JP3320184B2 - Manufacturing method of piezoelectric ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing piezoelectric ceramics, and more particularly to a method for producing a piezoelectric ceramic which includes a polarization treatment capable of suppressing deterioration of piezoelectricity due to heat.

[0002]

2. Description of the Related Art Piezoelectric ceramics such as lead zirconate titanate and lead titanate are used in communication fields such as various filters and resonators, sensor application fields such as ultrasonic sensors and angular velocity sensors, ultrasonic motors and various types. It is used in dynamic application fields such as actuator elements, and in very wide fields such as buzzers and piezoelectric transformers.

[0003] The piezoelectric properties of piezoelectric ceramics are developed by performing a polarization treatment after sintering. Piezoelectric ceramics fired by a solid-phase reaction are generally divided into a number of domains. Although the spontaneous polarization is aligned in a certain direction in each domain, the spontaneous polarization is hardly exhibited as a whole. For this reason, in order to utilize the piezoelectricity, it is necessary to apply a process of applying an electric field to the ceramics from the outside to make the spontaneous polarization of all domains uniform in one direction. This process is a polarization process, and the piezoelectric ceramic exhibits the largest piezoelectricity in a state where spontaneous polarization of all domains is completely aligned in one direction. The polarization treatment of the piezoelectric ceramic is usually performed by immersing the piezoelectric ceramic in silicon oil and applying an electric field at a temperature lower than the Curie temperature of the piezoelectric ceramic (hereinafter, usually referred to as polarization treatment). The reason for immersion in silicon oil is that it is necessary to apply a high voltage during the polarization treatment. Electric field application time during polarization processing,
An optimum combination of the electric field strength and the processing temperature is selected according to the material of the piezoelectric ceramic.

As a polarization method for improving piezoelectricity, for example, a method of applying a DC pulse voltage at a breakdown voltage or less (Japanese Patent Laid-Open No. 61-268085) or a method of utilizing heat generated by dielectric loss when an AC electric field is applied is used. To make the domain arrangement unstable and then apply a DC electric field (JP-B-46-4)
No. 710) has been proposed. In the examples of these publications, a high electric field of 3 kV / mm or more is applied at room temperature or room temperature.

Japanese Patent Publication No. 43-1349 discloses that, in a lead zirconate titanate-based porcelain composition, an antiferroelectric porcelain exhibiting an antiferroelectric phase at room temperature and a ferroelectric phase at a high temperature has an antiferroelectric property. A method of applying a DC voltage while maintaining the temperature at or above the transition point at which a transition from a dielectric phase to a ferroelectric phase occurs ”is disclosed. In this method, 20 kV / cm, that is, 2 kV
/ mm DC electric field is applied.

Japanese Patent Application Laid-Open No. 3-150880 discloses a polarization method in which, in a polarization step of a piezoelectric element, the piezoelectric element is heated to a temperature higher than the Curie temperature and cooled while applying an electric field. This polarization method is for suppressing the piezoelectric displacement hysteresis of the piezoelectric ceramic. According to the publication, a preferable range of the applied electric field is 1 to 5 kV / mm.
And that the polarization is incomplete if the applied electric field is too low. In the example of the publication, a device having a Curie temperature of 260 ° C. and a
A high electric field of V / mm or 3.0 kV / mm is applied.

[0007] All of the above-mentioned conventional polarization methods aim at improving the performance of a single piezoelectric ceramic, such as improving the piezoelectricity. However, as explained below,
In recent years, piezoelectric ceramics have been required to have improved heat resistance, that is, not to degrade piezoelectricity due to heating.

In recent years, as electronic devices have become smaller and thinner, the transition from conventional discrete components to SMDs (Surface Mount Components) has been urgently required for electronic components. S
MD requires high heat resistance because it passes through a solder reflow furnace when it is mounted on a printed circuit board, that is, it is required that the electrical characteristics of the MD be less deteriorated by passing through the solder reflow furnace. In the case of discrete parts, only heat of not more than 200 ° C is applied at the time of bonding at the processing stage, heat treatment at the film forming step or product production stage, bonding, etc. Heat is applied for about 5 to 30 seconds in the range of about 200 to 260 ° C. when passing through, so when ordinary piezoelectric ceramics developed for discrete parts is applied to SMD parts, significant degradation of piezoelectricity occurs. . For this reason, piezoelectric ceramics having a composition having good heat resistance have been developed for SMD parts (JP-A-5-17218, JP-A-5-139829, etc.). However, the development of a new composition results in high costs.

[0009]

The present invention has been made under such circumstances. An object of the present invention is to provide a piezoelectric ceramic in which the degradation of piezoelectricity is remarkably small even when exposed to high temperatures due to solder reflow treatment or the like, and to develop a new composition having such high heat resistance. Without the conventional composition for leaded components.

[0010]

This and other objects are achieved by the present invention which is defined below as (1) to (8). (1) In the polarization process of piezoelectric ceramics, Tc
(Curie temperature) is 1.0 × 10 ⁷ Ω ·
cm, the temperature of the piezoelectric ceramic is raised to a temperature equal to or higher than Tc and then cooled to room temperature. When the temperature of the piezoelectric ceramic is at least Tc, the intensity of the electric field applied to the piezoelectric ceramic is 0.8 kV. A method for producing a piezoelectric ceramic, characterized in that an electric current is applied so as to be not more than / mm. (2) The method for producing a piezoelectric ceramic according to the above (1), wherein the ultimate temperature of the piezoelectric ceramic is Tc + 100 ° C. or more. (3) The method for producing a piezoelectric ceramic according to the above (1) or (2), wherein the piezoelectric ceramic is cooled by applying an electric field when the temperature of the piezoelectric ceramic is Tc + 50 ° C. or higher. (4) The method for producing a piezoelectric ceramic according to any one of the above (1) to (3), wherein an electric field is continuously applied to the piezoelectric ceramic until the temperature of the piezoelectric ceramic becomes Tc-50 ° C. or lower. (5) The method for producing a piezoelectric ceramic according to any one of the above (1) to (4), wherein cooling is performed while a constant current is applied after the temperature of the piezoelectric ceramic reaches Tc. (6) The cooling is performed while maintaining the electric field intensity applied to the piezoelectric ceramic at a constant value equal to or higher than the electric field intensity at Tc at least in part after the temperature of the piezoelectric ceramic reaches Tc. The method for producing a piezoelectric ceramic according to any one of the above. (7) The method for producing a piezoelectric ceramic according to any one of the above (1) to (6), wherein the piezoelectric ceramic is a lead zirconate titanate or a lead titanate. (8) The method for producing a piezoelectric ceramic according to any one of the above (1) to (7), wherein Tc of the piezoelectric ceramic is 280 ° C. or more.

[0011]

Conventionally, the polarization treatment of piezoelectric ceramics is performed by applying a high electric field of 3 kV / mm or more in silicon oil having a temperature lower than the Curie temperature (Tc) of the piezoelectric ceramics, as described above. According to the present invention, however, the piezoelectric ceramic is heated to at least Tc in air, and when the temperature is lowered to room temperature, an electric current is applied to the piezoelectric ceramic at least at Tc. Such a polarization processing method in the present invention is hereinafter referred to as a high-temperature polarization processing. The piezoelectric ceramics targeted by the present invention have low resistivity and can be controlled at a constant current. Specifically, the resistivity at Tc is 1.0%.
× 10 ⁷ Ω · cm or less.

In the present invention, by performing such a polarization treatment, the heat resistance of the piezoelectric ceramic can be remarkably improved. That is, conventionally, even if the composition has a remarkable deterioration in piezoelectricity after passing through a solder reflow furnace, the deterioration in piezoelectricity is remarkably small when subjected to a high-temperature polarization treatment according to the present invention. . Therefore, the piezoelectric ceramics polarized according to the present invention,
Suitable for SMD. In addition, since the deterioration due to heat is small, the width of adjustment of the resonance frequency in the subsequent process can be reduced, and the process can be simplified. The high-temperature polarization is also suitable for a high-power piezoelectric ceramic for an actuator that generates a large amount of heat. In this case, characteristic deterioration due to heat generation during use can be prevented.

In the ordinary polarization treatment, sufficient polarization is normally impossible unless a high electric field of 3 kV / mm or more is applied. However, in the present invention, the intensity of the electric field applied to the piezoelectric ceramic at Tc is reduced. Since it is as low as 0.8 kV / mm or less, the cost of the power supply device can be reduced. Further, in the case of a product using the thickness shear vibration mode, a voltage is applied in the longitudinal direction of the piezoelectric ceramics during polarization, so that it is a great advantage that the electric field strength can be reduced.

When the present invention is applied to a piezoelectric ceramic such as a lead titanate having a large spontaneous polarization, the piezoelectricity can be improved as compared with the ordinary polarization.

When a conventional ordinary polarization process is applied to a piezoelectric ceramic having a relatively low resistivity at Tc, the polarization process is performed at a low temperature at which the resistivity becomes high in order to avoid local heat generation and dielectric breakdown. However, sufficient polarization may be difficult. In the high-temperature polarization treatment according to the present invention, a sufficient polarization is possible because a current flows, and furthermore, dielectric breakdown does not occur.

In the present invention, since the piezoelectric ceramic is not immersed in silicon oil, cleaning with an organic solvent is not required after the polarization treatment, and there is no risk of environmental pollution.

In the above-mentioned Japanese Patent Application Laid-Open No. 3-150880, a method is described in which a piezoelectric ceramic is heated above Tc and cooled while applying an electric field. The Tc of the piezoelectric ceramic described in this publication is 260 ° C., but when Tc is such a low value, the resistivity at Tc is generally 1 × 1.
0 ⁸ Ω · cm or more, the effect of the present invention is not realized. Further, in this method, since the applied electric field is too strong, there is a danger in operation as described above. When a high voltage of 1 kV / mm or more is applied to a piezoelectric ceramic having a resistivity of 1.0 × 10 ⁷ Ω · cm or less as in the publication, current does not flow uniformly and heat is locally generated. . Further, since a voltage is not uniformly applied to the entire piezoelectric ceramic, polarization is insufficient, and good piezoelectricity cannot be obtained.

The method of performing the polarization treatment by heating to a temperature equal to or higher than Tc is generally used for a single crystal piezoelectric material such as LiTaO ₃ or LiNbO ₃ . Since these single-crystal piezoelectric materials have high insulation resistance at low temperatures, about 600 ° C. for LiTaO ₃ and about 1200 ° C. for LiNbO _3.
Heat to about the temperature and apply voltage. Each of these temperatures is several tens degrees higher than Tc. Because of the need for such high temperatures, it is impossible to perform the polarization treatment of the single crystal piezoelectric material in silicon oil. The polarization treatment of the single crystal piezoelectric material is similar to the high temperature polarization treatment in the present invention in that a voltage is applied at a temperature exceeding Tc. However, a single crystal piezoelectric material is a single domain, which is completely different from the structure of piezoelectric ceramics. According to the present invention, in order to improve the heat resistance of the piezoelectric ceramics, a piezoelectric ceramic having a low resistivity is selected and an electric current is supplied to the piezoelectric ceramics near Tc. It is not simply diverted to polarization processing.

[0019]

[Specific Configuration] Hereinafter, a specific configuration of the present invention will be described in detail.

The piezoelectric ceramics targeted by the present invention are:
The resistivity at the Curie temperature Tc is 1.0 × 10 ⁷ Ω ·
cm or less, preferably 8 × 10 ⁶ Ω · cm or less. In a piezoelectric ceramic having a resistivity of 1.0 × 10 ⁷ Ω · cm or less at Tc, a voltage substantially corresponding to the applied current can be generated at Tc to perform favorable polarization processing. However, the resistivity at Tc is 1.0 × 10
^If it exceeds ⁷ Ω · cm, almost no current can flow at Tc, and a voltage is directly applied to the piezoelectric ceramics, and the effect of the present invention is not realized. That is, in the present invention, a current is passed through a piezoelectric ceramic having a relatively small electric resistivity, and polarization is performed by a voltage generated by the current. The resistivity at Tc is preferably 1.
0 × 10 ⁻³ Ω · cm or more, more preferably 1.0 × 10 ⁻⁴
Ω · cm or more. If the resistivity at Tc is too low,
It is difficult to obtain a sufficient electric field strength, and it is difficult to obtain good piezoelectricity.

In the present invention, the composition of the piezoelectric ceramic is not particularly limited as long as the resistivity at Tc is within the above range.
For example, the present invention can be applied to the polarization treatment of various piezoelectric ceramics such as lead zirconate titanate and lead titanate. Specifically, in the lead zirconate titanate,
PbTiO ₃ -PbZrO ₃ or PbT as a basic composition system
iO ₃ —PbZrO ₃ —Pb (B ¹ · B ² ) O ₃ (where B ¹ · B ² is Zn _1/3 Nb _2/3 , Co _1/3 Nb
_2/3 , Mg _1/3 Nb _2/3 , Mn _1/3 Nb _2/3 , Ni _1/3
Nb _2/3 , Sn _1/3 Nb _2/3 , Sn _1/3 Sb _2/3 , Mn
_1/3 Sb _2/3 ), with a small amount of Pb at the A site replaced with an alkaline earth element or a rare earth element, or Cr ₂ O ₃ , WO ₃ , MnO ₂
For example, a composition in which a small amount of an element not included in the basic composition system is added. In lead titanate, PbT
A material obtained by substituting Ca with another alkaline earth element or a rare earth element centering on iO ₃ —CaTiO ₃ , and adding Cr ₂ O ₃ , Nb ₂ O ₅ , MnO ₂ , Bi
A composition in which a small amount of an element not included in the basic composition system, such as ₂ O ₃ , is added.

The Tc of the piezoelectric ceramic to which the present invention is applied is not particularly limited, but the Tc of the piezoelectric ceramic having a resistivity in the above-mentioned range is usually 280 ° C. or higher. When Tc is at this level, it is possible to sufficiently withstand high-temperature treatment in a solder reflow furnace.

According to the present invention, in such a polarization treatment step of the piezoelectric ceramic, the piezoelectric ceramic is
After heating to the above temperature, it is cooled to room temperature. During this cooling, an electric current is applied to the piezoelectric ceramic at least when the temperature of the piezoelectric ceramic is Tc.

At Tc, the intensity of the electric field applied to the piezoelectric ceramic is 0.8 kV / mm or less, preferably 0.5 kV / mm.
V / mm or less, and preferably 0.02 kV / mm or more,
More preferably, it is 0.05 kV / mm or more. When an electric field exceeding 0.8 kV / mm is applied to the low-resistivity piezoelectric ceramics used in the present invention, current does not flow uniformly in the piezoelectric ceramics, causing local heat generation or dielectric breakdown. . On the other hand, if the intensity of the electric field is too weak, the polarization becomes insufficient.

In the polarization process, first, the temperature of the piezoelectric ceramic is raised to a temperature equal to or higher than Tc. The ultimate temperature of the piezoelectric ceramics at this time is preferably Tc + 100 ° C. or more, more preferably Tc + 200 ° C. or more. Piezoelectric ceramics remain piezoelectric near Tc, so if the temperature reached is too close to Tc, the spontaneous polarization cannot be sufficiently aligned during the polarization process, resulting in insufficient piezoelectricity. Is likely to be thermally degraded.

In order to make the spontaneous polarization of the piezoelectric ceramic heated to a temperature equal to or higher than Tc uniform, it is sufficient to supply a current so that a predetermined electric field is applied when the temperature reaches Tc during cooling. . In this case, a configuration may be adopted in which a current flows only at the time of Tc.
More preferably, a current is supplied when the temperature is Tc + 100 ° C. or higher. In order to prevent randomization of domains due to thermal disturbance when cooling from Tc to room temperature, it is preferable to continue applying an electric field even after the temperature of the piezoelectric ceramic becomes lower than Tc. Is T
It is preferable that the electric field is continuously applied until the temperature becomes lower than or equal to −50 ° C.

Although the cooling rate of the piezoelectric ceramic is not particularly limited, it is usually preferable to be about 60 to 600 ° C./hour.

When the electric field application is continued even after the temperature of the piezoelectric ceramics reaches Tc, constant current control may be performed so that the current density becomes a constant value, and at least a part after the temperature reaches Tc. Constant voltage control for maintaining the electric field strength at a constant value may be performed. Performing constant voltage control at least in part means switching from constant current control to constant voltage control when the temperature of the piezoelectric ceramics is lower than Tc, or performing constant voltage control after reaching Tc. means. The electric field intensity applied to the piezoelectric ceramics when performing the constant voltage control is preferably higher than the electric field intensity applied at Tc. If the constant voltage control is performed with such an electric field strength, the influence of thermal disturbance can be sufficiently prevented.

The reason why the constant voltage control is performed is to avoid excessive heat generation of the piezoelectric ceramics. Since the resistivity of the piezoelectric ceramic increases with a decrease in temperature, heat generation increases when constant current control is continued even below Tc. In order to obtain good piezoelectricity and heat resistance, a polarization current with the optimum strength is applied to the piezoelectric ceramics according to the composition. The amount may be too large, which may impair the piezoelectricity. Therefore, in such a case, it is preferable to use constant voltage control.

The preferred range of the current density in Tc varies depending on the composition of the piezoelectric ceramics and the like, and may be determined by experiments so as to obtain good piezoelectricity, but is usually 0.3 to 36 mA / cm ² . If the current density is not such that good piezoelectricity can be obtained immediately after the polarization treatment, the heat resistance will also be low.

The method for producing the piezoelectric ceramic to be polarized is not particularly limited, and a method commonly used for producing general ceramics may be used. That is, P
Compounds of constituent elements of piezoelectric ceramics such as b, Ti, and Zr are blended at a predetermined ratio, and sufficiently pulverized and mixed using a ball mill or the like. As the compound, an oxide or a compound which can be converted to an oxide by firing, for example, a carbonate, a hydroxide, an oxalate, a nitrate, or the like can be used.
The obtained mixture is calcined at about 700 to 900 ° C.
Next, the calcined product is pulverized, kneaded by adding water and a binder, and then pressure-molded into a predetermined shape by applying a pressure of about 0.2 to 5 ton / cm ² . The obtained molded body was 1150-
Sinter at a temperature of about 1250 ° C. for about 2 to 4 hours. Although sintering can be performed in the air, a more dense sintered body can be obtained by sintering in an oxygen atmosphere by hot pressing or hot isostatic pressing. After sintering, the above-described polarization treatment is performed.

[0032]

EXAMPLES Hereinafter, the present invention will be described in more detail by showing specific examples of the present invention.

Example 1 PbO, Z as starting materials
rO ₂ , TiO ₂ , MgCO ₃ , Nb ₂ O ₅ , Cr ₂ O
₃ was weighed so as to have the following composition, and mixed and pulverized for 24 hours by a ball mill.

Pb ｛(Mg _1/3 −Nb _2/3 ) _0.05 Ti
_0.45 Zr _0.50 @O ₃ + 0.2% by weight Cr ₂ O ₃

After the mixture was dehydrated and dried, it was calcined at 850 ° C. for 2 hours in the air, and the calcined powder was again ground by a ball mill for 24 hours and dehydrated and dried. To the obtained powder, 2% by weight of PVA was added as a binder, and after granulation, 4 t /
It was molded under a pressure of cm ² to obtain a molded body having a diameter of 20 mm and a thickness of 0.5 mm. This compact was placed in a firing sliver and fired in air. The heating rate during firing was 200 ° C./hour, and the temperature was maintained at 1200 ° C. for 2 hours. The fired body has a length of 6.
After processing into a plate having a size of 4 mm, a width of 3.9 mm, and a thickness of 0.23 mm, the plate was sufficiently washed, and a gold electrode (5 mm × 3 mm) was formed by a film forming apparatus to obtain a sample for evaluating electrical characteristics.

An Au terminal was connected to this sample, and the sample was placed in a high-temperature polarizer having the structure shown in FIG.
The temperature characteristics of the capacitance were measured up to 600 ° C. with an R meter, and the Tc of this sample was determined. Further, a resistance value at Tc was determined by a four-terminal method using a multimeter.
As a result, Tc was 365 ° C., and the resistance value at Tc was 61 kΩ (0.4 × 10 ⁶ Ω · cm).

Next, the sample was cooled by flowing a current from 600 ° C. to room temperature by a high-temperature polarizing device to perform polarization. The current passed through the sample was 0.45 mA (3.
0 mA / cm ² ) and maintained this current until room temperature. The cooling rate was 5 ° C./min. The electric field strength at Tc was 0.12 kV / mm. Note that the value of the current flowing through the sample is the highest electromechanical coupling coefficient (K
t) is a value obtained and was determined by conducting experiments in advance. The same applies to the following embodiments.

After leaving the sample subjected to the polarization treatment for one day and night, a solder heat resistance test was performed. First, the resonator characteristics (K
t) was evaluated. This was taken as the initial characteristic. Next, the sample was wrapped in aluminum foil and immersed in a solder bath set at 250 ° C. for 30 seconds to perform a solder heat resistance test. After allowing the sample to stand for one day and night, the resonator characteristics in the thickness longitudinal vibration mode were evaluated again by an impedance analyzer. This solder heat test is repeated a total of five times, and the initial Kt is determined from the Kt after each test.
The rate of change with respect to t was determined. The results are shown in FIG.

<Comparative Example 1> The sample for evaluating electrical characteristics prepared in Example 1 was immersed in silicon oil, and maintained at 120 ° C., and an electric field of 3.0 kV / mm was applied for 20 minutes. Processing was performed.

The silicon oil on the surface of the sample after the polarization treatment was sufficiently washed, and allowed to stand for one day and night. Then, a solder heat resistance test was performed in the same manner as in Example 1 to determine the Kt change rate. The results are shown in FIG.

As shown in FIG. 2, in Example 1 in which a constant current was applied when cooling from a temperature exceeding Tc, polarization was performed in Comparative Example 1 in which silicon was polarized below Tc in silicon oil.
The Kt deterioration after the soldering heat test is remarkably small.

Example 2 PbO, Z as starting materials
rO ₂ , TiO ₂ , MgCO ₃ , Nb ₂ O ₅ , MnCO
₃ , CoO, and Cr ₂ O ₃ were weighed to have the following composition, and were mixed and pulverized for 24 hours by a ball mill.

Pb ｛(Mg _1/3 −Nb _2/3 ) _0.05 Ti
_0.47 Zr _0.48 ｝ O ₃ + 0.2% by weight Cr ₂ O ₃ +0.2
Wt% MnCO ₃ +0.2 wt% CoO

After the mixture was dehydrated and dried, it was calcined at 850 ° C. for 2 hours in the air, and the calcined powder was again ground by a ball mill for 24 hours and dehydrated and dried. To the obtained powder, 2% by weight of PVA was added as a binder, and after granulation, 4 t /
It was molded under a pressure of cm ² to obtain a molded body having a diameter of 20 mm and a thickness of 1.0 mm. This compact was placed in a firing sliver and fired in air. The heating rate during firing was 200 ° C./hour, and the temperature was maintained at 1200 ° C. for 2 hours. The fired body has a diameter of 15
After processing into a disk having a thickness of 0.5 mm and a thickness of 0.5 mm, and washing sufficiently, a gold electrode (diameter: 13 mm) was formed by a film forming apparatus to obtain a sample for evaluating electrical characteristics.

An Au terminal was connected to this sample, and Tc and the resistance value at Tc were determined in the same manner as in Example 1. As a result, Tc was 365 ° C., and the resistance value at Tc was 82.9 kΩ (2.2 × 10 ⁶ Ω · cm).

Next, the sample was cooled by flowing a current from 600 ° C. to room temperature using a high-temperature polarizing device in the same manner as in Example 1 to perform polarization. The current flowing through the sample was set to 1.51 mA (1.14 mA / cm ² )
This current was maintained until room temperature. The cooling rate was 5 ° C./min. The electric field strength at Tc was 0.25 kV / mm.

After the polarized sample was allowed to stand for one day and night, a solder heat resistance test was performed in the same manner as in Example 1.
However, in this example, the number of times of immersion in the solder bath was set to one, and the temperature of the solder bath was changed as shown in FIG. 3, and Kt was determined after each test. The results are shown in FIG.

<Comparative Example 2> The sample for evaluating electrical characteristics prepared in Example 2 was immersed in silicon oil, and maintained at 120 ° C., and an electric field of 3.0 kV / mm was applied for 20 minutes. Processing was performed.

For the sample thus polarized,
A solder heat resistance test was performed in the same manner as in Example 2, and Kt was determined. The results are shown in FIG.

As shown in FIG. 3, in Example 2 polarized according to the present invention, the degradation of Kt due to high temperature is significantly smaller than that in Comparative Example 2 polarized by the conventional method. In Example 2, when the temperature of the sample was lowered to 250 ° C., the control was switched to the constant voltage control of 600 V (1.2 kV / mm), and the sample was cooled to room temperature while supplying a current in the state of the constant voltage control. However, solder heat resistance equivalent to that of Example 2 was shown.

Example 3 A fired body was produced in the same manner as in Example 2. However, the dimensions are 15mm long and 0.5m wide
m and a thickness of 5 mm. First, 0.5mm x 5m
The end face of m was mirror-finished. Next, an Au paste was applied to an end surface of 15 mm × 0.5 mm, and baked at 850 ° C. to form an electrode, and subjected to high-temperature polarization treatment as in Example 2. However,
The current flowing through the sample was set at 0.09 mA (1.14 mA / cm ² ), and this current was maintained until room temperature. Cooling rate is 5 ℃
/ Min. The electric field strength at Tc is 0.25 kV / mm
Met. The polarization direction (electric field application direction) is the thickness direction. After polarization treatment, Kt was measured to be 43%.

Next, the mirror-finished surface (the surface parallel to the polarization direction) of the fired body was etched with an etching solution (HCl: HF: H ₂ O =
1: 1: 100) for 5 seconds, and then SEM of the treated surface
(Scanning electron microscope) A photograph was taken. This SEM photograph is shown in FIG.

The same applies to the fired body subjected to a solder heat test at 250 ° C. after the high-temperature polarization treatment.
M photos were taken. This SEM photograph is shown in FIG.

<Comparative Example 3> A fired body was prepared in the same manner as in Example 3, and was subjected to ordinary polarization treatment in the same manner as in Comparative Example 2. Kt after the polarization treatment was 43%, which was the same as in Example 3. After the polarization treatment, an SEM photograph was taken in the same manner as in Example 3. FIG. 6 shows the SEM photograph. In addition, an SEM photograph was also taken of the fired body which was subjected to the solder heat test in the same manner as in Example 3 after the normal polarization treatment. This SEM photograph is shown in FIG.

As shown in FIG. 4, in the crystal grains of the fired body subjected to the high-temperature polarization treatment according to the present invention in Example 3, a plane perpendicular to the electric field application direction (horizontal direction in the figure) during polarization is formed. A streak-like structure oriented as the center is clearly observed. This streak-like structure is presumed to indicate a domain. In FIG. 5 after the solder heat resistance test, a streak-like structure in the crystal grain is observed with almost no decrease in clarity.

On the other hand, a clear streak-like structure was hardly observed in the crystal grains of the fired body of Comparative Example 3 shown in FIG. 6, and in FIG. 7 after the solder heat test, the streak-like structure was observed. Has almost completely disappeared.

In order to examine the details of the streak structure in the crystal grains, the fired body after polarization (before the solder heat resistance test) of Example 3 and the fired body after polarization (before the solder heat resistance test) of Comparative Example 3 were used. First, after polishing down to several hundred microns leaving a mirror-finished surface, and then performing ion etching, TE
An M (transmission electron microscope) photograph was taken. FIG. 8 shows a TEM photograph of the fired body of Example 3, and FIG. 9 shows a TEM photograph of Comparative Example 3. In FIG. 8, the streak structure is clearly recognized, but in FIG. 9, the streak structure is hardly recognized.

Example 4 As starting materials, PbO, T
iO ₂ , Bi ₂ O ₃ , CaCO ₃ , MnO ₂ , Nb ₂ O
With _5, it was weighed so become following composition (molar ratio), were mixed and pulverized for 24 hours by a ball mill.

0.76 PbTiO ₃ +0.22 CaTi
O ₃ +0.01 Bi ₂ O ₃ +0.02 MnO ₂ +0.0
15Nb ₂ O ₅

After the mixture was dehydrated and dried, it was calcined at 900 ° C. for 2 hours in the air, and the calcined powder was again pulverized by a ball mill for 24 hours and dehydrated and dried. To the obtained powder, 2% by weight of PVA was added as a binder, and after granulation, 2 t /
It was molded at a pressure of cm ² to obtain a molded body of 35 mm × 10 mm × 10 mm. This compact was placed in a firing sliver and fired in air. The heating rate during firing was 200 ° C / hour,
It was kept at 1200 ° C. for 2 hours. The fired body is 30 mm × 0.
After processing to 41mm x 7.3mm and washing thoroughly, 30mm
Apply Au paste to both ends of × 0.41 mm, and apply 850
The electrode was baked at ℃. An Au terminal was connected to this electrode, and Tc and the resistance value at Tc of the fired body were determined in the same manner as in Example 1. As a result, Tc was 310 ° C., and the resistance value at Tc was 23.7 MΩ (4 × 10 ⁶ Ω).
Cm).

Next, as in the first embodiment, the limit value of the voltage of the power supply was set to 1.97 kV using a high-temperature polarization device.
While supplying a constant current of 0.07 mA (0.6 mA / cm ² ),
The temperature of the fired body was started to fall from 0 ° C. The voltage at Tc (310 ° C.) is 1.83 kV (0.25 kV / mm),
At 300 ° C., the voltage limit reached 1.97 kV, and the control was switched to constant voltage control. Thereafter, a polarization treatment was performed at a constant voltage until room temperature. The cooling rate was 5 ° C./min.

The fired body polarized in this way is
After being left day and night, as shown in FIG. 10, an electrode was formed by shape processing to obtain a sample for measuring electrical characteristics. For this sample, the resonator characteristics in the thickness shear vibration mode were measured by an impedance analyzer. Table 1 shows the results
Shown in In Table 1, Fr is the resonance frequency, t is the thickness of the sample, RO is the resonance impedance, Za is the anti-resonance impedance, q is the peak-to-valley ratio of the resonance impedance {q = 20 · log (Za / RO)}, K Reference numeral ₁₅ denotes an electromechanical coupling coefficient in the thickness shear vibration mode, and Cd denotes a capacity.
After that, the sample was wrapped in aluminum foil, immersed in a solder bath set at 250 ° C. for 30 seconds, and left for one day and night.
The resonator characteristics in the thickness shear vibration mode were measured again by the impedance analyzer. K after solder heat test
₁₅ the rate of change of the ([Delta] K ₁₅₎ and capacitance change rate (? Cd) shown in Table 1.

<Comparative Example 4> The fired body produced in Example 4 was immersed in silicon oil and kept at 150 ° C to 4.5 kV / mm.
The polarization process was performed by applying an electric field of 20 minutes. The polarization direction was the same as in Example 4. The sample after the polarization treatment was processed in the same manner as in Example 4 to form an electrode, and was used as a sample for measuring electric characteristics having the structure shown in FIG. The same measurement as in Example 4 was performed on this sample. Table 1 shows the results.

[0064]

[Table 1]

As shown in Table 1, q of the sample of Example 4 was larger than that of the sample of Comparative Example 4, and K ₁₅ was also improved. In Comparative Example 3, characteristic fluctuations due to the solder heat resistance test are observed, but in Example 3, no characteristic fluctuations are observed. From these results, it can be seen that in the lead titanate-based piezoelectric ceramics, the high-temperature polarization treatment can prevent the piezoelectric thermal deterioration and also improve the piezoelectricity itself.

Comparative Example 5 PbO, Z
rO ₂ , TiO ₂ , SrCO ₃ , and Nb ₂ O ₅ were weighed to have the following composition, and were mixed and pulverized for 24 hours by a ball mill.

Pb _0.9 Sr _0.10 (Ti _0.45 Zr _0.55 ) O
₃ + 0.5% by weight Nb ₂ O ₅

After the mixture was dehydrated and dried, it was calcined at 850 ° C. for 2 hours in the air, and the calcined powder was again pulverized by a ball mill for 24 hours and dehydrated and dried. To the obtained powder, 2% by weight of PVA was added as a binder, and after granulation, 4 t /
It was molded under a pressure of cm ² to obtain a molded body having a diameter of 20 mm and a thickness of 1.0 mm. This compact was placed in a firing sliver and fired in air. The heating rate during firing was 200 ° C./hour, and the temperature was maintained at 1200 ° C. for 2 hours. The fired body has a diameter of 12
After processing into a disk having a thickness of 0.5 mm and a thickness of 0.5 mm and sufficiently washing, a gold electrode (diameter of 10.1 mm) was 700
It was baked at ℃ to make a sample for evaluating electrical characteristics.

An Au terminal was connected to this sample, and Tc was determined in the same manner as in Example 1. The result was 230 ° C. The temperature characteristic measurement of the capacitance for determining Tc is 4
Up to 00 ° C. When the resistance value at Tc was determined by an insulation resistance meter, it was 100 MΩ (5 × 10 ⁸ Ω · cm).

Next, the temperature of the sample was started to decrease from 400 ° C. while the constant current of 0.77 mA was applied while setting the limit value of the voltage of the power supply to 711 V using a high-temperature polarizing device. As a result, the voltage reached 711 V at a temperature higher than Tc, passed through Tc as it was, and was subjected to a polarization treatment at room temperature to room temperature. The electric field strength at Tc is 1.4
It was 22 kV / mm.

After the sample thus polarized was allowed to stand for one day and night, the resonator characteristics in the radial vibration mode were measured by an impedance analyzer. afterwards,
The sample was wrapped in aluminum foil, immersed in a solder bath set at 200 ° C. for 30 seconds, allowed to stand for a day and a night, and then the resonator characteristics in the radial vibration mode were measured again by an impedance analyzer. Table 2 shows the results. In Table 2, Fr is the resonance frequency, Fa is the anti-resonance frequency, and Kr
Is the electromechanical coupling coefficient in the radial vibration mode, Qm is the mechanical quality coefficient, and Cd is the capacity.

<Comparative Example 6> The sample for electrical property evaluation prepared in Comparative Example 5 was immersed in silicon oil, and maintained at 120 ° C., and an electric field of 3.0 kV / mm was applied for 20 minutes to cause polarization. Processing was performed. Next, after sufficiently washing the silicon oil on the sample surface after the polarization treatment, the sample was allowed to stand for one day and night, and the resonator characteristics in the radially widened vibration mode were measured in the same manner as in Comparative Example 5, and the solder heat resistance at 200 ° C. Similar measurements were made after the test. Table 2 shows the results.

[0073]

[Table 2]

As shown in Table 2, with respect to piezoelectric ceramics having a resistivity at Tc of more than 1.0 × 10 ⁷ Ω · cm, the soldering was performed by the high-temperature polarization treatment of Comparative Example 5 and the normal polarization treatment of Comparative Example 6. No significant difference is observed in the rate of change of Kr (electromechanical coupling coefficient in the radial vibration mode) and ΔFr (shift of resonance frequency) after the heat resistance test.

From the above results, piezoelectric ceramics having an electrical resistivity of 1.0 × 10 ⁷ Ω · cm or less at Tc were obtained.
The effect of the present invention in which the temperature is raised to the above-mentioned temperature and cooling is performed while applying a current so as to obtain a predetermined electric field strength to perform the polarization treatment is apparent.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram when a polarization process is performed on a piezoelectric ceramic by a high-temperature polarization device.

FIG. 2 is a graph showing the relationship between the number of repetitions of a solder heat test and the rate of change of Kt (electromechanical coupling coefficient).

FIG. 3 is a graph showing a relationship between a temperature of a solder bath in a solder heat resistance test and Kt (electromechanical coupling coefficient) after the solder heat resistance test.

4 is a drawing-substituting photograph showing a crystal structure, and is a SEM photograph of a cross section of a sample after a polarization treatment in Example 3. FIG.

5 is a drawing-substituting photograph showing a crystal structure, and is an SEM photograph of a cross section of a sample after a solder heat test in Example 3. FIG.

6 is a drawing-substitute photograph showing a crystal structure, and is an SEM photograph of a cross section of a sample after a polarization treatment in Comparative Example 3. FIG.

FIG. 7 is a drawing substitute photograph showing a crystal structure, and is an SEM photograph of a cross section of a sample after a solder heat resistance test in Comparative Example 3.

8 is a drawing-substituting photograph showing a crystal structure, and is a TEM photograph of a cross section of a sample after a polarization treatment in Example 3. FIG.

9 is a drawing-substitute photograph showing a crystal structure, and is a TEM photograph of a cross section of a sample after a polarization treatment in Comparative Example 3. FIG.

FIG. 10 is a perspective view illustrating a sample for evaluating electrical characteristics in a thickness shear vibration mode.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomohisa Higashi 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDK Corporation (72) Inventor Kazushi Tatemoto 1-13-1 Nihonbashi, Chuo-ku, Tokyo (56) References JP-A-3-150880 (JP, A) JP-A-4-179287 (JP, A) JP-A-2-5583 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) H01L 41/22

Claims (8)

(57) [Claims] In a polarization step of a piezoelectric ceramic, the resistivity at Tc (Curie temperature) is 1.0 × 1.
The 0 ⁷ Ω · cm or less in piezoelectric ceramics, after heating to a temperature above Tc, when cooled to room temperature, when the temperature of at least the piezoelectric ceramic is Tc, the strength of the electric field applied to the piezoelectric ceramic A method for producing a piezoelectric ceramic, characterized in that an electric current is applied so as to be 0.8 kV / mm or less. 2. The temperature reached by the piezoelectric ceramic is Tc + 1.
The method for producing a piezoelectric ceramic according to claim 1, wherein the temperature is not lower than 00 ° C. 3. The temperature of the piezoelectric ceramic is Tc + 50 ° C.
3. The method for producing a piezoelectric ceramic according to claim 1, wherein an electric field is applied to the piezoelectric ceramic to cool the piezoelectric ceramic. 4. The temperature of the piezoelectric ceramic is Tc-50 ° C.
The method for producing a piezoelectric ceramic according to any one of claims 1 to 3, wherein an electric field is continuously applied to the piezoelectric ceramic until: 5. The method for manufacturing a piezoelectric ceramic according to claim 1, wherein the cooling is performed while a constant current is applied after the temperature of the piezoelectric ceramic reaches Tc. 6. The method according to claim 1, wherein at least part of the time after the temperature of the piezoelectric ceramic reaches Tc, cooling is performed while maintaining the electric field strength applied to the piezoelectric ceramic at a constant value equal to or higher than the electric field strength at Tc. A method of manufacturing any of the piezoelectric ceramics. 7. The method for producing a piezoelectric ceramic according to claim 1, wherein the piezoelectric ceramic is a lead zirconate titanate or a lead titanate. 8. The method according to claim 1, wherein Tc of the piezoelectric ceramic is 280 ° C. or higher.