Charge storage in polyvinyledene fluoride electret

It is well established that pyroelectricity in polyvinylidene fluoride (PVF
2
) is significiantly enhanced on the application of a suitably high electric field at an elevated temperature for an extended period of time (i.e. poling). This induced effect, which may be quite stable at ordinary temperatures, will be superimposed on any inherent pyroelectricity of the materia11 A pyroelectric coefficient, as high as 4 × 10
⁻⁵
Cm
⁻²
K
⁻¹
may be obtained with prestretched and poled PVF
2
. The possible origins of pyroelectricity in PVF
2
are
¹
: (i) the temperature dependence of spontaneous polarization, (ii) a coupling of non-uniform temperature dependence of permittivity with the trapped charges and (iii) a combination of (i) and (ii). The time dependent charge transport properties of insulating polymers may be studied by the transient charging and discharging currents at different temperatures and fields. On the application of a high dc step field, the charging (polarizing)current decay as a function of t
⁻ⁿ
where t is time and n is a number, until a quasi-steady state conduction current is reached. If the sample is then short circuited, then the discharge (depolarizing) current tends to be a mirror image of the charging current for low temperatures and fields. .For high temperatures and fields this mirror image nature of the charging and discharging currents is no longer maintained. The charge transport and the polarising mechanisms in the low temperature range, including the glass transition temperature are of considerable interest in semi-crystalline polymers. The present work reports results of a study of slow pyroelectric responses during the depolarization (short circuited condition) of a previously charged PVF
2
electret in the temperature range of −60°c to +20°c.