MICROWAVE JOINING OF SILICON CARBIDE FOR FABRICATION OF CERAMIC HEAT EXCHANGER ASSEMBLIES

THE USE OF CERAMIC COMPONENTS IN ADVANCED ENERGY CONVERSION SYSTEMS PROVIDES IMPROVED EFFICIENCY BY ALLOWING HIGHER OPERATING TEMPERATURES. CERAMIC HEAT EXCHANGER TUBES ARE CRITICAL COMPONENTS IN MANY SYSTEMS THAT USE COMBINED CYCLES TO ACHIEVE HIGHER EFFICIENCY WITH REDUCED AIR POLLUTION. THIS PROJECT WILL ADDRESS A CRITICAL PROBLEM IN THE FABRICATION OF THESE CERAMIC HEAT EXCHANGERS-THE JOINTS BETWEEN THE CERAMIC TUBES AND MANIFOLD STRUCTURES. BECAUSE OF ITS COMBINATION OF HIGH TEMPERATURE STRENGTH, CORROSION RESISTANCE, HIGH THERMAL CONDUCTIVITY, AND EXCELLENT THERMAL SHOCK RESISTANCE, SILICON CARBIDE (SIC) IS THE MATERIAL OF CHOICE FOR MANY CERAMIC HEAT EXCHANGER APPLICATIONS. PHASE I OF THIS PROJECT IS TO DEMONSTRATE THE FEASIBILITY OF JOINING MULTIPLE TUBES OF SIC TO A PLATE OF REACTION-BONDED SILICON CARBIDE (RBSC), A MATERIAL PRODUCED THROUGH INFILTRATION OF A CARBON-SIC PREFORM WITH MOLTEN SILICON. THIS TYPE OF SIC-RBSC ASSEMBLY IS THE BUILDING BLOCK OF A NOVEL CERAMIC HEAT EXCHANGER FOR ADVANCED COAL-FIRED POWER SYSTEMS. IN PHASE I, SIC-RBSC JOINTS ARE BEING MADE USING A MULTIMODE MICROWAVE APPLICATOR, WITH AN INSULATED ENCLOSURE THAT PROVIDES RADIANT AS WELL AS MICROWAVE HEATING. THE EFFECT OF HEATING TIME AND JOINING TEMPERATURE ON THE MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF THE JOINED SPECIMENS ARE BEING STUDIED; THE JOINTS WILL BE SUBJECTED TO TEMPERATURE AND PRESSURE CYCLING TO SIMULATE SERVICE CONDITIONS.