Microwave sintering of ceramics, composites and metallic materials, and melting of glasses

Though microwaves have been in use for many applications for over 60 years, their application in ceramic processing involving synthesis, sintering, melting, joining, surface modifications, etc, has developed only in the last twenty years. Microwave materials processing is recognized for many advantages, namely, substantial reduction in cycle time resulting in large energy savings, selective and volumetric heating, providing fine microstructures, improved mechanical properties, and eco-friendliness. In the last two decades, various researchers have worked with a variety of traditional and advanced ceramics such as alumina, zirconia. hydroxyapatites, transparent ceramics, electroceramics, ceramic superconductors, glass-ceramics, non-oxide ceramics including Si₃N₄, AIN, SiC and WC/Co, etc. In all of these materials, substantial improvements in their properties over conventional products, as well as enhancements in the diffusion and reaction kinetics, were reported. Recently, microwave melting of glasses has also been achieved with many advantages over conventional melting. It was generally believed that metals reflect microwaves, and hence cannot be processed in a microwave field like ceramics. However, recently it has been discovered that if the metals are in powder form, they will also absorb microwaves and will get heated very effectively. Almost all metals, including refractory metals (W, Re, Mo, etc), alloys, steels, have now been sintered successfully in 5-15 min. The selective heating feature of microwaves has led to effective brazing and joining of metal parts. Even bulk metals can be heated and melted in a microwave field and the melt can be cast into useful products.