Mechanism of Microwave heating
1 Microwave heating:
MW heating occurrence is based on three mechanisms, which are dipolar polarization, ionic conduction, and interfacial polarization which is a combination of the first two mechanisms.
1.1 Dipolar polarization:
Dipolar polarization is a mechanism of the generation of heat in polar molecules. In the presence of an electromagnetic oscillating field having particular frequency po-lar molecules arrange themselves in this field. Polar molecule experience inertia due to the presence of intermolecular force so inadequate to follow the oscillating field, resulting in random motion of particle which produces heat. Generation of heat in dipolar polarization takes place by one or both the following methods:
* Force of attraction present between solvent molecules for example methanol, water, and ethanol.
* Force of attraction present between solute molecules for example formic acid and ammonia.
To allow sufficient inter-particle interaction frequency range of the electro-magnetic field must be suitable. For higher frequency range inter-molecular forces cease molecular motion before it attempts to follow the oscillating field which results is an insufficient inter-particle interaction. While in the case of lower frequency range molecules have enough time to arrange themselves in the oscillating field, resulting cease of random interaction between neighboring particles. MW radiation has a suitable frequency for oscillation of polar particles and permits sufficient inter-particle interaction. So it is considered ideal for polar solution heating. Besides MW photons energy is low which is approximately equal to 0.037kcal/mol comparatively of the energy needed to break the bond between molecules. So, MW excitation doesn’t affect a molecule’s structure and interactions are completely kinetics.
For example, water molecules are excited just because of a change in dipole moment, resulting in an electronegativity difference between hydrogen and oxygen atoms. The time requires by an electric field to alter direction is prolong than that of dipoles response time at low frequency and dielectric polarization remains in phase within the electric field. This electric field is a source of energy essential for arrangement by rotation of molecules. A portion of the energy is transferred toward random motion every time a dipole is knock out from arrangement and then rear-range. This energy transfer is so small that hardly temperature increases. If the oscillation of the electric field takes place rapidly it alters direction faster than dipole response time. As no rotation take place in dipole, energy doesn’t absorb and water doesn’t heat. In MW frequency range response time for field change and the dipole is the same. So rotation is due to inertia they experienced, but the result is lag in po-larization behind the alteration of the electric field. When the strength of the electric field is at max, consider polarization still as low. This lag depicts the absorption of energy by water molecules and becomes heated.
1.2 Conduction:
Conduction is the mechanism of generation of heat via resistance to electric cur-rent. A conductor electromagnetic oscillating field produces oscillation of ions or electrons which results in an electric current. The conductor is heated when current experience internal resistance because charged species collide with neighboring molecules. In the case of heat-producing capacity, conduction has a stronger effect as compare to dipolar polarization. The limitation of this mechanism is that it is not favorable for high conductivity materials as these materials transmit a major amount of energy that strikes on them. Conducting materials doesn’t directly heat by micro-waves rather induction of electrons takes place by electric field resulting heating of the sample just because of electrical resistance. For this case term of complex permittivity is introduce for the incorporation of conductivity losses.
1.3 Interfacial Polarization:
The interfacial Polarization is the mechanism that is the blend of dipolar polarization and conduction mechanisms. Interfacial Polarization plays an important role in those heating systems in which the conducting medium spread in a non-conducting medium. Such as scattering of particles of metal in sulfur. It shows zero response to MWs and metal transmit a major portion of MW energy to which they are exposed but a blend of two generates better MWs absorption material. To make this possible metal should be taken in powder form because powdered metal has a larg-er surface area for MWs absorption in comparison to the metal surface. Absorbed radiation cause heating which is alike of dipolar polarization. Powdered metals be-have as a solvent for those molecules which are polar and retard the motion of the ions by a force that is equal to inter-particle interactions present in polar solvents. Under the influence of the oscillating field, this restriction force induces the lag phase in the movement of ions. This phase lag produces random motion between ions, resulting in heating of the system.
