New paper published in Journal Materials

logo_journal_materialsA recent work developed by Microwave Division has been published in the Journal Materials, as a part of the Special Issue “Microwave Materials Processing”.

Microwave-assisted processes have recognized advantages over more conventional heating techniques. However, the effects on the materials microstructure are still a matter of study, due to the complexity of the microwaves-matter interaction, especially at high temperatures. Recently developed advanced microwave instrumentation allows the study of high temperature microwave heating processes in a way that was not possible before.

In this paper, different materials and thermal processes induced by microwaves have been studied through the in situ characterization of their dielectric properties with temperature.

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The figure shows an example of the dielectric properties of a ceramic frit sample together with the sample height up to 1200ºC. Important variations in the sample height are also observed during the process, due to different reactions and transformations. At temperatures above 450ºC the water inside the sample passes to a gas state. The gas tries to exit leading to the expansion of the sample. On the contrary, a contraction takes place above 900ºC, due to the melting of the sample.

The dielectric properties of the sample slightly increase with increasing temperature up to 700ºC. At this temperature, the dolomite decomposition starts, and a sharp change is observed in dielectric constant and loss factor.  A second increase starts about 900-1000ºC, due to the melting of the sample. The effect of the melting is more pronounced in the sample loss factor, reaching from very low values at room temperature to high values at the end of the process.

This and other examples presented in the paper illustrate how the dielectric properties correlate very well to the material transformations at increasing temperatures. This knowledge is crucial in several aspects:

  • to analyze the effects of the microwave field on the reaction pathways
  • to design and optimize microwave-assisted processes
  • to predict the behavior of materials leading to repeatable and reliable heating processes

 

Best Poster Award in GCMEA Conference

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DiMaS group has participated in the Third Global Congress on Microwave Energy Applications -GCMEA-. Our contribution entitled “Study of microwave thermal processes through in situ Raman and dielectric spectroscopy” was awarded with the Best Poster Award of the conference. We congratulate our researcher J.D. Gutiérrez for the quality of his work.

In the poster, it was explained that the use of microwave energy for materials processing is well known, however, the heating mechanisms and the particular thermal behavior of materials under high frequency electromagnetic fields is still a matter of research.  Recent works highlight the need of further investigations to understand the microwave-matter interactions behind the materials thermal processes.

The strategy presented in the poster consists of linking the information given by dielectric properties to the thermal processes that occur in the material. To this end, Raman spectroscopy is applied to obtain information about the microstructure and the chemo-physical changes in the materials. The in situ combination of both techniques (dielectric and Raman) in real time during microwave processing gives useful and innovative information about the heating mechanisms of different materials.