Joule Heating in Carbon Fiber w/ DC Current

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Discussion Overview

The discussion revolves around the calculations and considerations related to Joule heating in carbon fiber when subjected to a direct current (DC) in an argon environment. Participants explore the implications of resistivity, resistance, and the practical challenges of achieving high temperatures through resistive heating.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant calculates Joule heating using the formula I^2*R and expresses concern over an unexpectedly high power dissipation value of 14.2 megawatts.
  • Another participant clarifies the distinction between resistivity and resistance, emphasizing that resistance depends on the dimensions of the material.
  • A participant suggests that the heating will be limited by the current supply capability of the voltage source and requests dimensions to verify calculations.
  • One participant estimates the dimensions of the carbon fiber as approximately 2" by 3".
  • Another participant provides a calculation using a resistivity value and arrives at a power dissipation of 36 megawatts, indicating that the original estimate is in the right range.
  • There is a discussion about the need for current regulation due to the negative temperature coefficient of resistivity in carbon fiber, which could lead to thermal runaway.
  • Participants discuss the feasibility of using a current regulator with an adjustable current source to control heating.
  • One participant expresses the intention to use large blocks of graphite as electrodes and questions whether 35 amps would be sufficient to melt the carbon fiber.
  • Another participant suggests that using carbon-carbon contacts could avoid hot spots, while also recommending copper blocks for better heat dissipation.
  • Concerns are raised about the practicality of achieving high temperatures, with one participant suggesting that the project may be unfeasible due to the high power requirements and potential for melting electrical connections.
  • Participants discuss the specific heat of carbon fiber and the amount of energy required to reach high temperatures, with one stating that only 9 joules are needed to reach 3500 degrees Celsius.
  • There is a suggestion to consider alternative methods of heating, as achieving 3000 degrees Celsius through resistive heating may not be practical.
  • One participant shares a link to an experiment they wish to replicate, which involves heating carbon fiber to facilitate a reaction without a catalyst.

Areas of Agreement / Disagreement

Participants express various viewpoints on the feasibility and calculations related to Joule heating in carbon fiber, with no consensus reached on the practicality of the proposed heating methods or the specific requirements for achieving the desired temperatures.

Contextual Notes

Participants highlight the importance of considering the cross-sectional area of the carbon fiber, the limitations of current supply capabilities, and the potential for thermal runaway. There are unresolved questions regarding the specific dimensions and properties of the materials involved.

Warpspeed13
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Ok so as fare as I understand the equation for Joule heating in a material with a DC current is I^2*R.
I am trying to calculate for the heating of carbon fiber in an argon environment. Carbon fiber has a resistivity similar to graphite (roughly 5.0*10^-7) I am trying to calculate for 3.3VDc. However I am getting a massive value of 14291338.58 watts dissipated. To get the value of I I'm using the equation V/R=I. What am I doing wrong? I feel 14.2 megawatts is a bit off.
 
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You seem to be confusing resistivity, which is a property of a material, with resistance, which depends of the size and shape of a particular piece of the material. Resistivity and resistance are measured in different units.

That is rather like confusing the density of steel with the mass of a particular piece of steel.

See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html
 
Oh I need to edit that I factored in the dimensions of the material to convert resistivity to resistance
 
It doesn't sound off to me. The resistivity of CF is a very small number, the resistance value for a macroscopic CF conductor will also be a very small number.

In reality the heating will be limited by the current supply capability of your voltage source.

If you add the dimensions of the CF someone could check your calculation.
 
It was a guesstimate at around 2" by 3"
 
Giving better information will lead to better help.

I guessed it's a square bar with voltage applied across it's length and using :

ρ = 1 x10^-8 (Ωm)

For CF

(From wiki, as you didn't give units)

And

R = ρl/A

P=V^2/R

=> P = AV^2/ρl

I got 36 MW so you are in the right ball park.

Now all you have to do is find 3.3 VDC supply that'll put out 11 MILLION amps ;-)
 
Thank you for the help. I actually think I'll be building an adjustable current supply, I'd rather not fuse any atoms.
 
Power circuits with carbon elements often need current regulation in them because of the Negative Temperature Coefficient of the resistivity, which can give you thermal runaway if you aren't careful.
 
So would a current regulator paired with an adjustable current source allow me to gradually increase how hot it is?
 
  • #10
Warpspeed13 said:
So would a current regulator paired with an adjustable current source allow me to gradually increase how hot it is?

Yes.

CF is such a good conductor that unless your CF has a very small CSA most of the power will be dissipated in the connecting cables and the CF will not heat up much.
ie your wires will get hot and the CF will stay cool.
 
  • #11
billy_joule said:
Yes.

CF is such a good conductor that unless your CF has a very small CSA most of the power will be dissipated in the connecting cables and the CF will not heat up much.
ie your wires will get hot and the CF will stay cool.

Carbon filaments were commonly used for lighting before tungsten established itself as a better material. They worked at mains voltage (you can still buy them, in fact). They are pretty fragile, though, because of their necessary small cross sectional area.

Here's a historical link.
 
  • #12
Cool thanks. I'm going to be using large blocks of graphite to act as the cathode and anode and to sandwich the carbon fiber at both ends. Do you think with a maximum of 35Amps I could get the carbon fiber to melt? The melting point of carbon is 3500 degrees C. All in an inert environment of course.
 
  • #13
Using carbon / carbon contact could possibly be a good idea as it may avoid hot spots on contact but I would just have chosen copper blocks as they would dissipate any locally generated heat due to uneven contact. But under pressure, the copper would (?) mould around the fibres to give a good contact.
 
  • #14
Cool so the 35 Amps should be sufficient
 
  • #15
That's a good idea with the carbon carbon
 
  • #16
Warpspeed13 said:
Cool so the 35 Amps should be sufficient

It depends entirely upon the cross sectional area of your carbon.
 
  • #17
6" by 6"
 
  • #18
OMG
Your figure of MW in the OP sounds quite a reasonable answer - it's just an unfeasible project if you don't have a lot of money, I think. Source resistance will really prove to be a problem with such a low resistance load.
Would it be possible to take a step backward and to approach your requirement in a different way? Could you provide the electrical heating in a more conventional way, perhaps? But what is the CSA of your carbon Fibre? Aren't they very thin and not 6" X 6"?
14MW would melt your whole lab!
 
  • #19
Ya I don't want to get it nearly that hot it would vaporize I want like 3500 degrees Celsius so I won't be using nearly 14mw
 
  • #20
It would be a group of fibers in a cloth. I only need to get it to absorb about 9 jules in the form of heat to get it to 3500 degrees Celsius based off its specific heat
 
  • #21
Warpspeed13 said:
It would be a group of fibers in a cloth. I only need to get it to absorb about 9 jules in the form of heat to get it to 3500 degrees Celsius based off its specific heat

You need to consider heat transfer too...Objects that are 3500 deg C transfer a lot of heat to their surroundings...

You really need to do this:

sophiecentaur said:
take a step backward and to approach your requirement in a different way?
 
  • #22
Any ideas on how to get it that hot other than resistive heating? As fare as I know nothing except possibly some forms of thermite burns that hot. And anything but carbon would liquefy so I can't insulate it with anything.
 
  • #23
I don't know of any method.

If you can explain why you want to heat CF and what you intend to do to it once it's heated it may help.
Why do you want to heat a large piece of cloth?
 
  • #24
I have just been searching around for the specs of available kilns and furnaces. I couldn't find anything higher than 2k celsius. So aiming at 3k celsius is clearly not trivial. You are talking in terms of lamp filament temperatures and that is a seriously clever area of technology.
I think you would need to consider a much smaller scale and very short lived experiment, operating inside a muffle furnace, to increase the ambient temperature to something manageable. All your electrical connections would be very likely to melt / vaporise. A bit of a nightmare.
As billy_joule says, it may help if you tell us the actual purpose and context of this idea. You may be able to achieve what you want in another way.
 
  • #25
Hmmm we'll I want to replicate this experiment http://www.rsc.org/chemistryworld/News/2010/March/16031001.asp. It sounded interesting so I wanted to make some to incorporate in a radiation/ micro meteoroid shielding idea I had. I couldn't find what specific nickel catalyst they were using (in less of course they literally mean pure nickel in which case I'm over complicating things). My solution therefore was to use resistive heating to bring the material to the 3000 degrees centigrade required to make it react without a catalyst.
 

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