Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 394

with respect to the temperature should be large. Furthermore, the electrocaloric
material should be subjected to a large change in the electric eld.

Ferroelectrics are a group of dielectric materials that possess a large rate of
change of polarization with respect to the temperature. One of their characteristics is
that they possess a spontaneous polarization (polarization at zero electric

eld)
within a certain temperature interval. In this interval, the dielectric material is said
to be in a ferroelectric phase. However, there exists a critical temperature, referred
to as the Curie temperature, above which the spontaneous polarization vanishes and
the material is said to be in a paraelectric phase. The transformation of a dielectric
material from the ferroelectric phase to the paraelectric phase is called the phase
transition. With regard to this phase transition, ferroelectrics can be divided into

rst-
order phase transition is a sudden drop in the polarization around the Curie tem-
perature (Fig. 10.2 a). On the other hand, ferroelectric materials with a second-order
phase transition undergo a continuous change of polarization from the ferroelectric
to the paraelectric phase (Fig. 10.2 b). In both cases, a relatively large change in the
polarization occurs in the vicinity of the Curie temperature. Therefore, in this
temperature range, a relatively large electrocaloric effect is expected.
An interesting group of the dielectric materials are the so-called relaxor ferro-
electrics, which can be either polymers or ceramics. They usually possess no
spontaneous polarization; however,

rst-order and second-order phase transition materials. The characteristic of a

they undergo a phase transition under the

in eld-induced phase transition) [ 3 ]. As in the case of

common ferroelectric materials, relaxor ferroelectric materials can either possess a

fl uence of an electric eld (

rst- or second-order phase transition. A special characteristic of relaxor ferro-
electrics is that the maximum value of the electrocaloric effect is usually observed
over a broad temperature interval (up to a few tens of degrees kelvin). On the other
hand, the maximum electrocaloric effect of common ferroelectrics is observed over
a much narrower temperature interval, usually in the range of a few degrees kelvin.
Detailed information about the electrocaloric effect in relaxor ferroelectrics can be
found in Pirc et al. [ 3 ].
Besides the magnitude of the polarization change as a function of temperature,
the electrocaloric effect is de

ned by the magnitude of the electric eld change. In

general, a larger electric eld change induces a larger electrocaloric effect, though

this is only true until the saturation point is reached. At the saturation point, the
dipoles are perfectly aligned along the eld, the

dipolar entropy of the dielectric material remains the same [ 4 ]. However, in most
cases, the dielectric material breaks down before the electric

eld and by further increasing the

eld is increased up to

the saturation point [ 4 ].
The equations for the adiabatic temperature change (Eq. 10.7 ) and the isothermal
entropy change (Eq. 10.9 ) for an electrocaloric material give a good insight into the
main factors that in

uence the magnitude of the electrocaloric effect. However, it is
important to stress that these expressions do not always correctly predict the values
of the electrocaloric effect [ 2 , 5 ]. The reason for this is in the hysteresis behaviour
as the ferroelectric materials have two equilibrium states for every

fl

eld applied