JP2002207019A - Method for inspecting piezoelectric ceramic element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inspecting a piezoelectric ceramic element, capable of detecting the internal flaw in the piezoelectric ceramic element, which can not be detected by the temperature rise due to heating from the outside, in a non-destructive manner. SOLUTION: A high frequency measuring signal having a level higher than the rated level of the piezoelectric ceramic element is applied to the piezoelectric ceramic element and at least one of the phase characteristics and impedance characteristics of the piezoelectric ceramic element accompanied by the application of the high frequency measuring signal is measured. This measured value is compared with characteristics becoming a standard and the presence of the flaw in the piezoelectric ceramic element is detected from the comparing result. The piezoelectric ceramic element is strongly excited by the application of the high frequency measuring signal and generates heat itself by dielectric heating and the internal flaw of the piezoelectric ceramic element can be certainly detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for non-destructively inspecting internal defects such as micro cracks which affect the quality of piezoelectric ceramic elements such as oscillators and filters.

[0002]

2. Description of the Related Art Conventionally, as a method for non-destructively inspecting internal defects of a piezoelectric ceramic element, as described in JP-A-6-3305, impedance characteristics and / or phase characteristics of a piezoelectric ceramic element are measured. There is known an inspection method in which a curve pattern showing this characteristic is compared with a reference curve pattern, and when the two curve patterns are different, it is determined that a micro crack exists in the piezoelectric ceramic element. In such an inspection method, the automatic determination is possible, so that the quality can be determined in a large amount in a short time, and the inspection accuracy is high because the determination is not made by visual inspection.

In the above inspection method, electrical characteristics are measured at room temperature and compared with reference characteristics. However, at room temperature, there is little difference in characteristics between good and bad,
Or, in many cases, there are no such cases, and this alone cannot completely detect an internal defect such as an internal micro crack.

FIG. 1 shows a ceramic oscillator (oscillation frequency:
25MHz) to the oscillation circuit and oscillate,
The results of measuring the behavior of the oscillating voltage at that time when only the oscillator is placed in an atmosphere of 200 ° C. or higher are shown. These oscillators have no difference in characteristics at room temperature. As can be seen from FIG. 1, the oscillating voltage gradually decreases as the temperature rises. However, the oscillating voltage decreases to near 0 V at 150 ° C. or less, and the oscillation stops (NG). And (G) where oscillation does not stop.

[0005] When these oscillators were opened and the internal elements were observed with a microscope, it was confirmed that microcracks had occurred in the element NG whose oscillation stopped at a low temperature. From this, it was found that some of the devices that normally oscillate at room temperature and have normal characteristics have microcracks inside, and by measuring the characteristics when heated, such internal defects can be detected. did.

[0006] Based on the above findings, the present inventors have proposed a method for inspecting a piezoelectric ceramic element which can reliably and nondestructively detect an internal defect that cannot be detected at room temperature (Japanese Patent Application No. 2000-213). -36909). In this inspection method, the temperature of the piezoelectric ceramic element is increased by heating, the phase characteristic or the impedance characteristic of the piezoelectric ceramic element is measured in a state where the temperature is increased, and the phase characteristic or the impedance characteristic is compared with a reference characteristic.
The presence or absence of internal defects is detected. The temperature at the time of heating and raising the temperature is desirably a temperature near the maximum temperature at which the piezoelectric characteristics return to almost the same piezoelectric characteristics as before the temperature rise when the temperature of the piezoelectric ceramic element is returned to room temperature after the temperature is raised.

[0007]

However, the present inventors have conducted experiments on various piezoelectric ceramic elements,
It was discovered that, depending on the type of piezoelectric ceramic element, abnormalities in phase characteristics and impedance characteristics did not appear when measured in a heated state, but abnormalities appeared when a high-level high-frequency signal was applied. Observation of the piezoelectric ceramic element having these abnormal characteristics with a microscope revealed that microcracks were also present.

FIG. 2 shows an external heating of the piezoelectric ceramic element.
And the phase characteristic (f ₀ Maximum phase near
Angle) for two types of products (G, NG)
It is. FIG. 3 shows products (G, NG) similar to FIG.
To change the level of the high-frequency signal (applied voltage)
Phase characteristics (f₀ Near the maximum phase angle).
You. The ambient temperature of both products was 20 ° C and -20 ° C.
° C. As is clear from FIG.
There is almost no difference in the phase characteristics between the product G and the product NG. I
However, when a high level measurement signal is applied, FIG.
As can be seen, a remarkable difference in phase characteristics appears between the two products G and NG.
I have. Open inspection of both products G and NG
Of course, there was no microcrack in product G,
The product NG had microcracks.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for inspecting a piezoelectric ceramic element which can non-destructively detect internal defects of the piezoelectric ceramic element which cannot be detected by external heating and heating.

[0010]

In order to achieve the above object, the invention according to claim 1 comprises a step of applying a high-frequency measurement signal having a level higher than a rated level to a piezoelectric ceramic element, and a step of applying the high-frequency signal. A step of measuring at least one of the phase characteristic and the impedance characteristic of the piezoelectric ceramic element, a step of comparing at least one of the measured phase characteristic and the impedance characteristic with a reference characteristic, and Detecting a presence or absence of an internal defect.

First, a high-frequency measurement signal having a power level higher than the rated level is applied to the piezoelectric ceramic element. When such a high-level high-frequency signal is applied, the piezoelectric ceramic element is excited more strongly than when a normal-level signal is applied, and self-heats due to dielectric heating. Due to the influence, internal defects such as micro cracks cause a large change which does not appear when a signal of a normal level is applied or when the temperature is increased by heating from the outside (see FIG. 3). Therefore,
At least one of the measured phase characteristic and impedance characteristic is compared with a reference characteristic. The reference characteristic may be obtained, for example, from a phase characteristic or an impedance characteristic when a high-level high-frequency measurement signal is applied to a non-defective (no internal defect) piezoelectric ceramic element. As a result of the above comparison, if the reference characteristic differs from the reference characteristic by a certain range or more, it is determined that the piezoelectric ceramic element has an internal defect. In the present invention, it is possible to determine not only the microcracks but also the displacement of the electrodes and the case where foreign matter is attached to the electrodes.

The high-frequency measurement signal is preferably a signal near the highest level at which the piezoelectric characteristics of the piezoelectric ceramic element return to almost the same piezoelectric characteristics as before the application after the application. By applying a signal of a level as high as possible within a range in which the piezoelectric characteristics return, internal defects that cannot be detected at a normal level can be reliably detected. If the power level of the measurement signal is too high, the piezoelectric characteristics of the piezoelectric ceramic element itself change irreversibly, which is not preferable.

Specifically, the signal near the highest level is a signal having a level near the highest temperature at which the piezoelectric characteristics do not depole because the piezoelectric ceramic element is dielectrically heated by the high frequency measurement signal. Good.
FIG. 4 shows a temperature rise curve (calculated value) when the element is heated by dielectric heating. Each curve is obtained by gradually changing the power level of the measurement signal from 30 dBm to 40 dBm. As is clear from the figures, in all cases, the temperature has increased to near the maximum temperature after about 400 msec has elapsed from the application. As described above, when the element is heated by the dielectric heating, the temperature can be raised to the target temperature in about several hundred msec, so that the time of the temperature rise can be greatly reduced, and the time for detecting the internal defect can also be reduced. In addition, since the heating is instantaneous and local, there is an advantage that the time required for the temperature of the element to return to its original state after application of the high-frequency signal is short.

As the phase characteristic to be measured, the maximum phase angle θmax can be used. While the normal level measurement signal having a phase characteristic as shown by a solid line P ₁ in FIG. 5, at the high level measurement signal of the phase is lowered as indicated by the dashed line P _2, reduction of more elements with internal defect large. Claim 3 uses the decrease in phase to determine an internal defect.

Further, the impedance characteristic to be measured includes an anti-resonance impedance value Za
And the resonance impedance value Zr can be used. Impedance characteristics, although the normal level measurement signal having a characteristic as shown by the solid line I ₁ in FIG. 5, the anti-resonance point and the resonance point as indicated by the broken line I ₂ upon application of a high-level measurement signal are both high frequency side And the impedance value difference Za-Zr becomes smaller. In claim 4, an internal defect is determined using this characteristic. Z
Other than a-Zr, the values of Za and Zr themselves, the frequency change rate at the anti-resonance point (dZa / df), the frequency change rate at the resonance point (dZr / df), and the oscillation frequency fosc
, Anti-resonance frequency fa and resonance frequency fr may be used. The phase characteristics and impedance characteristics to be measured are not limited to these.

By applying a high-level high-frequency signal to the piezoelectric ceramic element and measuring the phase characteristics and impedance characteristics at the time of application, it is possible to detect a defect that could not be detected by applying a normal-level signal. With some elements, the phase or impedance temporarily deviates from the reference value range with the passage of time of application of a high-level signal, and returns to the reference value range after an internal defect is found. Such a piezoelectric ceramic element is also a defective product.

FIG. 6 shows the relationship between the elapsed time when a high-level high-frequency signal is applied to the piezoelectric ceramic element and the maximum phase angle θmax. As is clear from FIG. 6, in the non-defective product G, the phase angle hardly changes over time. In NG1, the phase angle is reduced in about 150 msec and a defect is found, and thereafter, the characteristics are restored to normal. 300 msec for NG2
Previously, it was normal, but after 300 msec, a defect was found. It is considered that the appearance of the defect differs depending on the defective products NG1 and NG2 due to the difference in the location of the microcrack.

As described above, if the inspection is performed only at a certain time, it is not possible to detect a defect of the piezoelectric ceramic element that returns to the reference characteristic. Therefore, claim 5
In this method, at least one of a phase characteristic and an impedance characteristic of a piezoelectric ceramic element is measured at a plurality of elapsed times after application of a high-frequency signal having a level higher than a rated level. When a high-level high-frequency signal is applied, as shown in FIG. 4, the internal temperature of the element rises according to the elapsed time. Therefore, if a plurality of elapsed times are measured, a defect can be determined only at a certain time. Even with piezoelectric ceramic elements,
Internal defects can be reliably detected.

[0019]

FIG. 7 shows an example of an inspection apparatus for performing an inspection method according to the present invention. Here, the ceramic oscillator 2 was used as the piezoelectric ceramic element. 1
Is a network analyzer for measuring and analyzing the electrical characteristics of the ceramic oscillator 2 to be measured as a function of frequency. A high-frequency measurement signal is output from the built-in sine-wave sweep oscillator from the output terminal 1a and applied to the ceramic oscillator 2 via the measurement terminals 3a and 3b, thereby measuring the phase characteristics and impedance characteristics of the ceramic oscillator 2. . Although FIG. 7 shows only one channel,
It is also possible to have multiple channels.

An RF power amplifier 4 for amplifying applied power is connected between the output terminal 1a of the network analyzer 1 and the measurement terminal 3a, and the amplified power is supplied to the measurement terminal 3a.
Is applied to the ceramic oscillator 2. Specifically, a signal level of about 0 dBm is usually used when measuring the impedance characteristic, but in the present invention, the output level of the power amplifier 4 is set to be 20 to 40 dBm. The signal flowing through the ceramic oscillator 2 is sent from the measurement terminal 3b to the attenuator 5, attenuated to the original power by the attenuator 5, and input to the input terminal 1b of the network analyzer 1.

The network analyzer 1 applies a high-level high-frequency measurement signal for several hundred ms per ceramic oscillator 2 and measures impedance characteristics and phase characteristics in that state. The phase characteristic expresses the frequency characteristic of the phase difference (phase angle) from the value obtained by averaging the measured values of the phase difference (phase angle) between the current and the voltage, as is well known. From this phase characteristic, for example, the oscillation frequency fosc
The nearby maximum phase angle θmax is measured. This maximum phase angle θm
If ax is equal to or more than the reference value, it is selected as a good product, and if it is smaller than the reference value, it is selected as a defective product having an internal defect.

FIG. 8 shows a maximum phase angle θmax near fosc when a measurement signal of 34 dBm is applied to the device shown in FIG.
FIG. 2 is a diagram showing the relationship between the oscillation stop temperature shown in FIG. 1 and FIG. Referring to FIG. 8, when a measurement signal of 34 dBm is added, if the reference value of the phase angle is set to 40 ° to 50 °,
It can be seen that the good product G and the defective product NG can be clearly distinguished.

As described with reference to FIG. 6, some of the piezoelectric ceramic elements having an internal defect return to the reference value range after the phase or impedance has deviated from the reference value range with the application of the high-frequency measurement signal. Some things get lost. FIG. 9 shows an example of a method for inspecting such a piezoelectric ceramic element.
Shown in 9, when the measurement is started, the first time a high frequency measurement signal than the rated level of applied only T ₀ seconds piezoelectric ceramic element (step S1). Since the rated level of the piezoelectric ceramic element used here is Vpp (peak to peak) = 15 V (AC), the rated level is 5.3 Vrms in terms of an effective value. A signal having a level higher than the rated level is applied. As a result, the piezoelectric ceramic element is strongly excited and generates self-heating by dielectric heating. Then, after T ₀ seconds, the maximum phase angle θ is compared with the reference value θs (step S2). By comparison, if θ <θs, it is determined that the product is defective (step S3). In step S2, if theta ≧ [theta] s, the second round of the high-frequency measurement signal is applied only T ₀ seconds piezoelectric ceramic element (step S4). This allows
Since the piezoelectric ceramic element is excited again and further generates heat, the maximum phase angle θ is compared with the reference value θs after T ₀ seconds (step S5). If θ <θs, it is determined that the product is defective (step S6). Thereafter, similar steps are repeated. Finally, the n-th piezoelectric ceramic element
The times th high-frequency measuring signal is applied only T ₀ seconds (step S7). At this point, since the piezoelectric ceramic element is heated to near the maximum temperature, the maximum phase angle θ is compared with the reference value θs after T ₀ seconds (step S8). Where θ <θ
If s, it is determined that the product is defective (step S9), and θ
If ≧ θs, it is determined that the product is good (step S1).
0). Note that a pause period is not provided between the first to nth application of the high-frequency signal, and the high-frequency signal is applied continuously.

In FIG. 9, the application time (inspection time) T ₀ of the high-frequency signal is fixed from the first time to the n-th time, but the application time T ₀ may be arbitrarily changed. In particular, the application time T ₀ may be lengthened in a time zone in which the defect does not appear remarkably, and the application time T ₀ may be shortened in a time zone in which the defect becomes remarkable. Also,
If the inspection time T ₀ is shortened, it becomes almost the same as that of the inspection, and it is possible to reliably detect the phase angle of the piezoelectric ceramic element regardless of the temperature at any temperature.

The present invention is not limited to the above embodiment. In the above embodiment, the maximum phase angle θmax is set to the reference value θs
Although the internal defect was determined by comparing with, the internal defect may be determined using the impedance values Za and Zr,
The determination may be made using both the phase angle and the impedance value. The piezoelectric ceramic element of the present invention is not limited to a ceramic oscillator, but may be any piezoelectric ceramic element such as a ceramic filter, a discriminator, and a trap. Further, the internal defect can be detected irrespective of a difference in a vibration mode such as a thickness longitudinal vibration, a thickness shear vibration, and an area vibration.

[0026]

As is apparent from the above description, claim 1
According to the invention described in the above, a high-frequency signal of a level higher than the rated level is applied to the piezoelectric ceramic element, and at least one of the phase characteristic and the impedance characteristic is measured.
Internal defects are determined by comparing the measured values with the reference characteristics, so that non-destructive internal defects that cannot be detected with normal level signal application or internal defects that cannot be detected with external heating and heating. Can be detected.
In addition, since the piezoelectric ceramic element can be strongly excited by application of a high-frequency signal and self-heated by dielectric heating, internal defects of the piezoelectric ceramic element can be detected in a short time.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in oscillation voltage when a temperature of a resonator having an internal defect and a resonator having no internal defect are raised.

FIG. 2 is a phase characteristic diagram when an element having an internal defect and an element having no internal defect are heated and heated from the outside.

FIG. 3 is a phase characteristic diagram when an applied voltage of a high-frequency signal is changed with respect to the element shown in FIG. 2;

FIG. 4 is a time change diagram of an application time of a high-frequency signal and a temperature of an element.

FIG. 5 is a diagram illustrating impedance characteristics and phase characteristics of a ceramic oscillator.

FIG. 6 is a change diagram of the maximum phase angle of an element having an internal defect and an element having no internal defect when a high-level signal is applied.

FIG. 7 is a configuration diagram of an example of an apparatus for performing the inspection method of the present invention.

FIG. 8 is a correlation diagram between the oscillation stop temperature of an element having an internal defect and an element having no internal defect and the maximum phase angle when a high-level signal is applied.

FIG. 9 is a flowchart illustrating a pass / fail determination method based on a plurality of inspections.

[Explanation of symbols]

 Reference Signs List 1 network analyzer 2 ceramic oscillator (piezoceramic element) 4 RF power amplifier 5 attenuator

Claims (5)

[Claims] A step of applying a high-frequency measurement signal having a level higher than a rated level to the piezoelectric ceramic element; and a step of measuring at least one of a phase characteristic and an impedance characteristic of the piezoelectric ceramic element accompanying the application of the high-frequency measurement signal. A method for comparing at least one of the measured phase characteristic and impedance characteristic with a reference characteristic; and detecting the presence or absence of an internal defect of the piezoelectric ceramic element from the comparison result. 2. The piezoelectric device according to claim 1, wherein the high-frequency measurement signal is a signal near a maximum level at which the piezoelectric characteristics of the piezoelectric ceramic element return to almost the same piezoelectric characteristics as before the application after the application. Inspection method for ceramic elements. 3. The measured phase characteristic is a maximum phase angle θma.
3. The method for testing a piezoelectric ceramic element according to claim 1, wherein x is x. 4. The method according to claim 1, wherein the measured impedance characteristic is a difference Za-Zr between the anti-resonance impedance value Za and the resonance impedance value Zr. 5. The method according to claim 1, wherein at least one of a phase characteristic and an impedance characteristic of said piezoelectric ceramic element is measured at a plurality of elapsed times after application of a high-frequency measurement signal having a level higher than a rated level. The method for inspecting a piezoelectric ceramic element according to any one of the above.