Investigations of Non-thermal Microwave Effects Using Hybrid Conventional/microwave Heating Calorimetry

A hybrid calorimeter is described whereby the test specimen may be heated using either hot air and/or microwave radiation. Thus it is possible to study phase transitions of materials under conditions ranging from purely conventional heating to pure microwave heating or mixtures thereof. Measurements using silver iodide showed that its phase change from the low temperature β-phase to the high temperature α-phase (which normally occurs at 147°C) was shifted to a progressively lower temperature with increasing microwave power. Furthermore, quasi-isothermal studies suggested that silver iodide could be transformed between states without changing its temperature by irradiating it with microwave energy. Examination of specimens heated purely by microwave energy using a thermal imaging camera indicated that large temperature gradients occurred in the sample after it had partially transformed to the α-phase. It is therefore possible that any microwave effect could be a consequence of these temperature gradients.