Microwave Drill
In conventional microwave heating, energy is directly introduced into the material, resulting in rapid volumetric heating with controlled thermal gradients. A phenomenon referred to as ‘thermal runaway instability’, however, can accidentally concentrate the microwave heating energy, causing a rapid local rise in temperature, that produces a ‘hotspot’. In a non-uniform electromagnetic field, this phenomenon can be caused by the temperature-dependence of material properties. It is generally viewed as a drawback in conventional microwave heating that seeks to minimize thermal gradients.
The ‘microwave drill’ makes intentional use of the thermal runaway instability in order to concentrate the microwave energy for rapid heating and melting of a confined hotspot. Microwave energy is directed to the drilled region by a coaxial waveguide with a movable center electrode. Insertion of the movable center electrode into the molten hotspot forms a hole. A fraction of the molten material would be moved towards and used to line the periphery of the hole. This concept has been reduced to practice for drilling into concrete, ceramics and rocks, and quantitative progress has been made towards increasing the diameter and depth of the holes drilled using this concept.
