-> The vanes are typically made of aluminum or tin. They both conduct 
-> heat but that doesn't matter because the heating is very localized and 
-> very brief. Remember that the incident photon energy is transformed 
-> into a mechanical torque via the gas molecules so that the vanes don't 
-> actually get hotter and hotter. As such there is a net transfer back 
-> out of the partially evacuated bulb. I think there is an electron 
-> cloud boiling off the surface of the vanes (or thermal emission just 
-> like in old radio tubes). These free electrons readily give up their 
-> energy to any gas molecule that comes into their vicinity and then 
-> plunge back into the metal lattice. That is a property of conductive 
-> metals in that they have free electrons which are responsible for 
-> electrical conduction. These can readily escape the surface if they 
-> are excited with the least amount of photon quanta required. 
  
The vanes that I have seen have been made of mica or some similar 
material. Why do you say that the heating is very brief? As I 
understand it, the black sides of the vanes are heated to a 
temperature significantly above that of the shiny sides. When gas 
molecules bounce off the dark sides, they gain energy from the rapidly 
moving molecules of the material of the vane, and thus fly off faster 
than molecules that hit the shiny sides. This gives the net force that 
drives the vane around. 
  
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