Searching for an additive to increase PCL's thermal conductivity (Unpolished)

Places to find candidate materials

Informally, things I'm looking for are:

thermal conductivity — high
hardness — low (Brinell hardness < 150)
- alternately, the hardness can be high, as long as you can buy them for cheap in ball form
- (because the plastic is hand-moldable, and I don't want the additive to really scratch the user's hands)
toxicity — very low
cost — low
doesn't react with water (this means iron is ruled out, because it becomes iron oxide, and so it loses its thermal conductivity)

On Matweb, try this specific query:

Metals that are "silvery":

aluminum [amazon] [elsewhere]
- pro — you can buy them in ball form
stainless steel
- pro — you can buy them in ball form
- pro — not too expensive (in ball form)
- information — Thermal conductivity at 20°C — 25 W/m K
- TODO: figure out its exact price-per-gram, and then figure out how that compares to competing alternatives
silver [amazon]
- cons — expensive, with a spot-price of ~$600 / lb
magnesium
- cons — very flammable if it can be ignited (which is hard to do), so it's not great to store a lot in one place
tin [amazon] [elsewhere]
- pro — softer than most metals (Mohs 1.5), and thermal conductivity isn't terrible (67 W/(m⋅K))
- con — a little bit expensive
silicon, elemental, powdered [amazon]
- con — high hardness (Mohs 6.5)

Other:

graphite, powdered [amazon]
- pro — very high conductivity, comes in fine enough particles that it won't feel "gritty" to the hand
- cons — it can probably really stain fingers (TODO: confirm this)
- pro — cheap
silicon carbide [amazon]
- pro — cheap
- pro — available in many particle sizes
- con — so hard that it's literally used as rock tumbler media
copper [amazon] [elsewhere]

Disqualified:

Iron — It reacts with water, forming iron oxide, which causes its thermal conductivity to plummet.

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