Re: emmisive black paint

Message posted by JB737 on October 26, 2007 at 14:02:58 PST:

This one has me scratching my head, but I'd like to throw several ideas out there.

I'm not sure what problem the paint had, or what goal they were trying to accomplish with paint additives. But if I'd been the one tasked with altering paint properties by adding crystal powder to it, diamond would have been my first attempt, closely followed by different types of synthetic sapphire, synthetic emerald, etc.

Diamond has very low emissivity, and is also a fantastic conductor of heat. The same is true of other crystals generally used as gemstones and abrasives.

Most types of paint have high emissivity. The exceptions are certain metallic paints, clearly not the case here. So if they added crystals to change its emissivity, on first glance it would seem intended to lower the emissivity. Maybe good for IR signature issues, but certainly causing a problem of where to send the heat instead. I read somewhere that the SR-71 was designed to sink a lot of the aerodynamic heat into its fuel supply. If true, then paint additives to increase thermal conductivity, and/or to act as area-increasing "heat sinks" makes some sense.

But there are many possibilities:
1. Increasing the microscopic-level surface area by introducing roughness.
2. Increasing thermal conductivity
3. Changing thermal expansion properties
(anti-crack/peel expansion control)
4. Increasing abrasion/erosion resistance
5. Controlling surface roughness for aerodynamic reasons (to optimize the Reynolds number at a certain speed, for instance).
6. Stealth effects at various wavelengths

If the crystals protrude above the paint, then they are acting as a heat sink (or reverse heat sink, gathering heat to send into the airframe, fuel, etc)

Because the thread was probably named "emissive paint" for a reason, I'll get back to emissivity:

If the crystals are recessed (like at the bottom of waffle indentations) and the microscopic peaks are the black paint, then they are increasing the exposed total area, and it would require detailed analysis to see if the added surface area makes up for the lower emissivity of the crystal portion of it. The bottom-of-waffle crystal would also give directionality to the IR emissions: low emissivity perpendicular to the surface, and the IR would be emitted more strongly in other directions. Maybe useful if trying to reduce the IR signature of the plane from directly below it (where missiles come from).


In Reply to: emmisive black paint posted by habu-e6 on October 24, 2007 at 17:36:58 PST:


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