Permittivity - It's Not Just for Electrical Engineers Anymore

The permittivity (dielectric properties) of a material is one of the factors that determine how the material interacts with an electromagnetic field. The knowledge of the dielectric properties of materials and their frequency and temperature dependence is of great importance in various areas of science and engineering in both basic and applied research. It has always been an important quantity to electrical engineers and physicists involved in the design and application of circuit components. Over the past several decades the knowledge of permittivity has become an important property to scientists and engineers involved in the design of stealth vehicles. These applications are most often associated with the defense industry. For the typical electrical engineer permittivity is a number that is needed to solve Maxwell’s equations. One of the purposes of this presentation is to give an explanation of why a material has a particular permittivity. The short answer is that a material has a particular permittivity because of its molecular structure. Another is how the permittivity can be related to other physical material properties.

The knowledge of permittivity has become increasingly important to agricultural engineers, biological engineers and food scientists. The most obvious application of this knowledge is in microwave and RF heating of food products. Here the knowledge of the dielectric properties is important in determining how long a food item needs to be exposed to the RF or microwave energy for proper cooking. For prepackaged food items, the knowledge of the dielectric properties of the packaging materials is also important. The interaction with the packaging material also determines the cooking time. Besides these obvious applications there are also numerous not-so-obvious applications. Dielectric properties can often be related to a physical parameter of interest. A change in the molecular structure or composition of a material results in a change in its permittivity. It has been demonstrated that material properties such as moisture content, fruit ripeness, bacterial content, mechanical stress, tissue health and other seemingly unrelated parameters are related to the dielectric properties or permittivity of the material. Many key parameters of colloids such as structure, consistency and concentration are directly related to the dielectric properties. Yeast concentration in a fermentation process, bacterial count in milk, and the detection and monitoring of microorganisms are a few examples on which research has been performed. Diseased tissue has a different permittivity from healthy tissue. Accurate measurements of these properties can provide scientists and engineers with valuable information that allows them to properly use the material in its intended application or to monitor a process for improved quality control. Techniques for measurement techniques will be reviewed. These techniques cover the frequency range from DC to 1 THz.