Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Measuring – testing – or inspecting
Reexamination Certificate
2000-08-11
2003-02-18
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Measuring, testing, or inspecting
C264S430000, C264S436000, C324S727000
Reexamination Certificate
active
06521166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of polarization-treating a piezoelectric body for use in a ceramic filter, a ceramic oscillator, and other suitable components.
2. Description of the Related Art
For polarization-treatment of piezoelectric ceramic substrates (block, unit, or other suitable substrates) of PZT and PT types, conventionally, after a piezoelectric ceramic substrate is fired, electrodes made of Ag or other suitable materials are provided on both of the opposite sides of the piezoelectric ceramic substrate. Plural piezoelectric ceramic substrates are simultaneously dipped into a polarization liquid at a temperature of 60 to 100° C., and a voltage of 2 to 8 kV/mm is applied for about 10 to 30 minutes to obtain a desired polarization degree.
After the polarization treatment, to prevent the deterioration of characteristics caused by heat treatment as an after-process, the piezoelectric body is left to stand still in an atmosphere at about 150° C. for 20 to 30 minutes (aging), so that the characteristics of the piezoelectric body are caused to deteriorate, and thereby, the time-dependent characteristics of the piezoelectric body are stabilized.
The polarization treatment of a piezoelectric body can be done by the in-liquid polarization which is carried out in a polarization liquid having insulation properties as described above, and the in-air polarization which is conducted in the atmosphere or in an gas atmosphere. In the in-air polarization, a desired electric field intensity cannot be attained because discharge occurs at a voltage of about at least 1 kV/mm. For this reason, in general, the in-liquid polarization is used to obtain a high polarization degree.
However, in the case of the in-liquid polarization, the polarization degree of the piezoelectric body cannot be measured during polarization. This is because the vibration of the piezoelectric body placed in the liquid is damped due to the liquid, and the frequency characteristic cannot be measured. For this reason, conventionally, the in-liquid polarization is the constant-time polarization in which the polarization is carried out for a desired time period. As a result, a problem arises in that the polarization degree cannot be exactly controlled, causing the firing and composition of the piezoelectric body to be dispersed, which results in dispersion of the polarization degree.
In Japanese Patent No. 2656041, a polarization method proposed is in which a piezoelectric constant (for example, electromechanical coupling coefficient K) is measured during polarization, and application of a voltage is stopped when the constant reaches the desired level which is determined by a correlation between the value K obtained immediately after the polarization is stopped and the stable value K obtained after a lapse of time. As a result, dispersion of the polarization degree caused by dispersion of materials and firing conditions is reduced. Accordingly, a piezoelectric body having constant qualities is produced.
According to the above-described method, the piezoelectric constant value is measured during polarization. Accordingly, it is necessary to carry out the polarization treatment in the air. However, in the in-air polarization, discharge occurs at a voltage of about at least 1 kV/mm, so that a high voltage cannot be applied . This causes the problem that a long polarization treatment time is needed to obtain a polarization degree comparable to that by the in-liquid polarization.
Moreover, according to the above-described method, the set level is determined based on the correlation between the value of K obtained immediately after the polarization is stopped and the stable value of K obtained after a lapse of time. However, if the piezoelectric body is aged after the polarization treatment, the polarization degree varies with the value of K. For this reason, if the aging is carried out after the polarization treatment, the set level cannot be determined based on the correlation between the value of K obtained immediately after the polarization is stopped and the stable value of K obtained after a lapse of time.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a method of polarization-treating a piezoelectric body in which a polarization degree comparable to that obtained by in-liquid polarization is obtained in a short period of time by in-air polarization.
Preferred embodiments of the present invention also provide a method of polarization-treating a piezoelectric body in which dispersion of the polarization degrees of respective piezoelectric bodies is reduced, and a target polarization degree is very precisely attained.
According to a first preferred embodiment of the present invention, a method of polarization-treating a piezoelectric body includes the steps of applying a DC voltage to the piezoelectric body for polarization in the air and in an atmosphere with a temperature at least as high as an aging temperature, measuring a polarization degree of the piezoelectric body while the piezoelectric body is polarized, stopping the application of the DC voltage at the time when the measured polarization degree reaches a set level, and aging the piezoelectric body at the aging temperature after the application of the voltage is stopped.
In the case of polarization treatment in the air, a voltage of about at least 1 kV/mm cannot be applied. However, a piezoelectric body can be polarization-treated at a higher temperature as compared with the polarization in a liquid. Accordingly, the polarization is conducted at a low voltage, and a desired polarization degree is attained in a short time. Moreover, since the aging of the piezoelectric body proceeds simultaneously with the polarization, the aging time after the application of the voltage is stopped is substantially shortened. For example, conventionally, for aging in an atmosphere at about 150° C., an aging time of about 20 to 30 minutes is required. According to preferred embodiments of the present invention, the aging is performed in only several minutes. That is, the aging time is reduced to about one tenth of the conventional aging time. Consequently, the polarization time and the aging time are greatly shortened. Thus, the overall time required for the polarization treatment is significantly reduced.
Further, the polarization is performed at a relatively low voltage. Therefore, a charge to the piezoelectric body is reduced, and problems such as cracking, chipping, and other common problems, which are caused by the polarization, are eliminated.
As the piezoelectric body is polarized in the air, the frequency characteristic is measured during the polarization. That is, the polarization degree is easily measured. With the polarization degree being measured, the application of the voltage is stopped at the time when the measured polarization degree reaches a set level, and after the application of the voltage is stopped, the piezoelectric body is aged at an aging temperature. Therefore, dispersion of the polarization degree is greatly reduced, and a target polarization degree is very precisely attained. The set level is a value that is determined depending on piezoelectric materials. Further, the set value may be a value determined corresponding to polarization conditions such as polarization temperature, polarization voltage, and other conditions.
FIG. 1
shows the variation of the polarization degree of a piezoelectric body in the process of polarization to aging to ordinary temperature restoration.
As seen in
FIG. 1
, the polarization degree rises to the maximum during the polarization, and is decreased by the aging, and then is partially restored by the ordinary temperature restoration to become stabilized. The maximum polarization degree &Dgr;f
1
in the polarization and the stable polarization degree obtained after the ordinary temperature restoration have a high correlation.
In the above example, the polarization degree of the piezoe
Fujii Naoki
Tomohiro Hiroshi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Tentoni Leo B.
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