Well I finally experimented with the Aerogel. Conclusion: non ideal, at least as i had it set up. Very messy, as particles get everywhere, and are very abrasive. they also bounce very far (just like silica desiccant. if you haven't played with it, try bouncing it on a hard surface! it's fun!)
I needed something to contain the Aerogel pieces, so i made a box out of copper foil, with a hole in the bottom that allows me to screw it onto the threaded hot end. the box has an open top, so i just poured the aerogel bits in 'til it was full.
Copper is a bad choice. the box gets very hot. it acts like a big heat sink, pulling the heat away from the place where i want heat concentrated; the nozzle tip. So, while the rest of the hot end is insulated, the nozzle gets cooled.
This may result in a more consistent temp at the nozzle than if it was uninsulated, but other insulation will do that with less effort or mess.
The box approach also leads to the insulation surrounding the ptfe spacer, making the melt zone larger, which is the last thing we want to do.
The only other option i see to using the aerogel scraps is making a kapton-aerogel-kapton package, and wrapping it around the hot end. again, i don't know if this will lead to a better solution than just fiberglass, and doubt it will.
