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Which metal for an Electrothermal gun?

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LAN_403

kromsson

Fri Jun 26 2009, 10:33AM

Registered Member #1444 Joined: Tue Apr 15 2008, 06:55AM
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Posts: 20

Which is the best metal to use for vaporising?

I understand that most hobbyists use aluminum (esp. foil) but this maybe a poor solution.

Aluminum needs 294 kJ/mol to vaporize, and the vapors would reach pressure of 1 Pa at 1482K.

Lead seems much more promising, needing only 180 kJ/mol to vaporize, and a would have a pressure of more than 1 kPa at the above temp (1482K).

More research might give even better metals for this job - only problem is lead toxicity, of course...

Electroholic

Fri Jun 26 2009, 01:16PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Al is turning to Al2O3 also.

kromsson

Fri Jun 26 2009, 03:57PM

Registered Member #1444 Joined: Tue Apr 15 2008, 06:55AM
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Posts: 20

The more I think of it, the more I realise that the importance of this matter.

The most important aspect of an ETG design is the piece of metal (or other material) that will vaporize.

The energy required for its vaporization, the pressure of the gas, and even the shape it will have before vaporization (you want the most of it turned into steam, if some part stays in solid form you get less pressure).

Does anyone have any info, experimentation, literature, whatever?

DYI

Mon Jul 06 2009, 04:16AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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If you are using any metal as the main propellant in your ETG, then you are going to run into trouble. Metals have high molecular weights, and they don't like staying in gaseous form. The propellant in an ETG should be something that will disassociate into a gas with a high SOS, to minimize the pressure gradient between breach and projectile base. This will maximize efficiency for a given energy input, and allow the gun to reach higher speeds. Water and some plastics, like polyethylene, are suitable propellants. I imagine that acetone and methanol might work as well, but you'd have to look into the enthalpies of vaporization and disassociation to be sure.

Remember, vaporizing the metal just gives you the plasma jet (if done properly, at least...). After that, you need to direct the plasma jet at the working fluid to convert it to a GAS, which you then use to propel your projectile. If the actual propellant gas is all plasma, the barrel won't last long regardless of what it is made of, and efficiency will be awful because of heat loss (i.e., under 5%, perhaps a lot less depending on the design). In small ETGs, using only a few kilojoules, design is tricky, as your working fluid will consist of much less than a gram of material. In this case, simply coating the chamber walls with it is probably an acceptable solution. As my tests have shown me, the capillary tube plasma generator is an absolutely essential part of good ETG design, as is the presence of a working fluid to absorb the very high temperature of the plasma.

kromsson

Mon Jul 06 2009, 05:57AM

Registered Member #1444 Joined: Tue Apr 15 2008, 06:55AM
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Posts: 20

I understand that you are right.

By vaporizing a metal, the projectile will start to move, leaving a LOT of more room for the metal gases to go to the solid metal state again thus destroying the efficiency and the barrel.

Water, I understand, is a very good solution instead of metals. But plastics ?!?! I would tend to think they will create a mess inside the barrel after they become solid again, let alone they will not totally evaporate as they are a bad conductor of electricity...

DYI

Mon Jul 06 2009, 03:58PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Water is a dielectric material just as much as plastics are. The conductive properties do not matter, as electricity does not pass through the working fluid. Basically anything you direct the plasma jet at will vaporize to some degree, it's really a matter of configuring the geometry so that it will vaporize as fully as possible. However, the only paper I've read that did a comparison test between water and a plastic (polyethylene in this case), found water to be easier to work with, and to achieve a greater degree of disassociation than the plastic did. Remember that in the ETGs most of us can build, the working fluid will be less than 100mg of material.

rp181

Tue Jul 07 2009, 01:09PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

With very high temperatures, water breaks down to hydrogen and oxygen. Wouldn't that mean the H and O would just combust with the heat, and go back to the water stage, and repeat, until the temperature is too low, and than turn to steam? That would mean more water, but it would be very hard to tell you need more water (or would it?).

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