Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Piezoelectric crystal testing
Reexamination Certificate
2000-12-22
2002-11-12
Oda, Christine K. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Piezoelectric crystal testing
C324S765010
Reexamination Certificate
active
06480010
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of non-destructively inspecting internal defects such as microcracks or other defects which may occur in a piezoelectric ceramic device such as an oscillator, a filter, or other such device, which defects affect the qualities and characteristics of the piezoelectric ceramic device.
2. Description of the Related Art
In a conventional method of non-destructively inspecting an internal defect of a piezoelectric ceramic device, the impedance and/or phase characteristic of a piezoelectric ceramic device is measured, the curve pattern representing the characteristic is compared with a standard curve pattern, and if the curve patterns are different from each other, it is judged that microcracks are present in the piezoelectric ceramic substrate, as described in Japanese Unexamined Patent Application Publication No. 6-3305.
According to such an inspection method, automatic judgement is possible. Thus, advantageously, judgement as to whether lots of piezoelectric ceramic devices are non-defective or defective can be performed quickly and efficiently. Moreover, the inspection accuracy is high, since the inspection is not visually performed.
According to the above-described inspection method, an electrical characteristic is measured at an ordinary temperature and compared with a standard characteristic. However, there are many cases in which a difference between non-defective components and defective components is small or is non-existent at ordinary temperatures. Thus, using the above-described inspection method, it is impossible to detect an internal defect such as microcracks or other defects completely.
In one example, each of a plurality of ceramic oscillators (oscillation frequency: 25 MHz) was incorporated in an oscillation circuit, the oscillator was placed in an atmosphere at a temperature of 200° C. while the oscillator was being oscillated, and then, characteristics of the oscillation voltage were measured.
FIG. 1
shows the measurement results. The characteristics of these oscillators are not different at ordinary temperature.
As seen in
FIG. 1
, the oscillation voltages are slightly decreased with the increase of the temperature. Some of the oscillators (NG) are decreased to about OV, and the oscillation is stopped. For the other oscillators (G), the oscillation is not stopped even at 200° C. or higher.
These oscillators were opened, and the internal devices were observed with a microscope. For the oscillators (NG) that had their oscillation stopped at a low temperature, it was determined that microcracks were formed inside of the devices. Accordingly, it has been discovered that some oscillators that normally oscillate and exhibit normal characteristics at ordinary temperatures have microcracks inside thereof, and such an internal defect can be detected by measuring the characteristic of oscillators while they are heated according to preferred embodiments of the present invention described below.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of inspecting a piezoelectric ceramic device in which an internal defect, which may be undetectable at ordinary temperature, can be accurately and reliably detected non-destructively.
In addition, preferred embodiments of the present invention provide a method of inspecting a piezoelectric ceramic device in which an internal defect can be detected at high speed using a simple instrument.
According to a preferred embodiment of the present invention, a method of inspecting a piezoelectric ceramic device includes the steps of heating and increasing the temperature of a piezoelectric ceramic device to an increased temperature that is in the vicinity of a maximum temperature at which the piezoelectric ceramic device, when the temperature of the device is returned to ordinary temperature, is returned to substantially the same piezoelectric characteristic as that before heating, measuring at least one of the piezoelectric phase characteristic and the impedance characteristic of the piezoelectric ceramic device while the device is heated and the temperature thereof is increased, comparing at least one of the measured piezoelectric phase characteristic and the measured impedance characteristic with a standard characteristic, and detecting the presence or absence of an internal defect of the piezoelectric ceramic device based on results of the step of comparing.
In this preferred embodiment of the present invention, first, the piezoelectric ceramic device is heated so that the temperature thereof is increased.
Next, when at least one of the phase characteristic and the impedance characteristic of the piezoelectric ceramic device in a state of heating and increasing the temperature, a large change is shown with the piezoelectric ceramic device having internal defects in accordance with the temperature increase, such large change is not shown in the normal temperature. Then, one of the measured phase characteristic and the measured impedance characteristic is compared with the standard characteristic. The standard characteristic may be attained from the phase characteristic or the impedance characteristic of the good piezoelectric ceramic device (without internal defects), for example.
As a result of the above-described comparison, if the measured characteristic is different from the standard characteristic so as to exceed a predetermined range of the characteristic, it is judged that the piezoelectric ceramic device has an internal defect.
In addition, according to preferred embodiments of the present invention, not only microcracks but also foreign matters stuck to an electrode can be detected.
It would be preferable to set the increased temperature of heating to the temperature which is in the vicinity of the maximum temperature in which the piezoelectric characteristic of the piezoelectric ceramic device returns to substantially the same as that of before heating when the piezoelectric ceramic device is back to the normal temperature after heating. The internal defects, which cannot be inspected in the normal temperature, can be securely inspected by heating at as high temperature as possible as long as the piezoelectric characteristic can come back. When the piezoelectric ceramic device is heated at higher temperature than the above described temperature, the piezoelectric characteristic of the piezoelectric ceramic device itself is changed non-reversibly, thereby being not preferable.
Preferably, as the phase characteristic to be measured, a maximum phase angle &thgr;
max
is used. At ordinary temperature, the piezoelectric ceramic device presents the phase characteristic shown by solid line P
1
in FIG.
2
. At a higher temperature, the phase is reduced as shown by broken line P
2
. The larger the internal defect of the piezoelectric ceramic device becomes, the more the amount of phase reduction increases. Preferably, an internal defect is judged by utilization of the phase reduction.
FIG. 3
shows the results obtained when devices NG having an internal defect and devices G having no internal defect are heated in the same manner, and the maximum phase angles in the vicinity of the oscillation frequency f
osc
are measured. As seen in
FIG. 3
, the devices NG having an internal defect present a larger reduction in maximum phase angle, as compared with the devices G having no internal defect. Thus, it can be seen that there is a correlation between the internal defect and the maximum phase angle.
For the devices NG having an internal defect, the phases are not more than about 60°. On the other hand, for the devices G having no internal defect, the phases are not less than about 70°. In the case of these devices, it can be securely detected whether an internal defect is present or not by setting the raised temperature for use in judgement of a non-defective or a defective at about 150° or higher, and moreover, setting the maximum phase angle as a threshol
Ikuta Masato
Nishimura Masao
Tabata Toshinari
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Oda Christine K.
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