True or False?
At Crescend, we actively try to debunk microwave myths and misconceptions and educate our customers. We feel that microwaves are grossly underutilized in the industrial and scientific sector partly because they are often misunderstood. There are many uses of microwaves which either have not been fully explored or on a very limited basis.
Mysterious Magic Box
According to the US bureau of Labor and statistics, over 90% of households in the US have a microwave oven. However, to most, this appliance remains a mysterious magic box. A majority of people have very limited knowledge about how microwave ovens actually work. Many people even have strong misconceptions about microwaves. Such as the phrase ‘nuking a potato’, this misnomer often implies to people that microwaves somehow use nuclear radiation or are radioactive. This is far from the truth, as microwaves are non-ionizing electromagnetic waves. Many people also learn half-truths, such as “do not put metal in a microwave”. In general this is true, but the damaging arcing associated with metal has more to do with the shape of the metal then the metal itself. Many modern food packages actually safely use metal in a microwave to help heat the food or increase convenience. A common example of this is crisping sleeves that come with Hot Pockets ™ brand frozen sandwiches. They work by mixing metallic powder into the paper sleeves which converts the microwave energy into radiant heat, crisping the outside of the food while the microwaves cook the inside. This metallic material in this application is called a susceptor. Another instance of safe metal in a microwave is the microwave safe pull top bowls sold by Chef Boyardee™. They have a metal rim that remains after the pull top has been removed. This pull top is a convenient feature as the meal can be made without needing to use a can opener. The metal rim goes into the microwave with no ill effects.
Another basic misunderstanding that we often come across is that microwaves heat foods from the inside out. While there is indeed volumetric heating occurring you have to consider the scale. Microwaves will get absorbed as they pass through a material, so if the material is large enough or the microwaves are too weak or a high frequency it will not heat the core. We call this penetration depth. Practically speaking, volumetric heating is a big advantage of microwave heating.
The majority of people who do have a basic understanding of how microwaves work will often remember what they were taught from their high school physics class, which usually gets shortened to; “the microwave causes water molecules to vibrate which causes heat”. This sentence is technically correct; however a lot of information is missing in this answer.
Microwaves are Radio Waves
Microwaves are radio waves, like the radio waves used in cell phones for communication just many magnitudes stronger. This basic understanding that microwaves cause water molecules to vibrate is also an oversimplification that causes many people to believe microwaves can only heat stuff that has water in it, that simply isn’t true. All materials can react to microwave fields depending on the molecular structure of the material. When microwaves encounter a material, the waves can behave three different ways; they can be reflected, they can be absorbed, or they can be pass through or any combination of these. Some materials are completely invisible to microwaves and show practically no interaction such as HDPE, Polypropylene, PTFE (Teflon™), fused quarts and more. The microwaves pass through the material and none of the energy is converted to heat. This can be extremely useful; our conveyor belts are often made from polypropylene! This allows us to carry materials through a microwave oven on industrial scales without wasting energy heating the conveyor belt. Some materials reflect microwave energy and also show little interaction. Most sheets of metal reflect the majority of the microwaves and show little heating. This is also very useful as this allows us to contain the microwaves inside metal boxes to concentrate their effect and increase efficiency. Some materials, like water, easily convert the microwave energy to heat. However, the energy that gets absorbed can actually happen through four different mechanisms.
This is the most common form of heating. This is how water molecules are heated in your home microwave. However, this mechanism can act on any polar molecule that is the correct shape. Alcohols are a good example of other polar molecules that heat in microwave fields.
This mechanism of heating is less common but still frequent. This occurs when a material has disassociated ions. The free ions do not try to rotate with the microwave fields like dipole rotation instead they try to moved back and forth through the material as the waves pass by. One simple way you can see this phenomenon is by adding table salt to a glass of water and heating it in a microwave next to some distilled water. The water with the salt will have Na+ and Cl- ions in suspension. When the microwave field is applied these ions also react and the solution will heat faster than the water without the ions (the distilled water).
This mechanism of action is much less common and occurs when the microwave fields induce an electrical current passing through the material. This generally only happens in materials that are electrically conductive. In conductive powders such as carbon this can also cause micro arcs between each small particle. These micro arcs are converting large electrical voltages which jump from one particle to another and convert a lot of that energy ionizing the air into plasma!
This is the least common mechanism of action and is usually only seen when materials that have a magnetic field are exposed to microwaves. Magnetic fields can also induce electrical current in conductive materials called Eddy currents. These currents will contribute to resistive heating (mechanism 2). Ferrite is a good example of this mechanism. Ferrite is often used as noise chokes on electronics. Many electronics such as laptops will have a bump near the plug. This bump is a chunk of ferrite aimed at filtering out unwanted noise (radio waves). When the ferrite is exposed to microwave fields the magnetic fields resist the microwaves and convert some of this energy into heat.
These four mechanisms are not fully understood for all materials and sometimes the best way to categorize how a particular material behaves in a microwave is by empirical testing. The field of microwave enhanced chemistry is also booming at the moment due to these phenomena and their poorly understood interaction with chemical reactions.
Pictured below is a graph of many common materials and how well they heat in a microwave relative to water. This is not a comprehensive list, but it allows people to quickly get a sense of how well a particular material should heat in a microwave from dipole rotation. Materials on the left have poor coupling and materials on the right couple very well and heat quickly.
We implore you to do your research and be an informed consumer; don’t believe everything you read about microwaves out there, much of it is false. Crescend will continue trying to debunk these and other myths.