Which Materials would be best for IR Ceramic Coating

[Josah]

Which materials would be best for an IR reflective/opaque coating to improve the efficiency of ceramic insulation materials for use in kilns and furnaces, meant for continuous use at up to 2500 deg F?

I understand that materials might be useful for different wavelengths of IR, so details regarding which wavelengths are reflected would be helpful.

 

[Baluncore]

A kiln at 1370°C radiates broadband IR centred on about 2 um. Any insulation will need to be broadband, which precludes the use of tuned λ/n reflective layers. It is probable that an IR-reflective, electrically conductive metallic mirror would be an advantage as an internal surface as it is broadband in it's reflectivity.

You have considered radiation, but conduction and convection will also be costing energy.

To reduce conduction losses requires low thermal conductivity materials be used for wall structure where the inner envelope wall is connected to the external wall. Filling spaces between the walls with material having a low thermal conductivity suggests having many layers of internal IR reflectance = mismatched impedance between the walls. Avoid homogeneous solids. That is usually done in bulk by filling with vermiculite or making the walls from a ceramic foam.

Convection can be reduced by having a vacuum envelope, or by filling the envelope with vermiculite to reduce convective airflow inside the wall.

 

[Benbenben]

A kiln at 1370°C radiates broadband IR centred on about 2 um. Any insulation will need to be broadband, which precludes the use of tuned λ/n reflective layers. It is probable that an IR-reflective, electrically conductive metallic mirror would be an advantage as an internal surface as it is broadband in it's reflectivity.

You have considered radiation, but conduction and convection will also be costing energy.

To reduce conduction losses requires low thermal conductivity materials be used for wall structure where the inner envelope wall is connected to the external wall. Filling spaces between the walls with material having a low thermal conductivity suggests having many layers of internal IR reflectance = mismatched impedance between the walls. Avoid homogeneous solids. That is usually done in bulk by filling with vermiculite or making the walls from a ceramic foam.

Convection can be reduced by having a vacuum envelope, or by filling the envelope with vermiculite to reduce convective airflow inside the wall.

Vermiculite does not handle temperatures that high very well; around 760°C is typical max working temp.

 

[Benbenben]

Which materials would be best for an IR reflective/opaque coating to improve the efficiency of ceramic insulation materials for use in kilns and furnaces, meant for continuous use at up to 2500 deg F?

I understand that materials might be useful for different wavelengths of IR, so details regarding which wavelengths are reflected would be helpful.

If you want to improve the efficiency of your 2500 deg F continuous use kiln or furnace, highly reflective material might not necessarily be ideal.
Continuous use suggests product is regularly being transported and that there are openings continually or regularly. Reflected IR might head out the openings rather than heat the work piece. In those cases, having a high emissivity coating might work better.
Highly reflective materials are typically low emissivity. If product is often moving through the space, better to have the walls stay at high temperature and provide more uniform emitted IR.
Here is a table of material emissivity at various temps.

 

[Josah]

Thank you all for your responses. Somehow I didn't see the notice that anyone had replied.

"You have considered radiation, but conduction and convection will also be costing energy."
Assuming that the walls will be composed of an excellent conductive/convective insulator, what I am interested in is a thin coating of 1-3mm, composed of a combination of non-exotic materials that can be applied via simple spray or painting process. This coating would ideally be stable for continuous use and/or cycling to 2500 F, and would limit the radiation heat penetration to the walls.

"Continuous use suggests product is regularly being transported and that there are openings continually or regularly."
Continuous use in this case refers to extended soaks at temperature, but not continuous processing with parts moving through the space. So loss of IR radiation through openings is not likely to be a large factor in this application. And in fact one objective is to minimize the thermal mass and heat storage of the walls, as well as minimizing heat loss through them.

Possible good candidates might be:
Aluminum Oxide
Zirconium Silicate
Molochite
Mullite

The table of material emissivity is very interesting. Would you happen to be aware of any tables regarding the IR Opacity/reflectivity of various materials?

"any insulation will need to be broadband, which precludes the use of tuned λ/n reflective layers"
My thinking here is that perhaps some materials, while being broadband, might have greater opacity at various wavelengths than others, and that a combination of complimentary materials may yield the optimal results.

 

[Tom.G]

For what it's worth, Diffusion Furnaces used in semiconductor processing operate around 1200°C (2200°F) and use 12 to 18 inches of firebrick as insulation. This keeps the exterior skin temperature low enough that people aren't cooked when they come in contact with it. (Keeps the airconditioning costs down too.)

For a rough rule of thumb, the radiative plus convective thermal transfer of a metal enclosure is 1 °F/BTU/Hr/Sq.Ft. This is convenient because you can divide the high temperature side by the desired external rise above ambient to find the 'R' value of the insulation.

Example: Req'd. Thermal Resistance = (HiTemp - Ambient) / (Desired_Ext_Temp - Ambient)

Lets say you want to limit the exterior temperature to 90°F, i.e. 20°F above ambient of 70°F.

'R'_value = (2500 - 70) / (90 - 70) = 2430/20 = 121.5
So the insulation wiil need an 'R' of 121.5 -- or thermal conductivity of 0.008 BTU/Hr/Sq.Ft./°F
(I'll leave conversion to SI units to others. It's getting late here.)

Have Fun!

 

[Josah]

Does anyone have any advice about how to get started with this? Looking for information about the IR reflective qualities of various ceramic materials, at high temperatures, relative to one another. Possible candidates include:
Aluminum Oxide
Zirconium Silicate
Molochite
Mullite

Someone posted a document with IR emissivity values. Can anything be inferred regarding about absorption/reflection from a material's emissivity?

 

[Tom.G]

Someone posted a document with IR emissivity values. Can anything be inferred regarding about absorption/reflection from a material's emissivity?

Absorption = Emissitivity

Absorption + Reflection = 1