The Microwave Heating Principle
Microwave heating is a multiphysics phenomenon that involves electromagnetic waves and heat transfer; any material that is exposed to electromagnetic radiation will be heated up. The rapidly varying electric and magnetic fields lead to four sources of heating. Any electric field applied to a conductive material will cause current to flow. In addition, a time-varying electric field will cause dipolar molecules, such as water, to oscillate back and forth. A time-varying magnetic field applied to a conductive material will also induce current flow. There can also be hysteresis losses in certain types of magnetic materials.
These phenomena will lead to current flow and oscillation of the molecules in the material. These losses will dissipate as heat and so cause a rise in temperature of the material. As the temperature of the material changes, the properties that govern how much heat is generated, such as the material's conductivity and the dielectric loss tangent, will change. In addition, the material-specific heat and thermal conductivity will also change. Microwave heating considers the coupling between all of these phenomena.
Applications of Microwave Heating
One obvious example of microwave heating is in a microwave oven. When you place food in a microwave oven and press the "start" button, electromagnetic waves oscillate within the oven at a frequency of 2.45 GHz. These fields interact with the food, leading to heat generation and a rise in temperature.
The efficiency of microwave heating depends upon the material properties. For example, if you place foods with varying water content in a microwave oven, they will heat up at different rates. A dinner plate may come out with some food on it that is very hot while the rest of it is still cold. Furthermore, the position of food relative to each other will also affect the electromagnetic field within the oven. That is why most microwave ovens have turntables to rotate the food and promote even heating.
Another application that leverages the effects of microwave heating is cancer treatment, in particular hyperthermic oncology. This type of cancer therapy involves subjecting tumor tissue to localized heating, without damaging the healthy tissue around it.
Doctors performing microwave coagulation therapy insert a thin microwave antenna directly into the tumor and heat it up. The microwave heating generates a coagulated region, killing the cancerous cells. This treatment method requires controlling the spatial distribution and heating power. The temperature sensors must be both well designed and strategically placed in order to avoid harming healthy tissue.