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JPH06269093

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DESCRIPTION JPH06269093
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
receiving type piezoelectric device having an enhanced receiving sensitivity of sound waves.
[0002]
2. Description of the Related Art Microphones usually used by being placed in these media to
receive sound waves propagating in air or in gas, and usually these media to receive sound waves
propagating in water or liquid 2. Description of the Related Art A receiving type piezoelectric
element such as a hydrophone, which is used by being placed inside, is known.
[0003]
The reception sensitivity of the sound wave in these piezoelectric elements is represented by the
hydrostatic pressure piezoelectric strain constant (dh constant) when the size of the piezoelectric
element is sufficiently smaller than the wavelength of the sound wave, and naturally the larger
the dh constant, the better the sensitivity. .
[0004]
Conventionally, this dh constant is considered to be unique to the piezoelectric body provided
with piezoelectric characteristics under certain conditions, and there is no example in which this
is grasped from the structural aspect of the element and linked to the improvement of the
sensitivity of the piezoelectric element. be equivalent to.
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[0005]
SUMMARY OF THE INVENTION An object of the present invention is to realize a high sensitivity
receiving type piezoelectric element from a certain piezoelectric material.
[0006]
According to the study of the present inventors, the surface of a piezoelectric body provided with
a recess as a through-hole from the surface to the thickness direction by embossing or the like. It
has been found that, by arranging the rigid member so as to cover, the sound pressure received
on the outer surface of the rigid member converges and concentrates on the surface of the
piezoelectric material, so that a piezoelectric element with significantly enhanced receiving
sensitivity can be realized. The
[0007]
That is, the wave receiving type piezoelectric element of the present invention has two surfaces
having a certain thickness, and at least one surface of which has a recess formed in the thickness
direction, and the recess is formed And a rigid member disposed so as to cover the surface of the
piezoelectric body so as to make the recess airtight. The sound pressure received by the outer
surface of the rigid member is converged on the surface of the piezoelectric body. It is a thing.
[0008]
In the present invention, the "recess" provided on the surface of the piezoelectric body includes a
through hole to another surface in addition to the recess in the vicinity of the surface.
In the case of a recess near the surface (non-through hole), it may be provided on either or both
of the two surfaces of interest of the piezoelectric body.
When a recess is formed only on one surface, only the one surface may be covered with a rigid
plate, but in order to prevent the bending deformation of the piezoelectric body itself which is
not normally undesirable, also in this case, the two surfaces are rigid members It is generally
desirable to cover with
[0009]
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Also, the sound wave referred to in the present invention should be interpreted as a pressure
vibration wave, and is not limited to the sound wave in the audible range.
More precisely, the sound waves of the invention are waves of pressure oscillations that are
longer than their wavelength can be compared with the size of the rigid member.
Moreover, sound pressure is the pressure of the said vibration.
[0010]
As described above, in the present invention, at least one surface is provided with a recess and
the surface is covered with the rigid member, so that the sound pressure acting on the rigid
member forms a recess in contact with the rigid member. It concentrates on the remaining
convex portion having a smaller surface area, is amplified as the sound pressure acting on the
piezoelectric body, and the piezoelectric distortion constant at the high sound pressure thus
amplified is extracted.
Also, since the piezoelectric recess is made airtight by covering it with a rigid member, in many
cases among the dh constants of the piezoelectric, the polarization direction component (d33
component of the piezoelectric, the polarization is in the thickness direction) Among the
contributions of the components (d31 component and d32 component) orthogonal to the
polarization direction, which often has an opposite action, and the contribution of the sound
pressure acting on the side surface of the piezoelectric material that causes in-plane deformation
to the piezoelectric material, It is understood that the pressure is relatively lowered by the
presence of the blocked recess, and this aspect also contributes to the apparent increase of the
dh constant (see the applicant's application filed on March 10, 1993).
The increase with the porosity of the below-mentioned piezoelectric characteristic (2) supports
this.
[0011]
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DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of the wave receiving type
piezoelectric element of the present invention will be described below with reference to the
drawings. In the drawings, the same reference numerals used in the description of different
embodiments indicate similar parts.
[0012]
FIG. 1 (a) is a plan view of an embodiment of a wave receiving type piezoelectric element
(hereinafter simply referred to as "piezoelectric element") of the present invention, and FIG. 1 (b)
is a B-B view of FIG. It is sectional drawing taken along a line. Referring to FIG. 1, this
piezoelectric element 10 has two surfaces 1a and 1b each having a certain thickness t and side
surfaces 1c substantially orthogonal to these two surfaces, respectively, on the two surfaces 1a
and 1b. The rectangular sheet-like piezoelectric body 1 in which the electrodes 5a and 5b are
formed is held by a pair of rigid plate members 2 and 3 having the same surface shape as the
piezoelectric body 1. Then, in the piezoelectric body 1 on which the surface electrodes 5a and 5b
are stacked, a large number of small through holes 4 penetrating in the thickness direction are
provided with a substantially uniform density, and the rigid plate 2 as a rigid member of the
present invention , 3 are arranged such that the through hole 4 is blocked from the influence of
the sound pressure of the received sound wave. In practice, the rigid plate members 2 and 3 are
bonded to the electrode surface, whereby the through hole 4 is an airtight internal space. In the
piezoelectric element of the present invention provided with a large number of small through
holes 4 as shown, the polarization direction of the piezoelectric body 1 is preferably the
thickness t direction.
[0013]
According to the piezoelectric element 10 of the above configuration, since the through holes 4
covered by the rigid plate members 2 and 3 form an airtight internal space, the inner surfaces 2b
and 3b of the rigid plate members at the internal space It is shielded from pressure. Therefore,
the sound pressure received by the outer surfaces 2a and 3a of the rigid plate is the rigidity of
the rigid plate because the pressure area (the area of the surface of the piezoelectric material
clamped by the rigid member) is reduced due to the presence of the through holes 4. It is
assisted and converges on the piezoelectric surfaces 1a and 1b. As a result, the stress (load)
applied to the piezoelectric body 1 is increased, and a piezoelectric element having high
sensitivity can be obtained as compared with the conventional one having no combination of the
through hole 4 and the rigid plate members 2 and 3 of the present invention. . Also, as described
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later, this sensitivity improvement is due to the contribution of the d31 component and the d32
component to the dh constant, that is, the influence of the sound pressure acting on the
piezoelectric body side surface 1c causing the in-plane deformation to the piezoelectric body 1
blocks the sound pressure. It is considered that the relative decrease is caused by the presence of
the through holes 4. In the embodiment of FIG. 1, the sound pressure converged as described
above is applied to the piezoelectric body 1 through the electrodes 5a and 5b.
[0014]
FIGS. 2 and 3 are sectional views of another piezoelectric body 21 and a piezoelectric body 31
used in place of the piezoelectric body 1 in the embodiment of FIG. That is, the piezoelectric body
21 is an example in which the non-penetrating recess 14 is provided on one surface thereof and
the convex portion 15 which is concentrated and receives the sound pressure is left. The
piezoelectric body 31 is an example in which non-penetrating recesses 14 are provided on the
two surfaces. The recess 14 is provided by embossing or the like.
[0015]
That is, as described above, in the present invention, the recess provided on at least one surface
of the piezoelectric body may be penetrated to the other surface or non-penetrated, and in any
case, the remaining recess may be formed. The concentrated sound pressure is applied in the
thickness direction of the piezoelectric body 1, 21 or 31 to extract the piezoelectric property
under the increased stress, and the effect of the sound pressure applied to the side wall direction
in the closed recess Is blocked.
[0016]
The surface shapes of the piezoelectric members 1, 21 and 31 are arbitrary, and may be other
shapes such as a circle or a polygon other than a rectangle.
Furthermore, the planar shapes of the through hole 4 and the non-through recess 14 are also
arbitrary, and may be polygonal, slit-like, or closed-loop-like grooves other than circular. Further,
as shown in FIG. 1, it is preferable to place a rigid member having the same planar shape as that
of the piezoelectric body 1 so as to face the entire surface of the piezoelectric surfaces 1a and 1b.
It may be partial as long as it includes a part of
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[0017]
In the above embodiment, air is sealed in a so-called recess to form an airtight internal space, but
unless the displacement of the piezoelectric body in the thickness direction due to the sound
pressure is substantially prevented, for example, a vacuum may be used. Alternatively, a filler
material such as other gas, elastomer resin or foamed resin having a deformation rate larger than
the compression deformation rate of the piezoelectric body can be filled therein.
[0018]
Examples of the constituent materials of the piezoelectric bodies 1, 21 and 31 are polymer
piezoelectric bodies or ceramic piezoelectric bodies such as PZT, and a hydrophone and a
microphone can be configured using them.
In particular, a polymer-based piezoelectric material is generally suitably used as a hydrophone
since the reflection of sound waves is small (the sound transmission is good) due to the
relationship between the piezoelectric material and the sound propagation medium. Alternatively,
stacked piezoelectrics may be used. The polarization direction of the piezoelectric body may be
the thickness direction as in the above embodiment, or may be the plane direction (in this case,
the electrodes are generally disposed to face the side surface of the piezoelectric body). In the
vertical direction, a larger dh constant can often be obtained.
[0019]
As the polymer-based piezoelectric material, a vinylidene cyanide-vinyl acetate copolymer having
relatively high heat resistance is suitably used, and a vinylidene fluoride-based resin piezoelectric
material having excellent piezoelectric properties is preferable, and in particular, piezoelectricity
is exhibited. VDF-based copolymers capable of β-type crystallization under ordinary
crystallization conditions (eg, superior amount as compared to vinylidene fluoride (VDF)
homopolymer which requires uniaxial stretching for β-type crystallization suitable for Of VDF
and inferior amounts of vinyl fluoride (VF), a copolymer of trifluoroethylene (TrFE) or
tetrafluoroethylene (TFE)), and further a superior amount (especially 70 to 80 mol%) of VDF and
Copolymers with inferior amounts (in particular 30 to 20 mol%) of TrFE are most preferably
used.
[0020]
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These polymer piezoelectric materials are formed into films or sheets after film formation by
melt extrusion etc., if necessary, subjected to polarization treatment by uniaxial stretching or heat
treatment at a softening temperature or less, and application of an electric field at a softening
temperature or less. .
The thickness of the polymer-based piezoelectric material used in the present invention is not
particularly limited, but is generally supplied at about 1 to 2000 μm (2 mm). In addition to
using the film or sheet described above as a single layer, it may be used as a laminate of, for
example, about 2 to 20 layers by laminating in the same direction as the polarization direction or
in the opposite direction through the intermediate electrode layer.
[0021]
The rigid member is generally formed of a hard resin material, a metal material, a porcelain
material or the like, depending on the magnitude of the pressure of the received sound wave. The
degree of rigidity of the rigid member is that the sound pressure received on the outer surface of
the rigid member is not effectively transmitted to the piezoelectric surface or it is airtight due to
the bending deformation at the site of the recess due to the sound pressure. The pressure in the
site may be in the range of stiffness where the pressure does not fluctuate following the sound
pressure. For example, in the case of a plastic material such as a vinyl chloride resin or an acrylic
resin, a rigid plate material having a thickness of 1/10, more preferably about 1/2 or more of
that of the representative dimension of the recess is preferably used. In addition, the difference in
specific acoustic impedance with the sound wave propagation medium, the relationship between
the natural frequency of the piezoelectric element including the rigid member and the frequency
of the sound wave, etc. are taken into consideration in determining the material of the rigid
member and the degree of rigidity. May be
[0022]
As the electrodes 5a and 5b, metal-sprayed electrodes as described in the specification of
Japanese Patent Application No. 3-536668 and Japanese Patent Application No. 4-158844 other
than foil electrodes attached with known vapor deposition electrodes and adhesives. A porous
sheet-like electrode embedded in the surface layer of the piezoelectric body as described in the
specification is preferably used. In the piezoelectric element 10 in which the electrodes 5a and
5b are provided on the surfaces 1a and 1b of the piezoelectric body 1, the through holes may not
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be formed in the electrodes 5a and 5b corresponding to the through holes 4. In this case, a rigid
plate electrode may be used to have the role of combining the electrodes 5a and 5b and the rigid
plate members 2 and 3.
[0023]
As shown in FIG. 1, the rigid plate members 2 and 3 are generally provided on the surface of the
piezoelectric body 1 or in the case where electrodes 5a and 5b and the like are formed thereon
by bonding or abutting on the electrode surface. However, for example, in the case where the
piezoelectric body 1 (or the electrodes 5a and 5b provided thereon) has a curved surface, the
rigid plate and the piezoelectric body surface so that the deformation stress uniformly acts on the
piezoelectric surface. Alternatively, a stress dispersion layer such as an elastomer resin may be
formed between the electrode surface and the electrode surface. An example of such an
elastomeric resin is an elastomeric resin such as silicone rubber, urethane rubber, fluoroprene
rubber, butyl rubber, or an adhesive thereof.
[0024]
In the receiving type piezoelectric element of the present invention including the example of FIG.
1 mentioned above, it has a function of focusing the sound pressure received on the outer
surface on the surface of the piezoelectric body whose pressing area is reduced due to the
presence of the recess. The rigid member of the present invention can also be regarded as a
sound pressure amplifier. In this case, one of the main parameters determining the amplification
factor is the porosity or open area ratio defined as the area ratio of the recess facing it to the
surface area of the rigid member. This porosity is generally in the range of 10 to 90%, due to a
significant increase in the sound wave reception sensitivity and the difficulty of recess formation.
[0025]
FIG. 4 (a) is a sectional view in the thickness direction of another embodiment of the wave
receiving type piezoelectric element of the present invention, and FIG. 4 (b) is a sectional view
taken along line B-B in FIG. 4 (a). is there. This piezoelectric element 20 is the same as the
piezoelectric element 10 of the embodiment of FIG. 1 except that a single through hole 4 is
formed instead of a large number of through holes.
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[0026]
Also in the piezoelectric element 20, the through holes 4 are also formed penetrating the
piezoelectric body 1 and the electrodes 5a and 5b. In the piezoelectric element 20, the
polarization direction of the piezoelectric body 1 is not limited to the thickness t direction, but
may be a surface direction perpendicular to that. The electrodes 5a and 5b do not necessarily
have to be provided on the piezoelectric surfaces 1a and 1b, and may be formed to face the side
surface 1c and the inner surface 1d of the piezoelectric body 1. However, in this case,
consideration should be given to employing a thin metal foil or a vapor deposition electrode so
that the electrode does not inhibit the displacement of the piezoelectric body 1 in the thickness
direction.
[0027]
FIG. 5 is a sectional view in the thickness direction of still another embodiment of the wave
receiving type piezoelectric element of the present invention, and corresponds to FIG. 4 (a). This
piezoelectric element 30 is the embodiment of FIG. 2 except that the piezoelectric body 1 is held
by a pair of bowl-shaped (bowl-shaped) rigid plates 12 and 13 instead of the plate-shaped rigid
plates 2 and 3. Is the same as
[0028]
In the piezoelectric elements 20 and 30 shown in FIG. 4 and FIG. 5, the through holes 4 have a
large diameter when the porosity is increased to obtain high reception sensitivity using the largearea piezoelectric body 1. However, in this case, in the piezoelectric element 20 in which the flat
plate-like rigid plate members 2 and 3 are used, as long as it is not so rigid and thick, it is due to
its bending deformation at the portion of the through hole 4 There is a problem that the pressure
received by the outer surfaces 2a and 3a of the rigid plate is not effectively transmitted to the
surface of the piezoelectric body. The piezoelectric element 30 solves this point by preventing
the bending deformation of the rigid plate by adopting the bowl-shaped rigid plates 12 and 13.
The shape of the rigid member according to the present invention does not have to be a plate
that can be recognized as having two substantially parallel surfaces, and any shape including a
bowl shape or irregularities and irregular shapes shown in FIG. It may be adopted.
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[0029]
FIG. 6 is a plan view of a piezoelectric body combination 11 used in place of the piezoelectric
body 1 in the embodiment of FIG. 4 or 5. The piezoelectric bonded body 11 is formed by joining
four strips of a strip-shaped piezoelectric body 1 alternately with an adhesive so as to form a
through hole 4 inside. Thus, the piezoelectric body used in the present invention may not be a
continuous body cut out of one piezoelectric material.
[0030]
Manufacturing Example Hereinafter, a manufacturing example and a comparative manufacturing
example of a hydrophone as a specific example of the wave receiving type piezoelectric element
of the present invention will be described.
[0031]
The produced hydrophone was determined by measuring the hydrostatic pressure piezoelectric
strain constant (dh constant) by the following method.
Immerse the sample in an insulating liquid such as silicone oil contained in a pressure-resistant
container, apply a pressure P (newton (N) / m 2) from a nitrogen gas source to the container and
charge amount Q (coulomb (C)) of the sample taking measurement. Then, the amount of increase
dQ of the charge with respect to the pressure rise dP in the vicinity of the gauge pressure 2 kg /
cm 2 was obtained and calculated by the following equation: dh = (dQ / dP) / A unit is C / N.
Here, A is the electrode area (m2).
[0032]
Comparative Example 1 A conventional sheet-like piezoelectric element was manufactured as
follows.
[0033]
First, a VDF / TrFE (75/25 molar ratio) copolymer (manufactured by Toha Chemical Industry Co.,
Ltd.) is sheet extruded at a die temperature of 265 ° C. and heat treated at 125 ° C. for 13
hours, then 75 MV / m in the sheet thickness direction Under an electric field of 125 ° C. for 5
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minutes and an elevation time including polarization time, polarization treatment was performed
for a total of 1 hour to obtain a 500 μm thick polymer piezoelectric sheet.
[0034]
Subsequently, an alumina-based abrasive having a particle size of # 220 was sandblasted on both
sides of the sheet under the conditions of an air pressure of 4.0 kg / cm 2 and a distance of 15
cm, and a 70 μm thick copper foil was coated with SBR adhesive (Sumitomo 3 M ( Co., Ltd.
<4693 Scotch grip>, affixing with a solvent 1, 2- dichloroethane 10%-20% concentration
solution) to form an electrode, and forming this electrode 5a, 5b, the planar shape 6 cm square
The sheet-like piezoelectric element (hydrophone) was manufactured by connecting the lead
wires to one of the corner portions of the both sides.
[0035]
Example 1 A piezoelectric element 10 as a hydrophone substantially as shown in FIG. 1 was
obtained as follows.
[0036]
First, in a sheet-like piezoelectric element similar to that obtained in Comparative Example 1, a
large number of through holes (diameter: 3.6 mmφ) penetrating in the thickness direction are
drilled, and subsequently, Comparative Example 1 is formed on the electrode surface. Apply the
same SBR-based adhesive as that used for electrode attachment, and have acrylic plates 2 and 3
with a thickness of 2 mm and a planar shape of 6 cm square (with a defect for lead wire
extraction at one corner of the corner Then, the piezoelectric element 10 was obtained by
pressure bonding under the conditions of preheating: 90 DEG C. for 4 minutes and
pressurization: 90 DEG C. for 4 minutes-150 kg / cm @ 2.
[0037]
The above-mentioned drilling was performed so that the density was substantially uniform on the
surface of the sheet-like piezoelectric element, and piezoelectric elements 10 having respective
void ratios (opening ratios) shown in Table 1 were produced.
In addition, the porosity was calculated | required by the weight ratio of the sheet-like
piezoelectric element before and behind drilling processing.
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[0038]
Example 2 A piezoelectric element 20 as a hydrophone substantially as shown in FIG. 4 was
obtained as follows.
[0039]
First, in the same sheet-like piezoelectric element as obtained in Comparative Example 1, a hole
having a diameter of 4 cmφ was formed by punching by aligning the center.
The adhesion of the acrylic plates 2 and 3 with the subsequent SBR-based adhesive was
performed in the same manner as in Example 1 except that the plate thickness of the acrylic
plate was 7.5 mm, and a piezoelectric element 20 with a porosity of 34.9% was produced.
[0040]
Table 1 summarizes the results of measuring the above-described piezoelectric characteristics of
various piezoelectric elements (hydrophones) obtained in this manner.
The piezoelectric characteristics (1) in Table 1 are calculated by setting the electrode area A
according to the above equation of the dh constant as the remaining electrode area A1 = A0
(100−χ) / 100 taking the porosity 空隙 (%) into consideration. The constant, the piezoelectric
characteristic (2), is a pseudo-piezoelectric constant calculated as the electrode area A0 = 36 cm
@ 2 before drilling.
The signs of the dh constants thus obtained are all minus, but in Table 1 only absolute values are
shown.
[0042]
The measurement results described above show that the piezoelectric constant of the receiving
type piezoelectric element of the present invention is an air gap relative to a blank (a
04-05-2019
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piezoelectric element in a state in which no drilling process is performed and no rigid member is
arranged, Comparative Example 1). It shows that it increases remarkably with the rate.
Furthermore, in the pseudo-piezoelectric constant calculated as the electrode area A as the
electrode area A0 before drilling, in Example 1, a significant increase is observed as the porosity
increases.
That is, as the porosity dh increases, the increase in the dh constant proportional to the stress
increase caused by the convergence of the sound pressure on the surface of the piezoelectric
body (when considering only this increase, the piezoelectric characteristic (2) is substantially
constant It is surprising for the present inventors that the above increase was observed.
[0043]
This extra increase is linear with respect to the porosity χ and the increased stress preferentially
contributes to the increase of the d33 component or, in some cases, the contribution of the d31
and d32 components to the dh constant, ie piezoelectric It is presumed that the influence of the
sound pressure acting on the side surface of the piezoelectric body which causes in-plane
deformation to the body is brought about by being limited to the peripheral portion of the
piezoelectric body by the presence of the through holes 4 where the sound pressure is blocked.
In any case, the pseudo-piezoelectric constant is theoretically equal to the value of the d33
component at a porosity of 100% (the extrapolation of the experimental value is a value slightly
lower than the value of d33 of about -40 pC / N). Although shown). The inventors of the present
invention have previously shown, in an application dated March 10, 1993, that a piezoelectric
performance close to the d33 constant can be obtained by arranging a sound pressure blocking
member opposite to the side surface of the piezoelectric body. However, according to the present
invention, it can be said that the same is realized in the pseudo piezoelectric constant. However,
in Example 2 in which a large hole is provided, a slight decrease is observed in the pseudopiezoelectric constant. The cause of this decrease is that the pressure received on the outer
surface of the rigid member is not sufficiently transmitted to the surface of the piezoelectric
member due to the bending deformation of the acrylic plate which is the rigid member of the
present invention holding the piezoelectric member. Conceivable. In this regard, it is possible to
improve by using a rigid member having an overall flexural deformation stiffness as shown in
FIG.
[0044]
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As described above, according to the present invention, the recess is provided in a piezoelectric
body having two surfaces having a certain thickness and having a recess formed on at least one
surface. By covering the surface with a rigid member and making the recess airtight, piezoelectric
characteristics under application of increased sound pressure are extracted, and a reception type
piezoelectric element with improved reception sensitivity can be obtained.
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