12 Lecture 1] 
highest for the ferroelectric ceramics, particularly PZT-4, and for quartz. 
However, with low-dielectric-constant materials such as quartz, the shunting 
effect of even short lengths of cable significantly reduces the effective sensi- 
tivity of the unit. Care should be exercised in the use of any simple criterion 
such as k?/tan§; in practical cases the whole receiving and recording system 
must be considered. 
For projectors againthe coupling factor andtan §, particularly at high driving 
fields, are important. In addition, it is desirableto have a low-impedance device 
to avoid high voltages, and this is achieved by using high-dielectric-constant 
materials. With quartz, the impedance of the units is so high that voltage break- 
down across the crystal surfaces limits the power output. The most interesting 
materials for high-power projectors are PZT-4 and the cobalt additive barium 
titanate, NRE-4 [12]. Among the other properties which affect the choice of ma- 
terial are the mechanical strength, the aging of the parameters, the variation of 
parameters with temperature, and the maximum temperature at which the ma- 
terial can be used. 
There is one big difference between the crystal piezoelectric materials and 
the ceramic pseudopiezoelectric materials. Because of the crystalline nature of 
the former, the impurities are small and the values of the parameters are 
reasonably constant and subject to little modification. The ceramics are not pure 
materials, and compositions consisting of mixtures of chemical compounds have 
been developed with particular characteristics. Small percentages of additives 
can have a pronounced effect on some characteristics. For example, Fig. 1.3 
shows the effect of adding small quantities of cobalt to a barium titanate com- 
position on the dielectric loss tangent as a function of exciting field. A corollary 
is, of course, that small changes incompositionand manufacturing technique can 
have large effects on the material parameters. Reproducibility on a production 
scale has presented problems with some compositions. 
Some characteristics of the more important magnetostrictive transducer 
materials are given in Table 1.II. Only the 4% cobalt—nickel alloy [13] has a cou- 
pling coefficient comparable with the coupling factors of the ferroelectric 
ceramics. With the exception of the ferrites, the main advantage of the mag- 
netostriction unit is ruggedness. It is a low-impedance device and there are 
Fig. 1.3. Tan § vs ac field 
for barium-calcium titanate 
with additions of cobalt. 
DIELECTRIC LOSS TANGENT 
{e) 0.5 1.0 15 2.0 2.5 3.0 3.5 4.0 
A.C. FIELD, KV/cm 
