FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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HOT PRESSED PIEZOELECTRIC CERAMIC ELEMENTS FOR ULTRASONIC
TRANSDUCERS
U.V. Patdt, J. van den Ande.1 and P.S. Nicholson
HamiCtcr., Ontanic, Canada
ABSTRACT
Piezoelectric ceramics (PZT4, PZT5, PZT7 and SPN) were investigated for
development of high frequency (30-100 MHz) normal and focussed transducers. Hot
pressed piezoelectric materials with a controlled grain size of ~2jum and
approaching theoretical density can be lapped down to 30 jum thickness without
structural damage whereas conventional sintered piezoelectric material with grain
sizes ranging frcm 2-15jum and at least 3% porosity can only be lapped down to
100>um. Hot pressed piezoelectric ceramics when compared with conventional
piezoelectric ceramics, show higher values of coupling coefficient (10-20%),
elastic compliance ^12% and moderate increases in mechanical quality factor
(Q ) and dielectric constant (€ ). The ccmpressional sound velocity measured
along the poling direction is very sensitive to the switching of dipoles other
than 180 ones. The velocity increases as the polarization increases and this
constitutes a useful method for quality control of the piezoelectric ceramic as
it gives the correct thickness resonant frequency. By accurately measuring the
change by a comparison method, the dipole behaviour can be better understood and
the degree of polarization and depolarization can be established. Temperature
dependent polarization and depolarization is also discussed in the light of
dipole switching.
Hot pressed SPN and PZT5 are sufficiently transparent to be considered for
use in opto-ultrasonic applications such as in medicine.
Because of the high value of the thickness-frequency constant and low O ,
SPN seems a better choice as a high frequency transducer material. The signal
spectra of a high frequency transducer constructed frcm these materials is
presented.
I INTRODUCTION
High frequency transducers in the range of 30 to 100 MHz are of great
importance for detection of minute flaws (~-'5/*m) in high performance ceramics.
The detection of these flaws require powerful high-frequency transducers which
are now being developed. As the frequency increases, the absorption in the
medium being investigated also increases. Therefore, to compensate this loss,
the transducer piezoelectric element should have a high transmitting coefficient
d,. and receiving constant g„. Also, the sound beam should be capable of