High thermal conductivity, lossy aluminum nitride dielectrics

Sienna Technologies, Inc., has successfully demonstrated that high thermal conductivity of aluminum nitride (AlN) can be alloyed or composited to increase its microwave losses without significant reduction in thermal conductivity to substitute for BeO-SiC composites in microwave tubes. Doped AlN and AlN composites were fabricated and their thermal conductivities and complex permittivities from 9 GHz to 12 GHz were measured and compared to those of BeO-SiC composites. AlN composites displayed the same loss tangent (0.455 at 9 GHz, and 0.588 at 12 GHz) and a comparable thermal conductivity (l05 W/m.K) as the 60BeO-40SiC composite (0.493 at 9 GHz, and 0.587 at 12 GHz). Doped AlN had a loss tangent of between 0.37 and 0.75 at 12 GHz with a dielectric constant of between 19 and 37, depending on dopant concentration. The thermal conductivity of doped AlN (150 W/m.K) was higher than that of 60BeO-40SiC composite (130 W/m.K).