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Abstract

The discovery of the “giant” electrocaloric (EC) effect (Mischenko et al., Science, 2006)
triggered the interest of the scientific community due to its potential use in refrigeration
technology. The magnitude of the EC effect, that is, the EC temperature change (𝜟TEC),
depends on the chemical composition, applied electric field and temperature. The
(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-100xPT) solid solution has been studied for its
excellent dielectric and piezoelectric properties, and it has been shown that PMN and
PMN-rich solid solutions with dielectric permittivity maxima close to room temperature
exhibit TEC that are among the largest reported for relaxor ferroelectric ceramics. The
latter are in the range of a few K for electric fields close to the dielectric breakdown strength
at about 100 kV/cm.
In the first part of the thesis we investigate the relation between the temperature of
the maximum 𝜟TEC (TEC max) and the temperature of the permittivity peak (Tm) as a
function of the electric field (E) in PMN, PMN-5PT and PMN-10PT bulk ceramics,
prepared from mechanochemically synthesized powders. Both maxima, 𝜟TEC and TEC max,
increase with increasing PT content at a given field. By applying the DC bias above the
threshold field the ferroelectric state is induced, and consequently the Tm is strongly upshifted
with increasing electric field. Since the TEC max is always lower than Tm and the
temperature gap between both maxima progressively increases with increasing DC bias,
the Tm cannot be used as the estimation of the TEC max. In contrast, Tm can only roughly
mark the temperature of the upper boundary of the temperature-electric-field window
where the EC responsivity (𝜟TEC/𝜟E) is the highest.
In multilayer (ML) elements the thickness of individual layers is in the range of a few
tens of µm, i.e., much lower than the thickness of bulk ceramic samples. The PMN and
PMN-10PT ML elements were fabricated by tape casting. The phase composition,
microstructure, low- and high-field dielectric properties of ML elements are comparable to
those of bulk. Moreover, the EC responsivity of the PMN and PMN-10PT ML elements is
comparable to their bulk counterparts; however, here significantly lower voltages are
needed. The maximum 𝜟TEC of 1.99 K and 2.72 K at 120 kV/cm at 62 °C and 99 °C were
obtained for PMN and PMN-10PT, respectively, by both direct and indirect EC
measurements. Nevertheless, at room temperature the PMN and PMN-10PT ML elements
exhibit almost the same 𝜟TEC as a consequence of the lower TEC max of the former. Both
PMN and PMN-10PT ML elements exhibit an almost fatigue-less EC behaviour after 106
electric field cycles at ~100 kV/cm, indicating that they are a viable choice for EC cooling
applications.