DFliyerz
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Thermite in Vacuum?
Since thermite can burn without air and therefore in a vacuum, what would it look like? Has anyone conducted such an experiment before?
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aga
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Never heard of it.
What would be the point ?
The fuel component provides the oxygen, so technically Air isn't required, and thermite welding underwater works ok.
In a confined space, the main problem would be if the reaction reached too high a temperature, boiling off the fuel or the aluminium, creating rather
a lot of gas rather quickly = explosion.
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Bert
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Why do you think thermite can burn in a vacuum? Black powder will not. Fireworks stars won't. Various gas generator mixtures used as actuators in
aerospace applications won't (they must be put in pressurized "cans"). Quite famously, the nitrocellulose based propellant in the mooring "harpoons"
of the Philae comet lander wouldn't- Amazed that the German engineers involved didn't know that, I have known since high school.
There have been numerous experiments on behavior of pyrotechnic mixtures in vacuum.
Gasseous or liquid reactants expand/boil off into the vacuum too quickly to alow enough heat transfer into the remaining reactants for continuing the
burn. The reaction is not self sustaining-
Thermite ignites at the temperature where Aluminum becomes molten and Oxygen is liberated from the Iron oxide. Molten Aluminum has a significant vapor
pressure, so does Oxygen.
[Edited on 5-1-2015 by Bert]
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Bert
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Thread Moved
5-1-2015 at 16:01 |
aga
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So the iron oxide isn't the Fuel ?
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Bert
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No, it's the oxidizer in this case. Reaction products would be Aluminum oxide and some VERY hot Iron-
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j_sum1
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Quote: Originally posted by Bert  | | Quite famously, the nitrocellulose based propellant in the mooring "harpoons" of the Philae comet lander wouldn't- Amazed that the German engineers
involved didn't know that. |
I did not know that that was what happened. That's a pretty serious oops. If you are designing a propulsion system to work in space, it really helps
to have one that works in the vacuum of space.
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careysub
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| Quote: Originally posted by Bert | Why do you think thermite can burn in a vacuum?
...
There have been numerous experiments on behavior of pyrotechnic mixtures in vacuum.
Gasseous or liquid reactants expand/boil off into the vacuum too quickly to alow enough heat transfer into the remaining reactants for continuing the
burn. The reaction is not self sustaining-
Thermite ignites at the temperature where Aluminum becomes molten and Oxygen is liberated from the Iron oxide. Molten Aluminum has a significant vapor
pressure, so does Oxygen.
[Edited on 5-1-2015 by Bert] |
Good golly, here is an (old) literature citation backing that up:
https://books.google.com/books?id=RBfzAAAAMAAJ&pg=RA1-PA...
Results vary with the reaction, but generally they are harder to start, less vigorous, and less complete. Some mixtures do go to completion, some
don't react at all. They don't look at, or report on, the classic iron oxide-aluminum thermite reaction.
But this survey on the classic thermite reaction does:
"Thermite reactions: their utilization, synthesis and processing of materials"
L. L. Wang, Z. A. Munir, Y. M. Maximov
Journal Of Materials Science 28 (1993) 3693-3708
"Romodanov and Pokil studied the effect of pressure, in vacuum range, on the combustion of the thermite mixture, Fe2O3-Al-Al2O3. The combustion of
this mixture has been shown earlier to be independent of inert gas pressure for pressures higher than 1 atm. However, as the pressure decreases, the
boiling point of aluminium can become lower than the combustion temperature (e.g. at P = 10 -2 mm Hg,the boiling point of aluminium is 1148 C)
combustion rate of this thermite system at low ambient gas pressure was found to depend on the level of pressure, Fig. 9. Combustion under vacuum
proceeds inthe presence of a gas phase (aluminium vapour)."
Here is the complete page with that Figure 9.
So it does burn, but at a diminished rate (about 2.5 times slower).
There is a classic theoretical thermodynamic thermite thtudy that (sorry, couldn't resist) has been uploaded to this site elsewhere :
http://www.dtic.mil/dtic/tr/fulltext/u2/a419762.pdf
It has a great table showing the reaction results: adiabatic temp with and without phase changes, the state of the products, the gas produced, and the
energy produced.
Looking at this it is easy to visualize the effects of lowering pressure - if a product is at the boiling state at STP, which limits the reaction
product temperature, then that temp will drop with pressure. If a liquid, non-boiling product is formed then it may begin to boil thus creating a
temperature ceiling where there was not one.
I think that if extremely low pressures are required to boil something then the corresponding very low gas pressure produced will at some point make
it cease to matter.
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Bert
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Thanks careysub, I'm beat and didn't have the energy to dig out the literature- Time to move this thread into the thermite sticky?
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Pasrules
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Would it be possible to mix thermite into activated carbon similar to the way black powder was made in a powder mill as an attempt to keep oxygen gas
from escaping into the vacuum for your thermite powder? or would it just lead to the same result as gun powder mentioned earlier.
[Edited on 6-1-2015 by Pasrules]
Atropine, Bicarb, Calcium.
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blogfast25
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| Quote: Originally posted by Pasrules | Would it be possible to mix thermite into activated carbon similar to the way black powder was made in a powder mill as an attempt to keep oxygen gas
from escaping into the vacuum for your thermite powder? or would it just lead to the same result as gun powder mentioned earlier.
[Edited on 6-1-2015 by Pasrules] |
I don't know but years ago I tried lighting a Classic Thermite with a few percent carbon mixed in. Could not light this mixture for love nor money.
Never understood why either...
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Pasrules
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With that said I suppose the carbon would behave similar to black aluminium flash powder where the carbon keeps the aluminium from oxidising in air
and possibly the same in thermite mixture.
Atropine, Bicarb, Calcium.
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j_sum1
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Pasrules | Would it be possible to mix thermite into activated carbon similar to the way black powder was made in a powder mill as an attempt to keep oxygen gas
from escaping into the vacuum for your thermite powder? or would it just lead to the same result as gun powder mentioned earlier.
[Edited on 6-1-2015 by Pasrules] |
I don't know but years ago I tried lighting a Classic Thermite with a few percent carbon mixed in. Could not light this mixture for love nor money.
Never understood why either... |
I have done thermites with unpurified battery gunk at about 70% MnO2 and 30%
graphite plus other impurities. Difficult to ignite, poor quality product and low yield. But it does go.
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careysub
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| Quote: Originally posted by Pasrules | Would it be possible to mix thermite into activated carbon similar to the way black powder was made in a powder mill as an attempt to keep oxygen gas
from escaping into the vacuum for your thermite powder? or would it just lead to the same result as gun powder mentioned earlier.
[Edited on 6-1-2015 by Pasrules] |
Oxygen gas does not escape from the reaction mixture in a vacuum.
However oxygen that is present in air is absent, which may affect the ease of ignition of some thermite type mixtures (but the fact that regular
thermite burns in inert atmospheres shows that this is hardly a major effect).
The aluminum fuel itself evaporates which could tend to quench the reaction. Al boils 2743 K at STP, well below the calculated product temp for many
thermite mixtures. See the article page I posted (drops to around 1400 K at 10 microbar).
Iron also boils at typical STP thermite reaction temperatures, so loss of iron vapor may be significant in a vacuum.
The chart with the linear pressure scale is not ideal to see where in progressively harder vacuums the effect of low pressure stops affecting the
burn, you can see it starts to steepen right at the end as you go down the logarithmic scale. But at some point the very low vapor pressure means that
little material will be evaporating.
[Edited on 6-1-2015 by careysub]
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zed
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How about a Helium atmosphere, at STP? Might not the mixture burn hotter, in that less heat will be dispersed to the atmosphere? Since pressure is
maintained, the low molecular wt. of Helium might serve to act as a form of insulation.
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Bert
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I would suggest Argon. It's cheaper, easier to handle and has superior insulating characteristics to either air or Helium.
http://www.decompression.org/maiken/Why_Argon.htm
[Edited on 10-1-2015 by Bert]
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careysub
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| Quote: Originally posted by zed | | ...Since pressure is maintained, the low molecular wt. of Helium might serve to act as a form of insulation. |
Works the other way around.
According to the kinetic theory of gases heat transport is proportional to particle velocity and to molar heat capacity. Since argon and helium are
both monoatomic gases (with just 3 translational degrees of freedom) their molar heat capacity is the same so the heavier gas, having slower
particles, has the lower heat conduction (as Bert's chart shows).
CO2 has even better insulating properties (marginally) than argon, even though it is triatomic with additional degrees of freedom (for vibration and
rotation) and a correspondingly higher heat capacity (more than twice argon and helium) since it is so dang heavy.
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deltaH
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| Quote: Originally posted by careysub | | Quote: Originally posted by zed | | ...Since pressure is maintained, the low molecular wt. of Helium might serve to act as a form of insulation. |
Works the other way around.
According to the kinetic theory of gases heat transport is proportional to particle velocity and to molar heat capacity. Since argon and helium are
both monoatomic gases (with just 3 translational degrees of freedom) their molar heat capacity is the same so the heavier gas, having slower
particles, has the lower heat conduction (as Bert's chart shows).
CO2 has even better insulating properties (marginally) than argon, even though it is triatomic with additional degrees of freedom (for vibration and
rotation) and a correspondingly higher heat capacity (more than twice argon and helium) since it is so dang heavy.
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I have seen and felt this in action. An in situ IR cell I once used a long time ago consisted of a tiny ceramic pot surrounded by a heater coil. This
say at the centre of a much larger steel cylinder that had a IR transparent window so the beams could hit catalyst placed in the little pot. One could
then fluch the atmosphere inside by reaction gases.
When it was flushed with Ar or CO and reaction pot in the centre kept at a fixed and hot temperature, the outer metal cylinder still felt cool to the
touch. When flushed with hydrogen, however, the outer cylinder got really hot.
Hydrogen and helium have very high thermal conductivities for the reason careysub mentioned.
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careysub
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I can throw in another historical tidbit about this.
When Cousteau and company were perfecting really deep sea diving, using heliox mixtures, they discovered that their wet suits did not provide the
thermal protection they were used to. They had to develop active heating methods to compensate.
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Bert
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What's your source on Cousteau's work? I'd like to look at that myself.
If they were truly using wet suits, that's quite expected. The closed cell foam gets squashed thinner and thinner by pressure at depth, fits looser so
more water circulates- And the thinner foam has less insulating value too, of course. A wet suit that works well on the surface is pretty darn loose
by 150'.
If they were pressurizing dry suits with their breathing mix, blame the Helium... We use small Argon bottles to pressurize our dry suits around here
(upper midwest, for Great Lakes wreck diving).
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careysub
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It would have been in connection with the Conshelf 2 and 3 underwater habitats that Cousteau organized in the early-mid 1960s.
Two documentaries about his activities (World Without Sun and the World of Jacques Cousteau) were done after Conshelf 2 and 3 respectively.
I saw both of these 45 years ago, and have picked up additional information about heliox diving in the years since (I don't dive but have an interest
in undersea research, have visited several marine science labs, toured hyperbaric chambers, etc.).
I think they were pressurizing dry suits, as you say - sorry for my inaccurate terminology.
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zed
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So, igniting Thermite under vacuum, is counter productive. It slows the reaction.
And igniting Thermite under Helium might also temper the reaction, due to Helium's high thermal conductivity.
BUT, igniting Thermite under Argon might be useful?
[Edited on 16-1-2015 by zed]
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CHughes
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Careysub. I hope you're still on here. Can you tell me where you found the report that you posted the page from? I would really appreciate it. Thanks.
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careysub
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Still here.
Which page are you referring to, as I posted two.
There is the complete Wang report, if you are speaking of that one.
Of the other: just type "vapor pressure curves of the elements" into Google image search and take your pick. There are scores of them.
Attachment: wang1993.pdf (2MB)
This file has been downloaded 656 times
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