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I need to thermally insulate two metal (say aluminum) parts, the first of which is at 200-400 °C (473-673 K), and the second one is at room temperature. It should be possible to screw the parts together or something, to form one solid assembly. The typical dimensions are like 10-40 mm. The insulator should be able to be created in a DIY setting.

The insulation material has to be reasonably stiff/strong to enable a rigid compound construction, so all plastic foams etc. are out and, due to the high temperatures, almost all (easily available) solid plastics are also out.

Comparing thermal conductivities I have found that all mineral-based materials are reasonably insulating at ~1 W/(mK) when compared to metals (~10^2 W/(mK)).

What has come to my mind (in order of ideation):

  1. porcelain
  2. glass
  3. gypsum
  4. fine-grained concrete

Anything obvious I have missed?

Gypsum seems very practical in that it can be easily cast with very fine structures. However I am not so sure whether it resists the heat (crystal water? gets more brittle at higher temp?). Anyway it is certainly a bad material for threading a metal screw in.

Glass is stiff and temperature resistant, but it is also sensitive to high temperature gradients and breaks easily under mechanic stress.

Fine-grained concrete (don't know if this exists at all as a product) - probably similar to gypsum, but a little stronger. But the question remains, if it can resist repeated heating/cooling/dehydration.

Porcelain (or other sintered minerals): sounds perfect, but can I reasonably expect to create a shaped part with threads/holes etc. at home?

Are there any best industry practices for parts like that?

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  • $\begingroup$ Considering wood as a second layer in combination with another layer is also a very good option. Wood is cheap, accessible ... . Other options are brick (not the best one but cheap), vermiculite and of course cement scores higher, cheap, accessible and good thermal insulator. $\endgroup$ – Sam Farjamirad Aug 29 '18 at 15:16
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    $\begingroup$ @SamFarjamirad: Wood? I think it decomposes above 300°C or even lower, doesn't it. At least that is what I see when I put my soldering iron on a wood plate. Vermiculite is grainy, isn't it? So hard to imagine how to fix a screw into it. Brick is definitely one thing I will consider! $\endgroup$ – oliver Aug 29 '18 at 15:24
  • $\begingroup$ I suggested the wood as the second layer, i mean outer layer, i don't quite sure if you can buy sheets of compressed vermiculite, i did once. i'm keep looking. $\endgroup$ – Sam Farjamirad Aug 29 '18 at 15:27
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have you considered firebrick, a product designed to be used in high temperature kilns, for example? It's light weight and easily cut with ordinary hand tools. It may not tolerate severe compression, but should bond well with aluminum and high-temperature adhesive.

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  • $\begingroup$ Very interesting hint. "Easily cut", does it mean that I don't need an angle grinder or something to cut it? I have also found mortar for firebrick - couldn't I just cast the mortar into shape and then bake it (it sets at ~800°C according to the data sheet, could be achieved with a gas torch...)? Remember: pretty small parts... $\endgroup$ – oliver Aug 29 '18 at 16:05
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    $\begingroup$ There are videos on the YouTube of people creating kilns for melting iron constructed of firebrick cut with ordinary handsaws often used for wood working. The mortar for firebrick will not have as great an insulating property as firebrick and may have very little structural strength. Use caution when heating the mortar as any moisture will explode if heated too rapidly. $\endgroup$ – fred_dot_u Aug 29 '18 at 16:43
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The aluminum can barely support itself at 400 C."Lightweight castable" refractory ( can be handled like concrete) should do the job; but you would need to find a supplier .It is what a petrochemical or power-plant might use . Portland cement/concrete type material will not tolerate temperatures of 400 C. Johns-Manville and Harbison-Walker made product like this in the past.

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  • $\begingroup$ The problem is I have to find a german supplier. And there seems to be a wide variation among product names, so I don't know what "lightweight castable refractory" is called in german. I have bought a refractory mortar at the local hardware store, and the manufacturer says, it's up to 1000°C. But the label says it is based on portland cement. I think I will just try. I have already prepared a small mold with my 3d printer. Let's see what happens with the final part when subjected to a gas torch... $\endgroup$ – oliver Aug 30 '18 at 18:18
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Don't consider this as an answer, but since i can't post a photo in comments, here i post it, i generated this diagram using CES, level two, i marked the most suitable and accessible materials. enter image description here

The disadvantage of all the materials here, is that they are brittle.

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