JP7667664B2 - Piezoelectric vibrator and its manufacturing method - Google Patents

Description

The present invention relates to a piezoelectric transducer used in ultrasonic probes of ultrasonic imaging inspection devices such as medical ultrasonic diagnostic devices, fish finders, and sonars, and a method for manufacturing the same.

Medical ultrasound diagnostic equipment, fish finders, sonars, and other ultrasound imaging devices irradiate an object with ultrasound waves via an ultrasound probe, and create an image of the inside of the object based on the reflected signal (echo signal) generated by the reflected wave from inside the object. Medical ultrasound diagnostic equipment and ultrasound imaging devices primarily use electronically scanned array-type ultrasound probes with ultrasound transmission and reception capabilities, and high-performance piezoelectric transducers are used for this purpose.

A typical ultrasonic probe has a backing material, a piezoelectric transducer bonded onto the backing material with electrodes formed on both sides of the piezoelectric body, and an acoustic matching layer bonded onto the piezoelectric transducer. The piezoelectric transducer and acoustic matching layer are formed into multiple channels by array processing. An acoustic lens is formed on the acoustic matching layer. The electrodes of the piezoelectric transducer corresponding to each channel are connected to the main body of the medical ultrasonic diagnostic device or ultrasonic imaging device via a control signal board (flexible printed circuit, FPC) and a cable.

The piezoelectric transducer used here is generally subjected to DC polarization. DC polarization is performed by forming electrodes on the top and bottom surfaces of the piezoelectric ceramics or single crystal plate, and applying a DC voltage in air or silicone oil for about 1 to 30 minutes. In the array-type ultrasonic probe of a medical ultrasonic diagnostic device, which is the mainstream for this application, a high-performance piezoelectric material with a high relative dielectric constant and an electromechanical coupling coefficient k 33 and a piezoelectric constant d ₃₃ in the length direction of the rod is desired in order to match the electrical impedance of a fine rectangular piezoelectric transducer (for example, width 0.15 mm, height 0.3 mm, length 10 mm) to about ₅₀ Ω of the connection cable. For this reason, a high-performance piezoelectric material, lead magnesium niobate Pb (Mg _1/3 Nb _2/3 ) O ₃ (PMN)-lead titanate PbTiO ₃ (PT) solid solution single crystal, has been widely used since 2000.

In addition, efforts are being made to improve the polarization process in order to enhance the performance of piezoelectric vibrators. For example, Patent Document 1 shows that the electromechanical coupling coefficient and dielectric constant of a piezoelectric element can be improved by performing an electric field cooling polarization process on a two-component piezoelectric material based on lead zinc niobate and lead titanate while applying a DC voltage from high to low temperatures.

On the other hand, Patent Documents 2 and 3 show that by manufacturing a piezoelectric element by subjecting a PMN-PT piezoelectric vibrator to an AC polarization process in which an AC electric field is applied instead of a DC electric field, the relative dielectric constant, coupling coefficient, and piezoelectric constant of the element can be improved. In addition, Non-Patent Document 1 shows that high piezoelectric properties can be obtained with a low AC electric field by performing AC polarization at a high temperature of around 80°C.

JP 7-50898 A JP 2014-045411 A JP 2014-187285 A

Yiqin Sun et al. Jpn.J. Apply. Phys. 59, SPPD08 (2020)

However, even in the ultrasonic transducers using the piezoelectric single crystal materials disclosed in the above-mentioned conventional prior art documents, further improvement in performance is required in order to obtain ultrasonic probes with higher performance, and there has been a demand for a piezoelectric transducer that reduces the dielectric loss that causes heat generation without generating unnecessary vibrations, and has improved relative dielectric constant and piezoelectric constant _d33 , etc.

The present invention was made in consideration of the above background technology, and aims to provide a piezoelectric vibrator that has higher dielectric and piezoelectric properties and a high manufacturing yield, and a method for manufacturing the same.

The present invention provides a piezoelectric vibrator formed of a rectangular single crystal plate composed of a lead complex perovskite compound containing magnesium oxide, niobium oxide, and lead titanate, and having electrodes on both sides, wherein the single crystal plate has a crystal orientation [001] plane as its main surface and [100] and [010] planes as its side surfaces, a ratio L/W of the length L of the single crystal plate in the [100] direction to the width W of the single crystal plate in the [010] direction is 3 to 10, and a ratio Nt/ _N31 of a frequency constant N31 expressed by the product of a resonance frequency fr of a transverse vibration mode perpendicular to the polarization direction and the length L, to a frequency constant _Nt expressed by the product of a resonance frequency fr of a longitudinal vibration mode parallel to the polarization _direction and a thickness t of the _single crystal plate is 2.75 to 3.9 at 25°C. In particular, the ratio of the frequency constants N _t /N ₃₁ is preferably 2.8 to 3.6 at 25° C., and more preferably 2.9 to 3.2.

The lead complex perovskite compound is a piezoelectric vibrator made of a _two -component system of lead magnesium niobate (100-x)Pb(Mg1 _/ 3Nb2 _/3 ) _O3 and lead titanate xPbTiO3, containing x mol % (x is a positive value) of lead titanate and (100-x) mol % of lead magnesium niobate, and has a temperature Tm showing a maximum dielectric constant of 125°C to 150°C, with x being 27 or more and 32 or less. Furthermore, the phase transition temperature _Trt of the crystal structure of the lead complex perovskite compound is preferably 60°C or more and 110°C or less.

The present invention also provides a method for manufacturing a piezoelectric vibrator, which includes a step of subjecting the single crystal plate that has been initially AC polarized to a heat treatment, AC polarization, and depolarization at a temperature of 150° C. or higher and 300° C. or lower, and repeating the above steps at least two to four times, after which the single crystal plate is subjected to a final AC polarization.

The AC electric field in the AC polarization is 1.0 kVrms/cm to 10 kVrms/cm (rms: Root Mean Square), and an AC electric field having a frequency of 0.01 Hz to 200 Hz using a triangular wave or a sine wave is applied for 4 to 200 cycles. The temperature in the AC polarization is preferably in the range of 20°C to 90°C.

After the final AC polarization , a DC electric field in the range of 2.0 kV/cm to 10 kV/cm is applied between the front electrode and the back electrode of the single crystal plate for 6 seconds to 1 minute. The strength of the DC electric field is preferably 0.8 to 1.5 times the strength of the AC electric field.

Furthermore, it is preferable that the AC electric field is 2.0 kVrms/cm or more and 4.0 kVrms/cm or less, the DC electric field is 3.0 kV/cm or more and 5.0 kV/cm or less, and the polarization temperature is in the range of 40°C to 70°C.

According to the piezoelectric vibrator and its manufacturing method of the present invention, it is possible to easily and reliably manufacture a piezoelectric vibrator that suppresses the generation of unwanted vibrations, has small dielectric loss, and has a high relative dielectric constant, coupling coefficient _k33 , and piezoelectric constant _d33 . This makes it possible to stably provide an ultrasonic device that has high resolution, high sensitivity, and low heat generation.

1 is a perspective view showing an example of the appearance of a piezoelectric vibrator according to an embodiment and example of the present invention; 1A is a process diagram showing a manufacturing process of a piezoelectric vibrator according to an embodiment and example of the present invention, and FIG. 1B is a process diagram showing a manufacturing process of a conventional piezoelectric vibrator. This is a graph showing an example of a process for manufacturing a piezoelectric vibrator according to an embodiment and example of the present invention, in which after electrodes are formed, AC polarization is performed, and then heat treatment depolarization is performed at 150° C. or higher, and this process is repeated 2-3 times. 11 is a graph showing the relationship between the frequency constant _N31 in the length direction and _Nt / _N31 , which is the ratio of the frequency constant _Nt in the thickness direction to the frequency constant N31 in the length direction, in a piezoelectric vibrator according to _an embodiment and example of the present invention.

One embodiment of the present invention will be described below with reference to Figures 1 to 4. First, the piezoelectric vibrator of this embodiment and a method for manufacturing the same will be described below.

First, piezoelectric single crystals are produced that contain lead magnesium niobate (Pb(Mg1 _/ 3Nb2 _/3 ) _O3 (hereinafter referred to as PMN), lead indium niobate (Pb(In1 _/ 2Nb1 _/2 ) _O3 (hereinafter referred to as PIN), lead titanate _PbTiO3 (hereinafter referred to as PT), etc. For this purpose, the following multiple raw materials are mixed in a predetermined ratio. The raw materials are _Pb3O4 , MgO, _Nb2O5 , _In2O3 , _{TiO2, ZrO} , etc., all _with a purity of 99.9 _% or more. ₂ is used. These raw materials are weighed in a predetermined ratio. The weighed raw materials are mixed in a wet state using a ball mill, zirconia balls, and distilled water. The mixed raw materials (hereinafter referred to as the mixed raw materials) are dried for a predetermined time. The dried mixed raw materials are pre-fired several times at 850 to 950 ° C. This pre-fire produces raw material powder of the mixed raw materials. In addition, some raw materials, especially MgO or In ₂ O ₃ , and Nb ₂ O ₅ are mixed in advance to produce MgNb ₂ O ₆ or InNbO ₄ with a columbite structure at about 1200 ° C., and another material such as TiO ₂ or Pb ₃ O ₄ may be mixed with this.

A water-soluble binding agent (binder) such as polyvinyl alcohol (PVA) is added to the raw powder produced at 0.5 to 10% of the raw powder. The raw powder to which the binder has been added is molded into a specified shape using a press machine. After molding, a binder removal process is carried out at 500°C for several hours. The molded body from which the binder removal process has been carried out is fired at 1100 to 1250°C for several hours. Hereinafter, the fired molded body will be referred to as a single crystal ceramic.

The ceramics for single crystals are placed in a platinum crucible with a diameter and length of 25 to 100 mm and a diameter of 50 to 200 mm. In some cases, a small amount of lead oxide or boron oxide is added to lower the melting point of the ceramics. A seed single crystal with the same composition and crystal orientation of [001] or [110] is placed in the lower part of the platinum crucible. The length of the [001] or [110] plate of the seed single crystal is 20 to 70 mm. The upper part of the platinum crucible containing the ceramics and seeds is sealed by welding. The sealed platinum crucible is kept at a temperature of 1100 to 1400°C for 5 to 15 hours. This allows the ceramics in the platinum crucible to completely melt. At this time, in order to prevent the seeds placed at the bottom of the platinum crucible from melting, a temperature gradient is provided from the bottom to the top of the platinum crucible, with the temperature increasing at 20 to 60°C/cm.

Then, in order to grow a long single crystal, the platinum crucible is lowered at 0.2 to 0.6 mm/hour in accordance with the temperature gradient and the growth of the single crystal. It is also possible to produce a large, long ingot with high uniformity by adding ceramic raw material during the crystal growth. In order to increase the uniformity of the temperature and composition inside the crucible during growth, the platinum crucible is rotated at a speed of 3 to 30 times per minute. Through the above steps, the growth of the single crystal is carried out over a total of 10 to 30 days. Through the above manufacturing steps of the Bridgman method, a piezoelectric single crystal ingot with a diameter of 25 to 100 mm and a length of 50 to 150 mm is produced.

That is, the piezoelectric single crystal ingot to be produced contains at least lead titanate ( _PbTiO3 ) and a relaxor-type lead complex perovskite compound (Pb( _B1 , _B2 ) _O3 ): ( _B1 is at least one of magnesium and indium, and _B2 is niobium). In addition to the Bridgman method, other methods for producing the piezoelectric single crystal ingot include the flux method, the molten Bridgman method, the TSSG method (Top Seeded Solution Groth), the horizontal melting Bridgman method, and the CZ method (Czochralski method). The present invention is not limited to the above-mentioned method for producing the piezoelectric single crystal ingot. It is sufficient to produce a piezoelectric single crystal by any of the above methods.

The lead complex perovskite compound has a rhombohedral to tetragonal phase transition temperature (hereinafter referred to as T _rt ), a rhombohedral to monoclinic phase transition temperature (hereinafter referred to as T rm ), and a monoclinic to tetragonal phase transition temperature (hereinafter referred to as T _mt ) in the range of 60°C to 110°C. Here, when the phase transition temperature T _rt is less than 60°C, as described later, the relative dielectric constant and coupling coefficient near room temperature are high, but the temperature dependence of these electrical properties becomes significant. Also, when the phase transition temperature T _rt exceeds 110°C, as described later, the desired relative dielectric constant and piezoelectric properties cannot be obtained at room temperature. From the above, it is desirable that the temperature range of _{the phase transition temperature T rt is} 60°C to 110°C. In order to combine both excellent piezoelectric properties and temperature characteristics, the phase transition temperature is more preferably from 80°C to 95°C.

Specifically, the lead complex perovskite compound has 68 mol% or more and 73 mol% or less of lead magnesium niobate or lead indium niobate, and 27 mol% or more and 32 mol% or less of lead titanate. This is because, when the ratio of lead titanate to the lead complex perovskite compound is less than 27 mol%, high relative dielectric constant, coupling coefficient, and piezoelectric constant cannot be obtained. Also, when the ratio of lead titanate to the lead complex perovskite compound exceeds 32 mol%, the phase transition temperature T _rm becomes 60°C or less, and the temperature dependence characteristics of the relative dielectric constant and coupling coefficient become remarkable, especially at room temperature to 50°C. Therefore, in order to maintain a high relative dielectric constant, coupling coefficient, and piezoelectric constant, and to reduce the above-mentioned temperature dependence characteristics at room temperature to 50°C, it is necessary to make the ratio of lead titanate in the lead complex perovskite compound 27 mol% or more and 32 mol% or less. More preferably, it is 29 mol% or more and 31 mol% or less.

As described above, the piezoelectric single crystal manufactured in this embodiment has a phase transition temperature T _rt of the crystal structure of the lead complex perovskite compound of 60° C. or more and 110° C. or less, and this lead complex perovskite compound is a two-component piezoelectric body of lead magnesium niobate (100-x)Pb(Mg _1/3 Nb _2/3 )O ₃ and lead titanate xPbTiO ₃ system. Here, the compound contains x mol % (x is a positive value) of lead titanate and (100-x) mol % of lead magnesium niobate. The temperature Tm showing the maximum relative dielectric constant at this time is 125° C. to 150° C., and x is 27 or more and 32 or less.

The lead complex perovskite compound may further contain lead indium niobate and lead zirconate. If the ratio of lead indium niobate to the lead complex perovskite compound exceeds 30 mol%, it becomes difficult to produce a single crystal of the lead complex perovskite compound, and a single crystal of the lead complex perovskite compound composed of three components (lead indium niobate, lead magnesium niobate, lead titanate) with high uniformity may not be obtained.

Furthermore, in order to maintain a high relative dielectric constant and coupling coefficient and reduce the above-mentioned temperature-dependent characteristics at room temperature to 50°C, the lead complex perovskite compound may contain 0 mol% to 30 mol% lead indium niobate, 36 mol% to 68 mol% lead magnesium niobate, and 27 mol% to 32 mol% lead titanate, the total of which may be 100 mol%. Furthermore, the piezoelectric single crystal of this embodiment may further contain 15 mol% or less of lead zirconate.

Here, the crystal orientation of the piezoelectric vibrator used in the ultrasonic probe is mainly a piezoelectric single crystal with a [001] orientation, which has excellent piezoelectric properties. Furthermore, a small amount (2.0 mol% or less) of manganese oxide or the like may be added to the piezoelectric single crystal of these piezoelectric vibrators. Here, in this embodiment and the present invention, the crystal orientation is the orientation shown by the arrow in Figure 1, and the length L, width W, and thickness t of the piezoelectric vibrator 1 also correspond to the orientation shown in Figure 1.

In order to set the driving center frequency of the ultrasonic probe to 2 MHz or more and 12 MHz or less, the piezoelectric transducer of the ultrasonic probe used in medical ultrasonic diagnostic equipment and ultrasonic image inspection equipment has a thickness of, for example, 0.05 mm or more and 0.5 mm or less.

Using the single crystal ingot obtained by the above method, a diamond blade or wire saw having a thickness of 0.1 to 0.5 mm is used to produce multiple wafers (hereinafter referred to as single crystal wafers) having a thickness of 0.2 to 0.7 mm from near the center of the single crystal ingot. As shown in Figure 1, the thickness direction of the single crystal wafer is the crystal orientation [001], the length direction L is [100], and the width direction W is [010]. Next, by lapping or polishing, a single crystal plate having a thickness of, for example, 0.05 mm to 0.5 mm and a crystal orientation of [001] on the surface on which the electrode is to be produced is produced.

Electrodes are then formed on the surface of the single crystal plate. The electrodes are made of baked silver or gold, or gold, platinum, or nickel made by sputtering or plating, and are formed on the front and back of the single crystal plate with a thickness of about 100 nm to 5000 nm. Hereinafter, the electrode provided on the front of the single crystal plate is called the front electrode, and the electrode provided on the back of the single crystal plate is called the back electrode. When attaching electrodes by sputtering, vapor deposition, or plating, it is desirable to apply chromium (Cr), nickel (Ni), titanium (Ti), palladium (Pd), etc., as a base electrode with a thickness of about 10 nm to 100 nm in order to improve adhesion with the single crystal plate. Hereinafter, the single crystal plate provided with electrodes is called a piezoelectric single crystal vibrator.

The next AC polarization process is carried out on this unpolarized piezoelectric single crystal vibrator. The polarization electric field in the AC polarization process is an AC electric field of a sine wave or triangular wave with a frequency of 0.01 Hz to 200 Hz without offset (the absolute value of the maximum voltage is equal to the absolute value of the minimum voltage). A frequency of less than 0.01 Hz can provide the effects of the characteristics of the present application described below, but is not preferable because it increases the working time. Furthermore, a frequency of more than 200 Hz is likely to cause fine cracks in the piezoelectric single crystal vibrator and insulation breakdown due to heat generation. As a result, the piezoelectric single crystal vibrator becomes more fragile and the manufacturing yield decreases.

For these reasons, the frequency of the AC electric field must be in the range of 0.01 Hz to 200 Hz. The electric field in this AC electric field is expressed as the root-mean-square (rms), which is the most commonly used value of the AC electric field. The electric field of the AC polarization required here is 1 to 5 times the coercive electric field Ec of the piezoelectric vibrator at the temperature. In an AC electric field equivalent to the coercive electric field, the increase rate of the relative dielectric constant and the piezoelectric constant in AC polarization is 10% or less compared to DC polarization. In addition, in an AC electric field exceeding 5 times the coercive electric field Ec, the piezoelectric single crystal vibrator is prone to unwanted vibrations, fine cracks, and insulation breakdown due to heat generation and deformation. As a result, the piezoelectric single crystal vibrator becomes more likely to break. For these reasons, the electric field of the AC polarization must be in the range of 1 to 5 times the coercive electric field.

Specifically, the AC electric field is 1.0 kVrms/cm or more and 10 kVrms/cm or less. If the AC electric field is less than 1.0 kVrms/cm, it is difficult to obtain the characteristic improvement effect of the present application described later. Furthermore, if the AC electric field exceeds 10 kVrms/cm, heat is generated in the single crystal oscillator, which makes the single crystal oscillator more likely to break. More preferably, the AC electric field is in the range of 2.0 kVrms/cm or more and 4.0 kVrms/cm or less. One cycle of the AC electric field is the process in which the AC electric field starts at 0 kVrms/cm, goes through one wavelength (one period), and ends at 0 kV/cm.

The AC polarization process is a process of applying the polarization signal through the electrodes (front electrode and back electrode) in the thickness direction of the piezoelectric single crystal vibrator for 4 to 200 cycles. If the number of cycles is less than 4, it is difficult to obtain the characteristic improvement effect of the present invention described later. Furthermore, if the number of cycles exceeds 200, heat is generated in the piezoelectric single crystal vibrator, and as a result, large piezoelectric single crystal vibrators, particularly those with an electrode area exceeding 2.0 ^cm2 , become easily broken. The number of cycles and electric field also depend on the polarization temperature, and a high voltage and number of cycles are required near room temperature, while a relatively low polarization voltage and number of cycles can be obtained at a high temperature of around 80°C.

For this reason, more preferably, the polarization temperature is 40°C to 70°C, the AC electric field is in the range of 2.0 kVrms/cm to 4.0 kVrms/cm, and the number of cycles is 10 to 50. The number of applications (cycles) of the AC electric field may be determined according to the composition and dimensions of the single crystal material. In addition, the AC polarization step and the final DC polarization step are preferably performed in a thermostatic chamber at a temperature (e.g., 50°C) below the phase transition temperature ( _Trt , _Trm , _Tmt ) in order to maintain the polarization state. That is, in order to avoid partial reversal of polarization or reduction in piezoelectricity when the phase transition temperature is exceeded, it is necessary to perform the polarization step in a stable temperature environment below the phase transition temperature.

As shown in Figures 2 and 3, the AC-polarized piezoelectric single crystal vibrator is heat-treated at a temperature of 150°C to 300°C, depolarized, and this process is repeated at least two to four times before the final AC polarization. Furthermore, DC polarization may be performed after the final AC polarization, and it is preferable that DC polarization is performed continuously after the final AC polarization process. This is mainly because when a vibrator is produced using only AC polarization, uneven polarization processing occurs within the surface, leading to the generation of unnecessary vibrations. For this reason, for example, the electric field used in DC polarization is preferably 0.8 to 1.5 times the AC polarization electric field Vrms/cm. In this way, by combining multiple AC polarizations and the final DC polarization, a piezoelectric vibrator with excellent electrical properties can be stably produced with a high manufacturing yield. Specifically, the electric field used in DC polarization is preferably 3.0 kV/cm to 5.0 kV/cm. The time for which DC polarization is performed is, for example, from room temperature (15 to 25°C) to 50°C, and generally from 10 seconds to 1 minute.

The piezoelectric constant d ₃₃ was measured using a Berlin Court type Piezod ₃₃ Meter, ZJ-4D, from the Institute of
Measurement was performed using an Acoustic of Academia Sinica at 25° C. Measurement of the relative dielectric constant and dielectric loss was performed using an LCR meter manufactured by Hioki Electronics Corporation at 1 kHz, 1 Vrms, and 25° C.

(Examples 1 to 10)
A single crystal [001] plate of lead magnesium niobate-lead titanate (Pb(Mg _1/3 , Nb _2/3 )O ₃ -PbTiO ₃ ) with a PbTiO ₃ content of 25 to 35 mol % and an outer dimension of 12×4 mm to 8×80 mm and a thickness of 0.2 mm to 0.5 mm is prepared by polishing and cutting. Then, chromium (Cr) is provided to a thickness of 20 nm on the upper and lower surfaces of the [001] plate (e.g., 15×4.0 mm) by a sputtering device. Gold is provided to a thickness of 200 nm on the chromium by a sputtering device to form electrodes. Next, the single crystal [001] plate with the electrodes provided thereon is cut using a dicer blade to produce a piezoelectric vibrator.

1 is a schematic diagram of the appearance of a piezoelectric vibrator, which is a piezoelectric single crystal vibrator with a length and width of 12 mm x 60 mm and a thickness of 0.3 mm. The phase transition temperature T _rt of this piezoelectric vibrator is about 60°C to 100°C. The Curie temperature Tc is 110°C to 165°C. The coercive field Ec measured using a Sawyer Tower circuit is 2.0 to 3.5 kV/cm at room temperature. AC polarization and DC polarization were performed between the electrodes of this piezoelectric vibrator under the conditions shown in Table 1.

FIG. 2 is a process diagram (a) of the repeated AC polarization of the present invention, and a process diagram (b) of the conventional AC polarization performed only once. FIG. 3 is a schematic diagram showing the AC polarization-depolarization-AC polarization of the present invention. In FIG. 3, the triangular waveform indicates temperature on the left vertical axis, and the horizontal axis indicates the change in time. Furthermore, in the ranges a, b, and c enclosed by squares in the triangular waveform, the value of the waveform is shown on the right vertical axis corresponding to the voltage during AC polarization. In this embodiment, the number of repetitions is shown as three, but it is the same for two to five times. The voltage, frequency, and number of cycles used here can be changed as appropriate according to the piezoelectric vibrator material, shape, and polarization temperature. FIG. 4 shows the relationship between the frequency constant N _t in the thickness direction of the piezoelectric vibrator of one embodiment of the present invention and N _t /N ₃₁ , which is the ratio of the frequency constant N ₃₁ in the length direction.

In this example, 24 hours after the polarization treatment, the room temperature dielectric constant, dielectric loss DF (%), piezoelectric constant d33 (pC/N), resonant frequency (fr) and antiresonant frequency (fa) in the thickness and length directions were measured, and the frequency constants _Nt and _N31 (and their ratio _Nt / _N31 ) were calculated, respectively.

The following Tables 1 and 2 show the various characteristics of a piezoelectric vibrator polarized by applying an AC electric field, along with the various characteristics of a PMN-PT piezoelectric vibrator of the same shape to which an AC electric field is not applied. The values in the tables are the average values of four samples for each example. In Table 1, the length L and width W of the piezoelectric vibrator are 12 mm and 4 mm (L/W = 3) for Examples 1-6, 60 mm x 12 mm (L/W = 5) for Examples 7-8, and 60 mm x 7 mm (L/W = 8.6) for Examples 9-10. All reference examples are 60 mm x 12 mm (L/W = 5). All thicknesses are 0.3 mm.

As is clear from the results of Table 1, when AC polarization is performed under the conditions of the present invention, the ratio Nt/ _N31 of the frequency constant N31 expressed by the product of the resonance frequency fr in the transverse vibration mode perpendicular to the polarization direction and the length L of the longest side in the vibration direction, which is the length of the [ ₁₀₀ ] direction of the single crystal plate, and the frequency constant _Nt expressed by the product of the resonance frequency fr in the longitudinal vibration mode parallel to the _polarization direction and the thickness t in the vibration direction of the single crystal plate, is 2.75 to 3.9 at 25 ° C., and it has become clear that high electrical characteristics can be manufactured with a high manufacturing yield. Figure 4 shows the preferred range of the frequency constant _N31 in the length direction of this piezoelectric vibrator and the ratio _Nt / _N31 of the frequency constant _Nt in the thickness direction and the frequency constant _N31 in the length direction. The value of _Nt / _N31 in the range indicated by α in Figure 4 is the preferred range, and more preferably the range indicated by β.

In the present invention, the ratio _{Nt/N31 of the frequency constants Nt and N31 is} specified to be 2.75 to 3.9 at 25° C., and more preferably 2.8 to 3.6, because a piezoelectric vibrator having satisfactory electrical characteristics cannot be obtained at 2.75 or less, and the phase transition temperature becomes 60° C. or less at 3.9 or more, resulting in large temperature characteristics of the piezoelectric characteristics. Therefore, from both of the above viewpoints, the ratio _Nt / _N31 of the frequency constants is preferably 2.8 to 3.6 at 25° C., and more preferably 2.9 to 3.2.

As shown in Tables 1 and 2, it is clear that the piezoelectric vibrators of Examples 1-10 having an appropriate frequency constant ratio _Nt / _N31 exhibit superior electrical characteristics compared to vibrators outside these ranges. In addition, when the presence or absence of unwanted vibrations in the examples was confirmed, all were good without any problems. Furthermore, the dielectric breakdown of the piezoelectric vibrators of the examples was 5% or less, and a high manufacturing yield of 95% or more was obtained. However, half of the samples of Reference Example II (2/4) experienced dielectric breakdown, and the remaining two also showed large unwanted vibrations. As shown in Table 2, in the present invention, the value of the relative dielectric constant is a high value of 8000 or more and 15000 or less, and there is a feature that electrical matching with the connection cable is easy, especially when a small rectangular vibrator is produced.

The present invention can be implemented in forms other than those described above without departing from the spirit of the invention. The embodiments disclosed in this application are merely examples, and the present invention is not limited to these embodiments. The technical scope of the present invention is supported by the description in this specification, is interpreted based on the description of the claims, and includes all modifications within the scope equivalent to the claims.

1 Piezoelectric vibrator 2 Electrode E Electric field application direction

Claims (10)

A piezoelectric vibrator formed of a rectangular single crystal plate composed of a lead complex perovskite compound containing magnesium oxide, niobium oxide, and lead titanate, and having electrodes on both sides,
the single crystal substrate has a [001] crystal orientation plane as a main surface, a [100] plane and a [010] plane as side surfaces, and a ratio L/W of a length L of the single crystal substrate in the [100] orientation to a width W of the single crystal substrate in the [010] orientation is 3 to 8.6 ;
A piezoelectric vibrator that maintains a polarization state due to AC polarization and DC polarization, and is characterized in that the ratio Nt/ _N31 of a frequency constant N31 expressed by the product of a resonance frequency fr in a transverse vibration mode perpendicular to the polarization direction and the length L, to a frequency constant _Nt expressed by the product of a resonance frequency fr in a longitudinal vibration mode parallel to the polarization _direction and the thickness t of the _single crystal plate, is 2.77 to 3.86 at 25°C. 2. The piezoelectric vibrator of claim 1, wherein the ratio of said frequency constants _Nt / _N31 is 2.8 to 3.6 at 25°C. The lead complex perovskite compound is a piezoelectric vibrator made of a two-component system of lead magnesium niobate (100-x)Pb(Mg _1/3 Nb _2/3 )O ₃ and lead titanate xPbTiO ₃ , and contains x mol % (x is a positive value) of lead titanate and (100-x) mol % of lead magnesium niobate, and has a temperature Tm showing a maximum relative dielectric constant of 125°C to 150°C, and x = 27 or more and 32 or less. 4. The piezoelectric vibrator according to claim 3, wherein the phase transition temperature _Trt of the crystal structure of the lead complex perovskite compound is 60° C. or higher and 110° C. or lower. 5. A method for manufacturing a piezoelectric vibrator according to any one of claims 1 to 4, comprising the steps of: subjecting the single crystal plate, which has been initially AC polarized, to a heat treatment and AC polarization and depolarization at a temperature of 150°C or higher and 300°C or lower; and repeating the steps at least two to four times, after which the single crystal plate is subjected to a final AC polarization. The method for manufacturing a piezoelectric vibrator according to claim 5, wherein the AC electric field in the AC polarization is 1.0 kVrms/cm or more and 10 kVrms/cm or less (rms: Root Mean Square), and an AC electric field having a frequency of 0.01 Hz to 200 Hz using a triangular wave or a sine wave is applied for 4 to 200 cycles. 7. The method for manufacturing a piezoelectric vibrator according to claim 5, wherein the AC polarization is performed at a temperature in the range of 40 ° C. to 70 ° C. 8. The method for manufacturing a piezoelectric vibrator according to claim 5, wherein after the final AC polarization , a DC electric field in the range of 3.0 kV/cm to 5 kV/cm is applied between the front electrode and the back electrode of the single crystal plate for 10 seconds to 1 minute. The method for manufacturing a piezoelectric vibrator according to claim 8, wherein the intensity of the DC electric field is 0.8 to 1.5 times the intensity of the AC electric field. 10. The method for manufacturing a piezoelectric vibrator according to claim 9, wherein the AC electric field is 2.0 kVrms/cm or more and 4.0 kVrms/cm or less, the DC electric field is 3.0 kV/cm or more and 5.0 kV/cm or less, and the polarization temperature is in the range of 40° C. to 70° C.

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