The Principle and Application of Microwave Heating
Microwave radiation is triggered by two mechanisms: dipole polarization and ionic conduction. At the same time, dipoles (such as polar solvent molecules or reagents) in the reaction mixture are involved in the dipolar polarization effect, and the charged particles (usually ions) in the sample lead to ion conduction.
When the microwave frequency begins to radiate, the dipoles in the sample are arranged in the direction of the applied electric field. When the electric field oscillates, the molecular dipole tries to readjust itself along the alternating electric field line. In this process, the energy is dissipated in the form of heat through molecular friction and dielectric loss (dielectric heating).
In the process of ion conduction, charged particles (usually ions) dissolved in the sample will collide with adjacent molecules or atoms under the influence of the microwave field oscillating back and forth. These collisions cause intense motion, which then generates heat, and these ion conduction pathways provide more heat than dipole rotation. These effects are more obvious when ionic liquids are heated in microwave fields.
At the frequency of the field applied by microwave, the heat produced is directly related to the ability of the dipole itself. If the dipole does not have sufficient time to adjust the field (high frequency radiation) or fast adaptation (low frequency radiation), no heating will occur.
In all systems, 2.45 GHz is between the two extremes of high frequency and low frequency, and the dipoles are given enough time to be arranged in the field, but they do not follow the field accurately.
The mw energy transfer rate in a particular material or solvent is determined by a so-called loss factor（tan δ） at a given frequency and temperature.The loss coefficient tan δ=ε〞/ε′, where ε" is the dielectric loss, which refers to the efficiency of electromagnetic radiation conversion into heat, and ε′is the dielectric constant, which is used to explain the degree of polarization of molecules in an electric field.
Generally speaking, in the standard operating frequency (2.45GHz) microwave reaction, the reaction medium with large loss coefficient is required to be effectively heated.
In general, the solvent media used in microwave chemistry can be classified into three types according to the loss factor:
High loss coefficient dielectric（tanδ>0.5）；
Medium loss coefficient（0.1<tanδ<0.5）；
Low loss coefficient dielectric（tanδ<0.1；）.
The laboratory uses solvents that cover the entire microwave absorption spectrum, from strong absorption solvents (eg ionic liquids, ethylene glycol) to moderately absorbed solvents (eg water, N-methylpyrrolidone, benzyl alcohol) to almost no solvent absorption. (for example, non-polar alkanes and alkenes).
Polar additives, such as ionic liquids or heating elements made of strong microwave absorbers, can be specifically used to increase the absorption level of low absorption solvents.
It is to be noted that the dielectric properties of most of the solvents (and generally other materials) are a function of temperature variations. For example, ethanol is a strong microwave absorbing solvent at room temperature, at this time, tan = 0.941, 100 ℃, tan = 0.270, 200 ℃,tanδ=0.080.
The reason for this is that most organic solvents, such as ethanol, are primarily heated by the even-polar mechanism, but at 2. 5GHz, as the temperature increases, the decrease in the viscosity of the solvent leads to a decrease in molecular friction, and the ability to absorb microwave radiation is also reduced. In contrast, the ionic liquid 1-butyl-3-medetomorhexis ([BMIM][PF 6]) is heated by an ion-conducting mechanism, so that its absorption of the microwave power is increased as the temperature increases.For [BMIM] [PF 6], tan δ = 0.185 at 20 ℃, tan δ = 1.804 at 100 ℃, tan δ = 3.592 at 200 ℃.
Therefore, ionic liquids are extreme microwave absorbers at higher temperatures, so it is difficult to accurately measure the temperature and control the reaction by microwave heating.
In addition to the effect of the electric field component of microwave radiation on the above materials, when the magnetic material is exposed to microwave radiation, the effect of magnetic field on the material should also be taken into account. In this case, the corresponding term is the permeability μundefined, the magnetic loss factor μ ", where μ" denotes the magnetic loss from relaxation to resonance under the influence of alternating magnetic field.
Although the magnetic field is not related to organic chemistry/ polymer chemistry, the composition of the magnetic field has an important influence on the microwave-assisted synthesis.