Interaction of microwaves with different materials

Dariusz Bogdal    Department of Chemistry, Politechnika Krakowska, Krakow, Poland

Microwaves are electromagnetic radiation placed between infrared radiation and radio frequencies, with wavelengths of 1 mm to 1 m, which corresponds to the frequencies of 300 GHz to 300 MHz, respectively. The extensive application of microwaves in the field of telecommunications means that only specially assigned frequencies are allowed to be allocated for industrial, scientific or medical applications (e.g., most of wavelength of the range between 1 and 25 cm is used for mobile phones, radar and radio-line transmissions). Currently, in order not to cause interference with telecommunication devices, household and industrial microwave ovens (applicators) are operated at either 12.2 cm (2.45 GHz) or 32.7 cm (915 MHz). However, some other frequencies are also available for heating [1]. Most common domestic microwave ovens utilize the frequency of 2.45 GHz, and this may be a reason that all commercially available microwave reactors for chemical use operate at the same frequency.

Heating in microwave cavities is based upon the ability of some liquids and solids to absorb and transform electromagnetic energy into heat. In general, during the interaction of microwaves with materials three different behaviors of a material can be observed depending whether the material is counted among:

• electrical conductors (e.g, metals, graphite - Fig. 1.2a)

• insulators, which are considered as materials with good dielectric properties (extremly poor conductors)(e.g., quartz glass, porcelain, ceramics, Teflon - Fig. 1.2b)

• lossy dielectrics, which are materials that exhibit so called dielectric losses, which in turn results in heat generation in an oscillating electromagnetic field (e.g., water - Fig. 1.2c)

When a strongly conducting material (e.g., a metal) is exposed to microwave radiation, microwaves are largely reflected from its surface (Fig. 1.2a). However, the material is not effectively heated by microwaves, in response to the electric field of microwave radiation, electrons move freely on the surface of the material, and the flow of electrons can heat the material through a resistive (ohmic) heating mechanism. In opposite, in the case of insulators (e.g., porcelain), microwaves can penetrate through the material without any absorption, losses or heat generation. They are transparent to microwaves (Fig. 1.2b).

For some dielectrics, the reorientation of either permanent or induced dipoles during passage of microwave radiation which is electromagnetic in nature can give rise to absorption of microwave energy and heat generation due to the so called dielectric heating mechanism (Fig. 1.2c). Dependent on the frequency the dipole may move in time to the field, lag behind it or remain apparently unaffected. When the dipole lags behind the field (polarization losses) then interactions between the dipole and the field leads to an energy loss by heating (i.e., by dielectric heating mechanism), the extent of which is dependent on the phase difference of these fields.

In practice, most good dielectric materials are solid and examples include ceramics, mica, glass, plastics, and the oxides of various metals, but some liquids and gases can serve as good dielectric materials as well. For example, distilled water is a fairly good dielectric; however, possesing polar molecules (i.e., a dipol moment) can couple efficiently with microwaves to lead to heat generation due to polarization losses. Thus, such substances that are counted among dielectrics but exhibit some polarization losses that result in the dielectric heating are also called dielectric lossy materials or in general lossy materials. On the other hand, n-hexane, which having a symmetrical molecule, does not possess a dipole moment and does not absorb microwaves.

Microwave radiation, as all radiation of an electromagnetic nature, consists of two components, i.e. magnetic and electric field components (Fig. 1.3). The electric field component is responsible for dielectric heating mechanism since it can cause molecular motion either by migration of ionic species (conduction mechanism) or rotation of dipolar species (dipolar polarization mechanism). In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz the field oscillates 4.9 × 109 times per second), and the strong agitation, provided by cyclic reorientation of molecules, can result in an intense internal heating with heating rates in excess of 10 °C per second when microwave radiation of a kilowatt-capacity source is used [1]. Therefore, to apply microwaves to organic reactions, it is most important to find at least one reaction component that is polarizable and whose dipoles can reorient (couple) rapidly in response to changing electric field of microwave radiation. Fortunately, a number of organic compounds and solvents fulfill these requirements and are the best candidates for microwave applications.

To consider the application of microwave irradiation for organic synthesis, the first step is to analyze the reaction components together with their dielectric properties among which of the greatest importance is dielectric constant (r) sometimes called electric permeability. Dielectric constant (r) is defined as the ratio of the electric permeability of the material to the electric permeability of free space (i.e., vacuum) and its value can derived from a simplified capacitor model (Fig. 1.4).

When the material is introduced between two plates of a capacitor, the total charge (Co) stored in the capacitor will change (C)(Eq. 1.1). The change depends on the ability of the material to resist the formation of an electric field within it and, finally, to get polarized under the electric field of the capacitor.

r=CC0

where:

• Co - the capacitance of the capacitor with vacuum

• C - the capacitance of the capacitor with the material

Thus, dielectric constants(r) that determine the charge holding ability of the materials are characteristic for each substance and its state, and vary with temperature, voltage, and, finally, frequency of the electric field. Dielectric constants for some common materials are given in Table 1.1.

Air has nearly the same dielectric constant as vacuum (r = 1.00059 and 1, respectively). Polar organic solvents (i.e., water, acetonitrile, ethyl alcohol) are characterized by relatively high values of dielectric constants and, in turn, can be heated by dielectric heating mechanism under microwave irradiation. Non-polar organic solvents (i.e., benzene, carbon tetrachloride, n-hexane) have low dielectric constants and, in fact, show negligible heating effects under microwave irradiation. Most plastics range in the low values of dielectric...