system
Given the following post, respond as if you're commenting on Reddit:
human
Why are ceramics used for things like heat shielding?

I was thinking about how ceramics are used in shielding and things like tiles on the Space a shuttle and realized that I don't know why they are so good at dissipating heat.
gpt
There are a few things you want in a heat shield. You want something that can withstand high temperatures, absorb a lot of heat, but also not cook your astronauts. 

Ceramics tend to be very good thermal insulators. They have a high thermal capacity, but do not conduct the heat they absorb very well which is good because you need to keep the heat away from the space craft. So you make ceramic tiles. But since they don’t conduct heat very well, they have no choice but to keep heating up since they aren’t dissipating much heat. 

You solve this by painting them black with a layer of what is essentially glass. This gives the tiles a very high emissivity. Emissivity is a measure of a materials ability to reject heat as radiation. The black ceramic tiles were able to reject up to 90% of the re-entry heating back into the atmosphere. 

So you have 90% being dissipated as IR radiation, and the remaining 10% being absorbed by the tiles and staying there due to the low thermal conductivity. These are simply properties of ceramics that make them desirable as heat shields. You could go deeper, but it’s akin to asking why copper makes good electrical wire. Since radiation is very slow for small temperature deltas it takes a hours to cool all the way to ambient but that doesn’t matter because even though the tiles will still be hot at landing they don’t transfer that heat. 

Let’s say you used a metal heat shield instead. Well metal also has a high heat capacity. But its also an excellent thermal conductor. It will transfer that heat into the aluminum frame of the spacecraft during re-entry, which could damage it. It also means you will be hearing the interior of the craft, since the metal frame is now conducting heat further inward.