JPH09321365A - Manufacture of piezoelectric element for ultrasonic probe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the sensitivity of a probe by ensuring a correct dielectric constant and increasing electromechanical coupling coefficient. SOLUTION: A piezoelectric ceramic element 1 is provided by polarizing a rectangular board element. The rear face of the piezoelectric ceramic element 1 is mounted on a backing material 6, and an acoustically matching layer 7 is mounted on the front face. The piezoelectric ceramic element 1 is polarized within a range of a temperature T1, where the dielectric constant of the element becomes maximum, to a temperature T2, where the electromechanical coupling coefficient becomes maximum. The piezoelectric element with a higher electromechanical coupling coefficient compared with the conventional coefficients and with a correctly controlled dielectric constant is obtained by such polarization.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a piezoelectric element, for example, a piezoelectric ceramic element used in a probe for improving the performance of an ultrasonic probe device.

[0002]

2. Description of the Related Art In recent years, ultrasonic transmission / reception devices using piezoelectric ceramic elements have been used for various purposes. Among them, in medical equipment, an ultrasonic diagnostic method that allows observation of the inside of the body from the surface without causing any inconvenience to the human body is widespread. The ultrasonic medical diagnostic device used for this ultrasonic diagnostic method,
Piezoelectric ceramic oscillators are used as ultrasonic transducers. In this case, a probe composed of a single piezoelectric ceramic oscillator and a plurality of small piezoelectric ceramic oscillators are arranged in parallel to form an array piezoelectric ceramic oscillator in order to form a tomographic image of the inside of the body for diagnosis. The so-called scanning probe is used.

The ultrasonic diagnostic probe radiates ultrasonic waves to a human body, and the radiated ultrasonic waves receive reflected waves from various tissue parts in the body. The sensitivity of these probes is generally represented by the ratio of the reception level to the radiation level, and the higher the probe sensitivity, the deeper the diagnosis in the body becomes possible. Further, in ultrasonic diagnosis, the higher the frequency of the emitted ultrasonic waves, the higher the resolution, which can contribute to proper diagnosis. Therefore, ultrasonic waves applied to the probe tend to use higher frequencies. On the other hand, however, high-frequency ultrasonic waves have large attenuation in the body, weak reflected waves and low sensitivity, making it difficult to diagnose deep inside the body. Particularly, in a medical ultrasonic diagnostic probe, accurate and appropriate diagnosis is required for each part in the body to be diagnosed, and a highly sensitive and high-resolution probe is required.

On the other hand, the sensitivity of a probe using a piezoelectric ceramic vibrator can be expressed by an electromechanical coupling coefficient (K), which is widely used to show the magnitude of the piezoelectric effect. This piezoelectric ceramic element is imparted with piezoelectricity by applying a DC voltage between electrodes formed on the surface thereof, that is, by a polarization treatment. The polarization treatment is generally performed by applying an electric field of 2 to 3 KV / mm at a high temperature below the Curie temperature.

In the piezoelectric ceramic element thus polarized, the characteristics of the ceramic vibrating element are the same as those of the rectangular plate-shaped element as shown in FIG. 3, and the electromechanical coupling of the vibration mode in the thickness (t) direction. It depends on the coefficient (Kp). The larger the electromechanical coupling coefficient, the better the sensitivity.
Known from simulation results.

In order to improve the sensitivity of the probe, it is important to match the electrical impedances of the probe and the electric circuit of the device. For this purpose, there is a method of attaching an impedance converter to the device side or adjusting the electrical impedance of the probe. In the latter case, it is effective to adjust the capacitance of the element itself. However, the capacitance of the element is determined by the area, thickness, and dielectric constant of the material, but the shape is often restricted by the design of the probe, so elements with different dielectric constants should be used. It is generally manufactured.

[0007]

By the way, in order to change the dielectric constant of a material, a device of changing the composition of its components has been devised. In lead zirconate titanate (PZT), which is a general piezoelectric ceramic, the dielectric constant can be adjusted by adding components such as strontium (Sr), barium (Ba), and calcium (Ca), and changing the amount of addition. However, this method cannot obtain a material having a dielectric constant lower than that of undoped PZT, and if the composition is changed, the electromechanical coupling coefficient may be lowered.

In the present invention, in order to adjust the dielectric constant and increase the electromechanical coupling coefficient, the means such as changing the composition of the element material as described above is not adopted, but the temperature at the time of polarization treatment is within a predetermined range. To provide an appropriate dielectric constant and to increase the electromechanical coupling coefficient to increase the sensitivity of the probe, and to provide a method for manufacturing a piezoelectric element for an ultrasonic probe. The purpose is.

[0009]

The method for manufacturing a piezoelectric element for an ultrasonic probe according to the present invention, which has been made in order to achieve the above object, includes a method for applying a high direct current voltage to an element so that the piezoelectric element In the polarization treatment for imparting the property, the polarization treatment is performed within a range from a temperature T1 at which the dielectric constant of the element is maximum to a temperature T2 at which the electromechanical coupling coefficient is maximum. The material composition of the element is preferably lead zirconate titanate (PZ
T) is mainly used, and antimony (S
b), niobium (Nb) and strontium (Sr) are added.

As described above, according to the present invention, by setting the temperature during the polarization treatment within a predetermined range, it is possible to secure an appropriate dielectric constant and increase the electromechanical coupling coefficient. In this case, in a general piezoelectric material, the polarization temperature T1 at which the dielectric constant becomes maximum exists at 60 to 120 ° C., although it depends on the Curie temperature of the material. At temperatures higher than this, both the dielectric constant and the electromechanical coupling coefficient decrease. At temperatures below T1, the dielectric constant decreases, the electromechanical coupling coefficient increases, and it saturates at a certain temperature T2. The degree of decrease in dielectric constant due to the decrease in T2 and the polarization temperature changes depending on the Curie temperature. In general, a material having a low Curie temperature has a low T2 and a large decrease in dielectric constant due to a decrease in polarization temperature. Based on such physical properties, the present invention sets the polarization temperature within the range of T1 to T2 and performs the polarization treatment. By combining the control of the polarization temperature and the composition of the element, the electromechanical coupling higher than the conventional one is achieved. It is possible to provide a piezoelectric element having a coefficient and a controlled capacitance.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A method for manufacturing a piezoelectric element for an ultrasonic probe according to the present invention will be described below. Prior to that, an embodiment in which the piezoelectric element for an ultrasonic probe is applied to an ultrasonic diagnostic apparatus. Will be described.

FIG. 1 shows an assembling process of a probe for a scanning ultrasonic diagnostic apparatus. In FIG. 1, (1)
As shown in (1), the rectangular thin plate piezoelectric ceramic vibrating element 1 is usually formed by joining electrodes 2 and 3 such as silver films on both surfaces thereof. In this case, the electrode has a partially wound structure so that the lead wire can be pulled out from one side as described later.
After electrodes are formed on both surfaces of the piezoelectric ceramic vibrating element 1, they are polarized in the thickness direction. Polarization is usually performed in insulating oil such as silicone oil at a temperature above the Curie temperature.
It is performed by applying a high voltage of about several thousand volts (v) per mm. The polarization treatment temperature at that time will be described later, but at a predetermined polarization temperature, about 30 minutes to 1 hour, 2 to 3 k
An electric field of v / mm is applied.

Next, as shown in (2), after polarization treatment,
Each lead wire is connected to both long side directions of the rectangular piezoelectric ceramic vibrating element 1 on which electrodes are formed. In this case, a copper foil as the ground side lead wire 4 on one side and a flexible lead wire 5 on the other side are partially wrapped around, respectively, and joined to the electrodes 2 and 3 drawn out on one side by soldering. To be done.

Then, as shown as (3), the lead wire 4
The acoustic wave absorbing material (backing material) 6 such as a rubber material is attached to the lead wire soldering side of the ceramic vibrating element 1 in which
Is heat-pressed with an organic adhesive or the like.

After the sound absorbing material 6 is bonded in this manner, the acoustic matching layer 7 is bonded and formed on the surface of the ceramic vibrating element 1 opposite to the bonding surface, as indicated by (4). The acoustic matching layer 7 is an organic resin such as an epoxy resin, a filler-containing epoxy resin, or a film so that the acoustic impedance can be matched so that the sound wave from the ceramic vibrating element 1 efficiently propagates to a diagnostic object such as a human body. The material is used. The thickness of the acoustic matching layer 7 is generally adjusted to be ¼ of the wavelength λ of the radiation frequency. Further, the acoustic matching layer 7 is formed by appropriately selecting one layer or two layers according to the usage conditions, depending on the required characteristics of the probe and the like.

After forming the acoustic matching layer 7 as described above,
In order to obtain the scanning ultrasonic diagnostic probe as shown as (5), in the long side direction of the ceramic vibrating element 1,
The acoustic matching layer 7 on the outer surface, the ceramic vibrating element 1 layer, and a part of the backing material 6 layer are continuously cut and cut at right angles and at predetermined intervals. By this series of dicing, it is possible to form the array-shaped vibrating element 10 in which a large number of rectangular plate-shaped elements 8 are arranged in parallel. In this case, the acoustic matching layer 7
It is also possible to form the array-shaped vibrating element 10 by similarly cutting a part of the ceramic vibrating element 1 layer and a part of the backing material 6 layer without forming the above.

FIG. 2 is an enlarged view showing a part of the array-like vibrating element 10 formed as described above.
FIG. 3 shows a rectangular plate-shaped element 8 forming the array-shaped vibrating element 10.
FIG. 2 and 3, the same reference numerals as those in FIG. 1 indicate the same parts. As shown in FIG.
The flexible lead wire 5 soldered to the electrode 2 or 3 formed on the surface of the ceramic piezoelectric vibrating element is designed in advance so that the conductor portion 9 matches the width W of the rectangular plate element 8. Has been done. In FIG. 3, the ratio (W / t) of the width W after cutting to the thickness t of the ceramic vibrating element is set to 0.6 or less so that unnecessary vibration other than vibration in the thickness direction does not occur. Composed.

In the piezoelectric ceramic vibrator for a probe manufactured by the above process, an acoustic lens is usually installed on the front surface of the acoustic matching layer, and the flexible lead wire 5 is used.
Can be connected to a cable connected to the main body of the diagnostic apparatus, a connector can be further attached, and the case can be housed in a case to be a probe for a scanning ultrasonic diagnostic apparatus component.

[0019]

EXAMPLES Next, a method of manufacturing a piezoelectric element in the ultrasonic probe configured as described above will be described. In a system in which a fixed amount of antimony (Sb) and niobium (Nb) were added to lead zirconate titanate (PZT), the polarization temperature of a piezoelectric material in which the addition amount of strontium (Sr) was adjusted to control the Curie temperature was measured. After changing the treatment, the polarization temperature dependence of the dielectric constant ε and the electromechanical coupling coefficient Kp was investigated. Table 1 shows the evaluation results.

[0020]

[Table 1]

In Examples 1 to 4 shown in Table 1, each dimension of the rectangular plate-shaped element 8 as the piezoelectric element for the probe has a width W of 5 m shown in FIG.
m, the thickness t was 10 mm, the length was 15 mm, the electric field applied in the polarization treatment was 2 kv / mm, and the application time was 30 minutes.

Here, as understood from the evaluation results shown in Table 1, the curie temperature is 260 ° C. by adjusting the addition amount of Sr.
In the piezoelectric material of Example 1, the temperature T1 at which the dielectric constant (ε) of the element is maximum is 100 ° C., and the temperature T2 at which the electromechanical coupling coefficient (Kp) is maximum is 0.
° C.

In addition, in the piezoelectric materials of Examples 2 and 3 in which the Curie temperatures are 210 ° C. and 160 ° C. by adjusting the Sr addition amount, the temperature T1 at which the dielectric constant (ε) of the element is maximum is
The temperature T2 is 100 ° C. and the maximum electromechanical coupling coefficient (Kp) is 25 ° C. Similarly, in the piezoelectric material of Example 4 in which the Curie temperature was set to 130 ° C. by adjusting the addition amount of Sr, the temperature T1 at which the dielectric constant of the element becomes maximum.
Is 60 ° C., and the temperature T2 at which the electromechanical coupling coefficient is maximum is 25 ° C.

As described above, by adopting the polarization treatment according to the method of the present invention, the dielectric constant ε can be controlled within the range of 5 to 15% without changing the composition. Further, the electromechanical coupling coefficient Kp can be improved by 2 to 3% without significantly impairing the dielectric constant.

Then, the polarization treatment temperature 2 in Example 1
A 2 MHz probe was manufactured according to the embodiment shown in FIG. 1 using a 5 ° C. element and a device that was polarized at 100 ° C. as a comparative example so that the usual dielectric constant was maximized.
Sound waves were radiated in water and the reflection characteristics from the acrylic plate were measured to compare the two. As a result, the sensitivity of the probe of Example 1 could be improved by 2 to 2.5 dB compared to that of Comparative Example.

[0026]

As is apparent from the above description, according to the method of manufacturing a piezoelectric element for an ultrasonic probe of the present invention, it is possible to obtain higher electric power than conventional by controlling the polarization temperature and the composition of the element. A piezoelectric element having a mechanical coupling coefficient and a controlled dielectric constant (capacitance) can be obtained.

Particularly, the electromechanical coupling coefficient can be improved by 2 to 3% as compared with the conventional one, and as a result, the sensitivity of the probe can be improved by 2 to 2.5 dB. In other words, it is possible to lower the polarization voltage corresponding to the increase in the sensitivity of the probe,
By lowering the polarization voltage, it is possible to prevent the occurrence of so-called edge discharge, which is effective for the polarization treatment of the element having a complicated shape.

Further, since the dielectric constant can be controlled within the range of 5 to 15%, the change in capacitance due to the change in the driving frequency of the probe can be canceled by adjusting the dielectric constant. . Further, it becomes possible to absorb the variation in the characteristics of each raw material production lot by adjusting the polarization temperature.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a process of assembling a probe for a scanning ultrasonic diagnostic apparatus using a piezoelectric element obtained by a manufacturing method of the present invention.

FIG. 2 is a partially enlarged view of an array-shaped vibrating element that constitutes the probe shown in FIG.

FIG. 3 is an explanatory diagram of a rectangular plate element that constitutes the array-shaped vibrating element shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric ceramic element 2,3 Electrode 4 Lead wire 5 Flexible lead wire 6 Backing material 7 Acoustic matching layer 8 Rectangular plate element 9 Flexible lead wire conductor portion 10 Array vibration element

Claims (2)

[Claims] 1. In a polarization treatment for imparting piezoelectricity to an element by applying a high DC voltage to the element, a temperature T1 at which the dielectric constant of the element becomes maximum to a temperature T2 at which the electromechanical coupling coefficient becomes maximum. A method for manufacturing a piezoelectric element for an ultrasonic probe, characterized in that polarization treatment is performed within the range. 2. As a material for forming the device, antimony (S) is added to lead zirconate titanate (PZT).
The method for producing a piezoelectric element for an ultrasonic probe according to claim 1, wherein b), niobium (Nb) and strontium (Sr) are added.