Microwave sintering of ceramics

Microwave radiation is an electromagnetic wave with frequencies ranging from 0.3 to 300 GHz. Until the mid-1940s, this radiation was used in radar. It was only during this period – and by chance – that the ability of microwaves to heat matter was discovered. Some thirty years later, this process of converting electromagnetic energy into heat was of interest to ceramists for sintering materials. Sintering is a key stage in the manufacture of ceramics, enabling powdered materials to be consolidated and densified by heat treatment, often at high temperatures, through the activation of material transport mechanisms. The first studies on sintering ceramics using microwave heating were carried out by American and French groups. From the 1970s onwards, American and Canadian groups studied the sintering of structural ceramics (groups from Los Alamos National Laboratory-T.T. Meek, Oak Ridge National Laboratory-H.D. Kimrey and the University of Alberta-W.R. Tinga). Simultaneously, French research was initiated at the Université Pierre et Marie Curie in Paris (J.C. Badot and A.J. Berteaud's group), the ENSPCI (P. Piluso, N. Lequeux and P. Boch), the Université de Nancy (G. Roussy) and the Université de Caen Normandie (G. Desgardin and B. Raveau). In the early 1980s, the U.S. space agency (NASA) turned its attention to this technology: the aim was to develop in situ – in this case on the Moon – structural materials by simple microwave sintering of lunar materials. It was soon demonstrated that lunar soil rocks, rich in iron oxides, were ideally suited to microwave heating.

Since the heating mechanism results from the movement of electrically charged species in solids, it is important to understand these microwave/material interactions by assessing the dielectric properties of solids subjected to microwave frequencies, if possible as a function of temperature. The microwave heating mechanism is linked to the phenomenon of alternating electrical polarization; the material's complex dielectric constant is therefore the most relevant physical property for anticipating its suitability for microwave heating. The transfer of energy within the part generally takes place within its volume, at least in the case of electrical insulators: the penetration depth of the wave is then of the order of a few centimeters. This process is therefore frequently referred to as "volume heating". This property is also accompanied by high heating speeds (> 200°C/min), resulting from a privileged interaction between the material and the microwave radiation. Only the workpiece and its immediate surroundings are heated, resulting in a significant saving in energy consumption. Microwave heating can also be used to reduce sintering temperatures and/or increase material densification (and hence diffusion) rates. These phenomena...