Thanks for this query.
The best insulation, when weight is a concern, is a vacuum.
Let's look at the physical principles and construction of a vacuum flask
Second best, next to the vacuum, are plastic or ceramic foams, which also are
light. For example
"polyimide foam as the thermal insulation material for spacecraft thermal design.
This material has characteristics of high heat resistance, high
ultraviolet resistance, electrical insulation, and lightweight"
Both the vacuum and these electrically non-conductive materials will
allow electromagnetic signals to pass through.
Qualitatively, we see already that it should be possible to fulfill
your requirements of weight and the ability to maintain temperature
difference 260 - 80 = 180 C for 2 hours. I must make the assumption
here that the electronics itself does not produce a significant amount
of heat. I will also not address the
issue of mechanical stability, since no data were given on that requirement.
For polyimid foam
thermal conductivity is 90 mW / m * K , and density is about 8g/ml
Here ml Milli-liter is same as cc (centimeter cubed). The formula for heat
J = A * lambda * (T1 - T2) / length
which in our case means, flow of heat J, measured in Joules/second i.e. Watts W)
,assuming steady state, is 18 W * A / l.
Area A can be made small. E.g. the vacuum flask might hang on polyimid threads,
and the length of the thread could be made long - so that the amount
of heat which would travel from ambient to flask in 7200 seconds could
be made very small. Just for comparison, let's consider each thread 1
mm square and 5 cm long.
The amount of heat would be
72 * 18 = 13 mJ
That is 13 miliJoules, which would increase temperature of the board,
depending on its mass, and specific heat very little, less than a degree
for typical electronics materials.
So, it certainly is possible to do what you need, and could be done
by less extreme means. (Instead of threads, it can sit on thin
stands.) This depends on requirements for mechanical stiffness and
strength. It might also be possible to add a few grams of 'dry ice'
or liquid nitrogen close to the electronics, provided that vacuum
could be actively maintained.
In some cases piece of common 'blue ice', placed with the
electronics into a stainless steel thermos, and wrapped inro heat
resistant fabric can do the job.
As you may already know, Paladinmed, this Google service will close
in about five days. I do hope that you will ask fo clarification, if
it is needed, before that. If you miss that Google deadline, this
does provide tips on research techniques and info on some of the other
research services or individual researchers who can help.
If you have additional data on parameters and requirements, it would be
possible to make these estimates tighter. I would be pleased to
provide such additional calculation, if you come back in time.