Ultrasonic transducer

Abstract

The present invention comprises a case (120), a piezoelectric vibrator (110), and wiring (130, 140). The case (120) has a bottomed cylindrical shape with a bottom section and a side wall section. The piezoelectric vibrator (110) comprises a piezoelectric ceramic that includes titanium and zirconium and is affixed to the bottom section on the inner side of the case (120). The wiring (130, 140) is connected to the piezoelectric vibrator (110) and is drawn out to the outer side of the case (120). The temperature at which the resonance frequency of a spreading vibration mode of the piezoelectric ceramic when not affixed to the bottom section is the lowest is within a range of −30°C to 10°C. The cross-sectional void ratio in an arbitrary vertical cross-section of the piezoelectric ceramic is 1% or less.

Description

Technical Field

[0001] The present invention relates to an ultrasonic transducer and a method for manufacturing the same.

Background Art

[0002] As a prior art document disclosing the configuration of an ultrasonic transducer, there is Japanese Patent No. 2651140 (Patent Document 1). The ultrasonic transducer described in Patent Document 1 includes a piezoelectric vibrator, a 1 / 4 wavelength acoustic matching layer, and a metal case. The piezoelectric vibrator utilizes the spreading vibration mode of a circular piezoelectric substrate.

[0003] As a prior art document disclosing a high heat-resistant piezoelectric element and a piezoelectric device using the same, there is Japanese Patent Application Laid-Open No. 2003-23187 (Patent Document 2). In the high heat-resistant piezoelectric element described in Patent Document 2, the temperature at which the resonance frequency of piezoelectric resonance in the piezoelectric element before heat treatment shows a minimum exists in the range of 60°C or higher and 200°C or lower.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In an ultrasonic transducer, it is required that the shortest detection distance capable of detecting a detection target is 20 cm or less.

[0006] The present invention has been made in view of the above problems, and an object thereof is to provide an ultrasonic transducer and a method for manufacturing the same that can make the shortest detection distance capable of detecting a detection target 20 cm or less.

Means for Solving the Problems

[0007] The ultrasonic transducer according to the present invention includes a case, a piezoelectric vibrator, and wiring. The case is a bottomed cylindrical shape having a bottom portion and a side wall portion. The piezoelectric vibrator has a piezoelectric ceramic containing Ti and Zr, and is attached to the bottom portion inside the case. The wiring is connected to the piezoelectric vibrator and is drawn out to the outside of the case. The temperature at which the resonance frequency of the spreading vibration mode of the piezoelectric ceramic in a state where it is not attached to the bottom portion is minimized is in the range of -30°C or higher and 10°C or lower. The cross-sectional porosity in any longitudinal section of the piezoelectric ceramic is 1% or less.

Effect of the Invention

[0008] According to the present invention, the shortest detection distance capable of detecting a detected object can be made 20 cm or less.

Brief Description of the Drawings

[0009]

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Embodiments for Carrying Out the Invention

[0010] Hereinafter, an ultrasonic transducer and a method for manufacturing the same according to an embodiment of the present invention will be described with reference to the drawings. In the following description of the embodiments, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

[0011] FIG. 1 is a longitudinal sectional view showing the configuration of an ultrasonic transducer according to an embodiment of the present invention. As shown in FIG. 1, an ultrasonic transducer 100 according to an embodiment of the present invention includes a case 120, a piezoelectric vibrator 110, a first wiring 130, and a second wiring 140. The ultrasonic transducer 100 further includes a bonding material 150 and a sealing material 160. Note that the sealing material 160 does not necessarily have to be provided.

[0012] The case 120 has a bottomed cylindrical shape having a bottom portion and a side wall portion. The upper end portion of the case 120 on the side opposite to the bottom portion is open. The bottom portion of the case 120 has a disc shape. Note that the shape of the bottom portion of the case 120 is not limited to a disc shape, and may be a rectangular plate shape or a polygonal plate shape. The side wall portion of the case 120 is erected vertically from the peripheral edge of the bottom portion to the bottom portion. The case 120 is formed of, for example, aluminum. The case 120 is grounded.

[0013] The piezoelectric vibrator 110 is attached to the bottom portion of the case 120 by an insulating adhesive such as an epoxy resin inside the case 120. The piezoelectric vibrator 110 has a flat piezoelectric ceramic. When viewed from a direction perpendicular to the bottom portion of the case 120, the piezoelectric ceramic has a square shape. The piezoelectric ceramic contains Ti and Zr. The piezoelectric ceramic is composed of PZT (lead zirconate titanate) - based ceramics. Electrodes are provided on each of the main surfaces of the piezoelectric ceramic facing each other.

[0014] Each of the first wiring 130 and the second wiring 140 is connected to the piezoelectric vibrator 110 and drawn out to the outside of the case 120. Specifically, the first wiring 130 is electrically connected to the electrodes on one main surface of the piezoelectric ceramic by a bonding material 150 such as solder. The second wiring 140 is electrically connected to the electrodes on the other main surface of the piezoelectric ceramic by a bonding material 150 such as solder. In FIG. 1, the state where the second wiring 140 is joined by the bonding material 150 to the lead-out electrode connected to the electrode on the other main surface of the piezoelectric ceramic and drawn out to one main surface is shown. In the present embodiment, the first wiring 130 and the second wiring 140 are lead wires, but may be composed of an FPC (Flexible Printed Circuit).

[0015] When a drive voltage is applied between the electrodes of the piezoelectric ceramic through the first wiring 130 and the second wiring 140, the piezoelectric vibrator 110 expands and vibrates in the in-plane direction. When the piezoelectric vibrator 110 vibrates, the bottom of the case 120 vibrates.

[0016] When the bottom of the case 120 vibrates by receiving ultrasonic waves from the outside, the piezoelectric vibrator 110 also vibrates along with this vibration. By generating charges along with the vibration of the piezoelectric vibrator 110, the ultrasonic waves are converted into electrical signals by the piezoelectric vibrator 110. The electrical signals are transmitted to the outside through the first wiring 130 and the second wiring 140 from the electrodes provided on the piezoelectric ceramic.

[0017] The sealing material 160 is filled in the case 120. The space inside the case 120 is filled by the sealing material 160. The sealing material 160 is made of, for example, rubber such as silicone rubber or urethane rubber, or resin such as epoxy resin, and has sound insulation properties and adhesiveness. Note that a sound-absorbing material made of a material having a lower elastic modulus than the sealing material 160 may be disposed so as to cover the piezoelectric vibrator 110. In this case, the sealing material 160 covers the sound-absorbing material.

[0018] (Experimental Example) Here, an experimental example for verifying the correlation between the characteristics of the piezoelectric ceramic and the detection distance of the ultrasonic transducer will be described. In this experimental example, 11 types of ultrasonic transducers of Examples 1 to 5 and Comparative Examples 1 to 6 were fabricated, and their characteristics were verified.

[0019] Table 1 summarizes the characteristic evaluation results of the piezoelectric ceramic and the ultrasonic transducer according to Examples 1 to 5 and Comparative Examples 1 to 6.

[0020]

Table 1

[0021] A lead zirconate titanate-based piezoelectric material that is the material of the piezoelectric ceramic was fabricated by the following method. PbO powder, TiO2 powder, ZrO2 powder, water, a dispersant, and media for mixing and grinding were each put into a container in a predetermined amount so that the content ratio of Ti to Zr was in the range of 0.89 or more and 0.95 or less, and mixing and grinding were performed over 24 hours. After mixing and grinding, filtration was performed to remove water, and drying was performed at 100 °C to obtain a powder. The obtained powder was put into a sheath made of Al2O3, and synthesis was performed at a temperature of about 900 °C to 1000 °C. After the synthesized powder was dry-ground, piezoelectric materials according to Examples 1 to 5 and Comparative Examples 1 to 6 were prepared by various methods. The content ratio of Ti to Zr in the piezoelectric materials according to Examples 1 to 5 and Comparative Examples 1 to 6 is as shown in Table 1.

[0022] In Examples 1 to 5 and Comparative Examples 1 to 3, the piezoelectric material was formed into a sheet. Specifically, a binder, a dispersant, and an antifoaming agent were added to the piezoelectric material to prepare a slurry, and a green sheet was prepared from the slurry using the doctor blade method. The obtained green sheets were laminated and pressure-bonded to form a laminate, and the laminate was fired to fabricate a piezoelectric ceramic.

[0023] In Comparative Example 4 and Comparative Example 5, the piezoelectric material was extrusion-molded. Specifically, a binder and a small amount of water were added to the piezoelectric material to make it clay-like, and then a molded body was formed from the clay-like piezoelectric material using an extrusion molding machine, and the molded body was fired to produce a piezoelectric ceramic.

[0024] In Comparative Example 6, the piezoelectric material was press-molded. Specifically, a binder and a dispersant were added to the dry powder of the piezoelectric material, and then a molded body was formed from the dry powder using a press molding machine, and the molded body was fired to produce a piezoelectric ceramic.

[0025] In Examples 1 to 5 and Comparative Examples 1 to 6, the obtained piezoelectric ceramics were processed into a rectangular parallelepiped shape with a side length of 5 mm and a thickness of 0.2 mm. Subsequently, electrodes made of Ag were formed on both main surfaces of the piezoelectric ceramic by sputtering, and then a DC voltage was applied between these electrodes for polarization. After the polarized piezoelectric ceramic was attached to the bottom of the case 120 with an adhesive, each of the first wiring 130 and the second wiring 140 was electrically connected to the electrode of the piezoelectric ceramic by a bonding material 150. After each of the first wiring 130 and the second wiring 140 was drawn out from the case 120, the inside of the case 120 was filled with a resin as a sealing material 160 to produce ultrasonic transducers according to Examples 1 to 5 and Comparative Examples 1 to 6.

[0026] Also, as a test piece for obtaining the temperature at which the resonance frequency of the piezoelectric ceramics in Examples 1 to 5 and Comparative Examples 1 to 6 is minimized, after forming electrodes made of Ag on both main surfaces of the piezoelectric ceramic prepared under the same conditions as above by sputtering, a DC voltage was applied between these electrodes for polarization, and then a test piece with a long side length of 4 mm, a short side length of 1 mm, and a thickness of 0.2 mm was prepared.

[0027] The ultrasonic transducers according to Examples 1 to 5 and Comparative Examples 1 to 6 were driven to evaluate the shortest detection distance. Also, the equivalent circuit constants of the main vibration modes of the ultrasonic transducers according to Examples 1 to 5 and Comparative Examples 1 to 6 were measured using an impedance analyzer, and the electromechanical coupling coefficient of the piezoelectric ceramic after being attached to the bottom of Case 120 was obtained from the square root of the value obtained by dividing the equivalent capacitance by the driving capacitance.

[0028] In addition, the test pieces of the piezoelectric ceramics according to Examples 1 to 5 and Comparative Examples 1 to 6 were put into a temperature bath, and the resonance frequency of the extensional vibration mode (31 mode) of the test pieces when the temperature was changed in 10 °C increments within the range from -50 °C to 200 °C was obtained, and the temperature Tfm at which this resonance frequency was minimized was obtained.

[0029] Furthermore, after polishing the longitudinal sections of the piezoelectric ceramics according to Examples 1 to 5 and Comparative Examples 1 to 6, the cross sections were observed with an SEM (Scanning Electron Microscope), and the cross-sectional porosity, which is the ratio of the area occupied by voids in the piezoelectric ceramic within the field of view, was evaluated.

[0030] Figure 2 is an image obtained by observing the polished longitudinal section of the piezoelectric ceramic according to Example 1 with an SEM. Figure 3 is an image obtained by observing the polished longitudinal section of the piezoelectric ceramic according to Comparative Example 3 with an SEM. Figure 4 is an image obtained by observing the polished longitudinal section of the piezoelectric ceramic according to Comparative Example 5 with an SEM. In Figures 2 to 4, the black dot-like portions are voids V. As shown in Figures 2 to 4, there are few voids V in the piezoelectric ceramic according to Example 1 in which the piezoelectric material was formed into a sheet, there are many voids V in the piezoelectric ceramic according to Comparative Example 3 in which the piezoelectric material was extrusion-molded, and there are many and large voids V in the piezoelectric ceramic according to Comparative Example 5 in which the piezoelectric material was press-molded.

[0031] As shown in Table 1, there is a strong correlation between the shortest detection distance of the ultrasonic transducer and the electromechanical coupling coefficient of the piezoelectric ceramic after attachment, and it was found that the higher the electromechanical coupling coefficient of the piezoelectric ceramic after attachment, the shorter the shortest detection distance of the ultrasonic transducer can be.

[0032] Therefore, when considering increasing the electromechanical coupling factor of the piezoelectric ceramic after attachment, it was found that the electromechanical coupling factor of the piezoelectric ceramic after attachment has a correlation with the electromechanical coupling factor of the piezoelectric ceramic before attachment and the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized.

[0033] FIG. 5 is a graph showing the relationship between the electromechanical coupling factor of the piezoelectric ceramic before attachment and the electromechanical coupling factor of the piezoelectric ceramic after attachment, and the temperature at which the resonance frequency of the piezoelectric ceramic is minimized, for the piezoelectric ceramics of Examples 1 to 5 and Comparative Examples 1 to 3. In FIG. 5, on the left vertical axis, the electromechanical coupling factor (%) of the piezoelectric ceramic before attachment is shown, on the right vertical axis, the electromechanical coupling factor (%) of the piezoelectric ceramic after attachment is shown, and on the horizontal axis, the temperature (° C.) at which the resonance frequency of the piezoelectric ceramic is minimized is shown. Also, the electromechanical coupling factor of the piezoelectric ceramic before attachment is indicated by a circle, the electromechanical coupling factor of the piezoelectric ceramic after attachment is indicated by a triangle, the approximate curve of the transition of the electromechanical coupling factor of the piezoelectric ceramic before attachment is indicated by a dotted line L1, and the approximate curve of the transition of the electromechanical coupling factor of the piezoelectric ceramic after attachment is indicated by a solid line L2.

[0034] As shown in FIG. 5, it was found that the transitions of the electromechanical coupling factor of the piezoelectric ceramic before attachment and the electromechanical coupling factor of the piezoelectric ceramic after attachment with respect to the change in the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized are different. Specifically, the temperature Tfm at which each of the electromechanical coupling factor of the piezoelectric ceramic before attachment and the electromechanical coupling factor of the piezoelectric ceramic after attachment peaks was different. Therefore, while the temperature Tfm at which the electromechanical coupling factor of the piezoelectric ceramic before attachment becomes high is room temperature (about 30° C.), the temperature Tfm at which the electromechanical coupling factor of the piezoelectric ceramic after attachment becomes high was about -15° C. From this, as will be described later, it was found that the composition of the piezoelectric ceramic at which the electromechanical coupling factor of the piezoelectric ceramic is maximized before and after attachment is different.

[0035] Here, the relationship between the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized and the composition of the piezoelectric ceramic will be described. FIG. 6 is a diagram showing the relationship between the temperature at which the resonance frequency of the piezoelectric ceramic is minimized and the composition of the piezoelectric ceramic in the crystal state diagram of the piezoelectric material. In FIG. 6, the vertical axis represents the temperature (°C), and the horizontal axis represents the molar fraction of PbTiO3 in PZT. The room temperature Tr is indicated by a dotted line. When the piezoelectric ceramic has the composition indicated by the dashed-dotted line La, the temperature at the intersection of the morphotropic phase boundary MPB, which is the phase boundary between the rhombohedral crystal and the tetragonal crystal, and the dashed-dotted line La is the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic having the composition is minimized. That is, by changing the content ratio of Ti to Zr in the piezoelectric material, the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized can be adjusted.

[0036] As shown in FIG. 5, the crystal structure of the piezoelectric ceramic in the composition range T1 where the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized is lower than the room temperature indicated by the dashed-dotted line becomes tetragonal at room temperature, and the crystal structure of the piezoelectric ceramic in the composition range T2 where the temperature Tfm is higher than the room temperature indicated by the dashed-dotted line becomes rhombohedral at room temperature.

[0037] As shown in FIG. 5, for the piezoelectric ceramics of Examples 1 to 5 in which the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized is in the range of -30°C or higher and 10°C or lower, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment was maintained as high as 18% or more.

[0038] The reason for this is that when the piezoelectric ceramic is attached to the case 120, the electromechanical coupling coefficient of the piezoelectric ceramic itself decreases due to the stress applied to the piezoelectric ceramic. However, by setting the composition of the piezoelectric ceramic such that the temperature Tfm is in the range of -30°C or higher and 10°C or lower, the crystal structure of the piezoelectric ceramic at room temperature becomes a stable tetragonal crystal, and it is considered that the decrease in the electromechanical coupling coefficient of the piezoelectric ceramic when it is attached to the case 120 can be suppressed.

[0039] Conventionally, the composition of the piezoelectric ceramic has been determined such that the electromechanical coupling coefficient of the piezoelectric ceramic before attachment is maximized at room temperature, that is, the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized is room temperature. In such a case, as shown in FIG. 5, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment decreases, and the shortest detection distance of the ultrasonic transducer cannot be shortened.

[0040] As shown in Table 1, in the ultrasonic transducers of Examples 1 to 5, as a result of the electromechanical coupling coefficient of the piezoelectric ceramic after attachment being maintained as high as 18% or more, the shortest detection distance of the ultrasonic transducer could be made 20 cm or less. On the other hand, in Comparative Examples 1 to 3 in which the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized is outside the range of -30°C or more and 10°C or less, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment was less than 18%, and the shortest detection distance of the ultrasonic transducer could not be made 20 cm or less.

[0041] As shown in Table 1, in the ultrasonic transducers according to Comparative Examples 4 to 6, the temperature Tfm at which the resonance frequency of the extensional vibration mode (31 mode) of the piezoelectric ceramic is minimized is within the range of -30°C or more and 10°C or less, but the electromechanical coupling coefficient of the piezoelectric ceramic after attachment was less than 18%, and the shortest detection distance of the ultrasonic transducer could not be made 20 cm or less.

[0042] This is presumably because the cross-sectional porosity of the piezoelectric ceramics according to Comparative Examples 4 to 6 is larger than 1%, resulting in a large decrease in the electromechanical coupling coefficient of the piezoelectric ceramic when the piezoelectric ceramic is attached to the case 120.

[0043] In the ultrasonic transducers of Examples 1 to 5, since the cross-sectional porosity of the piezoelectric ceramic is 1% or less, this also suppresses a decrease in the electromechanical coupling coefficient of the piezoelectric ceramic when the piezoelectric ceramic is attached to the case 120. As a result, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment is maintained as high as 18% or more, and the shortest detection distance of the ultrasonic transducer could be made 20 cm or less.

[0044] As can be seen from the above experimental results, in the ultrasonic transducer 100 according to an embodiment of the present invention, the temperature Tfm at which the resonance frequency of the spreading vibration mode (31 mode) of the piezoelectric ceramic in a state where it is not attached to the bottom of the case 120 is minimized is in the range of -30°C or more and 10°C or less. The cross-sectional porosity in any longitudinal cross-section of the piezoelectric ceramic is 1% or less. Thereby, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment can be maintained as high as 18% or more, and the shortest detection distance of the ultrasonic transducer 100 can be made 20 cm or less.

[0045] Preferably, the temperature at which the resonance frequency of the spreading vibration mode of the piezoelectric ceramic in a state where it is not attached to the bottom of the case 120 is minimized is in the range of -20°C or more and 0°C or less. Thereby, the electromechanical coupling coefficient of the piezoelectric ceramic after attachment can be maintained as high as 20% or more, and the shortest detection distance of the ultrasonic transducer 100 can be made 12 cm or less.

[0046] Conventionally, piezoelectric ceramics manufactured by a low-cost process such as extrusion molding or press molding have been used in ultrasonic transducers. However, the ultrasonic transducer according to the present embodiment can be manufactured by the following steps to have a shortest detection distance of 20 cm or less.

[0047] FIG. 7 is a flowchart showing a method for manufacturing an ultrasonic transducer according to an embodiment of the present invention. As shown in FIG. 7 and Table 1, in the method for manufacturing an ultrasonic transducer according to an embodiment of the present invention, a piezoelectric ceramic is produced by firing a laminate obtained by laminating and pressing a plurality of green sheets produced by sheet forming of a lead zirconate titanate-based material in which the content ratio of Ti to Zr is in the range of 0.915 or more and 0.935 or less (step S1); a step (S2) of attaching a piezoelectric vibrator having the piezoelectric ceramic to the bottom of a bottomed cylindrical case; and a step (S3) of connecting a wiring to the piezoelectric vibrator.

[0048] In the description of the above-described embodiments, combinable configurations may be combined with each other.

[0049] The embodiments and examples disclosed this time should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the above description, and it is intended that all modifications within the meaning and scope equivalent to the scope of claims be included.

Description of Reference Numerals

[0050] 100 Ultrasonic transducer, 110 Piezoelectric vibrator, 120 Case, 130 First wiring, 140 Second wiring, 150 Bonding material, 160 Sealing material.

Claims

1. A bottomed cylindrical case having a bottom and side walls, a piezoelectric vibrator having a piezoelectric ceramic containing Ti and Zr, and attached to the bottom inside the case, a wiring connected to the piezoelectric vibrator and drawn out to the outside of the case, the temperature at which the resonance frequency of the spreading vibration mode of the piezoelectric ceramic in a state where it is not attached to the bottom is minimized is in the range of -30°C or higher and 10°C or lower, an ultrasonic transducer in which the cross-sectional porosity in any longitudinal section of the piezoelectric ceramic is 1% or less.

2. The ultrasonic transducer according to claim 1, wherein the temperature at which the resonance frequency of the spreading vibration mode of the piezoelectric ceramic in a state where it is not attached to the bottom is minimized is in the range of -20°C or higher and 0°C or lower.

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