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DESCRIPTION JP2004029038

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DESCRIPTION JP2004029038
PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter equipped with an acoustic
matching layer capable of transmitting and receiving ultrasonic waves with high sensitivity by
matching with a gas which is sufficiently small in acoustic impedance and an ultrasonic radiation
medium, and capable of improving the rise response of signals. provide. SOLUTION: A
measurement is performed to measure an ultrasonic wave propagation time between a flow rate
measuring unit through which a fluid to be measured flows, a pair of ultrasonic transducers
provided in the flow rate measuring unit for transmitting and receiving ultrasonic signals, and an
ultrasonic transducer. An ultrasonic flow meter comprising a circuit and a flow rate calculation
circuit that calculates a flow rate based on a signal from the measurement circuit, wherein each
of the pair of ultrasonic transducers includes a piezoelectric layer 4 and a piezoelectric layer 4
And an acoustic matching layer 1 provided thereon, the acoustic matching layer 1 having a
density of 50 kg / m to 500 kg / m, and a first acoustic matching layer 2 having a density of 400
kg / m to 1500 kg / m. and the density of the first acoustic matching layer 2 is smaller than the
density of the second acoustic matching layer 3, and the second acoustic matching layer 3 is a
piezoelectric body. It is arranged on the layer 4 side. [Selected figure] Figure 2
Ultrasonic flow meter
[0001]
The present invention relates to an acoustic matching layer used for an acoustic matching layer
of an ultrasonic sensor, an ultrasonic transducer for transmitting and receiving ultrasonic waves,
a method of manufacturing the ultrasonic transducer, and the ultrasonic transducer. The present
invention relates to an ultrasonic flowmeter.
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1
[0002]
BACKGROUND ART In recent years, an ultrasonic flowmeter that measures the time in which an
ultrasonic wave is transmitted through a propagation path and measures the moving speed of a
fluid to measure the flow rate is being used as a gas meter or the like.
[0003]
FIG. 12 shows the measurement principle of the ultrasonic flowmeter.
As shown in FIG. 12, a fluid is flowing at a velocity V in the direction shown in the figure in the
pipe.
A pair of ultrasonic transducers 101 and 102 are installed opposite to each other on the tube
wall 103. The ultrasonic transducers 101 and 102 are configured using a piezoelectric body such
as a piezoelectric ceramic as an electrical energy / mechanical energy conversion element, and
exhibit resonance characteristics as with a piezoelectric buzzer and a piezoelectric oscillator.
Here, the ultrasonic transducer 101 is used as an ultrasonic wave transmitter, and the ultrasonic
transducer 102 is used as an ultrasonic wave receiver.
[0004]
The operation is as follows: When an AC voltage of 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 wave transmitter, and the same figure as in the external fluid. The
ultrasonic wave is emitted to the propagation path indicated by L1 in the inside, and the
ultrasonic wave transmitter / receiver 102 receives the ultrasonic wave propagated and converts
it into a voltage. Subsequently, conversely, the ultrasonic transducer 102 is used as an ultrasonic
wave transmitter, and the ultrasonic transducer 101 is used as an ultrasonic wave receiver. By
applying an AC voltage of a frequency near the resonance frequency of the ultrasonic transducer
102 to the piezoelectric vibrator, the ultrasonic transducer 102 transmits an ultrasonic wave in
the external fluid to the propagation path indicated by L2 in the same figure. The ultrasonic
transducer 101 receives the propagated ultrasonic wave and converts it into a voltage. Thus,
since the ultrasonic transducers 101 and 102 serve as a receiver and a transmitter, they are
generally referred to as ultrasonic transducers.
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[0005]
In addition, in such an ultrasonic flowmeter, when an AC voltage is continuously applied,
ultrasonic waves are emitted continuously from the ultrasonic transducer, making it difficult to
measure the propagation time, so it is usually a pulse signal. Using a burst voltage signal as a
carrier voltage. Hereinafter, the measurement principle will be described in more detail.
[0006]
When a burst voltage signal for driving is applied to the ultrasonic transducer 101 and an
ultrasonic burst signal is emitted from the ultrasonic transducer 101, this ultrasonic burst signal
propagates the propagation path L1 with a distance of L to t The ultrasonic transducer 102
arrives after time. The ultrasonic transducer 102 can convert only the transmitted ultrasonic
burst signal into an electrical burst signal at a high S / N ratio. The electrical 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 propagate from the ultrasonic transducer 101 and propagate in the propagation path to
reach the ultrasonic transducer 102 is called a sing-around period, The inverse is called the singaround frequency.
[0007]
In FIG. 12, let V be the flow velocity of the fluid flowing in the tube, C be the velocity of
ultrasonic waves in the fluid, and θ be the angle between the fluid flow direction and the
ultrasonic pulse propagation direction. When the ultrasonic transducer 101 is used as an
ultrasonic wave transmitter and the ultrasonic transducer 102 is used as an ultrasonic wave
receiver, ultrasonic pulses emitted from the ultrasonic transducer 101 are ultrasonic wave
transducers 102. The following equation (1) is established, assuming that the time around the
time to arrive at is a time around t1 and the time around frequency f1.
[0008]
f1 = 1 / t1 = (C + V cos θ) / L (1)
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[0009]
Conversely, when the ultrasonic transducer 102 is used as an ultrasonic wave transmitter and
the ultrasonic wave transmitter / receiver 101 is used as an ultrasonic wave receiver, assuming
that the sing around period is t2 and the sing around frequency f2: The relationship of the
following equation (2) is established.
[0010]
f2 = 1 / t2 = (C−V cos θ) / L (2)
[0011]
Therefore, the frequency difference Δf of both sing-around frequencies is expressed by the
following equation (3), and the flow velocity V of the fluid can be obtained from the distance L of
the propagation path of the ultrasonic wave and the frequency difference Δf.
[0012]
Δ f = f 1-f 2 = 2 V cos θ / L (3)
[0013]
That is, the flow velocity V of the fluid can be determined from the distance L of the propagation
path of the ultrasonic wave and the frequency difference Δf, and the flow rate can be checked
from the flow velocity V.
[0014]
Such an ultrasonic flowmeter is required to have high accuracy, and in order to improve the
accuracy, an ultrasonic transducer for transmitting ultrasonic waves to gas or receiving
ultrasonic waves that have propagated gas is configured. The acoustic impedance of the acoustic
matching layer formed on the transmission / reception wavefront of the ultrasonic wave in the
piezoelectric vibrator that is being used is important.
[0015]
FIG. 10 is a cross-sectional view showing the structure of a conventional ultrasonic transducer 10
'.
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The ultrasonic transducer 10 'is referred to as a piezoelectric layer (vibration means) 4 and an
acoustic impedance matching layer (acoustic matching means, hereinafter referred to as
"acoustic matching layer".
1) and case 5 are included.
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 type).
The ultrasonic wave vibrated by the piezoelectric layer 4 vibrates at a specific frequency (for
example, 500 kHz), and the vibration is transmitted to the case through the adhesive layer (not
shown) and further transmitted to the acoustic matching layer 1 through the adhesive layer. .
The matched vibrations propagate as sound waves to the gas, which is the medium present in
space.
[0016]
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 velocity of sound C and the density ρ in the
substance as shown in equation (4).
[0017]
Z = ρ × C (4)
[0018]
The acoustic impedance is largely different between the piezoelectric layer 4 and the gas which is
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a radiation medium of ultrasonic waves.
For example, the acoustic impedance Z1 of a piezoelectric ceramic 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>. .
Also, the acoustic impedance Z3 of the gas which is the radiation medium, for example, air, is
approximately 400 kg / s · m <2>. On such interfaces of different acoustic impedances, the
propagation of the acoustic wave is reflected, and the intensity of the transmitted acoustic wave
is reduced. As a method of solving this, for the acoustic impedances Z1 and Z3 of the
piezoelectric body and the gas, reflection of sound is achieved by inserting a substance having an
acoustic impedance having the relationship of the equation (5) between the two. It is generally
known how to reduce and increase the transmission strength of sound waves.
[0019]
Z2=(Z1×Z3)<(1/2)>・・・(5)
[0020]
The optimum value when the acoustic impedance matching this condition is matched is
approximately 11 × 10 <4> kg / s · m <2>.
The substance satisfying the acoustic impedance is required to be a solid, low in density and low
in sound velocity as understood from the equation (4). As a material generally used, a material
obtained by solidifying a glass balloon (hollow minute glass sphere) or a plastic balloon with a
resin material is also referred to as a piezoelectric layer ("ultrasonic transducer"). It is used by
forming it on the vibration surface of). Further, a method of thermally compressing a glass
balloon or a method of foaming a molten material is also used. This is disclosed, for example, in
Japanese Patent No. 2559144.
[0021]
However, the acoustic impedance of these materials is a value larger than 50 × 10 <4> kg / s · m
<2>, and in order to obtain high sensitivity by matching with a gas, a material having a smaller
acoustic impedance. is necessary.
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[0022]
The present applicant further describes an acoustic matching layer made of a dried gel according
to Japanese Patent Application No. 2001-56501 (filing date: February 28, 2001), so that the
acoustic matching layer is made more acoustic than the conventional glass balloon-containing
epoxy resin system. It is disclosed that the impedance can be reduced and that the durability can
be improved by hydrophobizing the dried gel.
[0023]
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 ultrasonic waves, the
sensitivity as an ultrasonic transducer becomes very high.
However, when ultrasonic waves are transmitted and received using a pulse signal as a carrier
wave, as in the case of measuring the propagation time by ultrasonic waves in a flow meter, the
rise response of the signals is degraded, making it difficult to determine the arrival time.
That is, normally, the signal of the wave front which became more than a certain arrival detection
level is detected with respect to the received signal of an ultrasonic wave, and determination of
arrival is performed. 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 determined to arrive
can be recognized well, and the arrival determination can be made without error. On the other
hand, if the rise of the reception signal of the ultrasonic wave is not good, the difference of the
wave front of the reception ultrasonic wave output becomes small, it becomes difficult to identify
the wave front to be judged to arrive, and error in detection tends to occur.
[0024]
The present invention has been made in view of the above problems, and is capable of
transmitting and receiving ultrasonic waves with high sensitivity by matching with the gas which
is a sufficiently small acoustic impedance and ultrasonic radiation medium, and has good rise
response of the signal. It is a primary object to provide an acoustic matching layer for an
ultrasonic transducer that can be used. Furthermore, the ultrasonic transducer which applied it,
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and a flow meter using it are provided.
[0025]
The acoustic matching layer of the present invention is an acoustic matching layer for matching
the acoustic impedances of 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. An acoustic matching layer; and a second
acoustic matching layer having a density in the range of 400 kg / m <3> to 1500 kg / m <3>, and
the density of the first acoustic matching layer is the second Less than the density of the acoustic
matching layer.
[0026]
In one embodiment, the density of the first acoustic matching layer is in the range of 50 kg / m
<3> to 400 kg / m <3>, and the density of the second acoustic matching layer is 400 kg / m <3>
or more. It is in the range of 800 kg / m <3> or less.
[0027]
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.
[0028]
In one embodiment, a 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.
[0029]
In one embodiment, the acoustic impedance of the first acoustic matching layer is in the range of
5 × 10 <4> kg / s · m <2> to 20 × 10 <4> kg / s · m <2>. .
[0030]
In one embodiment, a 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.
[0031]
In one embodiment, the first acoustic matching layer and the second acoustic matching layer
both include an inorganic oxide.
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[0032]
In one embodiment, the first acoustic matching layer comprises a dry gel.
[0033]
In one embodiment, the first acoustic matching layer comprises a dry gel powder.
[0034]
In one embodiment, the solid skeleton of the dry gel comprises an inorganic oxide.
[0035]
In one embodiment, the inorganic oxide is silicon oxide.
[0036]
In one embodiment, the solid skeleton of the inorganic oxide is hydrophobized.
[0037]
In one embodiment, the first acoustic matching layer and the second acoustic matching layer are
directly coupled.
[0038]
In one embodiment, a structural support layer is further provided between the first acoustic
matching layer and the second acoustic matching layer, and the density of the structural support
layer is 1000 kg / m <3> or more, the structural support The thickness of the layer is less than
one-eighth of the wavelength λ of the sound wave propagating in the structural support layer.
[0039]
The ultrasonic transducer according to the present invention comprises a piezoelectric layer and
any one of the above-mentioned acoustic matching layers provided on the piezoelectric layer, and
the second acoustic matching layer is disposed on the piezoelectric layer side. It is done.
[0040]
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In one embodiment, the acoustic matching layer is directly bonded onto the piezoelectric layer.
[0041]
In one embodiment, the piezoelectric device further includes a case having an upper plate
forming a recess containing the piezoelectric layer, and a bottom plate arranged to seal a space
in the recess, the piezoelectric layer being 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.
[0042]
In one embodiment, the case is formed of a metal material.
[0043]
In one embodiment, the upper plate of the case is integrally formed with the second acoustic
matching layer.
[0044]
The method of manufacturing an ultrasonic transducer according to the present invention is any
of the methods of manufacturing an ultrasonic transducer described above, wherein the
piezoelectric layer or the upper plate having the piezoelectric layer joined to the inner surface is
provided. Forming the second acoustic matching layer; and thereafter forming the first acoustic
matching layer of dry gel on the second acoustic matching layer.
Alternatively, the first acoustic matching layer made of dry gel is formed on the second acoustic
matching layer, the step of obtaining the acoustic matching layer, and the piezoelectric layer or
the piezoelectric layer is bonded to the inner surface. Bonding the acoustic matching layer onto
the top plate.
[0045]
The ultrasonic flowmeter according to the present invention includes a flow rate measuring unit
through which a fluid to be measured flows, a pair of ultrasonic transducers provided in the flow
rate measuring unit for transmitting and receiving ultrasonic signals, and an ultrasonic
transducer between the ultrasonic transducers. An ultrasonic flow meter comprising: a
measurement circuit that measures an acoustic wave propagation time; and a flow rate
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calculation circuit that calculates a flow rate based on a signal from the measurement circuit,
wherein each of the pair of ultrasonic transducers is It is comprised by one of ultrasonic
transducer.
[0046]
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 acoustic velocity and a
second acoustic matching layer having a higher density and a high acoustic velocity.
By applying it to an ultrasonic transducer, and disposing a first acoustic matching layer on the
radiation medium side that is acoustically impedance-matched with the radiation medium of
ultrasonic waves, the acoustic impedance is sufficiently small and the gas is a radiation medium
of ultrasonic waves Thus, it is possible to obtain an excellent ultrasonic transducer having high
sensitivity of signal transmission and reception as well as highly sensitive ultrasonic wave
transmission and reception.
[0047]
Further, in the ultrasonic transducer obtained by using the manufacturing method of the present
invention, high sensitivity and stable characteristics can be achieved by the acoustic matching
layer with low acoustic impedance.
[0048]
Furthermore, 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
less characteristic variation.
Also, by making the acoustic matching layer of an inorganic oxide, it is possible to obtain
characteristics excellent in temperature characteristics of flow measurement, and by making the
acoustic matching layer hydrophobic, a highly reliable ultrasonic flowmeter excellent in moisture
resistance. Can provide
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[0049]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings.
[0050]
The acoustic matching layer 1 according to the embodiment of the present invention includes, as
shown in FIG. 1, a first acoustic matching layer 2 having a low density and a low sound velocity,
and a second acoustic matching layer 3 having a higher density and a high sound velocity.
Prepare.
The density of the first acoustic matching layer 2 is in the range of 50 kg / m <3> to 500 kg / m
<3>, and the density of the second acoustic matching layer 3 is 400 kg / m <3> to 1500 kg / m
<3. And the density of the first acoustic matching layer 2 is smaller 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 400 kg / m <3>
more than 800 kg / m <3> It is below.
[0051]
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.
Thus, by using the piezoelectric vibrator 8 provided with the acoustic matching layer 1 according
to the present invention, the sensitivity of the ultrasonic transducer can be enhanced.
[0052]
For example, the ultrasonic transducer 10A according to the embodiment of the present
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12
invention shown in FIG. 3 is replaced with the acoustic matching layer 1 ′ of the conventional
ultrasonic transducer 10 ′ shown in FIG. It has the acoustic matching layer 1 of the
embodiment of the invention.
A first acoustic matching layer 2 acoustically impedance-matched to the ultrasonic radiation
medium is disposed on the radiation medium side.
With this configuration, it is possible to transmit and receive high-sensitivity ultrasonic waves
with a sufficiently small acoustic impedance, matched to the gas that is the radiation medium of
ultrasonic waves, and to have an excellent ultrasonic transducer with excellent signal rise
response. can get.
[0053]
Hereinafter, the 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. 8 (a)
to 8 (c) and 9 (a) to 9 (c).
[0054]
FIGS. 8A to 8C show the reception output characteristics of the ultrasonic wave of the ultrasonic
transducer, and show the reception waveform in each acoustic matching layer.
[0055]
FIGS. 8A and 8B use the ultrasonic transducer 10 'of the conventional configuration shown in
FIG. 10 in which the acoustic matching layer is a single layer.
FIG. 8 (a) shows the case of using a glass balloon / epoxy acoustic matching layer (thickness 1.25
mm, speed of sound 2500 m / s, density 500 kg / m <3>), and FIG. 8 (b) It shows the case where
an acoustic matching layer of a silica dry gel (thickness 90 μm, speed of sound 180 m / s,
density 200 kg / m <3>) is used.
[0056]
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FIG. 8C is a view showing the 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 of silica dry gel (thickness 90
μm, speed of sound 180 m / s, density 200 kg / m <3>) is used, and as the second acoustic
matching layer 3, acoustic matching of porous silica The case where a layer (thickness 750 μm,
speed of sound 1500 m / s, density 570 kg / m <3>) is used is shown.
[0057]
First, from the comparison between FIGS. 8 (a) and 8 (b), by using a low density dried gel as the
acoustic matching layer, it is possible to use a glass balloon / epoxy system generally used
conventionally. It can be seen that the maximum amplitude width (peak-to-peak voltage) of the
reception output voltage is large, and the sensitivity is improved.
[0058]
However, in FIG. 8 (b), it can be seen that the rising of the reception signal is duller than in FIG. 8
(a).
Furthermore, since the difference between the output values of each wave front of the 500 kHz
ultrasonic signal at the rise and the wave fronts before and after that is small, the allowable
range of propagation time detection by the arrival detection level is small and error detection is
likely to occur. It is getting harder.
From this, in the ultrasonic transducer using silica dry gel as the acoustic matching layer,
although the sensitivity is high, it is necessary to improve the rising characteristics.
[0059]
As shown in FIG. 8C, the peak-to-peak voltage becomes large and the sensitivity becomes high by
using the acoustic matching layer having a two-layer structure consisting of a silica dry gel and a
silica porous body manufactured by firing silicon oxide. The rising characteristics are also good.
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This is because the first acoustic matching layer arranged on the gas side has a low density and a
low sound velocity, and can achieve high sensitivity by achieving acoustic impedance matching
with the gas that is the radiation medium of ultrasonic waves, It is considered that this is because
good rising characteristics are secured by the second acoustic matching layer which is disposed
on the body side and has high density and high sound velocity.
[0060]
The reason why such good characteristics are obtained will be further described using FIGS. 9 (a)
to 9 (c).
FIGS. 9 (a) to 9 (c) are diagrams showing vibration displacement frequency characteristics of the
ultrasonic transducer corresponding to FIGS. 8 (a) to 8 (c), respectively.
[0061]
As shown in FIG. 9 (a), the acoustic matching layer of the conventional glass balloon / epoxy
system exhibits two-pole characteristics and a wide frequency band because the matching of the
acoustic impedance with the gas is not sufficient yet.
Therefore, the response to the pulse signal of the ultrasonic wave is well raised and the rising
characteristic is improved.
On the other hand, in the acoustic matching layer of the silica dry gel shown in FIG. 9 (b), in
order to match the acoustic impedance with the gas, a unipolar characteristic is exhibited to
narrow the band.
As a result, although the sensitivity is high, the rise characteristics of the pulse signal are
degraded because it is difficult to respond to changes faster than the resonance frequency.
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[0062]
When the configuration of the acoustic matching layer of the present invention shown in FIG. 9C
is used for the acoustic matching layer in these single layers, it has a two-layer structure of the
first acoustic matching layer and the second acoustic matching layer. In addition, the frequency
characteristic of the vibration displacement is to exhibit a three-pole characteristic, and the band
is broadened.
For this reason, the rise response is quickened, and the first acoustic matching layer facing it
matches the acoustic impedance with the gas which is the radiation medium, so that the
attenuation is small and the sensitivity is high.
[0063]
In an ultrasonic transducer used for emitting ultrasonic waves to gas and performing
measurement or the like by using the acoustic matching layer having a two-layer structure
according to the embodiment of the present invention, a conventional single layer acoustic
matching layer is used. It is possible to transmit and receive highly sensitive and responsive
ultrasound waves that could not be achieved.
Furthermore, by using such an ultrasonic transducer, it is possible to obtain an ultrasonic
flowmeter capable of improving the stability of flow rate measurement with high sensitivity and
small characteristic variations.
Although the acoustic matching layer according to the embodiment of the present invention is
typically a two-layer structure, the density of the acoustic matching layer is higher as it is closer
to the piezoelectric layer, and the acoustic matching layer is closer to the surface on the radiation
medium side. Three or more acoustic matching layers may be arranged to lower the density of
[0064]
Hereinafter, specific embodiments of the present invention will be described with reference to
the drawings.
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[0065]
First Embodiment FIG. 1 schematically shows the structure of an acoustic matching layer 1
according to an embodiment of the present invention.
[0066]
The acoustic matching layer 1 has a density of 50 kg / m <3> to 500 kg / m <3> and a density of
400 kg / m <3> to 1500 kg / m <3>. It has a structure in which the second acoustic matching
layer 3 within the range is stacked.
The density of the second acoustic matching layer 3 is higher than the density of the first
acoustic matching layer 2.
[0067]
The first acoustic matching layer 2 has a role of matching the acoustic impedance with respect to
the gas which is the radiation medium of ultrasonic waves to achieve high sensitivity.
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 be Za <Zb.
The value of the acoustic impedance of the first acoustic matching layer 2 is, for example, about
11 × 10 <4> kg / s · m <2> which is a value for matching the acoustic impedance between air
and the ceramic piezoelectric material. Is preferred.
However, taking as an example the case of using the ultrasonic transducer using the acoustic
matching layer of the present invention for measuring the flow rate of combustible gas as
another gas, the acoustic impedance of the first acoustic matching layer is, for example, 5 for
hydrogen. It is preferable to have a value of about × 10 <4> kg / s · m <2> to a value of about 12
× 10 <4> kg / s · m <2> to propane.
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Also, when other gases and mixed gases are considered, they are in the range of 5 × 10 <4> kg /
s · m <2> or more and 20 × 10 <4> kg / s · m <2> or less Is particularly preferred.
In addition, even in the state where the matching of the acoustic impedance with the gas of
interest is slightly shifted, high sensitivity is obtained in the region of the acoustic impedance
obtained by the first acoustic matching layer 2, so 50 × 10 <4> kg / s · m < 2> or less, preferably
0.5 × 10 <4> kg / s · m <2> or more and 50 × 10 <4> kg / s · m <2> or less.
[0068]
In order for the first acoustic matching layer 2 to obtain the above-described acoustic impedance,
one having a density in the range of 50 kg / m <3> to 500 kg / m <3> and a sound velocity of
less than 500 m / s is used .
At this time, it is preferable that the second acoustic matching layer 3 has a density of 400 kg /
m <3> or more and 1500 kg / m <3> or less and a sound velocity of 500 m / s or more. However,
it sets so that the relation of Za <Zb may be established. In addition, it is preferable that the
acoustic impedance Zb of the 2nd acoustic matching layer 3 is smaller than the acoustic
impedance of the piezoelectric material layer which transmits an ultrasonic wave.
[0069]
It also relates to the thickness of the acoustic matching layer in order to match the acoustic
impedance and improve the sensitivity. The reflectivity of the ultrasonic wave determined in
consideration of the reflection coefficient at the interface between the acoustic matching layer
and the radiation medium of the acoustic matching layer and the interface between the acoustic
matching layer and the ultrasonic transducer, and the ultrasonic wave transmitted through the
acoustic matching layer Is the smallest condition, ie, when the thickness of the acoustic matching
layer is a quarter of the ultrasonic oscillation wavelength, the transmission intensity is maximum.
Therefore, it is effective to increase the sensitivity that the thickness of the first acoustic
matching layer 2 is approximately one-fourth of the ultrasonic oscillation wavelength passing
through the acoustic matching layer. Furthermore, it is also effective to make the thickness of the
second acoustic matching layer 3 approximately one-fourth of the wavelength of the ultrasonic
oscillation passing through the acoustic matching layer. It is most effective to make both of the
two acoustic matching layers 3 have substantially a quarter wavelength. In addition, about 1/4 of
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the oscillation wavelength of an ultrasonic wave is about the range of 1/8 wavelength to 3/8
wavelength. In other words, if it is smaller than that, it does not work as an acoustic matching
layer, and if it is larger than that, the sensitivity approaches to the half wavelength where the
reflectance becomes maximum, so the sensitivity is lowered.
[0070]
As a material of the acoustic matching layer 1 of the present invention, 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
speed of sound of less than 500 m / s. Materials are preferred. Furthermore, the second acoustic
matching layer 3 preferably has a density of 400 kg / m <3> or more and 1500 kg / m <3> or
less and a material having a speed of sound of 500 m / s or more.
[0071]
As specific materials for the first acoustic matching layer 2, organic polymers, fibrous bodies of
inorganic materials, foams, sintered porous bodies, dried gels and the like are candidates, but it is
particularly preferable to use dried gels.
[0072]
Here, “dry gel” is a porous body formed by sol-gel reaction, and after a wet gel in which a solid
skeleton portion solidified by reaction of the gel raw material liquid contains a solvent, it is dried
and dried. It is formed by removing.
[0073]
In order to obtain a dried gel, as a method of removing the solvent from the wet gel and drying, a
drying method of special conditions such as supercritical drying and lyophilization, or ordinary
drying such as heat drying, reduced pressure drying and natural leave drying Methods can be
used.
[0074]
Supercritical drying is a method of removing the solvent in the supercritical state under the
temperature and pressure conditions above its critical point, and there is no gas-liquid interface,
so shrinkage does not occur because it gives dry stress to the solid skeleton of the gel. Very low
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density of dried gel can be obtained without doing.
On the other hand, the dried gel obtained by supercritical drying may be affected by stress in the
use environment, such as dew condensation, heat stress, chemical stress, mechanical stress and
the like.
[0075]
On the other hand, the dried gel obtained by the usual drying method is characterized in that it is
highly resistant to the stress in the subsequent use environment in order to withstand the drying
stress.
In order to obtain a low density dried gel by such a conventional drying method, it is necessary to
make the solid skeleton resistant to stress at the wet gel stage before drying.
For example, temperature conditions or a polyfunctional hydrophobizing agent that is easily
polymerized may be applied to strengthen the solid skeleton by aging the solid skeleton to
strengthen the solid skeleton, or the pore size may be increased. It can be realized by controlling.
In particular, when measuring the flow rate of gas, it is preferable to obtain the acoustic
matching layer with a dried gel produced by a common drying method, since it may be used in
various environments. Moreover, when applying the usual drying method, since it is not a high
pressure process such as supercritical drying, there are advantages such as facility simplification
and easy handling.
[0076]
The dried gel obtained by the above-described 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 of nanometer size. Therefore, in the 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 forming the unique network skeleton of the dried gel is extreme. And the speed of sound
propagating through the gas portion in the porous body by the pores is also extremely reduced.
Therefore, it has a feature that it exhibits a very slow value of 500 m / s or less as the sound
velocity and can obtain low acoustic impedance.
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20
[0077]
In the nanometer-sized pore portion, the pore size is equal to or less than the mean free path of
the gas molecules, and the pressure loss of the gas is large. It also has the feature that it can be
radiated by pressure.
[0078]
In addition, an inorganic material, an organic polymer material, etc. can be used as a material of a
dry gel.
The solid skeleton of the dry gel of the inorganic oxide can be applied as a component a general
ceramic obtained by sol-gel reaction such as silicon oxide (silica) or aluminum oxide (alumina).
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 curing
resin, polyacrylamide, polymethyl methacrylate and the like can be applied.
[0079]
Moreover, you may use the powder (powder dry gel) obtained by grind | pulverizing these dry
gels.
[0080]
Materials for the second acoustic matching layer 3 include organic polymers, fibrous bodies of
inorganic materials, foams, sintered porous bodies, materials obtained by solidifying glass
balloons and plastic balloons with resin materials, materials obtained by heat-compressing glass
balloons, etc. Can be used.
[0081]
The second acoustic matching layer 3 preferably has a density higher than that of the first
acoustic matching layer 2 and has a high sound velocity and a high acoustic impedance.
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21
More specifically, one having a density in the range of 400 kg / m <3> or more and 1500 kg / m
<3> or less is used.
The density in this range can obtain sufficient sensitivity to transmit and receive ultrasonic waves
without greatly shifting the matching of the acoustic impedance to the gas that is the radiation
medium of ultrasonic waves, and also has excellent response characteristics. You can get it. If the
density is higher than this, the acoustic impedance of the piezoelectric body tends to be close,
and the effect of the configuration of the acoustic matching layer of the present invention is
reduced. It becomes difficult to obtain. The upper limit of the density of the second acoustic
matching layer 3 may be 800 kg / m <3>.
[0082]
As the second acoustic matching layer 3, for example, an acoustic matching layer formed by
molding a glass balloon with a thermosetting resin, or a silicon oxide porous acoustic matching
layer obtained by mixing and firing a silicon oxide raw material and a polymer bead and
removing the polymer An acoustic matching layer formed by thermally bonding (thermally
compressing) a glass balloon can be suitably used.
[0083]
When the second acoustic matching layer 3 has a continuous pore structure, in particular,
penetration of the raw material liquid may occur when forming the first acoustic matching layer
2 made of a dry gel.
In this case, the first acoustic matching layer 2 can be formed while permeation occurs, but a
structural support layer may be formed on the surface of the second acoustic matching layer 3 in
order to avoid the penetration. However, in the case where the first acoustic matching layer 2
partially penetrates the second acoustic matching layer 3, there is also an effect that the
adhesion between the both increases. Therefore, the configuration may be determined by the
combination of the first acoustic matching layer 2 and the second acoustic matching layer 3.
[0084]
When the first acoustic matching layer 2 and the second acoustic matching layer 3 are both
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inorganic oxides, they are excellent in moisture resistance and chemical resistance, and in
temperature characteristics of acoustic impedance. That is, when the inorganic oxide dry gel is
used, the temperature change rate of the acoustic impedance in the range of 25 ° C. to 70 ° C.
is −0.04% / ° C. or less (absolute value is 0.04% / Acoustic matching layer) can be obtained. On
the other hand, when the conventional epoxy / glass balloon system or organic polymer gel is
used, it is difficult to set the absolute value of the temperature change rate of the acoustic
impedance to 0.04% / ° C. or less.
[0085]
When the temperature change rate of the acoustic impedance is small, for example, when used in
an ultrasonic flow meter to be described later, high measurement accuracy can be obtained over
a wide temperature range.
[0086]
Preferably, the first acoustic matching layer and the second acoustic matching layer in the
present invention are chemically combined.
This is effective for securing adhesion to ultrasonic vibration, ease of handling, durability to
vibration during use of the ultrasonic transducer, and the like. At this time, when the inorganic
oxide of the dry gel of the first acoustic matching layer is silicon oxide, it is preferable because
formation of the acoustic matching layer by sol-gel reaction is easily performed. Furthermore, it
is thought that if the second acoustic matching layer 3 is also made of silicon oxide, the influence
of the difference in material on the characteristics can be reduced. In this configuration, the
surface hydroxyl group of silicon oxide of the second acoustic matching layer 3 is easily
chemically bonded to the silanol group present when the first acoustic matching layer 2 is
formed by the sol-gel reaction, and thus a preferable effect is obtained. .
[0087]
Further, when an acoustic matching layer is formed using an inorganic oxide, it is preferable that
the solid skeleton portion of the inorganic oxide is hydrophobized (water repellent), because
there is a concern about the problem of moisture resistance due to moisture absorption. By
hydrophobizing, for example, when water or an impurity is present in the gas to be measured, it
can be made less susceptible to the influence of adsorption or adhesion thereof, so that a more
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reliable acoustic matching layer can be obtained.
[0088]
Hydrophobization of the solid skeleton of the inorganic oxide is performed using a surface
treatment agent such as a silane coupling agent, for example. As surface treatment agents,
halogen based silane treatment agents such as trimethylchlorosilane, dimethyldichlorosilane,
methyltrichlorosilane and ethyltrichlorosilane, and alkoxy compounds such as
trimethylmethoxysilane, trimethylethoxysilane, dimethyldimethoxysilane and
methyltriethoxysilane Silane treating agent, silicone based silane treating agent such as
hexamethyldisiloxane, dimethylsiloxane oligomer, amine based silane treating agent such as
hexamethyldisilazane, alcohol based treating agent such as propyl alcohol, butyl alcohol, hexyl
alcohol, octanol, decanol Etc. can be used.
[0089]
In addition to hydrophobization (water repellency), if a fluorinated treating agent is used in
which part or all of the hydrogen in the alkyl group possessed by these treating agents is
substituted with fluorine, it is further excellent in oil repellency, antifouling property, etc. Effect
can be obtained.
[0090]
Second Embodiment FIG. 2 schematically shows the cross-sectional structure of a piezoelectric
vibrator 8 used in an ultrasonic transducer according to an embodiment of the present invention.
The piezoelectric vibrator 8 is used for an ultrasonic transducer of an ultrasonic flow meter.
[0091]
The piezoelectric vibrator 8 for performing the electric-ultrasonic mutual conversion is composed
of 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
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24
piezoelectric single crystal or the like, is polarized in the thickness direction, and has electrodes
(not shown) on the upper and lower surfaces. The acoustic matching layer 1 is for transmitting
an ultrasonic wave to a gas or receiving an ultrasonic wave that has propagated a gas as
described above, and is a machine of the piezoelectric layer 4 excited by a driving AC voltage.
Vibration is efficiently radiated as an ultrasonic wave to an external medium, and the function is
such that the incoming ultrasonic wave is efficiently converted to a voltage, and the piezoelectric
body 4 is made to form a transmission / reception wave plane of the ultrasonic wave. It is formed
on one side of the layer 4.
[0092]
A structural support layer may be provided between the first acoustic matching layer 2 of the
acoustic matching layer 1 and the second acoustic matching layer 3 in order 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 one-eighth of the wavelength λ of the sound
wave propagating in the structural support layer. Preferably less than one. That is, because the
structure 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 wave becomes extremely small. As a material for
forming the structural support layer, a metal material, 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 the adhesive layer (adhesive or
adhesive sheet), the adhesive layer functions as a structural support layer.
[0093]
In the case where 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, the upper plate constituting
the case existing between the piezoelectric layer 4 and the acoustic matching layer 1 Functions
as a structural support layer.
[0094]
Furthermore, a structural support layer may be formed on the surface (gas side) of the first
acoustic matching layer 2.
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Since the material is supported by a high density material, the handleability of the acoustic
matching layer 1 is improved, and the adhesion is improved, whereby the preferable effect of
improving the durability is obtained.
[0095]
Third Embodiment FIG. 3 shows a schematic cross-sectional view of an ultrasonic transducer
according to an embodiment of the present invention.
[0096]
The ultrasonic transducer 10A shown in FIG. 3 is an ultrasonic transducer in which a
piezoelectric vibrator is configured using the acoustic matching layer 1 and the piezoelectric
layer 4 of the first embodiment.
[0097]
The ultrasonic transducer 10A further includes a case (a closed container) 5 having an upper
plate 5a forming a recess containing the piezoelectric layer 4 and a bottom plate 5b disposed so
as to seal the space in the recess. doing.
The piezoelectric layer 4 is bonded (bonded) 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. (Bonded).
[0098]
The upper plate 5a present 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 preferably less than one-eighth of the wavelength λ of the sound
wave propagating in the upper plate 5a. The density of the upper plate 5a is preferably 800 kg /
m <3> or more, and more preferably 1000 kg / m <3> or more.
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[0099]
By making the case 5 of a conductive material (for example, a metal material), the case 5
functions as a structural support member, and of the electrode (wiring) for detecting the
ultrasonic wave that oscillates or receives the piezoelectric layer 4. It also works. Electrodes (not
shown) formed on the pair of main surfaces of the piezoelectric layer 4 are connected to one of
the terminals 7 through the case 5, and the other is connected to the other terminal 7 by a wire
or the like. Accordingly, the case 5 is generally formed of a conductive metal. The other terminal
7 is insulated from the case 5 by the insulator 6.
[0100]
The acoustic matching layer 1 disposed so as to face the piezoelectric layer 4 via the upper plate
5a of the case 5 has a second acoustic matching layer 3 and a second acoustic matching layer 3
facing the medium that emits ultrasonic waves from the piezoelectric layer 4 side. 1 and the
acoustic matching layer 2 are stacked in this order. By arranging the acoustic matching layer 1 in
this manner, as described above with reference to FIGS. 8C and 9C, a highly sensitive and
responsive ultrasonic transducer 10 can be obtained. .
[0101]
When flammable 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. The inside (recess) of the case 5 is preferably
purged with an inert gas such as nitrogen. In this way, when using it for the ultrasonic flowmeter
for combustible gas, the advantage that safety is high is acquired. Further, it is preferable that the
constituent material of the acoustic matching layer in contact with the flammable gas does not
react with the gas or burn. Also from this viewpoint, the acoustic matching layer is preferably
made of an inorganic oxide.
[0102]
In the ultrasonic transducer 10A configured as described above, when a burst signal voltage
having an AC signal component of a frequency near the resonance frequency of the ultrasonic
transducer is applied to the drive terminal 7, the piezoelectric layer 4 has a thickness It vibrates
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in vibration mode and will emit bursts of ultrasound into the fluid, such as in gas or liquid.
[0103]
Fourth Embodiment FIG. 4 is a cross-sectional view of an ultrasonic transducer according to an
embodiment of the present invention.
[0104]
In the ultrasonic transducer 10B shown in FIG. 4, the case 15 is partially 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 opposite to the
placement position of the piezoelectric layer 4.
The second acoustic matching layer 13 also plays a role as a structural support layer.
Therefore, it is preferable that the second acoustic matching layer 13 be made of a material
having a relatively high density, and it is difficult to achieve matching of the acoustic impedance
with a gas that is a radiation medium of ultrasonic waves by using only the second acoustic
matching layer 13. However, as shown in FIG. 4, by further laminating the first acoustic matching
layer 12 on the second acoustic matching layer 13, the acoustic impedance can be matched with
that of the gas to achieve high sensitivity.
[0105]
Fifth Embodiment FIG. 5 is an explanatory view of a method of manufacturing an ultrasonic
transducer according to an embodiment of the present invention.
[0106]
In the method of manufacturing an ultrasonic transducer according to the present embodiment,
after the second acoustic matching layer is formed in the case where the piezoelectric layer or
the piezoelectric layer is disposed on the inner surface, the first acoustic matching layer made of
dry gel is laminated. It is a method including a process.
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[0107]
Step (a): The second acoustic matching layer 3 is prepared.
[0108]
Step (b): The piezoelectric layer 4 and the case 5 are prepared.
[0109]
Step (c): The piezoelectric layer 4 and the second acoustic matching layer 3 are bonded to the
case 5 using an adhesive or the like.
[0110]
Step (d): forming a first acoustic matching layer made of a dry gel on the second acoustic
matching layer 3;
[0111]
Step (e): The electrode and the terminal plate (bottom plate of the case 5) 5b are attached to
obtain an ultrasonic transducer.
[0112]
The step (d) of forming the first acoustic matching layer 2 is a film forming step of applying a gel
material solution on the second acoustic matching layer 3, a solidifying step of obtaining a wet
gel from the gel material solution, Removing the solvent in the wet gel layer to obtain a dry gel
layer.
Alternatively, the first acoustic matching layer 2 made of dry gel may be formed in advance, and
the first acoustic matching layer 3 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 not via an adhesive layer).
[0113]
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29
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 can also be chemically bonded.
For example, when the first acoustic matching layer 2 is made of an inorganic oxide and the
second acoustic matching layer 3 is treated so as to have hydroxyl groups on the surface by
washing or the like, the first acoustic matching layer 2 is made of an inorganic oxide Chemical
bonds can be formed during the formation of the dried gel.
As a treatment method for producing a hydroxyl group on the surface, washing with acid or
alkali, water washing, ultraviolet irradiation, ozone treatment, oxygen plasma treatment or the
like can be used.
[0114]
In addition, when the second acoustic matching layer 3 is a continuous pore body, a gel stock
solution for forming the first acoustic matching layer 2 can penetrate, and a stronger chemical
bond can be formed.
At this time, it is preferable to form the first acoustic matching layer 2 and the second acoustic
matching layer 3 with the same inorganic oxide.
Chemically bonding the first acoustic matching layer 2 and the second acoustic matching layer 3
is preferable because the acoustic coupling becomes strong, the sensitivity is improved, and the
stability and reliability of the characteristics are improved.
[0115]
Sixth Embodiment FIG. 6 is an explanatory view of a method of manufacturing an ultrasonic
transducer according to another embodiment of the present invention.
[0116]
In the method of manufacturing an ultrasonic transducer according to the present embodiment,
after the acoustic matching layer 1 in which the first acoustic matching layer 2 made of dry gel is
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laminated on one surface of the second acoustic matching layer 3, a piezoelectric body is formed.
This is a manufacturing method including the step of bonding 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.
[0117]
Step (a): The second acoustic matching layer 3 is prepared.
[0118]
Step (b): The first acoustic matching layer 2 is laminated on one side of the second acoustic
matching layer 3.
In this lamination method, a film forming process of applying a gel material solution on the
second acoustic matching layer 3, a solidification process of obtaining a wet gel from the gel
material solution, and removal of the solvent in the obtained wet gel layer And drying to obtain a
layer of dry gel.
Alternatively, the first acoustic matching layer 2 made of dry gel may be formed in advance, and
the first acoustic matching layer 3 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 not via an adhesive layer).
Also, in order to improve the durability of the laminated structure of the first acoustic matching
layer and the second acoustic matching layer, the same method as that of the sixth embodiment
can be used.
[0119]
Step (c): The piezoelectric layer 4 and the case 5 are prepared.
[0120]
Step (d): The acoustic matching layer 1 in which the first acoustic matching layer 2 and the
second acoustic matching layer 3 are stacked, and the piezoelectric layer 4 and the case 5 (c) are
bonded with an adhesive or the like.
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[0121]
Step (e): The electrode and the terminal plate (bottom plate of the case 5) 5b are attached to
obtain an ultrasonic transducer.
[0122]
Seventh Embodiment The first acoustic matching layer 2 can also be formed using a dry gel
powder.
The first acoustic matching layer 2A shown in FIG. 7A may be referred to as a dry gel powder
(hereinafter referred to as "powder dry gel").
And 2) and an additive 2b.
By forming the first acoustic matching layer 2A using a dry gel powder, the variation of the
characteristics due to the nonuniformity of the wet gel drying process is suppressed.
In addition, since the powder dry gel 2a can be prepared in advance by using the powder dry gel
2a, the advantage that the productivity of the ultrasonic transducer can be improved is also
obtained.
That is, in the above-described manufacturing process of the ultrasonic transducer, the step of
solidifying the gel raw material liquid to obtain the wet gel and the step of drying the gel can be
executed in advance, so that the ultrasonic transducer is manufactured. Throughput can be
improved.
[0123]
The average particle size of the powder dry 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 be reduced and
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the characteristic effect of the dried gel will be reduced, and the required amount of additives at
the time of molding will be increased. It can be difficult to get. If the average particle size of the
powder dry gel 2a is larger than the upper limit value, thickness control of the acoustic matching
layer becomes difficult, and it becomes difficult to form an acoustic matching layer with
sufficient thickness uniformity and surface flatness. Sometimes.
[0124]
As an additive (binder) 2b for bonding the powder dry gels 2a to each other to improve the
mechanical strength of the acoustic matching layer 2A, a polymer powder having thermal
bonding properties can be suitably used. When a liquid material is used, it may penetrate inside
the pores of the dried gel to change the acoustic properties or reduce the formability, so it is
preferable to use a solid material, particularly a powder.
[0125]
Here, the “heat-binding polymer” refers to a polymer that is solid at room temperature, melts
or softens by heating, and then solidifies. Thermosetting polymers are not only common
thermoplastic resins (eg, engineering plastics such as polyethylene and polypropylene) but, for
example, thermosetting resins which are solid at room temperature and temporarily soften by
heating and then crosslink and cure (For example, phenol resin, epoxy resin, urethane resin) can
be used. Moreover, when a thermosetting resin contains a main ingredient and a hardening
agent, you may add each as another powder. Of course, a thermoplastic resin and a
thermosetting resin may be mixed and used. The melting (softening) temperature of the heat
binding polymer powder is preferably in the range of 80 ° C. or more and 250 ° C. or less.
[0126]
When a heat-binding polymer is used as an additive, it is typically melted (softened) when
pressure-molding while heating a mixed powder of the powder-dried gel 2a and the additive, as
described later. The additive plays a role of bonding the powder-dried gels 2a together by
solidifying with cooling and / or crosslinking and curing.
[0127]
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The average particle size of the thermally adhesive polymer powder is preferably 0.1 μm or
more and 50 μm or less.
If it is smaller than this lower limit, it will be close to the pore diameter of the powder dry gel, so
the binding property may be reduced or the formability may be reduced. On the other hand, if it
is larger than the upper limit value, it may be difficult to obtain a low density acoustic matching
layer because the amount of addition required for molding increases.
[0128]
Further, the addition amount of the heat binding polymer powder is preferably 40% by mass or
less of the whole. If it exceeds 40 mass% of the whole, the density at the time of shaping |
molding may become high. Moreover, in order to obtain sufficient mechanical strength, for
example, it is preferable to add 5% by mass or more of the whole.
[0129]
The above additive (sometimes referred to as “additive A”. Fibers (inorganic fibers (eg, glass
wool) or organic fibers), whiskers, etc., as in the acoustic matching layer 2B schematically shown
in FIG. You may add further (it may be called "the additive B"). In the acoustic matching layer 2B
of FIG. 7 (b), the additive 2b is a powder of the same thermal bonding polymer as described
above, and the additive 2c is a short fiber. The range of the preferred diameter 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 several μm to several mm.
[0130]
It is preferable that the addition amount of two types of additives is 40 mass% or less with
respect to the whole, and a compounding ratio is suitably set as needed.
[0131]
The acoustic matching layer using a powder dry gel has the further advantage that the acoustic
impedance can be easily adjusted.
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For example, acoustic impedance can be adjusted by mixing multiple types of powder dry gels
having different densities from one another. Furthermore, the acoustic impedance can be
adjusted by adjusting the amount of the above-mentioned additive A (optionally additive B). Of
course, the addition amounts of the additives A and B are preferably in the above range in
consideration of moldability and the like.
[0132]
The first acoustic matching layer 2B including the powder dry gel can be formed, for example, by
the following method.
[0133]
Step (a): Low-density powdery dry gel (density: about 200 kg / m <3> to 400 kg / m <3>)
consisting of porous material and about 10% by mass (relative to the whole) of additive A and
additives Prepare B and.
The dried gel prepared here does not have to be a powder. It may be block-like. 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 glass wool having a fiber diameter of about 10 μm.
[0134]
Step (b): These are placed in the same container and mixed and ground to produce a fine powder.
It is typically carried out using a mill. Here, the grinding conditions are adjusted so as to obtain a
powder dry gel of the desired average particle size described above. Moreover, you may classify
as needed. Of course, the step of grinding the dry gel and the step of mixing may be performed
separately.
[0135]
Step (c): A desired amount of a mixed powder consisting of a low-density powdery dry gel, an
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additive A and an additive B is weighed and pressure-formed while heating. At this time, the first
acoustic matching layer 2 can be directly bonded to the second acoustic matching layer 3 by
directly pressing on the surface of the second acoustic matching layer 3.
[0136]
In addition, before pressing the powder dry gel and the mixed powder of the additives A and B, it
is preferable to flatten the upper surface of the mixed powder layer by, for example, applying
vibration to the layer of the mixed powder. By doing so, the characteristics of the obtained first
acoustic matching layer 2A can be made more uniform.
[0137]
Eighth Embodiment FIG. 11 shows a block diagram of an ultrasonic flowmeter using an
ultrasonic transducer according to an embodiment of the present invention.
[0138]
The ultrasonic flowmeter in FIG. 11 is installed so that the fluid to be measured flows at a
velocity V in the direction shown in the figure, which is a pipe that is the flow rate measuring
unit 51.
On a tube wall 52 of the flow rate measuring unit 51, piezoelectric vibrators 101 and 102 as an
ultrasonic transducer according to the present invention are disposed opposite to each other.
Here, the piezoelectric vibrator 101 is used as an ultrasonic wave transmitter, and the
piezoelectric vibrator 102 is used as an ultrasonic wave receiver. In addition, the ultrasonic wave
transmitter 101 and the ultrasonic wave receiver 102 are provided with a drive circuit 54 for
driving the ultrasonic transducers 101 and 102 and an ultrasonic pulse via the switching circuit
55 for switching between transmission and reception of these. A reception detection circuit 56
for detecting, a timer 57 for measuring propagation time of an ultrasonic pulse, an arithmetic
circuit 58 for calculating the flow rate from the output of the timer 57, and a control circuit 59
for outputting a control signal to the drive circuit 54 and the timer 57 ing.
[0139]
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The operation of the ultrasonic flowmeter configured as described above 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, starts the time measurement of the timer 57.
When 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 rate
measuring 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.
[0140]
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
again outputs a transmission start signal to the drive circuit 54, and at the same time measures
the time of the timer 57. To start. Contrary to the measurement of the propagation time t1,
ultrasonic pulses are transmitted by the ultrasonic transducer 102, received by the ultrasonic
transducer 101, and the propagation time t2 is calculated by the arithmetic circuit 58.
[0141]
Here, the distance connecting the centers of the ultrasonic transducer 101 and the ultrasonic
wave transfer transducer 102 is L, the speed of sound of the LP gas in a windless state is C, the
flow velocity in the flow rate measuring unit 51 is V, and the fluid to be measured The
propagation times t1 and t2 can be respectively determined, where θ is the angle between the
direction of the flow of the wave and the line connecting the centers of the ultrasonic transducers
101 and 102. Further, since the distance L is known, the flow velocity V can be obtained by
measuring the times t1 and t2, and the flow can be checked from the flow velocity V.
[0142]
EXAMPLES Specific examples of the present invention will be described below.
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[0143]
Example 1 The ultrasonic transducer of the present invention was manufactured as follows.
[0144]
(A) Production of Second Acoustic Matching Layer (Glass Epoxy) A jig was filled with a glass
balloon, then impregnated with an epoxy solution, and thermally cured at 120 ° C.
The cured molded product was cut to a thickness of one-fourth the ultrasonic oscillation
wavelength.
[0145]
The ultrasonic velocity was about 500 kHz, the velocity of sound 2500 m / s, the density 500 kg
/ m <3>, and the thickness 1.25 mm.
[0146]
(B) Bonding of Second Acoustic Matching Layer to Piezoelectric Body and 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 put
together and heated and cured while being pressed.
[0147]
(C) Lamination of first acoustic matching layer First, electrodialysis of sodium silicate is carried
out to prepare a pH 9 to 10 aqueous solution of silicic acid (14 wt% of silica component
concentration in aqueous solution).
After adjusting the pH of the aqueous silicic acid solution to 5.5, coating was performed to a
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thickness of 90 μm on the second acoustic matching layer washed to form hydroxyl groups on
the surface in advance by ultraviolet irradiation. After this, the coating film was gelled to obtain a
solidified silica wet gel layer. This container is supercritically dried using carbon dioxide at 12
MPa and 50 ° C. to form an acoustic matching layer in which a silica dry gel first acoustic
matching layer and a glass epoxy second acoustic matching layer are laminated. I got a child
case.
[0148]
The first acoustic matching layer made of the silica dry gel had an acoustic velocity of 180 m / s
and a density of 200 kg / m <3> with respect to an ultrasonic wave of about 500 kHz.
[0149]
(D) Formation of ultrasonic transducer A cover plate, a drive terminal and the like were attached
to a case in which an acoustic matching layer was formed, to obtain an ultrasonic transducer.
[0150]
Example 2 The ultrasonic transducer of the present invention was manufactured as follows.
[0151]
(A) Production of Second Acoustic Matching Layer (Silica Porous Body) After molding a spherical
acrylic resin having a diameter of several tens of μm and a sintered silica powder having a
diameter of 1 μm or less, compression molding was performed.
After drying the molded body, it was fired at 900 ° C. to form a porous silica body.
Thereafter, the thickness was adjusted to be a quarter of the ultrasonic oscillation wavelength.
[0152]
The ultrasonic velocity was about 500 kHz, the sound velocity was 1500 m / s, the density was
570 kg / m <3>, and the thickness was 750 μm.
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[0153]
(B) Lamination of second acoustic matching layer and first acoustic matching layer A gel material
solution prepared so that the molar ratio of tetramethoxysilane, ethanol and aqueous ammonia
solution (0.1 N) is 1: 3 to 4, Coating was performed to a thickness of 90 μm on the second
acoustic matching layer previously cleaned to form hydroxyl groups on the surface by plasma
cleaning.
After this, the coating film was gelled to obtain a solidified silica wet gel layer.
[0154]
The second acoustic matching layer on which the silica wet gel layer is formed is subjected to a
hydrophobization treatment in a 5 wt% hexane solution of trimethylethoxysilane, and then
subjected to supercritical drying (12 MPa, 50 ° C.) with carbon dioxide, The acoustic matching
layer in which the silica dry gel and the second acoustic matching layer were laminated was
obtained.
[0155]
In addition, since the hydroxyl group on the second acoustic matching layer and the alkoxy
group of tetramethoxysilane react to form a chemical bond, it is possible to provide an acoustic
matching layer with good adhesion.
[0156]
The first acoustic matching layer made of the silica dry gel had an acoustic velocity of 180 m / s
and a density of 200 kg / m <3> with respect to an ultrasonic wave of about 500 kHz.
[0157]
(C) Bonding of the acoustic matching layer and the case and the piezoelectric layer After
temporarily bonding the epoxy adhesive sheet on both sides of the top surface of the case, one
side of the piezoelectric body and the second acoustic matching layer and the case are combined
It heated and pressure-hardened and joined.
[0158]
(D) Formation of an ultrasonic transducer A cover plate, a drive terminal and the like were
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40
attached to a case to obtain an ultrasonic transducer.
[0159]
Example 3 The ultrasonic transducer of the present invention was manufactured as follows.
[0160]
(A) Production of Second Acoustic Matching Layer (Silica Porous Body) A sintered silica powder
having a particle diameter of several μm to several tens of μm is molded, and the obtained
molded body is fired at 900 ° C. A porous silica of about one-fourth the ultrasonic oscillation
wavelength was formed.
The second acoustic matching layer made of this porous silica material had an acoustic velocity
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). .
[0161]
A 3 μm thick glass layer (density: about 3000 kg / m <3>) was formed on one side of this
second acoustic matching layer (porous silica) as a structural support layer.
Since the speed of sound of this glass layer is approximately 5000 m / s or more, the wavelength
of the propagating sound wave for ultrasonic waves of approximately 500 kHz is greater than 1
cm.
Since the thickness of the glass layer formed is sufficiently smaller than one-eighth of the
wavelength, it has no effect on acoustic matching.
[0162]
(B) Lamination of second acoustic matching layer and first acoustic matching layer The silica wet
gel formed in the same manner as in Example 2 was heated to 40 ° C. to 70 ° C. on the glass
layer surface of the porous silica body formed in (a). After making it hydrophobic while heating,
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41
it was dried by heating at 80 ° C. in a nitrogen stream to obtain an acoustic matching layer in
which a first acoustic matching layer made of silica dry gel was laminated on a second acoustic
matching layer.
[0163]
The first acoustic matching layer made of the silica dry gel had an acoustic velocity of 180 m / s
and a density of 200 kg / m <3> with respect to an ultrasonic wave of about 500 kHz.
[0164]
(C) Joining the acoustic matching layer and the case and the piezoelectric body After temporarily
bonding the epoxy adhesive sheet on both sides of the top surface of the case, apply pressure to
the surface of the piezoelectric body and the second acoustic matching layer side and the case
together. While heating, it was cured and bonded.
[0165]
(D) Formation of an ultrasonic transducer A cover plate, a drive terminal and the like were
attached to a case to obtain an ultrasonic transducer.
[0166]
(Comparative example 1) The ultrasonic transducer of only the 2nd acoustic matching layer
(glass epoxy) produced in Example 1 was formed.
[0167]
(Comparative example 2) The ultrasonic transducer which produced only the silica dry gel as an
acoustic matching layer on the case of the piezoelectric vibrator by the method described in
Example 1 was obtained.
[0168]
Hereinafter, transmission and reception characteristics at 500 kHz ultrasonic waves of the abovedescribed Examples 1 to 3 and Comparative Examples 1 and 2 are compared.
An ultrasonic flowmeter was formed by facing each other the ultrasonic transducers
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manufactured in the respective examples and comparative examples.
At that time, evaluation was performed using the output waveform when the sound wave
transmitted from one ultrasonic transducer was received by the other ultrasonic transducer.
[0169]
8 (a) to 8 (c) show an example (Comparative Example 1, Comparative Example 2, Example 2).
[0170]
Sensitivity: (Example 2) ((Example 3)> (Example 1)> (Comparative Example 2) »(Comparative
Example 1) Rising response: (Example 1) ((Example 2) ((Example 3) ((Comparative Example 1)
»(Comparative Example 2)
[0171]
As described above, with respect to the sensitivity, the excellent characteristics of about 10 times
in Example 1 and about 20 times in Example 2 and Example 3 as compared with the acoustic
matching layer of Comparative Example 1 conventionally and generally used. showed that.
In addition, regarding the rise response, the characteristics are the same as or slightly better than
those of the acoustic matching layer of Comparative Example 1 generally used conventionally in
the first, second, and third embodiments.
That is, while the wave front of the ultrasonic wave peaks at the fifth wave in Comparative
Example 1 of FIG. 8 (a), the wave front of the ultrasonic wave peaks at the fourth wave in
Example 2 of FIG. 8 (c). It had become.
Therefore, it was found that in the ultrasonic transducer of the present invention manufactured
in the example, both the sensitivity and the rise response were superior to those of the
conventional one.
[0172]
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Example 4 The ultrasonic transducer of the present invention was manufactured as follows.
[0173]
(A) Production of Second Acoustic Matching Layer (Silica Porous Body) A sintered silica powder
having a diameter of several μm to several tens of μm is molded in the same manner as in
Example 3, and the obtained molded body is fired at 900 ° C. Thus, a silica porous body having
a thickness of about one-fourth the ultrasonic oscillation wavelength was formed.
The second acoustic matching layer made of this porous silica material had an acoustic velocity
of about 4000 m / s, a density of about 1200 kg / m <3> and a thickness of about 2 mm with
respect to an ultrasonic wave of about 500 kHz.
[0174]
(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 by (a) using an oligomer of tetraethoxysilane
silicone oligomer as a raw material in an isopropyl alcohol solvent and ammonia catalyst did.
After aging the wet gel at 70 ° C., hydrophobization treatment with dimethyldimethoxysilane
was performed.
Thereafter, the solvent was removed by natural standing to dry and remove, to obtain an acoustic
matching layer in which the first acoustic matching layer made of silica dry gel was laminated on
the second acoustic matching layer.
[0175]
The first acoustic matching layer made of silica dry gel had an acoustic velocity of about 300 m /
s and a density of about 420 kg / m <3> with respect to ultrasonic waves (about 500 kHz).
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A portion of the first acoustic matching layer was processed to a thickness of 150 μm to make a
final acoustic matching layer.
[0176]
(C) Bonding of 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
The acoustic matching layer and the piezoelectric layer were bonded to the upper plate of the
case by bonding the surface of the upper layer to the upper plate of the case via the adhesive
layer and heating and curing the adhesive while pressing.
[0177]
(D) Formation of ultrasonic transducer A cover plate (bottom plate) of the case, a drive terminal
and the like were assembled to obtain an ultrasonic transducer.
[0178]
The transmission / reception characteristics of the ultrasonic transducer manufactured in this
manner were also evaluated as a pair. As a result, characteristics superior to Comparative
Example 1 and Comparative Example 2 were also obtained with regard to sensitivity and rise
response.
[0179]
It is a typical sectional view of the acoustic matching layer of the 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 sectional drawing of the ultrasonic transducer shown as a 4th form of this invention.
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It is a figure which shows typically processes (a) to (e) in the manufacturing method of the
ultrasonic transducer of embodiment by this invention.
It is a figure which shows typically processes (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 suitably used for the acoustic matching layer of embodiment of
this invention.
(A) to (c) are diagrams showing the reception output characteristics of the ultrasonic transducer
used in the present invention, and (a) is a characteristic diagram when a single acoustic matching
layer (glass epoxy) is used. (B) is a characteristic diagram in the case of using a single acoustic
matching layer (silica dry gel), (c) is a characteristic diagram in the case of using a two-layer
acoustic matching layer (silica dry gel, porous silica). (A) to (c) are diagrams showing vibration
displacement frequency characteristics of the ultrasonic transducer used in the present
invention, and (a) is a case where a single acoustic matching layer (glass balloon / epoxy system)
is used (B) shows the characteristics when a single acoustic matching layer (silica dry gel) is used,
(c) shows the characteristics 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 an ultrasonic
transducer of the present invention. It is sectional drawing for demonstrating the measurement
principle of a general ultrasonic flowmeter.
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