Bond Valence Structure Analysis of Doped Bismuth Sodium Titanate

Abstract

This study focuses on a lead free, high temperature ceramic capacitor material having the base composition of (Bi0.5 Na0.5) TiO3. The goal is to modify this base composition to create a material that has X7R-like dielectric behavior, while maintaining its inherently good high temperature dielectric properties. This will alleviate some circuit design problems, and will create a component that is less susceptible to drastic environmental changes. Areas of interest include aerospace and weapons system applications, motor control, geological down hole drilling equipment and many more. An extensive experimental compositional matrix, along with theoretical modeling, has been investigated to modify the base material to attain the goals set forth. Additions of Ba2+, Sr2+, Ca2+, Zr4+ and Sn4+ were investigated by the bond valence method and dielectric constant measurements. Strontium and tin additions were also studied using the Rietveld refinement method. Many other additions were made to the structure to study the modification of the dielectric response. Both single and multicomponent dopant systems were studied to try and create a material that would meet the goal of the project. Barium additions in bismuth sodium titanate (BNT) raised the value of relative permittivity and lowered the peak temperature to a minimum of 150°C. Strontium additions raised the relative permittivity value and lowered the peak temperature, while tin additions suppressed the peak relative permittivity and maintained a constant peak temperature. Both additions increased the lattice parameters as predicted by the bond valence method and shown experimentally by the Rietveld refinements. Calcium additions resulted in a decrease in Curie temperature. Calcium is a very small cation that has been found to substitute for B-site cations in some situations. This occurrence is difficult to determine, however the electrical behavior of the calcium doped system may give some insight to this problem. Zirconium additions up to about 5 mol % increased the shoulder in the curve near 200° C. Above this percentage, the peak relative permittivity was suppressed similarly to the tin doped system. The multi-component systems that were studied exhibited results that combined the behavior of each of the xiii individual components. This trend, along with the bond valence modeling was used to guide the direction of the project. The bond valence method was used to compare the measured and calculated lattice parameters for the strontium and tin doped samples. The results show that the theoretical calculations are within a few hundredths of an angstrom of the measured lattice parameters for the samples. This can be useful to calculate how the size of the unit cell will be modified by adding various dopants and for tolerance factor calculations. The tolerance factor calculations are useful for determining compositions to test the electrical properties of. From the tolerance factor calculations, a range has been determined that describes the composition level of morphotropic phase boundary compositions in bismuth sodium titanate.