Interaction of microwave radiation with other materials

Microwave radiation as an electromagnetic wave can interact with materials in three different ways:

• Reflection: Microwave radiation is reflected off the surface of the material and no energy is absorbed by the material. Reflecting materials include metals among others. Because of these reflective properties, microwave ovens are made of stainless steel. In addition to the material, the oven design is also crucial. To reduce the risk of an inhomogeneous microwave field, round ovens have become established.
• Absorption: Reagents and samples that absorb microwave radiation cause a rapid increase in temperature. In this case, different acids and sample materials have different properties as described above. Due to the rapid heating and the risk of exothermic reactions, monitoring the parameters of pressure and temperature with modern sensors is a must. At the same time, the sensors must be shielded and must not be affected by the microwave field. It is also necessary that the samples are not contaminated by the sensors and that the sensors are corrosion resistant to the reagents used. Non-contact optical sensors have become established to allow complete reaction control without time delay.
• Transmission: Certain materials are transparent to microwave radiation. This allows direct and instantaneous heating of the sample-acid mixture. In practice, PTFE and quartz are the materials of choice. Due to their resistance to chemicals, including acids such as HF, fluoropolymers (PTFE, TFM, PFA) are particularly suitable for the manufacturing of digestion vessels. In addition, PTFE has insulating properties. The material is only heated indirectly via the sample, which minimizes material stress and eliminates the need for long cooling times. Solid plastics such as PEEK are only microwave-transparent to a limited extent and are therefore less suitable.