Liquid non metal conductor for railgun

[zappyguy111]

Hey all!

I've return back to my year 12 assignment, the railgun. I got a working model back then, but unfortunately didn't write up a good report. I have the videos on youtube if anyone is interested.

The reason for the conductive liquids is because I want to tackle the arcing issue.

I won't have enough power to simply force out the arcs and the quality of the rails isn't sufficient to place the gun under vacuum to make a plasma armature. So I have considered the next best thing, conductive liquids. Yet the problem is that a conductive oil is far, far too expensive, water solutions will cause the copper rails to corrode and metals are poisonous to inhaled, I was wondering if there was a conductive liquid that doesn't have the above disadvantages?

Zap

 

[dotancohen]

Have you looked at anti-corrosive/anti-boil additives for water? You can buy them by the gallon at many auto parts stores. Note that I am _not_ talking about anti-freeze (which also has corrosion inhibitors) but anti-corrosive additives which are designed for use in copper radiators.

 

[0xDEADBEEF]

Why do you say that liquid metals are toxic? Gallistan isn't http://en.wikipedia.org/wiki/Galinstan I do not know how much it costs though.

 

[zappyguy111]

Have you looked at anti-corrosive/anti-boil additives for water? You can buy them by the gallon at many auto parts stores. Note that I am _not_ talking about anti-freeze (which also has corrosion inhibitors) but anti-corrosive additives which are designed for use in copper radiators.

Yes I do recall buying some of this for my cooling system after I rusted out my thermostat and its housing... :P
I don't know why I didn't think of that earlier!

And in respects to galinstan I can't find any places to buy it, but I don't think it would be cheap. Especially adding in the fact that I live in Australia and we don't have access to the variety of stuff they have in the USA.
One of the downsides of living in a country with a low population density in the middle of no where.

But I will be certain to do some research and some hydrolysis experiments regarding the anti rust.

Many thanks!
Zap

 

[Dmytry]

I won't have enough power to simply force out the arcs and the quality of the rails isn't sufficient to place the gun under vacuum to make a plasma armature.

What do you mean? Arcs are plasma, and arcs are very conductive.

 

[zappyguy111]

But the process of getting the gas hot enough to generate a plasma is not, to make a low resistance plasma it needs to be at a very low pressure AND room pressure plasmas have the unwanted factor of melting the rails acting some what like a spot welder.

 

[Dmytry]

But the process of getting the gas hot enough to generate a plasma is not, to make a low resistance plasma it needs to be at a very low pressure

well actually at very low pressure you won't be able to have high current density.

AND room pressure plasmas have the unwanted factor of melting the rails acting some what like a spot welder.

Well, yes. You can try shaping your projectile so that the force expands the projectile to remain in contact with the rails to minimize the arc heating. Also, you can pre-accelerate the projectile pneumatically to eliminate the spot-welding on the start.

Bottom line is, you need to pass a lot of current, through small surface area. If you look at conductivity of liquid non-metals (electrolytes), you see that them are nowhere near conductive enough, a couple orders of magnitude short. E.g. some table:
http://www.smartmeasurement.com/en/wizards/flowmeter/flmtr_mag_conductivity.asp
0.146 siemens/cm for some potassium hydroxide solution, meaning that for distance 0.1cm and surface area of 1cm^2, you have the conductivity of 0.146*10 siemens , i.e. resistance of 1/1.46 ohm, which at current of 1000 amps will be a voltage drop of 680 volts and the heating power of 680 kilowatts.
You can probably have more luck shooting your projectile by exploding the liquid with current.

 

[zappyguy111]

Pressure inside a fluorescent lamp kept at 0.003 atmospheres and have the problem of excessive conductivity?

Regardless of how hard you try to force the projectile inside the rails there will always be arcing according to research done by Sam Barro.
http://www.powerlabs.org/emguns.html"
Also, forcing the projectile against the rails will lead to friction, causing similar effects to arcing.

If I could find a liquid with a low enough conductivity so that evaporation due to massive heating will not be a major factor, corrosion resistant properties to protect the rails against corrosion caused by the surge of ions and still be dirt cheap (I'd be over optimistic if I expect get all three) that would be absolutely fantastic. But it seems I'd have to use liquid metals...

On a lighter note:
An electrically powered steam cannon you say? :P AWWWEEE YEAH!

Oh yes,
Just somethings I found while researching.
http://www.dow.com/ethyleneglycol/about/properties.html" ) is not suitable as a film of 10 microns will have a resistance of at least 10KOhms...

It appears that I'll be returning these copper bars to they're prior owners and moving onto more civil projects... A digital peltier temperature control device for my fish tank with light control... :D
-or-
I could use high frequency oscillations, insulate the rails and use the magnetic field from the changing electric field to propel the projectile... BUT high frequency implies the use of semiconductors... very big and very, very expensive semiconductors... progresses to place face in hands.

Thank you for the link! I'll be certain to hold onto it for future projects.

Zap!

 

[Dmytry]

Pressure and plasma conductivity, that has to do with number of ions per volume... look at how ark lamps have to use high pressure gas to be able to achieve high current density.
The coin shrinker
http://www.capturedlightning.com/frames/shrinkergallery.html
uses in air spark gap to power the shrinker. The coin shrinker shots off fragments of wire at very high velocity, it is a quite dangerous thing. Air spark gap does actually have VERY low on resistance when its under high current.

I think the arcing is something you have to live with... DARPA build some serious big railgun but it still has problem with rail erosion and such.
http://boingboing.net/2011/04/20/railgun-fires-hugo-a.html
Ahh, by the way... even if you use liquid metal, it may not be able to wet the rails fast enough.

Some interesting experiment I did with high voltage a while back (unrelated to railguns). Big high voltage capacitor connected to the air spark gap in series with underwater spark gap. A small high voltage capacitor connected across the air spark gap. All connections really thick wire. Regular water, not de-ionized. Sequence of events: spark gap fires, small capacitor very quickly warms up the plasma, the big capacitor gets connected to the water, and you get underwater spark, extremely high pressure pulse. The shockwave from that spark cracks glass nearby, etc. Also shoots out the water from the cup some. I was trying it with spark energy of just few joules, and it made interesting tiny cocentric spherical cracks in a piece of window glass held next to the spark.

 

[Bob S]

I have read about liquid sodium-potassium jets being used on the mega-amp currents in large homopolar generators, like

http://en.wikipedia.org/wiki/Homopolar_generator

Apparently, special solid brushes were found to be acceptable substitutes. See

http://www.nature.com/nature/journal/v204/n4963/abs/2041079a0.html

http://books.google.com/books?id=v9...omopolar generator liquid sodium jets&f=false

Bob S

 

[0xDEADBEEF]

Wow NaK is vicious stuff igniting spontaneously and cannot be extinguished with water. Who would work with that if you are not forced to.