“The thermal conductivity of SiC single crystal has been reported as high as 500 W/m⋅K. However, most commercial SiC grades have thermal conductivity in the range 50-120 W/m⋅K.”
Silicon Carbide isn’t looking too bad if it can be made glass smooth.
edit
"Moissanite costs 1/10th the price of diamonds and sparkles brighter because of its higher refractive index. The occurrence of moissanite (silicon carbide) in nature is very rare, but it can now be manufactured in factories to be used as an imitation for diamonds.
“Good thermal conductivity, comparable to diamond.”
Moissanite {man made} is used in jewelry just like synthetic ruby.
I enjoy reading your discussion, lots of good ideas and questions, here are some comments:
There are several things to take into consideration regarding the material. You might not necessarily want the thermal conductivity as high as possible throughout the whole nozzle for example.
Diamond would of course beat the ruby in hardness, but I have already printed plastic with hight loading of boron carbide, which is the third hardest known substance (harder than ruby) with very little wear on the ruby.
Since I believe people are unlikely to print diamond- or cubic boron nitride-filled plastics, I would personally consider ruby/sapphire “hard enough”.
The most important question though, when manufacturing these in small series like we are talking about here, is: What hard materials can be readily sourced with suitable dimensions and a suitable hole size for a reasonable price?
For a steady state system like an extruder, you don’t need high thermal conductivity, as you just increase the temperature on the other side of the metal. High conductivity may actually hurt you as it would be more sensitive to drafts.
What you want in an extruder is a moderate thermal conductivity and high heat capacity. Stainless steel, IMHO is the right choice. Think of a vast iron skillet and a steak. Takes a long time to heat up, but when it’s hot, it stays hot, even after throwing a steak on it. An aluminum pan drops temperature quickly once a steak is on it and must hear back up. So it’s a combination of conductivity and capacity.
Hope that helps