Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-06-25
2001-05-08
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C356S032000
Reexamination Certificate
active
06227052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for testing the domain switching induced in ferroelectric ceramics upon light irradiation, with the aid of an acoustic emission technique. Since photoinduced domain switching causes a change in the remanent polarization and strain of ferroelectric ceramics, the test thereof is very important in controlling optical properties such as photovoltaic currents and photostrains.
2. Description of the Prior Art
Being defined as the phenomenon of transient elastic-wave generation due to a rapid release of strain energy caused by a certain event, such as a structural alteration, in a solid material, acoustic emission (hereinafter referred to as “AE”) is useful to non-destructively analyze the dynamic action generated in the material, in real time. AE is known to be used for monitoring the domain switching generated in ferroelectric materials under the influence of external electric fields, as reported by W. Pan and H. Cao (Ferroelectrics, 129, 119 (1992)) and Y. Saito and S. Hori (Jpn. J. Appl. Phys., 33, 5555 (1994)).
Generally, photoinduced domain switching phenomena are observed by generating and switching 180° domains in ferroelectric single crystals, such as SbSI, BaTiO
3
and PbTiO
3
, with the aid of an optical microscope. They are caused by the screening effect of photoinduced electrons on spontaneous polarization, as analyzed in reports (V. M. Fridikin, A. A. Grekov, N. A. Kosonogov and T. R. Volk, Ferroelectrics, 4, 169 (1972); A. Semenchev, V. Gavrilyatchenko and E. Fesenko, Ferroelectrcs, 157, 135 (1994)).
In ferroelectric polycrystal ceramics, on the other hand, there exist grain boundaries in which photoinduced non-equilibrial electrons can be entrapped, forming a space charge field over each grain, as reported by Land et al. (C. E. Land and P. S. Peercy, Ferroelectrics, 22, 677 (1978)). Expectedly, the space charge field is sufficient to cause the domain switching. However, since ferroelectric ceramics are composed of grains as small as several microns and the photoinduced domain switching is generated within a very shallow depth limited to the light-absorbed range, the domain switching is virtually difficult to observe with the aid of microscopes or by use of X-ray diffraction. In fact, research on the domain switching during the application of light is scarcely conducted. Related to the present invention is U.S. Pat. No. 4,344,326 with a disclosure of non-destructive testing of ferroelectric capacitors, but it is not concerned with domain switching.
SUMMARY OF THE INVENTION
With the aim of solving the problems encountered in prior arts and making it easy to observe the domain switching generated when irradiating ferroelectric ceramics, the present invention is directed to in-situ testing of the domain switching by use of an AE technique. In the present invention, the AE event counts generated when light is illuminated on a tetragonal structure of (Pb
1−z
La
x
) TiO
3
ferroelectric ceramic, are measured to test the activity of photoinduced domain switching against various experimental parameters. At the same time, the change in photovoltaic current of the material is monitored to confirm the reliability of the AE signals detected. Further, a tetragonal structure is found to retain two domains: 90° domain and 180° domain. On the basis of this finding, what the domain switching is, which is caused upon the application of light, can be determined through AE energy distributions using an AE technique.
Therefore, it is an object of the present invention to provide a method for testing the photoinduced domain switching of ferroelectric ceramics, which is simple and convenient.
In accordance with the present intention, the above object could be accomplished by a provision of a method for testing the photoinduced domain switching of ferroelectric ceramics by use of acoustic emission, comprising the steps of: focusing on a ferroelectric ceramic sample of a tetragonal structure a light beam which has an energy greater than or as great as a band gap, so as to induce electrons into conduction bands; detecting acoustic emission signals from the sample; filtering the raw acoustic emission signals; amplifying the filtered acoustic emission signals; and analyzing the amplified acoustic emission signals while measuring photovoltaic currents.
REFERENCES:
patent: 4344326 (1982-08-01), Kahn
patent: 4562736 (1986-01-01), Iwasaki et al.
patent: 5164669 (1992-11-01), Namkung et al.
patent: 359224660A (1984-12-01), None
patent: 358221162A (1984-12-01), None
Choi Dong Gu
Choi Si Kyung
Kim Sung Ryul
Korea Advanced Institute of Science and Technology
Miller Rose M.
Williams Hezron
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