JPH07108038B2 - Ultrasonic probe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an ultrasonic probe that is used in a medical ultrasonic diagnostic apparatus or the like and that transmits and receives ultrasonic waves.

(Prior Art) Generally, as a piezoelectric transducer material of an ultrasonic probe, a zircon / lead titanate-based piezoelectric material is used. In these materials, the coupling coefficient k of the lateral effect when used as a longitudinal wave converter
₃₁ was as large as 0.2 to 0.3, and therefore unwanted vibrations tended to occur, making it difficult to obtain good characteristics. Particularly in an array type probe used in an ultrasonic diagnostic apparatus or the like, in order to avoid unnecessary vibration due to the lateral effect, as shown in FIG. 6, the ratio W of the width W to the plate thickness T of the rectangular plate-shaped vibrator 61 is Since / T needs to be 0.6 or less, there is a problem that the width of the transducer becomes small and manufacturing becomes difficult when the frequency of the probe is increased. The structure of a general ultrasonic probe will be described with reference to FIG. 6. Reference numeral 61 is a rectangular plate transducer made of piezoelectric ceramics of the zircon / lead titanate system for electromechanical energy conversion. Reference numerals 62 and 63 are acoustic matching layers, which are provided to match the acoustic impedance of a subject (human body, water, or steel) whose acoustic impedance is significantly different from that of the piezoelectric ceramics. This contributes to lower loss. The acoustic impedance of the matching layer is usually set to an intermediate value between the acoustic impedances of the piezoelectric ceramic and the subject. The overall configuration is a configuration in which acoustic matching layers 62, 63 are formed on a plurality of rectangular plate-shaped vibrators 61 arranged at a predetermined interval.

On the other hand, Honda, Yamashita, Uchida "Ultrasonic transducer using lead titanate-based piezoelectric ceramic material" (Technical report of the Institute of Electronics and Communication Engineers US81-20 (1981)) or Takeuchi, Nakatani, Sadamura "PbTiO. Application of ₃ series ceramics to high frequency ultrasonic probe ”(Technical report of the Institute of Electronics and Communication Engineers US84-7 (1984)), a probe using a PbTiO ₃ series piezoelectric ceramic material was replaced with conventional zircon / lead titanate. It is stated that it is superior to the one used. That is, in the PbTiO ₃ -based piezoelectric ceramic material, since the coupling coefficient of the lateral effect is extremely small,
When unnecessary vibrations are drastically reduced, and when this material is applied to an ultrasonic probe, the vibration energy due to the lateral effect is considerably weak and practically used thickness transmission and reception that has almost no effect on vertical vibration. The characteristics can be expected. PbTiO ₃ -based piezoelectric ceramics can be used not only when the W / T is 0.6 or less, but also when the W / T is 1.2 to 2.0. This is extremely advantageous when trying to increase the frequency of the array type probe that has been used.

(Problems of the prior art) As described above, the PbTiO ₃ -based piezoelectric ceramics have a large anisotropy in the electromechanical coupling coefficient, and therefore the thickness vertical coupling coefficient actually used.
It is said that a material having a k _t of 0.50 or more and a lateral effect coupling coefficient k ₃₁ of 0.05 or less is obtained, which is an extremely excellent material for an ultrasonic probe. There is only about 200, which is only about one-tenth of zircon / lead titanate-based piezoelectric ceramics. In particular, the electrode area of the electronic scanning array probe is small, and when using PbTiO ₃ -based piezoelectric ceramics Is small, the impedance of the probe is about 10 times higher than that of the probe using zircon / lead titanate piezoelectric ceramics. Since the ultrasonic probe is usually connected to the diagnostic device main body via a cable, if the material has a small dielectric constant, such as PbTiO ₃ -based piezoelectric ceramics, firstly, it is affected by the capacitance of the cable. There is a drawback that the S / N ratio deteriorates. Secondly, the PbTiO ₃ -based piezoelectric ceramic has a small piezoelectric d constant due to its small dielectric constant, and therefore has a drawback that sufficient vibration amplitude cannot be obtained unless the driving voltage is increased. That is, the PbTiO ₃ -based piezoelectric ceramics have such drawbacks, which has been an obstacle to practical use of the probe. Furthermore, the actual ultrasonic probe is 3.5MH
A certain fixed center frequency such as z or 7.5 MHz is required, but the conventional transducer has a drawback that the frequency cannot be adjusted because the surface has electrodes.

(Object of the Invention) The present invention solves the above-mentioned drawbacks of the PbTiO ₃ -based piezoelectric ceramics, has a small impedance, is excellent in spurious characteristics, has a large effective electromechanical coupling coefficient, and can easily adjust the frequency. The purpose is to obtain an ultrasonic probe.

(Structure of the invention) In order to achieve the above object, the present invention has a plurality of planar internal electrodes are formed at equal intervals inside a PbTiO ₃ -based piezoelectric ceramics plate, and the plurality of planar internal electrodes are PbTi
The laminated structure has two side surfaces of the O ₃ -based piezoelectric ceramic plate connected to the external electrodes formed on the two side surfaces every other layer, and is sandwiched between the plurality of planar internal electrodes.
The PbTiO ₃ -based piezoelectric ceramic plate has a structure in which the layers are alternately polarized in opposite directions, and a piezoelectric transducer provided with a piezoelectrically inactive ceramic layer on both end faces in the stacking direction, and the piezoelectric transducer. And an acoustic matching layer formed on one surface perpendicular to the stacking direction.

(Detailed Description of Configuration) The ultrasonic probe using the PbTiO ₃ -based piezoelectric ceramics of the present invention solves various problems of the conventional technique by having the electrode inside the ceramics as described above. Hereinafter, description will be given with reference to the drawings.

FIG. 2 shows an example of the laminated structure of the PbTiO ₃ -based piezoelectric ceramic converter used in the probe of the present invention in the state of a green sheet. The structure of the converter and an example of the manufacturing method will be described in detail below. In FIG. 2, 20 is an organic binder
A green sheet made of PbTiO ₃ -based ceramic powder, and 21 is a conductive paste, which becomes an internal electrode after firing. Such green sheets are laminated as shown in the figure and pressure-bonded to obtain a green sheet laminated body. in this case,
The electrode 21 is left with a gap portion 22 not coated with a conductive paste so that the electric terminals can be taken out in parallel for every other layer. Next, the green sheet laminate is fired. After that, Pb fired as shown in FIG.
Electrodes are formed on the two side surfaces parallel to the yz plane of the TiO ₃ -based piezoelectric ceramic laminate by a method such as baking, vapor deposition, or plating. In this case, the internal electrode has a four-layer structure, but other structures such as a three-layer structure, a five-layer structure, and a six-layer structure may be used. In FIG. 3, 30 is a PbTiO ₃ -based piezoelectric ceramic, 31 is an internal electrode, and 32,3
2'is an external electrode provided on the side surface after firing, and 33, 33 'are surface ceramic layers. By applying a high DC electric field between the external electrodes 32 and 32 'to perform polarization, each layer of the PbTiO ₃ -based ceramics sandwiched between the internal electrodes is polarized to impart piezoelectricity. At this time, a sufficient voltage is applied between the layers, and the layer thicknesses d ₁ , d ₂ ,
It is desirable to set d ₃ smaller than the gaps t ₁ and t ₂ . The arrow indicates the polarization direction in FIG. The frequency of the transducer can be easily adjusted by polishing the ceramic layers 33, 33 'on the surface to reduce the thickness. The rectangular plate transducer used for the array type probe is xz in Fig. 3.
It can be easily obtained by cutting in the y direction parallel to the plane.

The converter according to the present invention as shown in FIG. 3 has the following advantages not found in the prior art.

First of all, the piezoelectric transducer of the present invention has an effective electromechanical coupling coefficient k _t as compared with the coupling coefficient k _t of the thickness vertical mode of the transducer in which an electrode is provided only on the surface having no internal electrode. _eff can be increased up to about 10%. This is due to the mass effect of the porcelain layer provided on the surface. The square of the electromechanical coupling coefficient k _eff is defined as the ratio of the mechanical vibration energy output to the electrical input energy, which is expressed by the following equation as another expression.

However, f _r ; resonance frequency f _a ; anti-resonance frequency Therefore, the converter according to the present invention can increase the resonance anti-resonance frequency difference f _a −f _r with respect to a constant resonance frequency,
This is especially effective when used in an ultrasonic probe that has a wide band and excellent pulse response characteristics.

Secondly, the piezoelectric transducer according to the present invention has a plurality of internal electrodes, and the internal electrodes are taken out in parallel every other layer. It is possible to realize a much smaller impedance than a transducer. Now, in order to simplify the description, as an example, a laminated transducer in which electrodes are provided at equal intervals will be specifically described. In the case of a transducer having (n + 1) layers of internal electrodes, the electrodes are connected in parallel by a simple calculation, so that the effective area becomes almost n times, and the electrode spacing is n times the plate thickness. Therefore, the free capacity is about n ² times that of a single plate. Since such a laminated structure does not impair the effective electromechanical coupling coefficient k _eff ,
The impedance of the transducer having the (n + 1) -layer internal electrodes is drastically reduced to about 1 / n ^{2 of} that of the piezoelectric single plate. In addition, this does not change the dielectric constant of PbTiO ₃ -based piezoelectric ceramics. , The effective permittivity of the transducer laminated at equal electrode intervals (Dielectric pole calculated by assuming that electrodes are only on both principal surfaces of the transducer) With 5 layers of electrodes With 6 layers of electrodes (N + 1) layer electrode And increases substantially in proportion to the square of (the number of electrode layers-1).

As mentioned above, the transducer in which the electrodes are laminated at equal intervals is described. However, the transducer in which the electrodes are laminated at unequal intervals is more effective than the transducer in which the electrodes are laminated at the same number of electrodes. Permittivity Is slightly larger. Further, a transducer having a narrower inner electrode distance in the central portion of the thickness of the transducer can obtain a larger k _eff than a transducer in which electrodes are laminated at equal intervals. However, this type of transducer is slightly disadvantageous as compared with a transducer in which electrodes are laminated at equal intervals, when a large voltage is applied and large-amplitude excitation is required.

Therefore, the piezoelectric transducer according to the present invention is a conventional zircon
Compared with a transducer made of lead titanate-based piezoelectric ceramics, an effective permittivity equal to or higher than that obtained, in other words, a small impedance equal to or lower than that can be obtained. Next, in the transducer used in the present invention, the frequency can be adjusted by polishing the surface porcelain layer. Furthermore, since PbTiO ₃ -based piezoelectric ceramics are used, there is an advantage that essentially unnecessary vibrations are reduced and a good thickness resonance response is obtained.

(Example) A center frequency having a triple matching layer shown in FIGS. 1A and 1B as an example of an ultrasonic probe according to the present invention 3.
A 5MHz linear array medical ultrasound probe is described.
FIG. 1 (A) is a side view of the probe, and FIG. 1 (A) is a sectional view of the probe. In this embodiment, the piezoelectric transducer 10 is Pb
It has a laminated structure in which four electrodes 31 are embedded inside the TiO ₃ -based piezoelectric ceramics. The ceramic is Pb _0.85
A composition of Ca _0.15 Ti _0.95 (Mn _1/3 Sb _2/3 ) _0.05 O ₃ was used, and the production was performed by the method using the green sheet described above. The polarization was performed by applying a high DC electric field between the outer electrodes 32, 32 '. In the transducer immediately after polarization, the thickness of the ceramic layers 33 and 33 'on the surface was about 40% of the total plate thickness.
A resonance frequency of 3.5 MHz was obtained by polishing 33, 33 '. At this time, the thickness of the ceramic layers 33 and 33 'on the surface is 23% of the total plate thickness. The k _{eff of} this converter is 0.553, and P
Electromechanical coupling coefficient of single plate of bTiO ₃ -based piezoelectric ceramics k _t
8% greater than. Also, effective relative permittivity Is 1790 and about 9 times the value of a single plate of PbTiO ₃ -based piezoelectric ceramics. In FIG. 1, 11, 12, 13 are acoustic matching layers, and 44 is a backing. The acoustic matching layer is about 1/4 of the resonance wavelength
Adjusted to. 11 is acoustic impedance 1.92 × 10 ⁶ Kg / m ²
sec urethane resin, 12 is an epoxy resin mixed with an appropriate amount of quartz glass fine powder, acoustic impedance 4.12 × 10
⁶ Kg / m ²・ sec, 13 is acoustic impedance 14.2 × 10 ⁶ Kg / m ²
・ Sec optical glass. The shape of one produced piezoelectric transducer is 0.6 mm × 0.3 mm × 13 mm. The transducers were arranged at a predetermined interval to form a linear array having a length of about 10 to 13 cm, and used as a probe.

Next, using this probe, the frequency characteristics (round trip insertion loss characteristics) when ultrasonic waves returning toward an Al reflector placed 3 cm in water are received by the same probe It is shown by a solid line in the figure. The dotted line in FIG. 4 shows the frequency characteristics of the probe when a zircon / lead titanate-based piezoelectric ceramic single plate piezoelectric transducer is used in the piezoelectric transducer portion.
Both passband characteristics are equivalent, but a large ripple is observed outside the band in a probe using zircon / lead titanate-based piezoelectric ceramics due to spurious lateral effects. Furthermore, in order to test the resolution of both probes, a nylon wire with a diameter of 0.3 mm embedded at a specified interval in a gel-like substance that has ultrasonic attenuation characteristics (0.7 dB / cm / MHz) and acoustic impedance equivalent to those of a living body. It was evaluated to what degree it was disassembled. In the probe using the conventional piezoelectric ceramic single plate, at the depth of 12 cm, the nylon wires embedded at intervals of 1.0 mm could be seen at most, but in the probe according to the present invention, the nylon wires were embedded at intervals of 0.7 mm. I could see the separated nylon wires clearly.

As another embodiment according to the present invention, a conductive paste is applied to the entire surface of the green sheet without leaving a gap 22, and then laminated in the same manner as shown in FIG. 2, followed by pressure bonding and firing to manufacture a PbTiO ₃ -based ceramic laminate. To do. Insulators on the side of the stack
51 is formed, external electrodes 32, 32 'are provided, and a DC high electric field is applied between 32, 32' to perform polarization. In this way, it is possible to obtain a piezoelectric transducer having a shape in which the internal electrodes are widened as shown in FIG. Since the piezoelectric transducer shown in FIG. 5 does not have the gap 22, the impedance can be slightly lowered as compared with the transducer shown in FIG. When a probe having a triple matching layer as shown in FIG. 1 was manufactured using the piezoelectric transducer shown in FIG. 5, the same characteristics as those shown by the solid line in FIG. 4 were obtained. It was As described above, the overlapping length of the internal electrodes is preferably about the same as the length of the piezoelectric transducer (at least 80% or more).

(Effects of the Invention) As described above, the probe according to the present invention retains the advantage that the energy of spurious vibration due to the lateral effect, which is a characteristic peculiar to PbTiO ₃ -based piezoelectric ceramics, is extremely small, and further, It is possible to eliminate the drawback that the capacitance of the cable is easily affected by the low dielectric constant. Furthermore, it has an excellent feature that the effective electromechanical coupling coefficient k _eff can be increased and the frequency can be adjusted. Obtainable.

[Brief description of drawings]

1 (A) and 1 (A) are schematic views of an ultrasonic probe showing an embodiment of the present invention, and FIG. 2 shows an example of a laminated structure of a piezoelectric transducer portion used in the probe of the present invention. Fig. FIG. 3 is a perspective view showing an example of a laminated piezoelectric transducer according to the present invention, FIG. 4 is a frequency characteristic diagram of an ultrasonic probe, FIG. 5 is a diagram showing another embodiment of the present invention, and FIG. FIG. 4 is a diagram showing an example of a conventional ultrasonic probe. In the figure, 10 is a laminated piezoelectric transducer, 11, 12 and 13 are acoustic matching layers, 14 is a backing, 20 is a green sheet, 21 is a conductive paste, 22 is a gap, 30 is a PbTiO ₃ -based piezoelectric ceramics, 31 is an internal electrode. , 32 and 32 'are external electrodes, 33 and 33' are surface porcelain layers, 51 is an insulator, 61 is a rectangular plate transducer, 62 and 63 are acoustic matching layers, and arrows indicate polarization directions.

Claims (1)

[Claims] 1. A plurality of planar internal electrodes are formed inside a PbTiO ₃ -based piezoelectric ceramic plate at equal intervals, and the plurality of planar internal electrodes are formed on two sides of the PbTiO ₃ -based piezoelectric ceramic plate. The PbTiO ₃ -based piezoelectric ceramics plate sandwiched between the plurality of planar internal electrodes has a laminated structure in which external electrodes formed on two sides are connected to every other layer, and the PbTiO ₃ -based piezoelectric ceramic plates are alternately reversed in the laminating direction. A piezoelectric transducer having a structure that is polarized in a vertical direction and provided with piezoelectrically inactive ceramic layers on both end surfaces in the stacking direction, and an acoustic wave formed on one surface perpendicular to the stacking direction of the piezoelectric converter. An ultrasonic probe having a matching layer.