Sec. 1-2] 



MECHANICAL IX PUT TRANSDUCERS 



si 



produced by subjecting the transducer to pressure from all sides 

 (hydrostatic pressure) causes an output voltage. 



The voltage-output versus strain-input characteristic is not linear, 

 except over a small operating range. A typical characteristic for 

 a cylindrical plate loaded axially 1 is shown in Fig. (1-2)63. The 

 linearity may be improved by prestressing of the transducer and 

 operating in the central part of the characteristic. 



sensitivity. The piezoelectric coefficient d, or charge sensitivity, 

 shown in Eqs. (1) and (2). varies between 1.4 and 6.3 X 10 -10 

 coulomb/newton. The charge sensitivity diminishes as the size of 

 the element decreases. 2 Since the 

 dielectric constant is very high, 

 about 1,200 to 1,700 at room 

 temperature, the coefficient g, or 

 voltage sensitivity, shown in Eq. 

 (3), is relatively small. Values 

 reported range from 1.2 x 10~ 2 to 

 6 x 10- 2 (volt/m)/(newton/m 2 .) 

 The piezoelectric constants can 

 vary considerably with the temper- 

 ature, the thermal history ("tem- 

 perature hysteresis"), and the 

 composition. In particular, addition ofleadtitanate(PbTi0 3 ) and cal- 

 cium titanate (CaTi0 3 ) changes the piezoelectric constants and their 

 variation with temperature, the dielectric constant and its variation 

 with temperature, and the mechanical properties, i.e., Young's 

 modulus and the resonance frequency of the piezoelectric elements. 

 The piezoelectric activity of barium titanate ceases at a temperature 

 above about 12o°C (curie point). Crystallographic transitions and 

 variations of the piezoelectric behavior also occur at about -10°C 

 and — 70°C. The variation occurring at — 10°C is less pronounced, 

 depending upon the composition of the ceramic body; with certain 

 compositions it can be made to disappear completely. 



The voltage sensitivity of barium titanate is less than that of 

 rochelle salt and ammonium dihydrogen phosphate, but its tempera- 

 ture range is wider, its mechanical strength higher, and its insulation 

 resistance is higher ( 10 13 ohm-cm). Compared with quartz, it has the 

 advantage of lower cost. It can be made in various forms and shapes 



1 From E. H. Mullins, Proc. Symposium on Barium Titanate Accelerometers, 

 Natl. Bur. Standards Rept. 2654, August, 1953. 



2 T. A. Perls, Proc. Symposium on Barium Titanate Transducers, Nail. Bur. 

 Standards Rept. 2654, p. 51, 1953. 



5 10 15 



x 10 3 lb/in 2 

 (Area -0.2 in 2 ) 



Fig. (1-2)63. Transfer characteristic of 

 bariumtitanate transducer. 



