Composite nanofiber piezoelectric material, method for manufacturing composite nanofiber piezoelectric material, pressure sensor, power generation device, and filter

Abstract

To provide a composite nanofiber piezoelectric material, which is a fiber piezoelectric material that can achieve high piezoelectric performance over a wide pressure range.SOLUTION: A composite nanofiber piezoelectric material includes a nanofiber membrane having a nonwoven structure and first nanofibers, and a microstructure formed in the pores of the nanofiber membrane having second nanofibers, and the first nanofibers contain a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer, and the second nanofibers contain a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer, and have an average fiber diameter smaller than that of the first nanofibers.SELECTED DRAWING: Figure 18

Description

[Technical Field]

[0001] The present invention relates to a composite nanofiber piezoelectric material, a method for producing a composite nanofiber piezoelectric material, a pressure sensor, a power generation device, and a filter. [Background technology]

[0002] Conventionally, materials that exhibit the piezoelectric effect (piezoelectric materials) are widely known to be made of quartz or ceramics. In addition, polymers (polymer materials) such as polyvinylidene fluoride (PVDF) that can be used as piezoelectric materials (piezoelectric polymers) are also known (see, for example, Patent Document 1 and Non-Patent Document 1).

[0003] By fabricating piezoelectric polymers into fibers, it is possible to produce highly flexible fiber piezoelectric materials. Such fiber piezoelectric materials can be used in the field of so-called flexible electronics. In other words, fiber piezoelectric materials are expected to be applicable to a variety of applications, including space exploration, interfaces for operating machines, and wearable electronics.

[0004] Specific examples of fibrous piezoelectric materials are as follows: Patent Document 1 describes a piezoelectric material in which nanofibers made of a piezoelectric polymer are aligned in the fiber axis direction to form a film. Patent Document 1 also describes polyvinylidene fluoride, poly(vinylidene-trifluoroethylene) copolymer, or poly-L-lactic acid as the piezoelectric polymer.

[0005] Furthermore, Non-Patent Document 1 describes a piezoelectric material in the form of a film of nanofibers using a piezoelectric polymer. The piezoelectric polymer described in Non-Patent Document 1 is polyvinylidene fluoride. Non-Patent Document 1 also describes the use of dopamine (DA) as an additive. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-109431 [Non-patent literature]

[0007] [Non-Patent Document 1] “High-Performance Poly(vinylidene difluoride) / Dopamine Core / Shell Piezoelectric Nanofiber and Its Application for Biomedical Sensors”,Advanced Materials,(Germany),Wiley-VCH,04 December 2020,Volume 33,Issue 3,2006093 Summary of the Invention [Problem to be solved by the invention]

[0008] However, conventionally known fiber piezoelectric materials have a problem in that they are unable to obtain high piezoelectric performance over a wide pressure range. For example, the piezoelectric material described in Patent Document 1 has an axial length of about 8 mm and a width of about 50 mm (approximately 400 mm). 2 ) the maximum output voltage in the sample was 37mV, which is a small value for the output voltage.

[0009] Furthermore, the piezoelectric material described in Non-Patent Document 1 is 10 mm x 20 mm (200 mm 2 ) sample, the output voltage was about 16V when a force of 1kPa was applied, and about 33V when a force of 6kPa was applied. This means that the sensitivity is 80VN. -1 ~27.5VN -1 , the pressure range is 0.2 N to 1.2 N. In other words, the piezoelectric material described in Non-Patent Document 1 has high sensitivity but a narrow pressure range (it can only demonstrate its performance under extremely low pressure conditions).

[0010] The present invention has been made to solve the above problems, and aims to provide a composite nanofiber piezoelectric material, which is a fiber piezoelectric material that can achieve high piezoelectric performance over a wide pressure range. Another aim of the present invention is to provide a method for producing the composite nanofiber piezoelectric material of the present invention. A further aim of the present invention is to provide a pressure sensor, a power generator, and a filter that use the composite nanofiber piezoelectric material of the present invention. [Means for solving the problem]

[0011] [1] A composite nanofiber piezoelectric material according to one embodiment of the present invention comprises a nanofiber membrane having a nonwoven structure and having first nanofibers, and a microstructure having second nanofibers formed within the pores of the nanofiber membrane, wherein the first nanofibers contain a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer, and the second nanofibers contain a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer, and the second nanofibers have an average fiber diameter smaller than that of the first nanofibers.

[0012] [2] In the composite nanofiber piezoelectric material according to one aspect of the present invention, the first piezoelectric polymer and the second piezoelectric polymer preferably contain a polyvinylidene fluoride (PVDF)-based polymer.

[0013] [3] In the composite nanofiber piezoelectric material according to one aspect of the present invention, the first additive and the second additive preferably contain catecholamine.

[0014] [4] In the composite nanofiber piezoelectric material according to one aspect of the present invention, the first additive and the second additive preferably contain dopamine (DA).

[0015] [5] In the composite nanofiber piezoelectric material according to one aspect of the present invention, the average fiber diameter of the first nanofibers is preferably within a range of 5 to 30 times the average fiber diameter of the second nanofibers.

[0016] [6] In one embodiment of the composite nanofiber piezoelectric material of the present invention, it is preferable that the average fiber diameter of the first nanofibers is in the range of 100 nm to 1,000 nm, and the average fiber diameter of the second nanofibers is in the range of 10 nm to 100 nm.

[0017] [7] In the composite nanofiber piezoelectric material according to one aspect of the present invention, the weight average molecular weight (Mw) of the first piezoelectric polymer is preferably smaller than the weight average molecular weight (Mw) of the second piezoelectric polymer.

[0018] [8] In one embodiment of the composite nanofiber piezoelectric material of the present invention, it is preferable that the weight average molecular weight (Mw) of the first piezoelectric polymer is in the range of 50,000 to 300,000, and the weight average molecular weight (Mw) of the second piezoelectric polymer is in the range of 500,000 to 1,000,000.

[0019] [9] In one embodiment of the composite nanofiber piezoelectric material of the present invention, it is preferable that the amount of the first additive added to the first nanofiber is in the range of 0.5 wt% to 5 wt% relative to the weight of the first piezoelectric polymer, and the amount of the second additive added to the second nanofiber is in the range of 0.5 wt% to 5 wt% relative to the weight of the second piezoelectric polymer.

[0020]

[10] A method for producing a composite nanofiber piezoelectric material according to one embodiment of the present invention includes a nanofiber membrane formation process in which a nanofiber membrane having a nonwoven structure is formed by electrospinning a first electrospinning solution containing a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer, and the first nanofiber is formed by electrospinning; and a microstructure formation process in which a second electrospinning solution containing a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer is used to form second nanofibers having an average fiber diameter smaller than that of the first nanofibers, and the microstructure is formed within the pores of the nanofiber membrane.

[0021]

[11] In one embodiment of the method for producing a composite nanofiber piezoelectric material according to the present invention, it is preferable that in the nanofiber membrane formation process, the nanofiber membrane is formed on the surface of a collector, and in the microstructure formation process, electrospinning is performed using the collector with the nanofiber membrane present on its surface.

[0022]

[12] A pressure sensor according to one embodiment of the present invention is characterized by comprising a piezoelectric portion having the composite nanofiber piezoelectric material described in [1] above, and a pair of electrodes arranged to sandwich the piezoelectric portion.

[0023]

[13] A power generating device according to one embodiment of the present invention is characterized by comprising a piezoelectric part having the composite nanofiber piezoelectric material described in [1] above, and a pair of electrodes arranged to sandwich the piezoelectric part.

[0024]

[14] A filter according to one embodiment of the present invention is characterized by comprising the composite nanofiber piezoelectric material described in [1] above. [Effects of the Invention]

[0025] The composite nanofiber piezoelectric material of the present invention comprises a "nanofiber membrane having a nonwoven structure, which has first nanofibers containing a first piezoelectric polymer and a first additive," and a "microstructure formed in the pores of the nanofiber membrane, which has second nanofibers containing a second piezoelectric polymer and a second additive." Therefore, as shown in the examples below, the composite nanofiber piezoelectric material of the present invention is a fiber piezoelectric material that can achieve high piezoelectric performance over a wide pressure range.

[0026] The method for producing a composite nanofiber piezoelectric material of the present invention includes a nanofiber membrane formation step of forming a nanofiber membrane and a microstructure formation step of forming microstructures in the pores of the nanofiber membrane, and therefore, the method for producing a composite nanofiber piezoelectric material of the present invention makes it possible to produce the composite nanofiber piezoelectric material of the present invention relatively easily.

[0027] The pressure sensor, power generation device, and filter of the present invention contain the composite nanofiber piezoelectric material of the present invention, and therefore are products with unprecedented performance (for example, high sensitivity, high power generation performance, and high collection performance). [Brief explanation of the drawings]

[0028] [Figure 1] 1 is an SEM image of a nanofiber membrane in an example. [Figure 2] 1 is a TEM image of a nanofiber membrane in an example. [Figure 3] 1 is an EDS image of a first nanofiber constituting PDS-2 in an example. [Figure 4] 1 shows an XPS spectrum of a nanofiber membrane in an example. [Figure 5] 1 shows an XRD spectrum of a nanofiber membrane in an example. [Figure 6] 1 shows an ATR-FTIR spectrum of a nanofiber membrane in an example. [Figure 7]1 is a bar graph showing the content of β-phase crystal structure in nanofiber membranes in Examples. [Figure 8] 1 is a DSC thermogram of a nanofiber membrane in an example. [Figure 9] 1 is a graph showing the crystallinity (Xc) and β-phase crystallinity of nanofiber membranes in Examples. [Figure 10] 1 is an SEM image of a microstructure in an example. [Figure 11] 1 is a TEM image of a microstructure in an example. [Figure 12] 1 is an EDS image of a second nanofiber constituting PDU-1 in an example. [Figure 13] 1 is an XPS spectrum of PDU-1 in an example. [Figure 14] 10 is a TEM image of a second nanofiber constituting a microstructure in an example. [Figure 15] 1 shows an ATR-FTIR spectrum of a microstructure in an example. [Figure 16] 1 shows an XRD spectrum of a microstructure in an example. [Figure 17] 1A and 1B are diagrams illustrating a piezoelectric element using a composite nanofiber piezoelectric material in an example. [Figure 18] 1 is an SEM image of a composite nanofiber piezoelectric material according to an example. [Figure 19] 1 is a graph showing the dielectric constant, dielectric loss, and piezoelectric coefficient of a composite nanofiber piezoelectric material according to an example. [Figure 20] 1 is a bar graph showing the piezoelectric coefficient (d33) of a composite nanofiber piezoelectric material according to an example. [Figure 21] 1 is a graph showing the output voltage of a piezoelectric element manufactured using a composite nanofiber piezoelectric material according to an example. [Figure 22] 1 is a graph showing the output current of a piezoelectric element manufactured using a composite nanofiber piezoelectric material according to an example. [Figure 23] 10 is a graph showing characteristics of a pressure sensor according to an example. [Figure 24] 10 is a graph showing an output voltage of a pressure sensor in response to speech (throat vibration) according to an example. [Figure 25] 10 is a graph showing an output voltage relative to the pulse rate of the pressure sensor according to the embodiment. [Figure 26] 10 is a graph showing an output voltage of a pressure sensor according to an embodiment in response to human body movement. [Figure 27] 10 is a photograph shown to explain an LED light emission test using a power generating device according to an example. [Figure 28] FIG. 10 is a diagram for explaining a capacitor charging test using the power generation device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, the composite nanofiber piezoelectric material, the method for producing the composite nanofiber piezoelectric material, the pressure sensor, the power generation device, and the filter of the present invention will be described based on embodiments.

[0030] [Embodiment] 1. Composite nanofiber piezoelectric material The composite nanofiber piezoelectric material according to the embodiment includes a nanofiber membrane (nanofiber layer) having a nonwoven structure and a second nanofiber, and a microstructure formed in the pores of the nanofiber membrane. Specific structures of the composite nanofiber piezoelectric material can be as shown in Figures 18(a) to 18(f) relating to the examples described below.

[0031] In this specification, the term "microstructure" refers to a mesh-like structure whose main structure is fine nanofibers. In the composite nanofiber piezoelectric material according to the embodiment, it can be said that a two-dimensional topological network structure is formed by combining (connecting) the nanofiber membrane and the microstructure.

[0032] The first nanofiber contains a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer, and the second nanofiber contains a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer, and has a smaller average fiber diameter than the first nanofiber.

[0033] In this specification, "piezoelectric polymer" refers to a polymer that exhibits the piezoelectric effect. Piezoelectric polymers also include those whose crystal structure determines whether they exhibit the piezoelectric effect. Furthermore, in this specification, "piezoelectric crystal structure" refers to a crystal structure that exhibits the piezoelectric effect, among the crystal structures that piezoelectric polymers can have. For example, in the case of polyvinylidene fluoride (PVDF), which will be described later, the β-phase crystal structure and the γ-phase crystal structure correspond to the piezoelectric crystal structure.

[0034] Furthermore, in this specification, "additives that stabilize the piezoelectric crystal structure of a piezoelectric polymer" refers to substances that have the effect of promoting the formation (transformation) of a piezoelectric crystal structure or the effect of maintaining a piezoelectric crystal structure during or after the formation of nanofibers (e.g., during electrospinning). Specific examples will be described later, but in many cases, the above effect is thought to be achieved by the interaction of the functional groups of the additive with the functional groups of the piezoelectric polymer. From this perspective, additives that contain at least one of an amino group, a carboxyl group, and a hydroxyl group, and preferably two or more of these, can be used.

[0035] The first piezoelectric polymer may be the same type of polymer as the second piezoelectric polymer, or a different polymer. Even if the first piezoelectric polymer is the same type of polymer as the second piezoelectric polymer, the physical properties, such as molecular weight, may be different. The first additive may be the same type of substance as the second additive, or a different substance.

[0036] The first piezoelectric polymer and the second piezoelectric polymer preferably contain a polyvinylidene fluoride (PVDF)-based polymer. Note that polyvinylidene fluoride may be simply referred to as "PVDF" in the following description. Furthermore, it is more preferable that the first piezoelectric polymer and the second piezoelectric polymer are made of a polyvinylidene fluoride-based polymer. In this specification, "polyvinylidene fluoride-based polymer" refers to a polymer produced using vinylidene fluoride as a material, and includes both homopolymers and copolymers.

[0037] The first additive and the second additive preferably contain a catecholamine. Catecholamine refers to a compound having a catechol and an amine in the molecule. Specific examples of catecholamine include dopamine, noradrenaline, and adrenaline. In the composite nanofiber piezoelectric material according to the embodiment, the first additive and the second additive preferably contain dopamine (DA). Note that dopamine may be referred to simply as "DA" in the following description.

[0038] The average fiber diameter of the first nanofibers is preferably within a range of 5 to 30 times the average fiber diameter of the second nanofibers. Also, the average fiber diameter of the first nanofibers is preferably within a range of 100 nm to 1,000 nm, and the average fiber diameter of the second nanofibers is preferably within a range of 10 nm to 100 nm.

[0039] The weight average molecular weight (Mw) of the first piezoelectric polymer is preferably smaller than the weight average molecular weight (Mw) of the second piezoelectric polymer. Preferably, the weight average molecular weight (Mw) of the first piezoelectric polymer is in the range of 50,000 to 300,000, and the weight average molecular weight (Mw) of the second piezoelectric polymer is in the range of 500,000 to 1,000,000.

[0040] It is preferable that the amount of the first additive added to the first nanofibers is in the range of 0.5 wt% to 5 wt% relative to the weight of the first piezoelectric polymer, and the amount of the second additive added to the second nanofibers is in the range of 0.5 wt% to 5 wt% relative to the weight of the second piezoelectric polymer.

[0041] The first nanofibers may contain an additive other than the first additive, and the second nanofibers may contain an additive other than the second additive.

[0042] 2. Manufacturing method for composite nanofiber piezoelectric material The method for producing a composite nanofiber piezoelectric material according to the embodiment includes a nanofiber membrane forming step and a microstructure forming step. Note that the method for producing a composite nanofiber piezoelectric material may further include steps other than those described above.

[0043] The nanofiber membrane formation process is a process for forming a nanofiber membrane having a nonwoven structure by forming first nanofibers by electrospinning using a first electrospinning solution containing a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer.

[0044] The microstructure formation process is a process of forming a microstructure within the pores of the nanofiber membrane by electrospinning second nanofibers having an average fiber diameter smaller than that of the first nanofibers using a second electrospinning solution containing a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer.

[0045] In the nanofiber membrane forming step, the nanofiber membrane is formed on the surface of a collector, and in the microstructure forming step, electrospinning is preferably carried out using a collector with the nanofiber membrane present on its surface.

[0046] The solvents for preparing the first electrospinning solution and the second electrospinning solution can be selected appropriately depending on the physical properties of the piezoelectric polymer used. Other parameters required for electrospinning can also be determined appropriately depending on the material used and the product to be manufactured. In order to form fine second nanofibers, it is preferable that the second electrospinning solution uses a second piezoelectric polymer with a higher molecular weight than the first electrospinning solution, and the concentration of the second piezoelectric polymer is lowered to increase the conductivity.

[0047] As shown in the examples below, in order to increase the conductivity of the second electrospinning solution, an additive other than the second additive (such as TBAC, which will be described later) may be added to the second electrospinning solution. The additive can be added to the second electrospinning solution in an amount ranging from 0.5 wt% to 5 wt% based on the weight of the second electrospinning solution, and in an amount ranging from 50 wt% to 400 wt% based on the weight of the second piezoelectric polymer.

[0048] 3. Pressure sensor The pressure sensor according to the embodiment includes a piezoelectric portion having the composite nanofiber piezoelectric material according to the embodiment, and a pair of electrodes arranged to sandwich the piezoelectric portion. Specific configurations of the pressure sensor may be as shown in Figures 17(a) to 17(c) relating to the examples described below. The pressure sensor may also include other components (e.g., electrical wires, protective layers, etc.) in addition to those described above.

[0049] 4. Power generation equipment The power generating device according to the embodiment includes a piezoelectric part having the composite nanofiber piezoelectric material according to the embodiment, and a pair of electrodes arranged to sandwich the piezoelectric part. Specific configurations of the power generating device can be as shown in Figures 17(a) to 17(c) relating to the examples described below. The power generating device may also include other components (e.g., electric wires, protective layers, rectifier circuits, capacitors, etc.) in addition to those described above.

[0050] 5. Filters The filter according to the embodiment includes the composite nanofiber piezoelectric material according to the embodiment. In this specification, the term "filter" refers to a filter for separating substances (e.g., impurities, solids, etc.) in a fluid by passing the fluid through it. The composite nanofiber piezoelectric material according to the embodiment has a porous structure and contains nanofibers with a large specific surface area as its main constituent, and therefore can be used as a constituent element (filtering medium) of a filter. Furthermore, in the filter according to the embodiment, the ability to capture substances can be improved by generating an electric charge due to the pressure generated when a fluid passes through it.

[0051] 6.Effects The effects of the composite nanofiber piezoelectric material, the method for manufacturing the composite nanofiber piezoelectric material, the pressure sensor, the power generation device, and the filter according to the embodiments will be described below.

[0052] The composite nanofiber piezoelectric material according to the embodiment comprises a "nanofiber membrane having a nonwoven structure, which has first nanofibers containing a first piezoelectric polymer and a first additive," and a "microstructure formed in the pores of the nanofiber membrane, which has second nanofibers containing a second piezoelectric polymer and a second additive." Therefore, the composite nanofiber piezoelectric material according to the embodiment is a fiber piezoelectric material that can achieve high piezoelectric performance over a wide pressure range, as will be shown in the examples described below.

[0053] Furthermore, the composite nanofiber piezoelectric material according to the embodiment has a composite structure comprising a nanofiber membrane and a microstructure, which allows for high strength, making it possible to achieve high durability capable of withstanding repeated pressure.

[0054] Furthermore, according to the composite nanofiber piezoelectric material of the embodiment, when the first piezoelectric polymer and the second piezoelectric polymer contain a polyvinylidene fluoride (PVDF)-based polymer, high piezoelectric performance can be achieved by using PVDF, which exhibits a high piezoelectric effect.

[0055] Furthermore, according to the composite nanofiber piezoelectric material of the embodiment, when the first additive and the second additive contain catecholamine, it is possible to increase the content of piezoelectric crystal structure (mainly β-phase crystal structure) in PVDF due to the interaction between the amine in the catecholamine and the fluorine in PVDF, etc.

[0056] Furthermore, according to the composite nanofiber piezoelectric material of the embodiment, when the first additive and the second additive contain dopamine (DA), it is possible to increase the content of piezoelectric crystal structure (mainly β-phase crystal structure) in PVDF, as shown in the examples described below.

[0057] Furthermore, in the composite nanofiber piezoelectric material according to the embodiment, when the average fiber diameter of the first nanofibers is within the range of 5 to 30 times the average fiber diameter of the second nanofibers, it is possible to stably form and maintain a microstructure. Furthermore, by adopting this numerical range, it is possible to prevent a decrease in the piezoelectric performance and sensitivity of the composite nanofiber piezoelectric material.

[0058] Furthermore, according to the composite nanofiber piezoelectric material of the embodiment, when the average fiber diameter of the first nanofibers is within the range of 100 nm to 1,000 nm and the average fiber diameter of the second nanofibers is within the range of 10 nm to 100 nm, it is possible to achieve both high strength (durability) and high piezoelectric performance as a whole in the composite nanofiber piezoelectric material. Furthermore, by adopting these numerical ranges, it is possible to prevent a decrease in the piezoelectric performance and sensitivity of the composite nanofiber piezoelectric material.

[0059] Furthermore, according to the composite nanofiber piezoelectric material of the embodiment, when the weight average molecular weight (Mw) of the first piezoelectric polymer is smaller than the weight average molecular weight (Mw) of the second piezoelectric polymer, it becomes relatively easy to achieve finer second nanofibers.

[0060] Furthermore, in the composite nanofiber piezoelectric material according to the embodiment, when the weight-average molecular weight (Mw) of the first piezoelectric polymer is within the range of 50,000 to 300,000 and the weight-average molecular weight (Mw) of the second piezoelectric polymer is within the range of 500,000 to 1,000,000, it is relatively easy to stabilize the structure of the first nanofibers and to refine the second nanofibers. Furthermore, by adopting these numerical ranges, it is possible to prevent a decrease in the piezoelectric performance and sensitivity of the composite nanofiber piezoelectric material.

[0061] When the amount of the first additive in the first nanofiber is within the range of 0.5 wt% to 5 wt% of the weight of the first piezoelectric polymer, and the amount of the second additive in the second nanofiber is within the range of 0.5 wt% to 5 wt% of the weight of the second piezoelectric polymer, the structure and physical properties of the composite nanofiber piezoelectric material can be stabilized. Furthermore, by adopting these numerical ranges, deterioration of the piezoelectric performance and sensitivity of the composite nanofiber piezoelectric material can be prevented.

[0062] The method for producing a composite nanofiber piezoelectric material according to the embodiment includes a nanofiber membrane formation step of forming a nanofiber membrane and a microstructure formation step of forming microstructures in the pores of the nanofiber membrane, and therefore, the method for producing a composite nanofiber piezoelectric material according to the embodiment makes it possible to relatively easily produce the composite nanofiber piezoelectric material according to the embodiment.

[0063] Furthermore, in the manufacturing method of the composite nanofiber piezoelectric material according to the embodiment, if a nanofiber membrane is formed on the surface of a collector in the nanofiber membrane formation process, and electrospinning is performed using a collector with the nanofiber membrane present on its surface in the microstructure formation process, the composite nanofiber piezoelectric material according to the embodiment can be manufactured in a simple procedure.

[0064] The pressure sensors, power generation devices, and filters according to the embodiments contain the composite nanofiber piezoelectric material according to the embodiments, and therefore are products with unprecedented performance (e.g., high sensitivity, high power generation performance, high collection performance).

[0065] [Example] The inventors of the present invention actually produced composite nanofiber piezoelectric materials according to the above-described embodiments and analyzed their morphology, etc. Furthermore, the inventors of the present invention measured the performance (piezoelectric performance) of the produced composite nanofiber piezoelectric materials as piezoelectric materials. Below, as examples, the test contents and results of the composite nanofiber piezoelectric materials are described.

[0066] 1. Materials and Equipment First, the materials and devices used in the examples will be described. Descriptions of general-purpose tools and devices will be omitted.

[0067] Granular polyvinylidene fluoride (PVDF) (Mw = 275,000), tetrabutylammonium chloride (TBAC), dopamine (DA), N,N-dimethylformamide (DMF) (≥ 99.9%), and acetone (≥ 99.9%) were purchased from Sigma-Aldrich Japan LLC. Powdered PVDF (Mw=500,000) was purchased from Arkema Inc. All compounds used were of analytical grade.

[0068] The scanning electron microscope (FE-SEM) used was a JSM-6010LA manufactured by JEOL Ltd. The scanning electron microscope was equipped with an energy dispersive X-ray spectrometer (EDX) (Inca) manufactured by Oxford Instruments, and the accelerating voltage was set to 10 kV. The transmission electron microscope (TEM) used was a JEM-2100 manufactured by JEOL Ltd.

[0069] The attenuated total reflectance-Fourier transform infrared spectrometer (ATR-FTIR) used was an IRPrestige-21 manufactured by Shimadzu Corporation. The X-ray diffractometer (XRD) used was a Miniflex 300 manufactured by Rigaku Corporation. Cu Kα radiation was used for the measurements. The differential scanning calorimeter (DSC) used was a DSC-8500 manufactured by PerkinElmer (USA). The heating rate during the measurement was 10°C min -1 The temperature range was 35°C to 300°C.

[0070] The X-ray photoelectron spectrometer (XPS) used was Axis-Ultra HSA SV manufactured by Kratos Analytical (UK). The digital thickness gauge used was the YHT780 (made in China). Measurement of the sample using the digital thickness gauge was performed by clamping the sample between two fixtures. Quasi-static piezoelectric effect method (d 33 Quasi-static d for measurement by the meter method 33 The measuring instrument used was the ZJ-3AN from the Chinese Academy of Sciences.

[0071] The precision impedance analyzer used was the 1260 / 1296 manufactured by Solartron Analytical (UK). The test range was 102 Hz to 106 Hz. The oscilloscope used was a ViewGo II DS-5414A manufactured by Iwasaki Electric Co., Ltd. The electrometer used was a 6514 model electrometer manufactured by Keithley Instruments (USA). The measurement results were recorded using LabVIEW software manufactured by National Instruments (USA).

[0072] 2. Formation and Analysis of Nanofiber Membranes and Microstructures 2-1. Preparation of the first and second electrospinning solutions First, a PVDF solution was prepared by dissolving 16 wt% PVDF (Mw=275,000) in a mixed solvent of DMF and acetone (DMF:acetone=3:2). DA was further added to the PVDF solution, and the mixture was stirred at 60°C for 2 hours to obtain a first electrospinning solution. In this example, four types of solutions with different amounts of DA added were prepared as the first electrospinning solution. The amounts of DA added were 0 wt%, 1 wt%, 2 wt%, or 3 wt% relative to the PVDF (Mw=275,000).

[0073] Next, 2.5 wt% TBAC was added to DMF and stirred at 40°C for 2 hours to prepare a TBAC solution. DA was then added to the TBAC solution and stirred at 50°C for 2 hours. Finally, 3 wt% PVDF (Mw=500,000) was dissolved in the TBAC solution to obtain a second electrospinning solution. In this example, four types of solutions with different amounts of DA were prepared as the second electrospinning solution. The amounts of DA added were 0 wt%, 0.5 wt%, 1 wt%, or 2 wt% relative to the PVDF (Mw=500,000).

[0074] 2-2. Formation of nanofiber membranes and microstructures (electrospinning) Electrospinning was performed using the first and second electrospinning solutions described above. The electrospinning method was basically the same for both nanofiber membranes and microstructures. Specifically, the electrospinning solution was placed in a 5 mL plastic syringe equipped with an 18 G or 20 G stainless steel nozzle, and electrospinning was performed by applying a voltage between the nozzle and a drum collector (rotation speed: 100 rpm).

[0075] When forming the nanofiber membrane (first nanofiber), the distance between the nozzle and collector was 20 cm, and the voltage between the nozzle and collector was 15 kV. When forming the microstructure (second nanofiber), the distance between the nozzle and collector was 22 cm, and the voltage between the nozzle and collector was 22 kV. The temperature during electrospinning was 25°C, and the humidity was 35% to 40%. When forming the microstructure, electrospinning was performed using a collector with a nanofiber membrane present on its surface.

[0076] 2-3. Nanofiber membrane morphology Hereinafter, the nanofiber membranes formed with 0 wt% DA added will be referred to as PDS-0, PDS-1, PDS-2, and PDS-3, respectively. Because PDS-0 does not contain DA, it is essentially a comparative example. Furthermore, PDS-0 is made of PVDF nanofibers that do not contain DA. Therefore, in the drawings (images, graphs, spectra, etc.) showing the results of the examples, "PVDF" may be used in the same sense as PDS-0.

[0077] Figure 1 shows SEM images of nanofiber membranes in the examples: Figure 1(a) is an SEM image of PDS-0, Figure 1(b) is an SEM image of PDS-1, Figure 1(c) is an SEM image of PDS-2, and Figure 1(d) is an SEM image of PDS-3. Figure 2 shows TEM images of nanofiber membranes in the examples: Figure 2(a) is a TEM image of PDS-1, Figure 2(b) is a TEM image of PDS-2, and Figure 2(c) is a TEM image of PDS-3. Figure 3 shows EDS images of the first nanofibers constituting PDS-2 in the example. Figure 3(a) is an SEM image of the first nanofibers, Figure 3(b) is an EDS image for fluorine (F), Figure 3(c) is an EDS image for nitrogen (N), and Figure 3(d) is an EDS image for oxygen (O). Figure 4 shows XPS spectra of nanofiber membranes in the example. Figure 4(a) shows the XPS spectra for the entire PDS-0 and PDS-2, and Figure 4(b) shows the XPS spectrum for N 1s of PDS-2.

[0078] Figure 5 shows XRD spectra of nanofiber membranes in the examples. Figure 5(a) shows the XRD spectra of each nanofiber membrane in the examples arranged vertically, Figure 5(b) shows the XRD spectrum of PDS-0, Figure 5(c) shows the XRD spectrum of PDS-1, Figure 5(d) shows the XRD spectrum of PDS-2, and Figure 5(e) shows the XRD spectrum of PDS-3. Figure 6 shows the ATR-FTIR spectra of the nanofiber membranes in the examples, with the results for each nanofiber membrane arranged vertically. FIG. 7 is a bar graph showing the content of β-phase crystal structure in the nanofiber membranes of the examples. Figure 8 shows DSC thermograms of the nanofiber membranes in the examples, with the results for each nanofiber membrane arranged vertically. FIG. 9 is a graph showing the crystallinity (Xc) and β-phase crystallinity of the nanofiber membranes in the examples.

[0079] First, SEM image observations confirmed that increasing the amount of DA added from 0 wt% to 2 wt% resulted in adhesion between fibers and an increase in the average fiber diameter (see Figures 1(a) to 1(c)). The average fiber diameter of PDS-0 was approximately 234 nm, that of PDS-1 approximately 257 nm, and that of PDS-2 approximately 268 nm. Referring to the TEM images, this phenomenon is thought to be due to the increase in DA concentration and the formation of a core-shell structure between PVDF and DA (see Figures 2(a) and 2(b)).

[0080] On the other hand, when the amount of DA added was increased to 3 wt%, the average fiber diameter became approximately 245 nm, which means that the average fiber diameter was smaller than when the amount of DA added was 2 wt% (see Figures 1(d) and 2(c)). This is thought to be mainly due to the penetration of DA into the interior of the PVDF.

[0081] Furthermore, EDS image analysis of PDS-2 confirmed that the distribution of fluorine (F) and nitrogen (N) in the core region was more uniform than that of oxygen (O) (see Figure 3). Furthermore, analysis of the XPS spectrum of PDS-2 confirmed the peaks due to NH and -NH2 in DA, as well as a peak (406.8 eV) due to NF (see Figure 4(b)). This result suggests that there is a strong interaction between the nitrogen in DA and the -CF2 moiety in PVDF.

[0082] From the above results, it is clear that the -NH of DA is formed under the influence of an electric field during electrospinning. 2 is positively charged, and the -NH 2 It is thought that the strong interaction between CF2 and PVDF favors the formation of a highly symmetric β-phase crystal structure in PVDF.

[0083] Next, the nanofiber membranes of the examples were analyzed by XRD spectroscopy. A prominent diffraction peak was observed at 2θ=20.5°, which is attributed to the β(110) crystal plane (see Figure 5(a)). Furthermore, the relatively weak diffraction peaks at 2θ=18.4° and 36° are attributed to the α(020) and α(200) crystal planes, respectively.

[0084] Here, the XRD spectra were checked for each nanofiber membrane, and it was confirmed that the proportion of peaks due to the α-phase crystalline structure reached a minimum in PDS-2 and then increased in PDS-3 in relation to the amount of DA added (see Figures 5(b) to 5(e)).

[0085] A similar trend was also confirmed in the ATR-FTIR spectrum (see Figure 6). In the ATR-FTIR spectrum, the peak at 840 cm due to the β-phase crystal structure was observed. -1 (-CH2 rocking), 1,176 cm -1 (CF telescopic), 1,275cm -1 In addition to peaks such as those due to the CF out-of-plane bending angle, there is a peak at approximately 763 cm due to the α-phase crystal structure. -1 A peak (characteristic peak of the α phase) due to (-CF2 deformation) was confirmed. Looking at the data in the order from PDS-0 to PDS-3, it was confirmed that the height of the characteristic peak of the α phase started to decrease and then increased at the boundary of PDS-2. This phenomenon is thought to be due to the fact that the increase in DA incorporated into PVDF reduces the regularity of the orientation and arrangement of PVDF, resulting in a decrease in crystallinity.

[0086] Furthermore, from the ATR-FTIR spectrum, the content of the β-phase crystal structure, F(β), can be calculated from the formula "F(β) = Aβ / (1.26Aα + Aβ)". Here, "Aα" and "Aβ" are the peaks at 763 cm -1 and 840cm -1 The absorbance at 1.26 is the coefficient for the electrospun nanofiber. As a result, it was confirmed that PDS-2 had the highest content of β-phase crystal structure (89%) among the nanofiber membranes in the examples (see Figure 6).

[0087] Furthermore, the results of the DSC thermograms confirmed that PDS-2 exhibited the highest melting temperature peak among the nanofiber membranes in the examples (see Figure 8). Since an increase in the content of the β-phase crystalline structure is thought to correspond to an increase in the melting temperature, the above results are consistent with those derived from the ATR-FTIR spectra.

[0088] Furthermore, the crystallinity (Xc) of PVDF was calculated from the DSC thermograms. As a result, PDS-2 had the highest crystallinity at 67.5% (see Figure 9). This crystallinity in PDS-2 is 22.14% higher than that of PDS-0 (pure PVDF).

[0089] Based on the above results, it was decided to use PDS-2 as the base for manufacturing the composite nanofiber piezoelectric material of the example.

[0090] 2-4. Morphology of microstructures Hereinafter, the microstructures formed with 0 wt% DA added will be referred to as PDU-0, 0.5 wt% as PDU-1, 1 wt% as PDU-2, and 2 wt% as PDU-3. Since PDU-0 does not contain DA, it is essentially a comparative example. Furthermore, PDU-0 is made of PVDF nanofibers that do not contain DA. Therefore, in the drawings (images, graphs, spectra, etc.) showing the results of the examples, "PVDF" may be used in the same sense as PDU-0.

[0091] Figure 10 shows SEM images of the microstructures in the examples: Figure 10(a) is an SEM image of PDU-0, Figure 10(b) is an SEM image of PDU-1, Figure 10(c) is an SEM image of PDU-2, and Figure 10(d) is an SEM image of PDU-3. Figure 11 shows TEM images of the microstructures in the example, where Figure 11(a) is a TEM image of PDU-1, and Figure 11(b) is a TEM image showing an enlarged portion of Figure 11(a). Figure 12 shows EDS images of the second nanofibers constituting PDU-1 in the example. Figure 12(a) is an SEM image of the second nanofibers, Figure 12(b) is an EDS image for nitrogen (N), Figure 12(c) is an EDS image for fluorine (F), and Figure 12(d) is an EDS image for oxygen (O). Figure 13 shows XPS spectra of PDU-1 in the example, where Figure 13(a) is the overall XPS spectrum and Figure 13(b) is the XPS spectrum for N 1s. Figure 14 shows TEM images of the second nanofibers constituting the microstructure in the example, where Figure 14(a) is a high-resolution TEM (HRTEM) image of PDU-1, and Figure 14(b) is an FFT image of PDU-1. Fig. 15 shows ATR-FTIR spectra of the microstructures in the examples, with the results for each microstructure arranged vertically. Fig. 16 shows XRD spectra of the microstructures in the examples, with the results for each microstructure arranged vertically.

[0092] First, SEM image observations confirmed that the average fiber diameter was approximately 35 nm for all microstructures, but that the fiber diameter tended to increase slightly as the amount of DA added increased (see Figures 10(a) and 10(b)). It was also confirmed that the addition of DA stabilized the twisting of the nanofiber network, increasing the droplet structure.

[0093] Unlike the nanofiber membrane, no clear core-shell structure was observed in the TEM images of the microstructure (see Figures 11(a) and 11(b)). On the other hand, EDS images confirmed the presence of DA (nitrogen and oxygen distributed) on the surface of the second nanofiber (see Figures 12(a) to 12(d)). On the other hand, the XPS spectrum for N 1s did not show any peaks due to NF, suggesting that there is no strong bond between -NH2 and -CF2. Therefore, the high content of β-phase crystal structure in the microstructure (especially PDU-1) (described below) is thought to be due to a mechanism different from that of the nanofiber membrane.

[0094] Furthermore, high-resolution TEM and FFT images of PDU-1 confirmed that the β-phase crystal structure was oriented in the microstructure (see Figures 13(a) and 13(b)). Specifically, the PVDF polymer chains were aligned parallel to the PVDF / DA interface. The interchain distance was 4.28 Å, corresponding to the (110) plane of the crystal structure. Because the second nanofiber in the microstructure did not undergo a mechanical stretching process, the orientation of the β-phase crystal structure described above can be said to be self-oriented.

[0095] The above structure was also confirmed by the ATR-FTIR spectrum and XRD spectrum of the microstructure. In the ATR-FTIR spectrum of the microstructure, the peak at 1,275 cm due to the β-phase crystal structure was higher than that of the nanofiber membrane. -1 On the other hand, in the ATR-FTIR spectrum of the microstructure, the peak intensity at 763 cm due to the α-phase crystal structure was significantly high (see Figure 14). -1 The peak intensity was very weak.

[0096] In the XRD spectrum of the microstructure, it was confirmed that the characteristic peak at 36° due to the α-phase crystal structure disappeared, and only the characteristic peak at 20.6° due to the β-phase crystal structure was present (see Figure 15). This indicates that the presence of β-phase crystal structure is high in the microstructure. The content of β-phase crystal structure in PDU-1 calculated from the analysis results was 98%, but this decreased with increasing the amount of DA added, reaching 88% in PDU-3. The results of ATR-FTIR analysis indicated that the above phenomenon was due to an increase in the γ-phase crystal structure (1,234 cm -1 It was confirmed that this is due to the increase in peak intensity at the γ phase. In PVDF, the γ phase crystal structure also exhibits piezoelectricity.

[0097] 3. Piezoelectric performance of composite nanofiber piezoelectric materials The composite nanofiber piezoelectric material of the present example was manufactured using the method described in 2-2 above. PDS-2 was used as the nanofiber membrane, and PDU-1 was used as the microstructure. Hereinafter, the composite nanofiber piezoelectric material manufactured using a 90-minute spinning time to form the microstructure will be referred to as MPD-1, MPD-2, and MPD-3, respectively.

[0098] Figure 17 is a diagram illustrating a piezoelectric element using a composite nanofiber piezoelectric material in an example. Figure 17(a) is an image of a piezoelectric element, Figure 17(b) is a photograph showing an example of the composite nanofiber piezoelectric material (left side of the photograph) and piezoelectric element (right side of the photograph) produced in an example, and Figure 17(c) is a photograph showing that the composite nanofiber piezoelectric material can be bent (left side of the photograph). In Figure 17(a), each component is shown as if it were separated, but this is to make the layered structure easier to understand; in an actual piezoelectric element, each component is in close contact. Figure 18 shows SEM images of the composite nanofiber piezoelectric material according to the example. Figures 18(a) and 18(b) are SEM images of MPD-1, Figures 18(c) and 18(d) are SEM images of MPD-2, and Figures 18(e) and 18(f) are SEM images of MPD-3. Note that the magnifications of the SEM images shown in Figures 18(a), 18(c), and 18(e) are different from those of the SEM images shown in Figures 18(b), 18(d), and 18(f). 19A and 19B are graphs showing the dielectric constant, dielectric loss, and piezoelectric coefficient of the composite nanofiber piezoelectric material according to the example, where Fig. 19A is a graph showing the relative dielectric constant, and Fig. 19B is a graph showing the dielectric loss. FIG. 20 shows the piezoelectric coefficient (d 33 ) is a bar graph showing Figure 21 shows graphs illustrating the output voltages of piezoelectric elements fabricated using composite nanofiber piezoelectric materials according to an example: Figure 21(a) is a graph of the output voltage for MPD-1, Figure 21(b) is a graph of the output voltage for MPD-2, Figure 21(c) is a graph of the output voltage for MPD-3, and Figure 21(d) is a bar graph comparing the output voltages for each composite nanofiber piezoelectric material. Figure 22 shows graphs showing the output current of piezoelectric elements manufactured using the composite nanofiber piezoelectric material according to the example, where Figure 22(a) is a graph of the output current, and Figure 22(b) is a bar graph showing a comparison of the output current for each composite nanofiber piezoelectric material.

[0099] When measuring the piezoelectric performance of the composite nanofiber piezoelectric material of the example, the composite nanofiber piezoelectric material 10 was measured in a size of 25 mm × 25 mm (effective area 625 mm 2 ) to form a piezoelectric part, which was then sandwiched between a pair of electrodes (copper layers) to produce a piezoelectric element (see Figure 17). Copper wire was soldered to each electrode, which was then reinforced with polyimide (PI) tape. Depending on the measurement item, the piezoelectric element was sealed with polyimide tape (PI layer) and appropriate pressure was applied to remove the interlayer gap. As a comparative example, a piezoelectric element was also produced using a nanofiber membrane (PDS-2) without a microstructure.

[0100] SEM images of the composite nanofiber piezoelectric material of the example confirmed that the density of the microstructure formed within the pores of the nanofiber membrane did not increase over time (see Figures 18(a) to 18(f)). It was also confirmed that the second nanofibers (ultrafine nanofibers) that make up the microstructure are closely connected to the first nanofibers (relatively thick nanofibers) that make up the nanofiber membrane, forming a two-dimensional topological network structure. The formation of the above structure is thought to be related to the presence of DA on the surface of the first nanofiber, as well as the formation of a thin film of DA during electrospinning of the second nanofiber.

[0101] Next, the dielectric constant and dielectric loss of the composite nanofiber piezoelectric material in the examples were measured. 2 Hz~10 6 Measurements were performed in the frequency range of 100 Hz. First, regarding the dielectric constant of the composite nanofiber piezoelectric material, it was confirmed that the longer the spinning time for forming the microstructure, the higher the dielectric constant at low frequencies, and then it decreased (see Figure 19(a)). Furthermore, regarding the dielectric loss of the composite nanofiber piezoelectric material, it was confirmed that it tended to remain in a low range overall compared to the dielectric loss of the nanofiber film alone (see Figure 19(b)).

[0102] In addition, the piezoelectric coefficient (d 33 ) was found to be affected by the spinning time, i.e., the content of microstructures (see Figure 20). In particular, the piezoelectric coefficient of MPD-2 was 52pCN. -1 This value is comparable to the piezoelectric coefficient of ceramic-based fibers.

[0103] Next, to more intuitively demonstrate the piezoelectric properties of the composite nanofiber piezoelectric material in the examples, we conducted a compression test on a piezoelectric element using a composite nanofiber piezoelectric material using a motor-controlled automatic stepper. The compression force was 1.5 N, and the compression frequency was approximately 1.2 Hz. As a result, the open-circuit voltage and short-circuit current of the piezoelectric element using the composite nanofiber piezoelectric material showed similar trends to the piezoelectric coefficient (see Figures 21(a) to 21(d) and Figures 22(a) and 22(b)). In particular, for the MPD-2, the open-circuit voltage output reached 14.3 ± 0.2 V, and the short-circuit current reached 1.01 ± 0.1 μA.

[0104] The piezoelectric properties of the composite nanofiber piezoelectric material are not necessarily proportional to the spinning time (amount of microstructures) required to form the microstructures. This is thought to be due to the charge density of the piezoelectric material and changes in the distribution of mechanical stress due to the increase in the number of microstructures (changes in structural properties).

[0105] 4. Pressure sensor The piezoelectric element using MPD-2 was tested for its performance as a pressure sensor. When applying piezoelectric materials to the field of pressure sensors, it is extremely important to evaluate the sensitivity and linear pressure range.

[0106] Fig. 23 is a graph showing the characteristics of a pressure sensor according to an example, in which Fig. 23(a) is a graph showing the difference in output voltage depending on pressure, Fig. 23(b) is a graph showing the correlation between pressure and output voltage, and Fig. 23(c) is a graph showing the stability (durability) of the output voltage over 10,000 operating cycles. 24 is a graph showing the output voltage of the pressure sensor according to the embodiment in response to speech (throat vibration). Note that the letters written above each graph indicate the content of the speech that is the source of the vibration. FIG. 25 is a graph showing the output voltage of the pressure sensor according to the example relative to the pulse rate. Fig. 26 is a graph showing output voltages of a pressure sensor according to an embodiment in response to human body movements, where Fig. 26(a) is a graph relating to finger tapping, Fig. 26(b) is a graph relating to elbow bending and straightening, Fig. 26(c) is a graph relating to knee bending and straightening, and Fig. 26(d) is a graph relating to foot stepping.

[0107] First, the difference in output voltage corresponding to the input force was measured for a pressure sensor using MPD-2 (pressure sensor according to the example). As a result, in the range of 1.5N to 40N, a large voltage amplitude was confirmed in the range of 1.5N to 10N, and a smaller voltage amplitude was confirmed in the range of 16.5N to 40N than in the range of 1.5N to 10N (see Figures 23(a) and 23(b)). Specifically, the sensitivity in the low pressure range of 0N to 4N was 7.29VN. -1 The sensitivity in the medium pressure range of 5N to 10N is 4.11VN -1 The sensitivity in the high pressure range of 30N to 40N is 1.41VN -1 It was.

[0108] The above results show that the composite nanofiber piezoelectric material (MPD-2) of the present example has higher sensitivity and a wider effective pressure range than conventionally known PVDF-based piezoelectric materials and piezoelectric materials made of conventional polymer composite materials. Specifically, conventionally known fiber-based piezoelectric materials generally have an effective pressure range of 10 N or less, and it is not uncommon for them to be below 1 N. Furthermore, even piezoelectric materials such as PVDF / Y-ZnO nanofiber, which are known to have experimental results of around 40 N, have a sensitivity of 1 VN. -1 was significantly lower (e.g., 0.3VN -1 degree).

[0109] Furthermore, in the composite nanofiber piezoelectric material of the example, the first nanofiber and the second nanofiber are tightly connected, and it was confirmed that a consistent voltage output was maintained even after 10,000 taps at 1.5 N and 2 Hz (see Figure 23(c)).

[0110] To commercialize pressure sensors, it is necessary to consider not only laboratory performance but also performance under practical conditions. The composite nanofiber piezoelectric material of the examples is flexible and lightweight due to its main structure being a nanofiber membrane. Therefore, pressure sensors using the composite nanofiber piezoelectric material of the examples can be suitably used to detect human movement and physiological activity (as wearable film sensors).

[0111] For example, by attaching a pressure sensor according to the embodiment to a human throat, it is possible to output subtle and complex shape changes, such as muscle movements and skin deformation during speech, as a voltage signal (see FIG. 24). The graph shown in FIG. 24 records consistent characteristic peaks for each pronunciation method, confirming that the pressure sensor according to the embodiment exhibits high sensitivity and reliability in the field of voice recognition. In other words, it is believed that the pressure sensor according to the embodiment can be used to rehabilitate the speaking ability of people with damaged vocal cords.

[0112] Furthermore, by attaching the pressure sensor according to the embodiment to a location where the pulse can be measured, such as a human wrist, the pulse can be output as a voltage signal (see FIG. 25). In the graph shown in FIG. 25, it was confirmed that the pulse frequency at the wrist was 65 beats per minute, and that the signal waveform resulting from the wrist pulse consisted of two distinct peaks, P1 and P2.

[0113] Of these, peak P1 corresponds to the pulse wave, and peak P2 corresponds to the reflected wave from the hand. The arterial augmentation index (P2 / P1) can be calculated based on peaks P1 and P2, and its average value was confirmed to be 0.448 (corresponding to a value for a healthy 26-year-old woman). Thus, pulse information can be used to detect high blood pressure and cardiovascular disease. The pressure sensor according to the embodiment has excellent sensitivity to weak force signals, and therefore has great potential for application in the fields of health monitoring and clinical diagnosis.

[0114] Furthermore, by attaching the pressure sensor according to the embodiment to a moving part of a person (such as a joint), it is possible to output a change in pressure due to human movement as a voltage signal (see FIGS. 26(a) to 26(d)). For example, by attaching the pressure sensor according to the embodiment to a fingertip, it is possible to accurately capture tapping motions, including strength and weakness (see FIG. 26(a)). In the embodiment, when tapping gently, the output voltage was 3.4 V, and when tapping strongly, the output voltage was 7.5 V.

[0115] Furthermore, by attaching the sensor according to the embodiment to the elbow, it was possible to detect when the elbow was bent from a straight position to 45 degrees and when it was bent to 90 degrees (see Figure 26(b)). The respective peak voltages were 7.1 V and 9.8 V. The positive and negative voltage values ​​correspond to the bending and straightening of the elbow, respectively.

[0116] Similarly, by attaching the sensors according to the embodiments to the knees or feet, bending and movement could be detected (see Figures 26(c) and 26(d)). The pressure sensors according to the embodiments exhibit high-speed response, so they can be applied to the fields of motion capture and tactile perception, and can be expected to be applied to intuitive remote operation and control of machines.

[0117] 5. Power generation equipment A test was conducted on the performance of a piezoelectric element using MPD-2 as a power generator. The test was conducted by connecting a piezoelectric element using MPD-2 (a power generator according to the embodiment) to a circuit equipped with a rectifier circuit (diode bridge), and using an LED and a capacitor as the target. The rectifier circuit was used to convert the output current (a current equivalent to AC current) obtained by tapping the piezoelectric element into DC current.

[0118] Figure 27 is a set of photographs used to explain an LED light emission test using a power generator according to an embodiment. Figure 27(a) is a photograph of the power generator (hidden by a hand) and the circuit connected to the power generator, Figure 27(b) is a photograph of the LED light emission when the power generator is tapped lightly, Figure 27(c) is a photograph of the LED light emission when the power generator is tapped hard, and Figure 27(d) is a photograph of the LED light emission when the power generator is tapped even harder. Fig. 28 is a diagram for explaining a capacitor charging test using a power generation device according to an embodiment. Fig. 28(a) and Fig. 28(b) are photographs showing the state of tapping the power generation device, and Fig. 28(c) is a graph of the capacitor charging curve.

[0119] As a result, it was confirmed that the LEDs emit light when the power generating device according to the embodiment is tapped (see Fig. 27(a) to Fig. 27(d)). It was also confirmed that the intensity of the LED light emission correlates with the strength of the tapping, and that by tapping the power generating device strongly, more than 20 LEDs emit light simultaneously.

[0120] It was also confirmed that tapping the power generation device according to the example enabled charging of a capacitor (33 μF) in the circuit (see Figures 28(a) to 28(c)). Specifically, when the tapping frequency was set to 2 Hz, a voltage of 3 V was able to be stored in the capacitor in 280 seconds. After charging, a timer connected to the circuit could be activated by discharging the capacitor. Considering the small size of the power generation device according to the example (25 mm x 25 mm in terms of the size of the composite nanofiber piezoelectric material), the power generation device according to the example is considered to be promising as a sustainable power source for portable electronic devices.

[0121] Although the present invention has been described above based on the above-mentioned embodiments and examples, the present invention is not limited to the above-mentioned embodiments and examples. The present invention can be embodied in various forms without departing from the spirit of the present invention, and for example, the following modifications are also possible.

[0122] (1) The materials and values ​​described in the above examples do not limit the present invention. Other materials and values ​​may be used as long as they do not impair the effects of the present invention.

[0123] (2) The composite nanofiber piezoelectric material of the present invention can also be applied to products (fields) other than pressure sensors, power generation devices, and filters.

Claims

1. a nanofiber membrane having a nonwoven fabric structure and including first nanofibers; a microstructure having second nanofibers and formed within the pores of the nanofiber membrane; the first nanofibers contain a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer; The second nanofibers contain a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer, and have an average fiber diameter smaller than that of the first nanofibers.

2. The composite nanofiber piezoelectric material of claim 1 , wherein the first piezoelectric polymer and the second piezoelectric polymer contain a polyvinylidene fluoride (PVDF) based polymer.

3. The composite nanofiber piezoelectric material of claim 2 , wherein the first additive and the second additive contain catecholamine.

4. The composite nanofiber piezoelectric material of claim 3 , wherein the first additive and the second additive contain dopamine (DA).

5. The composite nanofiber piezoelectric material described in claim 1, characterized in that the average fiber diameter of the first nanofibers is in the range of 5 to 30 times the average fiber diameter of the second nanofibers.

6. the first nanofibers have an average fiber diameter in the range of 100 nm to 1,000 nm; The composite nanofiber piezoelectric material according to claim 1, characterized in that the average fiber diameter of the second nanofibers is in the range of 10 nm to 100 nm.

7. The composite nanofiber piezoelectric material of claim 1 , wherein the weight average molecular weight (Mw) of the first piezoelectric polymer is smaller than the weight average molecular weight (Mw) of the second piezoelectric polymer.

8. the weight average molecular weight (Mw) of the first piezoelectric polymer is in the range of 50,000 to 300,000; The composite nanofiber piezoelectric material of claim 1, wherein the weight average molecular weight (Mw) of the second piezoelectric polymer is in the range of 500,000 to 1,000,000.

9. the amount of the first additive in the first nanofibers is in the range of 0.5 wt % to 5 wt % based on the weight of the first piezoelectric polymer; The composite nanofiber piezoelectric material described in claim 1, characterized in that the amount of the second additive added to the second nanofiber is in the range of 0.5 wt% to 5 wt% relative to the weight of the second piezoelectric polymer.

10. a nanofiber membrane formation step of forming a nanofiber membrane having a nonwoven fabric structure by forming first nanofibers by electrospinning using a first electrospinning solution containing a first piezoelectric polymer and a first additive that stabilizes the piezoelectric crystal structure of the first piezoelectric polymer; a microstructure formation step of forming a microstructure within the pores of the nanofiber membrane by electrospinning second nanofibers having an average fiber diameter smaller than that of the first nanofibers using a second electrospinning solution containing a second piezoelectric polymer and a second additive that stabilizes the piezoelectric crystal structure of the second piezoelectric polymer.

11. In the nanofiber membrane forming step, the nanofiber membrane is formed on a surface of a collector, The method for producing a composite nanofiber piezoelectric material described in claim 10, characterized in that in the microstructure formation process, electrospinning is performed using the collector with the nanofiber membrane present on its surface.

12. A piezoelectric part having the composite nanofiber piezoelectric material according to claim 1; a pair of electrodes arranged to sandwich the piezoelectric portion;

13. A piezoelectric part having the composite nanofiber piezoelectric material according to claim 1; a pair of electrodes arranged to sandwich the piezoelectric portion therebetween.

14. A filter comprising the composite nanofiber piezoelectric material of claim 1.

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