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Author: Subject: Simple exploding wire apparatus for nanopowders
Whathappensif
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[*] posted on 10-7-2020 at 23:18
Simple exploding wire apparatus for nanopowders


Has anyone set up a high power electrical discharge apparatus to make nanopowders?

According to this article, you need a current density of 104 - 106 A/mm2.

https://en.wikipedia.org/wiki/Exploding_wire_method

So doing some sums:
Resistivity Aluminum = 2.65e-8 Ω·m

Wire diameter = 0.1mm
Wire area = 7.9e-9 m2
Wire length = 10mm

Wire resistance = 0.034 Ω
Obviously all aluminum comes with a thin oxide coating, so the resistance will be higher than what I calculate

12V lead acid battery internal resistance = 0.01 Ω

Therefore discharge current = 274A
Current density = 34930 A/mm2 (exceeds the 10e4 value for the production of nano powders)

However, it seems this exceeds the maximum 10s discharge current of a lead acid battery, which is 25A.

https://electronics.stackexchange.com/questions/130580/what-...


So the question is whether a lead acid battery can provide 274A of current for <1s, and whether it can do so for what is a micro-batch process, for an extended duration, without killing the battery.

My idea is to extend a thin 0.1mm diameter wire through a holed plate (or even a titanium tube), offset from another titanium plate by a distance of 1cm. When the wire touches the other titanium plate, it completes an electrical circuit and in theory 274A of current gets dumped into it.

This apparatus would require:
1. Inert gas chamber + inert gas supply
2. Titanium plate x2, or titanium plate + >0.1mm inner diameter titanium tube
3. 12V lead acid battery
4. Advancement mechanism for the aluminum wire, which may initially be by hand

All are pretty easy to build/acquire, except for the inert gas supply.

exploding_nanowire_diagram.png - 11kB
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wg48temp9
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[*] posted on 11-7-2020 at 02:21


Your main problem is you need sufficient voltage to produce an arc. When the wire melts and forms a line of molten metal sphere an arc is required to continue the current flow. I doubt 12v would be sufficient to produce the arc. As per your wiki link its usual done with a charged capacitor and from my knowledge the voltage is usually in thousands of volts.
It may be possible to use an inductance in the circuit to strike and maintain the arc but I suspect it would be large (tens of kilo) if it has sufficient storage capacity and low resistance.

Car batteries are specified to have cranking currents of =>800A but at reduced voltage but you may need a lot of them in series.




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Sulaiman
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[*] posted on 11-7-2020 at 02:45


For a similar project I used the flash units from disposable cameras.
You can use multiple units in series and / or have multiple capacitors in parallel.
If you have access to old disposable cameras with flash units.




CAUTION : Hobby Chemist, not Professional or even Amateur
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Whathappensif
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[*] posted on 11-7-2020 at 03:13


OK, good point about needing to maintain the arc. Do you think keeping an electrical current flowing through the melted wire is essential to generating nanopowders? Otherwise the surface tension of the molten droplets will cause the metal to condense into micron or mm spheres? What is the resistance of the molten metal channel?

I found this article about making nanopowders by electrical explosion of wires. The key bit appears to be this:


Quote:

The tungsten and aluminium wires with diameter d = 0.2...0.3 mm and length l = 40...200 mm were used in the experiments. Electrical explosion of wires was carried out under conditions of “fast” explosion with an arc stage or with infinite current pause when almost all accumulated in capacitor energy was consumed by wire before explosion. Energy parameters of EEW were regulated with change of charging voltage and geometric characteristics of exploding wires. The specific electrical energy input in the wire (е) was changed from 0.4 to 1.8es (es is the sublimation energy of the wire material); the energy of the arc stage (еa) – (0.7...1.7)еs. O. Nazarenko Parameters of electric circuit: capacitance С = 2.25 μF; charging voltage U = 15...30 kV; inductivity L = 0.58 μH.





Attachment: nanopowders_by_electrical_explosion_of_wires_2007.pdf (215kB)
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wg48temp9
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[*] posted on 11-7-2020 at 04:48


Quote: Originally posted by Sulaiman  
For a similar project I used the flash units from disposable cameras.
You can use multiple units in series and / or have multiple capacitors in parallel.
If you have access to old disposable cameras with flash units.


You can use the mains power from a cooker outlet if you bypass the breakers or have an old fuse type distribution board. Be careful not to blow the line fuses. You can take pics of the exploding wire using the bulb setting on a camera. The explosion propels spinning blobs of molten metal some then spilt in two. You can also use the filament/electrode from a florescent tube. It produces a very bright flash.

You can explode the wire in water and it produce's a cavity which then collapses. That can break a small glass containers. That produces some powdered copper and small blobs of copper. It may be a simple (no inert gas required) method to start experimenting with. Perhaps paraffin oil could be used for aluminium.




i am wg48 but not on my usual pc hence the temp handle.

Thank goodness for Fleming and the fungi.

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wg48temp9
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[*] posted on 11-7-2020 at 06:49


Quote:
Quote: Originally posted by Whathappensif  


The tungsten and aluminium wires with diameter d = 0.2...0.3 mm and length l = 40...200 mm were used in the experiments. Electrical explosion of wires was carried out under conditions of “fast” explosion with an arc stage or with infinite current pause when almost all accumulated in capacitor energy was consumed by wire before explosion. Energy parameters of EEW were regulated with change of charging voltage and geometric characteristics of exploding wires. The specific electrical energy input in the wire (е) was changed from 0.4 to 1.8es (es is the sublimation energy of the wire material); the energy of the arc stage (еa) – (0.7...1.7)еs. O. Nazarenko Parameters of electric circuit: capacitance С = 2.25 μF; charging voltage U = 15...30 kV; inductivity L = 0.58 μH.


At 20kV thats about 450J. To get that amount of energy out of a car battery requires about 37.5A flowing for one second but its required in say 10ms so it would have to be 3,750A. It could be 1ms. 12 V cannot push that much current thru a thin bit of wire or an arc or perhaps not the stray inductance in 1ms.

PS Sorry I screwed up the quote formatting.


[Edited on 7/11/2020 by wg48temp9]




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[*] posted on 11-7-2020 at 13:07


Quote: Originally posted by wg48temp9  
Your main problem is you need sufficient voltage to produce an arc. When the wire melts and forms a line of molten metal sphere an arc is required to continue the current flow. I doubt 12v would be sufficient to produce the arc. As per your wiki link its usual done with a charged capacitor and from my knowledge the voltage is usually in thousands of volts.
It may be possible to use an inductance in the circuit to strike and maintain the arc but I suspect it would be large (tens of kilo) if it has sufficient storage capacity and low resistance.

Car batteries are specified to have cranking currents of =>800A but at reduced voltage but you may need a lot of them in series.


A fat inductor would easily maintain an arc at 12 v, though perhaps not 10 mm, but a quarter or third of that may work. For a 10 mm current sustained arc, I would expect to need at least 40 V. Combine that with ~10 mH and you should be OK. But then you may run into issues with the current rising too slow instead...
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[*] posted on 11-7-2020 at 15:23


You may have already read it, but if not you may find the following article by Steve Hansen interesting:
http://www.belljar.net/Exploding_Wires.pdf

Among other things, it describes the requirements for doing this, along with a circuit.




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[*] posted on 19-8-2020 at 06:52


Hello,

So I might be able to help you with this. I don't have an inert atmosphere but I can try things out in the open air. If I understand your requirements:
- High enough voltage to bridge the gaps that are created when the metal 'melts'.
- High enough current to vaporize the metal wire. (34930 A/mm2)
- Metal wire
- Material: Al
- Diameter: 0.1mm
- Area: 7.9e-9 m2
- Length: 10mm
- Resistance: 0.034 Ω

The combination of current and voltage is something that won't be easy to do with batteries.
So I will suggest to use capacitors.

You need to overcome the resistance of the complete circuit.
Capacitor internal resistance + resistance of wires used to connect everything + resistance of the 'exploding' wire.
All this has also some inductance, this will resist the change in current.
Therefore, when the capacitor is connected the current needs some time to rise.
Meanwhile, the capacitors are discharging while the current is rising.
Now there will be a point where the wire melts and the circuit is broken, you want to reach a high enough current before that point.

This can all be calculated using some differential equations.
Search for "RLC circuit discharge" or something like that.


I do have 3 big capacitors:
- 4700µ @3x => 14.100µF
- 350V @3x => 350V
- 35mOhm @3x => 11.67 mOhm

If you are interested I could do a couple of discharges trough some small wire at different voltages.


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[*] posted on 19-8-2020 at 09:02


I think a car battery will be fine to produce the amperage in most cases. I'm a little confused as to the length of time you need for the discharge, you say something about "exceeds the maximum 10s discharge current of a lead acid battery, which is 25A." I'm not sure what you are saying, maybe the max discharge for 10 seconds is 25A? That is certainly not true.

You can easily do full current of 250-400+ amps (depending on battery) for 10 seconds in most batteries.

You have other options as well such as the lithium ion emergency jump start packs or even the traditional lead acid ones (these have a 17 or 18AH battery usually and they can put out some serious amperage for their size).

You can also run a couple of the emergency battery packs in series to get the higher voltage to sustain an arc.
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[*] posted on 24-8-2020 at 17:56


I'm a bit sceptical. Wikipedia doesn't say the parameters for making nanoparticles, just the levels to create an "exploding wire" in general. 10^4 to 10^6 A/mm^2 only sounds like 2 orders of magnitude difference, but for any given material the power density will vary by 4 orders of magnitude. The duration of the process is between 10^-8 and 10^-5 seconds.

I'll try to find the real parameters for making nanoparticles in a little while, 10 minutes, maybe a week.

For now, Ill just say that a 100um 1cm aluminum wire weighs maybe 0.2 milligrams, based on cross sectional area density and length. Aluminum has a specific heat capacity of about .9j/gm-K and a heat of melting of about 400 j/gm. Assuming a temperature rise of 500K to melt it, that means about .18 j will have to be delivered to even melt the wire completely. With the power output of your theoretical setup, the wire will be partially solid until 5.5*10^-5 seconds, missing the very slowest time criterion. And that's before we add in the rise in resistance that happens when a metal gets hot.
I'm ignoring the 25kwh/kg requirement since that's probably as variable as the time and current, but if that rule applies, that slows everything down by another 2 orders of magnitude.




I now have a YouTube channel. So far just electronics and basic High Voltage experimentation, but I'll hopefully have some chemistry videos soon.
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