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Q: Thermal conductivity needed to insulate electronics, yet be able to transmit ( Answered,   0 Comments )
Subject: Thermal conductivity needed to insulate electronics, yet be able to transmit
Category: Science > Technology
Asked by: paladinmed-ga
List Price: $200.00
Posted: 25 Nov 2006 06:32 PST
Expires: 25 Dec 2006 06:32 PST
Question ID: 785440
We need to insulate electronics with dimensions of 100 X 50 X 50 mm.
We need to keep the temperature of the electronics below 85-deg
Centigrade when the ambient environment is heated to 260-deg
Centigrade for 2-hours.
(1) is this possible to do with insulation that weighs <100 gms?
(2) what would the thermal conductivity (lambda) of the insulating
material have to be (and how thick/weight)?
(3) is this possible to do with insulation that does not interfere
with signal transmission from the electronics inside?

Request for Question Clarification by hedgie-ga on 02 Dec 2006 21:42 PST
and what is the power consumption of 'eletronics' itself.

Request for Question Clarification by hedgie-ga on 16 Dec 2006 22:15 PST

                  It is unlikely that question can be answered 
if you will not respond to Requests for clarification.

  The electronics itself generates heat, and if you encase it in perfect
thermal insulation, it will get very very hot ..

There are ways to handle the problem, but we would need to know power
consumption and also size of the chip or PCB
Subject: Re: Thermal conductivity needed to insulate electronics, yet be able to transmit
Answered By: hedgie-ga on 25 Dec 2006 05:24 PST

           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
transfer is 

              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.

There are no comments at this time.

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