JPH06224486A - Polarizing method for piezoelectric ceramics - Google Patents

Abstract

PURPOSE:To easily form piezoelectric ceramics in which an aging change of a residual polarization is prevented. CONSTITUTION:A first polarizing step of polarizing piezoelectric ceramics by applying a voltage reverse to a voltage applying direction necessary to obtain a polarizing direction to be obtained to the ceramics, and a second polarizing step of polarizing the ceramics polarized in a reverse direction by applying a voltage of the voltage applying direction necessary to obtain a polarizing direction to be obtained are conducted. It is surmised that an internal bias electric field is generated by previously applying the voltage in the reverse direction, and it is considered that this field suppresses 90 deg. switching of a domain.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to a method of polarizing piezoelectric ceramics. The piezoelectric ceramic obtained by the polarization method of the present invention is optimal as a piezoelectric actuator used in automobiles and the like.

[0002]

2. Description of the Related Art In recent years, instead of an actuator utilizing electromagnetic force, for example, Japanese Patent Laid-Open No. 62-291187,
As disclosed in Japanese Utility Model Laid-Open No. 64-30865, a piezoelectric actuator using a piezoelectric ceramic such as lead zirconate titanate (hereinafter referred to as PZT) has been proposed.

This piezoelectric actuator is extremely promising as various mechanical drive elements because it has low power consumption, generates little heat, is small, and can be driven at high speed. However, since mechanical displacement due to the piezoelectric effect is essentially extremely small, it is provided as a piezoelectric laminate having a structure in which plate-shaped piezoelectric bodies and electrode plates are alternately laminated in order to obtain a large displacement amount.

Incidentally, spontaneous polarization such as PZT is usually
Isotropic. Therefore, in order to obtain a piezoelectric ceramic, a high voltage is applied to perform polarization processing for aligning the direction of spontaneous polarization in a specific direction so as to have piezoelectricity.
However, even if the piezoelectric ceramic material is polarized, the obtained residual polarization does not reach the polarization of the single crystal. Further, the remanent polarization is apt to gradually decrease with the passage of time, and the piezoelectric characteristic is deteriorated with the passage of time, or the piezoelectric characteristic becomes unstable in a high stress state due to use at high pressure, high temperature and high voltage. is there. Therefore, for example, in a piezoelectric actuator using PZT piezoelectric ceramics, a decrease in displacement amount (displacement deterioration) occurs particularly during use at high temperature for automobiles,
There is a problem that a desired amount of displacement cannot be obtained.

In view of this, Japanese Patent Application Laid-Open No. 62-234381 discloses a piezoelectric ceramic for applying an electric field in a direction perpendicular to the pressure direction during pressure sintering during pressure sintering or / and annealing treatment of piezoelectric ceramics. A method of manufacturing ceramics is disclosed.

[0006]

As described in the above publication, the piezoelectric ceramic had a considerable change in remanent polarization with time. Therefore, in order to keep the displacement of the piezoelectric actuator constant, a complicated control power supply circuit for controlling the voltage or current of the drive power supply, a cooling device, etc. are required. It is an obstacle.

Therefore, the inventors of the present invention have conducted extensive studies as to the cause of the decrease in displacement during use of the piezoelectric actuator.
It has been found that a major cause is that the electromechanical coupling coefficient Kp (hereinafter referred to as Kp) of the piezoelectric ceramic is lowered during use. It has also been found that Kp decreases due to loads such as pressure, temperature, and voltage during use, and this decrease in Kp is mainly due to 90 ° switching of domains. However, no means have so far been found to prevent 90 ° switching of the domains and prevent Kp degradation.

In the manufacturing method disclosed in JP-A-62-234381, it is necessary to perform heating and voltage application at the same time, and it is necessary to select a material that does not melt during heating. The degree is small and the man-hours are large. The present invention has been made in view of such circumstances, and an object of the present invention is to easily form a piezoelectric ceramic in which a change in remanent polarization with time is prevented.

[0009]

A method for polarizing a piezoelectric ceramics according to the present invention which solves the above-mentioned problems is a voltage application direction necessary for obtaining a desired polarization direction when a voltage is applied to the piezoelectric ceramics for polarization. A first polarization step of applying a voltage in the opposite direction to the piezoelectric ceramics to polarize it, and applying a voltage in the direction of voltage application necessary to obtain the desired polarization direction to the piezoelectric ceramics polarized in the opposite direction. And a second polarization step of polarization.

[0010]

In the polarization method of the present invention, first, a voltage in the direction opposite to the voltage application direction required to obtain the desired polarization direction in the first polarization step is applied, and the piezoelectric ceramic is polarized in the opposite direction. Then, the first polarization direction is set to the desired polarization direction.
The second polarization step is performed by applying a voltage in the opposite direction to the polarization step. As a result, a piezoelectric ceramic having a polarization state in which the change in residual polarization over time is suppressed can be obtained.

Although the mechanism of this is not clear yet, by applying a voltage in the opposite direction beforehand, an internal bias electric field is generated around the pores and inclusions inside the piezoelectric ceramic, and this internal bias electric field is generated in the domain. It is considered that 90 ° switching is suppressed.

[0012]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example 1) <Preparation of test piece> Pb as raw material powder
O, ZrO ₂ , TiO ₂ , Sr ₂ O ₃ , Nb ₂ O ₅
Pb _0.89 Sr _0.11 (Zr _0.55 Ti _0.44 Nb _0.01 ) O ₃ was weighed so as to have a composition, pulverized with a ball mill for 48 hours, baked at 900 ° C. for 1 hour, and then pulverized with a ball mill again.
Dried. About 5% by weight of PVA (polyvinyl alcohol) as a binder was added to this powder, and after granulation, 1 ton
Twenty cylindrical shaped bodies having a diameter of about 20 mm and a thickness of about 15 mm were formed at a pressure of / cm ² .

Each of the obtained compacts was mixed with Pb for Pb atmosphere adjustment.
The powder was put into an alumina crucible together with bZrO ₃ powder, heated in an electric furnace at a heating rate of 300 ° C./hr, and sintered at 1250 ° C. for 2 hours. This sintered body was cut into a disk shape having a diameter of 15 mm and a thickness of 1 mm, and silver paste was applied to the front and back surfaces of the disk, and then 550
After baking for 10 minutes at ℃ to form an electrode, the relative permittivity (ε
₃₃ ^T / ε ₀ ) and a test piece for Kp measurement. <First Polarizing Step> For each test piece 1, FIG.
As shown in (a), a polarization treatment was performed by applying a DC voltage of 3 kv / mm in silicon oil 2 at 100 ° C. for 30 minutes. <Second Polarizing Step> Next, as shown in FIG.
Polarization process is performed by arranging the anode and the cathode in reverse, that is, in such a manner that the voltage application directions are opposite, and similarly applying a DC voltage of 3 kv / mm for 30 minutes in 100 ° C. silicon oil 2 to perform polarization treatment. It was <Test> After standing for 24 hours after the second polarization process,
Kp and relative permittivity (ε ₃₃ ^T / ε ₀ ) were measured with the voltage application direction in the polarization step as the positive direction. The thickness of the test piece is 1 m when a voltage of 0 to 700 v is applied in the positive direction.
The displacement amount per m was measured. The results are shown in Table 1.

Furthermore, at a temperature of 100 ° C., the polarization direction is increased by 2
A durability test in which a voltage of 0 to 700 v is repeatedly applied 10 ¹ to 10 ⁸ times under a pressure of 0 MPa is performed, and changes in Kp and relative dielectric constant with respect to the number of repetitions are shown in FIG. The change in the amount of displacement is shown in FIG. (Example 2) A test piece was prepared in the same manner as in Example 1 except that the treatment temperature in the first polarization step was 30 ° C.
Similarly, Kp, relative permittivity and displacement amount were measured at the initial stage and the durability test, and the results are shown in Table 1, FIG. 2 and FIG. (Example 3) A test piece was prepared in the same manner as in Example 1 except that the treatment temperature in the second polarization step was 30 ° C.
Similarly, Kp, relative permittivity and displacement amount were measured at the initial stage and the durability test, and the results are shown in Table 1, FIG. 4 and FIG. Example 4 A test piece was prepared in the same manner as in Example 1 except that the treatment temperatures in the first polarization step and the second polarization step were 30 ° C., respectively, and Kp at the initial and endurance tests were similarly set. The relative permittivity and the amount of displacement were measured, and the results are shown in Table 1, FIG. 4 and FIG. (Conventional Example) A test piece was prepared in the same manner as in Example 1 except that the first polarization step was not performed and only the second polarization step was performed.
Similarly, Kp, relative permittivity and displacement amount were measured at the initial stage and the durability test, and the results are shown in Table 1, FIG. 2, FIG. 3, FIG. 4 and FIG.

[0015]

[Table 1]

(Examples 5 to 9) Test pieces were prepared in the same manner as in Example 1 except that the positive applied voltage in the second polarization step was variously selected from 3.6 to 1.2 kv / mm. It was prepared and K in the initial and 10 ⁸ times repeated durability test
p, relative permittivity and displacement were measured, and the results are shown in Table 2. (Examples 10 to 14) Test pieces were prepared in the same manner as in Example 1 except that the reverse applied voltage in the first polarization step was variously selected from 3.6 to 1.2 kv / mm. Similarly, Kp, relative permittivity, and displacement amount were measured at the initial stage and at the 10 ⁸ times repeated durability test, and the results are shown in Table 2.

[0017]

[Table 2]

(Examples 15 to 18) of piezoelectric ceramics
The composition is Pb_0.89Sr_0.11(Zr_0.56Ti_0.43Nb_0.01)
O ₃And the applied voltage in the positive direction in the second polarization step
Except that various selections were made between 3.6 and 2 kv / mm.
Test piece was prepared in the same manner as in Example 1, and
10⁸Kp, relative permittivity and displacement during repeated durability test
The amount was measured and the results are shown in Table 3.

The compositions of the piezoelectric ceramics of Examples 1 to 14 are tetragonal, the compositions of the piezoelectric ceramics of Examples 15 to 18 are rhombohedral, and Pb _0.89 Sr _0.11 (Zr
It is represented by _0.60 Ti _0.39 Nb _0.01 ) O ₃ .

[0020]

[Table 3]

(Evaluation) As is clear from the results of Tables 1 to 3 and FIGS. 2 to 5, the piezoelectric ceramics obtained in the examples have a large initial displacement amount as compared with those obtained in the conventional examples. The change rate of the displacement amount is small because the decrease in Kp accompanying the durability test is small. It is clear that this is due to the fact that the polarization treatment was performed in the reverse direction prior to the polarization treatment in the positive direction.

[0022]

That is, the piezoelectric ceramics obtained by the polarization method of the present invention exhibit a large amount of displacement and suppress the change in remanent polarization over time, and thus have excellent durability. Therefore, if a piezoelectric actuator is manufactured using this piezoelectric ceramic, a power supply circuit for displacement reduction correction becomes unnecessary, and a cooling device for displacement deterioration prevention becomes unnecessary, which leads to cost reduction and structure reduction. It is possible to contribute to the simplification of.

According to the polarization method of the present invention, the number of steps is almost the same as when the conventional polarization method is divided into two steps, and the piezoelectric ceramic can be formed extremely easily and stably. .

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram illustrating a polarization method according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in Kp and relative dielectric constant with respect to the number of durability tests.

FIG. 3 is a graph showing a change in displacement amount per 1 mm of thickness with respect to the number of durability tests.

FIG. 4 is a graph showing changes in Kp and relative dielectric constant with respect to the number of durability tests.

FIG. 5 is a graph showing changes in the amount of displacement per 1 mm of thickness with respect to the number of durability tests.

[Explanation of symbols]

1: Test piece (piezoelectric ceramics) 2: Silicon oil

Claims (1)

[Claims] 1. A first polarization for applying a voltage to a piezoelectric ceramic to polarize the piezoelectric ceramic by applying a voltage in a direction opposite to a voltage application direction necessary to obtain a target polarization direction. Piezoelectric ceramics characterized by performing a step and a second polarization step of applying a voltage in a voltage application direction necessary for obtaining a desired polarization direction to the piezoelectric ceramics polarized in the opposite direction Polarization method.