This beryllium graph is shocking. I decided to replace the mica insulators with beryllium oxide insulators.The first graph I showed two days ago comparing the sBitx DE continuous CW 30 W key down for 5 minutes, one thermometer chip on the metal tab of an IRFZ24N PA MOSFET and the other thermometer attached to the heat sink a few millimeters away. That configuration was with the old mica insulators. I was pleased to see it run hot without a PA failure. There was an understandable differential in temperature between the MOSFET and the heat sink.
After swapping out the mica insulators for beryllium oxide ceramic insulators the difference is astonishing. The thermal conductivity of BeO is about 200 times that of mica. The BeO discs are thicker by a factor of maybe 10 but look at the effect! The heat is conducted so much more efficiently that the MOSFET never gets much above 55 C, whereas with mica it got above 90 C.
I'm not sure it's worth changing to beryllium oxide but wow, what an effect. If you do get beryllium ceramic washers remember not to try grinding them. The dust is the worst type of poison because your body can't get rid of it. I snapped one washer in two to get it to fit both MOSFETs.
Jack, N6LN
John Terrell, N6LN