Study Notes on Basic Microwave Engineering

Microwaves are electromagnetic waves that have a frequency range from

around 0.3 GHz (there is no actual specified lower frequency limit) to 300 GHz with corresponding wavelengths ranging from 1m to 1mm. Microwaves are coherent and polarised in contrast to visible waves (apart from lasers). The generation of microwave in this  Fig. Everyone is familiar with the domestic microwave oven; indeed the majority of households contain one. The microwave heating process is however, fundamentally different from the heating process used in conventional ovens. With microwaves, heat is generated internally within the material as opposed to originating from
external heating sources. As a result, the thermal gradients and flow of heat is the reverse of those in materials heated by conventional means. A conventionally cooked Baked Alaska has the ice cream on the inside whereas a microwave cooked one has the ice cream on the outside! It is possible to heat both large and small shapes very rapidly and uniformly and as the absorption of microwave energy varies with composition and structure it is also possible to have selective heating.

Selective heating is desirable for palaeomagnetic purposes as the magnetic constituents can be specifically targeted. In fact it is possible to go one better by using ferromagnetic resonance (FMR) to demagnetize directly the magnetic particles with the microwave energy, before the energy is transferred to the lattice as heat. This leads to reduced heating of the bulk matrix of the sample and hence less alteration during experiments. As far as the magnetic particles are concerned, however, microwave heating is exactly the same as with conventional heat. In the demagnetization process, the spin system of the magnetic grains is excited (Walton, 1986, Section 1.1.3) both with microwaves and conventional heat. It is solely the method of getting the energy to the spin system that differs between the wo processes. In the Liverpool microwave technique the mechanism of FMR is used.There are different mechanisms by which microwaves (and lower frequency electromagnetic waves) can couple to a material and a whole host of ways that the microwave energy is subsequently lost to the system. The main loss mechanisms are electric, conduction (eddy current), hysteresis and resonance (domain wall and electron spin (FMR)). It is often difficult to ascertain which loss mechanism, or combination of mechanisms is occurring for a particular sample in given conditions. The different mechanisms do however have different dependencies on certain properties such as sample type and micro-structure, frequency and temperature. A brief description of these different loss mechanisms will be given below for background purposes before a detailed description of ferromagnetic resonance (FMR) phenomena including high power effects is given. Finally the role of microwave heat in industry will be touched upon with particular reference to magnetite.