Transformation of microwave energy into heat

Microwave energy is lost to the sample via two mechanisms ionic conduction and dipole rotation. Both occur simultaneously in many practical applications of microwave heating. 

Ionic conduction is the conductive migration of dissolved ions in the applied electromagnetic field. This ion migration is a flow of current that results in PR losses (heat production) due to resistance to ion flow. All ions in a solution contribute to the conduction processes however, the fraction of current carried by any given species is determined by its relative concentration and its inherent mobility in the medium. Therefore, the losses due to ionic migration depend on the size, charge and conductivity of the dissolved ions, and are subject to the effects of ion interaction with the solvent molecules [18]. 

Ion conduction is influenced by the ion concentration, ion mobility and solution temperature. Every ionic solution contains at least two ionic species and each species will conduct current according to its concentration and mobility. The dissipation factor of an ionic solution changes with temperature, which affects ion mobility and concentration. 

Dipole rotation refers to the alignment, by effect of the electric field, of molecules in the sample that have permanent or induced dipole moments. As the electric field of microwave energy increases, it aligns polarized molecules. As the field decreases, thermally induced disorder is restored. In fact, applied microwave fields cause molecules, on average, to temporarily spend very slightly more time pointing in one direction than in others. Associated with that very small fraction of preferred orientation there is another very small fraction of molecular order imposed and hence a tiny bit of energy. When the 

Temperature dictates to a great extent the relative contribution of each of the two energy conversion mechanisms (dipole rotation and ionic conduction). With small molecules such as water and other solvents, the dielectric loss to a sample due to the contribution of dipole rotation decreases as the sample temperature increases. By contrast, the dielectric loss due to ionic conduction increases as the sample temperature increases. Therefore, as an ionic sample is heated by microwave energy, the dielectric loss to the sample is initially dominated by the contribution of dipole rotation. As the temperature increases, the dielectric loss is dominated by ionic conduction. 

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