FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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deep penetration in same focussed fashion.
Most ccmmercially available piezoelectric discs are sintered. These
sintered materials have grain sizes between 2 and 10 /um with a few percent
porosity and occasionally have large flaws. In order to achieve high frequencies
in the range 30-100 MHz, these elements have to be lapped down to as thin as 20
jm without any structural danage, however, sintered piezoelectric ceramics, when
lapped below 100 /un thickness, receive structural damage, eg. grain pull-out and
have insufficient strength. Therefore, piezoelectric ceramics used for high
frequency transducers should have at least 7-12 grain layers devoid of porosity.
Large flaws also contribute to the electrical failure of piezoelectric ceramics
on polarisation.
It is veil known that grain size and bulk density markedly influence the
electrical properties of sintered piezoelectric ceramics. By choosing another
fabrication method, i.e., hot pressing, one can control the grain growth and
densification process. This method can yield 100% dense ceramics with
controlled grain sizes suitable for the extreme thinning required. The ccmpressional
wave sound velocity in the direction of polarisation for the hot pressed
material is higher than their sintered counterparts. This gives an extra layer
of thickness when compared with the sintered material for the same thickness
resonant frequency. The piezoelectric properties of hot pressed elements are
also superior to sintered elements.
Another important factor governing these ceramics is polarisation. One has
to optimise the poling condition for each composition when such ceramics are to
be lapped down to «OO/fm after polarisation. If one exceeds these cptimim
poling conditions (Ref.l) (e.g. temperature, poling field or poling time),
micrccracks will develop causing element failure when excited by the voltages
necessary to generate sufficiently powerful bursts of high frequency ultrasound.
Whereas the switching of normal dipoles (180 dipoles) does not involve
any structural changes, the alignment of "mechanical dipoles" (other than 180°)
during polarisation requires such changes. Since sound waves are associated with
interatomic forces, one can detect these mechanical dipoles by measuring sound
velocities in the material. Such measurements facilitate an understanding of
dipole behaviour during polarisation and depolarisation. It is the mechanical
dipole alignment associated with the thickness resonant frequency of the
piezoelectric ceramic which causes the change in sound velocity. Therefore, the
thickness resonant frequency will depend upon the number of mechanical dipoles
aligned during polarisation.
II SAMPLE PREPARATION
PZT 4, PZT 5 and PZT 7 calcined powders and SPN were pressed at 20 MPa
(3000 psi) into slugs ^15 nm long, isopressed at 340 MPa (50,000 psi) and fired
in a closed alunina crucible at 1270 -1350 c in a controlled atmosphere for
an hour. Heating and cooling rates of 450 c/h were maintained.
Samples of the same powders were pre-pressed to obtain slugs and these were
placed in a molybdenum mold and packed with zirconia/alunina (200 mesh) powder.
This composite was pressed in a vacuum induction furnace for ~10-20 minutes at
900-1200 C at pressures between 3 and 30 MPa (500-5000 psi) employing the same