Microwave Enhancement of Chemical and Physical Reactions
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MICROWAVE ENHANCEMENT OF CHEMICAL AND PHYSICAL REACTIONS Glenn E. Fanslow. EE and CprE. Coover Hall, Iowa State University. Ames, IA 50011 ABSTRACT Examples of the effects of microwave processing are presented that suggest that microwave "heating" creates higher excitation levels and increased chemical and physical reactions within a material than would be expected if the only change that had been produced by the microwaves was a rise in temperature. The examples include a chemical coal-cleaning process. the destruction of coronene in alcohol, heating of high-temperature superconducting compounds. and a microwaveinduced photovoltaic effect. INTRODUCTION The use of microwave energy for laboratory and industrial applications is based almost entirely on its ability to heat certain materials, and it is assumed that any physical or chemical reactions that take place are only due to the increase in temperature that the microwaves have produced. However, there are a number of examples [1-5] where the use of microwaves to provide for the "heat" for chemical experiments has resulted in such dramatic increases in the speeds of the reactions that it appears as though temperature is not the only excitation mechanism at work. It is the purpose of this paper to present experimental evidence that microwaves do indeed produce more than heat. In one example. microwave processing is compared with thermal processing in a chemical coalcleaning process. It was found that the microwave treatment took much less time than was required for a similar thermal treatment, and the temperatures reached during microwave processing were lower than those required for the thermal process. In another example, coronene in alcohol was subjected to a microwave treatment where temperatures were limited, to a degree, by an ice-water bath. UV-visible absorption spectrophotometry appears to indicate that microwaves have produced the breaking of chemical bonds, and possibly the destruction of the compound. 0 The making of high-temperature superconductors includes the baking of the materials at -950 C for approximately 12 hours. Preliminary experiments where these materials are treated with microwaves for minutes has produced results that. although limited, are similar to those obtained by the thermal treatment. Examples are also presented that appear to show that microwaves can induce a photovoltaic effect in solar cells. EXPERIMENTAL Microwave and Thermal Heating of Coal/Caustic Mixtures The microwave equipment used in the coal cleaning process is described by Fanslow et al. [6]. It consists of a cylindrical cavity. 10.2-cm (4-in) diameter by 61-cm (24-in) long, operating in the TM01 mode. There are choke assemblies on both ends of the cavity to prevent microwave leakage. To prevent oxidation and burning during microwave irradiation, a quartz tube is inserted through the cavity, and this tube is filled with an inert gas (argon). Samples are placed in quartz boats that ride on a support frame which centers the sample along the longitudinal axis in the region
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