When the thermal conductivity of copper and aluminum heat spreaders just
won't cut it, what's a space born NASA Laser Death Ray to do? The future it
seems, revolves around a material called Annealed Pyrolytic Graphite.
Pyrolytic Graphic is a carbon based material with exceptionally high thermal conductivity - second only to diamond - but only in specific axes; it's also relatively fragile. Thermacore recently sent out a PR about a
new hybrid material called 'K-Core Annealed Pyrolytic Graphite' it developed
which is essentially APG sandwiched between sheets of aluminum, or other
metals. The K-Core material was recently selected by NASA to cool
optics in the U.S. Geological Survey Landsat 8 satellite.
Example of K-Core APG material
Let's consider the numbers: where a solid aluminum heat spreader has a
thermal conductivity of 126W/mK, the same heatspreader with an Annealed
Pyrolytic Graphite core would see thermal
conductivity on the order of 1092W/mK. That's not a
typo.
What is Annealed Pyrolytic
Graphite?
Annealed Pyrolytic Graphite is a high conductivity material made from a "unique form of pyrolytic graphite manufactured by the decomposition of a hydrocarbon gas in a high vacuum furnace (by Chemical Vapor Deposition - CVD)." (source: Scott L. Kugler). Why isn't everything
made from this wonder material you ask? As you might guess, it's not quite as
simple as swapping metal for Pyrolytic Graphite; while thermal
conductivity in metals is not orientation specific, in APG thermal
conductivity is very much orthotropic. What that means is APG material possesses extremely
high thermal conductivity in its horizontal layers (a-axis, b-axis =
300-1700W/mK) and quite low thermal conductivity when moving from vertically
from layer to layer (c-axis = 3.5-10W/mK).
Writes ECNMag.com; "Annealed Pyrolytic
Graphite (APG) is orthotropic and as such the direction of the thermal
loading may also determine the thermal performance of the heat sink. The low
through-the-thickness thermal conductivity of the APG can lower the
effective thermal conductivity of the part when there is a thermal path normal
to the plane of the APG. For most applications, this characteristic is small
because the high in-plane conductivity of the APG quickly spreads the heat, thus
lowering the thermal density (Q/A). At the reduced thermal density, the
temperature rise through-the-thickness is significantly reduced. However,
designs that require a high-flux density energy(>10w/cm2) heat path normal to
the plane of the APG, the temperature rise can be significant."
Neat material, maybe one day soon it will trickle down to computer heatsinks...
