Accelerated and Decelerated Polarization Reversal in Thin Vinylidene Fluoride/Trifluoroethylene Copolymer Films

Ferroelectric switching characteristics during continuous polarization reversal were investigated in thin vinylidene fluoride/trifluoroethylene copolymer films by using a train of bipolar on-off electric field pulses. It was found that polarization reversal is accelerated as the pulse width Delta t is decreased whereas it is decelerated as Delta t is increased. For a given Delta t, the switching time tau(s) depends on the pulse height E in accordance with an exponential law tau(s infinity)e(Ea/E) in that E(a) increases from 0.5 GV/m to 1.2 GV/m as Delta t is increased from 10 mu s to 1000 s whereas tau(s infinity) remains constant at 8 ns. These results are interpreted in terms of the dependence of tau(s) on previous poling history that produces diverse polarization states containing various defects and disorder that affect the nucleation probability and growth velocity in the following switching process. Particular acceleration at short Delta t was attributed to unswitched polarization that aids in the generation of critical nuclei of reversed domains. Marked deceleration at long Delta t was attributed to structural stabilization observed as slow dielectric relaxation during the off period. The effects of ferroelectric heating and space charge redistribution are discussed in relation to acceleration and deceleration, respectively.