Abstract

Mixed Conduction in Rare-Earth Phosphates

by

Hannah Leung Ray

Doctor of Philosophy in Materials Science and Engineering

University of California, Berkeley

Professor Lutgard C. De Jonghe, Chair

The goal of this work is to gain a better fundamental understanding of mixed electronic and protonic conduction in rare earth phosphates. Specifically, the controlled introduction of a small amount of electronic conductivity is found to enhance the proton conductivity of the (La,Ce) orthophosphates. This discovery could change the way that materials for electrolyte applications are designed.

Rare earth orthophosphates for the past 20 years have been investigated for their applications as intermediate temperature fuel cell electrolytes. They are known to conduct protons, electron-holes, and oxygen vacancies, under different conditions. The rare earth phosphates are tunable in many different ways. The materials can be aliovalently doped; their metal to phosphorous ratio can be varied; and using different synthesis methods can have a large effect on their conduction properties. In addition, the (La,Ce) orthophosphates are an ideal model system for investigating mixed conduction. Systematically varying the Ce content tunes the contribution of electronic component of the total conduction.

In this work, the defect chemistry of the system is discussed; several synthesis methods for the rare earth phosphate compositions are carried out; different methods for identifying the dominant charge carriers are described; and the effect of increasing the Ce content upon the total conductivity in single atmospheres and in the fuel cell configuration is measured.