E3D's HotEnds come with one of these little sachets to be used between the heatbreak and heatsink, to improve thermal performance.
If you're pulling apart a HotEnd to clean or upgrade it, you may want to reapply the thermal paste.
Note that these are pretty much only good for one use, as the packet can't be resealed after opening.
Silicone Compounds 50%
Metal Oxide Compounds 30%
Compounds of Carbon 20%