Phase Boundary

The phase boundary describes the narrow band of material that is being heated by the microwave radiation at a time. A common misconception with microwave energy is a limited depth of penetration. At the frequency of a common microwave oven, 2.45 GHz, microwaves will penetrate into rock containing hydrocarbons and water only a few inches or centimeters. The power is coupled into this area because of the high permittivity values for water and hydrocarbons. Once the water and hydrocarbons are removed, then the permittivity for most types of rocks is very low, so microwave energy couples into the adjacent rock beyond deplete rock region, heats this region, liquefies or vaporizes the hydrocarbons, and then continues to migrate outward into the rock–hence the term migrating phase boundary.

Based on careful measurements of the background permittivity of rock, it is possible for the migrating phase boundary to reach distance of 25 meters or greater.