Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 396

10.1.2 Electrocaloric Materials

The rst report of a material exhibiting the electrocaloric effect was in 1930, when

Kobeko and Kurchatov [ 6 ] measured the electrocaloric effect in Rochell salt.
However, they did not report any values from the measurements. Later, in 1963,
Wiseman and Kuebler [ 7 ] repeated the experiment and measured an adiabatic
temperature change in a range of few millikelvins. During the 1960s and 1970s,
bulk materials exhibiting the electrocaloric effect were intensively studied, but then
the research was gradually abandoned since the values of the electrocaloric effect
did not exceed 1 K [ 4 ]. An important milestone came in 2006 when Mischenko
et al. [ 8 ] reported the so-called giant electrocaloric effect in PbZr 0.95 Ti 0.05 O 3 (PZT)
ceramic thin lms. Using indirect measurements, they demonstrated that the

material undergoes an adiabatic temperature change of 12 K for an electric eld

change of 48 MVm − 1 . After 2006 many researchers reported the discovery of new
materials, with the electrocaloric effect, including several ceramics and some

polymers [ 4 , 6 , 9 ].
In this section, the two largest groups, i.e. ceramic and polymer electrocaloric
materials, are further discussed. The general characteristics are outlined and for
each group a set of materials together with their electrocaloric properties are pre-
sented. The main parameter that characterizes an electrocaloric material is the
adiabatic temperature change. In most cases, this is obtained by indirect or direct
measurements. In the case of an indirect method, the polarization curves for dif-
ferent electric

elds at different temperatures are measured. The adiabatic temper-
ature change and isothermal entropy change of the material are then calculated
using Eqs. ( 10.7 ) and ( 10.9 ). Since indirect measurements do not always provide
accurate results [ 2 , 5 , 10 ], direct methods, such as differential scanning calorimetry
[ 11 ] and high-resolution calorimetry [ 12 ], should be used to determine the elect-
rocaloric effect. For a detailed description of measuring the electrocaloric effect, the
reader is referred to Correia et al. [ 13 ].
Besides the adiabatic temperature change, the isothermal entropy change gives
important information about the properties of an electrocaloric material. Further-
more, the maximum speci c cooling capacity, which is explained in detail in

Chap. 4 , can be used to characterize the cooling performance of the electrocaloric
material. It can be calculated as:

ð 2 T R þ D T ad Þ D s ist

q Rmax ¼ ð 10 : 10 Þ

2

where T R denotes the lowest temperature of the electrocaloric material in the cycle. In
cases where the Δ T ad at a speci c temperature was reported, T R can be expressed as:

T R ¼ T ref D T ad ð 10 : 11 Þ

where T ref is the temperature at which the electrocaloric effect was obtained.