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Abstract

Artificial refrigeration has influenced the growth patterns of settlements and altered demographics
across the world. The technology behind artificial refrigeration is vapour compression,
which has been serving now for two hundred years. The time has come to search
for alternatives, due to the several drawbacks of conventional refrigeration combined with
new requirements.
In this PhD study we made such an attempt by following the idea by Mischenko et al.
in 2007, who showed a great refrigeration potential by using the so-called electrocaloric
(EC) effect, i.e., the temperature change of dielectric materials induced by application of
an electric field. After the initial euphoria in the search for EC materials and ten years
of intense activity in this field, we still have no commercially available devices. The main
reason behind this is the low EC effect, i.e., low temperature change (up to 4.5 K), and high
electric fields needed to stimulate the dielectric refrigeration. We present an EC device
based on multifunctional cantilevers, which were designed in a cascade arrangement and
when an external electric field is applied to them they simultaneously change temperature
and bend. We have numerically modelled the behaviour of such a device and confirmed its
feasibility. Relative to the temperature change of a single cantilever, such an arrangement of
multifunctional elements can in principle increase the temperature change across the device
several fold. On the other hand, limitations popped-up, for example, the importance of
ensuring a good thermal contact between the elements.
Next, we have chosen the material system for our cantilevers, i.e., (1-x)Pb(Mg1=3Nb2=3)
O3-xPbTiO3 (PMN-100xPT) solid solution, which was shown to exhibit a giant EC and
electromechanical (EM) response. Six PMN-100xPT compositions were fabricated and
characterized. Compositions ranged from the archetypal relaxors (low x) to ferroelectric
samples (high x). This was reflected in their electrical, thermal, EM and EC properties.
Based on the measured properties, we selected an optimal composition by designing Figures
Of Merit. The results of the selection indicated that PMN is optimal for integration in the
multifunctional EC device. PMN ceramic was then investigated in terms of its EC response
under the same electric-field cycling conditions as expected in an EC cooling device. With
cycling, Joule heating emerges, which degrades the EC cooling ability of PMN. By carefully
examining the underlying mechanism and different cycling conditions, we propose ways to
avoid the degrading effects of fatigue with the help of the electric-field-temperature (E-T)
phase diagram of PMN.
In the last step we fabricated the PMN-based multifunctional cantilevers and tested
the performance of the multifunctional proof-of-concept device. We showed that the heat
transfer indeed occurs between the cantilevers and that the number of cantilevers increases
the temperature span in the device, which is consistent with the predictions of the numerical
model. However, the temperature span was not greater that the EC effect of the material
due to the poor thermal contacts. This observation is discussed and proposals for an
improvement to the thermal contacts are provided.