In-Situ Complete Monitoring of a MW Calcination: Dielectric Properties and Gas Evolution

Microwave calcination is extremely efficient when MW absorption of the material occurs during irradiation, and it is even more efficient when combustion synthesis is ignited. Yet, at these high temperatures the measurement of the dielectric properties of the sample undergoing calcination is very difficult to perform. This contribution presents a complete investigation of a complex calcination process for the preparation of N-doped TiO2 nano-powders where the cavity perturbation method (CPM) has been used to determine the in-situ MW absorption of the precursor compound from room temperature till 800°C. In this method, a sample of N-doped TiO2 containing also NH4Cl and citrate derivatives that arise from its synthesis is placed inside a quartz vial of approximately 15 mm height and 9.8 mm diameter. The latter is introduced in a cylindrical cavity where microwave heating and simultaneous measuring of dielectric properties is feasible with two different sources without interferences. The temperature of the sample is measured by an external IR pyrometer pointing to the surface of the quartz vial to track the variation of the loss factor with temperature. As another useful tool to study high-temperature processes, a simultaneous thermo-gravimetric and differential scanning calorimetry apparatuses operating with conventional resistance heating were combined with a FT-IR spectrometer to perform evolved gas analysis on titania precursors was performed. These data are combined with the dielectric behaviour observed in the MW cavity. Results indicate that after a first non-combustive decomposition, the EMW absorption decreases, so hybrid heating is needed to reach the desired calcination temperature of 375-400°C. During MW calcination it is crucial to know at which temperatures electromagnetic energy is better adsorbed by the sample thus to modulate MW power during the process for energy optimization and fine tuning of material’s properties.