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Resistivity of graphite sheets?

  1. May 26, 2016 #1
    I'm trying to calculate the possible resistance of a graphite sheet that I'm using to coat surfaces of metal conductors, protecting them from wear, welding, and other possible failures. I can't seem to find an accurate value for the Resistivity, some sites state it's 7.8E-06, while others 3 to 60E-05 ρ(ohm m). I'm not certain.

    I don't have the material and instruments to measure it either. Do any of you know the approximate value? Or a good reference that cites it?
     
  2. jcsd
  3. May 26, 2016 #2
    Surface resistivity or volume resistivity?
     
  4. May 26, 2016 #3
    Current will be flowing throughout the whole volume, and the contact will be on the surfaces.

    This diagram is perfect in helping me explain a bit more:

    bulk-resistivity-vs-surface-resistivity.jpg

    The left diagram is exactly the same configuration of the graphite sheet, where the top/bottom surface is connected to conductive metals, also the current is flowing in that manner.
     
  5. May 27, 2016 #4
    Depends on the amount of impurities in the graphite sheets.

    So if I were you in would assume a number of values to assess the sensitivity of the resistivity of graphite to your work. Then source the best, in terms of purity, material to suit your work
     
  6. May 31, 2016 #5

    Baluncore

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    It will depend on orientation, structure and fabrication of the sheet. How is it made, or where do you get this graphite sheet ?
     
  7. Jun 3, 2016 #6
    It would seem to me the supplier of the sheet should have this data handy.
     
  8. Jun 3, 2016 #7

    Baluncore

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    Maybe the OP has not realised that graphite is a sheet material at the molecular level and so will have anisotropic resistance.

    The conductivity will depend on the orientation and contacts between individual molecular sheet particles within the bulk fabric. If the material is flexible then it is certainly NOT 100% solid graphite. We have no idea if the sheets being used are a thick fabric or a thin surface dusting.

    If air and circuit inductance are present then I would be surprised if an arc would not start and burn some graphite. That would produce an explosion of combustion products that might contaminate the surface of nearby insulators with a destructive conductive coating.
     
  9. Jun 8, 2016 #8
    @Baluncore A potential product is this, at a smaller area, and indeed it is flexible the manufacture states:
    The reason why I'm trying to use graphite sheets over brushes is for a higher current carrying capacity, due to the low thickness(or L) and large area it would allow a higher current capacity, over a graphite&mix brush. From my basic understand of Magnetoresistance, having a strong magnetic field will reduce the resistivity, and since the sheet is perpendicular to a magnetic field I think it does? That helps the main goal. I'm still studying all the aspects related to the use of this sheet, I will contact the supplier for further data. Also, using the sheet in this manner.

    My major concern is calculating the pressure between the contact surfaces for a complete conductive path(0 gaps) and from that the wear the sheets would have. I worry that the sheet would heat up(do due to friction and dissipated power) and begin to wear off, creating an air gap, breaking the circuit. How I can calculate and analyze this before an experiment?
     
  10. Jun 8, 2016 #9

    Baluncore

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    The cloth has a density of about half that of solid graphite. The cloth will be held together only by weak bonds between sheets. Half of the volume will be air. It will make a good thermal insulator unless the cloth is clamped tight between fixed surfaces. Any movement of a contact against the graphite cloth will rapidly exfoliate flakes from the cloth.
     
  11. Jun 9, 2016 #10
    What would be a solution for my case? A thin(or short length) brush for low resistivity?
    The pressure between the surfaces when in contact (to complete the circuit) is something I'm trying to figure out, could you help me with that @Baluncore?
     
  12. Jun 9, 2016 #11

    Baluncore

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    Unfortunately you have not made it clear whether your application is a brush contact to a rapidly moving surface or a static connection between two fixed conductors. You know what you are thinking, I do not.
    You have first used the word “brush” there. But is the brush a static contact or clamped gasket, or is it a contact like a commutator or slip ring brush in rotating electrical machinery.
    What case is that ?

    Many small independent brushes will follow a surface better than one large brush.
    If there is any contact with a moving surface then pure graphite cloth will have a very short life.
    Brush replacement interval is determined by length of the brush and rate of consumption.
    If brush resistance is significantly less than the circuit resistance then brush resistance is not a problem.
    Higher voltages and lower currents are an alternative to more or bigger brushes.
     
  13. Jun 12, 2016 #12
    To explain the set-up simply, there is a copper bar that must transfer current to a moving array of copper bars of equal contact area, similar to a motor's set-up of having a brush connected to the commutator. A diagram:
    ghlOaNw.jpg
    Where the contact area is empty, and I cannot directly connect the two copper surfaces to avoid welding, and further damages due to the high velocity of the bars. Therefore, the use of thin graphite sheet with low resistivity would help in avoiding various problems related with motion and the friction generated from it(mechanical and electrical problems):

    WrexUIM.jpg
    The moving array of copper bars(A to B to C) are moving rapidly, therefore, to avoid wearing and any possible "break" of the circuit, having a graphite sheet in-front of each bar's surface(A,B,C) is a solution?

    Predicting the wear is important, so I'd know when the replacement is required so that current flow is constant throughout the change of bars A, to B , to C.
    I calculated the resistivity, it is higher than the circuit's load, therefore, adjusted the dimensions of the sheets thinner(reducing L) and increasing the area with respect to the bar's area resulted a lower resistance.
     
  14. Jun 12, 2016 #13

    Baluncore

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    If the contact is sliding then any cloth in the gap will be rapidly destroyed and short circuit everything nearby.
    What happens while two of the moving conductors are shorted by the fixed bar ?
    I really do not have your confidence that graphite cloth is a solution. It needs some bonded graphite block.
    Without numbers your plan is too abstract to explain the application.
    How high is high velocity?
    Is the high velocity a rotation or an oscillatory movement?
    How often does the sliding contact repeat, RPM?
    What fills the gaps between moving conductors?

    What approximate dimensions do these conductors have?
    What is the voltage between adjacent moving conductors?
    What current is flowing through the contact?

    Maybe you should study the materials and construction of a train pantograph.
    https://en.wikipedia.org/wiki/Pantograph_(transport)#Weaknesses
     
  15. Jun 16, 2016 #14
    The contact is of sliding motion between the fixed bar and the array of bars A through C. At what rate is the cloth rapidly destroyed? How can I start to calculate(or estimate it)? If the cloth(being extremely thin) can withstand the complete motion(or passing) of bar A at full conductivity then I could place a cloth for each bar(A - C) and for the fixed bar per each bar like so:


    fJnqvsX.jpg

    Could it be possible to constantly lubricate the surfaces with pure graphite powder? Instead of using the sheets.

    You mean when there is an air-gap in between when the cloth is destroyed? If so, the circuit breaks and the possibility of an electric arc(only at high voltages).

    A graphite block will last for a longer period, and multiple runs without an issue I agree, however, the block is quite high in resistance if my ultimate goal is to coat the surface and have the graphite sheets(or powder if considering a conductive lubricant) to protect the bars from welding, and other mechanical issues created from the high velocity sliding and it's resistance being negligible to the circuit. Using a large block will solve on problem(mechanical) while it creates another(electrical due to high resistance).

    I agree, I should've asked earlier what variables do we need? I don't know what they are to supply such values.

    3 - 5 m/s, if you supplied me with the proper mathematical equations(or process) I could know the maximum and minimum speeds.

    Translational and oscillating.

    Constantly, going back and forth from bars A to C then C to A again and so on.

    Between bar's A,B,C? Insulators only on the top and bottom sides. The ends coated with graphite.

    0.10m H x 0.10m W x 0.05m T per bar.

    I = 100A
    V = 3.67 x 10 ^-5


    Thank you for the help @Baluncore , I'll take a look at that link.
     
  16. Jun 16, 2016 #15

    Baluncore

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    No. I mean as the fixed bar is half way between A and B it is shorting A and B to each other.

    That is only 36.7 microvolts. Is that the voltage drop across the sliding contact, or the voltage between A and B and between B and C.

    You need to draw a circuit that shows what everything is connected to, and the full path taken by all currents.
    I can only guess that the device is a component for an over-unity or perpetual motion machine.
    You need to specify clearly why you are designing this. What is the application?
     
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