Radiative heat transfer in solids

[edd1eb]

I work in high pressure chemical synthesis, using the
same techniques as are used in industrial diamond making. I'm helping
to put together a review article about temperature control in these
experiments.

Over the years it has been common practice to use coloring dopants
(e.g. Cr2O3) to provide thermal insulation in the ceramic sample
holders at high temperature (above about 1600 C). The idea is that
photons coming out of the furnace are absorbed, then re-emitted in a
random direction - sometimes back towards the furnace. Thus heat
transfer away from the furnace is slowed.

Does this sound reasonable? Is there a review article or book I
could look at to get a better handle on the processes involved?

 

[unclesyd]

I haven't seen Chrome Oxide used in the manner you describe, but who knows. Here are two companies ZYP and Ultramet that are in the business of protecting components at very high temperatures.

http://www.ultramet.com/ceramic_protective_coatings.html

http://www.zypcoatings.com/ProductPages/BlackBody.htm

 

[Compositepro]

The principle you describe applies to all thermal insulation. To be an effective insulator a material must deal with all modes of heat transfer. In a high vacuum there is no conduction or convection so only radiation has to be dealt with. Multi-layer insulation is based on this. If you Google that you will probably find the information you are looking for.

 

[Ceramicguy]

Hi edd1eb,

Are you sure the Cr2O3 is added for the reason stated? Chrome refractories are/were commonplace but were used for their corrosion resistance and refractoriness. Perhaps the reasoning is anecdotal. If the sample holders are not load bearing e.g. heat shields, using a porous material may work. The scattering of photons in the bulk material will result in significant backscatter (oftentimes with nearly Lambertian behavior). The material requirements are transparency at the photon wavelengths a porous macrostructure and a refractive index different than the surrounding environment. This will minimize the thickness required and the temperature rise of the refractory. An effective thickness is 5+mm. A bisque fired alumina may work fine for you.
If you need a dense sample holder to contain reactants or bear a load then the above is not relevant.

Bruce

http://www.accuratus.com

 

[edd1eb]

Thanks for all the comments!

I should have been a little clearer. The material will not be porous - during use it will bear a load of up to several hundred tons over a few square milimeters. The exterior will be in contact with WC ceramic (cobalt binder) that applies the load. The interior will have a resistive heater (graphite or metal foil) and a sample inside that. The main heat loss will always be via conduction through the power supply electrodes that are in contact with two of the eight WC anvils.

The heat transfer I'm interested in is within the bulk of the ceramic, not across surfaces. The material most commonly used is MgO, but Al2O3, SiO2 or other oxides are often added. Coloured dopants are added as powder to the slurry before casting (or injection moulding) the ceramic sample assembly.

Thanks,

edd1eb.

 

[Compositepro]

Then I guess you are talking about solid, glassy materials. Transparency at any particular wavelength could allow significantly greater heat transfer.

 

[Ceramicguy]

Hi again edd1eb,

A search for radiative heat transfer in refractory materials will turn up a number of books related to the subject, many dealing with high temperatures. The refractory brick and ramming mix suppliers may be able to offer good advice also. That said, have you tried using just a cheap piece of pyrophyllite (grade A Lava) like the synthetic diamond guys do?

Bruce

http://www.accuratus.com