The emission of bursts of ultrasound accompanying polarisation reversal has been observed in a number of ferroelectric materials, including lead germanate, Rochelle salt, gadolinium molybdate and terbium molybdate. This acoustic emission (AE) occurs predominantly near the regions of saturation polarisation at the extremities of the hysteresis loop, and has been investigated as a function of sample size and temperature and of the magnitude and frequency of the electric field applied to cause the switching. These measurements indicate that the AE is closely connected with the process of polarisation reversal and therefore with the motion of domain walls within the crystal. In all the materials studied, a threshold electric field has been found below which no AE is produced, even though the hysteresis loop is still saturated and switching still occurs. This threshold field increases steeply as the Curie temperature of the material is approached, a finding which is attributed to a higher voltage being required to induce the AE generating process as the domain walls become more mobile. Measurements of the crystal polarisation reveal a one-to-one correspondence between this acoustic noise and the electrical Barkhausen pulses also associated with polarisation reversal in ferroelectries, which indicates that domain nucleation and collapse may be the source of AE. Simultaneous monitoring of the AE and visual observation of the domains in gadolinium molybdate and lead germanate confirm that these particular processes are indeed responsible for the AE produced by ferroelectric crystals.
|Date of Award||1982|