JP2004029038A - Ultrasonic flow meter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter that enables a highly sensitive ultrasonic wave transmission and reception with sufficiently small acoustic impedance, which is matched with gas being an emission medium of ultrasound, and also comprises a acoustic matching layer that can improve rising responsiveness of signals. <P>SOLUTION: The ultrasonic flowmeter is provided with a flow measurement part where the fluid to be measured flows; a pair of ultrasound transducer, which is arranged in the flow measurement part, for transmitting and receiving ultrasound signals; a measurement circuit for measuring an ultrasound propagation time between the ultrasound transducers; and a flow rate arithmetic circuit for computing the flow rate on the basis of signals from the measurement circuit. Each of the pair of ultrasound transducer has a piezoelectric layer 4 and the acoustic matching layer 1 arranged on the piezoelectric layer 4. The acoustic matching layer 1 has a first acoustic matching layer 2 of which density is within a range of 50 kg/m<SP>3</SP>to 500 kg/m<SP>3</SP>and a second acoustic matching layer 3 of which density is within a range of 400 kg/m<SP>3</SP>to 1,500 kg/m<SP>3</SP>. Also, the density of the first acoustic matching layer 2 is smaller than that of the second acoustic matching layer 3, and the second acoustic matching layer 3 is arranged at the side of the piezoelectric layer 4. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention uses an acoustic matching layer used as an acoustic matching layer of an ultrasonic sensor, an ultrasonic transducer for transmitting and receiving ultrasonic waves, a method for manufacturing the ultrasonic transducer, and the ultrasonic transducer. The present invention relates to an ultrasonic flowmeter which has been used.

In recent years, an ultrasonic flowmeter that measures the time for an ultrasonic wave to propagate through a propagation path, measures the moving speed of a fluid, and measures the flow rate has been used in gas meters and the like.

FIG. 12 shows the measurement principle of the ultrasonic flowmeter. As shown in FIG. 12, a fluid flows in the pipe at a speed V in the direction shown in the figure. A pair of ultrasonic transducers 101 and 102 are opposed to the tube wall 103. The ultrasonic transducers 101 and 102 are configured using a piezoelectric material such as a piezoelectric ceramic as an electric energy / mechanical energy conversion element, and exhibit resonance characteristics like a piezoelectric buzzer and a piezoelectric oscillator. Here, the ultrasonic transducer 101 is used as an ultrasonic transducer, and the ultrasonic transducer 102 is used as an ultrasonic transducer.

The operation is as follows. When an AC voltage having a frequency near the resonance frequency of the ultrasonic transducer 101 is applied to the piezoelectric vibrator, the ultrasonic transducer 101 acts as an ultrasonic transducer and is placed in an external fluid. Ultrasonic waves are radiated to the propagation path indicated by L1 in the middle, and the ultrasonic transducer 102 receives the transmitted ultrasonic waves and converts them into a voltage. Subsequently, on the contrary, the ultrasonic transducer 102 is used as an ultrasonic transducer, and the ultrasonic transducer 101 is used as an ultrasonic transducer. By applying an AC voltage having a frequency near the resonance frequency of the ultrasonic transducer 102 to the piezoelectric vibrator, the ultrasonic transducer 102 transmits the ultrasonic wave through a propagation path indicated by L2 in FIG. The ultrasonic transmitter / receiver 101 radiates and receives the transmitted ultrasonic wave and converts it into a voltage. As described above, the ultrasonic transducers 101 and 102 serve as a receiver and a transducer, respectively, and are therefore generally referred to as ultrasonic transducers.

Also, in such an ultrasonic flowmeter, if an AC voltage is applied continuously, ultrasonic waves are continuously emitted from the ultrasonic transducer and it becomes difficult to measure the propagation time. Is used as a driving voltage. Hereinafter, the measurement principle will be described in more detail.

When a driving burst voltage signal is applied to the ultrasonic transducer 101 and an ultrasonic burst signal is emitted from the ultrasonic transducer 101, the ultrasonic burst signal propagates along a propagation path L1 having a distance of L and becomes t. After a time, it reaches the ultrasonic transducer 102. The ultrasonic transducer 102 can convert only the transmitted ultrasonic burst signal into an electric burst signal with a high S / N ratio. This electric burst signal is electrically amplified and applied again to the ultrasonic transducer 101 to emit an ultrasonic burst signal. This device is called a sing-around device, and the time required for an ultrasonic pulse to be emitted from the ultrasonic transducer 101 and propagated along the propagation path to reach the ultrasonic transducer 102 is called a sing-around period. The reciprocal is called a single around frequency.

In FIG. 12, V is the flow velocity of the fluid flowing through the pipe, C is the velocity of the ultrasonic wave in the fluid, and θ is the angle between the flowing direction of the fluid and the propagation direction of the ultrasonic pulse. When the ultrasonic transducer 101 is used as an ultrasonic transducer and the ultrasonic transducer 102 is used as an ultrasonic transducer, an ultrasonic pulse emitted from the ultrasonic transducer 101 is used as an ultrasonic transducer 102. If the sing-around period, which is the time to reach, is t1 and the sing-around frequency f1 is, the following equation (1) is established.

F1 = 1 / t1 = (C + Vcos θ) / L (1)

Conversely, if the ultrasonic transducer 102 is used as an ultrasonic transducer and the ultrasonic transducer 101 is used as an ultrasonic receiver, the sing-around period is t2, and the sing-around frequency f2 is The following equation (2) holds.

F2 = 1 / t2 = (C−Vcos θ) / L (2)

Therefore, the frequency difference Δf between the two sing-around frequencies is given by the following equation (3), and the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic wave propagation path and the frequency difference Δf.

Δf = f1−f2 = 2Vcosθ / L (3)

That is, the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic wave propagation path and the frequency difference Δf, and the flow rate can be checked from the flow velocity V.

Such an ultrasonic flowmeter requires accuracy, and in order to improve the accuracy, an ultrasonic transducer that transmits ultrasonic waves to a gas or receives ultrasonic waves that have propagated a gas is configured. The acoustic impedance of the acoustic matching layer formed on the transmitting / receiving surface of the ultrasonic wave in the piezoelectric vibrator is important.

FIG. 10 is a cross-sectional view showing the configuration of a conventional ultrasonic transducer 10 '. The ultrasonic transducer 10 ′ includes a piezoelectric layer (vibration means) 4, an acoustic impedance matching layer (acoustic matching means, hereinafter, referred to as “acoustic matching layer”) 1, and a case 5. The case 5 and the acoustic matching layer 1 ', and the case 5 and the piezoelectric layer 4 are adhered by using an adhesive layer made of an adhesive (for example, epoxy). The ultrasonic waves vibrated by the piezoelectric layer 4 vibrate at a specific frequency (for example, 500 kHz), and the vibration is transmitted to the case via an adhesive layer (not shown), and further transmitted to the acoustic matching layer 1 via the adhesive layer. . The matched vibration propagates as sound waves to a gas that is a medium existing in the space.

The role of the acoustic matching layer 1 'is to efficiently propagate the vibration of the piezoelectric layer 4 to the gas. The acoustic impedance Z is defined by the sound velocity C and the density ρ in the substance as in equation (4).

{Z = ρ × C} (4)

The acoustic impedance differs greatly between the piezoelectric layer 4 and the gas that is the ultrasonic radiation medium. For example, the acoustic impedance Z1 of piezoelectric ceramics such as PZT (lead zirconate titanate), which is a general piezoelectric material constituting the piezoelectric layer 2, is about 30 × 10 ⁶ kg / s · m ² . Further, the acoustic impedance Z3 of a gas as a radiation medium, for example, air is about 400 kg / s · m ² . On the boundary surfaces having different acoustic impedances, the sound waves are reflected in the propagation of the sound waves, and the intensity of the transmitted sound waves is weakened. As a method of solving this, sound reflection is performed by inserting a substance having an acoustic impedance having a relationship represented by the formula (5) between the acoustic impedances Z1 and Z3 of the piezoelectric body and the gas, respectively. It is generally known to reduce the intensity to increase the intensity of transmitted sound waves.

Z2 = (Z1 × Z3) ^(1/2) (5)

The optimum value when the acoustic impedance that satisfies this condition is matched is about 11 × 10 ⁴ kg / s · m ² . The substance satisfying the acoustic impedance is required to be a solid substance having a low density and a low sound velocity, as can be seen from the equation (4). As a generally used material, a material obtained by solidifying a glass balloon (hollow minute glass sphere) or a plastic balloon with a resin material is formed on the vibration surface of the piezoelectric layer (also referred to as an “ultrasonic transducer”). Have been used. Further, a method of thermally compressing a glass balloon or a method of foaming a molten material is also used. This is disclosed in, for example, Japanese Patent No. 2559144.

However, the acoustic impedance of these materials is a value larger than 50 × 10 ⁴ kg / s · m ² , and a material having a lower acoustic impedance is necessary to obtain high sensitivity by matching with gas. .

By forming an acoustic matching layer using a dried gel in Japanese Patent Application No. 2001-56501 (filing date: February 28, 2001), the present applicant has further improved acoustic properties than a conventional epoxy resin system containing a glass balloon. It discloses that the impedance can be reduced and that the durability can be improved by making the dried gel hydrophobic.

As described above, if the acoustic impedance of the acoustic matching layer is reduced to improve the matching with the gas that is the radiation medium of the ultrasonic wave, the sensitivity as the ultrasonic transducer becomes very high. However, when ultrasonic waves are transmitted and received using a pulse signal as a carrier, as in the case where the propagation time is measured by ultrasonic waves in a flow meter, the rising response of the signal is deteriorated, and it is difficult to determine the arrival time. That is, usually, the determination of arrival is performed by detecting a signal of a wave front that has reached a certain detection level or higher in a reception signal of an ultrasonic wave. Therefore, if the rise of the signal output is good, the difference between the wave fronts of the ultrasonic waves is large, and the signal of the wave front to be determined can be recognized well, and the arrival can be determined without error. On the other hand, if the rise of the ultrasonic reception signal is not good, the difference between the wavefronts of the output of the received ultrasonic waves becomes small, so that it is difficult to identify the wavefront to be determined to arrive, and an error easily occurs in the detection.

The present invention has been made in view of the above-described problems, and has a sufficiently small acoustic impedance, can transmit and receive ultrasonic waves with high sensitivity in accordance with a gas that is a radiation medium of ultrasonic waves, and has a good signal rising response. It is a main object to provide an acoustic matching layer for an ultrasonic transducer that can be used. Further, the present invention provides an ultrasonic transducer using the same, and a flowmeter using the same.

The acoustic matching layer of the present invention is an acoustic matching layer for matching the acoustic impedance between the piezoelectric layer and the gas, and has a density in the range of 50 kg / m ³ or more and 500 kg / m ³ or less. And a second acoustic matching layer having a density in a range of 400 kg / m ³ or more and 1500 kg / m ³ or less, and a density of the first acoustic matching layer is smaller than a density of the second acoustic matching layer. .

In one embodiment, the density of the first acoustic matching layer is in a range of 50 kg / m ³ or more and 400 kg / m ³ or less, and the density of the second acoustic matching layer is more than 400 kg / m ^{3 and} 800 kg / m ³ or less. In range.

In one embodiment, the relationship between the acoustic impedance Za of the first acoustic matching layer and the acoustic impedance Zb of the second acoustic matching layer is Za <Zb.

In one embodiment, the thickness of the first acoustic matching layer is approximately one-fourth of the wavelength λ of the sound wave propagating in the first acoustic matching layer.

In one embodiment, an acoustic impedance of the first acoustic matching layer is in a range from 5 × 10 ⁴ kg / s · m ^{2 to} 20 × 10 ⁴ kg / s · m ² .

In one embodiment, the thickness of the second acoustic matching layer is approximately one quarter of the wavelength λ of the sound wave propagating in the second acoustic matching layer.

In one embodiment, both the first acoustic matching layer and the second acoustic matching layer include an inorganic oxide.

In one embodiment, the first acoustic matching layer includes a dry gel.

In one embodiment, the first acoustic matching layer includes a dry gel powder.

In one embodiment, the solid skeleton of the dried gel contains an inorganic oxide.

In one embodiment, the inorganic oxide is silicon oxide.

In one embodiment, the solid skeleton of the inorganic oxide is hydrophobized.

In one embodiment, the first acoustic matching layer and the second acoustic matching layer are directly coupled.

In one embodiment, a structure support layer is further provided between the first acoustic matching layer and the second acoustic matching layer, wherein the density of the structure support layer is 1000 kg / m ³ or more, and The thickness is less than one-eighth of the wavelength λ of the sound wave propagating in the structural support layer.

An ultrasonic transducer according to the present invention includes a piezoelectric layer, and any one of the above acoustic matching layers provided on the piezoelectric layer, and the second acoustic matching layer is disposed on the piezoelectric layer side. Have been.

In one embodiment, the acoustic matching layer is directly bonded on the piezoelectric layer.

In one embodiment, the piezoelectric device further includes a case having an upper plate forming a concave portion including the piezoelectric layer, and a bottom plate arranged to seal a space in the concave portion, wherein the piezoelectric layer is formed of the case. The acoustic matching layer is bonded to the upper surface of the upper plate so as to face the piezoelectric layer via the upper plate.

In one embodiment, the case is formed from a metal material.

In one embodiment, the upper plate of the case is formed integrally with the second acoustic matching layer.

The method for manufacturing an ultrasonic transducer according to the present invention is the method for manufacturing an ultrasonic transducer according to any one of the above, wherein the piezoelectric layer or the piezoelectric layer is bonded to the inner surface on the upper plate. Forming a second acoustic matching layer; and thereafter, forming the first acoustic matching layer made of dry gel on the second acoustic matching layer. Alternatively, a step of forming the first acoustic matching layer made of dry gel on the second acoustic matching layer and obtaining the acoustic matching layer, and joining the piezoelectric layer or the piezoelectric layer to the inner surface. Bonding the acoustic matching layer on the upper plate.

An ultrasonic flowmeter according to the present invention includes a flow measurement unit through which a fluid to be measured flows, a pair of ultrasonic transducers provided in the flow measurement unit for transmitting and receiving ultrasonic signals, and an ultrasonic transducer between the ultrasonic transducers. A measuring circuit for measuring the sound wave propagation time, and an ultrasonic flow meter including a flow rate calculating circuit for calculating a flow rate based on a signal from the measuring circuit, wherein each of the pair of ultrasonic transducers is It is composed of one of the ultrasonic transducers.

According to the present invention, there is provided an acoustic matching layer formed by laminating a first acoustic matching layer having a low density and a low sound speed and a second acoustic matching layer having a high density and a high sound speed. By applying it to an ultrasonic transducer and arranging a first acoustic matching layer acoustically matched with an ultrasonic wave radiating medium on the radiating medium side, a gas having a sufficiently small acoustic impedance and being an ultrasonic radiating medium In addition to this, it is possible to transmit and receive ultrasonic waves with high sensitivity in conformity with the above, and to obtain an excellent ultrasonic transmitter / receiver having a good signal responsiveness in rising.

超 Further, the ultrasonic transducer obtained by using the manufacturing method of the present invention can achieve high sensitivity and stable characteristics by using an acoustic matching layer having low acoustic impedance.

Further, the ultrasonic flowmeter of the present invention can improve the stability of flow measurement because the ultrasonic transducer of the present invention has high sensitivity and little variation in characteristics. In addition, the acoustic matching layer is composed of an inorganic oxide, so that excellent temperature characteristics of the flow rate measurement can be obtained. In addition, by making the acoustic matching layer hydrophobic, a highly reliable ultrasonic flow meter having excellent moisture resistance. Can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the acoustic matching layer 1 according to the embodiment of the present invention includes a first acoustic matching layer 2 having a low density and a low sound speed, and a second acoustic matching layer 3 having a higher density and a high sound speed. Prepare. The density of the first acoustic matching layer 2 is in the range of 50 kg / m ^{3 to} 500 kg / m ³ , the density of the second acoustic matching layer 3 is in the range of 400 kg / m ^{3 to} 1500 kg / m ³ , Further, the density of the first acoustic matching layer 2 is lower than the density of the second acoustic matching layer 3. For example, the density of the first acoustic matching layer 2 is 50 kg / m ³ or more and 400 kg / m ³ or less, and the density of the second acoustic matching layer 3 is more than 400 kg / m ^{3 and} 800 kg / m ³ or less.

In the piezoelectric vibrator 8 according to the embodiment of the present invention, as shown in FIG. 2, the first acoustic matching layer 2 is disposed on the radiation medium side, and the second acoustic matching layer 3 is disposed on the piezoelectric layer 4 side. As described above, by using the piezoelectric vibrator 8 including the acoustic matching layer 1 according to the present invention, it is possible to increase the sensitivity of the ultrasonic transducer.

For example, the ultrasonic transducer 10A according to the embodiment of the present invention shown in FIG. 3 is different from the conventional ultrasonic transducer 10 'shown in FIG. 10 in that the acoustic matching layer 1' is replaced with the book shown in FIG. The acoustic matching layer 1 according to the embodiment of the present invention is provided. The first acoustic matching layer 2, which is acoustically impedance-matched with the ultrasonic radiation medium, is disposed on the radiation medium side. By adopting this configuration, it is possible to transmit and receive high-sensitivity ultrasonic waves by matching the acoustic impedance to a gas that is a sufficiently small radiating medium of ultrasonic waves, and to provide an excellent ultrasonic transducer with good signal responsiveness. can get.

効果 Hereinafter, effects obtained by the configuration of the ultrasonic transducer according to the embodiment of the present invention will be described in detail with reference to FIGS. 8A to 8C and FIGS. 9A to 9C.

FIGS. 8 (a) to 8 (c) show the reception output characteristics of the ultrasonic wave of the ultrasonic transducer, and show the reception waveform in each acoustic matching layer.

FIGS. 8 (a) and 8 (b) show the use of the conventional ultrasonic transducer 10 'shown in FIG. 10 having a single acoustic matching layer. FIG. 8A shows a case where a glass balloon / epoxy acoustic matching layer (thickness: 1.25 mm, sound speed: 2500 m / s, density: 500 kg / m ³ ) is used, and FIG. This shows a case where a gel acoustic matching layer (thickness: 90 μm, sound velocity: 180 m / s, density: 200 kg / m ³ ) is used.

FIG. 8C is a diagram showing characteristics of the ultrasonic transducer 10A of the embodiment according to the present invention shown in FIG. As the first acoustic matching layer 2, an acoustic matching layer made of dried silica gel (thickness: 90 μm, sound speed: 180 m / s, density: 200 kg / m ³ ) is used, and as the second acoustic matching layer 3, a porous silica acoustic matching layer ( This figure shows a case where a thickness of 750 μm, a sound velocity of 1500 m / s, and a density of 570 kg / m ³ ) are used.

First, a comparison between FIGS. 8A and 8B shows that the use of a low-density dry gel as the acoustic matching layer makes it possible to reduce the use of a conventionally used glass balloon / epoxy system. It can be seen that the maximum amplitude (peak-to-peak voltage) of the reception output voltage is large, and the sensitivity is improved.

However, it can be seen that the rise of the received signal is slower in FIG. 8B than in FIG. 8A. Further, since the difference between the output value of each wavefront of the 500 kHz ultrasonic signal at the rising edge and the wavefronts before and after the wavefront is small, the permissible range of the propagation time detection based on the arrival detection level is small, and error detection is likely to occur, and the detection becomes difficult. It's getting harder. For this reason, the ultrasonic transducer using the silica dry gel as the acoustic matching layer has high sensitivity, but needs to improve the rising characteristics.

As shown in FIG. 8 (c), by using an acoustic matching layer having a two-layer structure composed of a dried silica gel and a porous silica prepared by firing silicon oxide, the peak-to-peak voltage is increased and the sensitivity is increased. Also, good rising characteristics are obtained. This is because the first acoustic matching layer having a low density and a low sound velocity arranged on the gas side can achieve high sensitivity by achieving acoustic impedance matching with the gas that is a radiation medium of ultrasonic waves, This is considered to be because good rising characteristics are secured by the second acoustic matching layer having a high density and a high sound velocity arranged on the body side.

(4) The reason why such good characteristics are obtained will be further described with reference to FIGS. FIGS. 9A to 9C are diagrams illustrating vibration displacement frequency characteristics of the ultrasonic transducer corresponding to FIGS. 8A to 8C, respectively.

で は As shown in FIG. 9 (a), the conventional glass balloon / epoxy acoustic matching layer still does not sufficiently match the acoustic impedance with the gas, so that it exhibits bipolar characteristics and has a wide frequency band. Therefore, the response to the ultrasonic pulse signal is good, and the rising characteristics are good. On the other hand, in the acoustic matching layer of the dried silica gel shown in FIG. 9B, since the acoustic impedance matches with the gas, it has a monopolar characteristic and the band is narrowed. Therefore, although the sensitivity is high, it is difficult to respond to a change faster than the resonance frequency, so that the rising characteristic with respect to the pulse signal is deteriorated.

When the single layer acoustic matching layer is configured as the acoustic matching layer of the present invention shown in FIG. 9C, it has a two-layer structure of the first acoustic matching layer and the second acoustic matching layer. In addition, the frequency characteristics of the vibration displacement show tripolar characteristics, and the band is widened. For this reason, the rising response is fast, and the acoustic impedance of the gas as the radiation medium is matched by the first acoustic matching layer facing the gas, so that the attenuation is small and the sensitivity is high.

By using the acoustic matching layer having a two-layer structure of the embodiment according to the present invention, in the ultrasonic transducer used for performing measurement and the like by radiating ultrasonic waves to a gas, in the conventional single-layer acoustic matching layer It is possible to transmit and receive highly sensitive and responsive ultrasonic waves that could not be achieved. Further, by using such an ultrasonic transducer, it is possible to obtain an ultrasonic flowmeter which is highly sensitive and has little variation in characteristics, thereby improving the stability of flow measurement. Although the acoustic matching layer of the embodiment according to the present invention typically has a two-layer structure, the density of the acoustic matching layer increases as the position is closer to the piezoelectric layer, and the acoustic matching layer increases as the position is closer to the surface on the radiation medium side. Three or more acoustic matching layers may be arranged so that the density of the acoustic matching layers becomes low.

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 schematically shows a configuration of an acoustic matching layer 1 according to an embodiment of the present invention.

The acoustic matching layer 1 has a first acoustic matching layer 2 having a density in a range of 50 kg / m ³ to 500 kg / m ³ and a second acoustic matching layer having a density in a range of 400 kg / m ³ to 1500 kg / m ^3. It has a structure in which the layer 3 is laminated. The density of the second acoustic matching layer 3 is higher than the density of the first acoustic matching layer 2.

The first acoustic matching layer 2 has a role of achieving high sensitivity by matching acoustic impedance with respect to a gas which is an ultrasonic wave radiation medium. At this time, it is preferable that the relationship between the acoustic impedance Za of the first acoustic matching layer 2 and the acoustic impedance Zb of the second acoustic matching layer 3 is Za <Zb. The value of the acoustic impedance of the first acoustic matching layer 2 is preferably, for example, about 11 × 10 ⁴ kg / s · m ² , which is a value for matching the acoustic impedance between the air and the ceramic piezoelectric material. However, taking as an example the case where the ultrasonic transducer using the acoustic matching layer of the present invention is used for measuring the flow rate of the flammable gas as another gas, the acoustic impedance of the first acoustic matching layer is, for example, 5 to hydrogen. It preferably has a value from about × 10 ⁴ kg / s · m ^{2 to} about 12 × 10 ⁴ kg / s · m ² for propane. In addition, when considering other gases and mixed gases, it is particularly preferable that it be in the range of 5 × 10 ⁴ kg / s · m ² or more and 20 × 10 ⁴ kg / s · m ² or less. In addition, even when the acoustic impedance matching with the target gas is slightly shifted, high sensitivity is obtained in the acoustic impedance region obtained by the first acoustic matching layer 2, so that 50 × 10 ⁴ kg / s · m ² or less. Preferably, it is in the range of 0.5 × 10 ⁴ kg / s · m ² or more and 50 × 10 ⁴ kg / s · m ² or less.

In order for the first acoustic matching layer 2 to obtain the above-described acoustic impedance, a material having a density in the range of 50 kg / m ³ or more and 500 kg / m ³ or less and a sound speed of less than 500 m / s is used. At this time, it is preferable that the second acoustic matching layer 3 has a density in the range of 400 kg / m ^{3 to} 1500 kg / m ³ and a sound speed of 500 m / s or more. However, it is set so that the relationship of Za <Zb is satisfied. The acoustic impedance Zb of the second acoustic matching layer 3 is preferably smaller than the acoustic impedance of the piezoelectric layer that emits ultrasonic waves.

In order to improve the sensitivity by matching the acoustic impedance, it also depends on the thickness of the acoustic matching layer. Reflectance of ultrasonic waves determined by considering the reflection coefficient at the interface between the acoustic matching layer and the radiating medium of the ultrasonic waves and the interface between the acoustic matching layer and the ultrasonic vibrator. Is minimum, that is, when the thickness of the acoustic matching layer is one quarter of the ultrasonic oscillation wavelength, the transmission intensity becomes maximum. Therefore, it is effective to increase the sensitivity if the thickness of the first acoustic matching layer 2 is approximately one-fourth of the ultrasonic oscillation wavelength passing through the acoustic matching layer. Further, it is also effective to adopt a configuration in which the thickness of the second acoustic matching layer 3 is approximately one-fourth of the ultrasonic oscillation wavelength passing through the acoustic matching layer. It is most effective to make both the two acoustic matching layers 3 have approximately a quarter wavelength. It should be noted that approximately one quarter of the oscillation wavelength of the ultrasonic wave is in the range of about one eighth to three eighths of the wavelength. That is, if it is smaller than this, it will not work as an acoustic matching layer, and if it is larger than that, it will approach the half wavelength where the reflectivity becomes maximum, and conversely the sensitivity will decrease.

As a material of the acoustic matching layer 1 of the present invention, a material having a condition that the first acoustic matching layer 2 has a density of 50 kg / m ³ or more and 500 kg / m ³ or less and a sound speed of less than 500 m / s is preferable. . Further, the second acoustic matching layer 3 is preferably made of a material having a density in a range of 400 kg / m ^{3 to} 1500 kg / m ³ and a sound speed of 500 m / s or more.

具体 Specific examples of the material of the first acoustic matching layer 2 include an organic polymer, an inorganic material such as a fibrous body, a foam body, a sintered porous body, and a dried gel, but it is particularly preferable to use a dried gel.

Here, the “dry gel” is a porous body formed by a sol-gel reaction, and a solid skeleton portion solidified by the reaction of the gel raw material liquid is dried through a wet gel including a solvent, and then dried. It is formed by removing.

In order to obtain a dried gel, the method of drying after removing the solvent from the wet gel is a drying method under special conditions such as supercritical drying or freeze drying, or a normal drying method such as heating drying, drying under reduced pressure, and drying by standing naturally. A method can be used.

Supercritical drying is a method in which the solvent is removed in a supercritical state at a temperature and pressure above its critical point, and there is no gas-liquid interface and no drying stress is applied to the solid skeleton of the gel, so it shrinks or shrinks. A very low-density dried gel can be obtained without performing. On the other hand, the dried gel obtained by supercritical drying may be affected by stress in the use environment, for example, dew condensation, heat stress, chemical stress, mechanical stress, and the like.

On the other hand, a dried gel obtained by a normal drying method has a characteristic of being highly durable even in a subsequent use environment stress because it can withstand drying stress. In order to obtain a low-density dry gel by such a normal drying method, it is necessary to make the solid skeleton part capable of withstanding stress in a wet gel stage before drying. For example, aging the solid skeleton to increase its strength, applying a temperature-dependent or polymerizable polyfunctional hydrophobizing agent to reinforce the solid skeleton when hydrophobizing, or reducing the pore size It can be realized by controlling. In particular, when measuring the flow rate of a gas, it may be used in various environments, so it is preferable to obtain the acoustic matching layer with a dried gel prepared by a normal drying method. In addition, when a normal drying method is applied, since the process is not a high-pressure process such as supercritical drying, there are advantages such as simplification of equipment and easy handling.

The dry gel obtained by the above method is a nanoporous body in which continuous pores having an average pore diameter in the range of 1 nm to 100 nm are formed by a solid skeleton having a nanometer size. Therefore, in a low-density state where the density is 500 kg / m ³ or less, preferably 400 kg / m ³ or less, the speed of sound propagating through the solid portion of the dried gel forming the unique network skeleton becomes extremely small. In addition, it has the property that the speed of sound propagating through the gas portion in the porous body due to the pores becomes extremely low. Therefore, it has a characteristic that a very low value of 500 m / s or less is exhibited as a sound velocity, and a low acoustic impedance can be obtained.

In the nanometer-sized pore portion, the pore size is less than or equal to the mean free path of the gas molecules, and the pressure loss of the gas is large. It also has the characteristic that it can be radiated by pressure.

無機 As a material of the dried gel, an inorganic material, an organic polymer material, or the like can be used. As the solid skeleton portion of the dried gel of the inorganic oxide, a general ceramic obtained by a sol-gel reaction such as silicon oxide (silica) or aluminum oxide (alumina) can be used as a component. The solid skeleton of the dried gel of the organic polymer can be made of a general thermosetting resin or thermoplastic resin. For example, polyurethane, polyurea, phenol cured resin, polyacrylamide, polymethyl methacrylate, or the like can be used.

粉末 Alternatively, a powder (powder dried gel) obtained by crushing these dried gels may be used.

Examples of the material of the second acoustic matching layer 3 include a fibrous body, a foam body, a sintered porous body, a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, a material obtained by thermally compressing a glass balloon, and the like. Can be used.

The second acoustic matching layer 3 preferably has a higher density than the first acoustic matching layer 2, has a higher sound velocity, and has a higher acoustic impedance. More specifically, a material having a density in the range of 400 kg / m ³ or more and 1500 kg / m ³ or less is used. With a density in this range, it is possible to obtain sufficient sensitivity to transmit and receive ultrasonic waves without significantly shifting the acoustic impedance matching with the gas that is the radiation medium of the ultrasonic waves, and to provide characteristics with excellent responsiveness. Obtainable. At higher densities, the acoustic impedance tends to be closer to the acoustic impedance of the piezoelectric body, and the effect of the acoustic matching layer of the present invention is reduced. It is difficult to obtain. The upper limit of the density of the second acoustic matching layer 3 may be 800 kg / m ³ .

As the second acoustic matching layer 3, for example, an acoustic matching layer formed by molding a glass balloon with a thermosetting resin, a silicon oxide porous material acoustic matching layer obtained by mixing a silicon oxide raw material and a polymer bead and firing to remove a polymer; An acoustic matching layer or the like formed by thermally bonding (thermally compressing) a glass balloon can be suitably used.

If the second acoustic matching layer 3 has a continuous pore structure, the permeation of the raw material liquid may occur particularly when the first acoustic matching layer 2 made of dried gel is formed. In this case, the first acoustic matching layer 2 can be formed with the permeation still occurring, but a structure support layer may be formed on the surface of the second acoustic matching layer 3 to avoid the permeation. However, when the first acoustic matching layer 2 partially penetrates into the second acoustic matching layer 3, there is also an effect that the adhesion between them increases. Therefore, the configuration may be determined by the combination of the first acoustic matching layer 2 and the second acoustic matching layer 3.

(4) When both the first acoustic matching layer 2 and the second acoustic matching layer 3 are made of an inorganic oxide, they have excellent moisture resistance reliability and chemical resistance, and also have excellent temperature characteristics of acoustic impedance. That is, when a dry gel of an inorganic oxide is used, the temperature change rate of the acoustic impedance within the range of 25 ° C. or more and 70 ° C. or less is −0.04% / ° C. or less (the absolute value is 0.04% / ° C. or less). Meaning) acoustic matching layer can be obtained. In contrast, when a conventional epoxy / glass balloon system or organic polymer gel is used, it is difficult to make the absolute value of the temperature change rate of the acoustic impedance 0.04% / ° C. or less.

(4) When the temperature change rate of the acoustic impedance is small, for example, when used in an ultrasonic flowmeter described later, high measurement accuracy can be obtained over a wide temperature range.

In the present invention, it is preferable that the first acoustic matching layer and the second acoustic matching layer are formed by chemical bonding. This is effective for ensuring adhesion to ultrasonic vibration, easy handling, durability against vibration during use of the ultrasonic transducer, and the like. At this time, when the inorganic oxide of the dried gel of the first acoustic matching layer is silicon oxide, it is preferable because the acoustic matching layer is easily formed by the sol-gel reaction. Furthermore, if silicon oxide is used for the second acoustic matching layer 3, it is considered that the influence on the characteristics due to the difference in the material can be reduced. In this configuration, a favorable effect is obtained because the surface hydroxyl groups of the silicon oxide of the second acoustic matching layer 3 are easily chemically bonded to the silanol groups present when the first acoustic matching layer 2 is formed by the sol-gel reaction. .

す る と Further, when the acoustic matching layer is formed using an inorganic oxide, the solid skeleton of the inorganic oxide is preferably made hydrophobic (water-repellent) because there is a concern about the problem of moisture resistance due to moisture absorption. By making the surface hydrophobic, for example, when moisture or impurities are present in the gas to be measured, the moisture or impurities can be made less susceptible to the influence of the adsorption or adhesion, so that a more reliable acoustic matching layer can be obtained.

(4) Hydrophobization of the solid skeleton of the inorganic oxide is performed using a surface treatment agent such as a silane coupling agent. Examples of surface treatment agents include halogen-based silane treatment agents such as trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, and ethyltrichlorosilane, and alkoxy-based agents such as trimethylmethoxysilane, trimethylethoxysilane, dimethyldimethoxysilane, and methyltriethoxysilane. Silane treating agents, silicone-based silane treating agents such as hexamethyldisiloxane and dimethylsiloxane oligomer, amine-based silane treating agents such as hexamethyldisilazane, alcohol-based treating agents such as propyl alcohol, butyl alcohol, hexyl alcohol, octanol, decanol Etc. can be used.

Further, by using a fluorinated treating agent in which a part or all of the hydrogen of the alkyl group of these treating agents is replaced by fluorine, in addition to hydrophobicity (water repellency), further excellent oil repellency, antifouling property, etc. The effect obtained is as follows.

(Embodiment 2)
FIG. 2 schematically shows a cross-sectional structure of a piezoelectric vibrator 8 used in the ultrasonic transducer according to the embodiment of the present invention. The piezoelectric vibrator 8 is used for an ultrasonic transducer of an ultrasonic flowmeter.

圧 電 The piezoelectric vibrator 8 that performs the electric-ultrasonic mutual conversion includes the piezoelectric layer 4 and the acoustic matching layer 1 described in the first embodiment. The piezoelectric layer 4 generates ultrasonic vibration, is made of a piezoelectric ceramic, a piezoelectric single crystal, or the like, is polarized in the thickness direction, and has electrodes (not shown) on upper and lower surfaces. The acoustic matching layer 1 is for transmitting an ultrasonic wave to a gas or receiving an ultrasonic wave transmitted through a gas as described above, and is a mechanical device of the piezoelectric layer 4 which is excited by a driving AC voltage. The mechanical vibration is efficiently radiated as ultrasonic waves to an external medium, and has a role of efficiently converting the arriving ultrasonic waves into a voltage. It is formed on one side of layer 4.

A structural support layer may be provided between the first acoustic matching layer 2 and the second acoustic matching layer 3 of the acoustic matching layer 1 to improve the mechanical strength of the acoustic matching layer and facilitate handling. . The structural support layer has a density of 800 kg / m ³ or more, preferably 1000 kg / m ³ or more, and the thickness of the structural support layer is less than one-eighth of the wavelength λ of the sound wave propagating in the structural support layer. Preferably, there is. That is, since the structural support material layer has a high density and a high sound velocity, when the thickness is sufficiently smaller than the ultrasonic oscillation wavelength, the influence on the transmission and reception of the ultrasonic waves is extremely small. As a material for forming the structural support layer, a metal material, or an inorganic sheet such as ceramic or glass, or a protective coat such as a plastic sheet can be used. When the first acoustic matching layer 2 and the second acoustic matching layer 3 are joined via an adhesive layer (adhesive or adhesive sheet), the adhesive layer functions as a structural support layer.

When employing a configuration in which the piezoelectric layer 4 is adhered to the inner surface of the case and the acoustic matching layer 1 is adhered to the outer surface of the case, an upper plate constituting the case existing between the piezoelectric layer 4 and the acoustic matching layer 1 Functions as a structural support layer.

Furthermore, a structural support layer may be formed on the surface (gas side) of the first acoustic matching layer 2. Since the acoustic matching layer 1 is supported by the high-density material, the favorable effect that the handleability of the acoustic matching layer 1 is improved and the durability is improved by improving the adhesion is obtained.

(Embodiment 3)
FIG. 3 is a schematic sectional view of an ultrasonic transducer according to an embodiment of the present invention.

超 The ultrasonic transducer 10A shown in FIG. 3 is an ultrasonic transducer using the acoustic matching layer 1 and the piezoelectric layer 4 of the first embodiment to constitute a piezoelectric vibrator.

The ultrasonic transducer 10A further includes a case (sealed container) 5 having an upper plate 5a forming a recess enclosing the piezoelectric layer 4 and a bottom plate 5b disposed so as to seal the space in the recess. are doing. The piezoelectric layer 4 is bonded (adhered) to the inner surface of the upper plate 5a of the case 5, and the acoustic matching layer 1 is bonded to the upper surface of the upper plate 5a at a position facing the piezoelectric layer 4 via the upper plate 5a. (Adhesive).

The upper plate 5a existing between the piezoelectric layer 4 and the acoustic matching layer 1 also functions as a structural support layer. The thickness of the upper plate 5a is preferably sufficiently smaller than the ultrasonic oscillation wavelength, and is preferably less than one-eighth of the wavelength λ of the sound wave propagating in the upper plate 5a. Further, the density of the upper plate 5a is preferably 800 kg / m ³ or more, more preferably 1000 kg / m ³ or more.

By forming the case 5 from a conductive material (for example, a metal material), the case 5 functions as a structural support member and oscillates the piezoelectric layer 4 or forms an electrode (wiring) for detecting received ultrasonic waves. Also works. Electrodes (not shown) formed on a pair of main surfaces of the piezoelectric layer 4 are connected to one terminal 7 via a case 5, and the other is connected to the other terminal 7 by a wire or the like. Therefore, the case 5 is generally formed from a conductive metal. Note that the other terminal 7 is insulated from the case 5 by the insulator 6.

The acoustic matching layer 1 disposed so as to face the piezoelectric layer 4 with the upper plate 5a of the case 5 interposed therebetween, the second acoustic matching layer 3 and the second acoustic matching layer 3 facing the medium that emits ultrasonic waves from the piezoelectric layer 4 side. One acoustic matching layer 2 is laminated in this order. By arranging the acoustic matching layer 1 in this way, as described above with reference to FIGS. 8C and 9C, the ultrasonic transducer 10 having high sensitivity and good responsiveness can be obtained. .

(4) When a combustible gas is to be detected, the piezoelectric layer 4 can be isolated from the gas by housing the piezoelectric layer 4 in the case 5. It is preferable to purge the inside (recess) of the case 5 with an inert gas such as nitrogen. In this way, when used in an ultrasonic flowmeter for combustible gas, an advantage of high safety can be obtained. Also, the constituent material of the acoustic matching layer in contact with the combustible gas is preferably one that does not react with the gas or burn. From this viewpoint, it is preferable that the acoustic matching layer is formed of an inorganic oxide.

In the ultrasonic transducer 10A configured as described above, when a burst signal voltage having an AC signal component having a frequency near the resonance frequency of the ultrasonic transducer is applied to the drive terminal 7, the piezoelectric layer 4 becomes thicker. It vibrates in a vibration mode, and emits a burst of ultrasonic waves into a fluid such as a gas or a liquid.

(Embodiment 4)
FIG. 4 is a sectional view of an ultrasonic transducer according to an embodiment of the present invention.

In the ultrasonic transducer 10B shown in FIG. 4, a part of the case 15 is formed of the second acoustic matching layer 13, and the piezoelectric layer 4 is disposed on the inner surface of the second acoustic matching layer 13 of the case 15. The first acoustic matching layer 12 is disposed on the outer surface of the second acoustic matching layer 13 facing the position where the piezoelectric layer 4 is disposed. The second acoustic matching layer 13 also serves as a structural support layer. Therefore, it is preferable that the second acoustic matching layer 13 is made of a material having a relatively high density, and it is difficult to match acoustic impedance with a gas that is a radiation medium of ultrasonic waves by using the second acoustic matching layer 13 alone. However, as shown in FIG. 4, by adopting a configuration in which the first acoustic matching layer 12 is further laminated on the second acoustic matching layer 13, acoustic impedance matching with gas can be achieved and high sensitivity can be achieved.

(Embodiment 5)
FIG. 5 is an explanatory diagram of a method for manufacturing an ultrasonic transducer according to an embodiment of the present invention.

In the method for manufacturing an ultrasonic transducer according to the present embodiment, the first acoustic matching layer made of dry gel is laminated after forming the second acoustic matching layer on the piezoelectric layer or the case where the piezoelectric layer is disposed on the inner surface. It is a method that includes a step.

Step (a): The second acoustic matching layer 3 is prepared.

Step (b): The piezoelectric layer 4 and the case 5 are prepared.

Step (c): The piezoelectric layer 4 and the second acoustic matching layer 3 are joined to the case 5 using an adhesive or the like.

Step (d): forming a first acoustic matching layer made of a dried gel on the second acoustic matching layer 3.

Step (e): Attach electrodes and terminal plates (bottom plate of case 5) 5b to obtain an ultrasonic transducer.

The step (d) of forming the first acoustic matching layer 2 includes a film forming step of applying a gel raw material liquid on the second acoustic matching layer 3 and a solidifying step of obtaining a wet gel from the gel raw material liquid. And removing the solvent in the wet gel layer to obtain a dry gel layer. Further, the first acoustic matching layer 2 made of a dried gel may be formed in advance, and the first acoustic matching layer 2 may be bonded to the second acoustic matching layer 3 using an adhesive or the like. It is preferable because the layer 2 and the second acoustic matching layer 3 can be directly bonded (bonded without using an adhesive layer).

第 In order to improve the durability of the laminated structure of the first acoustic matching layer 2 and the second acoustic matching layer 3, the first acoustic matching layer 2 and the second acoustic matching layer 3 may be chemically bonded. For example, if the first acoustic matching layer 2 is made of an inorganic oxide and the second acoustic matching layer 3 is treated by washing or the like so that hydroxyl groups are present on the surface, the first acoustic matching layer 2 is made of an inorganic oxide. When forming a dry gel, a chemical bond can be formed. As a treatment method for generating a hydroxyl group on the surface, washing with an acid or alkali, washing with water, ultraviolet irradiation, ozone treatment, oxygen plasma treatment, or the like can be used.

(4) When the second acoustic matching layer 3 is a continuous porous body, a gel stock solution for forming the first acoustic matching layer 2 penetrates, and a stronger chemical bond may be formed. At this time, it is preferable that the first acoustic matching layer 2 and the second acoustic matching layer 3 are formed of the same inorganic oxide. The chemical coupling between the first acoustic matching layer 2 and the second acoustic matching layer 3 is preferable because the acoustic coupling is strengthened and the sensitivity is improved, and the stability and reliability of the characteristics are improved.

(Embodiment 6)
FIG. 6 is an explanatory view of a method for manufacturing an ultrasonic transducer according to another embodiment of the present invention.

The method of manufacturing the ultrasonic transducer according to the present embodiment is configured such that, after forming the acoustic matching layer 1 in which the first acoustic matching layer 2 made of dry gel is laminated on one surface of the second acoustic matching layer 3, This is a manufacturing method including a step of attaching the acoustic matching layer 1 to the case 5 in which the layer 4 or the piezoelectric layer 4 is disposed on the inner surface.

Step (a): The second acoustic matching layer 3 is prepared.

Step (b): The first acoustic matching layer 2 is laminated on one side of the second acoustic matching layer 3. This lamination method includes a film forming step of applying a gel raw material liquid on the second acoustic matching layer 3, a solidifying step of obtaining a wet gel from the gel raw material liquid, and removing a solvent in the obtained wet gel layer. Drying to obtain a layer of dried gel. Further, the first acoustic matching layer 2 made of a dried gel may be formed in advance, and the first acoustic matching layer 2 may be bonded to the second acoustic matching layer 3 using an adhesive or the like. It is preferable because the layer 2 and the second acoustic matching layer 3 can be directly bonded (bonded without using an adhesive layer). Moreover, in order to improve the durability of the laminated structure of the first acoustic matching layer and the second acoustic matching layer, a method similar to that of the sixth embodiment can be used.

Step (c): The piezoelectric layer 4 and the case 5 are prepared.

Step (d): The acoustic matching layer 1 in which the first acoustic matching layer 2 and the second acoustic matching layer 3 are laminated, the piezoelectric layer 4 and the case 5 (c) are joined with an adhesive or the like.

Step (e): Attach electrodes and terminal plates (bottom plate of case 5) 5b to obtain an ultrasonic transducer.

(Embodiment 7)
The first acoustic matching layer 2 can also be formed using dry gel powder. The first acoustic matching layer 2A shown in FIG. 7A includes a powder of a dry gel (hereinafter, also referred to as “powder dry gel”) 2a and an additive 2b. By forming the first acoustic matching layer 2A using the powder of the dry gel, variation in characteristics due to unevenness in the drying process of the wet gel is suppressed. In addition, when the powder dry gel 2a is used, the powder dry gel 2a can be prepared in advance, so that there is an advantage that the productivity of the ultrasonic transducer can be improved. That is, in the above-described ultrasonic transducer manufacturing process, the step of solidifying the gel raw material liquid to obtain a wet gel and the step of drying the wet gel can be performed in advance, so that the manufacturing of the ultrasonic transducer is performed. Throughput can be improved.

(4) The average particle size of the dry powder gel 2a is preferably 1 μm or more and 100 μm or less. If it is smaller than this lower limit, the number of pores in the powder will decrease and the characteristic effect of the dried gel will be reduced, and the required amount of additives during molding will increase, so that a low-density acoustic matching layer is required. It can be difficult to obtain. When the average particle diameter of the powdered dry gel 2a is larger than the upper limit, it is difficult to control the thickness of the acoustic matching layer, and it is difficult to form an acoustic matching layer having sufficient thickness uniformity and sufficient surface flatness. Sometimes.

(4) As the additive (binder) 2b for bonding the powder dry gels 2a to each other and improving the mechanical strength of the acoustic matching layer 2A, a polymer powder having thermal binding properties can be suitably used. If a liquid material is used, it penetrates into the inside of the pores of the dried gel, which may change the acoustic characteristics or lower the moldability. Therefore, it is preferable to use a solid material, particularly a powder.

Here, the term “thermally binding polymer” refers to a polymer that is solid at room temperature, melts or softens by heating, and then solidifies. Thermo-binding polymers are not only general thermoplastic resins (for example, engineering plastics such as polyethylene and polypropylene), but also, for example, thermosetting resins that are solid at room temperature, soften once by heating, and then cross-link and harden. (For example, phenol resin, epoxy resin, urethane resin) can be used. When the thermosetting resin contains the main agent and the curing agent, each may be added as separate powders. Of course, a mixture of a thermoplastic resin and a thermosetting resin may be used. The melting (softening) temperature of the heat-binding polymer powder is preferably in the range of 80 ° C to 250 ° C.

When a heat-binding polymer is used as an additive, it typically melts (softens) when a mixed powder of the powdered dry gel 2a and the additive is pressed and molded while being heated, as described later. The additive plays a role of bonding the powder dry gels 2a by solidifying with cooling and / or by crosslinking and hardening.

平均 The average particle size of the heat-binding polymer powder is preferably 0.1 μm or more and 50 μm or less. If it is smaller than the lower limit, the particle diameter becomes close to the pore diameter of the powdery dried gel, so that the binding property may be reduced or the moldability may be reduced. On the other hand, if it is larger than the upper limit, the addition amount required for molding increases, so that it may be difficult to obtain a low-density acoustic matching layer.

(4) The amount of the heat-binding polymer powder added is preferably 40% by mass or less of the whole. If the total amount exceeds 40% by mass, the density at the time of molding may increase. In order to obtain sufficient mechanical strength, for example, it is preferable to add 5% by mass or more of the whole.

In order to strengthen the bonding between the above-mentioned additive (sometimes referred to as “additive A”) and the powdered dry gel, as shown in the acoustic matching layer 2B schematically shown in FIG. Inorganic fibers (eg, glass wool or organic fibers) and whiskers may be further added (sometimes referred to as “additive B”). In the acoustic matching layer 2B of FIG. 7B, the additive 2b is the same heat-binding polymer powder as described above, and the additive 2c is a short fiber. The preferred diameter range of the short fibers is about the same as the average particle diameter of the above-mentioned heat-binding polymer powder, and the length of the fibers is preferably about several μm to several mm.

添加 The addition amount of the two types of additives is preferably 40% by mass or less based on the whole, and the mixing ratio is appropriately set as necessary.

音響 The acoustic matching layer using the dry powder gel has an additional advantage that the acoustic impedance can be easily adjusted. For example, by mixing a plurality of types of powdered dry gels having different densities from each other, the acoustic impedance can be adjusted. Further, the acoustic impedance can be adjusted by adjusting the amount of the above-mentioned additive A (additive B if necessary). Of course, the amount of the additives A and B is preferably within the above range in consideration of moldability and the like.

第 The first acoustic matching layer 2B including the dry powder gel can be formed, for example, by the following method.

Step (a): providing a with Additive A composed of a porous low-density powder dry gel (density about ^{200kg / m 3 ~400kg / m 3} ) to about 10% by weight (based on total) and additive B I do. The dried gel prepared here does not necessarily need to be a powder. It may be in a block shape. The dry gel is, for example, a silica dry gel having an average pore diameter of 20 nm, the additive A is a polypropylene powder, and the additive B is a glass wool having a fiber diameter of about 10 μm.

Step (b): These are put in the same container, mixed and pulverized to produce a fine powder. It is typically performed using a mill. Here, the pulverization conditions are adjusted so that a powdery dry gel having the desired average particle size described above is obtained. Moreover, you may classify as needed. Of course, the step of pulverizing the dried gel and the step of mixing may be performed separately.

Step (c): A mixed powder composed of a low-density dry powder gel, additive A and additive B is weighed in a desired amount, and is molded under pressure while heating. At this time, the first acoustic matching layer 2 can be directly coupled to the second acoustic matching layer 3 by directly performing pressure molding on the surface of the second acoustic matching layer 3.

It is preferable that the upper surface of the layer of the mixed powder be flattened by, for example, applying vibration to the layer of the mixed powder before the powder dried gel and the mixed powder of the additives A and B are pressed. By doing so, the characteristics of the obtained first acoustic matching layer 2A can be made more uniform.

(Embodiment 8)
FIG. 11 is a block diagram of an ultrasonic flowmeter using the ultrasonic transducer according to the embodiment of the present invention.

超 The ultrasonic flow meter in FIG. 11 is installed so that the fluid to be measured flows in the pipe, which is the flow rate measuring section 51, at the speed V in the direction shown in the figure. Piezoelectric vibrators 101 and 102 as ultrasonic transducers of the present invention are disposed facing each other on a tube wall 52 of the flow rate measuring unit 51. Here, the piezoelectric vibrator 101 is used as an ultrasonic wave transmitter, and the piezoelectric vibrator 102 is used as an ultrasonic wave receiver. Further, a driving circuit 54 for driving the ultrasonic transmitters / receivers 101 and 102 is connected to the ultrasonic transmitter 101 and the ultrasonic receiver 102 via a switching circuit 55 for switching between transmission and reception. A reception detection circuit 56 for detecting, a timer 57 for measuring the propagation time of the ultrasonic pulse, an arithmetic circuit 58 for calculating the flow rate from the output of the timer 57, a drive circuit 54, and a control circuit 59 for outputting a control signal to the timer 57 are connected. ing.

The operation of the ultrasonic flowmeter thus configured will now be described. The fluid to be measured is, for example, LP gas, and the driving frequency of the ultrasonic transducers 101 and 102 is about 500 kHz. The control circuit 59 outputs a transmission start signal to the drive circuit 54 and, at the same time, causes the timer 57 to start measuring time. Upon receiving the transmission start signal, the drive circuit 54 drives the ultrasonic transducer 101 to transmit an ultrasonic pulse. The transmitted ultrasonic pulse propagates in the flow measurement unit and is received by the ultrasonic transducer 102. The received ultrasonic pulse is converted into an electric signal by the ultrasonic transducer 102 and output to the reception detection circuit 56. The reception detection circuit 56 determines the reception timing of the reception signal, stops the timer 57, and calculates the propagation time t1 by the calculation circuit 58.

Subsequently, the switching circuit 55 switches the ultrasonic transducers 101 and 102 connected to the drive unit 54 and the reception detection circuit 56, and the control circuit 59 outputs a transmission start signal to the drive circuit 54 and simultaneously measures the time of the timer 57. To start. Contrary to the measurement of the propagation time t1, an ultrasonic pulse is transmitted by the ultrasonic transducer 102, received by the ultrasonic transducer 101, and the arithmetic circuit 58 calculates the propagation time t2.

Here, the distance connecting the centers of the ultrasonic transducer 101 and the ultrasonic transducer 102 is L, the sound velocity of the LP gas in a windless state is C, the flow velocity in the flow measuring unit 51 is V, and the fluid to be measured is Assuming that the angle between the direction of the flow and the line connecting the centers of the ultrasonic transducers 101 and 102 is θ, the propagation times t1 and t2 are obtained. Further, since the distance L is known, the flow velocity V can be obtained by measuring the times t1 and t2, and the flow velocity can be checked from the flow velocity V.

[Example]
Hereinafter, specific examples of the present invention will be described.

(Example 1)
The ultrasonic transducer of the present invention was manufactured as follows.

(A) Production of Second Acoustic Matching Layer (Glass Epoxy) A jig was filled with a glass balloon, impregnated with an epoxy solution, and thermally cured at 120 ° C. This cured molded body was cut so as to have a thickness of one quarter of the ultrasonic oscillation wavelength.

The ultrasonic velocity was about 2500 m / s, the density was 500 kg / m ³ , and the thickness was 1.25 mm with respect to about 500 kHz.

(B) Joining of the second acoustic matching layer to the piezoelectric body and the case An adhesive was printed on both sides of the top surface of the case, and an adhesive was printed on one side of the piezoelectric layer and one side of the second acoustic matching layer. In this state, the piezoelectric body, the second acoustic matching layer, and the case were joined together by being heated while being pressed and cured.

(C) Lamination of the first acoustic matching layer First, sodium silicate is subjected to electrodialysis to prepare an aqueous solution of silicic acid having a pH of 9 to 10 (silica component concentration in the aqueous solution is 14% by weight). After the pH of the aqueous solution of silicic acid was adjusted to 5.5, it was applied to a thickness of 90 μm on the second acoustic matching layer which had been washed in advance so as to form hydroxyl groups on the surface by ultraviolet irradiation. Thereafter, the coating film gelled to obtain a solidified silica wet gel layer. This container is subjected to supercritical drying at 12 MPa and 50 ° C. using carbon dioxide, whereby a piezoelectric vibrator having an acoustic matching layer formed by laminating a silica dry gel first acoustic matching layer and a glass epoxy second acoustic matching layer is formed. Got the case.

The first acoustic matching layer made of dried silica gel had a sound velocity of 180 m / s and a density of 200 kg / m ³ with respect to about 500 kHz of ultrasonic waves.

(D) Formation of Ultrasonic Transceiver An ultrasonic transducer was obtained by assembling a cover plate, a drive terminal and the like into a case in which an acoustic matching layer was formed.

(Example 2)
The ultrasonic transducer of the present invention was manufactured as follows.

(A) Production of Second Acoustic Matching Layer (Porous Silica) Pressure molding was performed after mixing spherical acrylic resin having a diameter of several tens of μm and sintered silica powder having a diameter of 1 μm or less. After drying this molded body, it was fired at 900 ° C. to form a porous silica body. Thereafter, the thickness was adjusted to be one quarter of the ultrasonic oscillation wavelength.

The ultrasonic velocity was 1500 m / s, the density was 570 kg / m ³ , and the thickness was 750 μm with respect to the ultrasonic wave of about 500 kHz.

(B) Lamination of the second acoustic matching layer and the first acoustic matching layer A gel raw material liquid prepared by mixing tetramethoxysilane, ethanol, and an aqueous ammonia solution (0.1N) at a molar ratio of 1: 3: 4, On the second acoustic matching layer, which had been washed so as to form hydroxyl groups on the surface by plasma cleaning, coating was performed to a thickness of 90 μm. Thereafter, the coating film gelled to obtain a solidified silica wet gel layer.

The second acoustic matching layer having the silica wet gel layer formed thereon is subjected to a hydrophobizing treatment in a 5% by weight hexane solution of trimethylethoxysilane, followed by supercritical drying (12 MPa, 50 ° C.) with carbon dioxide to obtain silica. An acoustic matching layer in which the dried gel and the second acoustic matching layer were laminated was obtained.

In addition, since the hydroxyl group on the second acoustic matching layer reacts with the alkoxy group of tetramethoxysilane to form a chemical bond, an acoustic matching layer having good adhesion can be provided.

The first acoustic matching layer made of dried silica gel had a sound velocity of 180 m / s and a density of 200 kg / m ³ with respect to about 500 kHz of ultrasonic waves.

(C) Joining the acoustic matching layer to the case and the piezoelectric layer After the epoxy adhesive sheets are temporarily bonded to both sides of the top surface of the case, one side of the piezoelectric body and the surface of the second acoustic matching layer and the case are joined together. The coating was cured by heating while pressing.

(D) Formation of Ultrasonic Transducer An ultrasonic transducer was obtained by assembling a cover plate, a drive terminal and the like into a case.

(Example 3)
The ultrasonic transducer of the present invention was manufactured as follows.

(A) Production of second acoustic matching layer (porous silica) A sintered silica powder having a particle size of several μm to several tens μm is molded, and the obtained molded body is fired at 900 ° C. A silica porous material having about a quarter of the sound wave oscillation wavelength was formed. The second acoustic matching layer made of the porous silica had a sound speed of about 4000 m / s, a density of about 1200 kg / m ³ , and a thickness of about 2 mm with respect to ultrasonic waves (about 500 kHz).

A glass layer (thickness: about 3000 kg / m ³ ) having a thickness of 3 μm was formed as a structural support layer on one side of the second acoustic matching layer (porous silica). Since the sound speed of this glass layer is about 5000 m / s or more, the wavelength of the sound wave propagating with respect to the ultrasonic wave of about 500 kHz becomes larger than 1 cm. The thickness of the formed glass layer is well below one-eighth of the wavelength, so that there is no effect on acoustic matching.

(B) Lamination of the second acoustic matching layer and the first acoustic matching layer On the glass layer surface of the porous silica formed in (a), the silica wet gel formed in the same manner as in Example 2 was heated from 40 ° C to 70 ° C. After hydrophobizing while heating, the mixture was heated and dried at 80 ° C. under a nitrogen stream to obtain an acoustic matching layer in which a first acoustic matching layer made of dried silica gel was laminated on a second acoustic matching layer.

The first acoustic matching layer made of dried silica gel had a sound velocity of 180 m / s and a density of 200 kg / m ³ with respect to about 500 kHz of ultrasonic waves.

(C) Joining the acoustic matching layer to the case and the piezoelectric body After temporarily bonding an epoxy-based adhesive sheet to both sides of the top surface of the case, one side of the piezoelectric body and the surface of the second acoustic matching layer and the case are pressed together. While heating, it was cured and joined.

(D) Formation of Ultrasonic Transducer An ultrasonic transducer was obtained by assembling a cover plate, a drive terminal and the like into a case.

(Comparative Example 1)
An ultrasonic transducer including only the second acoustic matching layer (glass epoxy) manufactured in Example 1 was formed.

(Comparative Example 2)
According to the method described in Example 1, an ultrasonic transducer was produced on a piezoelectric vibrator case using only silica dry gel as an acoustic matching layer.

送 Hereinafter, the transmission and reception characteristics of the above Examples 1 to 3 and Comparative Examples 1 and 2 at an ultrasonic wave frequency of 500 kHz were compared. An ultrasonic flowmeter was formed by opposing a pair of ultrasonic transducers produced in each of Examples and Comparative Examples. At that time, an evaluation was performed using an output waveform when a sound wave transmitted from one ultrasonic transducer was received by another ultrasonic transducer.

8 (a) to 8 (c) show one example (Comparative Example 1, Comparative Example 2, and Example 2).

Sensitivity: (Example 2) ≒ (Example 3)> (Example 1)> (Comparative Example 2) ≫ (Comparative Example 1)
Rise response: (Example 1) １ (Example 2) ≒ (Example 3) ≧ (Comparative example 1) ≫ (Comparative example 2)

As described above, the sensitivity is about 10 times better in the first embodiment and about 20 times better in the examples 2 and 3 compared to the acoustic matching layer of the comparative example 1 which has been generally used. showed that. Further, with respect to the rising response, the acoustic matching layers of Comparative Example 1 which were conventionally generally used were equal to or slightly better in Examples 1, 2 and 3. That is, in Comparative Example 1 in FIG. 8A, the wave front of the ultrasonic wave peaks at the fifth wave, whereas in Example 2 in FIG. 8C, the wave front of the ultrasonic wave peaks at the fourth wave. Had become. Therefore, it was found that in the ultrasonic transducer according to the present invention manufactured in the examples, both the sensitivity and the rising response were superior to those of the related art.

(Example 4)
The ultrasonic transducer of the present invention was manufactured as follows.

(A) Production of Second Acoustic Matching Layer (Silica Porous Material) Sintered silica powder having a diameter of several μm to several tens μm was molded in the same manner as in Example 3, and the obtained molded body was fired at 900 ° C. A porous silica having a thickness of about one quarter of the ultrasonic oscillation wavelength was formed. The second acoustic matching layer made of the porous silica had a sound velocity of about 4000 m / s, a density of about 1200 kg / m ³ , and a thickness of about 2 mm for an ultrasonic wave of about 500 kHz.

(B) Lamination of the second acoustic matching layer and the first acoustic matching layer A silica wet gel is formed on the porous silica formed in (a) by using a silicone oligomer of tetraethoxysilane as a raw material in an isopropyl alcohol solvent with an ammonia catalyst. did. After aging the wet gel at 70 ° C., a hydrophobic treatment with dimethyldimethoxysilane was performed. After that, the solvent was dried and removed by standing naturally to obtain an acoustic matching layer in which the first acoustic matching layer made of the dried silica gel was laminated on the second acoustic matching layer.

The first acoustic matching layer made of the dried silica gel had a sound speed of about 300 m / s and a density of about 420 kg / m ³ with respect to ultrasonic waves (about 500 kHz). This portion of the first acoustic matching layer was processed to a thickness of 150 μm to obtain a final acoustic matching layer.

(C) Joining the acoustic matching layer to the case and the piezoelectric layer After applying an epoxy adhesive to both sides of the upper plate of the concave case, one side of the piezoelectric layer and the second acoustic matching layer side of the acoustic matching layer are applied. Was bonded to the upper plate of the case via an adhesive layer, and the adhesive was heated and cured while applying pressure, whereby the acoustic matching layer and the piezoelectric layer were joined to the upper plate of the case.

(D) Formation of Ultrasonic Transceiver An ultrasonic transducer was obtained by assembling the cover plate (bottom plate) of the case, the drive terminals and the like.

も The transmission / reception characteristics of the ultrasonic transducer thus manufactured were evaluated as a pair. As a result, characteristics superior to those of Comparative Examples 1 and 2 were obtained in terms of sensitivity and rising response.

It is a typical sectional view of an acoustic matching layer of an embodiment by the present invention. It is a typical sectional view of a piezoelectric vibrator of an embodiment by the present invention. It is sectional drawing of the ultrasonic transducer shown as a 3rd form of this invention. It is a sectional view of an ultrasonic wave transducer shown as a 4th form of the present invention. It is a figure which shows typically the process (a) to (e) in the manufacturing method of the ultrasonic transducer of embodiment of this invention. It is a figure which shows typically the process (a) to (e) in the manufacturing method of the ultrasonic transducer of other embodiment by this invention. (A) And (b) is a figure which shows typically the structure of the 1st acoustic matching layer containing the powder dry gel used suitably for the acoustic matching layer of embodiment of this invention. (A) to (c) are diagrams showing reception output characteristics of the ultrasonic transducer used in the present invention, (a) is a characteristic diagram when a single acoustic matching layer (glass epoxy) is used, (B) is a characteristic diagram when a single acoustic matching layer (dry silica gel) is used, and (c) is a characteristic diagram when a two-layer acoustic matching layer (dry silica gel and porous silica) is used. (A) to (c) are diagrams showing vibration displacement frequency characteristics of the ultrasonic transducer used in the present invention, where (a) is a case where a single acoustic matching layer (glass balloon / epoxy system) is used. (B) is a characteristic diagram when a single acoustic matching layer (silica dry gel) is used, and (c) is a characteristic diagram when a two-layer acoustic matching layer (silica dry gel, porous silica) is used. FIG. It is sectional drawing which shows the structure of the conventional ultrasonic transducer. It is a block diagram showing an ultrasonic flowmeter using the ultrasonic transducer of the present invention. FIG. 4 is a cross-sectional view for explaining a measurement principle of a general ultrasonic flowmeter.

Claims (18)

A flow measurement unit through which the fluid to be measured flows, a pair of ultrasonic transducers provided in the flow measurement unit for transmitting and receiving ultrasonic signals, and a measurement circuit for measuring an ultrasonic propagation time between the ultrasonic transducers An ultrasonic flowmeter comprising a flow rate calculation circuit that calculates a flow rate based on a signal from the measurement circuit,
Each of the pair of ultrasonic transducers includes a piezoelectric layer and an acoustic matching layer provided on the piezoelectric layer,
The acoustic matching layer has a first acoustic matching layer having a density of 50 kg / m ³ or more and 500 kg / m ³ or less, and a second acoustic matching layer having a density of 400 kg / m ³ or more and 1500 kg / m ³ or less. An ultrasonic wave having a matching layer, wherein a density of the first acoustic matching layer is smaller than a density of the second acoustic matching layer, and wherein the second acoustic matching layer is disposed on the piezoelectric layer side. Flowmeter. The density of the first acoustic matching layer is in a range from 50 kg / m ^{3 to} 400 kg / m ³ , and the density of the second acoustic matching layer is in a range from more than 400 kg / m ^{3 to} 800 kg / m ³ , The ultrasonic flowmeter according to claim 1. The ultrasonic flowmeter according to claim 1, wherein the relationship between the acoustic impedance Za of the first acoustic matching layer and the acoustic impedance Zb of the second acoustic matching layer is Za <Zb. 4. The ultrasonic flowmeter according to claim 1, wherein the thickness of the first acoustic matching layer is approximately one-fourth of a wavelength λ of a sound wave propagating in the first acoustic matching layer. 5. Ultrasonic according to any one of 4 claims 1 to be within the scope acoustic impedance of ^{5 × 10 4 kg / s ·} m 2 or more ^{20 × 10 4 kg / s ·} m 2 or less of the first acoustic matching layer Flowmeter. The ultrasonic flowmeter according to any one of claims 1 to 5, wherein the thickness of the second acoustic matching layer is approximately one-fourth of a wavelength λ of a sound wave propagating in the second acoustic matching layer. The ultrasonic flowmeter according to any one of claims 1 to 6, wherein both the first acoustic matching layer and the second acoustic matching layer include an inorganic oxide. The ultrasonic flowmeter according to any one of claims 1 to 7, wherein the first acoustic matching layer includes a dry gel. The ultrasonic flowmeter according to claim 8, wherein the first acoustic matching layer includes a dry gel powder. The ultrasonic flowmeter according to claim 8, wherein the solid skeleton of the dried gel contains an inorganic oxide. The ultrasonic flowmeter according to claim 10, wherein the inorganic oxide is silicon oxide. The ultrasonic flowmeter according to claim 10 or 11, wherein the solid skeleton of the inorganic oxide is hydrophobized. The ultrasonic flowmeter according to any one of claims 1 to 12, wherein the first acoustic matching layer and the second acoustic matching layer are directly coupled. The acoustic matching layer further includes a structural support layer between the first acoustic matching layer and the second acoustic matching layer,
13. The structure support layer according to claim 1, wherein the density of the structure support layer is 1000 kg / m ³ or more, and the thickness of the structure support layer is less than one-eighth of a wavelength λ of a sound wave propagating in the structure support layer. An ultrasonic flowmeter according to any of the claims. The ultrasonic flowmeter according to any one of claims 1 to 14, wherein the acoustic matching layer is directly bonded on the piezoelectric layer. The ultrasonic transducer further includes a case having an upper plate forming a recess enclosing the piezoelectric layer, and a bottom plate disposed to seal a space in the recess,
The piezoelectric layer is bonded to an inner surface of the upper plate of the case,
The ultrasonic flowmeter according to any one of claims 1 to 14, wherein the acoustic matching layer is coupled to an upper surface of the upper plate so as to face the piezoelectric layer via the upper plate. 17. The ultrasonic flowmeter according to claim 16, wherein the case is formed from a metal material. The ultrasonic flowmeter according to claim 18, wherein the upper plate of the case is formed integrally with the second acoustic matching layer.

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