Graphite for spark gaps? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Graphite for spark gaps?

1 2 3 next

Move Thread

LAN_403

jpsmith123

Sat Feb 16 2008, 04:01AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Hello,

I'm wondering if anyone here has tried using graphite as spark gap electrode material, and if so, how did it work? Thanks.

Chris

Sat Feb 16 2008, 04:49AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Graphite burns fairly readily in air at high temperatures, and it will get really hot since it is a poor thermal conductor. Even under inert gas it sublimes significantly. The moral of the story is, it wears out faster than tungsten or other refractory metals, but it's much cheaper too.

Bjørn

Sat Feb 16 2008, 06:19AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I don't think it will work very well for tesla coils. The characteristics of the gap will change a lot when it heats up and starts to wear.

Dr. Dark Current

Sat Feb 16 2008, 07:13AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

yeah, there were "carbon arc lamps" where the carbon burned out in 30 minutes...

jpsmith123

Sat Feb 16 2008, 04:31PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I'm not sure if a carbon arc lamp would be useful for comparison with a tesla coil spark gap or that of a Marx generator. The reason I say this is because carbon arc lamps are low voltage, high current devices, whereas tesla coils and marx generators are high voltage, low (average) current devices; and electrode wear is usually characterized in terms of mg of material lost/coulomb of charge transferred.

I've seen papers claiming that graphite is a very good material to use for a spark gap (see attached link), and I remember seeing commercial high energy spark gaps with graphite offered as an option to the nomally used copper-tungsten alloy. Since this goes against my intuition, that's why I'm wondering what experimental results people have been getting with them.

Firnagzen

Sun Feb 17 2008, 01:01AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Weeeell, my opinion is that it's a bad idea. I decided to use a mechanical pencil lead (graphite+clay) as an electrode for my ZVS flyback, and once the arc struck, the flame chewed up the electrode in seconds. Pencil lead isn't pure graphite, but then, pure graphite will burn even better.

Proud Mary

Sun Feb 17 2008, 10:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you are minded to give it a go, why not try using carbon welding electrodes, which have copper jackets to which one can readily solder?

HV Enthusiast

Sun Feb 17 2008, 03:54PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Graphite will not work for the reasons already stated.
Tungsten works best, but difficult to work with and not easily obtainable.
Other than that, just regular metal is fine.

Kizmo

Sun Feb 17 2008, 05:54PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Tungsten is quite easy to get, just buy it from welding equipment supplier as TIG welder electrodes (green ones are pure tungsten). They cost around 3-10EUR / piece depending thickness (1-4mm)

jpsmith123

Sun Feb 17 2008, 10:24PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

@Harry: Yes I suppose the only way I'll find out for sure is to rig something up and test it. What I'm aiming to build is actually a type of Marx generator and not a tesla coil. I posed the question in this particular group as I thought a 'coiler the most likely species of experimenter to have tried something with graphite.

I didn't build it and try it yet myself partly because, as I presently envision the design, there will be some investment in tooling to shape the graphite into a spherical or hemispherical shape. Thus I thought I'd ask first and try to get as much anecdotal information as possible before spending time and money.

@EastVoltResearch & Chris, I don't mean to sound rude, but I'm wondering if your opinions that graphite is a poor material for a spark gap electrode are based upon personal experience with it as such, or are you basically speculating? (All opinions are welcome and appreciated).

As I've mentioned, I've seen high power commercial spark gap switches offered with an option of graphite rather than copper-tungsten alloy material, and there have been several papers published which indicate that graphite may be superior to other materials, at least in certain cases. For example, the following study apparently found graphite to have a lower erosion rate, everything else being the same, than stainless steel or copper-tungsten alloy.

Electrode Erosion of a High Energy Impulse Spark Gap Switch
Yao Xueling et al 2005 Plasma Sci. Technol. 7 3157-3160 doi:10.1088/1009-0630/7/6/021

Yao Xueling1,2, Zeng Zhengzhong1 and Chen Jinliang2
1 Northwest Institute of Nuclear Technology, Xi'an 710024, China
2 School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China

Based on the principle of thermal conduction, three metal alloys (stainless steel, copper-tungsten and graphite) were chosen as the material of the high impulse current discharging switch. Experimental results indicate that the mass loss and surface erosion morphology of the electrode are related with the electrode material (conductivity Ïƒ, melting point Tm, density Ï and thermal capacity c) and the impulse transferred charge (or energy) per impulse for the same total impulse transferred charge. The experimental results indicate that the mass loss of stainless steel, copper-tungsten and graphite are 380.10 Î¼g/C, 118.10 Î¼g/C and 81.90 Î¼g/C respectively under the condition of a total impulse transferred charge of 525 C and a transferred charge per impulse of 10.5 C. Under the same impulse transferred charge, the mass loss of copper-tungsten (118.10 Î¼g/C) with the transferred charge per impulse at 10.5 C is far larger than the mass loss (38.61 Î¼g/C) at a 1.48 C transferred charge per impulse. The electrode erosion mechanism under high energy impulse arcs is analyzed briefly and it is suggested that by selecting high conductive metal or metal alloy as the electrode material of a high energy impulse spark gap switch and setting high erosion resistance material at the top of the electrode, the mass loss of the electrode can be reduced and the life of the switch prolonged.

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob