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JPS59171295

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DESCRIPTION JPS59171295
[0001]
TECHNICAL FIELD The present invention relates to an ultrasonic transducer used for an
ultrasonic sensor or the like for a distance measurement device. Configuration of the
Conventional Example and Problems Thereof Conventionally, when using a piezoelectric ceramic
signal element or a magnetostrictive vibrator as an ultrasonic transducer for air, these vibrators
have a large intrinsic acoustic impedance as compared with air, so the oscillators In order to
improve the inherent acoustic impedance mismatch with the air as an ultrasonic wave
propagation medium, as shown in FIG. 1, the base material such as an epoxy adhesive or a
silicone adhesive has a diameter of several dozen ˜ Several hundred μm or less of glass, carbon,
etc. micro hollow spheres (hereinafter referred to as microspheres). The thin layer 1 made of a
composite material filled with a layer not shown) or the thin layer 1 made of a silicone rubber or
the like is adhered to the sound wave emitting surface 3 of the vibrator 2 and used as a matching
layer. It was not possible to fully satisfy the condition for matching with the air. That is, the
sound velocity v1 of the piezoelectric ceramic vibrator 2 is about 3500 m / s and the density p1
is about 8000 kg / m3. Therefore, the specific acoustic impedance z1 represented by the product
of them is about 3 × 10 7 N · S / m 3 The characteristic acoustic impedance 2 of air at normal
temperature is about 400 Ns / m ', so when using a further matching layer, the matching layer
should have its specific acoustic impedance given the generally well-known matching conditions.
Assuming that zma, it is desirable that the thickness be a quarter wavelength. However, after
filling the above-mentioned microspheres in a silicone-based or epoxy-based adhesive agent, the
value of the specific acoustic impedance of the composite material or the silicone-based rubber
alone is about 9 to 13 × 10 5 N · S / m 5 and the piezoelectric ceramic vibration is The
characteristic acoustic impedance value required for the matching layer between the element 2
and the air was about one order of magnitude larger than the value LIX10 ′ ′ ′ N ′ ′-S / 、
6 and was not optimum as the matching layer material. Further, in order to realize a value of
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specific acoustic impedance smaller than that of the composite material in the above-mentioned
conventional example, the present applicant uses thermally expansive gas such as low boiling
point hydrocarbon as a base material such as epoxy adhesive or silicone adhesive. By heating a
composite material (hereinafter referred to as a thermally expandable composite material), which
is a mixture of thermally expandable micro hollow spheres (hereinafter referred to as a thermally
expandable balloon) made of encapsulated plastic etc., to about 100 ° C. It is proposed to use a
thin layer obtained by expanding a thermally expandable balloon mixed with an expandable
composite material as a matching layer material of a piezoelectric ceramic vibrator and air.
As a result of variously examining about this proposal, the following thing was understood. After
the thermally expandable composite material is heated to about 100 ° C. and returned to
normal temperature, the density and the speed of sound of the thermally expandable composite
material are, for example, 30% of the weight ratio of the thermally expandable balloon to the
base material. The heating has a tendency as shown in FIG. 2, and hence the value of the specific
acoustic impedance represented by the product of density and sound velocity changes as shown
in FIG. Therefore, according to FIG. 3, when the heating temperature is about 94 ° C., the value
of the specific acoustic impedance of the thermally expandable composite material is 1.1 × 10 5
N · S / m 3, which is the most suitable matching layer material for the piezoelectric ceramic
vibrator and air. It turns out that it is suitable. However, since the speed of sound of the
thermally expandable composite material changes with respect to the heating temperature as
shown in FIG. 2, the matching layer as a quarter wavelength at the operating frequency based on
the value of the speed of sound of the thermally expandable composite material at a certain
temperature. If the heating temperature is changed from a constant set value, the speed of sound
changes, so that the matching layer thickness causes an error from 1⁄4 wavelength, and the
matching condition is not satisfied. Become. Similarly, as shown in FIG. 3, the value of the specific
acoustic impedance of the thermally expandable composite material also changes due to the
variation of the heating temperature, so that the matching condition of the equation (1)
regarding the magnitude of the specific acoustic impedance is not satisfied. Had. Also, even if the
variation of the heating temperature is extremely small and the sound velocity or specific
acoustic impedance of the thermally expandable composite material is as per the set value, the
thickness of the matching layer is 1/1 only for a single operating frequency. It is difficult to
realize a sufficiently broadband matching layer ultrasonic transducer because the four
wavelength condition is satisfied. Further, by heating the thermally expandable composite
material, the thermally expandable balloon mixed therein is expanded, so that small irregularities
are generated on the surface of the thin layer formed as a composite layer to form a rough
surface. And the adhesion of the matching layer to the acoustic emission surface of a vibrator
such as a piezoelectric ceramic is not complete. SUMMARY OF THE INVENTION The present
invention is to improve the above-mentioned conventional drawbacks, and it is possible to
achieve sufficient acoustic matching between the piezoelectric ceramic vibrator or
magnetostrictive vibrator and air as an ultrasonic wave propagation medium, thereby broadening
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the bandwidth. The present invention is directed to providing an ultrasonic transducer having an
acoustic matching layer which is capable of forming a smooth interface surface with the
transducer. SUMMARY OF THE INVENTION In order to achieve the above object, according to
the present invention, the specific acoustic impedance or the speed of sound of the matching
layer is determined by continuously changing the particle size distribution of the hollow spheres
mixed in the synthetic resin with respect to the sound wave direction. Continuously changing
with respect to the sound wave propagation direction, absorbing the deviation from the matching
condition based on the variation of the inherent acoustic impedance or the velocity of sound
caused by the variation of the heating temperature of the thermal expansion composite used as
the matching layer Further, the bonding surface of the thin layer surface of the composite used
as the matching layer with the vibrator is smoothed to achieve more complete bonding.
Description of the Embodiments Embodiments of the present invention will be described below
with reference to the drawings. FIG. 4 is a cross-sectional view showing an embodiment of the
present invention, and is a composite material in which micro hollow spheres 5 are mixed with a
base material 4 such as an epoxy adhesive or silicone type adhesive. A thin layer 6 whose particle
size is continuously changed in the thickness direction of the composite material is adhered to
the sound wave emitting surface 3 of the piezoelectric ceramic vibrator or the magnetostrictive
vibrator 2 to form a matching layer, along the sound wave emitting direction The particle
diameter distribution is given such that the particle diameter of the micro hollow spheres 5 is
increased. Therefore, as can be seen from FIGS. 2 and 3, the speed of sound and the acoustic
impedance also decrease continuously along the sound wave radiation direction, so the
transducer 2 is spread over a wide frequency range (the particle size increases as the heating
temperature is higher). It becomes possible to match with the air which is the sound wave
propagation medium. The method for producing the matching layer as described above will be
described with reference to FIG. In FIG. 5, 6 is a thin layer made of a thermally expandable
composite material in which a thermally expandable balloon 5 is mixed with a matrix 4 such as
an epoxy adhesive or a silicone adhesive, and one side 7 of this thin layer 6 By making the thin
layer 6 have a temperature gradient by contacting the heat source 9 having the temperature T1
and contacting the heat reduction 10 (T1 <T2) having the temperature T2 on the other surface 8,
the thermally expandable balloon 5 The closer the side is, the higher the expansion rate and
therefore the lower the sound velocity and the acoustic impedance. As shown in FIG. 4, the thin
layer 6 obtained in this manner is bonded to the sound wave emitting surface 3 of the
piezoelectric ceramic vibrator or the magnetostrictive vibrator 2 as shown in FIG. The thickness
of the thin layer 6 is chosen to be at or near 1⁄4 of the average wavelength of the sound waves
propagating in the thin layer 6. As described above, according to the present invention, since the
particle diameter of the micro hollow spheres of the matching layer increases in the sound wave
emitting direction, the sound velocity or sound impedance of the matching layer from the side
closer to the ultrasonic transducer is the sound emitting direction. An ultrasonic transducer can
be realized that has a continuously smaller size and flat characteristics over a wide frequency
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range. In addition, since fine particles are distributed in the vicinity of the surface of the
matching layer on the side close to the sound emitting surface of the ultrasonic transducer, this
surface becomes smooth and adhesion with the ultrasonic transducer is easy. .
[0002]
Brief description of the drawings
[0003]
FIG. 1 is a cross-sectional view showing a conventional ultrasonic transducer, FIG. 2 is a view
showing temperature change of density and sound velocity in a thermally expandable composite
material, and FIG. 3 is a temperature change of specific acoustic impedance in thermally
expanded composite material FIG. 4 is a cross-sectional view showing an embodiment of the
present invention, and FIG. 5 is a cross-sectional view for explaining the manufacturing method
of the embodiment.
· 11 · · · base material, 5 thermally expandable balloon, 6 thermally expandable composite
material, 9, 10 · · · · heat source.
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