The application of microwave radiation in materials chemistry
Piña-Sandoval, Flora Marcela
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Microwave radiation is becoming an increasingly widely accepted source of energy in heating chemical reactions, producing remarkable increases in reaction rate, and sometimes better yields and product distributions compared to when they are heated conventionally. When microwave heating is applied to solutions, the reason for the improvement can mostly be attributed to the faster, and more usually direct delivery of energy to the reagents, often leading to superheating. The application of microwaves to solid-state and materials synthesis and processing is much more poorly understood. Where rate enhancements have been seen, they are often for reasons analogous to those recognized in wet media -direct and rapid transfer of heat to specific parts of the reactive system, sometimes leading to small, hot regions of material with a relatively high microwave susceptibility (so-called ‘hot spots’). There has been relatively little work to try to explore such effects. Here we present new microwave applicators or reactors that have been designed to be used in conjunction with X-ray and neutron diffractometers, and applied to a number of solid-state systems. In particular, we have developed tuned or tunable microwave cavities to be used in conjunction with an X-ray diffractometer working either in transmission mode with a capillary sample, or in reflection mode, with a flat plate. We have also developed further a microwave cavity that enables high-resolution neutron powder diffraction patterns to be taken, and tested it successfully on the High Resolution Powder Diffractometer (HRPD), at the ISIS Facility, UK. One application that was planned was the elucidation of synthetic steps in the microwave-assisted formation of the zeolite ZSM-5; we established a reproducible method for microwave-assisted synthesis that is appreciably faster than when it is heated conventionally, but found that progress would require the brightness of a synchrotron X-ray source. We also studied microwave-assisted processing of zeolites Na-Y and H-ZSM-5, accelerating the insertion of copper ions in the solid- state. Phase transitions in the ferroelectric materials BaTiO^ and KNbC>3 were studied by in situ diffraction to determine whether microwave irradiation can influence the transition between phases of different dielectric susceptibility, and evidence was found for a lower transition temperature in the later case compared to that of conventional heating. In situ neutron diffraction measurements have provided the first direct evidence of differential heating under microwave irradiation of heterogeneous catalysts in the form of M0S2 or Ni particles dispersed over a high surface area AUO3 support. This employed high-resolution measurements of the cell parameters at a series of temperatures achieved both under conventional and microwave heating, using the former to produce a calibration curve to establish a temperature scale, and the latter to determine the effective temperature of every crystalline phase under microwave heating. A differential temperature of the order of tens of Kelvin was found when the mean sample temperature was one or two hundred Kelvin above room temperature. Finally, we performed in situ X-ray diffraction on a sample of Agl held in a glass capillary to observe the transformation between the dense |3 phase, and the more open, fast-ion conducting a phase; this revealed a significant reduction in the transition temperature, possibly arising from a strong interaction between the microwave field and the defects or lattice modes implicated in the transition.