https://en.wikipedia.org/wiki/Thermal_barrier_coating

It’s important in attempting to answer questions such as this to understand the application and all applicable constraints. For example, where is the heat coming from? An intermittent exothermic chemical reaction, constant flame from fuel combustion, pressurized steam containment? What is the gas between the plates? What is your specification for “cool”? (In industrial applications we generally have a maximum heat to which workers or the public can be exposed to, and which we’ll call “acceptable”, even though it will still feel hot – it’s just not going to cause instant, damaging / crippling injury.) Is there any reason that the cool side of the cool plate can’t just be insulated via standard insulation? You haven’t given any specification for the thickness of the yellow insulating barrier. Given enough depth there, nearly anything will form a thick enough gas layer between the plates, and the gas itself will be the insulator.

Finally, what is the purpose of this construction?

Just make it out of Starlite, right?

Honestly if it has to be a void in the middle, surrounded by something, concentrating on making a vacuum there might be easiest to actually achieve on a budget.

ETA: a list of common materials

Ceramics. Like the stuff they use for atmospheric reentry.

how about asbestos?

@CWOTUS
The heat is coming from constant flame from propane combustion. Propane is dispersed evenly within the chamber and ignited at tiny holes on the top (red) plate. The top piece is 0.062” thick sheet metal. The bottom plate can be anything from a 0.062” – 0.250” thick plate. Probably go with .125”

The thinner the the yellow barrier can be, the better. I’m hoping not to have to go over .25”

As for the max temperature of the bottom plate – Trying to keep it below 130~°F.

Overall dimensions are 20” x 20” x ?? (depends on necessity) .500” – 1.0”
The thinner the better. The less space in the cavity, the more efficient/safer the gas usage.

How are you going to keep your gas from transferring the heat? If I’m understanding right, the majority of your “hot” surface is not going to touch the intermediate layer that your looking for, it’s going to touch the fuel that’s moving through the system, which will be heated, and transfer the heat to the outer plate.

Even if you find something to completely block the heat around the edges, you’re still going to transfer quite a bit through that gas. I would imagine less than a 600 degree drop in that inch between a hot plate and your cooler surface just based on that.

Unless that fuel is cooled, moves a lot faster than I’m imagining, or there is some piece I’m seriously misunderstanding, your “gasket” isn’t going to be the issue.

I agree with @funkdaddy – most of your heat will be going from the red layer to the blue layer via conduction, convection through the gas or radiation. You can reduce the radiative heat transfer by putting a shiny aluminum coating on the red side. Conduction and convection heat transfer can be reduced by flowing the gas really fast.