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Thermal conductivity (ceramics)

Discussion in 'Electronic Design' started by George Herold, Jan 2, 2013.

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  1. There's been a design for a little transistor/diode temp. sensor
    floating around in my head.

    In one version, there'll be a direct ground connection.

    I wanted to have an electrically floating version too.
    I want to try a circuit on flex (kapton).
    Using the kapton as the insulator looks like a good idea.
    But the thermal conductivity 'stinks'. (~0.15 W/m-K)
    So driving home tonight I was thinking about using
    an Al2O3 washer instead.
    "What's the thermal cond. I said to myself, 100, 50 W/m-K?"
    Googleing Al2O3, thermal conductivity Coorstek, I found this,

    http://www.coorstek.com/resource-library/library/8510-1042_ceramic_material_properties.pdf
    (I might have to print it out and pin it up.)

    So for this application I need a 'decent' electrical insulator.
    Which looks to rule out all the carbide's.
    But what about the direct sintered SiC?

    And then what about Aluminum and Silicon Nitrides?
    Has any one used those?

    I'm looking for a little thin insulating washer, that will go from
    77K to 400K. (Maybe 500?).
    A hole for a 2-56 screw.

    George H.
     
  2. Neat, I'm already using TO220 transistors as temp sensors.
    (Prepacked hole who could resist?)
    Interesting. Does anyone sell anodized Al washers?
    No, I need a sensor that students can move around.
    I'm thinking I need some sort of spring (Belville washer?) in the
    stack-up.

    George H.
     
  3. Jon Kirwan

    Jon Kirwan Guest

    If it is very thin (and I think you can get .0005" thickness
    tapes -- certainly .001" because I use that, myself) then it
    may not be so bad. What would be the area? (I can't work out
    the W/K without that.)
    Have you considered the usual suspects from Cotronics? Their
    810 potting compound is about 0.67 W/m-K, I think. It's
    volume resistivity is on the order of 10^11 (ohm-cm.)

    Jon
     
  4. Yeah, I ran the numbers with thin tape. I worry about it breaking
    with repeated use. Alumina has 100 times the conductivity, so the
    same area, but ten times thicker, (A 10 mil washer) still has 10 times
    the conductivity.

    I'd like something moveable.

    (Ge varnish is a classic low temp, thermal 'tie down'.)

    George H.
     
  5. Film? Adhesive on polyimide tapes is normally 1.5+ mils thick (per
    coated side, of course).

    AlO2 99.8% washers 0.25 O.D. 0.08" I.D. 0.03" thick $3 apiece
    Polyimide " 0.25 O.D. 0.09" I.D. 0.005" thick 70 cents ea.

    The AlO2 would have to be about 6x better conductivity to make up
    the increased thickness. Plus, it doesn't conform, so it won't
    make as good contact. (Un?) fortunately, it's about 60x better,
    so, ignoring contact issues, it will be 10x better overall, at
    4.3x the cost. If the surface is not very flat, and the applied
    pressure fairly even, the ceramic will probably break.



    Best regards,
    Spehro Pefhany
     
  6. Cracking the ceramic is an issue.
    For the tape, as far as I can tell the glue on the tape is worse than
    the tape itself.

    Any 'pinions on anodized Al?

    George H.
     
  7. Jon Kirwan

    Jon Kirwan Guest

    http://www.kaptontape.com/1_Mil_Kapton_tape.php

    I only barely recall seeing 1/2 mil somewhere (didn't want it
    at the time.) I'm using the 1 mil variety. You are right, as
    the silicone adhesive is way too thick and can be a problem.
    I just imagine it possible to get the film without the
    adhesive. Someone must have it.
    Pretty much agree. But without knowing exactly what George is
    about (I don't have a clear picture to be honest), I can't
    say if Cotronics stuff _could_ be useful or not. I've used
    it, though, and like it. And it is AlO2.

    Jon
     
  8. Jon Kirwan

    Jon Kirwan Guest

    Yes, the silicon glue is as thick or thicker than the film
    itself. As Spehro immediately pointed out. As I also said, I
    suspect that the film by itself should be available from
    someone. Just not "as available" as the silicone glued tapes
    are. Could mechanically thin the adhesive, I suppose. I might
    try that here.
    None from me except that I'd be worried about bumps and air
    acting as a thermal insulator.

    Jon
     
  9. miso

    miso Guest

    Glass bead is what they use in chips for electrical isolation with heat
    transfer. Though I've used it in production, I never opened a package to
    see what the bead material looks like.
     
  10. I like it, but don't know where to get hard anodized alumin(i)um
    washers in small quantities. I'm pretty sure my favorite OEM washer
    supplier could make them in moderate quantity, but I don't think that
    helps you.

    DIY is possible (these guys just the other side of Rochester have all
    the stuff) http://www.caswellplating.com but I bet your H&S folks
    would make things complicated.


    Best regards,
    Spehro Pefhany
     
  11. SoothSayer

    SoothSayer Guest

  12. Syd Rumpo

    Syd Rumpo Guest

    On 03/01/2013 03:14, Jon Kirwan wrote:

    RS Components, for one...

    http://uk.rs-online.com/web/p/thermal-insulating-films/5363946/

    ....that's one thou/mil and is sold as thermal insulating film!

    Cheers
     
  13. SoothSayer

    SoothSayer Guest

    There are tapes made for this. They are thermal paste coated.

    One applies the tape (a pre-cut piece), and then removes the film
    separator, and applies the device.

    The thermal coat melds into all the surface deformities. One ends up
    with a serviceable, replaceable 'gap tight' thermal mating.

    I will look at the branding on the roll at work and post it.
     
  14. Well I guess that depends on what's a 'moderate' quantity.
    Hundreds certainly and maybe 1,000's. But that's 'small potatoes' for
    washers.

    But we have a metal 'stamping' supplier that could do the aluminum and
    then have them anodized by someone else...
    Yuck... No, (but thanks) let someone else do the messy bits.

    George H.
     
  15. The Dupont site has all sorts of 'stuff'.
    http://www2.dupont.com/Kapton/en_US/products/index.html

    (I bet you didn't know there were some many types of Kapton.)

    George H.
     
  16. SoothSayer

    SoothSayer Guest


    Forgot to mention... The tape to which I refer is Aluminum.

    That should change your image of what I offered.
     
  17. Thanks John, I don't think I need the best, nor even the penultimate
    thermal conductor. (The MSDS is a bit off putting for student use.)

    George H.
     
  18. Hey, Seastrom will make Al washers with an anodized finish.
    http://www.seastrom-mfg.com/

    George H.
     
  19. You might want to try asking Bokers then- they can do that sort of
    small order, maybe they can do or arrange for the hard (Type III)
    anodize?

    http://www.bokers.com
     
  20. Thanks for the link.

    They don't list Type III (hard) anodize, but maybe they can do it.
     
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