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Box 5.1 Microwave Heating Historical Background
Dr Percy Spencer, a scientist with Raytheon Corporation, USA, was working on a radar-related project in 1946 when he noted something unusual. He was testing a new vacuum tube, called a magnetron, when he discovered that the candy bar in his pocket had melted. He went on to try some popcorn, which when placed close to the magnetron cracked and popped. From this curious beginning was discovered the microwave oven of today. In 1947, the first commercial microwave ovens (‘radarange’) for heating food appeared in the marketplace. These ovens were both very large and expensive. However, developments over the years have meant that both the price and size have been reduced considerably.
Microwave Interaction with Matter
Microwaves are high-frequency electromagnetic radiation with a typical wavelength of 1 mm to 1 m. Many microwaves systems, both industrial and domestic, operate at a wavelength of around 12.2 cm (or a frequency of 2.45 GHz) to prevent interference with radio transmissions . Microwaves are split into two parts, i.e. the electric-field component and the magnetic-field component. These are perpendicular to each other and the direction of propagation (travel) and vary sinusoidally. Microwaves are comparable to light in their characteristics. They are said to have particulate character as well as acting like waves. The ‘particles’ of microwave energy are known as photons. These photons are absorbed by the molecule in the lower-energy state (E0) and the energy raises an electron to a higher-energy level (?j). Since electrons occupy definite energy levels, changes in these levels are discrete and therefore do not occur continuously. The energy is said to be quantized. Only charged particles are affected by the electric-field part of the microwave. The Debye equation for the dielectric constant of a material determines the polarizability of the molecule. If these charged particles or polar molecules are free to move, this causes a current in the material. However, if they are bound strongly within the compound and consequently
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are not mobile within the material, a different effect occurs. The particles re-orientate themselves so they are in-phase with the electric-field. This is known as dielectric polarization .
The latter is split into four components, with each being based upon the four different types of charged particles that are found in matter. These are electrons, nuclei, permanent dipoles and charges at interfaces. The total dielectric polarization of a material is the sum of all four components:
«1 = ae + aa + ad + a
where a1 is the total dielectric polarization, ae is the electronic polarization (polarization of electrons round the nuclei), aa is the atomic polarization (polarization of the nuclei), ad is the dipolar polarization (polarization of permanent dipoles in the material), and ai is the interfacial polarization (polarization of charges at the material interfaces).
The electric field of the microwaves is in a state of flux, i.e. it is continually polarizing and depolarizing. These frequent changes in the electric field of the microwaves cause similar changes in the dielectric polarization. Electronic and atomic polarization and depolarization occur more rapidly than the variation in the electric field, and have no effect on the heating of the material. Interfacial polarization (also known as the Maxwell-Wagner effect) only has a significant effect on dielectric heating when charged particles are suspended in a non-conducting medium, and are subjected to microwave radiation. The time-period of oscillation of the permanent dipoles is similar to that of the electric field of microwaves. The resulting polarization lags behind the reversal of the electric field and causes heating in the substance. These phenomena are thought to be the main contributors to dielectric heating.
A possible reason for the reduced extraction times when using microwaves can be attributed to the different heating methods employed by microwave and conventional heating. The different heating profiles (Figure 5.5) obtained for water in a microwave and when using conventional methods show that liquid heated in a microwave reaches its boiling point much more rapidly than under conventional methods. In conventional heating, e.g. with a hot-plate, a finite period of time is required to heat the vessel before the heat is transferred to the solution. Thermal gradients are set up in the liquid due to convection currents. This means that only a small fraction of the liquid is at the required temperature.
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Methods for Environmental Trace Analysis
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Figure 5.5 Heating profiles for deionized water: ?, conventional heating; ?, microwave heating (cf. Box 5.1).
Microwaves heat the solution directly, without heating the vessel, and hence temperature gradients are kept to a minimum. Therefore, the rate of heating when using microwave radiation is faster than with conventional methods. Energy is not lost due to unnecessary heating of the vessel. Localized superheating can also occur .