A comparison of microwave and induction heating in the study of complex metal oxides

Although microwave and induction (magnetic) heating are widely used in industry, their potential as a tool for thermal analysis has not been fully studied. Microwave thermal analysis (MWTA), which uses a frequency of 2.45 GHz and was partly developed at the University of Huddersfield, has been successfully applied to a range of materials but research in this area has focussed primarily on the electrical (E-field) component of the electromagnetic wave with the magnetic component (H-field) being less studied. There is little previous research in the use of induction heating (utilising alternating magnetic fields at a frequency of around 150 kHz) for thermal analysis.

This work describes the adaptations made to the existing MWTA to allow for controlled magnetic heating and the development of IHTA (Induction Heating Thermal Analysis). The techniques were applied to a selection of compounds from simpler metal oxides to complex perovskite systems which were synthesised and characterised as part of this research.

The metal oxides and perovskite systems used were purchased from Sigma Aldrich and Fischer. The more complex systems (double perovskites) were synthesised via either a solid solution (dissolution in solvent) route or solid-state route (grinding with acetone) and then calcined at high temperatures. These are discussed in detail in Chapter 2. Chapter 3 outlines the changes made to the MWTA instrument and the development of the IHTA including sample cells available.

Chapters 4 and 5 discuss the results of this research. The instrumentation and method development are discussed in Chapter 4. Chapter 5 discusses how the adaptations made to the existing MWTA were successful and magnetic heating can be seen in several of the samples. This informed the decision of which samples were most likely to heat within the IHTA instrument. The IHTA showed promising results with numerous samples, as magnetic heating was evident, although some samples that had coupled well on the MWTA showed negligible coupling with the IHTA, suggesting a difference in the manner of heating taking place.

The simpler metal oxides behaved in a similar manner on both the MWTA and the IHTA and these were also used for Curie point determination and, in the case of Fe2O3/Fe3O4, the initial testing for percentage composition. It was also discovered that with manganese oxides, Mn2O3 heated better in the IHTA than the microwave, while MnO2 heated better in the MWTA. This also lends itself to the conclusion that another type of interaction is taking place, as it is possible the E-field in the MWTA may contribute to the coupling of the sample. The Sr2-xLaxFeMoO6 samples show similar power profiles, suggesting that even though the MWTA and IHTA have different input powers (2.45 GHz and 125kHz respectively), the power required to heat the samples increases at a similar rate throughout each experiment. This may allow the user to directly compare the two thermograms.

The results indicate that IHTA has the potential to be used as a complimentary thermal analysis technique alongside DSC/DTA and MWTA and is particularly suitable for materials that exhibit major changes in magnetic properties such as Curie points. However, the approach is still in its infancy and more work, particularly surround characterisation of Curie temperatures is still required. Further analysis into the more complex perovskite systems with current tachniques to observe similar changes with the IHTA would be beneficial for development of the instrumentation.