Mechanical guy needs help with endothermic reaction and heat decomposition

  • #1
sanchel
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0
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I am working on a project that requires us to protect components from high heat. We were thinking on using sodium bicarbonate, but our module needs to be exposed FIRST to 85C for long periods.... then the high heat exposure will be present. We believe this bicarbonate will be "depleted" after some weeks of exposure to 85C, so we need to use something that will decompose or collect heat at around 100C or just a little higher.... but not much higher than this. We need this compound or material to be a solid of some kind.

Google gave me some ideas, but the residues generated after heat exposure were acids or compounds that I would prefer not to have in my module. Sodium Bicarbonate was great.... but this 85C requirement is killing the idea. Do you have any suggestions? Thanks to all.
 
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  • #2
Welcome to PF! Got a few important questions to help us understand what's going on better, and the kinds of thermal loads we're talking about...

-What's the rough timeframe for the 85C exposure?
-How hot does it get at its peak, and for how long?
-What kind of temperature ramping schedule are you looking at?
-Can you use active cooling of any sort?
-What kind of components are you protecting?
 
  • #3
Flyboy,
Thanks. Here are some answers:
-The module will be exposed to 85C for about 30+ days
-The sample is then exposed to 500C + for 1/2 hour. The sample leaves on chamber and goes to the high heat without any ramping
-No active cooling
-The end item to protect will be encased either on a ceramic or an epoxy.

I am sorry that I cannot share more, but I would prefer not to disclose more details. The 30+days more likely are going to be a problem for the baking soda.... so we just want to replace it with another material... if there is one that could do the job....baking soda was working for the 500C+ fine....
 
  • #4
Whistles

That is a looooong dwell time at 85°C. If you’re looking for a sacrificial coating for the high temp cycle, that’s definitely more of a chemistry/materials science question than a mechanical one. If it’s not meant to be sacrificial, then a few more questions:

-Is the area needing protection able to withstand that extended heat soak at 85°C, and only needs protection from the 1/2 hour of extreme heat?
-Is the high temperature section in a neutral or reducing atmosphere?
-Is there any articulation required in the area needing protection?
 
  • #5
sanchel said:
I am working on a project that requires us to protect components from high heat.
sanchel said:
I am sorry that I cannot share more
But you can give us more information. If I understand your posts so far, you have an object of approximate dimensions X cm by Y cm by Z cm. That object is heat soaked at 85C for 30 days, then at 500C for 1/2 hour. Components inside the object must be kept below 100C to 110C.

If that correctly summarizes your problem, then we need more information. What is the approximate size, mass, and specific heat of the components to be protected? Is there space for thermal insulation and/or thermal mass between the object and the components?
 
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Likes Flyboy
  • #6
IMHO, internal fiber or foam insulation that can resist 500°C, and metal shielding from radiation are your best friends for passive protection of your components from high heat.
Research what it is used for protection of satellites' components from Sun radiation.
 
  • #7
Lnewqban said:
IMHO, internal fiber or foam insulation that can resist 500°C, and metal shielding from radiation are your best friends for passive protection of your components from high heat.
Research what it is used for protection of satellites' components from Sun radiation.
You’re thinking of Multi-Layer Insulation, which does wonders for radiant heat. But from my understanding of the situation, it’s going to be in an atmosphere, composition unknown, at elevated temperature for a month. MLI doesn’t solve the issue there, and I suspect that it’s going to be destroyed in the 500°C phase.

As for the fiber insulation… that might actually be more viable. Part of why I’m asking about any need for mechanical movement in the affected are is to figure out what kind of insulation is viable. If no movement is needed… I would argue something akin to the
LI-900 insulation in the shuttle tiles would work. Maybe not exactly that material, but something similar to it.

Movement significantly complicates things and would require a fabric type insulation.
 
  • #8
You need enough thermal resistance and thermal mass to delay the heat flow to your critical components for that 1/2 hour.

Sand or borosilicate glass beads or asbestos fiber or asbestos bricks. The looser the packing the better... up tp some limit. Loose enough to minimize conduction but avoid free convection.

I recommend some testing with a thermocouple in place of your critical components.

Cheers,
Tom
 
  • #9
Thanks to all. I will try some of these.....
 

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