WO2013190852A1

WO2013190852A1 - Acoustic matching member and method for manufacturing same, ultrasonic wave transmitter/receiver using same, and ultrasonic flow meter

Info

Publication number: WO2013190852A1
Application number: PCT/JP2013/003892
Authority: WO
Inventors: 永原　英知; 佐藤　真人; 足立　明久
Original assignee: パナソニック株式会社
Priority date: 2012-06-22
Filing date: 2013-06-21
Publication date: 2013-12-27
Also published as: JPWO2013190852A1

Abstract

An acoustic matching member (4) comprising dry gel particles (8) and binding particles (10), wherein the dry gel particles have micropores (7) therein and hold interparticle voids among the dry gel particles, the binding particles that are present among the dry gel particles bind the dry gel particles together, and the average particle diameter of the binding particles is larger than the average size of the micropores of the dry gel particles but smaller than the average size of the interparticle voids (9) of the dry gel particles.

Description

Acoustic matching member, manufacturing method therefor, ultrasonic transducer and ultrasonic flowmeter using the same

The present invention relates to an acoustic matching member for an ultrasonic transducer and a method for manufacturing the same. In particular, in order to transmit and receive ultrasonic waves to and from various gases with high sensitivity, an acoustic matching member to which dry gel is applied, a manufacturing method thereof, an ultrasonic transducer and an ultrasonic flowmeter using the acoustic matching member. About.

Patent Document 1 is an ultrasonic transducer including a piezoelectric body and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeleton portion of the dry gel is hydrophobized. An ultrasonic transmitter / receiver is disclosed.

JP 2002-262394 A

An object of the present invention is to provide an acoustic matching member having an acoustic impedance that is difficult to obtain with only a dry gel, using the dry gel.

To provide an acoustic matching member having various acoustic impedances, an ultrasonic transducer including the acoustic matching member, a manufacturing method thereof, and an ultrasonic flowmeter including the ultrasonic transducer is there.

The acoustic matching member of the present invention is an acoustic matching member composed of dry gel particles and binding particles, and the dry gel particles have fine pores therein and hold interparticle voids between the dry gel particles. The binding particles are present between the dry gel particles to bond the dry gel particles, and the average particle diameter of the binding particles is larger than the average size of the fine pores of the dry gel particles, It is smaller than the average size of the interparticle voids of the dried gel particles.

In the acoustic matching member, the dry gel particle may have a solid skeleton portion made of silica.

In the acoustic matching member, the average size of the fine holes may be 100 nm or less.

In the acoustic matching member, the average size of the fine holes may be less than 50 nm.

In the acoustic matching member, an average particle diameter of the dry gel particles may be 1 μm or more.

In the acoustic matching member, the average particle diameter of the binding particles may be 50 nm or more and 10 μm or less.

In the acoustic matching member, the binding particles may be an organic polymer.

In the acoustic matching member, the organic polymer may be epoxy.

In the acoustic matching member, the epoxy may have an average molecular weight of 1000 to 100,000.

Moreover, the ultrasonic transducer of the present invention includes an electromechanical transducer having electrodes on both surfaces, and any one of the acoustic matching members bonded to the acoustic wave emitting surface of the electromechanical transducer.

Also, the ultrasonic flowmeter of the present invention uses any of the ultrasonic transducers described above.

Also, the method for producing an acoustic matching member of the present invention includes a step of removing the aqueous solvent from the dispersion obtained by mixing the dried gel particles and the emulsion resin dispersed in the aqueous solvent, and performing a heat treatment to integrate them.

In the above production method, the average particle diameter of the emulsion resin may be smaller than the average particle diameter of the dried gel particles. In the above production method, the average size of the fine pores of the dried gel particles may be smaller than the average particle size of the emulsion resin.

The acoustic matching member of the present invention can provide an acoustic matching member having an acoustic impedance that is difficult to obtain by using only the dried gel, using the dried gel. An acoustic matching member having various acoustic impedances, an ultrasonic transducer including the acoustic matching member, a manufacturing method thereof, and an ultrasonic flowmeter including the ultrasonic transducer can be provided.

FIG. 1 is a perspective view showing the appearance of the ultrasonic transducer according to the first embodiment. FIG. 2 is a schematic diagram showing the fine structure of the acoustic matching member in the first embodiment. FIG. 3 is a diagram illustrating a manufacturing process procedure of the ultrasonic transducer according to the first embodiment. FIG. 4 is a diagram illustrating a manufacturing process procedure of the ultrasonic transducer according to the first embodiment. FIG. 5 is a diagram showing the relationship between the binding particle size and the molecular weight in the first embodiment. FIG. 6 is a diagram illustrating a casting jig for the acoustic matching member according to the first embodiment. FIG. 7A is a diagram showing the transmission / reception characteristics of the ultrasonic transducer with respect to 1 atmosphere of air. FIG. 7B is a diagram illustrating the transmission / reception characteristics of the ultrasonic transducer with respect to 1 atmosphere of air. FIG. 8A is a diagram showing the transmission / reception characteristics of the ultrasonic transducer with respect to air at 50 atmospheres. FIG. 8B is a diagram illustrating the transmission / reception characteristics of the ultrasonic transducer with respect to air at 50 atmospheres. FIG. 9 is a cross-sectional view showing the structure of a conventional ultrasonic flowmeter. FIG. 10 is a cross-sectional view showing the structure of a conventional ultrasonic transducer.

First, an ultrasonic flowmeter that is one application of an ultrasonic sensor equipped with the acoustic matching member of the present invention will be described.

In recent years, an ultrasonic flowmeter that measures the time during which ultrasonic waves travel through a propagation path, measures the moving speed of a fluid, and measures the flow rate is being used in gas meters and the like. FIG. 9 shows a cross-sectional configuration diagram of the main part of this type of ultrasonic flowmeter.

In the ultrasonic flow meter shown in FIG. 9, the fluid to be measured whose flow rate is to be measured is arranged to flow in the pipe. On the tube wall 102, a pair of

ultrasonic transducers

101a and 101b are installed facing each other. The

ultrasonic transducers

101a and 101b are configured using a piezoelectric material such as a piezoelectric ceramic as an electromechanical transducer, and exhibit resonance characteristics like a piezoelectric buzzer and a piezoelectric oscillator.

In the example of FIG. 9, at the first stage, the ultrasonic transducer 101a is used as an ultrasonic transmitter and the ultrasonic transducer 101b is used as an ultrasonic receiver. In this stage, an alternating voltage having a frequency near the resonance frequency of the ultrasonic transducer 101a is applied to the piezoelectric body in the ultrasonic transducer 101a. Then, the ultrasonic transducer 101a functions as an ultrasonic transducer and radiates ultrasonic waves into a fluid (for example, natural gas or hydrogen gas). The emitted ultrasonic wave propagates to the path L1 and reaches the ultrasonic wave receiver 101b. At this time, the ultrasonic transmitter / receiver 101b functions as a receiver, and receives the ultrasonic wave and converts it into a voltage.

Next, the ultrasonic transducer 101b functions as an ultrasonic transducer, and the ultrasonic transducer 101a functions as an ultrasonic receiver. That is, by applying an AC voltage having a frequency near the resonance frequency of the ultrasonic transducer 101b to the piezoelectric body in the ultrasonic transducer 101b, ultrasonic waves are radiated from the ultrasonic transducer 101b into the fluid. . The emitted ultrasonic wave propagates along the path L2 and reaches the ultrasonic transducer 101a. The ultrasonic transducer 101a receives the transmitted ultrasonic wave and converts it into a voltage.

Thus, the

ultrasonic transducers

101a and 101b are generally collectively referred to as “ultrasonic transducers” in order to alternately function as a transmitter and a function as a receiver.

In the ultrasonic flow meter shown in FIG. 9, when alternating voltage is continuously applied, ultrasonic waves are continuously emitted from the ultrasonic transducer and it becomes difficult to measure the propagation time. The burst voltage signal is used as the drive voltage so that only ultrasonic waves are transmitted.

Hereinafter, the measurement principle of the ultrasonic flowmeter will be described in more detail.

First, an ultrasonic burst signal is radiated from the ultrasonic transducer 101a by applying a burst voltage signal for driving to the ultrasonic transducer 101a. As a result, the ultrasonic burst signal propagates through the path L1 and reaches the ultrasonic transducer 101b after time t. The distance of the path L1 is equal to the distance of the path L2, and is L. The ultrasonic transducer 101b can convert only the transmitted ultrasonic burst signal into an electric burst signal with a high S / N ratio.

9, the flow velocity of the fluid flowing in the pipe is V, the velocity of the ultrasonic wave in the fluid is C, and the angle between the direction of flow of the fluid and the propagation direction of the ultrasonic pulse is θ. When the ultrasonic transducer 101a is used as an ultrasonic transmitter and the ultrasonic transducer 101b is used as an ultrasonic receiver, an ultrasonic pulse emitted from the ultrasonic transmitter / receiver 101a is converted into an ultrasonic transducer 101b. If the time to reach is t1, the following equation (Equation 1) holds.

t1 = L / (C + V cos θ) (1)
On the other hand, if the arrival time when the ultrasonic transducer 101b is used as the ultrasonic transmitter and the ultrasonic transducer 101 is used as the ultrasonic receiver is t2, the following equation (Equation 2) is satisfied. A relationship is established.

t2 = L / (C−Vcos θ) (2)
Here, the difference between the reciprocals of the propagation times t1 and t2 is expressed by the following equation (Equation 3).

1 / t1-1 / t2 = 2Vcos θ / L (3)
Further, when (Expression 3) is modified, the following expression (Expression 4) is obtained.

V = (L / 2 cos θ) · (1 / t1-1 / t2) (4)
According to (Formula 4), the flow velocity V of the fluid can be obtained from the distance L of the propagation path of ultrasonic waves and the propagation times t1 and t2. The flow rate can be determined from the flow velocity V and the cross-sectional area of the flow path.

Such an ultrasonic flowmeter requires high accuracy. In order to improve accuracy, it is necessary that the S / N ratio of the ultrasonic signal is high.

In order to increase the S / N ratio of the ultrasonic signal, the acoustic impedance of the acoustic matching member formed on the ultrasonic wave transmitting / receiving surface of the piezoelectric body in the ultrasonic wave transmitter / receiver is important. The acoustic matching member plays an important role when the ultrasonic transducer radiates (transmits) an ultrasonic wave to a gas and receives an ultrasonic wave that has propagated through the gas.

Hereinafter, the role of the acoustic matching member will be described with reference to FIG. FIG. 10 shows a cross-sectional configuration of a conventional ultrasonic transducer 103.

The illustrated ultrasonic transducer 103 includes a piezoelectric body 104 and an acoustic matching member 105 bonded to one surface of the piezoelectric body 103. The acoustic matching member 105 is bonded to one surface of the piezoelectric body 105 with an epoxy adhesive.

The ultrasonic vibration of the piezoelectric body 104 is transmitted to the acoustic matching member 105 through the adhesive layer made of this adhesive. Thereafter, the ultrasonic vibration is radiated as a sound wave to a fluid (ultrasonic propagation medium) such as a gas or a liquid in contact with the acoustic matching member 105.

The role of the acoustic matching member 105 is to efficiently propagate the vibration of the piezoelectric body to the fluid.

Hereinafter, this point will be described in more detail.

The acoustic impedance Z of a substance is defined by the following formula (Formula 5) using the speed of sound C in the substance and the density ρ of the substance.

Z = ρ × C (5)
The acoustic impedance of the gas to be radiated by ultrasonic waves is significantly different from the acoustic impedance of the piezoelectric body. An acoustic impedance Z ₁ of a piezoelectric ceramic such as PZT (lead zirconate titanate) which is a general piezoelectric body is about 2.9 × 10 ⁷ kg / m ² / sec. In contrast, the acoustic impedance _{Z 3} of air is 4.0 × ¹⁰ 2 kg / ^m of about ² / sec.

At the boundary surfaces with different acoustic impedances, the sound waves are easily reflected, and the intensity of the sound waves transmitted through the boundary surfaces is reduced. Therefore, between the piezoelectric body and the gas, and it is the practice to insert a material having an acoustic impedance Z ₂ represented by the following formula (6).

Z ₂ = (Z ₁ × Z ₃ ) ^ (1/2) (6)
Upon insertion of the substances with such acoustic impedance Z _2, is suppressed reflection at the interface, the transmittance of the wave are known to improve.

Suppose here that the flow rate of a gas such as air is measured.

When the acoustic impedance Z ₁ of the piezoelectric body is 2.9 × 10 ⁷ kg / m ² / sec and the acoustic impedance Z _{3 of the} air is 4.0 × 10 ² kg / m ² / sec, (Equation 6) is satisfied. The acoustic impedance Z ₂ is about 1.1 × 10 ⁵ kg / m ² / sec.

A substance with a value of 1.1 × 10 ⁵ kg / m ² / sec must of course satisfy (Equation 5), ie Z ₂ = ρ × C. It is extremely difficult to find such substances from solid materials. The reason is that it is required to have a sufficiently low density ρ and a low sound velocity C while being solid.

The acoustic matching material disclosed in Patent Document 1 sufficiently satisfies (Equation 6). This material is produced using a dry gel imparted with durability, has a low density ρ, and a low sound velocity C. An ultrasonic transducer equipped with an acoustic matching member made of a material with extremely low acoustic impedance, such as dry gel, can transmit and receive ultrasonic waves efficiently and with high sensitivity to gas. The gas flow rate can be measured with high accuracy.

The dry gel as described above is a porous body formed by a sol-gel reaction, and a network is formed in such a way that particles of several nm are joined to each other, and a skeleton of about several nm and a size of about several nm. It is formed from holes.

When a liquid substance enters and exits such fine pores on the order of several nanometers, a large force is generated by capillary force, which may destroy the skeleton structure of the dried gel body. If the skeletal structure is broken, the block shape cannot be maintained and the low density characteristics are lost. For this reason, it is very difficult to form a block using a liquid adhesive or to join it.

Also, there is an application for measuring a gas having a larger acoustic impedance than air. In such a case, an acoustic matching member having a larger acoustic impedance is required.

Specifically, there are gases with large specific gravity such as compressed air and bromine. In order to perform such gas measurement with high accuracy, an acoustic matching member suitable for such a gas is required, and this is an acoustic matching member having an acoustic impedance different from that of an acoustic matching member formed only of dry gel. Is required.

The present inventors have conceived the following embodiment as a result of the above examination.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 shows a section of the ultrasonic transducer according to the first embodiment. The illustrated ultrasonic transducer 1 includes a piezoelectric body 2 (electromechanical transducer), a pair of

electrodes

3a and 3b provided on both surfaces of the piezoelectric body 2, and the main body of the piezoelectric body 2 via the electrodes 3a. And an acoustic matching member 4 joined to a surface (ultrasonic wave transmitting / receiving surface).

The piezoelectric body 2 is formed of a piezoelectric material and is polarized in the thickness direction (vertical direction in FIG. 1). When a voltage signal is applied between the electrode 3 a provided on the upper surface side of the piezoelectric body 2 and the electrode 3 b provided on the lower surface side, the piezoelectric body 2 expands and contracts based on the voltage signal and Ultrasonic waves are emitted from the acoustic wave transmitting / receiving surface. The ultrasonic waves are radiated to an ultrasonic propagation medium (for example, gas) 5 through the acoustic matching member 4.

On the other hand, the ultrasonic wave propagating through the propagation medium 5 reaches the piezoelectric body 2 via the acoustic matching member 4, and generates a voltage signal between the

electrodes

3a and 3b. Thus, the ultrasonic transducer 1 of this Embodiment can perform both transmission and reception of an ultrasonic wave with one.

The material of the piezoelectric body 2 used in the present embodiment is arbitrary, and a known piezoelectric material can be used. Further, instead of the piezoelectric body 2, an electrostrictive body may be used. Also when using an electrostrictive body, the material is arbitrary and a well-known material can be used. The

electrodes

3a and 3b are also formed from a known conductive material.

The acoustic matching member 4 plays a role of efficiently propagating the ultrasonic waves generated in the piezoelectric body 2 to the propagation medium 5 and also plays a role of efficiently transmitting the ultrasonic waves propagated in the propagation medium 5 to the piezoelectric body 2. .

The acoustic matching member 4 of the present embodiment is composed of a block body in which dry gel particles 8 to which silica particles of about several nanometers are bonded while having pores of the same level are bonded by the bonded particles 10. The thickness of the acoustic matching member 4 in the sound wave propagation direction is set to be about ¼ of the wavelength of ultrasonic waves to be transmitted and received.

FIG. 2 shows a schematic diagram of the internal structure of the acoustic matching member 4 of the present embodiment. In FIG. 2, 6 is a solid particle constituting the skeleton portion, 7 is a void existing between the solid particles, 8 is a dry gel particle, and a plurality of solid particles 6 have voids. It is formed by being held and held.

9 is a void existing between the

dry gel particles

8, and 10 is a binding particle that binds the dry gel particles 8.

The binding particles 10 in the present embodiment have a size larger than the internal pores (fine pores) of the dry gel particles 8 and smaller than the gaps between the dry gel particles 8. Since the binding particles 10 having such a size are too small to enter the pores in the dry gel particles 8, the low density characteristics that are characteristic of the dry gel are not lost.

The average particle size of the bonded particles 10 is measured by a laser diffraction method (for example, SALD manufactured by Shimadzu Corporation). The average particle diameter of the dried gel particles 8 is measured by a laser diffraction method (for example, SALD manufactured by Shimadzu Corporation).

The average size of the internal pores of the dried gel particles 8 is measured by a gas adsorption method (for example, BELSORP-miniII manufactured by Nippon Bell Co., Ltd.). The intergranular space 9 of the dried gel particle 8 is measured by a mercury intrusion method (for example, Autopore manufactured by Shimadzu Corporation).

Further, since the size of the binding particle 10 is sufficiently small with respect to the inter-particle gap 9 of the dry gel particle 8, it is possible to easily enter and join the dry gel particles 8 to each other. The binding particles 10 in the present embodiment are made of a resin material.

Since the acoustic matching member 4 of the present embodiment can realize various acoustic impedances by adjusting the ratio of the binding particles 10 as compared with the acoustic matching member composed only of the dry gel, An ultrasonic transducer that efficiently transmits and receives ultrasonic waves can be realized by realizing an appropriate acoustic impedance according to the characteristics.

The acoustic matching member according to the present embodiment is a composite material in which dry gel particles having micro to meso-sized pores are bound together with binding particles, and the binding particles binding the dry gel particles are the internal voids of the dry gel particles. It has a size larger than the pore size and smaller than the gap between the dried gel particles.

For this reason, the binding particles cannot enter the pores in the dry gel particles, and can enter the gaps between the dry gel particles.

Since the binding particles cannot enter the fine pores of the dried gel, the low density characteristics of the dried gel are not lost. Moreover, since it can penetrate | invade into the space | gap between the particle | grains of a dry gel particle, it is possible to join dry gel particle | grains.

Furthermore, by adjusting the blending ratio of the particle resin that binds the dry gel particles, it is easy to change the density and sound speed of the acoustic matching member, so it is easy to form an acoustic matching member suitable for many applications. I can do it.

Hereinafter, a method for manufacturing the ultrasonic transducer 1 in the present embodiment will be described.

First, a piezoelectric body 2 is prepared according to the wavelength of ultrasonic waves to be transmitted and / or received. As the piezoelectric body 2, a material having high piezoelectricity such as piezoelectric ceramics or a piezoelectric single crystal may be used. As the piezoelectric ceramic, lead zirconate titanate, barium titanate, lead titanate, lead niobate, or the like can be used. As the piezoelectric single crystal, lead zirconate titanate single crystal, lithium niobate, quartz, or the like can be used.

In this embodiment, lead zirconate titanate ceramics are used as the piezoelectric body 2, and the frequency of ultrasonic waves to be transmitted and received is set to 500 kHz. In order for the piezoelectric body 2 to efficiently transmit and receive such ultrasonic waves, the resonance frequency of the piezoelectric body 2 is designed to be about 500 kHz.

The piezoelectric body 2 resonates strongly when its thickness is set to half the wavelength of the ultrasonic wave, and the transmission / reception efficiency of the ultrasonic wave is improved. Since the sound velocity of the lead zirconate titanate ceramic is about 3800 m / sec, the wavelength of the ultrasonic wave having a frequency of 500 kHz in the piezoelectric body 2 is 7.6 mm. For this reason, in this embodiment, the piezoelectric body 2 having a cylindrical shape with a thickness of about 3.8 mm and a diameter of 12 mm is used.

The upper and lower surfaces of the piezoelectric body 2 are provided with

silver electrodes

3a and 3b by baking, and the piezoelectric body 2 is polarized in this direction.

As shown in FIG. 1, the acoustic matching member 4 has the same diameter 12 mm as that of the piezoelectric body 2 and has a thickness of ¼ wavelength as described above. Since the sound velocity of the acoustic matching member used in the present embodiment is 1500 m / sec, it is about 0.75 mm, which is a quarter wavelength at 500 kHz. The density is 500 kg / m ³ and the acoustic impedance is 7.5 × 10 ⁵ kg / m ² / sec.

Hereinafter, a method for manufacturing the acoustic matching member 4 will be described with reference to FIGS. 3 and 4.

FIG. 3 shows a manufacturing flow of the dry gel particles 8 of the acoustic matching member 4, and FIG. 4 shows a manufacturing flow of bonding the dry gel particles 8 with the binding particles 10.

The gel particle dispersion obtained by the flow of FIG. 3 is an input of the production flow in FIG. 4, and the process of FIG. 4 is serially followed after the process of FIG.

First, a method for producing the dried gel particles 8 will be described with reference to FIG. The overall flow of FIG. 3 is to prepare dry gel particles 8 by preparing a gel-like raw material liquid and forming a block-shaped dry gel and then pulverizing it.

In the present embodiment, a dry gel is used in which the solid particles 6 that are solid skeleton portions are made of silica (SiO ₂ ). The size of the solid particles serving as the skeleton part of the dried gel is about several nm, and the pore size in the dried gel particles 8 has a relatively wide dispersion of about several nm to several tens of nm.

In addition to the above, the dried gel particles 8 may be alumina, chromium oxide, tin oxide, carbon, or a copolymer polymer of resorcinol and formaldehyde.

Further, the size of the dried gel particles 8 finally formed is about several to several tens of μm.

Hereinafter, a more specific manufacturing procedure will be described.

First, in STEP 1, a dry gel material solution is prepared. A raw material liquid is prepared by mixing alkoxysilane such as tetraethoxysilane, which is a raw material of the skeleton portion of the dried gel, with ethanol, water, and a catalyst (ammonia, hydrochloric acid, etc.).

As an example, the mixing ratio of the gel raw material liquid may be 6% tetraethoxysilane, 20% ethanol, and 74% 0.1N ammonia water. The volume ratio of tetraethoxysilane in the gel raw material solution directly affects the density of the dried gel formed.

When the volume ratio of tetraethoxysilane is small, a dry gel with a low density is generated, and when the volume ratio is large, a dry gel with a high density is generated.

In the dry gel of the present embodiment, for example, in order to take advantage of the low density characteristics of the dry gel, the blending ratio of the gel raw material liquid is set to a density of about 0.1. Further, this density is set so as to be a density that can be easily handled and can maintain strength when bonded by the binding particles 10 in this process and used as an acoustic matching member.

Next, in STEP 2, the prepared gel raw material solution is heated in a thermostatic chamber or the like to start and accelerate the polymerization reaction. As an example, the polymerization reaction is completed by leaving it in a constant temperature bath at 60 ° C. for 24 hours. Since the temperature and the rate of the polymerization reaction are in a substantially proportional relationship, and the rate of the polymerization reaction greatly affects the particle size of the gel raw material liquid, it is important to control the reaction temperature in order to obtain a desired gel. The polymerization reaction rate is also related to the stability of the properties of the gel.

When the polymerization rate is high, the particle size and the pore size tend to be small, and when the reaction rate is low, the particle size and the pore size tend to be large.

¡Dry gels with small particle size tend to have low sound speed, and conversely dry gels with large particle size tend to have high sound speed.

When using it as an acoustic matching member for gas, it is advantageous that the sound speed is slower, so that the reaction rate, that is, the heating temperature, may be controlled so that the particle size is about several nanometers.

As described above, by adjusting the blending ratio of the gel raw material liquid and the reaction temperature, the specific gravity of the gel skeleton and pore particles is appropriately set to 0.1, and the internal pore size is about 1 to 20 nm A gel can be obtained.

By the above process, a wet gel in which a liquid is filled between silica skeletons described in STEP 3 is obtained. Between the silica skeleton, ethanol, water, catalyst, and some unreacted alkoxysilane contained in the gel raw material liquid are present.

The alkoxysilane remaining in the wet gel may start a polymerization reaction in an environment such as an external temperature. In addition, decomposition and dissolution reactions may start together with the polymerization reaction in an environment where a catalyst such as ammonia exists. Therefore, these residual solutions may be removed.

In STEP4, in order to remove unstable elements that change the characteristics as described above, solvent replacement from the remaining gel raw material liquid and catalyst is performed.

The solvent to be substituted may be any solvent that does not affect the reaction of the gel skeleton, such as pure water, an alcohol solvent such as ethanol, or an organic solvent such as pentane or hexane, and any substitution solvent can be selected. .

In this embodiment, pure water is selected as a solvent in consideration of mixing with an aqueous emulsion resin used in a later process. The formed wet gel is taken out from the reaction vessel and left in a vessel containing pure water having a volume about 10 times that of the wet gel for 24 hours to remove residual alkoxysilane and catalyst in the wet gel.

In STEP5 and STEP6, the obtained block-shaped wet gel is pulverized by a ball mill in order to obtain particles having a desired size of about several μm.

The principle of pulverization with a ball mill is that it moves in a cylindrical container by filling a cylindrical container such as Teflon (registered trademark) with a wet gel and hard balls such as zirconia and rotating the cylindrical container. The gel is crushed by the collision of the zirconia balls.

In the case of pulverization with a ball mill, the particle size decreases with time. Further, when the rotational speed of the cylindrical container is increased, the size of particles obtained in the same processing time is reduced, that is, the pulverization efficiency is increased.

However, since the grinding effect cannot be obtained if the number of revolutions becomes so fast that the grinding balls in the cylindrical container rotate while sticking to the wall in the cylindrical container, the processing needs to be performed at a certain number of revolutions or less. is there.

Also, the size (diameter) of the ball for grinding affects the grinding efficiency and the particle size after grinding. When the ball is large, the impact at the time of the collision of the ball is large, so that the grinding efficiency is increased. That is, the change in particle size per unit time is large.

However, when the ball size is large, there is a tendency that the dispersion of the particle size that can be crushed tends to be large. In this embodiment, a zirconia ball having a diameter of 3 mm is used. Zirconia is suitable as a material for balls for grinding because it is hard and hard to wear. In addition to zirconia, alumina or the like can also be used.

Also, when the volume of the gel, solvent, and zirconia balls in the cylindrical container is less than a certain range with respect to the volume of the cylindrical container, the pulverization efficiency is improved. In this embodiment, it is about 60%.

As an example, 20% airgel and 20% zirconia balls are placed in a cylindrical container made of Teflon (registered trademark) having an inner diameter of 150 mm, and 20% zirconia balls are further added, followed by grinding at 90 rpm for 1 hour. Thus, dry gel particles 8 having an average particle diameter of about 8 μm can be obtained.

Here, the average particle diameter of the dried gel particles 8 was measured by a laser diffraction method (SALD manufactured by Shimadzu Corporation), and as a result, it was about 8 μm. The average size of the fine pores of the dried gel particles 8 was measured by a gas adsorption method (BELSORP-mini II manufactured by Nippon Bell Co., Ltd.) and found to be about 10 nm.

The balls used for pulverization are removed from the dried gel particles 8 thus obtained to obtain a gel particle-dispersed solution having a constant average particle diameter. Since the grinding balls used as described above have a diameter of 3 mm and gel particles have a size of about 10 μm, they can be easily separated with a mesh having an opening of about 1 mm.

The gel particle dispersion thus obtained is subjected to a binding process using the binding particles 10 as the next process.

FIG. 4 shows a coupling process using the coupled particles 10 and a process from the acoustic matching member to the ultrasonic transducer.

In STEP 8 of FIG. 4, an emulsion resin dispersion is added to the obtained gel particle dispersion. The emulsion resin dispersion is obtained by dispersing fine particulate resin in an aqueous solvent such as water.

In recent years, it is a material that is widely used as a paint or coating agent suitable for reducing environmental impact. Various materials can be used as the binding particles 10 in the emulsion resin dispersion. As an example, an acrylic resin, an epoxy resin, a urethane resin, a vinyl acetate resin, a styrene resin, an olefin resin, or the like can be used.

In this embodiment, an emulsion resin dispersion in which an epoxy resin having a particle size of about 800 nm is dispersed is used.

FIG. 5 shows the relationship between the molecular weight of the epoxy resin and the size of the particle diameter. In FIG. 5, the horizontal axis represents the particle diameter, and the vertical axis represents the molecular weight. FIG. 5 shows the maximum particle size that can be taken by the molecular weight. In other words, it shows the theoretical value when the chain molecule of epoxy, which is a polymer, is perfectly linear. In fact, the polymer is linear and hardly exists in the solution, and is bent. Or, it may be spherical like a rounded thread, and the size shown in FIG. 5 is the maximum size and is usually smaller than this.

As a result of measuring the average particle diameter of the epoxy resin by a laser diffraction method (SALD manufactured by Shimadzu Corporation), it was about 1 μm.

Since the epoxy resin is in a normal state and a liquid state when the average molecular weight is less than 600, it cannot be used in the acoustic matching member of the present embodiment. Further, there is a correlation between the molecular weight and the softening point (flow, curing), and the fluidity at the same temperature decreases as the molecular weight increases.

In STEP9, in order to improve the uniformity of the dispersion liquid in which the gel particles and the binding particles 10 are mixed, a homogenization process is performed. The homogenization process can be performed with a normal stirrer or the like. Alternatively, when the dispersion has a high viscosity, a rotary mixer or the like can be used. In this embodiment, the homogenization process is performed for 5 minutes using a rotary mixer.

The average molecular weight is measured by a liquid chromatographic method (for example, Nexera manufactured by Shimadzu Corporation).

In STEP 10, the dispersion of the gel particles and the binding particles 10 thus obtained is molded into a necessary shape, and an operation of removing excess water is performed. For this reason, in this embodiment, a gypsum mold that simultaneously performs the functions of molding and removing excessive water is used.

Specifically, as shown in FIG. 6, a hollow 12 having substantially the same shape as the size of the acoustic matching member 4 is provided on a plaster plaster plate 11, and a gel particle / binding particle 10 dispersion is cast into this portion. In the present embodiment, in order to obtain a pellet having a final shape of 12 mm in diameter and 1 mm in thickness, which is the size of the acoustic matching member 4, the diameter of the recess is 20 mm and the depth is 5 mm.

This is because the moisture is removed and the shrinking dimension, and finally the outer periphery and thickness need to be machined. This size was determined experimentally.

Since the mold made of gypsum does not completely block liquid such as water and has fine pores that block gel particles and binding particles 10, not only from the surface of the cast dispersion, Since it is possible to remove moisture from the gypsum body itself, it is possible to efficiently remove moisture.

This not only shortens the time of the drying process, but also reduces the distortion because the entire shape shrinks in the same way compared to when moisture is removed from the surface alone, for example during molding and assembly. It is difficult for cracks to occur.

In this embodiment, in order to remove moisture, the dispersion liquid cast into a plaster mold was left in a constant temperature bath at 40 ° C. for 2 days (STEP 11). By performing this treatment, 90% or more of the moisture can be removed. In this way, a dried product of the composite of the dried gel particles 8 and the binding particles 10 from which moisture has been removed can be obtained.

However, at this point, the dried gel particles 8 and the binding particles 10 are in a state of being joined only by a weak intermolecular force, and the strength is insufficient for use as an acoustic matching member.

In STEP12, in order to make this a stronger bond, the temperature of the thermostatic chamber is raised to 150 ° C. while being installed in the gypsum mold. When the temperature is about 150 ° C., a component having a particularly small particle size of the bonded particles 10 is melted to be in a liquid state. Since only such a resin having a small particle diameter, that is, a binding particle 10 having a small molecular weight, selectively flows on the surface of the large resin, it does not enter the fine pores inside the dry gel particle 8. .

When such an appropriate temperature is selected, the dried gel particles 8 can be integrated with each other with a strong bond without substantially destroying the pores inside the dried gel particles 8. After the heat treatment, if there is an abrupt temperature change, the composite that becomes the formed acoustic matching member is distorted, which may cause cracking or chipping.

In this embodiment, after heating at 150 ° C. and the effect, it was cooled down to room temperature over about 6 hours. The composite block thus formed had a thickness of 2 mm and a diameter of 15 mm.

In order to use the composite block body of the dry gel particles 8 and the binding particles 10 thus configured with strong bonds as acoustic matching, in STEP13, the outer periphery is processed and the thickness is adjusted.

First, the thickness of the sound wave emitting surface or the surface coupled with the piezoelectric body 2 is adjusted to a predetermined thickness of 1 mm by a polishing machine. Thereafter, the polished surface is held and outer periphery processing is performed. The outer peripheral machining was adjusted by holding with a jig slightly smaller than a disk-shaped acoustic matching member having a diameter of 12 mm, and polishing the portion exposed from the jig while rotating with a file.

As a result of measuring the average size of the interparticle voids of the dried gel particles 8 by a mercury intrusion method (Autopore manufactured by Shimadzu Corporation), it was 10 μm.

As described above, the acoustic matching member 4 thus obtained has a density of 0.50 and a sound velocity of 1500 m / s. The acoustic matching member 4 thus obtained is joined to the sound wave emitting surface of the piezoelectric body 2 in STEP14.

Also in this case, when an epoxy resin having a low molecular weight is used for bonding to the piezoelectric body 2, the resin before curing flows and enters the pores inside the dried gel particles 8, and the density of the obtained acoustic matching member Characteristics may vary. Moreover, when there is little adhesive agent, joining may not be implement | achieved.

For this reason, an emulsion resin may be used for bonding to the piezoelectric body 2.

First, an emulsion resin is applied to the upper surface of the piezoelectric body 2, and water is removed by heating at a low temperature of 60 ° C. A molded acoustic matching member is placed on the emulsion-like resin layer thus formed, and further heated to 150 ° C. while being pressurized. Thus, an ultrasonic transducer in which the piezoelectric body 2 is provided with an acoustic matching member composed of the dried gel particles 8 and the binding particles 10 is obtained.

An example of the characteristics of the ultrasonic transducer 1 formed in this way is shown in FIG. 7 and FIG. 7 and 8, the horizontal axis indicates time, and the vertical axis indicates the relative amplitude.

FIG. 7 is a diagram illustrating characteristics when ultrasonic waves are transmitted / received to / from air at 1 atmosphere. FIG. 7A shows the result of the acoustic matching member (acoustic impedance 0.1 × 10 ^ 6) formed only with the dried gel, and FIG. 7B shows the acoustic matching member (acoustic impedance) produced according to the present embodiment. 0.75 × 10 ^ 6). It can be seen that higher sensitivity can be obtained when using an acoustic matching member made of only a conventional dry gel for air at normal temperature and normal pressure.

On the other hand, FIG. 8 shows characteristics when ultrasonic waves are transmitted / received to / from air of 50 atm. At high pressure, the gas density increases proportionally. Since the speed of sound has no correlation with pressure, when the pressure increases 50 times, the acoustic impedance increases 50 times. For this reason, the optimum acoustic impedance as the acoustic matching member is larger than that in the case of atmospheric pressure air.

8A is a result of an acoustic matching member (acoustic impedance 0.1 × 10 ^ 6) formed only with a dry gel, and FIG. 8B is an acoustic matching produced by the present embodiment. It is a result by a member (acoustic impedance 0.75 × 10 ^ 6).

As can be seen from FIG. 8, the ultrasonic sensor using the acoustic matching member in the present embodiment can obtain higher sensitivity, that is, the measurement accuracy of the ultrasonic flowmeter can be improved.

As described above, when the acoustic matching member of the present embodiment is used, an acoustic matching member having a wide range of characteristics can be formed by using a dry gel material combined with another material. Alternatively, it is possible to realize an ultrasonic transducer and an ultrasonic flowmeter for use.

In the above, the process from the polymerization of the particulate dry gel has been described as an example, but a commercially available particulate dry gel can also be used.

In such a case, STEP 8 or later in which the particulate dry gel and the emulsion resin are mixed is performed. In this case, since the procedure up to STEP 7 can be omitted, such a method may be adopted in consideration of time cost. However, it is possible to select the optimum method in consideration of the total cost.

From the above description, many modifications and other embodiments of the present invention are apparent to persons skilled in the art. Accordingly, the foregoing description should be construed as illustrative only and is provided for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and / or function may be substantially changed without departing from the spirit of the invention.

The acoustic matching member of the present invention can be widely used in devices for transmitting and receiving ultrasonic waves between various media centered on gas, and in particular, sensing and flow rate required to detect ultrasonic waves with high accuracy. It can be suitably used for a measuring device.

1 Ultrasonic transducer 2 Piezoelectric body (electromechanical transducer)
3a, 3b Electrode 4 Acoustic matching member 5 Propagation medium 6 Solid particle 7 Fine pore 8 Dry gel particle 9 Interparticle void 10 Bonded particle

Claims

An acoustic matching member comprising dry gel particles and binding particles,
The dry gel particles have fine pores inside, and retain inter-particle voids between the dry gel particles,
The binding particles exist between the dry gel particles to bond the dry gel particles to each other,
The acoustic matching member, wherein an average particle diameter of the binding particles is larger than an average size of the fine pores of the dry gel particles and smaller than an average size of the interparticle voids of the dry gel particles.
The acoustic matching member according to claim 1, wherein the dry gel particles have a solid skeleton portion made of silica.
The acoustic matching member according to claim 1, wherein an average size of the fine holes is 100 nm or less.
The acoustic matching member according to claim 1, wherein an average particle diameter of the dry gel particles is 1 µm or more.
The acoustic matching member according to claim 1, wherein an average particle size of the binding particles is 50 nm or more and 10 µm or less.
The acoustic matching member according to claim 1, wherein the binding particle is an organic polymer.
The acoustic matching member according to claim 6, wherein the organic polymer is epoxy.
The acoustic matching member according to claim 7, wherein the epoxy has an average molecular weight of 1,000 to 100,000.
An ultrasonic transducer having the electromechanical transducer having electrodes on both surfaces and the acoustic matching member according to any one of claims 1 to 8 joined to a sound wave emitting surface of the electromechanical transducer.
10. The ultrasonic transducer according to claim 9, wherein the acoustic matching member is joined to an electromechanical transducer with the coupling particles.
An ultrasonic flowmeter using the ultrasonic transducer according to claim 9 or 10.
An ultrasonic flowmeter comprising the ultrasonic transducer according to claim 9 or 10.
A method for producing an acoustic matching member in which an aqueous solvent is removed from a dispersion obtained by mixing dry gel particles and an emulsion resin dispersed in an aqueous solvent, and heat treatment is performed for integration.