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Temperature Control Fabric

  1. Nov 14, 2017 at 7:20 PM #1

    BillTre

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    This article in Science mag news is about a fabric that can,
    • let more heat (via IR) out in hot weather, or
    • flipped over, keep more heat in the cold
    Here is the Science article (not free).

    The fabric also lets gasses and humidity pass through because it has nano-pores.
    It has 4 layers: 2 outside layers of polyethylene and 2 thin layers of carbon black and copper.

    • outside when cold
    PE layer 1: IR clear, 24 µm thick
    carbon black: absorbs and emits IR
    copper: weakly absorbs and emits IR, reflecting?
    PE layer 2: IR clear, 12 µm thick
    • out side when hot

    The copper and carbon layers are in contact, but distinct layers.

    What I don't understand:
    If the copper is reflecting the IR re-emitted by the carbon layer (as news article says), why is not the IR being absorbed by the carbon blocked to the same extent?
    Does the closeness of the carbon layer to the copper matter?
    Are there wavelength differences in what's absorbed and emitted by the carbon, that could interact with copper differently?
     
  2. jcsd
  3. Nov 14, 2017 at 10:32 PM #2

    Bystander

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    Is the copper in contact, or does a gap exist?
     
  4. Nov 14, 2017 at 11:40 PM #3

    BillTre

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    There is not a gap.
    The copper is sputter coated onto a layer of carbon that was dried down onto one of the PE layers.
     
  5. Nov 15, 2017 at 12:09 AM #4

    Bystander

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    What does direct contact imply for/about heat transfer/transport? Radiation? Or conduction?
     
  6. Nov 15, 2017 at 12:39 PM #5

    BillTre

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    Contact between the copper and carbon layers implies heat conduction (vibrating molecules causing neighboring molecules to vibrate more) could occur, however, the article focuses almost exclusively on IR (InfraRed) radiation, how it penetrates the materials and is absorbed and emitted.
    The copper layer's action is discussed as a reflector.

    I can see a few ways contact between the carbon and black layers could have an effect:
    • The optical interface between the carbon and copper layers gets different optical properties from the juxtaposition of the two layers due to their different optical properties (such as optical density). The layers together may act as a one-way mirror.
    • There is something like fluorescent transfer going on at the interface between the carbon and the copper. The carbon absorbs an IR photon and then readmits it (like a fluorescent molecule). Presumably the emitted photo would be lower energy and thus longer wavelength. The carbon emitter of the IR photon might instead undergo an energy transfer to the copper. This would not involve most of the material in the carbon layer (9 µm thick) from interacting in this way because most of the atoms would be too far away.
    • The geometry of the closeness of the carbon emitter of IR photons to the copper layer night result in more photons interacting with the copper at angles more likely to result in a reflection instead of refraction (have not fully thought this one through).
     
  7. Nov 15, 2017 at 1:42 PM #6

    Bystander

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    ... and, the temperature dependence at "ambient" conditions? T4 = what?
     
  8. Nov 15, 2017 at 2:58 PM #7

    OmCheeto

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    Has anyone else done the maths on the thermal conductivity for this material?
    I came up with 11,000 watts per Kelvin.

    After that, my head started exploding.

    This is what I came up with:

    Code (Text):
    radiant            net W/m^2   conditions  
    too cold            119.0       50 °F amb
    just right           51.0       72 °F amb
    too hot              47.0      100 °F amb
    external body temp              90 °F
     
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