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Name: Matthew
Status: other
Grade: other
Location: WI



Question:
I am in search of a metal that when heated, rapidly dissipates the heat, and cools quickly. When a direct heat source is applied to the material it would quickly expel the additional energy. Although I do not know the mechanisms responsible for those new soldering irons, that are cool to the touch within seconds, I am interested in finding a material that has similar capabilities.


Replies:
In terms of cooling quickly, what you want is a material with low heat capacity and high thermal conductivity, and you want to have as little of it as possible (low mass). There are lots of metals that fit that bill, but in terms of overall usefulness and cost, aluminum comes to mind. There are plenty of other options if that doesn't work. In the case of the fast-cooling soldering iron, I think it's just the very small tip that is heating (via resistive heating), not an entire large metal wand. Because such a small amount of the tip is being heated, it can cool quickly. The downside is that it can overheat and it's harder to control the temperature precisely. The larger, heavier ones heat more slowly, and hold the heat better, so you don't need as high a current, and you don't have as much temperature fluctuation.

Hope this helps,
Burr Zimmerman


Hi Matthew,

I think you misunderstand the concept of heat dissipation. By analogy, water draining from a tank does not just disappear; it has to flow somewhere. Similarly, heat in an object does not just disappear; it too needs to flow somewhere else. Heat dissipation in situations like you describe is primarily by conduction or convection. Conduction means that heat is conducted away to another object. Convection means that the hot object causes air to rise nearby, resulting in local air currents that carry away the heat and cool the hot object.

New soldering irons that cool quickly do not work on the principles of magic. They cool (and heat) quickly because the heated tips have very little mass; not because of some magic material. The less mass the hot tip has, the less total heat there is to get rid of, and the faster the tip will heat and cool. By contrast, imagine how long it would for a soldering iron tip to heat up and cool down, if the tip were the size of a brick!

Some materials are able to store more heat than others. Copper, for example, can absorb more heat than the same weight of (say) iron. We say that these materials have a higher "specific heat value". This makes them require more heat to reach a certain temperature, but it goes the other way too: because they have stored more heat, they have more heat to get rid of when cooling. Thus, using a metal like iron (steel) in your experiment, may result in a slightly faster rate of cooling, but as you can see below, the difference will not matter much.

Imagine you have just heated a small piece of metal that now is sitting on an insulating material (such as a piece of wood). What is going on when the metal is cooling? The hot metal contains a certain amount of heat that is proportional to its temperature, and its mass. The higher its temperature, and the greater its mass, the more total heat it contains, and the more heat it needs to get rid of to cool. The hot metal is sitting there, transferring some of its heat to the air next to it. The hot air rises, carrying away some of the heat, and allows more cool air to replace it. This is called convection, and works because hot air is lighter and therefore rises. The process repeats gradually cooling the outside layer of the metal.

The outside of the metal is now a little cooler than the inside, so heat from the inside now starts to flow by conduction to the outside, where it is (as before) dissipated to the air by convection.

It is important to understand that heat flows in any metal by conduction MUCH, MUCH faster than it can be removed by the convection process. That means that the characteristics of the metal (its ability to conduct heat) makes little difference to how fast it cools, since the slow process of convection is what limits the process of cooling, and convection is not affected by what material the metal object is made of.

To summarize, the soldering iron tips you mention are typically made of plain ordinary copper, that is electroplated with a thin layer of iron (to protect the copper from corrosion at high temperature). They cool fast, because they have very little mass (the tips are very small), and therefore have very little heat to dissipate. The heating elements inside the metal tip are usually a type of ceramic that conducts heat to the metal tip well, but which has very low "thermal mass" (that is, the ceramic stores very little heat because it too has very low mass). In short, there is no "magic bullet" here!

Regards,
Bob Wilson



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