REPOSITORY > RESULTS

Abstract

This thesis presents the results of our experimental work in the field of new advanced materials such as liquid crystal elastomers (LCEs), electrocalorics and magnetoelectrics.
In the first part, the conversion of thermal to mechanical energy is addressed in LCEs. The three important, newly discovered, chemical and physical parameters for controlling their critical behavior are presented. Via these parameters it is possible to influence the phase transition from an isotropic to a nematic phase. Since the nature of this transition is directly connected to the temperature profile of a thermomechanical or thermoelastic response, it is thus possible to influence the type of the thermoelastic response. Therefore, the LCE materials can expand/contract either quickly, i.e., discontinuously or slowly, i.e., continuously, depending on the proper tuning of the above-mentioned parameters. These scientific findings could help chemists in synthesizing the LCEs with proper thermomechanical properties, as well as engineers in constructing the final product of a particular application, such as, for instance, an artificial muscle by using the appropriately engineered LCEs. This part of the thesis has been done in collaboration with the research group of Prof. Dr. Heino Finkelmann, Institut für Makromolekulare Chemie, Freiburg, Germany, which kindly supplied most of the LCE samples we studied, and with Dr. George Cordoyiannis, EN-FIST Centre of Excellence, JSI, who made experiments related to the impact of the cross-linker concentration on the nature of the PN-N phase transition in LCEs. The NMR studies of LCEs were made by the research group of Prof. Dr. Boštjan Zalar, from the Condensed Matter Physics Department, JSI.
In the second part the electrocaloric effect (ECE) related to the conversion of the electrical to thermal energy is addressed. The section on electrocaloric materials presents the first direct measurements of the giant electrocaloric effect in several organic and inorganic materials. These studies have been done in collaboration with two research groups: the research group of Prof. Dr. Marija Kosec, from the Electronic Ceramics Department, JSI, Slovenia, and with the group of Prof. Dr. Qiming Zhang, from Pennsylvania State University, USA. These groups supplied inorganic and organic ECE materials, respectively. In addition, collaboration with Prof. Dr. Raša Pirc who has done the theoretical part of the ECE research studies is also acknowledged. Our direct observations of the ECE are compared with recent predictions based on indirect measurements in the above mentioned materials and moreover, they show a characteristic peak of the effective ECE responsiveness near the critical point (CP). This indicates the potential relevance of the CP for the enhancement of the giant ECE.
The last third part of the thesis addresses the magnetoelectric effect, i.e., the control of the magnetic properties via electrical ones, and vice versa. In this section the first ever prepared soft magnetoelectric material, i.e., a mixture of ferroelectric liquid crystal and ferromagnetic nanoparticles is presented. This research work has been done with Dr. Sašo Gyergyek and Prof. Dr. Miha Drofenik from the Materials Synthesis Department, JSI, with Dr. Marko Jagodiĉ and Prof. Dr. Zvonko Jagliĉić, the Institute of Mathematics, Physics, and Mechanics, Ljubljana, and with Prof. Dr. Samo Kralj from the Department of Physics, Faculty of Natural Science and Mathematics, University of Maribor.
To conclude, the main purpose of this dissertation is a contribution to a better understanding of the studied advanced materials, and, consequently, to help to
manufacture new, even more improved, advanced materials that will find a place in relevant applications, which could improve our lives.
The scientific accomplishments of thesis have been published in twenty-four scientific articles in various international scientific journals. Also, four new scientific articles have already been accepted for publication and one article is unpublished.