacitor is used in a power control unit (PCU) of a hybrid vehicle or an electric vehicle. Since the film capacitor is a large component of the PCU, miniaturization is desired. In order to miniaturize the film capacitor, it is necessary to improve the maximum energy density. The maximum energy density is positively correlated with the dielectric constant and dielectric breakdown strength of the material, and the higher the dielectric constant and/or the dielectric breakdown strength, the higher the maximum energy density.

In general, as one of the methods for improving the relative dielectric constant of a polymer film, there is a method of blending an inorganic filler having a high dielectric constant into the polymer film to form a composite. However, in general, when an inorganic filler is blended into a polymer, the inorganic filler aggregates in the polymer film, resulting in a large decrease in dielectric breakdown strength. As a result, the maximum energy density is not improved.
Furthermore, in order to improve performance by blending an inorganic filler, it is necessary to uniformly disperse the inorganic filler in the composite film. However, there is no example in which a method for uniformly dispersing an inorganic filler in a composite film has been proposed.

Japanese Patent No. 4893396 JP 2015-201513 A

The problem to be solved by the present invention is to provide a dielectric film comprising a composite of a polymer and an inorganic filler and having high dielectric breakdown strength.

In order to solve the above problems, a dielectric film according to the present invention is summarized as having the following configuration.
(1) The dielectric film is
a substrate made of a polymer;
and an inorganic filler dispersed in the base material.
(2) The dielectric film has a degree of dispersion represented by the following formula (1) of 0.45 or less.
Dispersion = AD _L /L _m (1)
however,
AD _L is the average deviation of the distance between the centers of gravity of the inorganic filler;
L _m is the average value of the distances between the centers of gravity of the inorganic fillers.
(3) The inorganic filler has an average particle aspect ratio of 1.8 or more.
(4) The dielectric film has a dielectric constant of 12 or more.

When producing a dielectric film containing an inorganic filler using a casting method, if there is a step of drying the inorganic filler between the production of the inorganic filler and the production of the dielectric film, the inorganic filler is dried during drying. agglomerate. Aggregation of the once aggregated inorganic filler cannot be completely eliminated even if dispersion treatment is performed in a subsequent step. Therefore, the dielectric film obtained by such a method has a low dielectric breakdown strength due to aggregation of the inorganic filler.

On the other hand, when a dielectric film is produced by a casting method, aggregation of the inorganic filler is suppressed if there is no process for drying the inorganic filler from the production of the inorganic filler to the production of the dielectric film. be. A dielectric film obtained by such a method exhibits high dielectric breakdown strength because the inorganic filler is uniformly dispersed.

It is a schematic diagram for demonstrating the definition of an average particle aspect-ratio. SEM images of samples obtained in Examples 1-2 and Comparative Examples 1-4, as well as dispersity, average particle aspect ratio, dielectric constant @ 10 kHz, and dielectric breakdown strength (BDS).

An embodiment of the present invention will be described in detail below.
[1. dielectric film]
The dielectric film according to the present invention is
a substrate made of a polymer;
and an inorganic filler dispersed in the base material.

[1.1. polymer]
In the present invention, the polymer material is not particularly limited. The polymer may be a thermoplastic resin, or it may be a thermoset resin.
Specific examples of polymers include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, polystyrene, polyphenylene ether, polyphenylene sulfide, polyfluorene, polysulfone, polyethyleneimide, polycarbonate, polyether ether ketone, polyethylene naphthalate, Examples include polyethylene terephthalate, polyimide, polyurethane, cellulose acetate, polyvinyl chloride, polysiloxane, and cyanoethyl cellulose. Any one of these may be used for the polymer, or two or more thereof may be used.

Among these, the polymer is preferably PVDF. This is because it has a higher dielectric constant than other polymers. That is, the dielectric constant of PVDF is 11 at maximum. Therefore, by combining this with an inorganic filler having a high dielectric constant, a dielectric film having a dielectric constant of 12 or more can be easily obtained even when the amount of the inorganic filler added is relatively small. can be done.

[1.2. inorganic filler]
[1.2.1. composition]
In the present invention, the inorganic filler is not particularly limited as long as it is an inorganic compound having dielectric properties. Examples of inorganic fillers include layered perovskite oxide dielectrics, perovskite oxide dielectrics, titanium oxide, niobium oxide, and tantalum oxide. Among these, layered perovskite-type oxide dielectrics are
(a) having a relatively high dielectric constant;
(b) easy synthesis of tabular particles due to the anisotropy of the crystal structure;
Because of these advantages, it is suitable as a material for inorganic fillers.
Here, the term "layered perovskite-type oxide dielectric material" means a perovskite-type structure in which at least four _BO6 octahedrons (where B is one or more transition metal ions) are included in a unit cell. Oxides that have B is particularly preferably Nb, Ta, or Ti.

Specific examples of layered perovskite oxide dielectrics include the following. The inorganic filler may contain any one of these, or may contain two or more.
(a) _{KCa2Nam - 3NbmO3m +1} (m is an integer of 3 or more ).
(b) _{Ca2Nam - 3NbmO3m +1} (m is an integer of 3 or more ).
(c) A Dion Jacobson (DJ) type compound represented by the general formula: A'·A _n-1 B _n O _3n+1 (n is an integer of 1 or more).
(d) A Rudolsden popper (RP) type compound represented by the general formula: _A'm ·A _n-1 B _n O _3n+1 (n and m are integers of 1 or more).
(e) Orlibilius-type compounds represented by the general formula: A' ₂ O ₂ ·A _n-1 B _n O _3n+1 (n and m are integers of 1 or more).
however,
A' is one or more alkali metal ions,
A is any one or more ions selected from the group consisting of alkali metal ions, Group 2 element ions, and rare earth metal ions;
B is one or more transition metal ions;

The inorganic filler is particularly _{KCa2Nam - 3NbmO3m+1} ( _m is an integer of 3 or more ) or _{Ca2Nam - 3NbmO3m+1 (} m is an integer of 3 or more ) is preferred. This is because it has a high relative dielectric constant and is relatively easy to produce a plate-like inorganic filler.

[1.2.2. Average particle aspect ratio]
FIG. 1 shows a schematic diagram for explaining the definition of the average particle aspect ratio.
"Particle aspect ratio" refers to the ratio (=D/t) of the main axis direction length (D) to the main axis direction width (t) of the inorganic filler appearing in the cross section of the dielectric film.
The “main axis direction length (D)” refers to the maximum length of the inorganic filler appearing in the cross section of the dielectric film.
"Principal axis direction width (t)" refers to the maximum value of the length in the direction perpendicular to the principal axis direction length.
"Average particle aspect ratio" refers to the average value of particle aspect ratios measured for 50 or more inorganic fillers appearing in the cross section of the dielectric film.

The average D/t ratio of inorganic fillers influences the relative figure of merit. Here, "relative figure of merit" refers to the ratio of the maximum energy density of a dielectric film containing rectangular fillers to the maximum energy density of a dielectric film containing spherical fillers.
In general, the higher the average D/t ratio, the better the relative figure of merit. To obtain a high relative figure of merit, the average D/t ratio should be 1.8 or greater.
On the other hand, if the average D/t ratio is too large, the flexibility of the film will decrease, making winding difficult. In addition, the slight inclination of the fillers makes it easier for the fillers to come into contact with each other. Therefore, the average D/t ratio is preferably 200 or less. The average D/t ratio is preferably 100 or less, more preferably 20 or less.

[1.2.3. Dielectric constant]
The dielectric constant of the inorganic compound that constitutes the inorganic filler affects the properties of the dielectric film. In general, the higher the dielectric constant of the inorganic compound, the higher the dielectric constant of the dielectric film containing it. In order to obtain a dielectric film having a high dielectric constant (≧12), the inorganic compound preferably has a dielectric constant of 120 or more.

[1.3. Characteristics of Dielectric Film]
[1.3.1. Dispersion]
"Dispersion" refers to a value represented by the following formula (1). The degree of dispersion represented by formula (1) is a measure of the degree of dispersion of the inorganic filler in the polymer, and the lower the degree of dispersion, the more uniformly the inorganic filler is dispersed.
Dispersion = AD _L /L _m (1)
however,
AD _L is the average deviation of the distance between the centers of gravity of the inorganic filler;
L _m is the average value of the distances between the centers of gravity of the inorganic fillers.
The term “distance between the centers of gravity of inorganic fillers” refers to the distance between the centers of gravity of adjacent particles.

The degree of dispersion of the inorganic filler affects the dielectric breakdown strength of the dielectric film. In general, the lower the degree of dispersion (the more uniformly the inorganic filler is dispersed), the higher the dielectric breakdown strength. In order to obtain high dielectric breakdown strength, the degree of dispersion should be 0.45 or less.

[1.3.2. Dielectric constant]
The maximum energy density has a positive correlation with the dielectric constant and dielectric breakdown strength of the dielectric film. Therefore, the higher the dielectric constant, the higher the maximum energy density. In order to improve the maximum energy density, the higher the dielectric constant of the dielectric film, the better.
By optimizing the composition and content of the inorganic filler contained in the dielectric film, the relative permittivity of the dielectric film becomes 12 or more. By further optimizing the manufacturing conditions, the dielectric constant of the dielectric film becomes 15 or more.

[1.3.3. Dielectric breakdown strength]
In order to improve the maximum energy density of the dielectric film, the dielectric breakdown strength of the dielectric film should also be as high as possible.
By optimizing the composition, content, etc. of the inorganic filler contained in the dielectric film, the dielectric breakdown strength of the dielectric film becomes 250 V/μm or more. If the manufacturing conditions are further optimized, the dielectric breakdown strength of the dielectric film will be 300 V/μm or more.

[1.3.4. Content of inorganic filler]
The content of the inorganic filler contained in the dielectric film is not particularly limited, and the optimum content can be selected depending on the purpose. Polymers have a smaller dielectric constant than inorganic fillers. Therefore, if the content of the inorganic filler is too small, the relative dielectric constant of the dielectric film will decrease. Therefore, the content of the inorganic filler is preferably 10 vol % or more. The content of inorganic filler is preferably 20 vol % or more.

On the other hand, if the inorganic filler content is excessive, the inorganic filler tends to aggregate in the film, resulting in a decrease in dielectric breakdown strength. Therefore, the content of the inorganic filler is preferably 40 vol % or less. The content of the inorganic filler is preferably 30 vol % or less.

[1.3.5. Thickness of dielectric film]
The thickness of the dielectric film is not particularly limited, and an optimum thickness can be selected depending on the purpose. In general, if the thickness of the dielectric film becomes too thin, it becomes difficult to handle as a self-supporting film. Therefore, the thickness of the dielectric film is preferably 2 μm or more.
On the other hand, if the thickness of the dielectric film becomes too thick, the capacitance density will decrease. Therefore, the thickness of the dielectric film is preferably 10 μm or less. The thickness of the dielectric film is preferably 6 μm or less, more preferably 4 μm or less.

[2. Manufacturing method of dielectric film]
The dielectric film according to the present invention is
(a) preparing an inorganic filler having a predetermined composition and average particle aspect ratio;
(b) dispersing a polymer and an inorganic filler in a solvent so as to have a predetermined composition to form a slurry;
(c) It can be produced by casting a slurry on the substrate surface and drying the coating film.

[2.1. Inorganic filler production process]
First, an inorganic filler having a predetermined composition and average particle aspect ratio is produced (inorganic filler production step).
The method for producing an inorganic filler that satisfies predetermined conditions is not particularly limited, and an optimum method can be selected according to the composition of the inorganic filler.

For example, when the inorganic filler is _{Ca2Nam - 3NbmO3m +1} (CNN), which is a type of layered perovskite oxide, the inorganic filler having a predetermined average particle aspect ratio is
(a) synthesizing a powder of _{KCa2Nam - 3NbmO3m +1} (KCNN) using a solid state reaction method;
(b) treating KCNN powder with nitric acid to produce _{HCa2Nam - 3NbmO3m +1} (HCNN) powder in which K ⁺ between the layers is exchanged with H ⁺ ;
(b) can be produced by delaminating HCNN powder;

In this case, when the synthesized inorganic filler is dried, the inorganic filler aggregates. Aggregation of the once aggregated inorganic filler cannot be completely eliminated even if dispersion treatment is performed in a subsequent step. Therefore, the inorganic filler thus obtained needs to be subjected to the next step in a state of being dispersed in the solvent (A) without being dried.

[2.2. Slurry preparation process]
Next, the polymer and the inorganic filler are dispersed in the solvent (B) so as to have a predetermined composition to form a slurry (slurry preparation step).
The type of solvent (B) is not particularly limited, and an optimum solvent can be used depending on the purpose. The solvent (B) for preparing the slurry may be the same as or different from the solvent (A) for dispersing the inorganic filler. When the solvent (B) is different from the solvent (A), solvent replacement is performed without drying the inorganic filler.
The polymer concentration and inorganic filler concentration in the slurry are not particularly limited, and optimum concentrations can be selected according to the purpose.

[2.3. Casting process]
Next, the slurry is cast on the substrate surface and the coating is dried (casting step). Thereby, the dielectric film according to the present invention is obtained.
The method and conditions for casting and the method and conditions for drying are not particularly limited, and optimum conditions can be selected according to the purpose.

[3. action]
[3.1. Improvement of dielectric breakdown strength by rectangular filler]
A film capacitor is used in a power control unit (PCU) of a hybrid vehicle or an electric vehicle. Since the film capacitor is a large component of the PCU, miniaturization is desired. In order to miniaturize the film capacitor, it is essential to improve the capacitance density of the material. The capacity is represented by the following formula (1).
Capacity density [F/m ³ ]=ε ₀ ε _r /d ² (1)
however,
ε ₀ : permittivity of vacuum, 8.854×10 ⁻¹² [F/m],
ε _r : Relative permittivity of material,
d: film thickness [m]

In order to improve capacity density, it is necessary to make the film thinner. Along with this, miniaturization of the inorganic filler in the film is also required. However, conventionally used spherical fillers have a size effect, and the smaller the average particle size, the lower the dielectric constant. Therefore, conventional dielectric films cannot provide high-performance capacitor films.

On the other hand, an inorganic filler made of an inorganic compound has a higher dielectric constant than a polymer. In particular, layered perovskite-type oxides have an anisotropic crystal structure, so plate-like particles can be synthesized relatively easily. Further, since the plate-like particles have no size effect, even if the particle size is reduced, the relative dielectric constant is less lowered. Therefore, when such plate-like particles are used as inorganic fillers, dielectric films with higher performance can be produced than when spherical fillers are used.

Specifically, a dielectric film using a plate-like inorganic filler has a higher energy density than a dielectric film using a spherical inorganic filler. As a result, high dielectric breakdown strength is exhibited even when the thickness of the film is reduced. In addition, plate-like inorganic fillers have no particle size effect, so even when the thickness of the film is made thin and the particle size is made small accordingly, a high dielectric constant is exhibited.

Furthermore, when the same volume of inorganic filler is blended in the dielectric film, the ratio of the inorganic filler present in the film thickness direction decreases as the average particle aspect ratio increases. Therefore, a decrease in dielectric breakdown strength in the film thickness direction is suppressed, and the relative figure of merit of the dielectric film is improved. In particular, when the average grain aspect ratio is 1.8 or more, the relative figure of merit is 1.02 or more.

[3.2. Improvement of dielectric breakdown strength by controlling degree of dispersion]
A dielectric film containing an inorganic filler can be produced by, for example, a method of dispersing an inorganic filler and a polymer in a solvent to form a slurry and casting the slurry onto a substrate (casting method). In this case, if there is a step of drying the inorganic filler between the production of the inorganic filler and the production of the dielectric film, the inorganic filler aggregates during drying. Aggregation of the once aggregated inorganic filler cannot be completely eliminated even if dispersion treatment is performed in a subsequent step. Therefore, the dielectric film obtained by such a method has a low dielectric breakdown strength due to aggregation of the inorganic filler.

On the other hand, when a dielectric film is produced by a casting method, aggregation of the inorganic filler is suppressed if there is no process for drying the inorganic filler from the production of the inorganic filler to the production of the dielectric film. be. A dielectric film obtained by such a method exhibits high dielectric breakdown strength because the inorganic filler is uniformly dispersed.
In particular, when the degree of dispersion is 0.45 or less, short-circuited portions due to aggregation of the inorganic filler are reduced. Therefore, a decrease in dielectric breakdown strength is suppressed.

(Examples 1-2, Comparative Examples 1-4)
[1. Preparation of sample]
[1.1. Example 1]
[1.1.1. Preparation of inorganic filler]
First, a powder of KCa ₂ Na _m−3 Nb _m O _3m+1 (KCNN), which is a type of layered perovskite oxide, was synthesized by a solid phase reaction method. The obtained KCNN powder was pulverized (average particle size: 200 nm). Then, the pulverized KCNN powder was treated with 5M nitric acid to obtain _{HCa2Nam - 3NbmO3m +1} (HCNN) powder in which K ⁺ between the layers was replaced with H ⁺ .
The HCNN powder was then treated with an aqueous tetrabutylammonium solution. The molar ratio of H and tetrabutylammonium in the HCNN powder was 1:1. As a result, the HCNN powder was delaminated to obtain a dispersion containing _{Ca2Nam - 3NbmO3m +1} (CNN) filler.
Furthermore, the solvent of the dispersion liquid containing the CNN filler was replaced with N,N-dimethylformamide (DMF) by centrifugation to obtain a DMF dispersion liquid of the CNN filler.

[1.1.2. Preparation of dielectric film]
A predetermined amount of PVDF was dissolved in DMF to obtain a DMF solution of PVDF. A DMF dispersion liquid of CNN filler was added to and mixed with this, and defoamed to obtain a slurry. A dielectric film was obtained by casting this slurry on a glass plate and drying it. The content of inorganic filler was 30 vol %.
In Example 1, the inorganic filler was not dried during the period from the production of the inorganic filler to the production of the dielectric film.

[1.2. Example 2]
A dielectric film was produced in the same manner as in Example 1, except that the DMF dispersion of the CNN filler and the DMF solution of PVDF were mixed so that the content of the inorganic filler was 25 vol %.

[1.3. Comparative Example 1]
A dielectric film was produced in the same manner as in Example 1, except that the CNN filler drying step was performed before the slurry preparation step.

[1.4. Comparative Example 2]
A dielectric film was produced in the same manner as in Comparative Example 1, except that an unpulverized KCNN powder (average particle size: 250 μm) was used as the starting material.

[1.5. Comparative Example 3]
A dielectric film was produced in the same manner as in Comparative Example 1, except that commercially available barium titanate (average particle size: 300 nm) was used as the starting material.

[1.6. Comparative Example 4]
Using FIG. 1 of Patent Document 2, the same evaluation as in Examples 1 and 2 was performed.

[2. Test method]
[2.1. Dispersion]
Photomicrographs of cross-sections of the dielectric films were taken. Next, the distance between the centers of gravity of the inorganic filler was measured using the distance between the centers of gravity method. Furthermore, the degree of dispersion was calculated using the formula (1).
[2.2. Average particle aspect ratio]
The average grain aspect ratio was determined from the photomicrographs taken.

[2.3. Dielectric constant]
Gold electrodes were formed on both sides of the film, and the dielectric constant at 10 kHz was measured using an impedance analyzer.
[2.4. Dielectric breakdown strength]
Gold electrodes were formed on both sides of the film, and the dielectric breakdown strength (BDS) was measured using an ultra-high voltage withstand tester. The voltage when a current of 1 mA flowed was taken as the dielectric breakdown voltage.

[3. result]
FIG. 2 shows SEM images of the samples obtained in Examples 1-2 and Comparative Examples 1-4, as well as the degree of dispersion, the average particle aspect ratio, the dielectric constant @ 10 kHz, and the dielectric breakdown strength (BDS). indicates From FIG. 2, the following can be understood.

(1) Examples 1 and 2 both have high relative permittivity and dielectric breakdown strength. This is probably because the inorganic filler has an average particle aspect ratio of 1.8 or more and a dispersity of 0.45 or less.
(2) Comparative Example 1 has a high dielectric constant but a low dielectric breakdown strength. This is probably because the inorganic filler aggregates and the degree of dispersion exceeds 0.45.
(3) Comparative Example 2 has a high dielectric constant but a low dielectric breakdown strength. This is probably because the average particle aspect ratio of the inorganic filler is high, but the degree of dispersion exceeds 0.45.
(4) Comparative Example 3 has a high dielectric constant but a low dielectric breakdown strength. This is believed to be due to the use of spherical fillers, that is, the average particle aspect ratio being less than 1.8.
(5) Comparative Example 4 has a high dielectric breakdown strength but a low dielectric constant. This is probably because the matrix polymer is polyethylene terephthalate, which has a lower dielectric constant than PVDF, and because the filler content is small.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The dielectric film according to the present invention can be used as a dielectric of capacitors used in PCUs of hybrid vehicles and HV vehicles.

Claims (3)

A dielectric film comprising:
(1) The dielectric film is
a substrate made of a polymer;
and an inorganic filler dispersed in the base material.
(2) The dielectric film has a degree of dispersion represented by the following formula (1) of 0.45 or less.
Dispersion = AD _L /L _m (1)
however,
AD _L is the average deviation of the distance between the centers of gravity of the inorganic filler;
L _m is the average value of the distances between the centers of gravity of the inorganic fillers.
(3) The inorganic filler has an average particle aspect ratio of 1.8 or more.
(4) The dielectric film has a dielectric constant of 12 or more.
(5) The content of the inorganic filler in the dielectric film is 10 vol % or more and 40 vol % or less. 2. The dielectric film of claim 1, wherein said polymer comprises PVDF. The inorganic filler is _{KCa2Nam - 3NbmO3m+1 (} m is an integer of 3 or more ) or _{Ca2Nam - 3NbmO3m+1 (} m is an integer of 3 or more ) The dielectric film according to claim 1 or 2, comprising:

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