Patent ID: 12191088

DESCRIPTION OF EMBODIMENTS

To begin with, examples of relevant techniques will be described.

As an energy harvesting technology for converting energy existing in the environment into electric power, practical application of an energy harvesting element using electrostatic induction of an electret has been studied. An organic polymer material such as a fluororesin is generally used as a constituent material of the electret. For example, a chain-like fluororesin or a polymer having a fluoroaliphatic ring structure in a main chain is used.

As an energy harvesting element using an electret, a vibration powered generator has been proposed. In the vibration powered generator, a movable electrode that faces an electret on a fixed electrode vibrates to change the amount of electric charge induced by the movable electrode, so that the vibration powered generator derives the energy as an electric power. Further, applications of such vibration powered generator to various fields have been studied. For example, an electret is formed by forming a fluororesin polymer film on facing surfaces of a pair of substrates, and electrostatic induction conversion device is configured such that one of the pair of substrates is used as a fixed substrate, and the other of the pair of substrates is used as a movable substrate. The movable substrate is configured to move relative to the fixed substrate. The electrostatic induction conversion device configured as described above is applied to a sensor and an actuator. In the electrostatic induction conversion device, the electret and a conductor are coexist on the facing surfaces and the area ratio between the electret and the conductor is defined.

In such vibration powered generator, the movable electrode facing the electret relatively moves by vibration, which is an external energy, and the capacitance between the electret and the movable electrode fluctuates, so that power can be generated. In other words, when a vibration powered generator is used as a power generator, the power generator cannot generate power during the period when kinetic energy such as vibration is not input into the power generator from the outside even when the power generator is connected to a load such as a sensor and an actuator. Thus, use environment of the power generator is limited. Alternatively, in order to efficiently utilize the environmental vibration, it is necessary to devise the arrangement of the electret and the conductor. Thus, an electrode configuration, a support structure of the movable electrode, and the like tend to be complicated.

It is an objective of the present disclosure to provide, using an electret, a power generator that can generate power even in a usage environment where kinetic energy such as vibration is not input to the power generator from the outside.

According to one aspect of the present disclosure, a power generator includes a power generating unit and at least one of a power storage unit or an output unit. The power generating unit includes an electret having a first charged surface and a second charged surface that are charged with opposite polarities, a first electrode formed on the first charged surface, and a second electrode formed on the second charged surface. The at least one of the power storage unit or the output unit is electrically connected to the power generating unit. The electret is formed by polarizing an electret material that includes an inorganic dielectric having a bandgap energy of 4 eV or more. At least one of the first electrode or the second electrode is partially disposed on the corresponding charged surface such that at least one of the first charged surface or the second charged surface has a portion as a current collecting surface that is exposed outward.

In the power generator having the above configuration, the inorganic dielectric constituting the electret has a high bandgap energy of 4 eV or more, so that the dielectric breakdown voltage during the polarization treatment can be increased. Therefore, for example, a high surface potential can be obtained through application of a high voltage under heating conditions. Furthermore, it was found that power can be taken out when electrodes are formed on opposite charged surfaces of the electret, one of the electrodes is partially arranged on the charged surface such that a part of the charged surface exposed outward, and the electrodes are connected to the power storage unit or the output unit.

The reason is not clear, but it is presumed that charge transfer be possible because the residual polarization of the inorganic dielectric constituting the electret changes over time while floating charges are fixed on the exposed portion of the charged surface, as a current collecting surface, due to the high surface potential of the electret. As a result, it is possible to generate power even during a period in which no external energy is input, and the power generating unit does not require a movable portion, so that a simpler configuration can be achieved.

As described above, according to the above aspect, it is possible to provide a power generator capable of generating power by using an electret even in a usage environment in which kinetic energy such as vibration is not input to the power generator from the outside.

First Embodiment

The first embodiment relating to a power generator will be described with reference to the drawings.

As shown inFIG.1, the power generator1of the present embodiment includes a power generating unit10using an electret2, and at least one of a power storage unit41and an output unit42that is electrically connected to the power generating unit10. The power storage unit41and the output unit42constitute a power supply unit4, and the power generated by the power generating unit10can be supplied to the outside via the power supply unit4.

The electret2of the power generating unit10has a first charged surface21and a second charged surface22that are charged with opposite polarities. A first electrode31is formed on the first charged surface21, and a second electrode32is formed on the second charged surface22.

The electret2is formed by polarizing an electret material that contains an inorganic dielectric having a bandgap energy of 4 eV or more. At least one of the first electrode31or the second electrode32is partially disposed on the corresponding one of the first charged surface21or the second charged surface22such that at least one of the first charged surface21or the second charged surface22has a portion as a current collecting surface20that is exposed outward.

At this time, the current collecting surface20is formed on the electret2, and the power storage unit41or the output unit42is electrically connected at a position between the second electrode32and the first electrode31in contact with the current collecting surface20. Thereby, electric charges accumulated on the current collecting surface20can be derived and stored in the power storage unit41, or output outward through the output unit42.

Hereinafter, specific examples of the electret2used in the power generator1will be described in detail.

Specific configuration examples of the power generating unit10and the power generator1will be described in detail later.

The electret material forming the electret2of the present embodiment contains an inorganic dielectric having a bandgap energy of 4 eV or more as a main component. Here, the “main component” means an electret material composed only of the inorganic dielectric material or an electret material to which some other components are added in the process of converting the inorganic dielectric into the electret material. Further, in the process of converting the electret material into the electret2, other components may be added to the electret material as long as the desired electret characteristics can be obtained.

The electret2is obtained by subjecting such electret material to a polarization treatment. In other words, the electret2is obtained through the polarization treatment of an electret material to develop a surface potential of the electret material and charge the electret material. Here, the electret material is, for example, molded into a plate shape having a predetermined thickness, and converted into an electret through polarization in the thickness direction as a polarization direction X. Thereby, a positive or negative charge is retained on the surface of the electret. As a result, the electret2that provides an electrostatic field around it is obtained.

Here, the up-down direction inFIG.1will be referred to as the polarization direction X of the electret2, and hereinafter, one surface facing in the polarization direction X will be referred to as an upper surface and the opposite surface will be referred to as a lower surface. At this time, the upper surface of the electret2is the first charged surface21and the lower surface of the electret2is the second charged surface22. The first charged surface21and the second charged surface22are charged with opposite polarities. Further, in the electret2, a part of the first charged surface21, which is the upper surface, is exposed to the atmosphere, and the exposed surface serves as the current collecting surface20. The second charged surface22on the lower surface does not have an exposed surface. However, a part of the second charged surface22may be exposed to the atmosphere.

The inorganic dielectric used as the electret material may be an inorganic compound that has an apatite structure containing phosphate ions and hydroxide ions. Apatite is a general term for compounds represented by the composition formula M10(ZO4)6(X)2, wherein ZO4and X in the composition formula correspond respectively to phosphate ion and hydroxide ion. Typically, apatite has a crystal structure whose unit cell is classified into a hexagonal system and space group is P63/m. Apatite often has a non-stoichiometric composition. The inorganic compound having an apatite structure containing phosphate ions and hydroxide ions is a compound represented by a composition formula M10(PO4)6(OH)2. Examples of the metal element M includes divalent alkaline earth metal elements such as Ca.

By using such inorganic compound, the electret2having a high surface potential can be obtained, which realizes a high collection amount of charge. Preferably, hydroxyapatite (HA) is used as the inorganic compound that has the apatite structure containing phosphate ions and hydroxide ions. Hydroxyapatite has a composition formula of (Ca10(PO4)6(OH)2) when satisfying stoichiometry, and has a crystal structure in which the unit cell is classified into the hexagonal system and the space group is P63/m.

It is preferable that the inorganic compound used as the inorganic dielectric material has a hydroxide ion content in the apatite structure less than the stoichiometric ratio. Hydroxyapatite with the hydroxide ion content less than the stoichiometric ratio can be synthesized by sintering a raw material powder having hydroxide ions and phosphate ions at a temperature of more than 1250° C. and less than 1500° C., for example, to have a crystal structure of hexagonal hydroxyapatite. In the process, the content of hydroxide ions becomes less than the stoichiometric ratio. This is due to dehydration from the hydroxyl group through heating of the hydroxyapatite, thereby oxyhydroxyapatite (OHA) is generated from the hydroxyapatite, and crystal defects occur.

Alternatively, as the inorganic dielectric, an oxide material containing two different metal elements A and B and having a composite oxide represented by the composition formula ABO3as a basic composition can also be used. It is preferably to use, as the composite oxide, one having an amorphous structure or one having a perovskite crystal structure. The composite oxide may have a structure having less oxygen content relative to the basic composition due to defects formed in the structure (i.e., ABOx, where x≤3). Preferably, when the amount of oxygen is less than the stoichiometric ratio of the basic composition, defects are likely to be introduced and the surface potential is likely to increase.

The presence of defects is important for the development of the surface potential in the electret2, and it is easy to introduce defects by using the composite oxide having an amorphous structure or the composite oxide having a perovskite structure among bandgap materials. With using these composite oxides, the electret2having a high surface potential can be obtained, and a high collection amount of charge can be realized.

In the composite oxide having an amorphous structure, defects due to dangling bonds in an unbonded state are likely to be formed compared with an oxide crystal having a perovskite structure of the same composition. In contrast, in the oxide crystal having a perovskite structure, defects can be introduced by element substitution which will be described later. When a composite oxide having an amorphous structure is used, the composite oxide can be processed at a lower temperature than the oxide crystal. Thus, thermal damage to wiring or the like can be suppressed during the device production.

The composite oxide having a perovskite structure is a composite oxide having a perovskite-type crystal structure represented by the composition formula ABO3, and typically has a cubic unit cell. The metal element A is located at each vertex of a cubic crystal, the metal element B is located at a center position of the cubic crystal, and an oxygen atom O is coordinated with respect to each of the metal elements A and B in a regular octahedron. In the perovskite structure, the composite oxide often has a non-stoichiometric composition due to the lack of oxygen atoms. In that case, the composite oxide can be expressed by the composition formula ABOx(x<3), and crystal defects occur because the amount of oxygen is less than the stoichiometric ratio. It is preferable to use a configuration in which the amount of oxygen is less than the stoichiometric ratio to improve the surface potential.

As a specific example of the composite oxide represented by the composition formula ABO3, the metal element A (A site) can be a rare earth element R selected from the group consisting of La, Y, Pr, Sm and Nd, and the metal element B (B site) can be Al. Since a perovskite type composite oxide containing a trivalent rare earth metal element R and a trivalent Al (i.e., RAlO3; rare earth aluminate) has a large bandgap energy of 4 eV or more and relatively small permittivity (e.g., 100 or less), a high surface potential can be realized. In addition, the rare earth aluminate can be manufactured using relatively inexpensive materials, which is advantageous in the manufacturing cost.

In the perovskite structure, the combination of the metal element A occupying the A site and the metal element B occupying the B site is not particularly limited as long as the combination satisfies the composition formula ABO3. In that case, for example, in addition to the combination of the trivalent metal element A and the trivalent metal element B, a combination of monovalent and pentavalent, and a combination of divalent and tetravalent can be used.

In the composition formula ABO3, a part of the metal element A at the A site, a part of the metal element B at the B site, or both of them may be substituted by a dopant element that is different from the metal element A and the metal element B. In that case, when the dopant element is a metal element having a lower valence than the metal elements A and B, defects due to oxygen vacancies are likely to occur in the structure. For example, when the metal element A is a trivalent rare earth element R, a divalent alkaline earth metal element (including Mg) is preferably used as the dopant element, and when the metal element B is a trivalent Al, one or more elements selected from the group consisting of divalent alkaline earth metal elements (including Mg) and Zn are preferably used.

The combination of the metal elements A and B is not particularly limited, and the dopant element may be any metal element having a valence lower than the valence of the metal elements A or B to be substituted by the dopant element. By substituting the metal element A and/or the metal element B by a lower valence dopant element, crystal defects due to oxygen deficiency occur in the perovskite structure in order to maintain electrical neutrality, which contributes to the improvement of the surface potential. Since there is a correlation between the substitution amount of the dopant element and the amount of defects, it is possible to control the amount of defects that affect the surface potential by controlling the introduced amount of the dopant element, so that stable surface potential characteristics can be obtained.

Specifically, lanthanum aluminate (LaAlO3) can be mentioned as a typical example of the rare earth aluminate, and a structure in which a part of La in the lanthanum aluminate is substituted by an alkaline earth metal element (for example, Ca) can be used. In that case, it can be represented by the composition formula (La, Ca)AlO3-δ, where δ represents the amount of oxygen defects. The amount of oxygen defects varies depending on the substitution amount of the dopant element, the atmosphere, and the like. When a substitution ratio by the dopant element is x (atomic %) and when the oxygen defect is due to the substitution, the composition formula is represented by La(1−x)CaxAlO3−x/2.

The substitution ratio of the dopant element that substitutes for the metal element A can be appropriately set in the range of, for example, 0.5 atomic % to 20 atomic %. Similarly, the substitution ratio of the dopant element that substitutes for the metal element B preferably falls within the range of, for example, 0.5 atomic % to 20 atomic %. When the substitution ratio is 0.5 atomic % or more, higher surface potential can be obtained as compared with the case where the dopant element is not introduced. Preferably, when the substitution ratio is 1 atomic % or more, the surface potential is significantly improved. However, when the substitution ratio approaches 20 atomic %, the effect obtained by introducing the dopant element tends to decrease. The reason for this is not entirely clear, but it is presumed that an increase in the relative permittivity acts to lower the surface potential. Therefore, it is preferable to appropriately set the substitution ratio within the range less than 20 atomic % so that the desired characteristics can be obtained.

In the present embodiment, the electret2is obtained by polarizing the above mentioned inorganic dielectric material, such as a predetermined-shaped sintered body of an inorganic compound having an apatite structure (hereinafter referred to as an inorganic compound sintered body) or a predetermined-shaped sintered body of a composite oxide having a perovskite structure (hereinafter referred to as a composite oxide sintered body). The inorganic compound sintered body or the composite oxide sintered body to be the electret2can have an arbitrary outer shape (for example, a rectangular flat plate shape or a disk shape).

Further, the electret2may be a film containing such inorganic dielectric material (hereinafter, referred to as an inorganic dielectric film). For example, when the inorganic dielectric material is an inorganic compound, it is an inorganic compound film, and when the inorganic dielectric material is a composite oxide, it is a composite oxide film. Such inorganic dielectric film can be formed as a thin film having a desired thickness by forming an inorganic dielectric material film on a substrate using an arbitrary film forming method such as a sputtering method. As the substrate, in addition to a conductive substrate such as a conductive Si substrate, an insulating substrate can also be used.

Further, the electret2may be a composite film in which particles of the inorganic dielectric material are dispersed in a base material film. In this case, using a mixed material of the inorganic dielectric particles and the base material, the composite film having a desired thickness can be formed on a substrate through an arbitrary method such as a printing method. The base material to be the base film may be any material having excellent heat resistance and voltage resistance. The base material may be an organic material such as polyimide or any material that is in a liquid phase to form a film.

As described above, the form of the electret2is not particularly limited. When the electret2is formed on the substrate, another layer such as a conductive film may be laminated between the electret2and the substrate. Alternatively, the electret2may be used after being peeled off from the substrate.

The polarization treatment method for forming an electret is not particularly limited, but the polarization treatment may be performed by forming electrodes on opposite surfaces of an electret material having a predetermined shape and applying a voltage to the electret material. As for the polarization treatment conditions, it is desirable to apply a direct-current voltage so that the electric field strength becomes 1 kV/mm or more at 100° C. or higher, for example. In order to realize efficient power generation of the power generator, a surface potential of 100 V or more is required. It is possible to realize a desired surface potential through a polarization treatment with an electric field strength of 1 kV/mm or more. In addition, by performing the polarization treatment at a temperature higher than room temperature, stable electret performance can be realized even in applications whose usage environment is at high temperature.

InFIG.1, the power generating unit10is formed by forming a pair of electrodes on opposite surfaces of the electret2obtained as described above. In the electret2, opposite surfaces in the polarization direction X are charged with opposite polarities. When the upper surface is referred to as the first charged surface21(for example, negatively charged surface) and the lower surface is referred to as the second charged surface22(for example, positively charged surface), the first electrode31and the second electrode32are arranged respectively on the first charged surface21and the second charged surface22.

The polarities of the first charged surface21and the second charged surface22are not limited to this example as long as the first charged surface21and the second charged surface22have different polarities.

In the power generating unit10, the first electrode31arranged on the first charged surface21is formed of a plurality of electrode portions30electrically connected to each other. An area of the first charged surface21where the plurality of electrode portions30are not arranged is exposed to the atmosphere, and this exposed surface serves as the current collecting surface20. On the current collecting surface20of the first charged surface21, an electric charge (for example, negative charge) corresponding to the internal polarization state of the electret2is stored. Thereby, a surface potential (for example, negative polarity) is expressed and a part of the electric charge can move through the adjacent first electrode31so that the current can be collected.

As shown inFIG.2A, the plurality of electrode portions30as the first electrode31may be formed of a plurality of stripe electrodes30aparallel to each other and an annular electrode30bsurrounding the stripe electrodes30a. In the example shown inFIG.2A, the electret2has a rectangular shape, and the annular electrode30bhas a rectangle annular shape having a predetermined width along the peripheral edge of the first charged surface21so as to correspond to the first charged surface21. The stripe electrodes30aare arranged at a predetermined interval between the stripe electrodes30aand connect two opposing sides of the annular electrode30b. Each of the stripe electrodes30ahas a predetermined width.

In the first charged surface21, the current collecting surface20is formed of a plurality of strip-shaped exposed surfaces surrounded by the stripe electrodes30aand the annular electrode30b. As described above, when the plurality of current collecting surfaces20are formed on the first charged surface21and the peripheral edges of the current collecting surfaces20are in contact with the stripe electrodes30aor the annular electrode30b, movable electric charge accumulated on the current collecting surface20can easily derived through the stripe electrodes30aand the annular electrode30b.

Alternatively, as shown inFIG.2B, when the electret2has a disk shape, a stripe electrode30cextending along the peripheral edge of the first charged surface21and connecting the plurality of stripe electrodes30amay be arranged. Alternatively, a circular annular electrode30b(not shown) having a predetermined width and surrounding the plurality of stripe electrodes30amay be formed along the peripheral edge of the first charged surface21. For example, each of the plurality of stripe electrodes30ahas a predetermined width, has one end connected to the stripe electrode30c, and arranged at predetermined interval therebetween. Alternatively, each of the plurality of stripe electrodes30ahas a predetermined width and arranged at predetermined interval therebetween inside the annular electrode30b.

In this embodiment, the second electrode32is formed on, for example, the entire surface of the second charged surface22. The area where the second electrode32is arranged is not always limited, and a part of the second charged surface22may be exposed to the outside. The first electrode31, the second electrode32are metal electrodes, and are made of, for example, an electrode material containing precious metals such as gold (Au) and platinum (Pt), or precious metal alloys and the like.

When the power storage unit41and the output unit42, which are the power supply unit4, are electrically connected between the first electrode31and the second electrode32of the power generating unit10A as shown inFIG.3A, the generated electric charge can be derived. The power storage unit41and the output unit42are connected in parallel between a first wiring L1connected to the first electrode31and a second wiring L2connected to the second electrode32. The connection between the power generating unit10and one of the power storage unit41and the output unit42can be switched by a switch unit5.

The power storage unit41stores the electric power generated by the power generating unit10, and can be configured by using, for example, a power storage element C such as a capacitor. The output unit42includes an output terminal portion42aconnected to an external load (load resistance R) or the like, and supplies the power generated by the power generating unit10to the outside through the output terminal portion42a. The output unit42may directly supply the power generated by the power generating unit10to the outside, or may supply the power stored in the power storage unit41to the outside.

As shown inFIG.3B, the switch unit5may be a combination of a plurality of open/close switches51to53. A branching wiring L11connects the first wiring L1to the power storage unit41or the output unit42. For example, the switch unit5includes an open/close switch51configured to open and close the first wiring L1, an open/close switch52configured to open and close a branching path of the branching wiring L11connected to one end of the power storage unit41(here a positive terminal side end), and an open/close switch53configured to open and close a branching path of the branching wiring L11connected to one end of the output unit42(here, a positive terminal side end). The other ends (here, the negative terminal side ends) of the power storage unit41and the output unit42are connected to the second wiring L2via a branching wiring L2.

The switch unit5can control on/off of the open/close switches51to53by a switch controller (not shown). For example, by closing the open/close switch51to connect the power generating unit10to the power supply unit4and closing the open/close switch52or the open/close switch53to connect the power generating unit10to the power storage unit41or the output unit42, the generated power can be supplied one of the power storage unit41and the output unit42. Further, by opening the open/close switch51to block the connection between the power generating unit10and the power supply unit4and by closing the open/close switch52and the open/close switch53, the stored power in the power storage unit41can be supplied to the output unit42.

The configuration of the switch unit5can be arbitrarily changed. For example, the open/close switch51may be omitted, or the switch unit5may have only a switch configured to switch a connection position of the power generating unit10to the power storage unit41or a connection position of the power generating unit10to the output unit42.

When the power storage unit41or the output unit42is connected between the first electrode31and the second electrode32, electric charges accumulated on the current collecting surface20(for example, negative charges) can move to the power storage unit41or the output unit42.

The mechanism of power generation in the power generating unit10configured in this way is not clear, but is considered, for example, as follows. The mechanism will be described with reference toFIG.4.

As shown in the left figure ofFIG.4, in the electret2after the polarization treatment, a large number of electric dipoles D existing inside the inorganic dielectric that is an electric material are arranged in the polarization direction X and a residual polarization P is generated. As a result, charged molecules, ions, etc. (suspended charges) in the atmosphere are adsorbed to the electret2according to the polarization direction X of the electret2. For example, as shown in the figure, when the first charged surface21on the upper surface is positively charged, negative charges N that have opposite polarity exist on the first charged surface21to cancel the residual polarization P, and these negative charges N are usually retained on the first charged surface21and cannot freely move.

In contrast, in the inorganic dielectric material used as the electret material of the present embodiment, it has been found that the electric charges can be derived by arranging the electrodes on opposite surfaces of the electret2as in test examples which will be described later. The reasons are not clear, but it is presumed that a movable negative charge N is generated as the magnitude of the residual polarization P changes over time due to the adsorption of atmospheric ions and the like.

For example, as shown in the right figure ofFIG.4, when the residual polarization is changed to P′ (P>P′), the electric charge required to cancel the residual polarization is changed. Thus, the electric charge on the first charged surface21(i.e., the current collecting surface20) becomes excessive. In that case, a part of the surplus negative charge N can be collected via the electrode or the like, and the power generating unit10can function.

Test Example 1

The electret2constituting the power generating unit10of the first embodiment was produced by the following method.

As the inorganic dielectric material constituting the electret2, oxyhydroxyapatite (hereinafter, referred to as OHA; bandgap energy of 5.3 eV), which is an inorganic compound having an apatite structure, was used. Then, a test sample of the electret2was prepared as a test example 1 by subjecting an OHA sintered body to a polarization treatment.

Preparation of Powder

First, hydroxyapatite (hereinafter, HA) powder described below was prepared as a raw material for the OHA sintered body, and weighed to have a predetermined sample size. Polyvinyl alcohol described below (hereinafter, PVA) was weighed to be 3.2 wt % with respect to the HA powder.

Apatite for biomaterial research HAP monoclinic Ca10(PO4)6(OH)2, manufactured by FUJIFILM Wako Pure Chemical Corporation.

Polyvinyl alcohol, degree of polymerization 1500; manufactured by FUJIFILM Wako Pure Chemical Corporation

Next, an appropriate amount of pure water was poured into a petri dish, and the weighed PVA was dissolved in the pure water while heating the petri dish on a hot stirrer. Next, the weighed HA was added to the PVA solution in the petri dish and mixed. This petri dish was covered with a wrap film and placed in a dryer pre-heated to 100° C. or higher, and the mixed solution was dried for 1 day or longer.

Powder Classification and Preparation of Molded Product

The dried sample was transferred from the petri dish to a mortar and pulverized into powder using a pestle. Next, the powder is classified into 30 μm to 38 μm. Then, about 0.3 g of the classified powder was measured and molded into a 13 mm ϕ disk shape using a uniaxial pressure molding machine. At this time, the pressure was sequentially applied at 30 MPa for 2 minutes at first, then at 60 MPa for 2 minutes, then at 90 MPa for 2 minutes, and finally at 120 MPa for 3 minutes to obtain a molded product including the HA/PVA mixed powder.

In the case of preparing a larger disk-shaped sample, about 0.55 g of the powder classified into 30 μm to 38 μm was measured and molded into a disk shape of 18 mmϕ using a uniaxial pressure molding machine. In this case, the pressure is sequentially applied at 60 MPa for 2 minutes at first, then at 120 MPa for 2 minutes, then at 180 MPa for 2 minutes, and finally at 240 MPa for 3 minutes to obtain a molded product including the HA/PVA mixed powder.

Preparation of OHA Sintered Body

The obtained molded product was placed on an alumina sintered board lined with a platinum mesh, and in this state, sintered in the air at 1400° C. for 2 hours using a tube furnace. As a result, a disk-shaped OHA sintered body was prepared.

Polarization Treatment

The OHA sintered body obtained as described above was subjected to a polarization treatment using a polarization treatment apparatus shown inFIG.5. InFIG.5, a pair of gold electrodes2aand2bwere baked in advance respectively on the upper and lower surfaces11aand11bof the OHA sintered body11in the thickness direction (i.e., the polarization direction X). Illustration of the gold electrode2bon the lower surface is omitted. The OHA sintered body11was sandwiched between two alumina rods13around which platinum wires12were wound, so that a voltage could be applied between the pair of gold electrodes2aand2b. The two alumina rods3were longer than the diameter of the OHA sintered body10and were fixed by being tied with threads14made of fluororesin (polytetrafluoroethylene) at both ends of the two alumina rods3in the longitudinal direction of the alumina rods3(i.e., the direction perpendicular to the polarization direction X). Next, this was wrapped around a polarization instrument, and the whole was coated with silicone oil15in order to prevent insulation breakdown of air.

This polarization instrument was placed in a box furnace and left until the temperature inside the furnace became stable at 200° C. Next, while keeping the temperature at 200° C., a direct-current electric field of 8.0 kV/mm was applied between the pair of gold electrodes2a,2bof the OHA sintered body11for 1 hour to perform the polarization treatment. After the lapse of a predetermined time, the sintered body was allowed to cool until the temperature became 40° C. or lower while the direct-current electric field was continuously applied.

Preparing an Electret

After the polarization treatment, the pair of gold electrodes2aand2bon the opposite surfaces of the OHA sintered body11were removed using a polishing sheet. Next, the OHA sintered body11was ultrasonically washed with ethanol and pure water for 10 minutes each. The OHA sintered body11was left in a dryer at 100° C. for 3 hours or more to obtain the electret2.

Surface Potential Measurement

As shown in the upper figure ofFIG.6, the electret2of the test example 1 obtained as described above was left on the stage16connected to the ground, and the surface potential was measured. For the measurement, a surface electrometer (MODEL341-B: manufactured by Trek Japan Co., Ltd.) was used to measure the surface potential in a contactless manner with a measurement probe17facing the electret2, and the value after the lapse of 5400 seconds was read. As the result is shown below, a high surface potential exceeding 3900 V (absolute value) was obtained.

Test Example 1: OHA electret surface potential: −3947 V

Repeated Surface Potential Measurement

Further, the measurement of the surface potential was repeated according to the procedure indicated by the arrows inFIG.6. First, as shown in the lower figure ofFIG.6, the measurement probe17is displaced from a position facing the electret2, and then, as shown in the middle figure ofFIG.6, the metal electrode plate18connected to the ground is brought into contact with the surface of the electret2to cause short-circuit. Then, by the method described above, the measurement probe17is positioned to face the electret2again and the surface potential was measured with the measurement probe17. This procedure was repeated. The change in the surface potential over time was shown inFIG.7.

As shown inFIG.7, by short-circuiting, the surface potential of the electret2was released and the surface potential was not detected once, but the surface potential was increased again thereafter. Further, by repeating short-circuit and release, the surface potential rises repeatedly. The surface potential decreases from the first time to the second and third times, but it stabilizes with the passage of time. This indicates that when the short-circuit is released, the surface of the electret2can be recharged, and the surface potential can be repeatedly recovered.

Measurement of Dielectric Charge

As schematically shown inFIG.8, the power generating unit10in which the first electrode31and the second electrode32were formed respectively on the first charged surface21and the second charged surface22of the electret2was formed and a known coulomb meter was connected to the power generating unit10. The first electrode31has an electrode pattern in which the plurality of electrode portions30are adjacent to each other with stripe-shaped current collecting surfaces20interposed therebetween. The electrode pattern to be the first electrode31was formed by arranging a metal mask processed to have a line of 100 μm and a space of 100 μm on the first charged surface21of the electret2and by sputtering with a gold (Au) target. The thickness of the obtained electrode pattern was about 30 nm.

The power generating unit10was left on a metal stage connected to the ground, and a copper electrode plate and a probe of the coulomb meter were simultaneously brought into contact with the first charged surface21of the electret2to measure dielectric charge. The change in the dielectric charge over time is shown inFIG.9.

As shown inFIG.9, by using the electret2of the test example 1, it can be seen that the dielectric charge increases with the passage of time, and the charge continues to be accumulated in the capacitor of the coulomb meter over time.

Comparative Example 1

For comparison, a fluororesin based organic polymer described below was used as an electret material and an electret film was formed on a substrate.

The surface potential and dielectric charge of the electret film were measured in a similar manner.

CYTOP (registered trademark) CTL-809M manufactured by AGC Inc.

Preparing the Electret Film

A film of the above-mentioned fluororesin-based electret material was formed on a copper substrate by spin coating under the conditions of 500 rpm and 30 sec. Then, the obtained film was treated with heat at 200° C. for 1 hour, to produce the fluororesin electret film as the comparative example.

Polarization Treatment

A corona discharge device was used for the polarization treatment of the obtained fluororesin electret film. The electret film was left on a stage heated to 120° C. and treated with a corona discharge beam by applying a direct current voltage of −6.44 kV to the electret film to polarize the electret film.

Surface Potential Measurement

The surface potential of the fluororesin electret film obtained as described above was measured in the same manner as in the test example 1. As the result is shown below, the surface potential of the fluororesin electret film was 300 V (absolute value), which was much lower than the surface potential of the test sample 1.

Comparative Example 1: Fluororesin Electret Film Surface Potential: −300 V

The results of the test example 1 and the comparative example 1 are shown in Table 1. Note that the magnitude of the surface potential in Table 1 is an absolute value, and in the following description, values without signs are absolute values unless otherwise specified.

TABLE 1Test example/ComparativeElectretCa substitutionSurfaceexamplematerialamount (atm %)potential (V)Test example 1OHA—3947Test example 2LAO0.5438ComparativeFluororesin—300example 1ComparativeLAO—20example 2ComparativeBaTiO3—4example 3

Measurement of Dielectric Charge

With respect to the obtained electret film, a pattern electrode was formed in the same manner as in the test sample 1, and the dielectric charge was measured. A pattern electrode made of a gold electrode film (thickness of about 300 nm) was formed on the surface of the fluororesin electret film by sputtering in the same manner as in the test example 1. The fluororesin electret film on which this electrode pattern was formed was left on a metal stage connected to the ground, and the dielectric charge was measured by contacting the probe of the coulomb meter with the electrode surface of the fluororesin electret film. The change in the dielectric charge over time is shown inFIG.9as the comparative example 1 together with the result of the test example 1.

Here, the result ofFIG.9shows the change in the dielectric charge that is corrected with reference to the surface potential of the comparative example 1 at the measurement start time. In the comparative example 1 using the fluororesin electret film, almost no change was observed in the dielectric charge over time, and it is assumed that the fluororesin electret film does not serve as a power generating unit. On the other hand, as described above, the power generating unit10using the electret2of the test example 1 had a higher dielectric charge than the comparative example 1, and the dielectric charge increases with the passage of time, so that the power generating unit10using the electret2of the test example 1 can be effectively used as a power generator1.

Test Example 2

The electret2constituting the power generating unit10of the first embodiment was produced by the following method.

As the inorganic dielectric material constituting the electret2, a LAO-based composite oxide having a composition in which a part of La in lanthanum aluminate (LaAlO3) having a perovskite structure was substituted by a dopant element was used. Here, the dopant element was Ca and raw materials were prepared to have a composition of (La0.995, Ca0.005)AlO3-δto obtain a composite oxide sintered body. The obtained composite oxide sintered body was polarized to be the electret2of the test example 2.

Lanthanum aluminate (LaAlO3), which is a typical composition of the LAO-based inorganic dielectric material, has a bandgap energy of 5.6 eV, and even a configuration in which a part of Al is substituted by Ca, which is a dopant element, has almost the same bandgap energy.

Preparation of Powder

First, as raw materials for the LAO-based composite oxide sintered body, the nitrate reagents described below were prepared and weighed so that the substitution amount of Ca was 0.5 atomic %. 20 ml of ultrapure water was added to a beaker containing the reagents to obtain a solution in which the reagents were dissolved.

La(NO3)3.6H2O(3N) 6.03 g manufactured by FUJIFILM Wako Pure Chemical Corporation

Al(NO3)3.9H2O(2N) 5.25 g manufactured by FUJIFILM Wako Pure Chemical Corporation

Ca(NO3)2.4H2O(3N) 16.5 mg manufactured by FUJIFILM Wako Pure Chemical Corporation

The obtained solution of the reagents was transferred to a plastic beaker and mixed with a stirrer. The stirring was performed by placing the stirrer in the plastic beaker and rotating the stirrer at 500 rpm. To the beaker containing this mixed solution, an aqueous NaOH solution having a molar concentration of 12 M was added little by little using a dropper to adjust pH of the solution to 10.5 while measuring pH with a pH meter. Then, a precipitate was filtered under reduced pressure and washed with about 100 ml of ethanol and ultrapure water.

The reagents described below were used for NaOH and ethanol for the NaOH aqueous solution.

NaOH special grade (granular) manufactured by Kanto Chemical Co., Inc.

Ethanol (99.5) manufactured by Kanto Chemical Co., Inc.

Next, the filter paper on which the washed sample was located was placed in a dryer at 120° C., and the sample was dried for 12 hours or more. The dried sample was placed in an agate mortar and pulverized, and was further classified (<100 μm).

Producing of Molded Product/Sintered Body

The powder obtained by classification was placed in an alumina boat and pre-burned. As pre-burning conditions, the temperature was raised to 1000° C. at a rate of 2.5° C./min, held at 1000° C. for 6 hours, and then lowered to room temperature at a rate of 2.5° C./min.

The pre-burned sample was placed in an agate mortar and crushed, and was further classified (<100 μm) to obtain a powder for molding. About 0.65 g of the powder was placed in a φ13 mm mold and pressed at a pressure of 250 MPa for 3 minutes to form a disk-shaped molded product.

The obtained molded product was sintered at the temperature above the sintering temperature to obtain an LAO sintered sample composed of an LAO-based composite oxide sintered body. As sintering conditions, the temperature was raised to 1650° C. at a rate of 2.5° C./min, held at 1650° C. for 2 hours, and then lowered to room temperature at a rate of 2.5° C./min. The diameter of the obtained sintered sample was about 1ϕ11 mm. The thickness was adjusted to 1 mm by polishing.

Further, the obtained LAO sintered sample was subjected to elemental analysis using ICP emission spectroscopy, and it was confirmed that the obtained sintered body had the desired composition (La0.995, Ca0.005)AlO3-δ. Specifically, the LAO sintered sample pulverized in the mortar was dissolved in a solvent as an analysis sample, and constituent elements of the sintered body were detected from emission lines generated by ICP (high frequency inductively coupled plasma). As a result, as shown below, almost the same analysis result (0.51 atomic %) was obtained with respect to 0.5 atomic %, which is the Ca substitution ratio at the time of raw material preparation (charged Ca substitution amount).

Charged Ca Substitution Amount: 0.5 Atomic %

ICP Analysis Result: 0.51 Atomic %

The analysis of the LAO sintered sample is not limited to the ICP emission spectroscopic analysis method, and any method such as XPS (X-ray photoelectron spectroscopic analysis) method and XRF (fluorescent X-ray analysis) method may be adopted. By introducing the dopant element in this way, the substitution amount can be easily controlled and the quantitative evaluation can be easily performed.

Polarization Treatment and Surface Potential Measurement

The LAO sintered sample obtained as described above was subjected to a polarization treatment in the same manner as in the test example 1 to obtain the electret2. The surface potential of this electret2was measured in the same manner. The results are shown in Table 1.

As shown in Table 1, in the test example 2 in which a part of La of LaAlO3was substituted by Ca, the surface potential was 438 V (absolute value). That is, by applying a direct-current electric field of 8.0 kV/mm to the test example 2, a high surface potential of 400 V or more was obtained.

Comparative Examples 2 and 3

For comparison, a commercially available LaAlO3(100) single crystal substrate (manufactured by Crystal Base Co., Ltd.) was used as a LaAlO3sintered body in which La was not substituted, and polarization treatment was performed for the LaAlO3(100) single crystal substrate in the same manner as in the test example 1 to obtain an electret of the comparative example 2. Further, a sintered body of barium titanate (BaTiO3), which is a composite oxide having a perovskite structure and a bandgap energy of 3.5 eV, was prepared and was subjected to the polarization treatment in the same manner as in the test example 1 to obtain an electret of the comparative example 3.

The surface potentials of the electrets of the comparative examples 2 and 3 were measured in the same manner as in the test example 1. The results are shown in Table 1.

As shown in Table 1, in the comparative example 2, which was composed of a single crystal of LaAlO3, wherein La in the LaAlO3was not substituted by a dopant element, the surface potential was significantly reduced to 20 V (absolute value). Further, in the comparative example 3 using BaTiO3having a bandgap energy of less than 4 eV, the surface potential was further reduced to 4 V.

Test Examples 3 to 6

In the same method as in the test example 2, electrets2each composed of LAO-based composite oxide sintered body was produced. The substitution ratio (x) of La with the dopant element (Ca) in the composition of La(1−x)Ca(1−x)CaxAlO3-δwas changed between the test examples 3 to 6 as follows.Example 3 (La0.99Ca0.01)AlO3-δExample 4 (La0.95Ca0.06)AlO3-δExample 5 (La0.9Ca0.1)AlO3-δExample 5 (La0.8Ca0.2)AlO3-δ

As shown in Table 2, the substitution ratio (x) of La by the dopant element (Ca) was varied in the range of 1 atomic % to 20 atomic %. The row materials were prepared to get the desired composition and molded products were prepared with the row materials. The obtained molded products were sintered to get sintered samples. The sintered samples were subjected to polarization treatment to obtain the electrets2in the test examples 3 to 6.

The surface potentials of the obtained electrets2in the test examples 3 to 6 were measured in the same manner as in the test example 1, and the results were shown in Table 2. Table 2 also shows the relative permittivity of the composite oxide sintered bodies.

TABLE 2Ca substitutionamountSurface potentialRelativeTest examples(atm %)(V)permittivityTest example 31343318Test example 45368839Test example 510211546Test example 620140072

As is clear from Table 2, the surface potentials in the test examples 3 to 6 are higher than the surface potential in the test example 2, and the surface potentials of 1400 V (test example 6) to 3688 V (test example 4), which are higher than 1000 V, were obtained. It is presumed that this is because, the substitution with the lower valence dopant element with respect to LaAlO3having the perovskite structure causes oxygen defects according to the amount of substitution, which contributes to the expression of the high surface potentials.

FIG.10shows the relationship between the Ca substitution amount and the surface potential based on the comparative example 2 and the test examples 2 to 6. According toFIG.10, a high surface potential is expressed when the Ca substitution amount is 0.5 atomic % or more. When the Ca substitution amount increases to 1 atomic % to 5 atomic %, the surface potential further increases significantly. Even in the range where the Ca substitution amount exceeds 5 atomic %, a high surface potential is expressed, but the surface potential tends to decrease as the Ca substitution amount increases.

On the other hand, as shown in Table 2, the relative permittivity of the composite oxide sintered body tends to increase as the amount of Ca substitution increases. It is presumed that this is because increase in the amount of oxygen defects according to the substitution ratio with the dopant element increases the surface potential while increase in the relative permittivity makes it easy for charge to leak and suppresses the increase in the surface potential.

From these results, it is desirable that the substitution ratio of the dopant element with respect to the metal elements A and B occupying the A site or the B site of the perovskite structure is preferably in the range of 0.5 atomic % to 20 atomic %.

As described above, by using the inorganic dielectric material made of the composite oxide sintered body, the electret2that has excellent thermal stability, whose amount of crystal defects can be controlled, and that has stable characteristics in the usage environment can be formed.

Second Embodiment

The second embodiment of the power generator1will be described with reference to the drawings. The basic configuration of this embodiment is the same as that of the first embodiment, and the electrode shape of the power generating unit10is modified. Hereinafter, the differences will be mainly described.

Those of reference numerals used in the second and subsequent embodiments which are the same reference numerals as those used in the above-described embodiments denote the same components as in the previous embodiments unless otherwise indicated.

In the first embodiment, as shown inFIGS.2A and2B, the first electrode31on the first charged surface21of the disk-shaped or rectangular-shaped electret2is formed of the stripe electrodes30a,30cand the annular electrode30b. However, these electrode shapes are examples and may have different shapes. For example, as shown inFIG.11, the electrode portions30may not be surrounded by the annular electrode30bas long as connected to each other.

Specifically, as shown in (A) ofFIG.11, a stripe electrode30cperpendicular to the stripe electrodes30athat are arranged parallel to each other may be formed along one side of the first charged surface21. The stripe electrode30cconnects one ends of the stripe electrodes30a. Further, as shown in (B), a grid-like electrode30dmay be used instead of the stripe electrodes30a, and as shown in (C), each of the stripe electrodes30amay not have a constant width. In that case, the both ends of each of the stripe electrodes30ahas, for example, a part protruding outward into an arc shape as an arc-shaped electrode30e.

Further, as shown inFIG.11, a shape of a combination of diamond-shaped electrodes30fconnected to each other at the apexes as shown in (D), a shape of a combination of a plurality of annular electrodes30bthat have coaxial circular shapes as shown in (E), and a shape of a circular electrode30gdefining a plurality of circular holes30has shown in (F) may be adopted. In any shape, exposed portions between the plurality of electrode portions30and exposed portions through the holes30hare used as the current collecting surface20. Further, the stripe electrode30cis formed at the peripheral edge of the first charged surface21so that the plurality of electrode portions30are connected to each other, and drawn out to the outside.

As described above, the shape of each of the electrode portions30or the shape of the electrode collective portion of the electrode portions30can be arbitrarily selected. Of course, the shape other than those shown inFIG.11can be used.

As shown inFIG.12, instead of forming the annular electrode30band the stripe electrode30cthat connect the plurality of electrode portions30to each other, wire bonding may be used to connect the plurality of electrode portions30. In that case, as shown in (A) ofFIG.12, a plurality of wires W are connected respectively to the plurality of stripe electrodes30a(see (A) inFIG.11). Further, in (B) ofFIG.12, a plurality of rectangular electrodes30iare arranged to be surrounded by a portion where the grid-like electrode30dis formed in (B) ofFIG.11while the portion is used as the current collecting surface20. Then, the plurality of rectangular electrodes30iare connected by a plurality of wires W.

Further, as shown in (C) to (E) ofFIG.12, each of the plurality of circular electrodes30g, the plurality of diamond-shaped electrodes30f, and the plurality of annular electrodes30bmay be connected by a plurality of wires W. In the case of the circular electrode30gshown in (F) ofFIG.12, a single wire W is connected to the circular electrode30gand drawn out to the outside.

At this time, in order to efficiently take out electric charges with the power generating unit10, it is desirable that at least a part of a distance between adjacent ones of the plurality of electrode portions30is equal to or less than 8 mm. In other words, it is desirable that the width of a part of the current collecting surface20between adjacent ones of the plurality of electrode portions30is equal to or less than 8 mm. Preferably, it is desired that an area where the electrode distance is equal to or less than 8 mm is as wide as possible. For example, it is better to design the electrode shape and the arrangement of the electrode such that the electrode distance of the first electrode31is about or less than 8 mm in a main area of the first charged surface including a center potion of the first charged surface. As the electrode distance decreases from 8 mm, the width of the current collecting surface20between the electrode portions is narrowed. Thus, electric charge accumulated on the current collecting surface20is more likely to be collected through the adjacent electrode portions30.

Test Example 3

As shown inFIG.13A, a plurality of electrode portions30to be the first electrode31are formed on the first charged surface21of the disk-shaped electret2that is formed in the same manner as in the test example 1, and test samples are prepared by varying the electrode distance between the electrode portions30. Each of the electrode portions30has a substantially semicircular shape. The electrode portions30are arranged so that their linear sides facing each other are parallel to each other. The electrode distance d that is the shortest distance between the electrode portions30is changed between 0.1 mm (100 μm), 1 mm, 2 mm, 4 mm, 6 mm and 8 mm to prepare the test samples (Samples 1 to 6).

These test samples are electrets2formed by preparing OHA sintered body in the same manner as the test sample 1 and applying a direct-current electric field of 8.0 kV/mm to the OHA sintered body at 200° C. for 1 hour for polarization treatment. After the gold electrodes for the polarization treatment are polished and removed, the electrets2are subjected to sputtering under masking in the same manner as in the test example 1, thereby gold electrode films to be the electrode portions30are formed on the electret2such that the electrode distance falls within a range between 0.1 mm (100 μm) to 8 mm.

The surface potentials of the test samples obtained as described above were measured in the same manner as in the test example 1, and the surface charge state was evaluated. At that time, the electric field of the electret2was partially electrostatically shielded by connecting the plurality of electrode portions30to the ground. The surface potential measurement results of the test samples are shown in Table 3 andFIG.13B.

TABLE 3Electrode distanceSample No.(mm)Surface potential (V)Sample 183670Sample 262930Sample 341813Sample 421163Sample 51400Sample 60.1105

As shown in Table 3, the surface potentials of Samples 1 to 6 are all less than that of the test example 1 (surface potential: 3947 V). Further, as shown inFIG.13B, in the sample 1 having the electrode distance d of 8 mm, the surface potential is as high as 3670 V, and the surface potential decreased as the electrode distance d decreases as shown in the sample 2 (d=6 mm), the sample 3 (d=4 mm), the sample 4 (d=2 mm) and the sample 5 (d=1 mm). In the sample 6 (d=0.1 mm), the surface potential is 105 V. The decrease in the surface potential occurs because, in the electret2of the test example 1, charges on the current collecting surface20move through the first electrode31arranged on the first charged surface21.

From these results, when the first electrode31is formed on the first charged surface21such that the first charged surface has the current collecting surface20and at least a part of the electrode distance d between the electrode portions30is 8 mm or less, the function of collecting charges can be obtained. Further, the smaller the electrode distance d is, the more easily electric charge on the current collecting surface20moves to the adjacent electrode portion30, so that the surface potential decreases more. Thus, it is preferable to set an area where the electrode distance is 8 mm or less is as wide as possible to improve power generation in the power generating unit10. Therefore, it is desirable to set the electrode distance between the electrode portions30to a value equal to or less than 8 mm in at least a part. More preferably, it is desirable that almost entirety of the electrode distance is about 8 mm or less.

As described above, the power generator1that can generate power even in an environment in which external force is not input into the power generator1is obtained by using an inorganic dielectric material having bandgap energy of 4 eV or more as the electret material, polarizing the electret material to form the electret2, and partially disposing an electrode on at least one surface of the electret2such that the at least one surface has a portion as a current collecting surface20that is exposed outward.

The present disclosure is not limited to the respective embodiments described above, and various modifications may be adopted within the scope of the present disclosure without departing from the spirit of the disclosure. For example, the power generator1may be configured to include a load connected to the output unit42of the power supply unit4that is connected to the power generating unit10, or the output unit42may not include a load and may be configured as an output terminal portion42afor external output. Further, the power supply unit4may be formed of only the power storage unit41or the output unit42.