Pressure wave generating element and manufacturing method thereof

Abstract

This pressure wave generating element comprises a support body 10 and a fiber layer 20 that is provided upon the support body 10 and that generates heat from the application of electric current. The fiber layer 20 includes fibers having a metal coating at least partially provided to the surfaces thereof. The fiber layer 20 is formed from a fiber film having an average pore size in the range of 0.1-1.0 μm. As a result of this configuration, a pressure wave generating element having improved sound pressure and suitable electrical resistance is obtained.

Description

[Technical Field]

[0001] The present invention relates to a pressure wave generating element that generates pressure waves by periodically heating air, and also to a method for manufacturing the pressure wave generating element. [Background technology]

[0002] Pressure wave generating elements, also known as thermophones, have, for example, a resistor layer provided on a support. When current flows through this resistor, it heats up, causing the air in contact with the resistor to thermally expand; when the current is subsequently stopped, the expanded air contracts. This cyclic heating generates sound waves. When the drive signal is set to an audio frequency, it can be used as an acoustic speaker. When the drive signal is set to an ultrasonic frequency, it can be used as an ultrasonic source. Because such thermophones do not use a resonance mechanism, they are capable of generating wideband and short-pulse sound waves. Because thermophones convert electrical energy into thermal energy before generating sound waves, improvements in energy conversion efficiency and sound pressure are desired.

[0003] In Patent Document 1, a carbon nanotube structure in which multiple carbon nanotubes are arranged in parallel with each other is provided as a resistor, thereby increasing the surface area in contact with air and reducing the heat capacity per unit area. In Patent Document 2, a silicon substrate is used as a heat dissipation layer, and porous silicon with low thermal conductivity is used as a heat insulating layer, thereby improving the heat insulating properties. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-296591 [Patent Document 2] Japanese Patent Application Publication No. 11-300274 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 considers reducing the heat capacity by using carbon nanotubes in the heat generating layer. Although carbon nanotubes have been put to practical use, they are expensive and difficult to handle during manufacturing, which is likely to pose problems when put to practical use. In addition, the resistivity (10 -5 ~10 -2 Ωcm) is the resistance of metal materials (10 -6 Since the resistivity is higher than that of the MOS transistor (Ωcm), it is necessary to drive the element at a higher voltage to input the same amount of power.

[0006] It is an object of the present invention to provide a pressure wave generating element having improved sound pressure and suitable electrical resistance, and a method for manufacturing such a pressure wave generating element. [Means for solving the problem]

[0007] A pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer has an average pore diameter 0.2 It consists of a fiber membrane with a thickness of ~1.0 μm. 、 The fiber membrane contains fibers having a fiber diameter of 1 nm to 100 nm. .

[0008] Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is Contains fibers with a fiber diameter of 1 nm to 100 nm, and The porosity 87 It is composed of a fiber membrane that is in the range of 100% to 95%. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm, The fibrous layer includes beads sandwiched between the fibers. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; The fibrous layer includes beads sandwiched between the fibers. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm, The fiber layer is made of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1<Φ2) larger than the first fiber diameter. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; The fiber layer is made of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1<Φ2) larger than the first fiber diameter. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm, The metal coating increases in thickness the further it is from the substrate. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; The metal coating increases in thickness the further it is from the substrate.

[0009] A method for manufacturing a pressure wave generating element according to another aspect of the present invention includes the steps of: providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; During spinning, two or more solutions with different concentrations are spun simultaneously to form a fiber membrane made of composite fibers.

[0010] A method for manufacturing a pressure wave generating element according to another aspect of the present invention includes the steps of: providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; During spinning, two or more different materials are spun simultaneously to form a fiber membrane made of composite fibers.

[0011] Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is made of a fiber membrane, The fiber membrane contains fibers having a fiber diameter of 1 nm to 100 nm, The penetration depth of the metal coating into the fiber layer is 2.2 μm or more. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fibrous layer includes beads sandwiched between the fibers. Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is made of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1<Φ2) larger than the first fiber diameter. [Effects of the Invention]

[0012] In the pressure wave generating element according to the present invention, the fiber layer contains fibers at least partially coated with a metal on its surface, thereby increasing the surface area in contact with air and improving sound pressure. Furthermore, the use of a metal material allows the electrical resistance of the fiber layer to be set to an appropriate value. Furthermore, the fiber layer is made of a fiber membrane with an average pore size in the range of 0.1 to 1.0 μm. Alternatively, the fiber layer is made of a fiber membrane with a porosity in the range of 70% to 95%. This increases the specific surface area of the fiber layer, improving acoustic conversion efficiency and improving sound pressure.

[0013] Furthermore, the manufacturing method of the pressure wave generating element according to the present invention makes it possible to realize a fiber layer with a large surface area in contact with air and with appropriate electrical resistance. Furthermore, by forming a fiber membrane made of composite fibers, the pore size and porosity of the fiber layer are increased, which improves the acoustic conversion efficiency and increases the sound pressure. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view showing an example of a pressure wave generating element according to a first embodiment of the present invention. [Figure 2] 1 is an electron microscope photograph showing the surface of a fiber layer. [Figure 3] FIG. 2 is a cross-sectional view showing the thickness distribution of a metal coating. [Figure 4] FIG. 2 is a plan view showing an example of electrode arrangement. [Figure 5] 1 is an electron microscope photograph showing an example of a fiber membrane on which beads are formed. [Figure 6] 10 is a flowchart showing an example of a method for manufacturing a pressure wave generating element. [Figure 7] 1 is an electron microscope photograph showing an example of measurement of the penetration depth into a metal-coated nonwoven fabric. DETAILED DESCRIPTION OF THE INVENTION

[0015] A pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is made of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm.

[0016] According to this configuration, the fiber layer includes fibers at least partially coated with a metal coating. This increases the surface area in contact with the air, improving the sound pressure per unit input power. The fibers can be arranged in the form of a nonwoven fabric, a woven fabric, a knitted fabric, or a mixture thereof, and the cavities around the fibers are interconnected, ensuring airflow between the internal cavities and the external space. This significantly increases the contact area between the porous structure formed by the fibers and the air compared to a non-porous, smooth surface. This increases the efficiency of heat transfer from the fiber layer to the air, improving the sound pressure.

[0017] Furthermore, by applying a metal coating to at least a portion of the fiber, the electrical resistance of the fiber layer can be easily set to an appropriate value by adjusting the coating thickness and selecting the coating material, thereby achieving the desired electrical resistance and optimizing the drive voltage.

[0018] Furthermore, if a low thermal conductivity material is used as the fiber, for example, heat conduction from the fiber layer to the support can be suppressed. This increases the temperature change on the surface of the fiber layer, improving the sound pressure per unit input power. Because the fiber layer containing such fibers has a porous structure, there is no need to introduce a heat insulating layer to improve sound pressure, as in Patent Document 2.

[0019] The fiber layer is made of a fiber membrane with an average pore size in the range of 0.1 to 1.0 μm, which increases the specific surface area of the fiber layer, improving acoustic conversion efficiency and sound pressure.

[0020] In the present invention, the fiber membrane preferably contains fibers having a fiber diameter of 1 nm to 100 nm and an average pore diameter of 0.2 μm or more, thereby increasing the specific surface area of the fiber layer, improving the acoustic conversion efficiency, and improving the sound pressure.

[0021] Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is made of a fiber membrane having a porosity in the range of 70% to 95%.

[0022] This increases the specific surface area of the fiber layer, improving the acoustic conversion efficiency and increasing the sound pressure.

[0023] In the present invention, the fiber membrane preferably contains fibers with a fiber diameter of 1 nm to 100 nm and has a porosity of 87% or more, which increases the specific surface area of the fiber layer, thereby improving the acoustic conversion efficiency and sound pressure.

[0024] In the present invention, the fiber layer is preferably made of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1<Φ2) larger than the first fiber diameter, which increases the pore size and porosity of the fiber layer, thereby improving the acoustic conversion efficiency and sound pressure.

[0025] In the present invention, it is preferable that the first fiber diameter Φ1 is in the range of 1 nm≦Φ1≦100 nm and the second fiber diameter Φ2 is in the range of 100 nm≦Φ2≦2000 nm, which increases the pore size and porosity of the fiber layer, thereby improving the acoustic conversion efficiency and sound pressure.

[0026] In the present invention, it is preferable that the fiber layer contains beads, and the beads are sandwiched between the fibers, thereby increasing the pore size and porosity of the fiber layer, thereby improving the acoustic conversion efficiency and sound pressure.

[0027] In the present invention, it is preferable that the thickness of the metal coating increases with increasing distance from the support.

[0028] This configuration suppresses heat generation on the support side inside the fiber layer while increasing heat generation on the opposite side of the support, thereby suppressing heat conduction from the fiber layer to the support, improving the efficiency of heating the air, and improving the sound pressure per unit input power.

[0029] In the present invention, the fiber layer is preferably made of a nonwoven fabric, which increases the specific surface area, pore size, porosity, etc. of the fiber layer, thereby improving the acoustic conversion efficiency and sound pressure.

[0030] A method for manufacturing a pressure wave generating element according to another aspect of the present invention includes the steps of: providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; During spinning, two or more solutions with different concentrations are spun simultaneously to form a fiber membrane made of composite fibers.

[0031] A method for manufacturing a pressure wave generating element according to another aspect of the present invention includes the steps of: providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; During spinning, two or more different materials are spun simultaneously to form a fiber membrane made of composite fibers.

[0032] According to these methods, the fiber layer contains fibers at least partially coated with a metal on its surface, and functions as a heater. This increases the surface area in contact with the air, improving the sound pressure per unit input power. Furthermore, it is easy to achieve a fiber layer with appropriate electrical resistance.

[0033] Furthermore, by using the electrospinning method, it is possible to realize fibers with diameters in the range of 1 nm to 2000 nm, such as nanofibers, submicron fibers, and micron fibers.

[0034] Furthermore, a fiber layer with a large surface area in contact with air and appropriate electrical resistance can be realized. Furthermore, by forming a fiber membrane made of composite fibers, the pore size and porosity of the fiber layer can be increased, which can improve the acoustic conversion efficiency and sound pressure.

[0035] Furthermore, a pressure wave generating element according to one aspect of the present invention includes: A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The penetration depth of the metal coating into the fiber layer is 1 μm or more.

[0036] This allows the pressure wave generating element to generate a large sound pressure per unit input power.

[0037] (Embodiment 1) FIG. 1 is a cross-sectional view showing an example of a pressure wave generating element 1 according to a first embodiment of the present invention.

[0038] The pressure wave generating element 1 comprises a support 10, a fiber layer 20, and a pair of electrodes D1, D2. The support 10 is formed of a semiconductor such as silicon, or an electrical insulator such as glass, ceramic, or polymer. A thermal insulating layer having a lower thermal conductivity than the support 10 may be provided on the support 10, thereby suppressing heat dissipation from the fiber layer 20 to the support 10. As will be described later, if the fiber layer 20 has a thermal insulating function, the above-mentioned thermal insulating layer may be omitted.

[0039] A fiber layer 20 is provided on the support 10. The fiber layer 20 is made of a conductive material and is electrically driven to generate heat when an electric current flows through it, radiating pressure waves caused by the cyclic expansion and contraction of air. A pair of electrodes D1, D2 are provided on both sides of the fiber layer 20. The electrodes D1, D2 have a single-layer or multi-layer structure made of a conductive material.

[0040] In this embodiment, the fiber layer 20 includes fibers with a metal coating at least partially applied to the surface. This increases the surface area in contact with the air, improving sound pressure. Furthermore, by applying a metal coating to the fibers, the electrical resistance of the fiber layer 20 can be set to an appropriate value by adjusting the coating thickness and selecting the coating material.

[0041] The fibers may be placed directly on the support 10 or may be placed via an adhesive layer such as a polymeric material.

[0042] Figure 2 is an electron microscope photograph showing the surface of a fiber layer 20. The photograph shows a case in which the fibers are randomly oriented and bonded or entangled by thermal, mechanical, or chemical action to form a sheet. A metal coating is applied to the surface of the fibers.

[0043] The fiber layer 20 may be in the form of such a nonwoven fabric, a woven fabric formed by combining warp and weft threads, a knitted fabric formed by knitting fibers, or a mixture of these.

[0044] The fibers can be selected from the group consisting of polymer fibers, glass fibers, carbon fibers, carbon nanotubes, metal fibers, and ceramic fibers. When low-thermal-conductivity materials such as polymers, glass, and ceramics are used as fibers, the fibers themselves have thermal insulation properties, thereby suppressing heat conduction from the fiber layer to the support. This increases the temperature change on the surface of the fiber layer, thereby improving the sound pressure per unit input power.

[0045] Specific examples of polymer materials that can be used include polyimide, polyamide, polyamideimide, polyethylene, polypropylene, acrylic resin, polyvinyl chloride, polystyrene, polyvinyl acetate, polytetrafluoroethylene, liquid crystal polymer, polyphenylene sulfide, polyether ether ketone, polyarylate, polysulfone, polyethersulfone, polyetherimide, polycarbonate, modified polyphenylene ether, polybutylene terephthalate, polyethylene terephthalate, polyacetal, polylactic acid, polyvinyl alcohol, ABS resin, polyvinylidene fluoride, cellulose, polyethylene oxide, polyethylene glycol, and polyurethane.

[0046] The metal coating is preferably formed of a metal material such as Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, or Al, or an alloy containing two or more of these metals. The metal coating may have a single layer structure or a multi-layer structure made of multiple materials.

[0047] (Embodiment 2) 6 is a flowchart showing an example of a method for manufacturing a pressure wave generating element. First, in step S1, a support 10 is prepared.

[0048] Next, in step S2, a fiber membrane is formed on the support 10 using the spun fibers. Spinning methods that can be used include meltblowing, flash spinning, centrifugal spinning, and melt spinning. Also, methods that involve crushing pulp and processing it into a sheet, such as cellulose nanofibers, can be used. In particular, electrospinning can be used to produce nanofibers, submicron fibers, micron fibers, and the like. The spun fibers may be placed directly on the support 10 in the form of a nonwoven fabric, or may be placed on the support 10 in the form of a woven fabric made by combining warp and weft threads, or in the form of a knitted fabric made by knitting the fibers.

[0049] Instead of spinning directly onto the support 10, it is also possible to spin onto another support, and then peel off the spun fibers and adhere them onto the support 10.

[0050] In step S2, two or more solutions with different concentrations may be used for spinning simultaneously from multiple spinning nozzles to form a fiber membrane made of composite fibers. Using a high-concentration solution results in a larger diameter of the spun fiber, while using a low-concentration solution results in a smaller diameter of the spun fiber. Therefore, spinning using two or more solutions with different concentrations results in a composite fiber made of multiple fibers with different fiber diameters. This increases the pore size and porosity of the fiber layer, improving the acoustic conversion efficiency and sound pressure.

[0051] Furthermore, in step S2, two or more different materials (e.g., polyimide fiber and acrylic fiber) may be simultaneously spun from multiple spinning nozzles to form a fiber membrane made of composite fibers. This allows various physical properties of the fiber, such as specific surface area, fineness, specific gravity, mechanical properties, degradation resistance, optical properties, moisture absorption and swelling, thermal properties, flammability, electrical properties, frictional properties, and dyeability, to be controlled to desired values. For example, increasing the specific surface area of the fiber layer can improve acoustic conversion efficiency and increase sound pressure.

[0052] Next, in step S3, a metal coating is applied to the obtained fiber membrane to form a fiber layer 20. Coating methods that can be used include vapor deposition, sputtering, electroplating, electroless plating, ion plating, and atomic layer deposition. Metal materials that are generally mentioned above can be used.

[0053] Next, in step S4, a pair of electrodes D1, D2 are formed on the resulting fiber layer 20. Methods for forming the electrodes include vapor deposition, sputtering, electroplating, electroless plating, ion plating, atomic layer deposition, printing, spray coating, and dip coating. The electrodes are preferably made of metals such as Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, Al, and Sn, or alloys containing two or more of these metals. The electrode structure may be a single-layer structure or a multi-layer structure made of multiple materials. [Example]

[0054] Example 1 (Sample preparation method) Pressure wave generating elements were fabricated in the following manner (samples 1 to 5).

[0055] A polyamic acid solution prepared using N,N-dimethylacetamide (DMAc) as a solvent was used as the spinning solution. The solution concentration was adjusted to 22 wt%.

[0056] Using this solution, polyamic acid fibers were spun onto an aluminum foil attached to the periphery of a drum collector by electrospinning. The drum collector used had a diameter of 200 mm, and spinning was carried out while rotating at 100 rpm.

[0057] The electrospinning conditions were an applied voltage of 23 kV, a nozzle-to-collector distance of 14 cm, and the deposition time was adjusted so that the fiber membrane thickness was approximately 1 to 80 μm. The obtained polyamic acid fibers were heat-treated (imidized) at 300 °C for 2 hours to obtain polyimide fibers. The fiber diameter of the produced polyimide was 157 nm. Because polyimide materials are heat-resistant, heat treatment processes can be applied.

[0058] Next, we will explain the composite fibers. Polyimide fiber membranes with different porosities and pore sizes were fabricated by simultaneously spinning a polyamic acid solution and an acrylic resin solution using a multi-nozzle during electrospinning, and then thermally decomposing only the acrylic fibers through heat treatment.

[0059] The acrylic resin solution was prepared as follows. The acrylic resin solution prepared using N,N-dimethylformamide (DMF) as a solvent was used as the spinning solution. The solution concentration was adjusted to 10 wt% to 25 wt%.

[0060] Using a 22 wt% polyamic acid solution and a 10-22 wt% acrylic resin solution, polyamic acid fibers and acrylic fibers were simultaneously spun onto aluminum foil attached to the periphery of a drum collector using the multi-nozzle electrospinning method. The solution discharge rate was 1:1. The discharge rate can be adjusted by the discharge speed and number of nozzles. The drum collector used had a diameter of 200 mm, and spinning was performed while rotating at 100 rpm.

[0061] The electrospinning conditions were an applied voltage of 23 kV, a nozzle-to-collector distance of 14 cm, and film formation time adjusted to achieve a fiber membrane thickness of approximately 1–80 μm. The fiber diameters of the acrylic resin produced were 210 nm at a solution concentration of 10 wt%, 615 nm at 15 wt%, 873 nm at 20 wt%, and 1025 nm at 22 wt%. The resulting fiber membrane, a mixture of polyamic acid and acrylic fibers, was heat-treated at 300°C for 2 hours, resulting in thermal decomposition of the acrylic fibers and imidization of the polyamic acid, yielding polyimide fibers. While polymeric materials with low thermal decomposition temperatures or melting points cannot be used to produce fiber membranes, the heat-treatment process is feasible for polyimide materials due to their heat resistance.

[0062] Each fiber membrane was peeled from the aluminum foil and attached to a silicon substrate (support). Adhesion to the substrate can be achieved by applying an adhesive such as epoxy to the substrate beforehand, or by using double-sided tape. The substrate can also be made of ceramics such as glass, alumina, zirconia, magnesium oxide, aluminum nitride, boron nitride, or silicon nitride, or flexible substrates such as PET film or polyimide film.

[0063] A gold film with a thickness ranging from 1 to 40 nm was formed on the fiber membrane formed on the substrate by sputtering. Methods for metal coating on the fiber include vapor deposition, ion plating, atomic layer deposition, and electroless plating. Metal species that can be used include Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, and Al.

[0064] The thickness of the metal coating may be uniform or non-uniform in the circumferential direction of the fiber. For example, the thickness may increase as it gets farther from the support. The metal coating has a thickness T1 at the position closest to the support side and a thickness T2 at the position farthest from the support side, and it may satisfy T1 < T2. The form of the metal coating on the fiber may, for example, as shown in FIG. 3, have a portion where the metal coating 22 is not applied at the lower part close to the support 10 on the circumferential surface of the fiber 21. This can suppress heat generation on the support side inside the fiber layer while enhancing heat generation on the side opposite to the support.

[0065] The coating state (cross-sectional image) of the metal-coated fiber can be analyzed as follows. For example, the sample is processed by a focused ion beam (FIB), and the coating state on the fiber can be analyzed by observation with a transmission electron microscope (JEM-F200 manufactured by JEOL) and elemental mapping analysis by energy-dispersive X-ray spectroscopy.

[0066] The fabricated element size was processed to be 5 mm × 6 mm. A pair of electrodes D1 and D2 were formed on both sides of the sample with electrodes having a size of 4 mm × 0.8 mm and an electrode distance of 3.4 mm (FIG. 4A). The electrode stack structure was Ti (10 nm thick), Cu (500 nm thick), and Au (100 nm thick) from the support side. Note that the electrodes D1 and D2 may have a comb-tooth electrode structure as shown in FIG. 4B to adjust the element resistance.

[0067] As the electrode film formation method, evaporation, sputtering, ion plating method, atomic layer deposition method, electroplating, electroless plating, spray coating, dip coating, printing, etc. can be adopted. As the electrode material, Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, Al, etc. can be used.

[0068] (Evaluation method) 1) Acoustic characteristics (sound pressure) The acoustic characteristics of the pressure wave generating element were measured using a MEMS microphone (Knowles SPU0410LR5H). The distance between the pressure wave generating element and the microphone was 6 cm, and the evaluation was performed by reading the microphone output voltage when the drive signal frequency was 60 kHz. The input voltage to the pressure wave generating element was 6 to 16 V.

[0069] Pressure wave generating elements generate pressure waves by heating the air with a heating element. Therefore, even with the same element, the more power input, the greater the sound pressure. To determine whether sound waves can be generated efficiently (acoustic conversion efficiency), it is necessary to compare sound pressure at the same power. As the power is increased, the output also increases linearly, but for example, if the acoustic conversion efficiency is good, the ratio of the increase in microphone output ΔV to the increase in power ΔW will be large. Here, the slope ΔV / ΔW was used as an index of sound pressure. The results of comparison sample 1 were used to compare the indexes.

[0070] 2) Fiber diameter The fiber diameters of polyimide and acrylic fibers were measured as follows. The fiber membranes were observed using a scanning electron microscope (Hitachi S-4800, accelerating voltage 5 kV, 3 k to 120 k magnifications) to obtain SEM images. The average fiber diameter was calculated by measuring the fiber diameters from the images. Specifically, 10 fibers were randomly sampled per field of view from the multiple fibers included in the images, excluding abnormal ones. This was repeated for five fields of view, resulting in a total of 50 sampled fibers. The diameters of these fibers were measured, and the average fiber diameter was calculated.

[0071] 3) Porosity The porosity of the polyimide fiber membrane was calculated using the following formula. Porosity (%) = {1 - (bulk density ÷ true density)} x 100 Another method for calculating porosity is to repeatedly perform cross-sectional processing with FIB and SEM observation to obtain 3D images. Specifically, FIB processing is performed using an FEI HELIOS NANORAB 660i, the SEM images are observed, and then the FIB is used to process the sample again by 10 nm in the depth direction, after which the SEM images are observed. By repeating this FIB processing and SEM observation, a total of 41 SEM images are obtained covering a depth of 400 nm. A 3D image of the fiber layer can be constructed from these 41 SEM images, and the porosity can be calculated.

[0072] 4) Average pore diameter The average pore size (through-pore size) of polyimide fiber membranes was calculated using a perm-porometer (CFP-1200AEL, manufactured by POROUS MATERIALS INC.). The average through-pore size was measured using the half-dry method (ASTM E1294-89). Galwick (manufactured by POROUS MATERIALS INC., surface tension 15.9 mN / m) was used as the liquid for sample impregnation. The average pore size after metal coating can be estimated from the thickness of the metal coating on the fiber. For example, if a polyimide nonwoven fabric fiber with an average pore size of X (μm) is surrounded by a metal coating with a thickness of Y (μm), the average pore size of the metal-coated fiber can be calculated as X-2Y.

[0073] 5) Penetration depth of metal-coated nonwoven fabric As shown in Figure 7, the penetration depth of the metal coating into the nonwoven fabric was measured by observing the cross section of the element using a scanning electron microscope (Hitachi S-4800, accelerating voltage 15 kV, 1k-20k magnification) and obtaining images using backscattered electron images or elemental mapping analysis using energy dispersive X-ray spectroscopy. The penetration depth of the metal coating from the surface of the metal-coated nonwoven fabric into the nonwoven fabric was measured from the obtained images. The sample to be observed was solidified with resin and then polished to expose the fiber layer. This preprocessing of the sample allowed for the acquisition of a cross-sectional image of the metal-coated area, and the area where the contrast with the resin was visible was defined as the penetration depth of the metal coating. Because the fiber layer has a porous structure and therefore has unevenness, the penetration depth was defined as the point where the penetration depth was greatest.

[0074] (Production method of comparative sample 1) Comparative sample 1 was prepared by forming a thin Au film (20 nm thick) on a 100 μm thick polyimide (PI) film by sputtering. The PI film had a substantial porosity of 0%, and its characteristics were compared with those of samples 1 to 5. The element size and electrode structure were the same as those of sample 1 above.

[0075] [Table 1] TIFF0007726264000001.tif53155

[0076] From the results in Table 1, it can be seen that the larger the pore size and porosity of the nonwoven fabric constituting the fiber layer, the larger the specific surface area of the fiber layer, which increases the acoustic conversion efficiency and improves the sound pressure. Furthermore, because the fibers are made of low-thermal-conductivity materials such as polymers, they have a heat insulating effect toward the substrate, which increases the temperature change on the surface of the heating element, allowing for greater sound pressure per unit input power. As an example, the thermal conductivity of polyimide is approximately 0.28 W / m·K, while that of SiO2 (the oxide layer on the surface of a silicon substrate) is approximately 1.3 W / m·K. Polyimide has lower thermal conductivity and a greater insulating effect toward the substrate, resulting in greater sound pressure.

[0077] Example 2 (Sample preparation method) Pressure wave generating elements were fabricated by the following method (Comparative Sample 2, Samples 6, 7, and 8).

[0078] (Production method of fiber membrane for comparison sample 2) A polyimide (PI) solution prepared using N,N-dimethylformamide (DMF) as a solvent was used as the spinning solution. The solution concentration was adjusted to 6.5 wt%, and 0.05 wt% lithium chloride was added to the solution. Other additives that can be used include tetrabutylammonium chloride and potassium trifluoromethanesulfonate.

[0079] Using this solution, polyamic acid fibers were spun onto an aluminum foil attached to the periphery of a drum collector by electrospinning. The drum collector used had a diameter of 200 mm, and spinning was carried out while rotating at 100 rpm.

[0080] The electrospinning conditions were an applied voltage of 29 kV, a nozzle-to-collector distance of 14 cm, and the deposition time was adjusted so that the fiber membrane thickness was approximately 1 to 80 μm. The average fiber diameter of the produced polyimide was 46 nm.

[0081] (Fabric membrane manufacturing method for samples 6, 7, and 8) When spinning using the electrospinning method, a multi-nozzle was used to simultaneously spin two types of polyimide solutions with different concentrations to produce a fiber membrane. Here, the 6.5 wt% polyimide (PI) solution used in Comparative Sample 2 and a 10 wt% polyimide (PI) solution prepared using N,N-dimethylformamide (DMF) as a solvent were used as spinning solutions.

[0082] These two types of polyimide solutions were simultaneously spun onto aluminum foil attached to the periphery of a drum collector using a multi-nozzle electrospinning method. The extrusion rates of the 6.5 wt% polyimide (PI) solution and the 10 wt% polyimide (PI) solution were 2:1 (Sample 6), 1:1 (Sample 7), and 1:2 (Sample 8). The extrusion rate can be adjusted by the extrusion speed and number of nozzles. The drum collector used had a diameter of 200 mm, and spinning was performed while rotating at 100 rpm.

[0083] The electrospinning conditions were an applied voltage of 29 kV, a nozzle-to-collector distance of 14 cm, and the deposition time was adjusted so that the fiber membrane thickness would be approximately 1 to 80 μm. The average fiber diameter of the fiber membrane prepared from a 10 wt% polyimide solution was 126 nm. In this way, composite fiber membranes with average fiber diameters of 126 nm and 46 nm, respectively, were obtained.

[0084] The fabricated fiber membrane was peeled off from the aluminum foil and adhered to a silicon substrate (support). Adhesion to the substrate can be achieved by applying an adhesive such as epoxy to the substrate beforehand, or by using double-sided tape. The substrate can also be made of ceramics such as glass, alumina, zirconia, magnesium oxide, aluminum nitride, boron nitride, or silicon nitride, or flexible substrates such as PET film or polyimide film.

[0085] A gold film with a thickness ranging from 1 to 40 nm was formed on the fiber membrane formed on the substrate by sputtering. Methods for metal coating on the fiber include vapor deposition, ion plating, atomic layer deposition, and electroless plating. Metal species that can be used include Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, and Al.

[0086] The thickness of the metal coating may be uniform or non-uniform in the circumferential direction of the fiber. For example, the thickness may increase as the distance from the support increases. The metal coating has a thickness T1 at the position closest to the support side and a thickness T2 at the position farthest from the support side, and T1 < T2 may be satisfied. The form of the metal coating on the fiber may be such that, as shown in FIG. 3, there may be a portion where the metal coating 22 is not applied at the lower part close to the support 10 on the circumferential surface of the fiber 21. This can suppress heat generation on the support side inside the fiber layer while enhancing heat generation on the side opposite to the support.

[0087] The coating state (cross-sectional image) of the metal-coated fiber can be analyzed as follows. For example, the sample can be processed by a focused ion beam (FIB), and the coating state on the fiber can be analyzed by observation with a transmission electron microscope (JEM-F200 manufactured by JEOL) and elemental mapping analysis by energy-dispersive X-ray spectroscopy.

[0088] The fabricated element size was processed to be 5 mm × 6 mm. A pair of electrodes D1 and D2 were formed on both sides of the sample with electrodes having a size of 4 mm × 0.8 mm and an electrode distance of 3. mm (FIG. 4A). The laminated structure of the electrodes was Ti (10 nm thick), Cu (500 nm thick), and Au (100 nm thick) from the support side. Note that the electrodes D1 and D2 may have a comb-tooth-shaped electrode structure as shown in FIG. 4B in order to adjust the element resistance.

[0089] As the film-forming method of the electrodes, vapor deposition, sputtering, ion plating method, atomic layer deposition method, electroplating, electroless plating, spray coating, dip coating, printing, etc. can be adopted. As the electrode material, Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, Al, etc. can be used.

[0090] The evaluation method is the same as the description in (Example 1).

[0091] [Table 2] TIFF0007726264000002.tif45155

[0092] The results in Table 2 show that the use of composite fibers results in larger pore diameters and void ratios compared to single fibers, which increases the acoustic conversion efficiency and improves sound pressure.

[0093] Furthermore, since the fibers are made of a low thermal conductivity material such as a polymer, there is a heat insulating effect in the substrate direction, and the temperature change on the surface of the heating element is large, so that the sound pressure per unit input power can be increased.

[0094] Example 3 (Sample preparation method) A pressure wave generating element was fabricated in the following manner (Sample 9).

[0095] When spinning using the electrospinning method, a multi-nozzle was used to simultaneously spin two types of polyimide solutions with different concentrations to produce a fiber membrane. Here, the 6.5 wt% polyimide (PI) solution used in Comparative Sample 2 and a 3 wt% polyimide (PI) solution prepared using N,N-dimethylformamide (DMF) as the solvent were used as spinning solutions.

[0096] These two types of polyimide solutions were simultaneously spun onto aluminum foil attached to the periphery of a drum collector using the multi-nozzle electrospinning method. The solution discharge rate was 1:1. The discharge rate can be adjusted by the discharge speed and number of nozzles. The drum collector used had a diameter of 200 mm, and spinning was carried out while rotating at 100 rpm.

[0097] The electrospinning conditions were an applied voltage of 29 kV, a nozzle-collector distance of 14 cm, and the film formation time was adjusted so that the thickness of the fiber film was approximately 1 to 80 μm.

[0098] When electrospinning is carried out with a 3 wt% polyimide solution, since the solution viscosity is low, fibrillation does not occur, and spherical or elongated spherical beads as shown in Fig. 5 are formed. The size of the beads has a minor axis of 0.5 to 3.0 μm. Also, those beads may be hollow spherical, elongated spherical, or in a shape where the sphere has collapsed.

[0099] That is, by simultaneously spinning from a multi-nozzle using a 6.5 wt% polyimide solution and a 3 wt% polyimide solution, a fiber membrane in which the above-mentioned beads and polyimide fibers with an average fiber diameter of 46 nm are combined can be obtained (Fig. 5).

[0100] The fabricated fiber membrane was peeled off from the aluminum foil and adhered onto a Si substrate (support). Adhesion to the substrate can be carried out by previously applying an adhesive such as epoxy to the substrate or using a double-sided tape, etc. Also, as the substrate, ceramic substrates such as glass, alumina, zirconia, magnesium oxide, aluminum nitride, boron nitride, silicon nitride, etc., or flexible substrates such as PET film and polyimide film can be used.

[0101] Au having a thickness distributed in the range of 1 to 40 nm was formed by sputtering on the fiber membrane formed on the substrate. As the method for metal coating on the fiber, methods such as vapor deposition method, ion plating method, atomic layer deposition method, electroless plating method, etc. may be used. Also, as the metal species, Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, Al, etc. can be used.

[0102] The thickness of the metal coating may be uniform or non-uniform in the circumferential direction of the fiber. For example, the thickness may increase as it gets farther from the support. The metal coating has a thickness T1 at the position closest to the support side and a thickness T2 at the position farthest from the support side, and it may satisfy T1 < T2. The form of the metal coating on the fiber may be such that, as shown in Fig. 3, there may be a portion where the metal coating 22 is not applied at the lower part close to the support 10 on the circumferential surface of the fiber 21. Thereby, while suppressing heat generation on the support side inside the fiber layer, heat generation on the side opposite to the support can be enhanced.

[0103] The coating state (cross-sectional image) of metal-coated fibers can be analyzed as follows: For example, the sample can be processed using a focused ion beam (FIB), and the coating state on the fiber can be analyzed by observing it with a transmission electron microscope (JEOL JEM-F200) and performing elemental mapping analysis using energy dispersive X-ray spectroscopy.

[0104] The fabricated element was processed to a size of 5 mm x 6 mm. A pair of electrodes D1 and D2 was formed on both sides of the sample, measuring 4 mm x 0.8 mm with an electrode separation distance of 3.4 mm (Figure 4A). The electrode layer structure was Ti (10 nm thick), Cu (500 nm thick), and Au (100 nm thick) from the support side. Note that electrodes D1 and D2 may have a comb-like electrode structure as shown in Figure 4B in order to adjust the element resistance.

[0105] Electrode film formation methods that can be used include vapor deposition, sputtering, ion plating, atomic layer deposition, electroplating, electroless plating, spray coating, dip coating, printing, etc. Electrode materials that can be used include Au, Ag, Cu, Pt, Rh, Pd, Ru, Ni, Ir, Cr, Mo, W, Ti, and Al.

[0106] The evaluation method is the same as that described in (Example 1).

[0107] [Table 3] TIFF0007726264000003.tif23155

[0108] The results in Table 3 show that the use of a composite fiber made of beads and fibers increases the pore size and porosity compared to single fibers, improving acoustic conversion efficiency and sound pressure. This phenomenon is thought to occur because, when beads are formed in the fiber membrane and sandwiched between metal-coated fibers, the beads act as spacers, increasing the pore size in the membrane and allowing heat generation not only in the layer near the surface but also in the layer near the substrate to be efficiently converted into acoustic output.

[0109] Furthermore, since the fibers are made of a low thermal conductivity material such as a polymer, there is a heat insulating effect in the substrate direction, and the temperature change on the surface of the heating element is large, so that the sound pressure per unit input power can be increased.

[0110] As described above, by including fibers with a metal coating at least partially on the surface of the fiber layer, the surface area in contact with the air is increased, thereby improving sound pressure. Furthermore, the use of a metal material allows the electrical resistance of the fiber layer to be set to an appropriate value.

[0111] The fiber layer is made of a fiber membrane with an average pore size in the range of 0.1 to 1.0 μm, which increases the specific surface area of the fiber layer, improving acoustic conversion efficiency and sound pressure.

[0112] The present invention will now be fully described in connection with preferred embodiments with reference to the accompanying drawings. However, various variations and modifications will be apparent to those skilled in the art. and modifications therein without departing from the scope of the present invention as defined by the appended claims. It should be understood to include. [Industrial Applicability]

[0113] The present invention is extremely useful industrially in that it can realize a pressure wave generating element having improved sound pressure and appropriate electrical resistance. [Explanation of symbols]

[0114] 1 Pressure wave generating element 10 Support 20 fiber layer 21. Fiber 22 Metallic Coating D1,D2 electrode

Claims

1. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; the fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm; A pressure wave generating element, characterized in that the fiber layer contains beads, and the beads are sandwiched between the fibers.

2. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; A pressure wave generating element, characterized in that the fiber layer contains beads, and the beads are sandwiched between the fibers.

3. 3. The pressure wave generating element according to claim 1, wherein the beads have a minor axis of 0.5 to 3.0 μm.

4. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; the fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm; A pressure wave generating element characterized in that the fiber layer is composed of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1 < Φ2) larger than the first fiber diameter.

5. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; A pressure wave generating element characterized in that the fiber layer is composed of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1 < Φ2) larger than the first fiber diameter.

6. 6. The pressure wave generating element according to claim 4, wherein the first fiber diameter Φ1 is in the range of 1 nm≦Φ1≦100 nm, and the second fiber diameter Φ2 is in the range of 100 nm≦Φ2≦2000 nm.

7. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; the fiber layer is composed of a fiber membrane having an average pore size in the range of 0.1 to 1.0 μm; The pressure wave generating element, wherein the thickness of the metal coating increases with increasing distance from the support.

8. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is composed of a fiber membrane having a porosity in the range of 70% to 95%; The pressure wave generating element, wherein the thickness of the metal coating increases with increasing distance from the support.

9. providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; A method for manufacturing a pressure wave generating element, in which two or more solutions with different concentrations are simultaneously spun during spinning to form a fiber membrane made of composite fibers.

10. providing a support; forming a fiber membrane on the support using fibers spun by an electrospinning method; and applying a metal coating onto the fiber membrane to form a fiber layer; A method for manufacturing a pressure wave generating element, in which two or more different materials are simultaneously spun during spinning to form a fiber membrane made of composite fibers.

11. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; The fiber layer is made of a fiber membrane, The fiber membrane contains fibers having a fiber diameter of 1 nm to 100 nm, A pressure wave generating element, wherein the penetration depth of the metal coating into the fiber layer is 2.2 μm or more.

12. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; A pressure wave generating element, characterized in that the fiber layer contains beads, and the beads are sandwiched between the fibers.

13. A support; a fiber layer provided on the support and capable of generating heat when an electric current is applied thereto; the fiber layer includes fibers having a metal coating at least partially disposed on a surface thereof; A pressure wave generating element characterized in that the fiber layer is composed of composite fibers including first fibers having a first fiber diameter Φ1 and second fibers having a second fiber diameter Φ2 (Φ1 < Φ2) larger than the first fiber diameter.

Images