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Abstract

Solid-state refrigeration technology based on caloric effects represents a promising alternative for the replacement of the less energy efficient and environmentally problematic vapor-compression technology. Furthermore, it can be employed in fields where vapor-compression technology cannot be used, such as the cooling of small electronic devices and their components. There is considerable interest in the use of the electrocaloric (EC) and magnetocaloric (MC) effects, which are well-known and studied phenomena. In most cases, however, the individual caloric effects exhibit small cooling effects, with the highest values located in narrow temperature ranges, which limits their commercial use. An idea of how to solve this problem is to combine the EC and MC effects in the same multicaloric material and thus enhance the cooling performance and/or extend the operating temperature range of a cooling device.
In the first part of the thesis, single-phase Pb(Fe0.5Nb0.5)O3 (PFN) ceramics that combine EC and MC properties were successfully prepared by mechanochemical synthesis followed by sintering. The enhanced electrical conductivity was controlled with a careful optimization of the sintering conditions, resulting in the lowest value (~7·10−12 Ω−1m−1) in a ceramic sintered at 1000 °C. In this sample, the maximum electrocaloric effect of ~0.9 °C at 28 °C and 100 kV·cm−1; and the maximum magnetocaloric effect of ~0.1 °C at −271 °C and 50 kOe were obtained. The EC as well as the MC properties of the PFN were further improved by doping with manganese. The addition of manganese successfully suppresses the excessive electrical conductivity at high temperatures, leading to superior EC effect of ~2.5 °C at 80 °C and 140 kV·cm−1. At the same time, the MC properties of the material were also improved, resulting in a maximum MC effect of ~0.4 °C at −271 °C and 50 kOe. Despite the promising room-temperature EC effect, the maximum value of the MC effect is still too small for practical usage.
In the second part, the ferroic properties of PFN were tailored by the addition of multiferroic BiFeO3 (BFO). Seven single-phase PFN–100xBFO (x = 0–0.5) compositions were successfully prepared using mechanochemical synthesis followed by sintering. With a gradual increase in the BFO content, the temperature of the permittivity peak of the PFN–100xBFO solid solutions first decreased (reaching RT when x = 0.2), while at higher BFO contents (x > 0.2) it increased and reached ~200 °C in the PFN–50BFO. Introducing BFO into the PFN–100xBFO solid solution leads to the formation of a pseudocubic phase, consequently strengthening the relaxor behavior. This is accompanied by a broadening of the peak in the temperature-dependent permittivity, narrowing of ferroelectric P–E hysteresis loops and decreasing the size of the ferroelectric domains into polar nanoregions. Among the prepared compositions, PFN–20BFO exhibited low ferroelectric hysteresis losses and peak-permittivity near ambient temperature, making it a promising candidate for room-temperature multicaloric applications.
In the last part of this work, we focused on studying the multicaloric properties of PFN–BFO-based solid solutions. In PFN–20BFO, both the peak-permittivity anomaly as well as the paramagnetic-to-antiferromagnetic phase transition coincided at RT, making it one of the first such single-phase room-temperature multiferroic materials. Despite the
favorable room-temperature position of both ferroic anomalies, the excessive electrical conductivity and poor magnetic response of the material limit its EC and MC cooling capability. The further doping of PFN–20BFO with manganese and gadolinium ions successfully suppressed the excessive electrical conductivity and enhanced the magnetic response, leading to the improvement of the EC and MC properties. This results in a PFN–BFO composition with negligible Joule heating up to 75 °C, a room-temperature EC effect of more than 1 °C at 140 kV·cm−1, a high MC effect above 3 °C at cryogenic temperatures and 50 kOe, and the highest room-temperature MC effect among all the prepared PFN-based multicalorics.