Aiming at the optimization of the electromechanical and electrocaloric properties of (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) ceramics for the efficient integration of these compositions into multifunctional cooling elements, we focused on the understanding of the relationship between the domain structure and macroscopic functional response of this material class. For this purpose, we investigated the domain-wall dynamics and phase transitions under an applied electric field in targeted PMN-xPT compositions with x = 0.3, 0.35 using combined “ex-situ” transmission electron microscopy (TEM) and piezo-response force microscopy (PFM). These analyses were supported by “in-situ” X-ray diffraction analysis performed under the collaboration with colleagues from North Carolina State University, USA. The results revealed different local responses to the applied electric field depending on the local structure and polar ordering in PMN-xPT. We found out that the major role in the macroscopic functional response of the monoclinic composition (x = 0.3) is likely played by the highly mobile domain walls present in a dense domain structure with hierarchical arrangement. In contrast, the morphotropic composition (x=0.35) is characterized by a field-induced phase transformation from the monoclinic to the tetragonal phase, which was also confirmed by “in-situ” XRD analyses. The results clarified a set of local mechanisms, pertaining to the development of domain structure and phase fractions under applied field, further elucidating the multiple and complex contributions to the large electromechanical response of PMN-xPT. The study was published in a high impact-factor journal, i.e., Acta Materialia (6.036 in 2017; source: COBISS+).
COBISS.SI-ID: 31410471
Mechanochemical synthesis is one of the most promising solid-state processing methods for the synthesis of ceramic powders of complex chemical compositions. The drawback of the method is the typical long milling times required for the synthesis of the target composition (even )100 hours in the case of the synthesis of (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) with x = 0.1). In the frame of the processing part of the project, we aimed at improving the synthesis conditions for PMN-xPT by an innovative approach using PT seeds in the initial powder mixture. We found out that the powder seeds indeed accelerate the mechanochemical reaction with the net result of the milling time, necessary to obtain a phase-pure perovskite, reduced by almost twice. Using a combination of quantitative X-ray diffraction analysis and high-resolution transmission electron microscopy analysis we explained the role of the seeds, which act as nucleation sites in the amorphous phase, effectively promoting the crystallization of the perovskite during milling. The study was published in J. Eur. Ceram. Soc., which was in 2017 ranked as the first journal in the field of ceramics (source: COBISS+).
COBISS.SI-ID: 31988263
To verify the main idea of the research project, we simulated by numerical methods the behavior of an electrocaloric solid-state cooler for its possible use in microelectronics. The essential parts of the cooler are multifunctional cantilevers made from (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) with x=0.1, which exhibit simultaneous bending and change of the temperature upon application of an electric field. The results of the simulations confirmed that by providing thermal contacts between the cantilevers a temperature gradient is established across the structure, resulting in a promising cooling capacity. The results, therefore, theoretically validated the concept of the cooling device proposed in this project. The paper was published in one of the most important journals in the field of applied physics (Applied Physics Letter), which is regularly read by our research community.
COBISS.SI-ID: 29824039
The origins of the large electromechanical and electrocaloric responsivity of the (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) system, on which we based our research project, are complex and not yet fully clarified. To gain insights into these origins and be thus able to efficiently optimize the multifunctional structures for the cooling device, developed within the project, we aimed at a better understanding of the mechanisms that contribute to the large functional responses of PMN-xPT ceramics. These mechanisms were studied using “in-situ” X-ray diffraction analysis by following the structural changes induced by an applied electric field to the samples. The results revealed two mechanisms: (i) the rotation of polarization and (ii) the phase transition between two monoclinic phases. Despite being extensively discussed in the literature, our analyses directly prove the polarization rotation in PMN-0.3PT on an experimental basis. The results, therefore, essentially contributes to the complex picture of the functional responses of PMN-xPT ceramics. The study was published in Physical Review B, one of the most read journals in the field of physics.
COBISS.SI-ID: 31471911
Despite great research efforts, several microscopic mechanisms that crucially affect the macroscopic electrocaloric (EC) response in ceramic materials remain unclarified. An example is charged point defects, which have the ability to locally interact with domain walls in ferroelectric materials, affecting the field-induced polarization response and potentially the EC response as well. One of the aims of the project was to clarify some of these mechanisms in light of improving the project compositions. These studies were performed on Pb(Zr,Ti)O3 (PZT) ceramics, in which the role of defects in the polarization response is well known, while their effect on EC response is still not clear. Invoking electrical and EC measurements on PZT samples with different dopants along with analytical modeling, we found out that the key role on the EC response is played by the hysteresis in the polarization-electric-field (P-E) response. This hysteresis, which is largely defined by the type, mobility and distribution of charged point defects within ceramic grains, represents losses and thus leads to self-heating effects, reducing the overall EC cooling performance. The results of this study pinpoint the key role of the losses in the selection of materials for EC applications. The losses (hysteresis) can be readily engineered by shaping the defect chemistry of the material using dopants (e.g., acceptor and donor dopants in PZT).
COBISS.SI-ID: 31696935