JP4269689B2 - Method for manufacturing piezoelectric element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a piezoelectric element, of which a piezoelectric body made of piezoelectric ceramics is polarized in the thickness direction, and more particularly to a block-shaped piezoelectric body polarization method.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent Publication No. 8-34650 [Patent Document 2]
JP, 2002-232032, A Conventionally, in a manufacturing process of a piezoelectric filter or an oscillator using piezoelectric ceramics, a block-like piezoelectric body having a side of about several centimeters is formed and fired, and then the front and back surfaces of the piezoelectric body are formed. After forming electrodes for polarization and applying a high voltage between these electrodes for polarization treatment, this block-shaped piezoelectric body is cut into tens to thousands of elements and processed into products. . The problem here is characteristic variation due to polarization degree variation between elements cut out from one piezoelectric body.
[0003]
As shown in FIG. 1, the polarization degree distribution of the piezoelectric body after polarization is low in the central part and high in the peripheral part, and in the central part as shown in FIG. May be high, and the degree of polarization may be low at the periphery. The polarization degree distribution as shown in FIG. 1 is likely to occur in the case of a thin piezoelectric body (for example, having a thickness of about 1 mm) that warps due to polarization, and the distribution as shown in FIG. 2 does not generate warp due to polarization. It is likely to occur in such a thick block piezoelectric body.
When an element is cut out from such a piezoelectric body having a variation in polarization degree, the element cut out from the vicinity of both ends cannot obtain the necessary piezoelectric characteristics, resulting in the occurrence of many elements that do not satisfy the characteristics.
[0004]
[Problems to be solved by the invention]
In Patent Document 1, in order to increase the degree of polarization at the central part of the disk and lower the degree of polarization at the peripheral part in the polarization of the disk-shaped piezoelectric body, the electrical resistance at the central part is lowered and the electrical resistance at the peripheral part is increased. A method has been proposed.
In this case, when a uniform voltage is applied to the electrodes of the piezoelectric body, the voltage applied to the piezoelectric body is high in the central portion and low in the peripheral portion due to the difference in resistance between the central portion and the peripheral portion. By this voltage distribution, a polarization degree distribution that is large in the central portion and small in the peripheral portion is obtained.
However, when the method of Patent Document 1 is used for polarization processing of a thick piezoelectric body, the difference in the degree of polarization between the central portion and the peripheral portion increases, and the variation in polarization degree between elements cut out from one piezoelectric body increases. There is a drawback. In addition, it is extremely difficult to continuously change the resistance value of the electrode for polarization between the central portion and the peripheral portion, it is difficult to realize an optimum resistance value distribution, and the obtained polarization degree distribution is not desirable. .
[0005]
In Patent Document 2, in order to make the polarization degree in the block uniform in the polarization of the laminated piezoelectric body, after performing the first polarization process, the secondary polarization is performed in the opposite direction to the first, By optimizing the voltage of the secondary polarization, the polarization degree variation in the block is reduced.
In the case of the target laminated piezoelectric material, the peripheral portion is likely to be polarized as in the case of a thin piezoelectric material. This property works to lower the degree of polarization at the center of the block during positive polarization and to increase the degree of polarization at the periphery, and to increase the degree of polarization at the center of the block during reverse polarization. Work to lower. By paying attention to the contradictory properties of the normal and reverse polarization, the polarization degree tendency in the block is canceled by the normal and reverse polarization to make the degree of polarization uniform.
However, in the case of a laminated piezoelectric material, since it has a polarization degree distribution opposite to that of a thick piezoelectric material, it cannot be directly applied to the polarization of a thick piezoelectric material. In addition, since the polarization degree after reverse polarization is lower than the saturation polarization degree of the material, the above method cannot be applied when the saturation polarization degree is required.
[0006]
Accordingly, an object of the present invention is to provide a method for manufacturing a piezoelectric element that can reduce variation in characteristics between elements cut out from a piezoelectric body by making the polarization degree distribution in the piezoelectric body uniform.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 has a high degree of polarization distribution at the center when a uniform electric field is applied to a piezoelectric body of a certain thickness made of piezoelectric ceramics and polarized in the thickness direction. In a method of manufacturing an element from a piezoelectric material having a property of being lowered at the periphery, the thickness distribution of the piezoelectric material to be processed from the polarization degree distribution when a uniform electric field is applied to the piezoelectric material of a constant thickness And processing the peripheral portion of at least one of the front and back surfaces of the piezoelectric body so that the thickness of the piezoelectric body gradually decreases from the central portion toward the peripheral portion based on the thickness distribution ; A step of forming a continuous electrode for polarization on the front and back surfaces of the piezoelectric body from the central portion to the peripheral portion, and a step of applying a direct current electric field between the polarization electrodes to perform a polarization treatment in the thickness direction of the piezoelectric material. If, double the piezoelectric body already above polarization treatment To provide a manufacturing method of a piezoelectric element and a step of cutting out the elements of the.
[0008]
When polarization electrodes are formed on the front and back surfaces of a block-shaped piezoelectric body having a constant thickness and a high voltage is applied between the electrodes, the polarization degree distribution is such that the polarization degree distribution is high in the central part and low in the peripheral part. . In the present invention, in consideration of such a polarization degree distribution in advance, the peripheral portion of at least one surface of the front and back is processed so that the thickness of the piezoelectric body gradually decreases from the central portion toward the peripheral portion. That is, the electric field distribution is optimized by reducing the thickness of the peripheral portion where the degree of polarization of the piezoelectric body tends to be low.
Therefore, the electric field strength in the peripheral portion having a small thickness is higher than that in the central portion, and the degree of polarization in the peripheral portion of the piezoelectric body having the property of lowering the degree of polarization can be increased. As a result, the non-uniformity of the degree of polarization between the central part and the peripheral part is corrected, and a substantially uniform degree of polarization can be obtained for the entire piezoelectric body.
Also, by continuously changing the thickness of the piezoelectric body according to the polarization degree distribution of the piezoelectric body having a constant thickness, there is no occurrence of a location where the polarization degree changes discontinuously in the middle of the piezoelectric body, and the degree of polarization is uniform. Can be realized accurately.
Further, since the present invention does not require additional polarization such as reverse polarization, it can be applied even when a saturation polarization degree is required.
[0009]
In the present invention, the shape of the piezoelectric body to be processed is preferably obtained from the degree of polarization distribution when a uniform electric field is applied to the piezoelectric body having a constant thickness.
When a uniform electric field is applied to a piezoelectric body having a constant thickness, the degree of polarization distribution is high at the center and low at the periphery. If the thickness distribution of the piezoelectric body is estimated from this distribution and processed according to this thickness distribution, a substantially uniform polarization degree distribution can be obtained.
[0010]
As a method of determining the shape of the processed portion, as in claim 2 , a step of obtaining a polarization degree distribution when a uniform electric field is applied to a piezoelectric body having a constant thickness, and an actual value from the polarization degree distribution The step of calculating the amount of deviation from the ideal value, the step of obtaining the slope near the polarization electric field strength to be used from the relationship curve between the electric field strength and the degree of polarization, and the product of the amount of deviation and the slope described above, The step of calculating the strength and the step of obtaining the thickness distribution of the piezoelectric body to be processed from the additional electric field strength can be used.
In this case, the shape of the processed part can be obtained quantitatively.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 3 shows an example of the manufacturing process of the piezoelectric body according to the present invention.
FIG. 3 (a) shows a rectangular parallelepiped block-like piezoelectric body 1 formed and fired from piezoelectric ceramics. As the piezoelectric ceramic, for example, a known piezoelectric material such as a PZT system can be used. The thickness T, width W, and length L of the piezoelectric body 1 are as follows, for example.
T = 10mm, W = 30mm, L = 20mm
FIG. 3B shows a piezoelectric body 2 in which a processed portion 3 is formed by cutting both sides of the surface of the piezoelectric body 1 in an oblique direction with a polishing machine or the like. If the width of the processed portion 3 is S and the thickness is D, the dimensions are set as follows, for example.
S = 6.0mm, D = 1.05mm
In FIG. 3C, polarization Ag electrodes 4, 5 are formed by sputtering or the like on the entire front and back surfaces of the piezoelectric body 2 on which the processed portion 3 is formed, and a DC high-voltage power source 6 (for example, between the electrodes 4, 5). 25 kV) is connected and a DC electric field is applied. The piezoelectric body 2 is polarized in the thickness direction as indicated by an arrow P. In FIG. 3C, the electrode 4 on the front surface was connected to the DC high voltage power source 6, the electrode 5 on the back side was connected to the ground, and an electric field was applied in 100 ° C. silicon oil for 10 minutes.
[0012]
FIG. 4 shows the polarization degree distribution after polarization of the piezoelectric body.
4A shows a polarization degree distribution when the rectangular parallelepiped piezoelectric body 1 is polarized under the same conditions as FIG. 3C, and FIG. 4B shows the piezoelectric body in which the processed portion 3 is formed. 2 is a degree of polarization distribution when the same polarization is performed on 2.
[0013]
In order to obtain the polarization degree distribution, as shown in FIG. 5A, the piezoelectric body 1 after polarization (the arrow P indicates the polarization direction) is fixed along a line 7 parallel to the length L direction ( For example, after forming a plurality of strips 8 by cutting at 0.3 mm, the strip 8 is further cut along vertical and horizontal lines 9 as shown in FIG. As shown in (c), electrodes were formed on the front and back surfaces of the element 10 and Δf (= fa−fr) was measured. The element 10 is, for example, a thickness shear vibration mode element having a resonance frequency fr of 4.0 MHz.
FIG. 4 shows an average value of Δf of a plurality of elements 10 obtained from each strip 7 among the elements 10 created as described above, in a bar graph. Here, Δf is used as a characteristic that is substantially proportional to the degree of polarization, but an electromechanical coupling coefficient may be used instead of Δf.
[0014]
As is clear from FIG. 4, in the case of the rectangular parallelepiped piezoelectric body 1 in which the processed portion 3 is not formed, the degree of polarization (Δf) decreases at both ends as shown in (a), and the range is 6 mm from both ends. Δf of the element 10 cut out from the above is 330 kHz or less. In particular, the element near both ends is 325 kHz or less, and the target piezoelectric characteristics are not obtained. On the other hand, in the case of the piezoelectric body 2 in which the processed portion 3 is formed, a decrease in the degree of polarization at both ends is suppressed as shown in (b), and Δf is 330 kHz or more in almost the entire width direction of the piezoelectric body 2. It has become. Therefore, the element 10 cut out from almost the entire area of the piezoelectric body 2 satisfies desired piezoelectric characteristics.
In addition, in the data of FIG. 4, it is missing in the range of 2 mm from both ends because the data is not obtained.
[0015]
Next, a specific method for determining the shape of the processed portion 3 will be described with reference to FIG. First, a polarization degree distribution when a uniform electric field is applied to a block-shaped piezoelectric body having a constant thickness is obtained (step S1). This polarization degree distribution is a Δf distribution as shown in FIG. 7, and is obtained by the same method as in FIG. A solid line indicates an actual value, and a broken line indicates an ideal value.
Next, a deviation amount between the actual value and the ideal value is calculated from the polarization degree distribution (step S2). FIG. 8 shows the amount of deviation, that is, the shortage of Δf, as a distribution in the width direction (W) of the piezoelectric body.
Next, a relational curve of electric field strength−Δf as shown in FIG. 9 is obtained in advance, and the inclination near the polarization electric field strength to be used is read (step S3). In this case, when a voltage of 30 kV is applied to a block having a thickness of 10 mm, that is, when the inclination is calculated for an electric field strength around 3000 V / mm, it is about 0.05 kHz / V / mm. In addition, the relational curve of FIG. 9 changes with ceramic materials, polarization temperature, and polarization time.
Next, the necessary additional electric field strength is calculated from the product of the deviation amount (Δf deficiency) obtained in step S2 and the slope obtained in step S3 (step S4). FIG. 10 shows this additional electric field strength distribution.
Finally, the thickness of the block piezoelectric body is calculated from the additional electric field strength obtained in step S4 (step S5). Ideally, the shape is as shown in FIG. 11, but in practice, the piezoelectric body may be processed so as to be as close as possible to this shape.
As described above, the ideal thickness of the piezoelectric body can be quantitatively calculated.
[0016]
FIG. 12 shows various forms of a piezoelectric body provided with a processed portion.
(A) is provided with inclined processed portions 3 along four sides of the surface of a rectangular parallelepiped piezoelectric body. The tendency for the degree of polarization at the end of the piezoelectric body to decrease occurs in both the W direction and the L direction. Therefore, by providing the processed portions 3 along the four sides, the uniformity of the polarization degree distribution is further improved.
(B) is one in which a conical taper-shaped processed portion 3 is provided at the periphery of the surface of a rectangular parallelepiped piezoelectric body. Accordingly, a circular top 2 a is left at the center of the piezoelectric body 2.
(C) is one in which a concentric taper-shaped processed portion 3 is provided at the periphery of the surface of a disk-shaped piezoelectric body.
(D) shows a cross section of the piezoelectric body 2, and is provided with a processed portion 3 that is inclined in a curved shape from the central portion to the peripheral portion of the surface of the piezoelectric body 2. The cross-sectional shape of the processed portion 3 approximates the block thickness distribution curve shown in FIG.
(E) also shows a cross section of the piezoelectric body 2, in which processed portions 3 inclined in a curved shape from the central portion to the peripheral portion are provided on both front and back surfaces of the piezoelectric body 2. The curvature of the processing part 3 on each of the front and back sides is about half of the curvature of the processing part 3 in (d).
[0017]
The present invention is not limited to the above embodiments.
In the above embodiment, the processed portion 3 is formed by cutting the end portion of the piezoelectric body that has been molded and fired to a constant thickness. However, the piezoelectric body is formed into a shape in which the end portion has been processed into a thin wall in advance and fired. May be. That is, you may provide the process part 3 in the stage of shaping | molding.
[0018]
【The invention's effect】
As apparent from the above description, according to the invention of claim 1, the thickness of the peripheral portion of the block-shaped piezoelectric body is thinned, and the electric field strength of the peripheral portion of the piezoelectric body is intentionally increased. It is possible to prevent a decrease in the degree of polarization at the periphery of the piezoelectric body, which has a low property, and to correct the nonuniformity of the degree of polarization in the plane of the piezoelectric body. Therefore, the characteristic variation between the elements cut out from the piezoelectric body can be reduced, and the situation that many elements that do not satisfy the characteristics as in the conventional case occur can be solved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a polarization degree distribution of a thin plate-like piezoelectric body.
FIG. 2 is a diagram showing a polarization degree distribution of a thick block piezoelectric body.
FIG. 3 is a manufacturing process diagram of a piezoelectric body according to the present invention.
FIG. 4 is an actual polarization degree (Δf) distribution diagram of a conventional product and a product of the present invention.
FIG. 5 is a diagram illustrating a method of cutting out an element from a piezoelectric body.
FIG. 6 is a flowchart showing a method for determining the thickness of a piezoelectric body according to the present invention.
FIG. 7 is a distribution diagram of the degree of polarization (Δf) in the piezoelectric body.
FIG. 8 is a distribution diagram of Δf deficiency in the piezoelectric body.
FIG. 9 is a relationship curve diagram of electric field strength−Δf.
FIG. 10 is a distribution diagram of additional electric field strength.
FIG. 11 is an ideal thickness distribution diagram of a piezoelectric body.
FIG. 12 is a diagram illustrating various forms of a piezoelectric body provided with a processing portion.
[Explanation of symbols]
1 Piezoelectric body (before processing)
2 Piezoelectric (after processing)
3 Processing parts 4 and 5 Polarizing electrode 6 DC high voltage power supply

Claims (2)

High polarization degree distribution when polarized in the thickness direction by applying a uniform electric field to the piezoelectric constant thickness made of piezoelectric ceramics in the central portion, the element of a piezoelectric material having a property such that lower at the peripheral portion In the method of manufacturing
The thickness distribution of the piezoelectric body to be processed is obtained from the polarization degree distribution when a uniform electric field is applied to the piezoelectric body having a constant thickness, and the peripheral portion of at least one of the front and back surfaces of the piezoelectric body is the thickness distribution. A step of processing so that the thickness of the piezoelectric body gradually decreases from the central part toward the peripheral part based on
A step of forming electrodes for polarization continuous from the center to the periphery on the front and back surfaces of the processed piezoelectric body;
Applying a direct current electric field between the electrodes for polarization to perform polarization treatment in the thickness direction of the piezoelectric body;
And a step of cutting out a plurality of elements from the polarization-treated piezoelectric body . Obtaining a polarization degree distribution when a uniform electric field is applied to a piezoelectric material having a constant thickness;
Calculating a deviation amount between the actual value and the ideal value from the polarization degree distribution;
From the curve of the relationship between the electric field strength and the degree of polarization, a step of obtaining a slope near the polarization electric field strength to be used
Calculating a required additional electric field strength by a product of the deviation amount and the slope;
The method for producing a piezoelectric element according to claim 1 , further comprising: obtaining a thickness distribution of the piezoelectric body to be processed from the additional electric field strength.

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