It used to be known that hollow microspheres were directly fished out from the combustion products of power plants, and they were called "floating beads" or "floating beads", which were quite cheap. Of course, you can also get "sinking beads".
I checked the patent for this direct manufacturing method.
Because what you said is not very clear, it is certain that graphite powder has nothing to do with heat conduction, and at which temperature thermal radiation is the main heat transfer mode. Glass melts into a sphere under surface tension.
Solid glass microspheres are relatively easy to manufacture and can be obtained by directly spraying molten glass. Larger ones can be made by ball milling. I can think of which one you said must be hollow, otherwise there is no need to go to so much trouble. Look at this.