In Situ Raman Spectroscopic Investigations of Microwave-Assisted Inorganic Transformations

Microwave heating is an intriguing nonconventional method for the preparation of inorganic solids with unique advantages. Understanding of the method remains poor due to the limited application of in situ methods to characterize the underlying formation process to date. Raman spectroscopy as an analysis technique is well suited to enable in situ studies of microwave heating at a larger scale. In this work, a Raman spectroscopic instrument for in situ monitoring of high temperature inorganic transformations under microwave heating was designed and constructed. Observation of the thermally induced phase transition of anatase to rutile titania under microwave heating demonstrated the ability to monitor inorganic transformations with high time resolution. The development of a method to determine temperature from the Raman spectra collected showed additional physical information could be derived by further analysis of the data. The developed instrument and analysis methods were then applied to the study of the microwave synthesis of several layered NaMO2 (M = Mn, Fe, Co, Ni) compounds. In combination with standard ex situ analyses, the microwave synthesis of each was compared to conventional furnace heating to explore potential impacts on the synthesis process and resulting products. In these materials, no significant effects specific to microwave heating were conclusively observed. Still, the ability to study microwave inorganic preparations through in situ Raman spectroscopy was clearly demonstrated.