Conductive Gel or Paste

Discussion in 'General Electronics Chat' started by sirch2, Jan 12, 2014.

  1. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    I am trying to measure the conductivity of some small (about 1" square) rock samples. I need to get a good contact between between the electrodes (pieces of copper clad board) and the rock. In order of apparent success I have tried dry, wet, copper filings, graphite, and brine.

    The graphite and brine give similar results but both are fairly variable in terms of the actual measured reading. The reading tends to rise with time (over a few seconds) and varies with pressure. By cleaning everything up and starting again I can get reasonably repeatable results, but then by fidling around I can get almost any reading I want.

    So is there any good conductive gel that would be better than graphite or brine that would give more consistent results?
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
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    At your local auto supply, find a product for repairing open traces on a rear-window defroster... works like a charm... let a drop dry on your object, and measure away... I have never encountered any appreciable error in measurement with the product
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Gallium metal and various gallium tin alloys 'wet' ceramics, glass and stone. For best accuracy, I would go this route. These are the alloys used to bond copper leads to the semiconductor silicon.

    You can find them with melting points in varying ranges from skin temp to 200c range.

    Indium metal is more expensive but may be included in the alloy. Check eBay for gallium samples. Check Indium Corporation for gallium- and indium-containing solders.
     
    Last edited: Jan 12, 2014
  4. PackratKing

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    Thank-you sir !!! That is likely the product / procedure they used to " dope " the ceramic substrates I was referring to... and answers a long-standing puzzle...

    Since camera mfgrs. at the time, considered some of their processes " trade secret " and still do for that matter... some camera repair people - at their own loss - refused to work on units with ceramic PCB's...
     
  5. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    Thanks guys, I couldn't find any obvious sources of gallium alloy (and I guessing it's going to be expensive).

    I'll may be try the conductive paint.
     
  6. shortbus

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  7. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    I don't really want to glue the electrodes on just get complete contact across the surface of the rock sample.
     
  8. djsfantasi

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    Apr 11, 2010
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    sirch2 - think outside the box. Applying a coating of glue will give you complete contact across the surface. Correct?
     
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  9. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
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    Yes, thanks, I had realized that. But like I said at the top I have already tried graphite powder and I'm assuming the glue is just carbon or in some kind adhesive so other than gluing the electrodes on I'm not sure of the advantage?
     
  10. shortbus

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    If the graphite you are using is the type for lubricating locks, it won't work too well. There are different types of graphite. I was a die maker and did EDM work that used graphite blocks, it was a very conductive form of graphite produced from petroleum refining waste. Lubricating and pencil lead graphite is a natural graphite mixed with a form of clay, and not as conductive.

    Have you tried the conductive paste/gel used for EEG and TENS units?
    http://www.amazon.com/Ten20-EEG-Conductive-Paste-Tube/dp/B002R16OUK


    The glue in my first answer gives this information - "Conductivity: 13.16 Ohms per cubic centimeter". Using a glue keeps all of the grains of the conductive material in contact with each other, unlike just using it in a powdered form.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    I don't see how such a measurement can be of any use unless you rigorously control the surface area of the contact with the electrodes, and the distance between and alignment of the electrodes. The ONLY variable then is the composition of the sample under test.

    With irregular contact patches and irregular sample geometry, you're just generating artifacts. I'm not sure how you would ever know for sure that the electrode:stone interface was not the limiting factor, as opposed to the rock itself. Wouldn't the degree of polishing of the rock surface impact the measurement?
     
  12. GopherT

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    Nov 23, 2012
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    Orientation of the crystals in the rock will also impact the conductivity. Ulexite is particularly noteworthy and visible example but many other igneous rocks also have crystals on the inch scale or greater and conductivity on the xy plane can be much different than the z-axis.
     
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