Development of a New Ceramic-to-Metal Brazing Technique for Oxygen Separation/Generation Applications

Mixed ionic/electronic conducting (MIEC) oxides, such as SrFeCo0.5Ox, (La0.6Sr0.4)(Co0.2Fe0.8)O3, and BaCeO3, are a class of ceramics that contain ionic and electronic carriers in high enough concentration that both forms of charge conduction are exhibited at high level, typically at temperatures in excess of 500°C. Because of their properties, the demand for MIEC oxide-based devices has grown considerably. The value of the present-day market is conservatively estimated to be $3 billion, with particularly high growth rates in automotive systems, environmental control, and energy generation technology where the devices are employed primarily as amperometric chemical sensors [1]. Solid oxide fuel cells (SOFCs) represent an even larger potential market than that established for chemical sensors. These electrochemical devices convert the chemical energy from fossil fuels into electricity in a highly efficient manner and may find application in a number of energy generation applications, from auxillary power units in automobiles and trucks to megawatt generators helping to power the electrical grid. MIEC oxides are employed in SOFCs as electrodes, carrying out charge separation and charge transfer at the electrolyte/electrode interface, and as agents to increase the electrocatalytic activity of the electrodes with respect to fuel reformation [2]. If the ionic conductivity of a given MIEC oxide is high enough, it can be employed in electrically driven oxygen-ion transport membranes for oxygen gas separation, partial hydrocarbon oxidation, and waste reduction and recovery [3]. MIEC oxide-based membrane technology offers the potential to separate oxygen from air with far greater efficiency and at one-third lower cost than the cryogenic processing technology used today. And unlike cryo-separation, oxygen transport membranes operate at high temperature, making them ideally suited for direct integration with coal gasification plants [4].