# Materials with good thermal insulating properties

I'm doing a research for a university project.

In particular I'm looking for a "commercial" material (so a material that is available on the market or can be home made) that has good thermal insulating properties.

In particular the material has to last for about 300sec in an environment up to 2000C and the inner part of it has to be as cold as possible.

I have to choose parts for something like a satellite that should rotate around the sun or a star.

I did a lot of research and in particular I focus on the NASA projects. I saw a lot of video of them about "insulation tiles" and some other thermal coating but I think a common person cannot buy them or make at home.

I hope that this is the right community where ask the question.

Edit

Are there no other constraints? Yes the material should be as light as possible.

Because then you could just use a very thick graphite (or copper, for that matter) layer, thick enough that after 300 s the inner part still was close to the original temperature. Most likely you want something a tad lighter. I think that the problem is related to the temperature of 2000C on the outer layer, a lot of materials have a melting point lower that 2000C. I was also thinking something like a layer of a very high melting point material covered by a protective coating, the problem is related to the 2000C.

How would you launch your satellite, if you are restricted to things a person can buy or make at home? This is the question that I try to answer with my research, is it possible to build "something like a satellite" with commercial products? My focus is only on the thermal aspects.

Are you thinking about the thermal shield of the satellite? Yes basically a thermal shield, the material should be used to protect the inner part of the satellite from the "heat radiation" which came from the sun, during the takeoff .... The first idea is something that completely envelops the satellite not considering "the engine part". I perform same calculation I find that the most critical condition are: temperature of 2000C for a time of 300s.

Is it going to be in some atmosphere or in vacuum during these 300 s? At the moment I'm considering both scenarios, but I think the most challenging is the one in which there is atmosphere.

• Are there no other constraints? Because then you could just use a very thick graphite (or copper, for that matter) layer, thick enough that after 300 s the inner part still was close to the original temperature. Most likely you want something a tad lighter. Going in that direction I would look at silica aerogel, which unfortunately has a melting point of 1,473 K but otherwise close to best available thermal conductivity, 0.03 W/(m·K) in air and better in vacuum. – Anders Sandberg Apr 19 '19 at 21:15
• How would you launch your satellite, if you are restricted to things a person can buy or make at home? And are thinking about the thermal shield of the satellite? Is it going to be in some atmosphere or in vacuum during these 300 s? – nasu Apr 20 '19 at 1:31
• I update my answer. – Ugo Mela Apr 20 '19 at 13:42
• Thank you! @AndersSandberg – Ugo Mela Apr 20 '19 at 14:24
• How did you get the 2000 degrees value? Before you look for an answer it helps a lot if you make sure you have the right question. Why do you even need this shield? For the launching phase? For the re-entry? What does it mean to have a satellite around the Sun? How far? And looking for a material that does not melt at the specific temperature may not be the best thing. Melting and evaporating the shield dissipates a lot of heat and it may keep the rest cool. It is a one time only shield or multiple uses? – nasu Apr 21 '19 at 15:37

You are interested in three different classes of properties. One is the ability to withstand high temperatures and low pressures. I would put this within a class called the environmental properties of the material. The second is the ability to maintain a low heat flux under a high temperature gradient. This is a true thermal property. The third is a physical characteristic or intrinsic property of the material ... its density. This is not truly a mechanical property because we just measure this value, we do not "do something" to the material to determine a property.

You could start with the environmental properties and then down-select to thermal and denisty. The better approach however is to start with the thermal insulation properties of materials and refine further based on the environmental. It is better because in this way, you make thermal insulation your go/no-go criteria and you make environmental integrity simply a max/min criteria. When the material does not meet your thermal performance goal, it fails. When it does not meet the environmental goals, it could still be viable if it is only used within a restricted (shorter) period of time.

How does density appear? It appears in one of two ways. For the case at hand, one way is to state that your material must have a density that is as low as possible. Then, density is a minimum selection criteria. Alternatively and perhaps better, you can fold density in to the thermal property. In this case, rather than searching for materials with the lowest thermal conductivity, you search for materials with an appropriate scaling of thermal conductivity and density. What is appropriate? The truth is that you want the lowest thermal conductivity per unit mass. In this case, take their ratio.

Start with research to find materials that have low thermal conductivity $$k$$. Add a column for density $$\rho$$ and the ratio $$k/\rho$$. Add columns to the list that define metrics for the thermal and vacuum integrity. The former could be the melting or decomposition temperature of the material. The latter could be the vapor pressure of the material (at room temperature).

To assess whether the material meets your thermal metric, find materials with the lowest specific thermal conductivity $$k/\rho$$. Then select viable materials that have acceptable metrics for the thermal and pressure stability.

Once you have a selection of candidate materials, you can proceed to search for commercial products that contain that material. Alternatively, you can search for commercial sources of the material in a form that allows you to make your own thermal insulation.

As a reference to the optimization approach, I suggest the book from Ashby on Materials Selection for Mechanical Designs.

Just to add to the accepted answer. A material that would work would be a ceramic. No synthesizing aerogels needed.

You can also buy single crystal sapphire for pretty cheap and the thermal conductivity goes down upon heating. I used these kinds of materials a lot for research under ultra high vacuum (10^-10 torr) with a sample reaching around 2000-2200 K. I had liquid nitrogen on top (77 K) and the heated sample on bottom separated by the sapphire. Be careful because it's brittle, but also transparent which is nice.

There you go, something you can buy. Also if you need to measure the temperature you will probably need a calibrated Type C thermocouple, since Type K (a very common one, Chromel Alumel) will melt!