Conductivity of Aluminium Oxide
The thermal conductivity of pure (99.5+%) aluminum oxide (alumina) is actually a function of temperature. Here are a few data points:
35.7 W/m-K at 20?
25.5 W/m-K at 100?
12.6 W/m-K at 400?
Note that the thermal conductivity of Al2O3 is about an order of magnitude lower than Al (~237 W/m-K around room temperature). Have you considered using aluminum instead alumina as a filler?