High-temperature microwave processing of materials

Abstract

This article reviews the physical aspects of a cross-disciplinary science and technology field: the microwave processing of materials. High-temperature microwave processing has a clear industrial perspective in such areas as the production of advanced ceramics, the deposition of thermal barrier coatings, the remediation of hazardous wastes etc. This review starts with the relevant fundamental notions regarding the absorption of electromagnetic waves, heat transfer and the electrodynamics of single-and multimode microwave cavities. Useful formulae, estimates, and interrelations between process variables are presented. This is followed by a review of process examples illustrating the specific features of microwave processing: reduction in energy consumption and process duration, rapid and controllable heating, peculiar temperature distribution, and selectivity of energy deposition. Much attention is given to the advantages of higher-frequency millimetre-wave processing, which include the enhanced absorption in many materials of industrial interest, improved uniformity of electromagnetic energy and temperature, and the possibility of surface treatment. The phenomenon of microwave process rate enhancement is addressed in connection with the problem of the non-thermal microwave effect on mass transport in solids. Both experimental and theoretical approaches to the identification of the mechanism responsible for this effect are illustrated. Finally, the physical and technical factors influencing microwave technology scaleup and transfer to industry are discussed.

Microwave Heating, Isothermal Sintering, and Mechanical Properties of Powder Metallurgy Titanium and Titanium Alloys

“…[1][2][3][4] To date, MW sintering is an established process for ceramics and has been realized in industrial production. In contrast, MW sintering of powder metals is only a recent development, [5] but it has attracted increasing attention because of its potential for both cost reduction and property improvements.…”

Development of a novel instrument for microwave dielectric thermal analysis

“…1,2,5,12,13 These are the imaginary and real components of the complex permittivity and their ratio, the loss tangent ͑tan ␦͒. The greater a material's loss tangent is, the greater will be the heating rate in a given microwave field.…”

“…[1][2][3][4] The extensive use of microwave energy in heating is due to the significant advantages it provides over conventional methods. 5 These include the potential for highly uniform heating, the ability to achieve exceptionally rapid heating rates in certain materials, and improved levels of process control over conventionally heated systems.…”

Surface oxygen exchange between yttria-stabilised zirconia and a low-temperature oxygen rf-plasma

“…The coupling of the electromagnetic field to the ionic material is relatively weak at the investigated temperatures. For example in the more powerful microwave heating an increase in the microwave absorption is only observed at temperatures of about 0.4-0.5 T m (where T m is the melting temperature of the material) [37]. For zirconia the temperature should be definitely higher than 1000 jC in order to obtain serious heating effects.…”