POLARIZATION REVERSAL AND ELECTROCALORIC MEASUREMENTS FOR FIELD-ENFORCED TRANSITIONS IN THE SYSTEM LEAD-ZIRCONATE - LEAD-TITANATE - LEAD-OXIDE : TIN-OXIDE.

摘要

The Pb(Zr,Sn,Ti)O(,3) crystalline solution series (PZST) has several phases of near equal free energy, allowing field-enforceable antiferroelectric (AFE) to ferroelectric (FE) and paraelectric (PE) to ferroelectric (FE) transitions. An understanding of the stability of FE, AFE, and PE phases with respect to chemical composition, electric field and temperature was developed from low frequency polarization vs. electric field measurements at different temperatures. All materials studied had a sequence of phase transitions FE-AFE-PE with increasing temperature. The temperature range of AFE phase stability increased with increasing Zr/Ti ratio, while SnO(,2) content increased the stability over a wider compositional range. Thus, through compositional modification, some materials were fabricated that had an AFE temperature range of stability of only 3(DEGREES)C, while for others the temperature range of AFE phase stability was in excess of 100(DEGREES)C.; The electrocaloric effect is the temperature change that results when an electric field is applied to a dielectric material under adiabatic conditions. Basic thermodynamic principles were used in conjunction with the total entropy differential to relate the electrocaloric temperature difference to the integrated pyroelectric coefficient. The integrated pyroelectric coefficient was calculated from polarization vs. electric field characteristics via numerical integration techniques. Thus, it was possible to relate experimentally determined electrocaloric measurements with theoretical quantities calculated from dielectric data.; The development of an understanding of phase transition behavior in the PZST system, coupled with the ability to relate experimentally determined electrocaloric measurements with thermodynamic relations, allowed systematic examination of the PZST system for the maximization of the electrocaloric effect. The effect of chemical composition and associated phase transitions on the electrocaloric property were as follows: (i) electrocaloric effects in the vicinity of the zero field FE to AFE transition temperature were found to be smaller than those measured near the AFE to PE transition temperature or Curie point, (ii) electrocaloric effects associated with field-enforced transitions near T(,c) increased with increasing SnO(,2) content, and (iii) PZST materials for which the AFE phase was stable over a small temperature range had larger values of the integrated pyroelectric coefficient, and produced larger electrocaloric effects near the Curie point, than those materials for which the AFE phase was stable over a wide temperature interval.; Polarization reversal studies were carried out on normal FE-FE and field-enforced AFE-FE transitions in the PZST system. Direct observation of domain dynamics under cross-polarized light revealed considerable domain wall motion during the switching process. All PZST materials studied had two different electric field regions of switching behavior. The first switching measurements of the field-enforced AFE-FE transition in the PZST system were analyzed in this thesis.; In summary, materials were tailor-made for the optimization of ferroelectric and electrocaloric properties through the interrelationship of material processing, microstructure development, and the systematic analysis of phase transition behavior. The culmination of this work was the largest electrocaloric effect measured to date.