Evidence for the Microwave Effect during the Hybrid Sintering of Zno

Many investigators have reported unexpected effects resulting from the use of microwave radiation as an alternative energy source during the processing of materials. This has included apparent evidence for accelerated kinetics for a range of processes in ceramic, polymeric and organic systems [1-10]; enhanced sintering of ceramic powder compacts, including lower sintering temperatures [1,2]; and reduced activation energies [1-3,11]. Whilst it is now probably true to say that it is generally, though not unanimously, accepted that a ‘microwave effect’ exists, uncertainty still occurs because of: • The inability to vary the energy source without simultaneously affecting a wide range of other variables. • Uncertainties associated with temperature measurement. Pyrometry is often used with microwave heating whilst thermocouples are used in the conventional experiments. When a single technique is used, it is usually a shielded thermocouple although the presence of the metallic shielding is known to distort the local microwave field. Finally, the surface temperature is usually measured. With conventional heating this will be the hottest part of the specimen, whilst with microwave heating it will be the coolest. This makes a direct comparison of results problematic at best.