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ABSTRACT Microwave heating is an intriguing method for the synthesis of inorganic solids offering a variety of advantages over conventional furnace heating, such as fast heating and cooling rates as well as volumetric and selective heating of precursors. However, there are many open questions regarding this “black‐box” process, and insights into the effect of microwave radiation on different types of solids are generally missing. In situ Raman spectroscopy is a powerful technique to unravel chemical transformations and identify intermediate species during microwave solid‐state syntheses. A major challenge is the temperature measurement under microwave conditions because (metallic) thermocouples cannot be used and optical pyrometry has significant drawbacks. In contrast, Raman thermometry is a viable method that relies on the temperature‐induced shift of Raman signals. Here, we use this method to estimate the temperature during microwave heating of a model system (titania) that undergoes a phase transition at temperatures >800°C. The estimation is derived from a flexible double exponential calibration function applied to Raman spectroscopic peak shifts in the temperature‐resolved furnace heating data, which was found to describe two titania modes (one anatase and one rutile) extremely well. Based on a detailed error and uncertainty analysis, we suggest options to further optimize Raman thermometry for use in high‐temperature microwave heating conditions. 

Abstract Microwave heating methods offer unique advantages in preparations of inorganic solids due to the high heating rates, potentially selective heating, and time/energy reductions. Understanding of these enhancements as well as involved mechanisms is poor due to the lack of available and easily applicable in situ monitoring methods, particularly for samples in the solid state. Existing in situ studies typically rely on access to beamline facilities as well as custom‐built microwave systems, which is in the best case inconvenient and in the worst case not achievable. In situ Raman spectroscopy is an ideal technique as it provides rapid and unambiguous phase identification by a noncontact method. Further, the instrument components are simple and compact, facilitating use in the typical synthetic laboratory. Only a few reports on using Raman spectroscopy for in situ measurements during microwave heating exist, and they all utilize specialized custom reactor setups. In this work, a new Raman measurement system designed to observe inorganic transformations in situ that is readily deployable in a standard, commercially available laboratory scale microwave reactor is described. As a simple demonstration, the anatase‐to‐rutile phase transition in TiO₂is monitored under both microwave and conventional furnace heating. The excellent time resolution achieved demonstrates the utility of the system in understanding microwave‐assisted methods for the preparation of inorganic compounds. The simplicity will encourage integration by the non‐specialist to understand microwave heating for synthetic preparations and promote wider application of the technique.