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continued to some considerable degree at other research installations
under various names. Most of the development work on lead metaniobate
has been done at General Electric. Both of these compositions show
much higher Curie points, 3800 and 5750C respectively, permitting
operation without much deterioration close to these temperature points.
Both have lower dielectric constants, 600 to 1000 for PZT, depending on
composition, and approximately 280 for the lead metaniobate. Both are
known to show rather high electrical conductivity in the upper tempera-
ture range. To date this conductivity at the higher temperature has been
somewhat of a handicap. It tends to limit the degree of poling or
activating by limiting the potential that may be applied at elevated
temperature, where the poling is usually done. This conductivity is
also detrimental to most of the higher temperature applications.
PZT when working into a high impedance, shows an efficiency of about
22 times that of barium titanate as either a voltage or energy generator
when stressed to 200 to 30,000 psi. Lead metaniobate is about equal in
efficiency to PZT as a voltage generator, under these conditions and in
this stress range, but drops to about } the efficiency of PZT as an
energy generator. (See attached table.)
Some other compositions have come forth to answer the current demands,
but they are at this time believed to be secondary to the two just
described,
When comparing efficiencies of these compositions, conditions were
specified because it has been shown that PZT when highly stressed
quasi-statically, while working into a large capacity, (500 x the element
cap.) will effectively produce about 5 times as much charge and resultant
voltage as the d3 measured at 200 psi (with a shunt cap. of from 50 to
150 times the elient cap.) indicates it should. If this charge and
resultant voltage are measured on release of stress they are somewhat
lower, about 3 times what they should be. This phenomenon has not
been fully explained, but apparently it is not due to depoling since the
original d value has been checked after a number of such stressing
operations 3 More work must be done to establish just which physical
constants are changed sufficiently by this relation between mechanical
and electrical stress to bring about this abnormal charge.
During Sandia's period of interest, close observations have been made to
check physical constants demonstrated by the 4Wo lead titanate barium
titanate specimens received from the suppliers. This composition has
been more or less standard for Sandiats applications. The dielectric
constant has increased from about 900 to 1600 or 1700. Electromechanical
coupling has varied somewhat; for the most part, some slight improvement
has been observed. The piezoelectric coefficient (d3 is our prime
interest) has also shown considerable increase over this period. Con-
sequently voltage and energy for a given stress has increased somewhat.
This is, in general, quite pleasing but it adds to the difficulties for
our suppliers to keep their products within specifications covering these
elements. To date, our specifications, wherever possible, have been
relaxed to ease this situation for the suppliers.
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