Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Measuring – testing – or inspecting
Reexamination Certificate
1999-12-07
2002-06-11
Fiorilla, Christopher A. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Measuring, testing, or inspecting
C264S435000, C264S436000
Reexamination Certificate
active
06403012
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of polarization-treating a piezoelectric material for use in ceramic filters, ceramic oscillators, and so forth.
2. Description of the Related Art
According to conventional methods of polarizing piezoelectric ceramics having a granular structure, typically in PZT type ceramics, desired polarization degrees are obtained by sputtering a silver electrode onto a firing material, and applying a direct voltage of about 2 to 5 kV/mm at a temperature ranging from room temperature to about 150° C. for a time of from several seconds to several minutes. However, the conventional methods have the drawback that the characteristics of the piezoelectric ceramics are degraded by a thermal shock (by soldering and so forth) generated in a process after the polarization treatment. One of the reasons is speculated to be as follows.
Polarization treatment means that spontaneous polarization is arranged in a predetermined direction. This movement of the spontaneous polarization is achieved by two types of rotation, i.e., 180° and 90° rotation, on the whole. The residual component of the spontaneous polarization achieved by 90° rotation has the property that it is restored to its original state by heating, while the residual component of the spontaneous polarization by 180° rotation has the property that the polarization state is maintained below the Curie temperature. That is, the spontaneous polarization achieved by 180° rotation is more stable to thermal shock as compared with the spontaneous polarization achieved by 90° rotation.
FIGS. 1A
,
1
B, and
1
C illustrate the change in directions of spontaneous polarization.
FIG. 1A
shows the directions of the spontaneous polarization before polarization treatment, which are different from each other. By the polarization treatment, all the spontaneous polarization is arranged in the same direction as shown in FIG.
1
B. After this, by suffering a thermal shock, only the spontaneous polarization achieved by 90° rotation is restored to the original state as shown in FIG.
1
C.
A polarization treatment method has been proposed in which the spontaneous polarization of a piezoelectric material achieved by 90° rotation is removed by aging treatment, utilizing the above-described properties, whereby the polarization degree is made up solely of the polarization degree achieved by 180° rotation. (Japanese Unexamined Patent Publication No. 7-172914). As an aging treatment method for this case, a method (first method) of heat-aging at a temperature of from 200 to 250° C., a method (second method) of repeating changing a temperature from −40° C. to 130° C., and then from 130° C. to −40° C. at about 50 cycles, a method (third method) of applying a compression stress of 50 MPa at about 100 cycles, and so forth are exemplified.
However, according to the conventional second and third aging treatment methods, since an excess load is applied to the piezoelectric material, there are problems that cause inconveniences such as cracking, breaking, or the like and the yield is reduced. On the other hand, according to the first aging treatment method, it is difficult to produce a piezoelectric material having a target polarization degree simply by carrying out heat-aging at a temperature of from 200 to 250° C., though the yield is high. In particular, conventionally, the polarization-treatment is carried out at a voltage, time, and a temperature selected appropriately, and thereafter, the aging is conducted until the polarization achieved by 90° rotation is completely removed. Therefore, obtained final polarization degrees are uneven, depending on how much the spontaneous polarization achieved by 180° rotation is present on completion of the polarization treatment.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of polarization-treating a piezoelectric material whereby a target polarization can be attained at a high precision, and the treated piezoelectric material is stable thermally with passage of time.
The inventors carried out polarization and aging under different conditions. As a result, they have found that the ratio of the polarization degree &Dgr;f
180
achieved by 180° rotation to the polarization degree &Dgr;f
90
achieved by 90° rotation in the polarization degree &Dgr;f is dominated by the temperature only, irrespective of the polarization voltage and the polarization time. Therefore, when the temperature is predetermined, the ratio of the polarization degree &Dgr;f
180
achieved by 180° rotation to the polarization degree &Dgr;f
90
achieved by 90° rotation can be principally known.
Throughout this specification, the term “polarization degree” refers to “residual polarization degree” which is a polarization degree at an ordinary temperature after the polarization treatment is completed.
The polarization degree of a piezoelectric material can be expressed by use of a frequency difference &Dgr;f between the resonance frequency f
r
and the anti-resonance frequency f
a
of the piezoelectric material, as is well known.
FIG. 2
shows the relation between the polarization temperatures and the polarization degrees achieved by 180° and 90° rotation in the polarization degrees &Dgr;f, which were experimentally determined by the inventors of this invention. The above-mentioned polarization degrees &Dgr;f are obtained by carrying out the polarization (hereinafter, referred to as total-polarization) by which the maximum polarization degree can be achieved at each polarization temperature. In the experiment, as a piezoelectric material, PZT ceramics were used comprising Pb(ZrTi)O
3
to which Sr, Cr were added.
For example, in the case such as in
FIG. 2
, the polarization temperatures T
1
through T
6
are 26° C., 50° C., 100° C., 150° C., 200° C., and 250° C., and the aging is carried out at the same temperatures as the polarization temperatures, respectively. The polarization degrees &Dgr;f
180
achieved by 180° rotation, after the aging, are constant, irrespective of the temperatures, while the polarization degrees &Dgr;f
90
achieved by 90° rotation are reduced with the temperatures being higher, and at the temperature T
6
, the residual polarization degree &Dgr;f is made up solely of the polarization degree achieved by 180° rotation. With the polarization temperatures being changed from T
1
to T
6
, the ratios of the polarization degrees achieved by 180° rotation to those by 90° rotation are changed from 77:23 to 78:22 to 81:19 to 85:15 to 90:10 to 100:0, for example. The ratios of the polarization degrees achieved by 180° rotation to the polarization degree achieved by 90° rotation depend on the temperatures only, and are not affected by the polarization voltages and the polarization times (total or half polarization). The polarization treatment may be carried out in the atmosphere or an insulating oil. Thus, the relation as shown in
FIG. 3
is obtained. Herein, the term “half polarization” means the polarization before it reaches the total polarization, which is carried out while time and voltage are controlled.
To determine the ratios of the polarization degrees achieved by 180° rotation to the polarization degrees achieved by 90° rotation in the polarization degrees &Dgr;f, the (002) lattice plane of a piezoelectric material is measured by XRD (X-Ray Diffraction Method), and then determination is made as to the polarization temperature at which the polarization degree &Dgr;f made up solely of the polarization degree achieved by 180° rotation can be obtained.
FIG. 4
shows the relation between the polarization temperatures and the X-ray intensities at the (002) crystal lattice plane. The X-ray intensities are obtained by the measurement of the piezoelectric material wherein the temperature has been restored to a room temperature after the polarization treatment. The term “the (002) lattice plane” means the lattice plane wherein the C axis is in parallel to the polarization direction. As seen in
FIG. 4
,
Fujii Naoki
Tomohiro Hiroshi
Burns Doane , Swecker, Mathis LLP
Fiorilla Christopher A.
Murata Manufacturing Co. Ltd.
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