Dielectric loss sample heating

Microwave energy is not transferred primarily by conduction or convection as with conventional heating, but by dielectric loss [28]. The dielectric loss factor (loss factor, e") and the dielectric constant (e ) of a material are two determinants of the efficiency of heat transfer to the sample. Their quotient is the dissipation factor (tan 8),... 

Differences in sample size, shape and composition can also affect heating rates. The last case particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. Therefore as an ionic sample is microwave-irradiated, the heating results predominantly from dielectric loss by dipole rotation initially, but the contribution from ionic conduction becomes more significant with temperature rise. 

Perhaps the best comparison is that of Alford and Dole (1955) who measured the specific heat of a sample of polyvinyl chloride from the same source as the polyvinyl chloride used by Fuoss (1941) in his extensive dielectric loss studies. The comparison between c and e", the dielectric loss factor measured at 60 cps, is shown in Fig. 15, and covers the glass transition range. Note that the peak of the dielectric loss curve is at 100° C, about 20 degrees higher than the inflection point of the c — T curve. 

The dielectric properties of materials are defined by two different parameters, namely the dielectric constant and the dielectric loss. The dielectric constant, e, describes the ability of a molecule to be polarized by the electric field. At low frequencies, e reaches a maximum as the maximum amount of energy can be stored in the material. The dielectric loss, s, measures the efficiency with which the energy of the electromagnetic radiation can be converted into heat. The dielectric loss goes through a maximum as the dielectric constant falls [16]. The dissipation factor (tan d) is the ratio of the dielectric loss of the sample, also called loss factor , to its dielectric constant tan 8 = e /s. 

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. 

In classical descriptions, thermal runaway is attributed to a strong increase of dielectric losses because of heating. So, the energy provided by microwave irradiation increases with temperature. The authors have shown it is possible to achieve thermal runaway with dielectric losses decreasing with temperature as a result of dimensional resonance or focusing effects of an electromagnetic held within the dielectric sample [128, 129]. 

---