JP2011217159A - Laminated piezoelectric body and ultrasonic transducer using the same, and method of manufacturing laminated piezoelectric body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a harmonic component, and at the same time, attenuates a fundamental wave component when transmitting/receiving the harmonic wave using a multilayer piezoelectric body.SOLUTION: When transceiving a third harmonic wave using the three-layer piezoelectric body, the three layers are not laminated arranging a polarization direction of the three layers in order, while letting a direction of a part be inverse direction, and electrode wiring is devised. As a concrete target, electrodes of separated sides of the adjoining piezoelectric bodies are connected each other, and respectively connected to two terminals. When making a direction of residual polarity of the piezoelectric body 1 (+P) as a reference, if the piezoelectric body 2 is of the same direction (+P), and the piezoelectric body 3 is of the inverse direction in parallel (-P), sensitivity is negated for λ/4 resonance, and enhanced for 3λ/4 resonance in an electric displacement between terminals. In the way, it is possible to suppress the fundamental wave, and extract the third harmonic waves without using a filter or an amplifier.

Description

  The present invention relates to an ultrasonic transducer used as a probe or the like of an ultrasonic diagnostic apparatus, and a piezoelectric material used therefor, in particular, a laminated piezoelectric material formed by laminating a plurality of layers, and a method for producing the same.

  In general, a piezoelectric body is used for an ultrasonic transducer. This is because the piezoelectric body converts mechanical energy into electrical energy, and vice versa, and has a so-called electric system-mechanical coupling action. The piezoelectric body used is in the form of a sheet, plate or rod provided with a pair of electrodes, one electrode is fixed to the back layer, and the other electrode is in contact with the medium via the acoustic lens or matching layer.

Many piezoelectric ultrasonic transducers radiate sound waves to the medium or detect sound waves propagating to the medium in the d ₃₃ mode or e ₃₃ mode. The d ₃₃ mode is generally called longitudinal vibration of the columnar vibrator, and the e ₃₃ mode is generally called thickness vibration of the plate vibrator. In ferroelectric materials such as PZT ceramics and PVDF, high dielectric materials such as P (VDCN / VAc), and porous polymer electret piezoelectric materials, the residual polarization is maintained by the orientation of the electric dipole by the poling process, and d ₃₃ and e ₃₃ are set. Show. On the other hand, in the case of a piezoelectric crystal having no remanent polarization, if the piezoelectric crystal such as ZnO, LiNbO ₃ , KNbO ₃ is oriented perpendicularly to the electrode surface with the C-axis in the case of quartz crystal or the A-axis in the case of quartz, d ₃₃ And e ₃₃ (d ₁₁ and e _{11 for} quartz). The piezoelectric composite material depends on the material used.

  Here, in the piezoelectric body constituting the ultrasonic transducer, the simplest mechanical boundary condition is a case where one end is a fixed end and the other end is a free end. Theoretically, the acoustic impedance Z (unit is MRayl.) Of the contacting object and the boundary condition have a relation that Z = 0 is a free end and Z = ∞ is a fixed end. It is not strict, and with the exception of the adhesive layer and electrode layer, the impedance Z of the piezoelectric material is regarded as a fixed end when the impedance Z is small or equivalent, and a free end when it is large. In addition, the resonance of the longitudinal vibration or the thickness vibration of the piezoelectric body is used for the transmission and reception of the ultrasonic transducer, and the resonance frequency fr depends mainly on the structure of the transducer and the pressing to the medium. It is determined by the physical properties and dimensions of the piezoelectric body. Therefore, in the present specification, factors that change the resonance frequency other than the dimensions and properties of the piezoelectric body are excluded.

First, the resonance frequency fr in the d ₃₃ mode and e ₃₃ mode of the piezoelectric body is determined from the sound velocity v and the height (thickness) h of the piezoelectric body.
fr = v / 4h (1)
It becomes. This is generally referred to as λ / 4 resonance. λ means the wavelength in the piezoelectric body. In addition, there is a λ / 2 resonance in which both ends are free. The resonance frequency is ½ of λ / 4 resonance.

On the other hand, the sound velocity v of the piezoelectric body is the longitudinal vibration of the columnar vibrator.
v = (1 / sρ) ^1/2 (2)
In the thickness vibration of the plate-shaped thickness vibrator,
v = (c / ρ) ^1/2 (3)
It becomes. Here, s is elastic compliance, c is elastic stiffness, and ρ is density.

  Thus, it is understood from the above formulas (1) to (3) that the transmission and reception frequencies of the transducer are mainly determined by the height (thickness) h, the elastic modulus s, and the density ρ of the piezoelectric body. .

  In the field of non-destructive image inspection using ultrasonic waves, such as the medical field and the architectural field, it is required to increase the frequency of the transducer and improve the transmission / reception performance in order to obtain a higher resolution image. In an ultrasonic transducer using a piezoelectric body, electrical impedance matching between the transducer and the electrical processing circuit is an important factor for transmitting an electrical signal at a high S / N ratio in order to improve transmission / reception performance. is there. In addition, when the frequency is increased, since the transmission / reception frequency is determined by the thickness of the piezoelectric body, it is necessary to make the piezoelectric body thinner. Since the piezoelectric thin film works in the direction of lowering the electrical impedance, it works favorably for impedance matching with the electric circuit, but the lowering width is at most the reciprocal of the thickness ratio. In addition, the thinning of the piezoelectric body makes the manufacturing process such as film thickness control and handling difficult.

Therefore, in the prior art, the harmonic component in the transmission / reception signal of the conventional λ / 4 resonant transducer is used for the purpose of obtaining a high-frequency signal. However, the harmonic component has a sensitivity lower than that of the fundamental wave component, and is easily attenuated by the damping of the piezoelectric body and the surrounding material, so that there is a problem that it is difficult to obtain a signal with a high S / N ratio. Therefore, as an example of transmitting and receiving ultrasonic waves using the harmonic, with reference to FIG. 1, described e ₃₃ thickness stretching mode. This FIG. 1 and the following description are shown in Non-Patent Document 1. The constants of the elements constituting the equivalent circuit of FIG.
C _n = p _n k _t ² C ₀ (4)
L = 1 / ω _p1 ² C ₁ (5)
p _n = (1 / n ² ) (8 / π ² ), n = 2m−1 (6)
It is. Here, C _n is the capacitance of the elements, L is the inductance, k _t is the electromechanical coupling coefficient of thickness stretching mode, the omega _p1 is the resonant frequency.

In the above equation (6), if approximated as p _n ≈1 / n ² , equation (4) becomes
^{_{C n / C 0 = k t}} 2 / n 2 ... (7)
It becomes. Equation (7) shows that the effective value of the electromechanical coupling coefficient at the nth harmonic is reduced to 1 / n. In the case of the primary mode, since n = 1, equation (7) is
_{C n = 1 / C 0 =} k t 2 ... (8)
It becomes. This equation, relation between _{k t} and a dielectric constant in the first order mode _{^{ε T / ε S = 1 +}} k t 2 ... (9)
Therefore, ε ^T = C ₀ + C _n , ε ^S = C ₀ , which is in agreement with the above equation. The epsilon ^S dielectric constant of the dielectric constant ,, epsilon ^T free conditions constraints, and C ₀ and C _n is the capacitance. For the d ₃₃ mode, the above equation (4) is
C _n = p _n (k ₃₃ ² / 1−k ₃₃ ² ) C ₀ (10)
The same result can be obtained by replacing.

The effective value of the electromechanical coupling coefficient when transmitting and receiving the third harmonic is given by n = 3 from Equation (7). If the apparent coupling coefficient is k _t ′, k _t '= K _t / n = k _t / 3. This result means that the apparent coupling coefficient is attenuated to 1/3 when transmitting and receiving the third harmonic.

FIG. 2 is a graph showing the frequency characteristic (calculated value) of the complex dielectric constant of a piezoelectric body exhibiting a primary mode of thickness resonance at 1 MHz. However, kt = 0.3, h / 2v = 2.485 × 10 ⁻⁷ (s), tan δ _m = 0.04.

The maximum / minimum of the real part (indicated by reference numeral α1) and the maximum of the imaginary part (indicated by reference numeral α2) seen at 1 MHz are due to the primary mode of thickness resonance. Thereafter, a third harmonic component is observed at 3 MHz and a fifth harmonic component is observed at 5 MHz. On the other hand, as shown in FIG. 3, when the third harmonic component shown in FIG. 2 is applied with a piezoelectric model having 3 MHz as the primary mode, the coupling coefficient and the thickness of the piezoelectric body are set to 1/3. Matched. These results are consistent with the above interpretation. FIG. 3 is a graph showing the frequency characteristic (calculated value) of the complex dielectric constant of a piezoelectric body exhibiting thickness resonance. However, the broken lines are kt = 0.3, h / 2v = 2.485 × 10 ⁻⁷ (s), tan δ _m = 0.04 as described above. On the other hand, the solid lines are kt = 0.1, h / 2v = 8.300 × 10 ⁻⁷ (s), tan δ _m = 0.04.

  As described above, the problems of the prior art are that, when detecting harmonics, the apparent electromechanical coupling coefficient is reduced to 1 / n and the electrical impedance is uniquely determined by the size of the piezoelectric body. There is to be.

  On the other hand, in a medical ultrasonic diagnostic apparatus, tissue harmonic imaging (THI) diagnosis using harmonic signals is a standard diagnostic modality because a clear diagnostic image that cannot be obtained by conventional B-mode diagnosis is obtained. It's getting on. If the frequency used is higher as in this harmonic imaging, the sidelobe level will be lower, the S / N will be better, the contrast resolution will be better, the beam width will be narrower and the lateral resolution will be better. Is small, and since there is little variation in sound pressure, it has many advantages such as no multiple reflection.

  Therefore, in Patent Document 1, the signals received by the piezoelectric elements of the ultrasonic transducer are added by the phasing addition circuit, and then input to the fundamental band filter and the harmonic band filter in common. Ultrasound diagnosis in which the output is weighted with a gain corresponding to the depth of the diagnostic region of the subject and then combined to interpolate the attenuation of harmonic components in the deep diagnostic region with the fundamental wave A device has been proposed. That is, when receiving harmonics, the decrease in the electromechanical coupling coefficient is compensated by using a filter and an amplifier.

  Similarly, in Patent Document 2, a fundamental piezoelectric element is formed by laminating a harmonic piezoelectric element on a fundamental wave piezoelectric element, radiating a transmission ultrasonic wave from the fundamental wave piezoelectric element, and receiving it by the fundamental wave piezoelectric element. Diagnose the signal component of the wave by passing the multiple harmonic components received by the piezoelectric element for harmonics through the band-pass filter and extracting the desired component, then adjusting the gain individually and adding them. There has been proposed an ultrasonic diagnostic apparatus that obtains a signal corresponding to the depth of a region.

JP 2002-11004 A Japanese Patent No. 4192598

Basics of Piezoelectric Materials by Takuro Ishida Ohmsha

  However, in the above-described conventional technology, it is necessary to insert filters and amplifiers in signal paths from a large number of piezoelectric elements.

  In addition, it is preferable to use an organic material such as PVDF for receiving a signal having a high frequency as compared with an inorganic material such as PZT. However, an inorganic material has a high dielectric constant, and therefore has a large capacitance and a low electrical impedance, so matching with a subsequent circuit is relatively easy, whereas an organic material has a low dielectric constant, For this reason, since the capacitance is low and the electrical impedance is high, there is a problem that matching with a subsequent circuit is difficult.

  An object of the present invention is to make it possible to make the output sound pressure at the time of transmitting a desired harmonic component or the output voltage at the time of receiving a wave higher than those in the primary mode, and to reduce the electrical impedance. Another object of the present invention is to provide a multilayer piezoelectric body that can be produced, an ultrasonic transducer using the same, and a method for producing the multilayer piezoelectric body.

  The multilayer piezoelectric body of the present invention is formed by laminating a plurality of piezoelectric bodies having the same thickness, and transmits and receives ultrasonic waves of the third harmonic component due to 3λ / 4 resonance generated by the thickness expansion and contraction of the piezoelectric body. The piezoelectric body is laminated in three layers, and has electrodes on the surface of the piezoelectric body between the layers and at both ends, and by connecting the electrodes on the separation side in the piezoelectric bodies adjacent to each other, The piezoelectric body includes two sets of interconnecting wires that connect the piezoelectric bodies in parallel to each other, and each piezoelectric body has an electric displacement due to a piezoelectric positive effect or a remanent polarization direction or a crystal axis related to a sign of an electric field, and a first end on the fixed end side. With reference to the axis of the piezoelectric element at the stage, the second stage piezoelectric element in contact therewith is arranged in the same direction, and the third stage piezoelectric element above it is arranged in the opposite direction. To do.

  The method for producing a laminated piezoelectric material according to the present invention is formed by laminating a plurality of piezoelectric materials having the same thickness, and the ultrasonic wave of the third harmonic component due to the 3λ / 4 resonance generated by the thickness expansion and contraction of the piezoelectric material. A method for producing a laminated piezoelectric material for transmitting and receiving waves, wherein three layers of the piezoelectric material are laminated, and electrodes formed on the surface of the piezoelectric material between layers and at both ends of each piezoelectric material are separated from each other in a piezoelectric material adjacent to each other. By connecting the electrodes of each of them with a connection wiring, the piezoelectric bodies are connected in parallel to each other, and the direction of the remanent polarization or the crystal axis related to the electrical displacement or the sign of the electric field due to the piezoelectric positive effect, Using the axis of the first-stage piezoelectric body on the fixed end side as a reference, the second-stage piezoelectric body in contact therewith is arranged in the same direction, and the third-stage piezoelectric body above it is arranged in the opposite direction. It is characterized by that.

  According to the above configuration, it is used as an ultrasonic transducer of an ultrasonic diagnostic apparatus, and is formed by stacking a plurality of piezoelectric bodies having the same thickness, and transmitting and receiving ultrasonic waves by expanding and contracting the thickness of the piezoelectric bodies. In the multilayer piezoelectric body and the method for producing the multilayer piezoelectric body that perform at least one of the above, in the present invention, when transmitting and receiving the ultrasonic wave of the third harmonic component due to 3λ / 4 resonance, the number of piezoelectric bodies stacked is three and each piezoelectric element is stacked. The bodies are connected in parallel, and in a predetermined manner (according to a certain rule), some of the piezoelectric bodies are laminated with their front and back reversed.

  Specifically, first, among the electrodes formed on the surface of the piezoelectric material between the layers and at both ends of each piezoelectric material laminated in the thickness direction as described above, the electrodes on the separation side of the piezoelectric materials adjacent to each other are arranged. Each piezoelectric body is electrically connected in parallel by being connected by two sets of connecting wires.

Next, by setting the number of stacked layers to 3, the nodes and antinodes of the elastic waves propagating in the piezoelectric body can be made to coincide with the boundary surface of the piezoelectric body, and the laminated piezoelectric body at this time Paying attention to the distortion distribution of the third harmonic component, the distortion of each piezoelectric body is reversed in phase by 180 degrees without changing the absolute value, and when the piezoelectric bodies are connected in parallel as described above, for example, the base end (rear side) If the distortion of the piezoelectric material on the (layer) side is +, “+, +, −” is obtained. Therefore, as the predetermined mode, the residual polarization of each piezoelectric body or the orientation of the C-axis or A-axis of the crystal (the axes that determine the signs of d ₃₃ , e ₃₃ , d ₁₁ , e ₁₁ ) The piezoelectric body at the mismatched portion (corresponding to the sign of “−”) is reversed and laminated so as to match the sign of displacement and electric field. In the above case, with respect to the axis of the first-stage piezoelectric body (in contact with the back layer) on the fixed end side, the second-stage piezoelectric body in contact with the axis is in the same direction, and the third-stage piezoelectric body thereon The body arranges in the opposite direction.

  Thereby, each piezoelectric body in the laminated piezoelectric body is made to coincide with the electric displacement due to the piezoelectric positive effect and the sign of the electric field, and the third harmonic is compared with the case of λ / 4 resonance without using a filter or an amplifier. The output sound pressure at the time of component transmission or the output voltage at the time of wave reception can be increased, and the signal of the primary mode can be attenuated. In addition, the electrical impedance of the piezoelectric body can be reduced to the parallel number, that is, 1/3, which is advantageous for a piezoelectric body having a low dielectric constant and a small capacitance such as an organic piezoelectric body.

  Furthermore, the laminated piezoelectric material of the present invention is formed by laminating two layers of piezoelectric materials having the same thickness, and transmitting and receiving ultrasonic waves of the third harmonic component due to 3λ / 4 resonance generated by the thickness expansion and contraction of the piezoelectric material. A laminated piezoelectric body that swells, and each piezoelectric body has electrodes on its interlayer and each outer surface, and the electrodes on the outer surface are connected to each other to connect the piezoelectric bodies in parallel with each other. The piezoelectric elements are provided with wirings, and the two piezoelectric bodies are arranged so that the directions of remanent polarization or crystal axes related to the electric displacement due to the piezoelectric positive effect or the sign of the electric field are in the same direction.

  In the method for producing a laminated piezoelectric material according to the present invention, two layers of piezoelectric materials having the same thickness are laminated, and the ultrasonic wave of the third harmonic component due to 3λ / 4 resonance generated by the thickness expansion and contraction of the piezoelectric material. A method for producing a laminated piezoelectric body that transmits and receives a wave, wherein the electrodes on the outer surface of each piezoelectric body are connected to each other by connecting wirings. Are connected in parallel to each other, and the two piezoelectric bodies are arranged so that the directions of remanent polarization or crystal axes related to the electric displacement due to the piezoelectric positive effect or the sign of the electric field are in the same direction.

  According to the above configuration, it is used as an ultrasonic transducer of an ultrasonic diagnostic apparatus, and is formed by laminating two layers of piezoelectric bodies having the same thickness, and transmitting and receiving ultrasonic waves by expanding and contracting the thickness of the piezoelectric bodies. In the laminated piezoelectric material that performs at least one of the above and a manufacturing method thereof, in the present invention, when transmitting and receiving an ultrasonic wave of the third harmonic component due to 3λ / 4 resonance, the two piezoelectric layers are connected in parallel, In addition, with respect to the two layers of piezoelectric bodies, the directions of remanent polarization or crystal axes are stacked in parallel with each other.

  Specifically, first, the electrodes on the outer surface among the electrodes formed on the interlayer and the outer surface of the piezoelectric body laminated in two layers in the thickness direction as described above are connected by connection wiring, thereby forming two layers. The piezoelectric body is electrically connected in parallel.

  Next, when the two-layer piezoelectric body is caused to resonate at 3λ / 4, the nodes and antinodes of the elastic waves propagating in the two-layer piezoelectric body do not coincide with the boundary surface of the two-layer piezoelectric body. When attention is paid to the strain distribution of the third-order harmonic component in the multilayer piezoelectric body at this time, a difference occurs in the absolute value of the strain of the piezoelectric material of the two layers, and the phase is inverted by 180 degrees, so that the base end (rear side) If the distortion of the piezoelectric material on the (layer) side is +, “+, −” is obtained. However, if the distortions are added in parallel connection, the polarity of 3λ / 4, that is, the third harmonic can be reversed and a large gain can be obtained, but the fundamental wave can be attenuated. .

Therefore, the remanent polarization of the two layers of piezoelectric materials or the directions of the C-axis and A-axis of the crystal (the axes that determine the signs of d ₃₃ , e ₃₃ , d ₁₁ , e ₁₁ ) are in the same direction as described above. By arranging in such a manner, the output sound pressure at the time of transmission of the third harmonic component by the 3λ / 4 resonance or at the time of reception can be compared with the case of λ / 4 resonance without using a filter or an amplifier. The output voltage can be increased and the primary mode signal can be attenuated. Further, since the number of parallel piezoelectric elements, that is, the number of stacked layers is 2, the electrical impedance can be reduced to ½, which is advantageous for a piezoelectric body having a low dielectric constant and a small capacitance such as an organic piezoelectric body.

  Furthermore, the laminated piezoelectric material of the present invention is formed by laminating four or more layers of piezoelectric materials having the same thickness, and has electrodes on the surfaces of the layers and at both ends of the piezoelectric material. By connecting the electrodes on the separation side, two sets of connecting wirings that connect the stacked piezoelectric bodies in parallel with each other are provided, and each of the piezoelectric bodies has a residual polarization related to an electric displacement due to a piezoelectric positive effect or a sign of an electric field. The direction or crystal axis of the second-stage piezoelectric body in contact with the first-stage piezoelectric body on the fixed end side is the same direction, and the third and fourth-stage piezoelectric elements on the same direction. The body is arranged so as to have periodicity in the opposite direction.

  The method for producing a laminated piezoelectric material according to the present invention is a method for producing a laminated piezoelectric material comprising four or more layers of piezoelectric materials having the same thickness. In the electrodes formed on the surfaces of the piezoelectric bodies at both ends, the plurality of piezoelectric bodies are connected to each other in parallel by connecting the electrodes on the separation side of the piezoelectric bodies adjacent to each other by a connection wiring. The direction of remanent polarization or the crystal axis related to the electrical displacement due to the positive piezoelectric effect or the sign of the electric field is the same for the second-stage piezoelectric body in contact with the axis of the first-stage piezoelectric body on the fixed end side. The piezoelectric elements in the third direction and the fourth stage above the direction are arranged so as to have periodicity in the opposite direction.

  According to the above configuration, it is used as an ultrasonic transducer of an ultrasonic diagnostic apparatus, and is formed by stacking a plurality of piezoelectric bodies having the same thickness, and transmitting and receiving ultrasonic waves by expanding and contracting the thickness of the piezoelectric bodies. According to the present invention, when transmitting and receiving ultrasonic waves in the third and higher resonance modes, four or more layers of piezoelectric bodies are stacked and the piezoelectric bodies are connected in parallel. In a predetermined manner (according to a certain rule), some piezoelectric bodies are laminated with the front and back reversed.

  Specifically, first, among the electrodes formed on the surface of the piezoelectric material between the layers and at both ends of each piezoelectric material laminated in the thickness direction as described above, two electrodes on the separation side in the piezoelectric materials adjacent to each other are provided. Each piezoelectric body is electrically connected in parallel by being connected by a set of connecting wires.

Next, when the number of stacked layers is the order number of the resonance mode or an integer multiple thereof, the nodes and antinodes of the elastic wave propagating in the piezoelectric body can be made to coincide with the boundary surface of the piezoelectric body. Focusing on the strain distribution of the desired harmonic component in the multilayer piezoelectric body, the phase of the distortion of each piezoelectric body is inverted 180 degrees without changing the absolute value, and the piezoelectric bodies are connected in parallel as described above. For example, if the distortion of the piezoelectric material on the base end (back layer) side is set to +, the periodicity of “+, +, −, −” is repeated every four layers. Therefore, as the predetermined mode, the residual polarization of each piezoelectric body or the orientation of the C-axis or A-axis of the crystal (the axes that determine the signs of d ₃₃ , e ₃₃ , d ₁₁ , e ₁₁ ) The piezoelectric body at the mismatched portion (corresponding to the sign of “−”) is reversed and laminated so as to match the sign of displacement and electric field. In the above case, with respect to the axis of the first-stage piezoelectric body on the fixed end side (in contact with the back layer), the second-stage piezoelectric body in contact therewith is in the same direction, and the third and fourth stages thereon In the piezoelectric body of the eye, they are arranged in the opposite direction. When the piezoelectric bodies are stacked by an integral multiple of the order, the piezoelectric bodies are arranged so as to have periodicity in the same direction, the same direction, the reverse direction, and the reverse direction thereafter for every four layers of piezoelectric bodies.

  Thereby, each piezoelectric body in the laminated piezoelectric body is made to coincide with the electric displacement or the sign of the electric field due to the piezoelectric positive effect, and, for example, 5λ / 4, compared to the case of λ / 4 resonance without using a filter or an amplifier. The output sound pressure at the time of transmission of the resonance frequency component or the output voltage at the time of reception can be increased, and the signal of the primary mode can be attenuated. Further, when the number of piezoelectric bodies in parallel, that is, the number of stacked layers is n, the electrical impedance can be reduced to 1 / n, which is advantageous for a piezoelectric body having a low dielectric constant such as an organic piezoelectric body and a small capacitance.

  Furthermore, in the laminated piezoelectric material of the present invention, each of the piezoelectric materials is further divided into two pieces in the thickness direction, and those obtained by connecting them in parallel to each other are laminated as a set, and the divided piezoelectric material is divided. With respect to each of the bodies, the direction of the remanent polarization or the crystal axis is inverted so that the sign of the electric displacement or electric field in the strain distribution in the multilayer piezoelectric body coincides.

  Further, in the method for producing a laminated piezoelectric material according to the present invention, each piezoelectric material is further divided into two pieces in the thickness direction, and those obtained by connecting them in parallel with each other are laminated as a set and divided. For each of the piezoelectric bodies, the direction of the remanent polarization or the crystal axis thereof is reversed so that the sign of the electric displacement or electric field in the strain distribution in the multilayer piezoelectric body coincides.

  According to said structure, each piezoelectric material used as the thickness of the said vibration node or abdomen is divided | segmented into 2 sheets, and what was mutually connected in parallel is handled as 1 set.

  Therefore, the electrical impedance can be further halved, and the electrodes at both ends can be at the same potential, and the entire laminated piezoelectric body can be electrically shielded.

  Furthermore, the ultrasonic transducer of the present invention is characterized by using the laminated piezoelectric material.

  According to the above configuration, it is possible to increase the output sound pressure at the time of transmitting a harmonic component or the output voltage at the time of reception, compared to the case of λ / 4 resonance, without using a filter or an amplifier. At the same time, it is possible to realize an ultrasonic transducer that can attenuate the signal in the first-order mode and further reduce the electrical impedance.

  As described above, the multilayer piezoelectric body, the ultrasonic transducer using the multilayer piezoelectric body, and the method for producing the multilayer piezoelectric body of the present invention perform expansion and contraction in the thickness direction when transmitting and receiving ultrasonic waves of a desired harmonic component. A multilayer piezoelectric ultrasonic transducer is formed by laminating a plurality of piezoelectric bodies in the thickness direction, and when the piezoelectric bodies are connected in parallel, paying attention to the distortion distribution of the harmonic component in the multilayer piezoelectric body, A part of the remanent polarization of the piezoelectric body or the direction of the C-axis or A-axis of the crystal is arranged in the reverse direction.

  Therefore, it is possible to increase the output sound pressure at the time of transmitting a desired harmonic component or the output voltage at the time of reception without using a filter or an amplifier, as compared with the case of λ / 4 resonance. The next mode signal can be attenuated. Further, the electrical impedance can be reduced to 1 / n according to the number n of parallel piezoelectric bodies.

It is an equivalent circuit diagram in the thickness expansion and contraction mode of the piezoelectric body. It is a graph which shows the frequency characteristic of the complex dielectric constant of the piezoelectric material which shows the primary mode of thickness resonance at 1 MHz. It is a graph which shows the frequency characteristic of the complex dielectric constant of the 3rd harmonic component in the piezoelectric material shown in the said FIG. 2, and the piezoelectric material which shows the 1st mode of thickness resonance in 3 MHz. FIG. 3 is a schematic cross-sectional view of a λ / 4 resonance state in a three-layer piezoelectric transducer. FIG. 5 is a schematic cross-sectional view of a 3λ / 4 resonance state in the three-layer piezoelectric transducer shown in FIG. 4. It is a typical sectional view of an n layer piezoelectric transducer. It is a figure which shows typically the displacement and distortion at the time of exciting or detecting an nth-order harmonic with the n layer piezoelectric transducer shown in FIG. FIG. 7 is a diagram showing the relationship between the residual polarization of each piezoelectric body when detecting nth-order harmonics or the orientation of the C-axis and A-axis of the crystal and the electrical displacement due to the piezoelectric positive effect in the multilayered piezoelectric body shown in FIG. It is. It is a figure which shows typically the structure of the 2n layer piezoelectric transducer for detecting an nth-order harmonic. FIG. 9 is an example of a detailed embodiment according to the concept of the present invention shown in FIG. 8, and is a cross-sectional view schematically showing the structure of a two-layer piezoelectric transducer. It is a figure for demonstrating the displacement and distortion at the time of the harmonic transmission / reception in the two-layer piezoelectric transducer shown in FIG. 12 is a diagram illustrating the relationship between the direction of remanent polarization of each piezoelectric body and the electrical displacement due to the piezoelectric positive effect when detecting the third harmonic in the multilayered piezoelectric body shown in FIGS. 10 and 11. FIG. It is a graph which shows an experimental data and a simulation result about the transmission / reception characteristic of the ultrasonic wave by the two-layer piezoelectric material shown in FIG. 10, and its comparative example. FIG. 8 is another example of the detailed embodiment according to the concept of the present invention shown in FIGS. 8 and 5 and is a cross-sectional view schematically showing the structure of a three-layer piezoelectric transducer. It is a figure for demonstrating the displacement and distortion at the time of harmonic transmission / reception in the three-layer piezoelectric transducer shown in FIG. FIG. 16 is a diagram illustrating the relationship between the direction of remanent polarization of each piezoelectric body and the electrical displacement due to the piezoelectric positive effect when the third harmonic is detected in the multilayered piezoelectric body shown in FIGS. 14 and 15. It is a graph which shows an experimental data and a simulation result about the transmission / reception characteristic of the ultrasonic wave by the 3 layer piezoelectric material shown in FIG. It is a graph which shows an experimental data and a simulation result about the ultrasonic wave transmission / reception characteristic by the comparative example of the three-layer piezoelectric material shown in FIG. FIG. 10 is still another example of the detailed embodiment according to the concept of the present invention shown in FIG. 8, and is a cross-sectional view schematically showing the structure of a six-layer piezoelectric transducer. It is a figure which shows the relationship between the direction of the remanent polarization of each piezoelectric material at the time of detecting a 3rd harmonic in the laminated piezoelectric material shown in FIG. 19, and the electric displacement by a piezoelectric positive effect. It is a graph which shows a simulation result about the transmission / reception characteristic of the ultrasonic wave by the 6 layer piezoelectric material shown in FIG.

  First, before explaining detailed embodiments of the present invention, the concept of the present invention will be described. The present invention is based on the knowledge of the inventor that the third and higher harmonics can be efficiently transmitted and received by laminating piezoelectric bodies having the same thickness according to a certain rule. Many methods for laminating piezoelectric materials have been reported so far, but the present invention focuses on the fact that there is a strain distribution in the piezoelectric material when harmonics are transmitted and received.

As an example, a λ / 4 vibrator in which three piezoelectric bodies A1, A2, and A3 shown in FIG. In this excited state at λ / 4 of the λ / 4 vibrator, the three piezoelectric bodies A1, A2, A3 are expanded and contracted synchronously, and the expansion and contraction of the maximum ΔZ is performed as a whole. If the coordinate of the back surface of the first layer of piezoelectric material A1 that is smaller than the fixed end, ie, 40 MRayl. But is in contact with the back layer having a sufficiently large acoustic impedance, is z ₀ , the displacement of the position accompanying the expansion and contraction is , Z ₀ = ΔZSsin0 ° = 0. On the other hand, the displacement of the coordinate z _{1 on} the surface of the piezoelectric material A1 of the first layer is z ₁ = ΔZSsin30 ° = 0.5ΔZ, and the displacement of the coordinate z _{2 on} the surface of the piezoelectric material A2 of the second layer is z ₂ = ΔZSsin 60 ° = 0.85ΔZ, and the displacement of the coordinate z _{3 on} the surface of the piezoelectric layer A3 of the third layer in contact with the free end, that is, the space having a sufficiently small acoustic impedance although it is larger than 0 is z ₃ = ΔZSsin90 ° = 1.0ΔZ. Here, the front and back sides of the piezoelectric bodies A1, A2, and A3 are formed by pressing the piezoelectric bodies in the thickness direction to generate a positive voltage, and the reverse side is generating a negative voltage.

  That is, in the case of the three-layer piezoelectric bodies A1, A2 and A3 in FIG. 4, the piezoelectric body A1 on the fixed end side of the entire expansion and contraction ΔZ is subjected to the expansion and contraction of 0.5ΔZ, and the second-layer piezoelectric body A2 is , 0.35ΔZ, and the third-layer piezoelectric body A3 expands and contracts only 0.15ΔZ. As described above, in the multilayer piezoelectric body, at the fundamental wave λ / 4 resonance, each piezoelectric body A1, A2, A3 expands and contracts synchronously (in the same direction), but each piezoelectric body A1, A2, A3 has one. It does not expand and contract in a similar manner, but has a non-uniform strain distribution.

On the other hand, when a similar multilayer piezoelectric body is caused to resonate at 3λ / 4, the result is as shown in FIG. That is, the coordinate z ₀ of the back surface of the first-layer piezoelectric body A1 is z ₀ = ΔZ sin 0 ° = 0, and the displacement of the surface coordinate z ₁ of the first-layer piezoelectric body A 1 is z ₁ = ΔZ sin 90 ° = The displacement of the coordinate z _{2 on} the surface of the piezoelectric material A2 of the second layer is z ₂ = ΔZ sin 180 ° = 0, and the displacement of the coordinate z ₃ of the surface of the piezoelectric material A3 of the third layer is z ₃ = ΔZ sin 270. ° = −ΔZ. Accordingly, the first-layer piezoelectric body A1 extends by ΔZ, while the second-layer and third-layer piezoelectric bodies A2 and A3 have a contraction of ΔZ.

Therefore, the present inventor pays attention to such non-uniform strain distribution, and remanent polarization (ferroelectric material such as PZT and PVDF) of each piezoelectric material, or the orientation of the C-axis and A-axis of a crystal (such as quartz) ( d ₃₃ , e ₃₃ , d ₁₁ , the axis that determines the sign of e ₁₁ ) corresponds to the location of the mismatch (sign of “−” so that the electrical displacement and the sign of the electric field in the distortion distribution at the time of harmonic transmission and reception match. The piezoelectric body was laminated with the front and back reversed. In the case of FIGS. 4 and 5, as shown by the arrows on the left side, the second and third layer piezoelectric bodies A2 and A3 are different from the first layer piezoelectric body A in the direction of remanent polarization or crystals. Laminate so that the axis is in the opposite direction. Thereby, the component of the fundamental wave λ is removed as 0.5 · ΔZ + 0.35 · (−ΔZ) + 0.15 · (−ΔZ) = 0, and at the same time, the component of the third harmonic 3λ is 1 · ΔZ + (− 1) · (−ΔZ) + (− 1) · (−ΔZ) = 3ΔZ can be extracted.

  On the other hand, with reference to FIG. 6, a case where an nth-order harmonic is transmitted and received in a λ / 4 vibrator in which n (n is an integer of 4 or more) piezoelectric bodies are stacked will be described. In order to simplify the model, one end of the laminated film was fixed to the base and the other end was a free end. The influence of the thickness of the adhesive layer, such as the impedance matching layer and the back (backing) layer, is excluded.

FIG. 7 is a diagram schematically showing displacement and distortion when the n-th order harmonic is excited or detected by the n-layer piezoelectric transducer shown in FIG. (A) is the state of lamination, (b) is the displacement of each layer at a certain moment, and (c) is the strain polarity. Similar to FIGS. 4 and 5, the boundary surface between the base and the piezoelectric body in contact with the base is the origin z ₀ , and the coordinates in the height (thickness) direction of the element are z ₁ , z ₂ , z ₃ ,. , Z _n . In the laminated piezoelectric body, the displacement of each piezoelectric layer forms a sine wave having the origin z0 at the boundary between the back layer and the first-stage piezoelectric body 1.

In general, when an n-th harmonic (n ≧ 1) is excited in the thickness direction, the displacement ξ (z) at the height z is
ξ (z, t) = ξ ₀ sin (nπ / 2 · z / h) (cosnω _r t + θ) (11)
(Fundamental physics selection book 8: vibration and vibration by Masataka Ariyama 裳 華 房). Here, ω _r is the resonance frequency 2πf _r , θ of the laminated piezoelectric material, and θ is a phase difference between stress and displacement when receiving voltage or sound wave. The coefficient ξ ₀ sin (nπ / 2 · z / h) means the amplitude of the displacement at the height z. Hereinafter, the time term is omitted.

In this case, the displacement ξ (z ₁ ) at the coordinate z ₁ is
ξ (z ₁ ) = ξ ₀ sin (nπ / 2 · z / h) (12)
It is. Here, the relationship between the displacement ξ (z) and the strain S is
dS = dξ / dz (13)
Therefore, the strain S _m of the piezoelectric material of the m-th layer is
S _m = [ξ (z _m ) −ξ (z _m−1 )] / (z _m −z _m−1 ) (14)
Can be written. However, m = 1 to n and z ₀ = 0.

Therefore, the strain S ₁ of the piezoelectric body ₁ is
S ₁ = Δh ₁ / h ₁ = ξ ₀ sin (nπ / 2 · h ₁ / h) / h ₁ (15)
It becomes. Here, Δh ₁ is a change in thickness of the piezoelectric body 1, and ξ (z ₁ ) −ξ (z = 0), h ₁ is a thickness of the piezoelectric body 1. Similarly for the piezoelectric body 2, the strain S ₂ is
S ₂ = Δh ₂ / h ₂
= Ξ ₀ {sin (nπ / 2 · (h ₁ + h ₂ ) / h)
-Sin (nπ / 2 · h ₁ / h)} / h ₂ (16)
It becomes. The strain S _m of the piezoelectric material of the m-th layer is
S _m = Δh _m / h _m
= Ξ ₀ {sin (nπ / 2 · z _m / h) −sin (nπ / 2 · z _m−1 / h)} / h _m
... (17)
It becomes.

In the above equation (17), the strain of the piezoelectric body in the m-th layer is determined by sin (np / 2 · z _m / h) −sin (np / 2 · z _m−1 / h). It means that it does not stretch uniformly. Therefore, if the front and back of the piezoelectric body are reversed between a piezoelectric body whose sign of the term is positive and a piezoelectric body which is negative, the sign of the electric displacement or electric field due to the piezoelectric positive effect can be made to coincide. It is understood that an n-order harmonic electrical signal can be obtained efficiently.

Furthermore, if the thickness of each piezoelectric body is made equal,
z _m = (m / n) h, z _m−1 = (m−1 / n) h, h _m = h / m (18)
And can be simplified. Substituting this into equation (17) gives
S _m = mξ ₀ {sin (mπ / 2) −sin [(m−1) π / 2]} / h (19)
It becomes.

となる。したがって、各圧電体の電気容量をＣとすれば、積層圧電体の容量はｎＣとなり、電気インピーダンスは圧電体１層の１／ｎに減少する。 Next, consider the electrical system. The electrical displacement (charge per unit electrode area) D _m generated by the piezoelectric material of the m-th layer is as follows:
D _m = e ₃₃ S _m or D _m = d ₃₃ S _m = d ₃₃ sT _m (20)
It is. Here, s is the elastic compliance of the piezoelectric body. If each piezoelectric body is electrically connected in parallel, the net electric displacement D _Total output from the laminated piezoelectric body is:

It becomes. Therefore, if the electric capacity of each piezoelectric material is C, the capacity of the laminated piezoelectric material is nC, and the electric impedance is reduced to 1 / n of one piezoelectric material layer.

  From the above formulas (19) and (21), the inventor of the present application realizes the simple parallel coupling and the remanent polarization that maximizes the net electric displacement output from the laminated piezoelectric material, or the rules for the arrangement of the crystal C axis and A axis. I found sex. Further, if the number n of layers is 4 or more and the number n of layers is matched with the harmonic order, the nodes and antinodes of the elastic waves propagating in the piezoelectric body can be made to coincide with the boundary surface of the piezoelectric body. At this time, the distortion of each piezoelectric body is reversed by 180 degrees without changing the absolute value. For example, in the case of FIG. 7 where each piezoelectric body is in series, if the distortion of the piezoelectric body 1 is +, “+, −, − , + "Periodicity can be found every four layers, and it has been found that the detection of n-order harmonics can be performed more efficiently. A theoretical explanation of this is as follows.

FIG. 8 shows the residual polarization of each piezoelectric body or the orientation of the C-axis and A-axis of the piezoelectric body when detecting n-order harmonics in the n-layer piezoelectric body according to the present invention, and the electric displacement D ( An example of the relationship with C / m ² ) is shown. Electrodes are provided on the surfaces of the piezoelectric bodies between the layers and at both ends of each piezoelectric body, and the electrodes on the far side of the adjacent piezoelectric bodies are connected by a connection wiring, and the piezoelectric bodies are connected in parallel. Further, the remanent polarization (C-axis of the crystal in the case of a piezoelectric body not having it (A-axis in the case of quartz)) is oriented in the z-axis direction. In the drawing, for the sake of convenience, the remanent polarization of the piezoelectric body 1 or the direction of the C-axis or A-axis of the crystal is represented as + P. This is to identify the residual polarization of another piezoelectric material or whether the C-axis or A-axis of the crystal is in the same direction or in the opposite direction to that of the piezoelectric material 1, and does not limit the polarization direction of the piezoelectric material 1.

  As shown in FIG. 7C, the distortion of each piezoelectric body has periodicity (in the case of FIG. 7C, each piezoelectric body is in series as described above). Therefore, in order to achieve high charge output or high potential output, which is the object of the present invention, when remanent polarization or crystal C-axis or A-axis direction is parallel, each electrode is electrically insulated, It must be wired independently, and the structural and manufacturing risks are extremely high.

  However, according to the present invention, in the case of the parallel connection shown in FIG. 8, the periodic polarization of “+ P, + P, −P, −P” is applied to the residual polarization of the piezoelectric material for every four layers or to the C-axis and A-axis of the crystal. In other words, the second-stage piezoelectric body in contact with the axis of the first-stage piezoelectric body in contact with the back layer is the same direction, and the third and fourth-stage piezoelectric bodies are If the piezoelectric elements are arranged in the reverse direction and the piezoelectric elements of the four layers are arranged so as to have periodicity in the same direction, the same direction, the reverse direction, and the reverse direction thereafter, the electric impedance is 1 / of that of the piezoelectric material of one layer. In addition to being reduced to n, the charge sensitivity between both terminals can be easily amplified n times.

In the above description, the case where the nth harmonic is detected has been described as an example. However, in the case where the nth harmonic is transmitted, the efficiency is improved compared to the conventional case by connecting an oscillator between the terminals in FIG. To do. In the case of transmission, the relationship between the strain S and the applied electric field E is
S = dE or S = (e / c) E (22)
It is. If a voltage generator is connected between both terminals of the laminated piezoelectric material shown in FIG. 8 and a voltage having a frequency corresponding to the nth harmonic is applied, the distortion shown in FIG. 8 and the displacement shown in FIG. Ultrasound can be excited in the medium. And from the relationship of the electrical impedance described above, the structure of the present invention provides low voltage and high current driving.

On the other hand, FIG. 9 is a diagram schematically showing the structure of a 2n-layer piezoelectric transducer for detecting n-order harmonics. (A) is the stacking condition, (b) is the displacement of each layer at a certain moment, and (c) and (d) are the coefficients of strain and electrical displacement, respectively. The coefficients of 0.3 and 0.7 represent the relative ratios of strain and electric displacement, respectively, and do not indicate absolute values. It was approximated as ^{1/2 1/2} = 0.7. Each piezoelectric body was provided with electrodes on the interlayer and both end faces, and the electrodes on the separation side of the piezoelectric bodies adjacent to each other were connected to form a parallel coupling. In this case, the electric displacement due to the piezoelectric positive effect is the same as that of the n-layer piezoelectric transducer described above, but the electric capacity is doubled and the electric impedance is halved. Residual polarization or the arrangement of the C-axis and A-axis is repeated for “+ P, −P, −P, + P, −P, + P, + P, −P” every 8 layers with reference to the piezoelectric body in contact with the back layer. Become.

The strain Sm ^nω of each piezoelectric body is given by the following equation.

Sm ^nω
= 2nξ ₀ ^nω {sin [m (π / 4)] − sin [(m−1) (π / 4)]} / h, m
= 1, 2,..., 2n (23)

  Below, the detailed Example of this invention according to the above-mentioned view is described. First, detection of 3λ / 4 harmonics by the two-layer piezoelectric transducer shown in FIG. 10 will be described as a first embodiment. A two-layer piezoelectric transducer has the simplest structure among laminated piezoelectric materials. In this case, the order of harmonics used for transmission / reception does not match the number of stacked layers. However, by applying the present invention as follows, the efficiency of transmission and reception can be increased.

That is, the strains S ₁ and S ₂ of the piezoelectric body 1 and the piezoelectric body 2 in FIG.
S ₁ = Δh ₁ / h ₁ = ξ ₀ sin (nπ / 2 · h ₁ / h) / h ₁ (24)
S ₂ = Δh ₂ / h ₂
= Ξ ₀ {sin (nπ / 2) −sin (nπ / 2 · h ₁ / h)} / h ₂ (25)
It becomes. Here, h is the height of the laminated piezoelectric material, h ₁ and h ₂ are the height of each piezoelectric material, and h ₁ = h ₂ = h / 2.

The response to the 3λ / 4 resonant harmonic is given by n = 3. In this case, the distortions S ₁ ^3ω and S ₂ ^3ω are
S ₁ ^3ω
= 2ξ ₀ ^3ω sin (3π / 4) / h = 2 (ξ ₀ ^3ω / 2 ^1/2 ) / h (26)
S ₂ ^3ω
= 2ξ ₀ ^3ω {sin (3π / 2) −sin (3π / 4)} / h
= -2ξ ₀ ^3ω (1 + ^{1/2 1/2} ) / h (27)
It becomes. The subscript 3ω indicates the displacement at the third harmonic. Here, when approximated as ^{1/2 1/2} ≈0.7, these equations indicate that the amplitude ratio between the distortion of the piezoelectric body 2 and the distortion of the piezoelectric body 1 is −1.7: +0.7 at the third harmonic wave. Indicates that

  FIG. 11 shows such displacement and distortion codes of each piezoelectric body. (A) is displacement and (b) is distortion. The strain ratio between the piezoelectric body 1 and the piezoelectric body 2 is as shown in FIG. The reason why the absolute values of the strains of the piezoelectric bodies do not match is that the boundaries of the piezoelectric bodies, the nodes of displacement, and the antinodes do not match as described above.

Therefore, as shown in FIG. 12, if the electrodes on the separation surfaces of adjacent piezoelectric materials are connected in parallel to form a parallel coupling, the electrical impedance of the laminated piezoelectric material becomes ½ of the impedance of one piezoelectric material, and the piezoelectric positive The electrical displacement D _{1, −1} ^3ω due to the effect can be obtained by parallel connection if the direction of remanent polarization of the piezoelectric body 1 or the direction of the C-axis or A-axis of the crystal and those directions of the piezoelectric body 2 are the same direction. Polarity is reversed and added together,
D _{1, -1} ^3ω = 2 (1 + 2 ^1/2 ) eξ ₀ ^3ω / h (28)
It becomes.

On the other hand, the response to the fundamental wave can be understood when n = 1 in equations (24) and (25). That is, the strains S ₁ ^ω and S ₂ ^ω are
S ₁ ^ω = 2ξ ₀ ^ω sin (π / 4) / h = 2 (ξ ₀ ^ω / 2 ^1/2 ) / h (29)
S ₂ ^ω = 2ξ ₀ ^ω {sin (π / 2) −sin (π / 4)} / h
= 2ξ ₀ ^ω {1-1 / 2 ^1/2 } / h (30)
It becomes.

Here, when parallel coupling as shown in FIG. 12 is performed and the remanent polarization or the direction of the C-axis or A-axis of the crystal is the same direction, the electric displacement D _1,1 ^ω is
D _1,1 ^ω = 2eξ ₀ ^ω (2 ^1/2 -1) / h (31)
It becomes. Here, the subscript 1 corresponds to the remanent polarization of each piezoelectric body or the direction of the C-axis or A-axis of the crystal, and from the left, the remanent polarization of the piezoelectric body 1 or 2 or the C-axis or A-axis of the crystal. Indicates the direction.

  From the above results, in consideration of the distortion of each piezoelectric body, in the case of parallel connection, the residual polarization or the C-axis and A-axis directions of the crystal are set in the same direction, so that 2.4 times for 3λ / 4 waves. The sensitivity can be amplified, and at the same time, the sensitivity can be reduced by a factor of 0.4 for λ / 4 waves. Therefore, if the present invention is applied to an ultrasonic transducer composed of a two-layer piezoelectric body, it is possible to transmit / receive a signal due to 3λ / 4 resonance at a high S / N ratio.

  FIG. 13 shows experimental results and simulation results of the transmission / reception sensitivity characteristics of an ultrasonic transducer composed of a two-layer piezoelectric material using P (VDF / TrFE). Solid lines are experimental results, and broken lines are simulation results. In both cases, the separated surfaces of the piezoelectric bodies adjacent to each other, that is, the electrode surfaces formed on the outer surface are connected to each other and connected in parallel, and the λ / 4 resonance frequency is 7 MHz and the 3λ / 4 resonance frequency is about 20 MHz. (A) is a result when the polarization direction is the same direction according to the present invention, and (b) is a result when the reverse direction is set for reference. As shown in (a), when the polarization direction based on the present invention and the wiring method are combined, the 3λ / 4 resonance peak seen at 20 MHz is larger than the λ / 4 resonance peak seen at 7 MHz, and (b) Compared to the results shown, the 3λ / 4 resonance peak is increased by 20 dB. As described above, by combining the polarization direction and the wiring method based on the method of the present invention for the two-layer piezoelectric body, the third harmonic component can be increased and the fundamental wave component can be attenuated at the same time.

  Next, detection of the third harmonic by the three-layer piezoelectric transducer will be described. FIG. 14 shows a schematic structure diagram. This transducer is also a λ / 4 vibrator in which one end of the laminated piezoelectric body is fixed to the base and the other end is a free end.

First, the design process according to the present invention will be described. In the same manner as described above, the boundary between the substrate and the piezoelectric body 1 is the origin, and the coordinate in the height (thickness) direction of the element is z. Next, the strain S generated by each piezoelectric body will be considered. The piezoelectric body 1, the piezoelectric body 2, and the piezoelectric body 3 are formed in this order from the substrate side. Coordinate z is, z = 0 at the boundary between the base and the piezoelectric element 1, z ₁ at the boundary between the piezoelectric body 1 and the piezoelectric body _2, z ₂ at the boundary between the piezoelectric body 2 and the piezoelectric element _3, the end of the piezoelectric element 3 the part and _{z 3.} The thickness of the piezoelectric body 1 is h ₁ , the thickness of the piezoelectric body 2 is h ₂ , the thickness of the piezoelectric body 3 is h _3, and the height of the laminated piezoelectric body is h.

Then, the displacement ξ (z ₁ ) at the coordinate z ₁ is
ξ (z ₁ ) = ξ ₀ sin (nπ / 2 · z ₁ / h) (32)
Therefore, the strain S ₁ of the piezoelectric body ₁ is
S ₁ = Δh ₁ / h ₁ = ξ ₀ sin (nπ / 2 · h ₁ / h) / h ₁ (33)
It becomes. Similarly, for piezoelectric bodies 2 and 3,
S ₂ = Δh ₂ / h ₂
= Ξ ₀ {sin (nπ / 2 · (h ₁ + h ₂ ) / h) −sin (nπ / 2 · h ₁ / h)} / h ₂
... (34)
S ₃ = Δh ₃ / h ₃
= Ξ ₀ {sin (nπ / 2) −sin (nπ / 2 · (h ₁ + h ₂ ) / h)} / h ₃
... (35)
It becomes.

The distortion of each piezoelectric body at the time of 3λ / 4 resonance is given by n = 3 in the above equations (33) to (35). When the thickness of each piezoelectric body is equal, that is, when h ₁ = h ₂ = h ₃ = h / 3 in the above equation, the strains S ₁ ^3ω , S ₂ ^3ω , and S ₃ ^3ω of each piezoelectric body are
S ₁ ^3ω = Δh ₁ ^3ω / h ₁ = 3ξ ₀ ^3ω sin (π / 2) / h)
= 3ξ ₀ ^3ω / h (36)
S ₂ ^3ω = Δh ₂ ^3ω / h ₂
= 3ξ ₀ ^3ω {sin (π) −sin (π / 2)} / h
= -3ξ ₀ ^3ω / h (37)
S ₃ ^3ω = Δh ₃ ^3ω / h ₃
= 3ξ ₀ ^3ω {sin (3π / 2) −sin (π)} / h
= -3ξ ₀ ^3ω / h (38)
It becomes. Here, the subscript 3ω means a response at the third harmonic. From these equations, the 3λ / 4 resonance indicates that the distortion of the piezoelectric body 2 and the piezoelectric body 3 and the distortion of the piezoelectric body 1 are in opposite phases.

  FIG. 15 shows the displacement and distortion sign of each piezoelectric body. (A) is displacement and (b) is distortion. In the present embodiment, transmission and reception of the third harmonic and three piezoelectric bodies are laminated. Therefore, as shown in FIG. 5A and FIG. 5 described above, the displacement nodes and antinodes coincide with the boundaries of each piezoelectric body. To do. At this time, the distortion of each piezoelectric body can be encoded with the distortion of the piezoelectric bodies 2 and 3 as-, as shown in FIG. 5B and FIG.

Based on the above behavior of the dynamic system, the optimum structure of the electrical system is derived. As shown in FIG. 16, if the electrodes on the surfaces on the separated side of the piezoelectric bodies adjacent to each other are connected to each other and the wiring is drawn out from the electrodes, the piezoelectric bodies can be easily electrically connected in parallel. At this time, the electrical impedance of the laminated piezoelectric material is reduced to 1/3 of one piezoelectric material. Further, the direction of remanent polarization of the piezoelectric body 1 or the direction of the C-axis or A-axis of the crystal is set as a reference (+ P), and the direction of remanent polarization of the piezoelectric body 2 or the crystal C-axis or A-axis is set in the same direction (+ P). If the piezoelectric body 3 is reverse (−P), the charge induced in the wiring is the sum of the charges generated by the piezoelectric positive effect. At this time, the electric displacement D _{1,1, -1} ^3ω induced between both terminals is
D _{1,1, −1} ^3ω = eS ₁ −e (S ₂ + S ₃ ) = 9 eξ ₀ ^3ω / h (39)
It becomes. Here, the subscripts 1 and -1 correspond to the residual polarization of each piezoelectric body or the direction of the C-axis or A-axis of the crystal, and the residual polarization or crystal of the piezoelectric body 1, the piezoelectric body 2 and the piezoelectric body 3 from the left. The direction of the C axis and the A axis is shown.

The response to the λ / 4 resonance is given by n = 1, and the strains S ₁ , S ₂ , S ₃ of each piezoelectric body are
S ₁ = Δh ₁ / h ₁ = 3ξ ₀ sin (π / 6) / h (40)
S ₂ = Δh ₂ / h ₂
= 3ξ ₀ {sin (π / 3) −sin (π / 6)} / h (41)
S ₃ = Δh ₃ / h ₃
= 3ξ ₀ {sin (π / 2) −sin (π / 3)} / h (42)
That is,
S ₁ = S ₂ + S ₃ (43)
It becomes.

On the other hand, in the parallel connection shown in FIG. 16, the electrical displacement D _{1,1, -1} ^{ω with} respect to the λ / 4 resonance is
D _{1,1, -1} ^ω = e (S ₁ + S ₂ + S ₃ ) = 0 (44)
In the transducer composed of the three-layer piezoelectric material of the present example, the sensitivity based on λ / 4 resonance is canceled out.

As a comparison, the transmission and reception of the third harmonic by simply using the residual polarization or the three-layer piezoelectric body in which the C-axis and A-axis of the crystal are parallel will be described. The electrical displacement D _1,1,1 ^3ω produced by the piezoelectric positive effect in the laminated piezoelectric body at 3λ / 4 resonance is
D _1,1,1 ^3ω = e (S ₁ + S ₂ + S ₃ ) = − ₃ eξ ₀ ^3ω / h (45)
It becomes. Therefore, in this embodiment, as shown in the above equation (45), it is understood that the electrical displacement between the terminals can be improved three times (about 10 dB) in the parallel connection.

  FIG. 17 shows experimental data and simulation results for the ultrasonic wave transmission / reception characteristics of the three-layer piezoelectric material shown in FIG. In the experiment, a copolymer (P (VDF / TrFE)) of vinylidene fluoride and ethylene trifluoride, which is a typical ferroelectric polymer, was used. The structural schematic diagram of the ultrasonic transducer is shown in the right figure of FIG. In accordance with the present invention, three-layer piezoelectric bodies are electrically connected in parallel, and the direction of polarization is the same as that shown in FIG. The height of the three-layer piezoelectric body is approximately 120 μm, and the λ / 4 resonance frequency is 4.5 MHz. The one-dot chain line on the left is the experimental result, and the broken line is the simulation result.

  From FIG. 17, at 20 MHz or less, no peak is shown in the vicinity of 4.5 MHz corresponding to the resonance frequency, and the λ / 4 resonance peak disappears, while the first peak in the transducer of this example is It is understood that the frequency is 13.5 MHz, which is 3λ / 4 resonance.

  As a comparative example, FIG. 18 shows characteristics of a laminated piezoelectric material in which the directions of remanent polarization of the piezoelectric materials 2 and 3 are opposite to those in FIG. As shown in the left figure, this three-layer piezoelectric body has peaks at both 4.5 MHz, which is λ / 4 resonance, and 13.5 MHz, which is the third harmonic component, and the third harmonic component. It is understood that the sensitivity is about −50 to −60 dB, which is about 10 to 20 dB smaller than the result of the present embodiment shown in FIG. From these experimental results, it is understood that by combining the polarization direction and the wiring method based on the method of the present invention, it is possible to increase the third harmonic component and simultaneously attenuate the fundamental wave component. .

  As described above, in the second embodiment, the method of transmitting and receiving the third harmonic using a three-layer piezoelectric material has been described. Here, the harmonic order of the laminated piezoelectric material is matched with the number of laminated layers. Thus, (i) the vibration mode of each piezoelectric body can be encoded and understood by matching the boundary surface of the piezoelectric body with the elastic wave nodes and antinodes of the piezoelectric body, and based on the sign and wiring method, (ii) The remanent polarization of each piezoelectric material or the direction of the C-axis or A-axis of the crystal is set in the same or opposite direction as appropriate, so that the remanent polarization of each piezoelectric material or the orientation of the C-axis or A-axis of the crystal is simply anisotropic Compared with the case of 3), it is possible not only to increase the sensitivity 3 times (about 10 dB) or more when transmitting / receiving 3λ / 4 waves, but also to have a function as a filter that cancels λ / 4 waves. Moreover, since it is electrically connected in parallel, the electrical impedance can be reduced to 1/3. As described above, when the 3λ / 4 wave is selectively transmitted and received at a high S / N ratio, the present invention is extremely effective. Thereby, in the case of reception, the band separation filter and the amplifier can be reduced. Or even if it does not lead to reduction, in the case of a filter, a loss can be suppressed by reducing the order, and in the case of an amplifier, the gain can be reduced.

  The third embodiment is transmission and reception of the third harmonic by a transducer in which six layers of piezoelectric bodies having the same thickness are laminated. In this embodiment, each piezoelectric body is divided into two piezoelectric bodies in the three-layer piezoelectric body shown in the first embodiment, and not only the electric impedance is further halved when parallel-coupled, but also the upper end. Since the electrode and the lower end electrode are communicated with each other, the entire laminated piezoelectric body can be electrically shielded.

  FIG. 19 is a cross-sectional view showing the structure of the six-layer piezoelectric body and schematically showing displacement and distortion when transmitting and receiving third-order harmonics. Like FIG. 9, (a) is a lamination | stacking condition, (b) is the displacement of each layer in a certain moment, (c) is a coefficient of distortion.

The distortion S ₁ , S ₂ , S ₃ , S ₄ , S ₅ , S ₆ of each piezoelectric body is
S ₁ = 6ξ ₀ sin (π / 4) / h = 6ξ ₀ (1/2 ^1/2 ) (46)
S ₂ = 6ξ ₀ {sin (π / 2) −sin (π / 4)} / h
= 6ξ ₀ (1-1 / 2 ^1/2 ) (47)
S ₃ = 6ξ ₀ {sin (3π / 4) −sin (π / 2)} / h
= 6ξ ₀ (1/2 ^1/2 −1) (48)
S ₄ = 6ξ ₀ {sin (π) −sin (3π / 4)} / h
= 6ξ ₀ (−1/2 ^1/2 ) (49)
S ₅ = 6ξ ₀ {sin (5π / 4) −sin (π)}
= 6ξ ₀ (−1/2 ^1/2 ) (50)
S ₆ = 6ξ ₀ {sin (3π / 2) −sin (5π / 4)}
= 6ξ ₀ (1/2 ^1/2 −1) (51)
It becomes. When approximated to 1/2 ^1/2 ≈0.7, the distortion ratio of each piezoelectric body is as shown in the schematic diagram of FIG.

  FIG. 20 shows a preferred arrangement of remanent polarization or crystal C-axis or A-axis when the electrodes on the separation side in adjacent piezoelectric bodies are connected and electrically connected in parallel. With this configuration, the total charge amount is the same as that in the case of stacking three sheets shown in FIG. 16, but the electrical impedance can be reduced to ½ of that.

  FIG. 21 shows the result of simulating the frequency characteristics of sensitivity in a six-layer piezoelectric body in which a polarization arrangement and wiring are combined as shown in FIG. The 6-layer piezoelectric body has a λ / 4 resonance frequency of 5 MHz. However, as shown in the right figure, by combining the polarization arrangement and the wiring based on the present invention, the sensitivity due to λ / 4 resonance is attenuated and the sensitivity due to 3λ / 4 resonance is maximized as shown in the left figure.

  The multilayer piezoelectric body of the present invention can be used not only for an ultrasonic transducer in an ultrasonic diagnostic apparatus, but also applicable to, for example, a fish detector, and transmission / reception signals are not limited to an ultrasonic band. The present invention can also be applied to a configuration that generates power by applying low-frequency stress.

A1 to A3 Piezoelectric material

Claims (10)

A laminated piezoelectric material that is formed by laminating a plurality of piezoelectric materials having the same thickness, and that transmits and receives ultrasonic waves of the third harmonic component due to 3λ / 4 resonance generated by thickness expansion and contraction of the piezoelectric material,
The piezoelectric body is laminated in three layers, and has electrodes on the surface of the piezoelectric body between the layers and at both ends,
By connecting the electrodes on the separation side of the piezoelectric bodies adjacent to each other, it has two sets of connecting wirings that connect the piezoelectric bodies in parallel with each other,
Each piezoelectric body has a second stage in contact with the direction of remanent polarization or crystal axis related to the electric displacement due to the positive piezoelectric effect or the sign of the electric field with respect to the axis of the first stage piezoelectric body on the fixed end side. A laminated piezoelectric material, wherein the piezoelectric material is arranged in the same direction in the piezoelectric body of the eye, and in the opposite direction in the piezoelectric material of the third stage above. A laminated piezoelectric material that is formed by laminating two layers of piezoelectric materials having the same thickness, and that transmits and receives ultrasonic waves of the third harmonic component due to 3λ / 4 resonance generated by thickness expansion and contraction of the piezoelectric material,
Each piezoelectric body has electrodes on its interlayer and on each outer surface, and is provided with a connection wiring for connecting each piezoelectric body in parallel by connecting the electrodes on the outer surface,
Both piezoelectric bodies are arranged so that the direction of remanent polarization or crystal axes related to the electrical displacement or the sign of the electric field due to the positive piezoelectric effect are in the same direction. It is formed by stacking four or more layers of piezoelectric bodies having the same thickness, and has electrodes on the surfaces of the layers and at both ends of the piezoelectric body,
By connecting the electrodes on the separation side of the piezoelectric bodies adjacent to each other, it is provided with two sets of connecting wirings that connect the stacked piezoelectric bodies in parallel with each other,
Each piezoelectric body has a second stage in contact with the direction of remanent polarization or crystal axis related to the electric displacement due to the positive piezoelectric effect or the sign of the electric field with respect to the axis of the first stage piezoelectric body on the fixed end side. A laminated piezoelectric material, wherein the piezoelectric materials are arranged so as to have periodicity in the same direction in the piezoelectric body of the eye, and in opposite directions in the third and fourth stage piezoelectric bodies above the piezoelectric body.   4. The multilayer piezoelectric body according to claim 3, wherein ultrasonic waves in a resonance mode of the third order or higher are transmitted and received by the thickness expansion and contraction of the piezoelectric body. Each of the piezoelectric bodies is further divided into two pieces in the thickness direction, and those that are connected in parallel to each other are laminated as a set,
For each of the divided piezoelectric bodies, the direction of the remanent polarization or the crystal axis is inverted so that the sign of the electric displacement or electric field in the strain distribution in the multilayer piezoelectric body matches. The multilayer piezoelectric body according to claim 1, 3 or 4, characterized by the above.   An ultrasonic transducer using the laminated piezoelectric material according to any one of claims 1 to 5. A method for producing a laminated piezoelectric material comprising a plurality of piezoelectric materials having mutually equal thicknesses, and transmitting and receiving ultrasonic waves of a third harmonic component due to 3λ / 4 resonance generated by thickness expansion and contraction of the piezoelectric materials,
Three layers of the piezoelectric bodies are laminated,
In the electrodes formed on the surfaces of the piezoelectric bodies between the layers and at both ends of each piezoelectric body, the piezoelectric bodies adjacent to each other are connected to each other by a connection wiring, thereby connecting the piezoelectric bodies in parallel with each other,
The second stage in which each piezoelectric body is in contact with the direction of remanent polarization or the crystal axis related to the electric displacement due to the positive piezoelectric effect or the sign of the electric field with respect to the axis of the first stage piezoelectric body on the fixed end side. A method for producing a laminated piezoelectric material, wherein the piezoelectric material is arranged in the same direction in the piezoelectric body of the eye and in the reverse direction in the piezoelectric material in the third stage above. A method for producing a laminated piezoelectric material comprising two layers of piezoelectric materials having the same thickness, and transmitting and receiving ultrasonic waves of a third harmonic component due to 3λ / 4 resonance generated by thickness expansion and contraction of the piezoelectric material. ,
In the electrodes formed between the layers of each piezoelectric body and on each outer surface, the electrodes on the outer surface are connected by a connection wiring to connect the piezoelectric bodies to each other in parallel.
A method for producing a laminated piezoelectric material, characterized in that the two piezoelectric materials are arranged so that the direction of remanent polarization or crystal axes related to the electrical displacement or the electric field sign due to the piezoelectric positive effect are in the same direction. A method for producing a laminated piezoelectric material comprising four or more layers of piezoelectric materials having equal thicknesses,
In the electrodes formed between the layers of the piezoelectric bodies and the surfaces of the piezoelectric bodies at both ends, the plurality of piezoelectric bodies are connected in parallel by connecting the electrodes on the separated side of the piezoelectric bodies adjacent to each other by connecting wires. connection,
The second stage in which each piezoelectric body is in contact with the direction of remanent polarization or the crystal axis related to the electric displacement due to the positive piezoelectric effect or the sign of the electric field with respect to the axis of the first stage piezoelectric body on the fixed end side. A method for producing a laminated piezoelectric material, wherein the piezoelectric materials are arranged so as to have periodicity in the same direction in the piezoelectric body of the eye, and in the reverse direction in the third and fourth stage piezoelectric bodies thereabove. Each of the piezoelectric bodies is further divided into two pieces in the thickness direction, and the ones connected in parallel to each other are laminated as one set,
With respect to each of the divided piezoelectric bodies, the reversal polarization direction or crystal axis is inverted so that the sign of the electric displacement or electric field in the strain distribution in the multilayer piezoelectric body is coincident. A method for producing a laminated piezoelectric material according to claim 7 or 9.

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