Development of Pyroelectric Ceramics for High Temperature Applications

Abstract

Bismuth Niobium Titanate (Bi3NbTiO9 or BNT) is a ferroelectric material with high Curie temperature which makes it attractive for high temperature applications. The premise of this work was the investigation of Bi3NbTiO9 ferroelectric ceramic and the possibility of its modification in order to convert it from a potential candidate into a practical material for high temperature pyroelectric applications. Major factors that degrade the pyroelectric properties of this material are its relatively high conductivity at high temperatures, low remnant polarization, and poor grain alignment if fabricated using conventional pressureless sintering. The approach taken was first to investigate the influence of microstructure and dopants on the electrical conductivity in Bi3NbTiO9 ceramic and the possibility of its reduction without adversely affecting the remnant polarization. The second part of the study was dedicated to the investigation of processing methods to promote improved grain alignment and thus enhanced remnant polarization. Finally a study was made of the feasibility of depositing BNT thick films using the airflow deposition method. If successfully applied, this method could be used to fabricate compositionally graded structures of Bi3NbTiO9-based materials in order to obtain enhanced pyroelectric coefficients over a wide temperature range. Another interesting phenomenon associated with ferroelectric graded structures is the "giant pseudo-pyroelectric coefficient" found in graded thin films of BaSrTiO3. This phenomenon in thick ferroelectric films and its applicability to high temperature pyroelectric sensors was also investigated. The study revealed that BNT showed ionic-p-type mixed conductivity above 500°C. The conductivity was successfully reduced by up to three orders of magnitude and the dielectric strength increased up to four times by donor W6+ substitution for Ti at the B site (Bi3Ti(1-x)WxNb09). The doping also increased the remnant polarization of the material by a factor of two. A high degree of grain alignment was achieved by hot isostatic pressing. Thick films of BNT ceramics with improved grain structure were fabricated using the airflow deposition method. The high green density of the resulting films enabled sintering to be carried out below the Curie point of the material. Electric field assisted sintering of the films resulted in improved grain alignment and accelerated grain growth. Thick graded films of BaSrTiO3 were prepared to study the "giant pseudo-pyroelectric effect". The existence of the ferroelectric hysteresis offset and associated "giant pseudo-pyroelectric coefficient" in thick films was confirmed. However, further investigation revealed that the origin of this effect was not related to the ferroelectric properties of the material. It was shown that the hysteresis offset is associated with asymmetrical conductance of the graded structures. This asymmetry is very low and its use for pyroelectric measurements seems impractical. On the other hand, the applicability of the airflow deposition method to graded film fabrication was confirmed. The graded films displayed enhanced conventional pyroelectric properties compared with bulk ceramics, over a larger range of temperatures. A simple semi-empirical model for calculation of the temperature dependence of the electric field distribution in ferroelectric graded structures was established. The attempted fabrication of a graded BNT film using the airflow deposition method was unsuccessful. The lamella-like shape of the precursor powder particles resulted in rapid loss of powder during deposition. Solving this problem required a substantial modification of the deposition setup which was outside the scope of this work. Based on this study an example of a graded BNT structure with enhanced pyroelectric properties at high temperatures was formulated.