Analysis of Ferroelectric Switching Process in VDF/TrFE Copolymers
- PDF / 1,090,751 Bytes
- 5 Pages / 417.6 x 639 pts Page_size
- 78 Downloads / 162 Views
89 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
EXPERIMENTAL SWITCHING CURVE
The samples used in this investigation were a VDF/TrFE copolymer with composition 65/35 mol% supplied by Daikin Kogyo Co. Ltd., Japan. Film samples were obtained by melt-extrusion and annealed at 147°C for one hour. Scanning electron microscope images showed that such samples consisted of regularly stacked large lamellae 60nm in thickness and 600 x 600nm 2 in area. Gold was evaporated on both surfaces to serve for electrode. The time domain measurements of polarization reversal consist of an application of a stepwise electric field in the direction opposite to the existing remanent polarization and a detection of charge response as a function of time. Details of the experimental setup have been described previously 5). As a result, we obtain a switching curve which is approximately expressed by (1) Psw = AP[1 - exp{-(t/'r)n}] Here AP is the amount of reversed polarization equaling double the remanent polarization 2 Pr and being independent of applied field strength E. The time constant r, called a switching time, rapidly decreases with increasing E obeying an exponential law. Exponent n is related to the shape of the switching curve. Its value is normally greater than unity. In practice, the observed switching curve D(t) contains considerable contributions from linear and nonlinear dielectric responses and is written as follows D(t) = Ps, +--1IE + e 3E3 +±...
+ (E
Here, e, is the linear permittivity and e, with n > 2 is the n-th order nonlinear permittivity. It should be noted here that the even order permittivity change sign in accord with polarization reversal. We have thus incorporated their contributions assuming that they are proportional to Psw - Pr. In order to extract Psw from the observed switching curve, we removed contributions from dielectric responses inrf4.2 a manner as de-
2
2 +E4E
.. .)
(PSW P-- 1
(2)
2
10.1 64
" s'
2
0 10
6 4 2
10.3
scribed inref. 4.
10-3
3 456
2
10-2
34568
101
t[s] Figure 1 shows a double logarithmic plot of Ps, vs time t obtained for a uniaxially oriented and annealed 65/35 Fig. 1 Two step character of Switching Curve for VDF(65) / mol% copolymer at 50MV/m. It is found that Psw consists of two components, an initial gradual increase in proportion to t0 5 and a latter rapid increase according to eq. (1) with exponent n = 6. Assuming the nucleation-growth mechanism, we can naturally attribute the latter to the growth process. However, such a large exponent is beyond the prediction of conventional theory. Its maximum value is 4 for the case of three dimensional growth of homogeneous nucleation. We have recently suggested that such a large exponent can be explained by introducing an incubation time3 ). In the following, we try to reproduce the observed Ps, by means of computer simulation.
90
COMPUTER SIMULATION
Computer simulation was made with respect to an array of N cells and N x N cells describing I-D and 2-D growth processes, respectivery. Each cell c
Data Loading...
