Free of air is a reasonable demand. In ceramics, it results from good sintering.
Is it aluminium nitrate Al(NO3)3 or nitride AlN? The nitride is a known technical ceramic.
550°C is damn hot for electronics. I know no semiconductor operating or even surviving at this temperature. Some resistors, vacuum valves, special-made capacitors may work. Or is the circuit cooler? In oilfield operations, electronics operates shortly at +180°C with special precautions. The usual uneasy limit is +125°C.
Do you expect some X-ray shielding properties from the material? AlN is transparent, BN more so. Heavy elements shield more at identical weight.
The permittivity changes with temperature. Shall it be kept, or even be constant, to 550°C? That would restrict the choice. For instance the titanates and niobates of Ba, Zr, Ba+Zr or Pb+Zr have ε>1000 but only at room temperature and low frequency, with big losses, microphonic and memory effect.
9 Al2O3, common ceramic
40-200 BN, available ceramic
85+ TiO2, available ceramic
29 ZrO2, available ceramic (contains also HfO2 usually, and must be stabilized with some Y2O3)