Organic-assisted solid-state reaction method for fabrication of PNN-PT ceramics with superior ferroelectric properties

Abstract

Relaxor ferroelectric ceramic materials have attracted considerable attentions in recent years due to their high dielectric constants and superior piezoelectric properties. Since such properties are only associated with pure perovskite phase, the synthesis process is of utmost importance. The traditional approaches mostly rely on the solid-state reaction but encounter many difficulties in eliminating the undesired pyrochlore phases. To synthesize pyrochlore-free Pb-based relaxor ferroelectric powders, many other methods have been proposed such as the columbite route, the chemical method and the mechanochemical reaction approach. However, they bring new problems of complicated processing procedures, high fabrication cost and inevitable contamination, making them inappropriate for industrial production. To tackle these problems, this study aims to develop a simple and economical method for synthesis of Pb-based relaxor ferroelectrics that can attain the desired structure of single perovskite phase, excellent electrical properties and high electric fatigue resistance while accommodating the intense demand of mass production. The key idea is to introduce organic materials into the ball-milling process of the conventional solid-state reaction route so that the oxygen atoms in the organics could effectively assist the formation of the perovskite phase. In this thesis work, 0.64Pb(Ni1₁/₃Nb₂/₃)O₃-0.36PbTiO₃powders and ceramics with pure perovskite structure have been successfully fabricated using a polyethylene glycol (PEG)-assisted method. The details of experimental studies on the improvements of various material properties such as phase structure, microstructure, electrical properties and electrical fatigue resistance have been presented. The major contributions of this research can be highlighted in three aspects: (1) the successful demonstration of organic-assisted solid-state reaction method by synthesizing pyrochlore-free 0.64Pb(Ni1₁/₃Nb₂/₃)O₃-0.36PbTiO₃powders via a polyethylene glycol (PEG)-assisted method, which yields drastically improved properties of ceramics in density, dielectrics and piezoelectrics as compared to those derived without PEG; (2) the study of chemical functional groups by comparing the assisting effects of polyalcohol and polyether, which finds that the hydroxyl oxygen atoms are more effective in assisting the formation of the perovskite phase than the ether oxygen atoms due to their different activities in interacting with PbO; and (3) the characterization of the 0.64Pb(Ni1₁/₃Nb₂/₃)O₃-0.36PbTiO₃ceramic samples fabricated by the organic-assisted method, which shows superior performance as compared to those fabricated without the organic materials, particularly for electrical properties and unipolar and bipolar electric fatigue characteristics. In summary, the organic-assisted solid-state reaction method developed in this research work has shown its great potential in fabricating single-phase perovskite Pb(Ni1₁/₃Nb₂/₃)O₃-PbTiO₃ceramics with superior electrical performance, high electric fatigue resistance and mass productability, making it suitable for wide industrial applications such as multilayer capacitors, actuators and transducers, etc.