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Igniting Magnesium

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DerAlbi

Mon Jun 17 2019, 10:57PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Guys, i would like to check if magnesium could be used as a structural material that supports power electronics.

Does anyone know if an exploding IGBT or SCR or a short circuit of a capacitor bank can ignite magnesium?
The thought is, that a magnesium heat sink would have a larger surface area and higher heat capacity compared to an equal-weight aluminum heat sink. Nop worries, i understand that the heat sink it self does not spread the heat as well.

..and there is an obvious risk like exploding components or a short circuit on the heat sink itself.

I only know from school that the activation energy to ignite magnesium requires a continuous flame even on a small low mass strip. I just wonder if, for example, a capacitor bank short circuit can have a high enough local energy density that the self sustaining reaction could be started :-/ That would be a no-go.

2Spoons

Mon Jun 17 2019, 11:05PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

A sustained arc might do it, but I doubt an exploding igbt would have enough energy to get the metal hot enough. It really depends on the form of the metal - thin ribbon is relatively easy to ignite but a solid block is a different story. A heatsink is designed to dissipate heat so I'd expect that to be even harder to light.

DerAlbi

Tue Jun 18 2019, 08:43AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I understad your argument. On the other hand, if a wire touches a magnesium block with a 1kJ, 1kV capacitor bank behind it, does it really matter? Magnesiums heat transfer capability issnt that great and in the end the contact point is so stupidly hot that... well what I want to say is: this is how welding works. It is all about energy density at some point - independent of material properties.

I agree on the expoding silicon - not enough heat transfer from the burning plasma to the metal.

But i dont know, if magnesium would ignite from a direct short circuit event. After all an arc would be hot enough to ignite the contact point and explode metall away (as even copper vaporizes) - in my mind this should be enough to ignite the flying metal pieces.
But the surrounding metal of te stock material wouldnt be preheated.. the question becomes if the oxidation can become self sustained under such conditions.

Lets say i talk about at least 3mm thick magnesium sheets..

Edit:
Well.. an article about magnesium welding:

I mean.. they of couse recommend inert gases as shield, but no single word is spent on dangers of fire/ignition. Is it really such insignifant problem? Would be nice ^_^

Sulaiman

Tue Jun 18 2019, 10:45AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Igniting fine magnesium powder is very easy
magnesium ribbon ignites fairly easiy with a candle flame or similar,
a solid lump of magnesium is surprisingly difficult to ignite.
(and even more difficult to extinguish)
Magnesium alloys are widely used in all kinds of vehicle with very few fire problems.
but as significantly;

. a magnesium heatsink would need some kind of surface passivation as magnesium oxidises (as in rusts) quite quickly in air,
and the elevated temperatures of a heatsink would increase the corrosiom rate.

. (thermal conductivity)/(density) for Mg and Al only differ by about 2% but Mg is significantly more expensive than Al.

DerAlbi

Tue Jun 18 2019, 11:21AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I just talked to a company (for high pressure die casting) and they said that the same part made from Al vs Mg is roughly the same price. I havent asked if their Al-technology is vastly too expensive however

I understand that the heat sink performance sucks, but in an application where you require thermal capacity, Mg outperforms Al by 15%. Alternatively the gain can be translated into a weight reduction. The same amount of mass would have potentially a higher surface area because the volume of Mg is just that much more. On the other side the heat wont spread that nicely in Mg. I understand the tradeoffs i guess. I am just really courious about the danger-aspect. But as you said: Mg seems to be used in aircrafts and everything. If it was a real problem, this wouldnt happen i guess.

Thank you all for your input

radiotech

Fri Jun 28 2019, 02:49PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

In an electrical arc explosion, damage occurs because of an
extremely rapid rate of rise of temperature of air.

It is enough to hurl iron cases fairly far and blow open doors of
switch gear.

These facts I know, from much time maintaining decrepit electrical systems.

Hot air in such cases includes super heated steam. (above mentioned was
also in boiler houses)

My ancient chemistry text shows:

Magnesium will burn brightly if heated in a current of steam.

Signification

Tue Jul 02 2019, 05:50AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@Albi,

I thought you wrote: "IGNITE MAGNETISM". It DID make me think wild thoughts!

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