The heat capacity of the gas depends on both mass and molecular structure. Atoms like argon have no structure, but molecules like O2 and N2 carry extra heat energy in their vibrating bond, so they have more heat capacity per unit mass.
That's a good point. Perhaps the result of that argon/nitrogen comparison was a coincidence, or perhaps heat capacity doesn't much affect thermal conductivity.
Here's an argument that heat capacity shouldn't affect thermal conductivity in a solid: imagine a block of material with a smooth temperature gradient through it and the temperature at any point remaining constant, so there is heat flowing through it. Now imagine you have some tiny beads of some other material with a super-high heat capacity. Distribute those beads throughout the block of material, preheating each bead to the correct local temperature before inserting it. Then intuitively it seems that neither any local temperature nor the rate of flow of heat through the block should change much. Yet the heat capacity of the new composite material is significantly higher than that of the original material. Does that argument make sense? I did physics at school but not since then!
Yes, I don't think they're strongly related in a solid.
But in a gas, heat transfer is mostly driven by convection. Gas near the hot surface heats up, rises, moves to the cold surface, falls, and repeats. So what matters is the specific heat capacity * the flow rate between hot and cold surfaces.
Thanks for that explanation. So the "thermal conductivities" I quoted above, if they don't take account of convection, are perhaps not even particularly relevant. The advantage of argon over nitrogen might be greater than those numbers suggest?
