476 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



be generated in a disk with the stress appHed uniformly. With this 

 factor the open circuit voltage per unit of force — which determines the 

 effective gus piezoelectric constant of the ceramic — agrees well with that 

 obtained by other methods of measurement. For the most desirable 

 ceramic obtained for the 1325°C l)ai:ing temperature the value of ^33 

 equals 



2 



^33 = 3.25 X 10"' ---^ r in c.g.s. units = 0.98 X 10"' 



stat-coulomb 



(1) 

 meters . , ' ., 



— ni m.k.s. units 



JN ewton 



The dielectric constant £ of this material is about 1500 so that the d33 

 piezoelectric constant is 



d33 = 9J^ = 390 X 10-« «^^^-^»^l»"^bs ^ ^3Q ^ ^^-u coulombs ^^^ 

 47r dyne Newton 



Since for some applications in this paper, high voltage gradients of 

 opposite sign to the poling voltage are applied to the ceramic, it is a 

 matter of importance to find out whether the ceramic will become de- 

 poled by the action of this voltage. To test out this feature the circuit 

 of Fig. 3 is equipped with a high voltage generator, which is applied to 

 the ceramic through 10-megohm resistors and the high voltage is kept 

 out of the measuring circuit by 4-microfarad condensers. The procedure 

 was to apply a negative voltage for 3 minutes, then to recalibrate the 

 voltage due to impact. This was repeated with a higher voltage each 

 time until the range was covered. 



The curves of Fig. 2 show that there is an optimum baking tempera- 

 ture for a large coercive field. Above this temperature larger sized crys- 

 tals grow in the ceramic and the coercive field decreases markedly. It 

 is thought that the smaller crystal size corresponds to a more strained 

 condition in the individual crystallites and it requires a higher field to 

 overcome the mechanical bias and change the direction of the ferroelec- 

 tric axis. A similar condition has been found by x-ray techniques for 

 single crystals where it has been found impossible to make a single 

 domain out of a multidomain crystal by the application of a field, if 

 the crystal is too highly strained. 



The effects of additives are also very marked on the properties of the 

 polarized ceramics. It has previously been reported' that the addition 

 of 4 per cent of lead titanate to the commercial barium titanate increases 

 the coercive field. This is confirmed by the curves of Fig. 4 which show 



