Composite material for power generation and method for producing the same

Abstract

To provide a composite material for power generation with high durability and relatively good piezoelectric properties, and a method for manufacturing composite material for power generation.SOLUTION: A composite material for power generation includes a piezoelectric ceramic having a plate shape with a predetermined thickness, and a polymer with piezoelectricity, and the ceramic content is configured to continuously and gradually change along the thickness direction. The composite material for power generation is manufactured such that multiple types of piezoelectric thin films containing piezoelectric ceramics and piezoelectric polymers and having different ceramic contents are laminated such that the ceramic contents gradually change along the thickness direction, and then are subjected to heat treatment.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a composite material for power generation and a method for producing the composite material for power generation. [Background technology]

[0002] In recent years, with the spread of the Internet of Things (IoT), energy harvesting devices that can convert minute amounts of energy from the surrounding environment, such as heat, wind, and vibration, into electrical energy have been attracting attention as a power supply source for IoT devices. One type of energy harvesting material is a piezoelectric material that can convert mechanical energy into electrical energy. Piezoelectric materials are relatively durable, sensitive to minute strains, and can be expected to produce high output power density and output voltage. In addition, piezoelectric materials are small and compact, and are not easily affected by environmental factors such as humidity, making them ideal as a power supply source for IoT devices.

[0003] Piezoelectric ceramics are known as materials with excellent piezoelectric properties, but they are susceptible to fatigue cracks when subjected to repeated high-frequency loads, making them difficult to use on their own. To solve this problem, piezoelectric power generation units have been widely used in which piezoelectric ceramics are attached to one or both surfaces of a substrate such as a metal elastic plate (see, for example, Patent Document 1). Piezoelectric composite materials, in which ceramics are added to flexible polymers, have also been developed (see, for example, Non-Patent Documents 1 to 3).

[0004] In addition, a composite piezoelectric element has been proposed that improves bending resistance. The composite piezoelectric element includes two first piezoelectric layers and a second piezoelectric layer disposed between the first piezoelectric layers, each containing a resin and piezoelectric particles. The volume percent concentration of the piezoelectric particles in the second piezoelectric layer is lower than that of each of the first piezoelectric layers (see, for example, Patent Document 2). In this composite piezoelectric element, the low volume percent concentration of piezoelectric particles in the second piezoelectric layer improves the bending resistance of the second piezoelectric layer, and the bending resistance of the entire piezoelectric layer can also be improved compared to a piezoelectric element composed of a single layer of piezoelectric with a high volume percent concentration of piezoelectric particles. Moreover, because the second piezoelectric layer is sandwiched between two first piezoelectric layers with a high volume percent concentration of piezoelectric particles, the performance of each first piezoelectric layer takes priority, preventing a significant decrease in the piezoelectric performance of the entire piezoelectric layer.

[0005] As one type of piezoelectric composite material, research is also being conducted on functionally graded materials (FGMs), which are ceramics with a concentration gradient. FGMs have been confirmed to have high strength, high toughness, and high fatigue strength (see, for example, Non-Patent Document 4), but their piezoelectric properties have not yet been confirmed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-250536 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-50432 [Non-patent literature]

[0007] [Non-Patent Document 1] Zhenjin Wang, Fumio Narita, “Corona Poling Conditions for Barium Titanate / Epoxy Composites and their Unsteady Wind Energy Harvesting Potential”, Advanced Engineering Materials, 2019, Volume 21, 1900169 [Non-Patent Document 2] Zhenjin Wang, Fumio Narita, “Fabrication of Potassium Sodium Niobate Nano-Particle / Polymer Composites with Piezoelectric Stability and Their Application to Unsteady Wind Energy Harvesters”, Journal of Applied Physics, 2019, Volume 126, 224501 [Non-Patent Document 3] Zhenjin Wang, Hiroki Kurita, Hiroaki Nagaoka, Fumio Narita, “Potassium Sodium Niobate Lead-Free Piezoelectric Nanocomposite Generators Based on Carbon-Fiber-Reinforced Polymer Electrodes for Energy-Harvesting Structures”, Composites Science and Technology, 2020, Volume 199, 108331 [Non-Patent Document 4] Saurav Sharma, Anuruddh Kumar, Rajeev Kumar, Mohammad Talha, Rahul Vaish, “Geometry Independent Direct and Converse Flexoelectric Effects in Functionally Graded Dielectrics: An Isogeometric Analysis”, Mechanics of Materials, 2020, Volume 148, 103456 Summary of the Invention [Problem to be solved by the invention]

[0008] Conventional piezoelectric generating units, such as those described in Patent Document 1, require piezoelectric ceramics to be attached to a substrate using adhesives or other means, which can cause peeling when subjected to repeated vibrations or impacts. Conventional piezoelectric composite materials also have the problem of poor piezoelectric properties. Furthermore, the composite piezoelectric element described in Patent Document 2 sandwiches a second piezoelectric layer, which has a different volume percent concentration of piezoelectric particles, between two first piezoelectric layers. Therefore, when a bending load is applied to the piezoelectric layers, strain and stress gaps occur at the interfaces between the first and second piezoelectric layers. This can lead to the piezoelectric layers peeling and breaking at these interfaces.

[0009] The present invention has been made in light of these problems, and aims to provide a power generating composite material that is highly durable and has relatively excellent piezoelectric properties, and a method for producing the power generating composite material. [Means for solving the problem]

[0010] In order to achieve the above object, the composite material for power generation according to the present invention is characterized in that it contains a ceramic having piezoelectricity and a polymer having piezoelectricity, and is configured so that the content of the ceramic changes continuously and gradually along a predetermined direction.

[0011] The method for producing a composite material for power generation according to the present invention is characterized in that it comprises stacking a plurality of types of piezoelectric thin films, each containing a piezoelectric ceramic and a piezoelectric polymer and having different ceramic contents, so that the ceramic contents gradually change along the film thickness direction, followed by heat treatment.

[0012] The method for producing a power-generating composite material according to the present invention can suitably produce the power-generating composite material according to the present invention. Because the power-generating composite material according to the present invention contains not only ceramic but also a flexible polymer, it is more durable than piezoelectric materials composed only of ceramic, which are weak against repeated loads. Furthermore, because it does not need to be attached to an elastic substrate, it is resistant to vibration and impact and has excellent durability. Furthermore, while cracks tend to occur at interfaces where the ceramic content changes suddenly, the power-generating composite material according to the present invention is configured so that the ceramic content changes continuously and gradually, and there are no interfaces where the ceramic content changes suddenly, making it less susceptible to cracking and highly durable.

[0013] The power generating composite material according to the present invention has superior piezoelectric properties compared to composite materials made of ceramic and non-piezoelectric materials, because not only the ceramic but also the polymer has piezoelectricity. Since the ceramic content of the power generating composite material according to the present invention gradually changes, the power generating composite material contains ceramic and polymer with different piezoelectric constants, and thus can have a variety of properties depending on the magnitude of the content value and the state of change in the content. In order to obtain particularly excellent piezoelectric properties, the power generating composite material according to the present invention preferably contains a ceramic with a positive piezoelectric constant and a polymer with a negative piezoelectric constant.

[0014] The method for producing a composite material for power generation according to the present invention involves stacking multiple types of piezoelectric thin films with different ceramic contents and then performing heat treatment, thereby enabling the ceramic content to change continuously and gradually at the boundaries of the piezoelectric thin films, rather than suddenly. This prevents the boundaries of the piezoelectric thin films from being interfaces where the ceramic content changes suddenly, preventing cracking or peeling at the boundaries of the piezoelectric thin films and improving durability.

[0015] In the method for producing a power generating composite material according to the present invention, the heat treatment may be performed by heating each of the stacked piezoelectric thin films while they are compressed in their thickness direction, or by heating each of the stacked piezoelectric thin films without compressing them. Heating at a temperature lower than the recrystallization temperature of each of the piezoelectric thin films is also preferred. In this case, deterioration of the piezoelectric properties due to recrystallization can be prevented. When compressing each of the stacked piezoelectric thin films, the heat treatment can be performed, for example, by hot pressing.

[0016] In the method for producing a power generating composite material according to the present invention, it is preferable to produce each piezoelectric thin film by spin coating using a mixed solution obtained by adding the ceramic and the polymer to a solvent and stirring the mixture. In this case, a thin piezoelectric thin film with a uniform concentration can be produced relatively easily. In addition to spin coating, the piezoelectric thin film can also be produced by, for example, solvent casting.

[0017] In the method for producing a composite material for power generation according to the present invention, each piezoelectric thin film may be produced and then laminated, or each piezoelectric thin film may be laminated by sequentially producing piezoelectric thin films on the produced piezoelectric thin film. In the latter case, the step of laminating each produced piezoelectric thin film can be omitted compared to the former case, and the composite material for power generation can be produced efficiently in a relatively short time.

[0018] The power-generating composite material according to the present invention may be configured so that the ceramic content gradually increases or decreases continuously along the predetermined direction. In this case, the power-generating composite material according to the present invention can be manufactured by stacking the piezoelectric thin films so that the ceramic content gradually increases or decreases along the film thickness direction, and then performing the heat treatment. Since the power-generating composite material according to the present invention has different piezoelectric constants on one surface and the other surface, it has high power generation efficiency when bent in a predetermined direction (stacking direction). Therefore, it can be effectively used, for example, for vibration power generation using vibrations in a predetermined direction.

[0019] Furthermore, the power generating composite material according to the present invention may be configured so that the distribution of the ceramic content along the predetermined direction is plane-symmetric with respect to a central plane in the predetermined direction. In this case, the power generating composite material according to the present invention can be produced by stacking the piezoelectric thin films and then performing the heat treatment so that the ceramic content along the stacking direction of the laminate formed by stacking the piezoelectric thin films is plane-symmetric with respect to a central plane in the stacking direction of the laminate. Because the power generating composite material according to the present invention has a distribution of piezoelectric constants plane-symmetric with respect to a central plane in the predetermined direction (stacking direction), it has high power generation efficiency when compressed in the predetermined direction. Therefore, it can be effectively used, for example, for impact power generation using an impact in a predetermined direction.

[0020] The power generating composite material according to the present invention may be configured so that the ceramic content varies in both increasing and decreasing ranges along the predetermined direction. In this case, the power generating composite material according to the present invention can be manufactured by stacking the piezoelectric thin films and then performing the heat treatment so that the ceramic content varies in both increasing and decreasing ranges along the stacking direction of the laminate obtained by stacking the piezoelectric thin films. The power generating composite material according to the present invention has a distribution in which the piezoelectric constant increases and decreases along the predetermined direction (stacking direction).

[0021] In the power generating composite material and the method for producing the power generating composite material according to the present invention, the ceramic may be, for example, barium titanate (BaTiO3; BTO), potassium sodium niobate [(K,Na)NbO3; KNN], bismuth sodium titanate [(Bi 1 / 2 Na 1 / 2 The piezoelectric material may be made of any material having piezoelectric properties, such as perovskite structures such as titanium dioxide (Ti3;BNT) and bismuth ferrite (BiFeO3;BF), but it is preferable that it does not contain toxic lead. Perovskite structures are ferroelectrics, and therefore exhibit piezoelectricity when polarized. The polymer may be made of any material having piezoelectric properties, such as polyvinylidene fluoride (PVDF) or P(VDF-TrFE), a copolymer of polyvinylidene fluoride and trifluoroethylene. When P(VDF-TrFE) is polarized, the difference in electronegativity between hydrogen and fluorine causes a charge imbalance, resulting in piezoelectricity. While PVDF must be polarized in a stretched state, P(VDF-TrFE) does not need to be stretched during polarization.

[0022] The power generating composite material according to the present invention is preferably plate-shaped, with the thickness direction being the predetermined direction. In the method for producing a power generating composite material according to the present invention, it is preferable that a laminate obtained by stacking each piezoelectric thin film is plate-shaped, with the thickness direction being the stacking direction of the laminate. Furthermore, the power generating composite material according to the present invention preferably has a ceramic content of 50% or less in any range along the predetermined direction. In this case, the power generating composite material can be produced by the method for producing a power generating composite material according to the present invention, by making the ceramic content of each piezoelectric thin film 50% or less. Because the ceramic content is 50% or less, the power generating composite material is less likely to crack when subjected to repeated loads, vibrations, impacts, etc., and is highly durable.

[0023] In the power generating composite material according to the present invention, the ceramic content preferably varies substantially continuously. For example, as long as the ceramic content varies continuously, the change may be somewhat abrupt. Furthermore, in the power generating composite material according to the present invention, the interface between the polymer and the ceramic may be subjected to physical or chemical treatment. Furthermore, in addition to the piezoelectric polymer and ceramic, the power generating composite material according to the present invention may contain an alloy powder having a positive magnetostrictive effect, such as iron-cobalt (FeCo) or iron-cobalt-vanadium (FeCoV), or a ceramic powder having a negative magnetostrictive effect, such as cobalt ferrite (CoFeO). Furthermore, the power generating composite material according to the present invention may be attached to an electrode, for example, made of carbon fiber reinforced plastic, for increased toughness. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a power generating composite material that is highly durable and has relatively excellent piezoelectric properties, and a method for producing the power generating composite material. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a perspective view showing the flow of Example 1 of a method for producing a composite material for power generation according to an embodiment of the present invention. [Figure 2] FIG. 1 shows a perspective view (top) and a side view (bottom) of Example 1 of the composite material for power generation according to an embodiment of the present invention, showing (a) a sample having a BTO volume fraction of 30 vol.% as a comparative example, (b) a sample (FGM1) in which the BTO volume fraction gradually increases, (c) a sample (FGM2) in which the BTO volume fraction gradually decreases from both surfaces toward the center plane, and (d) a sample (FGM3) in which the BTO volume fraction gradually increases from both surfaces toward the center plane. [Figure 3] (a) Scanning electron microscope (SEM) photographs of cross sections of the comparative sample shown in FIG. 2(a), (b) FGM1 shown in FIG. 2(b), (c) FGM2 shown in FIG. 2(c), and (d) FGM3 shown in FIG. 2(d). [Figure 4]1 is a graph showing DSC curves obtained by differential scanning calorimetry of samples having a constant volume fraction of BTO, which are comparative examples of Example 1 of the power generating composite material according to an embodiment of the present invention. [Figure 5] 1 is a graph showing the piezoelectric constant d33 of each sample of Example 1 of the power generating composite material according to an embodiment of the present invention. [Figure 6] 1 is an overall configuration diagram showing a test device for an impact power generation test of Example 1 of a power generation composite material according to an embodiment of the present invention. [Figure 7] 1 is a graph showing output voltages in an impact power generation test of samples FGM1, FGM2, and FGM3, which are comparative examples of Example 1 of the power generation composite material according to an embodiment of the present invention and have a BTO volume fraction of 30 vol. %. [Figure 8] 1 is a perspective view showing the overall configuration of a test device for a vibration power generation test of Example 1 of a power generating composite material according to an embodiment of the present invention. [Figure 9] 1 is a graph showing output voltages at each frequency in a vibration power generation test for samples FGM1, FGM2, and FGM3, which are comparative examples of Example 1 of the power generation composite material according to an embodiment of the present invention and have a BTO volume fraction of 30 vol.%. [Figure 10] 1A is a front view showing a finite element analysis model of a composite material for power generation according to an embodiment of the present invention, and FIG. 1B is a perspective view showing the finite element analysis model and boundary conditions. [Figure 11] Figure 10 shows the analysis results using the finite element analysis model: (a) six BTO volume fraction distribution patterns and the potential difference for each pattern; (b) six BTO volume fraction distribution patterns and the potential difference for each pattern. [Figure 12] FIG. 1 is a perspective view showing a flow of Example 3 of a method for producing a composite material for power generation according to an embodiment of the present invention. [Figure 13]FIG. 10 shows scanning electron microscope (SEM) photographs of the cross sections of (a) sample AFGC1, (b) sample AFGC2, and (c) sample AFGC3 of Example 3, in a method for producing a composite material for power generation according to an embodiment of the present invention; (d) an SEM photograph of an enlarged central portion of (b); and (e) an SEM photograph of an enlarged central area of ​​(b). [Figure 14] 1 is a graph showing the output voltage at each frequency in a vibration power generation test for sample AFGC1 and sample FGM1 (denoted as FGC1 in the figure) of Example 3, which are composite materials for power generation according to an embodiment of the present invention. [Figure 15] 1 is a graph showing the output voltage (real-time output voltage) at elapsed times (time) of (a) 0 to 4005 seconds and (b) 4000 to 8005 seconds in a durability test of sample AFGC1 of Example 3 of the power generating composite material according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present invention will be described based on examples. The power generating composite material according to the embodiment of the present invention is in the form of a plate having a predetermined thickness, and includes a ceramic having piezoelectric properties and a polymer having piezoelectric properties, with the ceramic content gradually varying continuously along the thickness direction.

[0027] Ceramics include, for example, barium titanate (BaTiO3; BTO), potassium sodium niobate [(K,Na)NbO3; KNN], bismuth sodium titanate [(Bi 1 / 2 Na 1 / 2 The materials used are those with perovskite structures such as titanium dioxide (Ti3; BNT) and bismuth ferrite (BiFeO3; BF), and polymers such as polyvinylidene fluoride (PVDF) and P(VDF-TrFE), a copolymer of polyvinylidene fluoride and trifluoroethylene.

[0028] Furthermore, the composite material for power generation according to an embodiment of the present invention may be configured, for example, so that the ceramic content gradually increases or decreases along the thickness direction, or so that the distribution of the ceramic content along the thickness direction is symmetrical with respect to the center plane in the thickness direction.

[0029] The power generating composite material according to the embodiment of the present invention can be manufactured by the method for manufacturing the power generating composite material according to the embodiment of the present invention. That is, in the method for manufacturing the power generating composite material according to the embodiment of the present invention, first, a plurality of types of piezoelectric thin films containing a piezoelectric ceramic and a piezoelectric polymer and having different ceramic contents are manufactured. In this case, the piezoelectric thin films may be manufactured by any method as long as they are thin and have a uniform concentration. For example, each piezoelectric thin film may be manufactured by spin coating using a mixed solution obtained by adding a ceramic and a polymer to a solvent and stirring the mixture.

[0030] After the piezoelectric thin films are fabricated, they are laminated so that the ceramic content gradually changes along their thickness direction, and then heat-treated. At this time, for example, each laminated piezoelectric thin film may be heated in a state compressed in the thickness direction by hot pressing, or each laminated piezoelectric thin film may be heated without being compressed. Furthermore, by heating at a temperature lower than the recrystallization temperature of each piezoelectric thin film, it is possible to prevent a decrease in piezoelectric properties due to recrystallization. Furthermore, heat treatment can be performed so that the ceramic content does not change suddenly at the boundaries of the piezoelectric thin films, but changes continuously and gradually. In this way, a composite material for power generation according to an embodiment of the present invention can be manufactured.

[0031] The power generating composite material according to the embodiment of the present invention contains not only ceramic but also a flexible polymer, making it more durable than piezoelectric materials composed solely of ceramic, which are weak against repeated loads. Furthermore, since it does not need to be attached to an elastic substrate, it is resistant to vibration and impact and has excellent durability. Furthermore, while cracking is likely to occur at interfaces where the ceramic content changes suddenly, the power generating composite material according to the embodiment of the present invention is configured so that the ceramic content changes continuously and gradually, eliminating any interfaces where the ceramic content changes suddenly. Therefore, cracking and peeling are less likely to occur at the boundaries of the piezoelectric thin film, making it highly durable.

[0032] The power generating composite material according to the embodiment of the present invention has superior piezoelectric properties compared to a composite material made of a ceramic and a non-piezoelectric material, because not only the ceramic but also the polymer has piezoelectricity. The power generating composite material according to the embodiment of the present invention contains a ceramic and a polymer with different piezoelectric constants, and is configured so that the ceramic content changes gradually. Therefore, the power generating composite material can have a variety of properties depending on the value of the content and the state of change in the content.

[0033] In the method for producing a power generating composite material according to an embodiment of the present invention, instead of producing multiple types of piezoelectric thin films and then laminating each of the piezoelectric thin films, multiple types of piezoelectric thin films may be laminated by sequentially producing piezoelectric thin films on the produced piezoelectric thin films. In this case, the step of laminating each of the produced piezoelectric thin films can be omitted, allowing for efficient production in a relatively short time. In this case, each piezoelectric thin film can also be produced by spin coating. [Example]

[0034] A power generating composite material was produced by the method for producing a power generating composite material according to an embodiment of the present invention, and the piezoelectric constant was measured and a power generation test was carried out.

[0035] [Manufacturing composite materials for power generation] Among the raw materials used to manufacture the power-generating composite material, BaTiO3 (BTO, size 1.04 μm; manufactured by Nippon Chemical Industry Co., Ltd.) was used as the piezoelectric ceramic, and polyvinylidene fluoride (PVDF) copolymer, polyvinylidene trifluoroethylene (P(VDF-TrFE)) (Ideal Star Co., Ltd., "ISPP015-1"), was used as the piezoelectric polymer. BTO does not contain toxic lead, and is a ferroelectric material with a perovskite structure that exhibits piezoelectricity when polarized. Furthermore, when P(VDF-TrFE) is polarized, the difference between the electronegativity of hydrogen and the electronegativity of fluorine creates a charge imbalance, resulting in piezoelectricity.

[0036] As shown in Figure 1, powdered P(VDF-TrFE) was first added to the solvent dimethylformamide (DMF, Fujifilm Corporation) in a P(VDF-TrFE):DMF = 10:90 weight ratio and stirred for 20 minutes at 60 °C using a mixer (IKA-Werke GmbH & Co. KG, C-MAG HS4). BTO nanoparticles were then added to the solution, which was then stirred for 1 hour at room temperature and then ultrasonically stirred for 2 hours at 40 °C. The mixed solution was then thinly spread on a 10 mm diameter silicon wafer using a spin coater (Mikasa Corporation, MS-B150). The solvent was evaporated by heating on a hot plate at 90 °C to produce a BaTiO / P(VDF-TrFE) piezoelectric thin film 11. In this spin coating method, the spin coater was rotated at a rotation speed of 1000 rpm / sec for 5 seconds, and then decelerated to -200 rpm / sec.

[0037] Six types of piezoelectric thin films 11 were fabricated with volume fractions of BTO relative to P(VDF-TrFE) of 0 vol.%, 10 vol.%, 20 vol.%, 30 vol.%, 40 vol.%, and 50 vol.%. The average thicknesses of the piezoelectric thin films 11 were 0.010 mm, 0.012 mm, 0.014 mm, 0.016 mm, 0.018 mm, and 0.020 mm, respectively.

[0038] The various piezoelectric thin films 11 thus produced were laminated in their thickness direction, and then heated at 130°C for 30 seconds in a hot press while being compressed in the thickness direction at 7.5 MPa. In this manner, samples 10 of various types of composite materials for power generation were produced. After hot pressing, each sample 10 was cut into a rectangular shape of the desired size. The thickness was 0.15 mm.

[0039] As shown in Figure 2(a), six types of samples 10 were fabricated, each with a constant BTO volume fraction (comparison example), by laminating only piezoelectric thin films 11 with the same BTO volume fraction: 0 vol.%, 10 vol.%, 20 vol.%, 30 vol.%, 40 vol.%, and 50 vol.%. Because the thickness of the piezoelectric thin films 11 differs depending on the BTO volume fraction, the number of layers stacked was set to 18, 14, 13, 12, 9, and 8, respectively, to achieve a thickness of 0.15 mm after hot pressing. Note that Figure 2(a) shows sample 10 with a BTO volume fraction of 30 vol.%.

[0040] 2(b)–(d), three types of samples 10 with gradually varying BTO volume fractions were fabricated: one in which piezoelectric thin films 11 were laminated so that the BTO volume fraction gradually increased from 0 vol.% to 50 vol.% from one surface of the laminate to the other (hereafter referred to as FGM1); one in which piezoelectric thin films 11 were laminated so that the BTO volume fraction gradually decreased from 50 vol.% to 0 vol.% from both surfaces to the center plane of the laminate, symmetrically with respect to the center plane in the thickness direction of the laminate (hereafter referred to as FGM2); and one in which piezoelectric thin films 11 were laminated so that the BTO volume fraction gradually increased from 0 vol.% to 50 vol.% from both surfaces to the center plane of the laminate, symmetrically with respect to the center plane in the thickness direction of the laminate (hereafter referred to as FGM3). The average BTO volume fraction of these three samples 10 was 30 vol.%.

[0041] Each of the manufactured samples 10 shown in Figures 2(a) to (d) was covered with epoxy resin and cross-sectional observation was performed using a scanning electron microscope (SEM; Hitachi High-Tech Corporation's "SU-70"). The results are shown in Figures 3(a) to (d), respectively. As shown in Figures 3(a) to (d), in each of the samples 10, no boundaries were observed between the laminated piezoelectric thin films 11, confirming that they were integrated. In particular, in each of the samples 10 shown in Figures 3(b) to (d), it was confirmed that the volume fraction of BTO gradually increased along the direction of the arrows in the figures.

[0042] Differential scanning calorimetry (DSC) was performed on each sample 10 with a constant BTO volume fraction using a differential scanning calorimeter (DSC 404F3 manufactured by Netsch Japan Co., Ltd.). The results are shown in Figure 4. As shown in Figure 4, it was confirmed that the melting points of each sample 10 were 150°C to 155°C, and that the melting points decreased as the BTO volume fraction increased. It was also confirmed that the recrystallization temperature was 140°C. This indicates that hot pressing at 130°C did not affect the crystalline structure of sample 10.

[0043] [Polarization treatment] To impart piezoelectricity to each of the manufactured samples 10, a corona polarization process was performed using a corona discharge system (ELC-01N manufactured by Element Co., Ltd.). In the corona polarization process, a high voltage was applied to a tungsten needle placed away from the surface of each sample 10 while the back surface of each sample 10 was grounded and heated on a hot plate, generating a corona discharge between each sample 10 and the tungsten needle. This sprays a charge from the tungsten needle onto the surface of each sample 10, generating an electric field between the front and back surfaces of each sample 10, resulting in polarization. The voltage applied to the tungsten needle was 7.0 kV, and the hot plate temperature was 65°C.

[0044] [Measurement of piezoelectric properties] In a piezoelectric material polarized in the thickness direction, when the electric field is zero and stress T is applied in the thickness direction, the electric field density D is calculated as follows: 33 It is expressed by equation (1) using the above formula. Also, from equation (1), the piezoelectric constant d 33 is calculated using equation (2). D=d 33 ×T (1) d 33 =D / T=(Q / A) / (F / A)=Q / F (2) where Q is the surface charge (C) and A is the electrode area (m 2 ), F is the applied alternating force (N).

[0045] An alternating force F is applied to the back surface of each manufactured sample 10, and the generated charge Q is measured using an electrode attached to the surface of each sample 10, and the piezoelectric constant d is calculated using equation (2). 33 The measurement was carried out using the piezoelectric 33 A piezoelectric meter (YE2730A manufactured by Sinocera Piezotronics Inc.) was used. The positive and negative piezoelectric constants were obtained from the measurement, and the average value of the positive and negative piezoelectric constants was calculated as the piezoelectric constant d 33 The obtained piezoelectric constant d of each sample 10 33 is shown in Figure 5.

[0046] As shown in Figure 5, for each sample 10 with a constant volume fraction of BTO, P(VDF-TrFE) has a negative piezoelectric constant d 33 BTO has a positive piezoelectric constant d 33 Therefore, when the volume fraction of BTO is 0 vol.%, the piezoelectric constant d 33 is negative, but as the volume fraction of BTO increases, the piezoelectric constant d 33 It was confirmed that the piezoelectric constant d 33 was confirmed to be zero.

[0047] Furthermore, as shown in Figure 5, in each sample (FGM1, FGM2, FGM3)10 in which the volume fraction of BTO gradually changes, the piezoelectric constant d 33 It was confirmed that FGM1 had a negative piezoelectric constant d that was almost the same as that of sample 10, which had a constant BTO volume fraction of 0 vol.%. 33 FGM2 and FGM3 have a larger negative piezoelectric constant d than FGM1. 33 It was confirmed that a temperature difference of 1000°C was obtained. This is thought to be due to the influence of internal stress generated during hot pressing in Sample 10, where the volume fraction of BTO changes gradually. In other words, the internal stress makes it easier to polarize the piezoelectric ceramic and piezoelectric polymer.

[0048] Next, the relative permittivity ε of each sample (FGM1, FGM2, FGM3) with gradually changing BTO volume fraction was γ was measured using an LCR meter ("ZM2371" manufactured by NF Corporation). As a result, the relative dielectric constant ε γ was about 17-18.

[0049] From the above results, the figure of merit (FoM) of each sample FGM1 to 3 was calculated. FoM is calculated as follows: d×g=d 2× ε, where d is the piezoelectric strain constant, g is the piezoelectric voltage constant, and ε is the dielectric constant. FoM represents the energy generation efficiency of the piezoelectric material in the 33 mode, and the larger the FoM, the higher the energy harvesting efficiency. The FoM for each sample was 279 for FGM1, 402 for FGM2, and 551 for FGM3. From this, it can be considered that of samples FGM1 to 3, FGM3 has the highest energy harvesting efficiency.

[0050] [Impact power generation test] An impact energy generation test was conducted on sample 10 with a constant BTO volume fraction of 30 vol.% and on each sample 10 with a gradually changing BTO volume fraction. As shown in Figure 6, in the test, to examine the amount of power generated by impact when striking the surface of each sample 10, each sample 10 was cut into a 10 mm x 20 mm rectangle and attached to the enter key 1a of a keyboard 1. The output voltage when the enter key 1a was pressed was measured using an oscilloscope ("DL850E" manufactured by Yokogawa Electric Corporation) 2. Measurements were conducted 200 times for each sample, and the average value was used as the measured value. The measurement results are shown in Figure 7.

[0051] As shown in Figure 7, it was confirmed that the samples 10 (FGM1, FGM2, FGM3) with gradually changing BTO volume fractions had higher output voltages and generated more power than the sample 10 with a constant BTO volume fraction of 30 vol.%. Furthermore, among the samples 10 with gradually changing BTO volume fractions, the output voltage of FGM3 was particularly high, and it was confirmed that it generated approximately twice the power of the sample 10 with a constant BTO volume fraction of 30 vol.%.

[0052] [Vibration power generation test] A vibration power generation test was conducted on sample 10 with a constant BTO volume fraction of 30 vol.% and on samples 10 with gradually varying BTO volume fractions. As shown in Figure 8, each sample 10 was cut into a 40 mm x 10 mm rectangular plate. One end of the sample was attached to a vibrator ("ET-132" manufactured by Labworks) 3, and a 1.5 g weight 4 was attached to the other end. Each sample 10 was vibrated at a desired frequency using a function generator ("FG-281" manufactured by Kenwood Corporation) 5, and the output voltage was measured using a data logger ("Keyence NR-500" manufactured by Keyence Corporation) 6. The amplitude was set to 0.9 mm, and the frequency was varied from 0 to 68 Hz, and the output voltage was measured at each frequency. The measurement results are shown in Figure 9.

[0053] As shown in Figure 9, it was confirmed that the output voltage of each sample 10 increased with increasing frequency. For example, at a frequency of 68 Hz, the output voltage of each sample 10, with a constant BTO volume fraction of 30 vol.%, was 0.973 mV, 9.024 mV for FGM1, 2.374 mV for FGM2, and 1.987 mV for FGM3. Furthermore, among sample 10 with gradually varying BTO volume fractions, the output voltage of FGM1 was significantly higher, and it was confirmed that it could generate 5 to 10 times more power than the other samples 10 at frequencies above approximately 30 Hz. This is thought to be because, in FGM1, the BTO volume fraction is asymmetric with respect to the center plane of the thickness, and therefore, when vibrated, the charges generated on one surface (e.g., the outer surface of the bend) and the other surface (e.g., the inner surface of the bend) do not cancel each other out, resulting in a high output voltage. In contrast, in each of the other samples 10, the volume fraction of BTO is symmetrical with respect to the center plane of the thickness, so it is thought that when vibrated, the charges generated on one surface and the charges generated on the other surface cancel each other out, resulting in a smaller output voltage. [Example]

[0054] Numerical calculations were carried out on the compressive stress-induced voltage of the power generating composite material according to the embodiment of the present invention using a simple finite element analysis model.

[0055] As shown in Figure 10(a), the finite element analysis model of the power-generating composite material was a thin rod containing BTO and P(VDF-TrFE), with a pattern in which the volume fraction of BTO gradually increased from both ends toward the center (the pattern of FGM3 in Example 1). In the finite element analysis model, the BTO was assumed to be spherical, and the radius of the BTO at each position was adjusted to obtain the desired BTO volume fraction. Tables 1 to 3 show the material properties of PVDF and BTO used in the finite element analysis, as well as the radius of BTO at each volume fraction.

[0056] [Table 1]

[0057] [Table 2]

[0058] [Table 3]

[0059] As shown in Figure 10(b), the finite element analysis (FEA) model was designed as a rectangular column with a base of 1 mm × 1 mm and a height of 12 mm, and the bottom surface (z = 0) of the model was fixed. Furthermore, symmetry conditions were used for the side surfaces at x = 0 and y = 0, and the potential at z = 0 was set to 0. Furthermore, the FEA was used to calculate the distribution of the BTO volume fraction for the six models shown in Figures 11(a) and (b), as shown in Figure 10(b). The potential distribution was calculated when a compressive stress P0 = 1 kPa was applied to the top surface (z = h) of each model. The potential difference resulting from a compressive stress of 1 kPa was calculated from the potential distribution. The distribution of the BTO volume fraction for each model is shown in Figure 11(a), and the potential difference calculated for each model is shown in Figures 11(a) and (b).

[0060] 11(a) and (b), it was confirmed that the output voltage differs depending on how the BTO volume fraction changes, even in a pattern in which the BTO volume fraction gradually increases from both ends toward the center (the pattern of FGM3 in Example 1). For example, it is expected that the amount of power generated will be greater if the BTO volume fraction is increased more at the center than at the ends. Furthermore, even if the overall BTO volume fraction is small, it is expected that the amount of power generated will be greater if the BTO volume fraction is rapidly increased toward the center. [Example]

[0061] A power generating composite material was produced by the method for producing a power generating composite material according to an embodiment of the present invention, and similarly to Example 1, the piezoelectric constant was measured and a power generation test was carried out.

[0062] [Manufacturing composite materials for power generation] Three samples, AFGC1 to AFGC3, having the same structure as FGM1 to FGM3 in Figure 2 were fabricated using the same materials and equipment as in Example 1 but a different process from Example 1. As shown in Figure 12, powdered P(VDF-TrFE) was first added to the DMF solvent in a weight ratio of P(VDF-TrFE):DMF = 10:90 and stirred at 60°C for 20 minutes. BTO nanoparticles were then added to the solution, which was then stirred at room temperature for 2 hours and then ultrasonically stirred at 40°C for 10 minutes. The stirred mixture was then thinly spread on a 4-inch diameter, 300 μm thick silicon wafer by spin coating and heated on a hot plate at 148°C for 15 minutes to evaporate the DMF solvent, resulting in the formation of the bottom piezoelectric thin film 11. Similarly, a total of 12 piezoelectric thin films 11 were laminated by sequentially fabricating another piezoelectric thin film 11 on top of the fabricated piezoelectric thin film 11. In the spin coating method, for each piezoelectric thin film 11 fabricated, the substrate was rotated at a rotation speed of 500 rpm / sec for 5 seconds, then at a rotation speed of 1000 rpm / sec for 5 seconds while accelerating at 100 rpm / sec, and then decelerated at -200 rpm / sec.

[0063] After all the piezoelectric thin films 11 were laminated, a heat treatment was performed at 148°C for 4 hours. After the heat treatment, each sample was immersed in deionized water (DI water) together with the wafer overnight, and then peeled off from the wafer and dried. In this way, samples 10 of AFGC1 to 3 were manufactured. The thicknesses of the manufactured samples AFGC1 to 3 are shown in Table 4.

[0064] [Table 4]

[0065] This manufacturing method can efficiently manufacture a power-generating composite material in a relatively short time by eliminating the step of laminating each of the fabricated piezoelectric thin films 11 one by one. Furthermore, by immersing each processed sample together with the wafer in deionized water overnight, each sample 10 can be peeled off from the wafer without being damaged by shrinkage of the bottom piezoelectric thin film 11.

[0066] As shown in Table 4, the edges of each of the samples AFGC1 to AFGC3 were thicker than the center due to the accumulation of the solution during the spin coating method. Therefore, when using each of the samples AFGC1 to AFGC3, a test piece was cut out from the center of each sample AFGC1 to AFGC3.

[0067] The cross sections of the manufactured samples AFGC1 to AFGC3 were observed with a scanning electron microscope, and the results are shown in Figures 13(a) to 13(e), respectively. As shown in Figures 13(a) to 13(c), in each of the samples AFGC1 to AFGC3, the laminated piezoelectric thin films were integrated without any discernible boundaries, and it was confirmed that the volume fraction of BTO gradually changed. Furthermore, as shown in Figures 13(d) and 13(e), it was confirmed that BTO was embedded in the dendritic P(VDF-TrFE) crystals, and that P(VDF-TrFE) and BTO were bonded to each other.

[0068] [Polarization treatment] In order to develop piezoelectricity in each of the manufactured samples AFGC1 to AFGC3, a polarization treatment was carried out by corona polarization in the same manner as in Example 1. The polarization treatment was carried out at 65°C and 52 kV / mm 2 Each sample was polarized in the thickness direction for 30 minutes.

[0069] [Measurement of piezoelectric properties] The piezoelectric constant d of each of the samples AFGC1 to AFGC3 manufactured in the same manner as in Example 1 was 33 and relative permittivity ε γ The measurement results are shown in Table 4. As shown in Table 4, the piezoelectric constant d 33 It was confirmed that AFGC1 had the largest negative value, and AFGC2 and AFGC3 had almost the same negative value. γIt was confirmed that the piezoelectric constant d of each sample AFGC1 to AFGC3 was approximately 11 to 14. 33 The value of the piezoelectric constant d 33 It was confirmed that AFGC2 and AFGC3 had almost the same values ​​as FGM2 and FGM3, which have the same structure, while AFGC1 had a value approximately 1.6 times that of FGM1, which has the same structure. This is thought to be due to differences in the manufacturing process of each sample, i.e., whether or not a hot pressing process was used, and the uniformity of heat transfer during the hot pressing process during the manufacturing of sample FGM1 in Example 1.

[0070] From the above results, the figure of merit (FoM) of each of the samples AFGC1 to 3 was calculated in the same manner as in Example 1. The FoM of each sample was 884 for AFGC1, 593 for AFGC2, and 679 for AFGC3. Comparing these FoM values ​​with the FoM values ​​of each of the samples FGM1 to 3 in Example 1, it was confirmed that each of the samples AFGC1 to 3 was larger than each of the samples FGM1 to 3 having the corresponding structure. From this, it is considered that each of the samples AFGC1 to 3 has a higher energy harvesting efficiency than each of the samples FGM1 to 3. Furthermore, among the samples AFGC1 to 3, AFGC1 has the largest FoM, and AFGC1 is considered to have the highest energy harvesting efficiency.

[0071] [Vibration power generation test] A vibration power generation test was conducted on sample AFGC1 in the same manner as in Example 1. The test was conducted using the same apparatus as in Figure 8, except that the length of sample AFGC1 was set to 30 mm. In the test, sinusoidal vibration with an amplitude of 0.9 mm and a frequency of 10 to 50 Hz was used, and the output voltage at each frequency was measured. Note that the same measurement was also conducted on sample FGM1 produced in Example 1. The measurement results are shown in Figure 14.

[0072] As shown in Figure 14, an output voltage peak was observed in both samples AFGC1 and FGM1 (referred to as FGC1 in the figure), and it was confirmed that the resonant frequency at that time was approximately 23 Hz in both cases. It was also confirmed that the output voltage at the resonant frequency was approximately 91 mV in sample AFGC1, which was approximately 10 times the output voltage (approximately 9 mV) of sample FGM1. This is thought to be due to differences in the manufacturing methods of samples AFGC1 and FGM1, and is thought to be because the piezoelectric thin films 11 in sample AFGC1 were more firmly bonded than in sample FGM1, making polarization easier and improving the piezoelectricity.

[0073] To investigate the durability of sample AFGC1, a vibration power generation test device was used to vibrate the sample at an amplitude of 0.9 mm and a frequency of 25 Hz, and the output voltage (real-time output voltage) was measured for 5 seconds (125 cycles) every 50,000 cycles (2,000 seconds) up to 200,000 cycles. The measurement results are shown in Figures 15(a) and 15(b). As shown in Figure 15, it was confirmed that the output voltage did not decrease even after 200,000 cycles. This indicates that sample AFGC1 has excellent durability. [Explanation of symbols]

[0074] 1 keyboard 1a Enter key 2 oscilloscopes 3. Vibrator 4 weight 5. Function Generator 6 Data Logger 10 (power generating composite material) samples 11 Piezoelectric thin film

Claims

1. A composite material for power generation comprising a piezoelectric ceramic and a piezoelectric polymer, wherein the ceramic content varies continuously and gradually along a predetermined direction, and the composite material is configured to be symmetrical with respect to a central plane in the predetermined direction.

2. 2. The composite material for power generation according to claim 1, wherein the ceramic content increases and decreases along the predetermined direction.

3. 3. The composite material for power generation according to claim 1, wherein the ceramic has a perovskite structure.

4. The ceramic is barium titanate (BaTiO ３ ; BTO), potassium sodium niobate [(K,Na)NbO ３ ;KNN], bismuth sodium titanate [(Bi １／２ Na １／２ ) Ti ３ ;BNT], and bismuth ferrite (BiFeO ３ BF), The polymer contains at least one of polyvinylidene fluoride (PVDF) and P(VDF-TrFE), which is a copolymer of polyvinylidene fluoride and trifluoroethylene.

3. The composite material for power generation according to claim 1 or 2.

5. 3. The composite material for power generation according to claim 1, which is in the form of a plate, and the thickness direction of the plate is the predetermined direction.

6. A method for producing a composite material for power generation, comprising stacking a plurality of types of piezoelectric thin films, each of which contains a piezoelectric ceramic and a piezoelectric polymer and has a different ceramic content, so that the ceramic content gradually changes along the film thickness direction, and then performing a heat treatment in which each of the stacked piezoelectric thin films is compressed in the film thickness direction and heated at a temperature lower than the recrystallization temperature of each piezoelectric thin film.

7. 7. The method for producing a composite material for generating electricity according to claim 6, wherein the ceramic and the polymer are added to a solvent, stirred, and then the resulting mixture is used to form each of the piezoelectric thin films by spin coating.

8. 8. The method for producing a composite material for power generation according to claim 6, wherein after each piezoelectric thin film is produced, the piezoelectric thin films are laminated.

9. 8. The method for producing a power generating composite material according to claim 6, wherein each piezoelectric thin film is laminated by successively producing a piezoelectric thin film on the produced piezoelectric thin film.

10. 8. The method for producing a composite material for power generation according to claim 6, wherein the ceramic has a perovskite structure.

11. The ceramic is barium titanate (BaTiO ３ ; BTO), potassium sodium niobate [(K,Na)NbO ３ ;KNN], bismuth sodium titanate [(Bi １／２ Na １／２ ) Ti ３ ;BNT], and bismuth ferrite (BiFeO ３ BF), The polymer contains at least one of polyvinylidene fluoride (PVDF) and P(VDF-TrFE), which is a copolymer of polyvinylidene fluoride and trifluoroethylene. The method for producing the composite material for power generation according to claim 6 or 7.

12. 8. The method for producing a composite material for power generation according to claim 6, wherein the laminate obtained by laminating each of the piezoelectric thin films is in the form of a plate, and the thickness direction of the laminate is the lamination direction of the laminate.

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