FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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the density by »w3 to 7%.
These increases are due to the increases in density, the lack of texturing
and the uniform grain size of the hot pressed materials. Such materials are
clearly advantageous for use in high frequency transducers. The higher sound
wave velocities allow extra thickness for the same frequency, e.g. for 100 MHz
longitudinal sound wave velocity, the required thickness for hot pressed PZT 7 is
•* 25 /«m whereas for the sintered material it is 23 yum. The hot pressed
piezoelectric ceramics drew less current during poling than the sintered ones
(Figure 1). For all materials the anount of current drawn increases with the
applied voltage. Current fluctuation or considerable increases in
voltage during poling, indicate the development of microcracks in the ceramic.
This causes a decrease in the coupling coefficient (9) as indicated in Figure 2
beyond 3,600 V/mm. If either time, temperature or field are increased, the
specimens fail. The dipole alignment on application of an electric field, leads
to the development of internal stresses which eventually leads to microcracking.
In the absence of microcracking, the dipole alignment increases k ; with microcracking,
k drops. p
In assembling high frequency transducers, either indium bonding or a
conducting epoxy is used to bond the piezoelectric discs to the backing (damper)
medium. For both cases, a temperature of 80-150°C is required to construct the
transducer. Therefore, the elements should be able to withstand these temperatures
without losing their piezoelectric properties. Figure 3 illustrates the effects
of overheating PZT 5 discs. The curves show the value of k as a function
of increasing temperature up to 175 C on the same test specBnen. It is clear
that k is reduced by «'Si when the piezoelectric element is heated to ~200 o c
No sucn property change was observed when specimens were heated to 140°C;
k peaks at «/HO C and this can be attributed to the transition frcm one
ferroelectric phase to another.
To construct a high frequency transducer it is necessary to know the correct
thickness resonant frequency of the element and this is difficult to measure on
thin discs. By measuring the sound velocity in the polarization direction,
a high degree of accuracy can be realized. Fran such data, the thickness
resonant frequency; (f = c/2t, c = velocity, f = frequency and t = thickness)
can be obtained and dipole behaviour during poling, elucidated.
During poling the 180 -demain reversal is complete and other discrete
angle dipoles are incompletely switched (the percentage of such switching depends
upon the phases involved and the impurities added to the ceramic). Stress free
piezoelectric ceramics would be those whose domains switch only by 180°, i.e.,
experience no structural changes. Therefore, the sound velocity measured along
the polarization direction for such a material, should be unchanged when compared
with the virgin ceramic. This was true for the initial polarization of PZT 4,
PZT 5 and PZT 7 in a low field, i.e., where only the 180 dipoles are involved
(Figures 4,5,6 and 7). The k values obtained frcm alignment of these normal
dipoles for PZT 4 (rhcmbohedral/tetragonal phase), sintered and hot pressed are
~23% and 29%, respectively; for PZT 5 (rhanbohedral), sintered and hot
pressed are 27% and 38%; for PZT 7 (tetragonal phase), sintered and hot
pressed are 18% and 23%, and for SPN, sintered and hot pressed are '"19% and
«/26%, respectively.