Sciencemadness Discussion Board - Suitable material for holding a plasma

Suitable material for holding a plasma

Fusionfire - 18-10-2011 at 00:09

Hello folks,

I am trying to assess the budget of how much it would cost to build the ducting of a high temperature plasma generator.

The basic idea is to start with an off-the-shelf propane torch to get around 1600-1800C (accounting for losses) and inject vaporised lithium and boron into the airflow.

Lithium metal which is cheaper to get than boron will initially react with oxygen, producing an oxygen deficient atmosphere while yielding 43.13 kJ/g in accordance to:
4Li + O2 → 2Li2O ΔfH° = -598.73 kJ/mol

Subsequently powdered boron will be injected into the flow, reacting with nitrogen in an anaerobic flow while yielding 23.21 kJ/g, in accordance to:
2B + N2 → 2BN ΔfH° = -250.91 kJ/mol

Burning boron the entire time would be thermodynamically ideal but expensive.

A later design may use oxy-acetylene instead of propane/air.

So the design requirements are:
1) Melting point in excess of 3000C
2) Chemically stable to attack from elemental boron and lithium dusts/gases.
3) As sensitive equipment will be next to the plasma generator it is preferable that the duct walls are not thermally conductive (pretty much narrows it down to the refractory ceramics and glasses).

IrC - 18-10-2011 at 04:31

Tantalum hafnium carbide is a ceramic with a melting point of 4215 C - the highest known. Good luck finding some. Or something on this page?

http://www.azom.com/article.aspx?ArticleID=5127

AndersHoveland - 18-10-2011 at 04:57

The plasma may still be highly corrosive to the walls, even below the melting point.
Sometimes plasmas are seeded with potassium compounds to lower the plasma temperatures.

Wizzard - 18-10-2011 at 05:24

Superconductive magnets. Or at least AlNiCo magnets (better heat tolerance). Plasmas are conductive, and if you can shape a very strong field, it will flow along it (and not along the walls necessarily).

watson.fawkes - 18-10-2011 at 06:27

Quote: Originally posted by Fusionfire

I am trying to assess the budget of how much it would cost to build the ducting of a high temperature plasma generator.

Why is it that you're seeking a chemically-generated plasma rather than a electrically-generated one? Your eventual goal could greatly affect your engineering choices.

If all you want is the flame, use a large steel chamber and a high speed vacuum pump to evacuate the reaction products. You'll need to refrigerate the structural elements, or at very least address where the heat goes.

Fusionfire - 18-10-2011 at 10:10

Quote: Originally posted by watson.fawkes

Quote: Originally posted by Fusionfire

I am trying to assess the budget of how much it would cost to build the ducting of a high temperature plasma generator.

Because most electrical energy you get off the grid is ultimately chemically generated (unless you live in certain areas like hydro-power rich Norway or near nuclear reactor rich Japan).

Chemically generated plasmas are much more convenient, compact and portable

. Boron has among the highest energy density of elements, too bad it is so expensive. Lithium has a more -ve oxidation enthalpy per gram of reactant than even magnesium. I haven't done an exhaustive survey but:

4B + 3O2 → 2B2O3 ΔfH° = -1271.94 kJ/mol
Heat per gram of B = 58.83 kJ/g

2Mg + O2 → 2MgO ΔfH° = -601.24 kJ/mol
Heat per gram of Mg = 24.74 kJ/g

4Li + O2 → 2Li2O ΔfH° = -598.73 kJ/mol
Heat per gram of Li = 43.13 kJ/g

Problem with lithium is that is sucks burning in anaerobic conditions once the oxygen is depleted...

6Li + N2 → 2Li3N ΔfH° = -164.56 kJ/mol
Heat per gram of Li = 7.903 kJ/g

watson.fawkes - 19-10-2011 at 04:39

Quote: Originally posted by Fusionfire

Chemically generated plasmas are much more convenient, compact and portable

Frankly, by the time you deal with an oxygen tank (to get your temperature up and avoid heating argon), chemical storage, metering devices, etc., you're not talking about a compact unit. Commercial plasma-based metallizing guns have small guns, but the supporting gear requirements aren't small at all. I do expect that you'll get a more compact and portable unit using an engine-powered generator and a few jerrycans of fuel.

Frankly, though, I feel you've dodged the question. You've talked about secondary properties (convenience, etc.) not about the primary purpose. What do you want to do with the plasma, regardless of how it's sited or transported?

Neil - 19-10-2011 at 05:53

Have you looked at plasma furnace technology? They have a variety of ways of containing the plasma without using crazy materials, some such as cool air vortex methods may be in reach of home science.

Neil - 19-10-2011 at 05:56

Have you looked at plasma furnace technology? They have a variety of ways of containing the plasma without using crazy materials, some such as cool air vortex methods may be in reach of home science.

metalresearcher - 19-10-2011 at 08:51

What about an electric arc furnace using a welder from the hardware shop around the corner ? You will get temperatures over 3000oC this way. The arc is a plasma which you can flush with argon gas.

Fusionfire - 23-10-2011 at 13:51

Quote: Originally posted by watson.fawkes

What do you want to do with the plasma, regardless of how it's sited or transported?

I want to inspect the augmentation of thrust on a test stand using powdered metal/metalloid fuels for afterburning, rather than JP8.

Which is why I am also asking about propane torch exhaust conditions. Some metal/metalloid fuels will also burn in the presence of water vapour or carbon dioxide; 19% oxygen is also great.

Sedit - 23-10-2011 at 17:37

Magnetic bottles as they are so called is your best (only)option by far. There is no need to mess around with anything else because as your experiments progress and temperatures become higher the ability to find suitable material will become impossible sooner or later. You may as well start on the right path now before you have to back track down the road when you have already made forward progress.

Fusionfire - 24-10-2011 at 07:19

Quote: Originally posted by Sedit

Magnetic bottles as they are so called is your best (only)option by far. There is no need to mess around with anything else because as your experiments progress and temperatures become higher the ability to find suitable material will become impossible sooner or later. You may as well start on the right path now before you have to back track down the road when you have already made forward progress.

I have thought about that but don't you still need a good heat shield between the permanent or electromagnets to protect them from conductive, convective or radiative heat?

It seems magnetic lensing or plasma filamentation approaches augment but do not replace high temperature refractories in plasma chambers.

Neil - 24-10-2011 at 14:18

A plasma spray gun?

Edit: I just noticed that it double posted up above, if some kind hearted admin would delete one of them I'd be grateful.

[Edited on 24-10-2011 by Neil]

Sedit - 24-10-2011 at 14:27

Thats funny Neil, What you just posted is what I do for a living yet our guns work slightly different except the 4000 gun which is exactly what you posted.

Fusionfire - 24-10-2011 at 14:30

Thanks Neil. But how will the above topology produce thrust augmentation by afterburning?

AIUI the afterburning has to be within the engine cavity to produce additional thrust by a pressure rise + faster ejected flow.

If the fuel is dumped outside the engine (e.g. "dump and burn") then it produces no thrust augmentation.
http://en.wikipedia.org/wiki/Fuel_dumping#Dump-and-burn

Neil - 24-10-2011 at 14:48

@Sedit Haha Ironic, You've got a wicked job.

@ fusionfire

Spray the fuel of choice into the engines exhaust gases, just replace a fuel nozzle with a plasma injector nozzle. A conventional plasma cutter with some kind of venturi may even work - Ask Sedit!

There are lots patents pertaining to exactly this in SCRAM jet starting and fuel seeding tech. You may even find the same for afterburners. I know I've seen articles on plasma igniters and seed plasma flame enhancement... If I was on my home computer I'd even have some at hand to link too.

So long as it is cooled; copper, alumina and tungsten may be all you need. No reason for you to keep a 'jug' of plasma sitting around. If injection is the game then it might as well be your aim.

If you get a afterburner to run on silicon carbide let me know, it was a childhood dream of mine

Sedit - 24-10-2011 at 16:27

Quote: Originally posted by Neil

@Sedit Haha Ironic, You've got a wicked job.

@ fusionfire

Spray the fuel of choice into the engines exhaust gases, just replace a fuel nozzle with a plasma injector nozzle. A conventional plasma cutter with some kind of venturi may even work - Ask Sedit!

LOL naaa don't ask Sedit, just because I do it don't make me an expert, I just started a few months back and I have only so far worked with the 3000 gun which is a duel Nichrome wire ( or whatever we are using to coat) feed where the arc is generated between the two wires and they are vaporized using electricity and blasted onto the surface using high pressure air or argon depending on the material being used. The 4000 gun is the one mentioned above and due to its light weight, ease of use, and relatively low efficiency that is for the higher ups or very special jobs like when we need to coat something with Titanium Nitride or another insainly hard material or ceramic.

PS: Its not a glamorous job as it sounds either, it is the hardest most grueling back breaking work I have ever done in my entire life and I have been in the construction industry for over a decade now. This job is a total nightmare other then the pay

[Edited on 25-10-2011 by Sedit]