Can Resistive Heaters Fail in Vacuum Due to Reasons Other Than Temperature?

[duhuhu]

I am working on a project for a cube-sat to go up on a balloon to 40 km. My team has been talking to some other groups who have worked on other cube-sats who say that our resistive heaters need to be Kapton heaters rather than 1D heaters like what we have designed for. They claim that our heaters will fail in the medium vacuum but haven't given us a reason as to why they would fail.

Through my research and analysis, I have found that we will not come even close to the operational temperature limits of the heaters, nor will we have any vibration that could cause issues with it.

My question is: Have any of you heard of any issues on restive heaters in vacuum unrelated to temperature that could cause the heaters to fail? and what would you recommend we do to mitigate the problem that doesn't require extremely expensive replacements?

The heater in question can be found at: http://www.digikey.com/product-detail/en/riedon/PF1262-15RF1/696-1682-5-ND/2447965

 

[rbelli1]

The two issues with using that resistor in a vacuum as a heater are that your heat loss will only go through the interface and not through the back side and your TIM will need to be vacuum stable.

Loss of back side heat loss should only be a problem with high temperatures and wattages.

If your TIM has any volatile active components they will out-gas and leave you with a melted heater.

That said I would want to get an explanation from those with experience in the field as to the details of heater failure before committing to a design.

What is a 1D heater?

BoB

 

[davenn]

. My team has been talking to some other groups who have worked on other cube-sats who say that our resistive heaters need to be Kapton heaters

well, I can see why they are liked by others ...
http://www.bucan.com/en/kapton_heaters

Kapton heaters are ultra-thin, ultra-flexible, semitransparent, lightweight heaters. They derive their name from Kapton, the polyimide film developed by Du Pont that is utilized to encapsulate the heating circuit of these heaters. Kapton heaters can operate within temperatures as low as -320°F and as high as 400°F, and have superior dielectric properties. Kapton heaters exhibit excellent tensile strength, durability, and dimensional stability. These heaters produce uniform heat with rapid heat-up and cool-down characteristics. Kapton heaters can endure radiation, resist many chemicals, and because they are very low in outgassing, they are ideal for vacuum applications. The heating elements of Kapton heaters are produced by chemically etching a resistive circuit in nickel alloy foils. This circuit is encapsulated between two layers of Kapton films utilizing either a Teflon® FEP adhesive or acrylic adhesive. These two adhesives define the thermal characteristics and the cost of Kapton heaters produced.

big advantages
1) wide operating temperature range
2) good tensile strength
3) radiation resistant
4) chemical resistant
5) very low outgassing

almost everything your resistor choice isn't
They are NOT designed to be used as heaters, they are just a plain high power resistorDave

 

[Rive]

My question is: Have any of you heard of any issues on restive heaters in vacuum unrelated to temperature that could cause the heaters to fail? and what would you recommend we do to mitigate the problem that doesn't require extremely expensive replacements?

Both of the issues were already mentioned. First is the casing of the resistor, which might not be ready for vac. Since all the 'cubesat' things are cheap and not meant to operate for too long (so they are mostly built from commercial components), I think this one alone is not a big issue.
Second is the vacuum itself: the spreading of heat is limited to direct contact or radiation only, so the surface available is a critical point. Vaacum is a very good heat insulator.

What you might try to do is to make the PCB with some heating wires. It would require some effort on layout, but has some advantages I think.

 

[duhuhu]

What is a 1D heater?

Sorry about that, I should have explained. We call it a 1D heater because my team mate who found it claimed that it was intended to be used as a heater and push heat out in only one direction (The side with the metal contact).

They are NOT designed to be used as heaters, they are just a plain high power resistor

That figures... I was told by the electronics team that it was designed to be a heater. We are trying to avoid the Kapton heaters simply because of the price point. These little ones cost us $2.50 each, and the cheapest Kapton heater we could find was almost $50. I as the thermal guy would love to have those, but its not in the budget unfortunately.

What you might try to do is to make the PCB with some heating wires. It would require some effort on layout, but has some advantages I think.

The our PCB is already going to stay warm enough because of some power converters we have on it. The heaters are for some other low power components on the other side of the cubesat. The plan was to bolt them down directly to the supporting aluminum with some vacuum grade thermal paste and running them at 5W to dump a bit of heat into that side.

 

[Rive]

The plan was to bolt them down directly to the supporting aluminum with some vacuum grade thermal paste and running them at 5W to dump a bit of heat into that side.

If it's correctly planned then this solution is as good (or better) than it would be with any Kapton stuff.

 

[davenn]

If it's correctly planned then this solution is as good (or better) than it would be with any Kapton stuff.

I would have to strongly disagree with that ... considering the spec's of the Kapton item!

maybe you didn't read the info sheet ??

 

[davenn]

These little ones cost us $2.50 each, and the cheapest Kapton heater we could find was almost $50. I as the thermal guy would love to have those, but its not in the budget unfortunately.

for very short term use, it would be maybe worth considering the resistors. I would be concerned with
will it actually supply enough heat to do the job you need ?

some basic testing ( monitoring) whilst in a very cold vacuum chamber would almost be a requirement
to ensure it does what you want without causing side effects

D

 

[Rive]

Maybe you didn't read the info sheet ??

I did read the info sheet, thank you.
For this application those film heaters would give only extra problems. To design the affected mechanical part as a heat spreader and fix one or two resistors to it would do nicely for a cube 'sat' on a balloon at 40km.

 

[davenn]

For this application those film heaters would give only extra problems.

again I strongly disagree with that considering they are designed for this purpose

 

[Rive]

That alone does not guarantee that it'll perform better in the given environment.

Would it be a real 'sat', designed for 1+ month lifetime, and I would actually agree with you. But it's for a balloon.

It's a waste to cut brick with diamond.

 

[f95toli]

almost everything your resistor choice isn't
They are NOT designed to be used as heaters, they are just a plain high power resistor
Dave

We (and almost everyone else I know in my field) use high power resistors of this type as heaters in our cryogenic systems (which are in much harder vacuum than you would find at 40km altitude) and they work fine. The only thing I would be concerned about would be the reliability after repeated thermal cycling, the plastic body could -in theory- crack. I tend to prefer power resistors with a metal body for this reason.

 

[davenn]

We (and almost everyone else I know in my field) use high power resistors of this type as heaters in our cryogenic systems (which are in much harder vacuum than you would find at 40km altitude) and they work fine.

yup

never said they couldn't ... just said that that WASNT what they are designed to do

The only thing I would be concerned about would be the reliability after repeated thermal cycling, the plastic body could -in theory- crack.

yes exactly, and that would be one of my major concerns as well

I tend to prefer power resistors with a metal body for this reason.

if the OP really wants to use resistors, then yes, that would be my choice as well

Dave