FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The alignment of the mechanical dipoles involves structural changes and
this causes internal stress; this will change the sound velocity in the polarization
direction. The velocity change is directly proportional to the extent of
mechanical dipole alignment during polarization. For hot pressed, PZT 4 the
change was ~9%, for PZT 5 «'10%, for PZT 7 "7% and for SPN ~8%. Their sintered
counterparts showed a few percent less sound velocity. The initial temperaturedependent
depolarization occurs via the mechanical dipoles as stresses are
relieved. For the PZT compositions and the SPN, the mechanical dipoles alone
switch back up to 250°C and 325°C respectively. As temperatures are increased
beyond 250 C for PZT canpositions and 325°C for SPN, the normal
dipoles also switch back. The time and frequency spectra of hot pressed elements
and transducers constructed fran them are shown in Figures 8 and 9.
SPN, with a high sound velocity, low dielectric constant and low O is
potentially useful for the construction of high frequency transducers. Also the
acoustic impedance for SPN (31.3 x 10 _ g/an sec) is low compared with that of
the PZT canpositions ( ~39 x 10 g/an sec). This property facilitates
matching with transducer backing materials.
Hot pressed PZT 5 and SPN can be sufficiently transparent for consideration
in opto-ultrasonic applications.
V CONCIDSIONS
As a result of the reported investigation, the following conclusions can be
made;
(1) hot pressed piezoelectric ceramics ( /v 100% dense) can be used to construct
high frequency transducers in the range 50-100 MHz.
(2) the piezoelectric properties of the hot pressed materials are considerably
better than their sintered counterparts.
(3) the high values of sound velocity along the polarisation direction (i.e.,
the high frequency constant) for hot pressed material allows an extra layer
of grain thickness for the same frequency as compared to similar sintered
materials.
(4) the grain size of the final ceramic can be controlled by hot pressing
techniques.
(5) the ccmpressional wave velocity measurements in the polarisation direction
facilitate an understanding of the behaviour of dipole switching during
polarisation and depolarisation.
REFERENCES
(1) R.W. Rice and P.C. Pohanka, J. An. Ceram. Soc., 56 (8) 420-23 (1973).
ACKNOWLEDGEMENT
SPN calcined powder was supplied by BM Hi Tech (Collingwood, Ontario).