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Is metal a good heat shield?
Category: Physics
Published: March 8, 2016
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
If used in a certain way, metal can indeed act as a good heat shield. At the same time, if metal is used in a different way, it can act as a good heat sink, which is the opposite of a heat shield.
There are three ways that heat can be transported from one place to another: through radiation, through conduction, and through convection. Thermal radiation consists of electromagnetic waves that are emitted by objects because of their temperature. Thermal radiation carries energy and therefore transports heat. Since thermal radiation is a form of light, it shoots out in straight lines, travels easily through free space, and is easily blocked by materials. The warmth that you feel when sitting near a campfire is due to the thermal radiation given off by the fire. Conduction is the transport of heat between two objects by direct contact. The warmth that you feel when you grab a mug of hot cocoa is due to conduction. Convection is the transport of heat through the bulk movement of fluid. The warmth that you feel when you place your face above a mug of hot cocoa is due to convection.
The key property that distinguishes a metal is the presence of a large number of electrons that are effectively free to move around. This property leads metals to be good electrical conductors, which is why there is so much metal in electrical circuits. This property also gives metals some interesting thermal properties. Since the conduction electrons in metals are free to move about, they are able to quickly and easily respond to the waving electromagnetic fields present in thermal radiation. This strong response leads to a strong reflection of the thermal radiation. It is somewhat similar to how an agile and unimpeded baseball outfielder is able to quickly and effectively return balls back to the infield. For this reason, metals are excellent shields to thermal radiation. If the main way that heat is being transported in a particular system is through thermal radiation, then metals are excellent heat shields. Note that a metal will only reflect most of the thermal radiation if the metal is thick enough; on the order of a millimeter thick or more. If the metal is too thin, thermal radiation can leak through the metal in a process called wave tunneling.
The free electrons in metals also make metals good thermal conductors. The electrons can absorb energy at one spot and then move freely to another spot, carrying the thermal energy with them. Therefore, if a metal is in direct contact with a hot object, it will quickly carry heat away from the object. If the main way that heat is being transported in a particular system is through conduction, and if the metal is in contact with the hot object, then metal is a poor heat shield.
We have already looked at thermal radiation and thermal conduction. In both cases, the free electrons in metals play an important role. For the last heat transport mechanism, convection, the free electrons do not play a role. Convection requires a fluid material that can move around. Typically, metals are solid, not fluid, and therefore do not participate in convection. Furthermore, even if air or water acts as a fluid that is enabling convection, large solid pieces of metal can block the air or water and stop it from moving around. If the main way that heat is being transported in a particular system is through thermal convection, then a large solid piece of metal can be an excellent heat shield. If the metal has many holes or channels that allow fluids to pass through, then it will be a poor heat shield in terms of convection.
In reality, all three heat transport mechanisms are typically present. This means that metal can be either a good heat shield, a mediocre heat shield, or a bad heat shield, depending on how we use it.
Based on the discussion above, we can now determine how to make metal into a good heat shield. We should use a large sheet of metal that does not have any holes or channels. By placing this sheet of metal around the heat source, it will reflect back the thermal radiation created by the heat source and block much of the thermal convection. At the same time, since metals are good thermal conductors, we should be careful that the metal does not come into direct contact with the heat source. There should be an air gap between the metal and the heat source. This is to prevent the metal from transporting heat away from the source via conduction. If all these steps are taken, then all three modes of heat transport will be blocked and the metal will act as an excellent heat shield.
For example, consider that you have just cooked a turkey and you want to keep it warm. By surrounding the turkey with a heat shield, you can prevent the heat from escaping and stop the turkey from getting cold. The best way to do this is to get a solid sheet of aluminum foil and wrap it around the turkey, being careful to ensure that there is a small gap of air between the turkey and the foil. The foil will reflect thermal radiation back into the turkey, will block thermal convection currents, and the air gap will ensure that the metal does not thermally conduct away heat.
In contrast, we can also construct metal to be a poor heat shield, or in other words a good heat sink. First of all, we should put many holes or channels in the metal to allow convection currents to pass through. Shaping the metal to have many fins, spikes, holes and channels promotes convection, and it also increases the surface area through which the metal can transfer heat to the air. Secondly, we should place the metal in direct contact with the heat source, so that we can take advantage of metal's high thermal conductivity. Lastly, we should make the metal very thin so that thermal radiation can leak through. However, very thin metal is typically too weak to hold its shape, so this last step is often skipped. When these approaches are applied, metal will act like a very good heat sink and a very poor heat shield.
For example, consider the central processing unit of a computer. Such a large amount of electricity passes through the main chips of a computer that they quickly heat up. If the heat is not efficiently transported away from the computer chips, they will burn. For this reason, a heat sink is typically attached to the main chips. The heat sink is typically just a chunk of metal that is shaped so that it has lots of fins and channels and is placed in direct contact with the computer chips. In addition, a fan is typically placed on top to ensure that air is constantly blowing through the metal, increasing the effect of convection.
In summary, because of metal's unique properties, it can be an excellent heat shield (such as when aluminum foil is wrapped around warm food) or an excellent heat sink (such as when metals fins are attached to computer chips).