High thermal conductivity aluminum nitride ceramics for high power microwave windows

Aluminum nitride (AlN) is a leading candidate for replacing BeO or high-purity alumina as RF window material for high power, multi-megawatt (20 MW-100 MW) microwave devices. However, there is a controversy around the dielectric loss of AlN. This controversy, in our opinion, arises from the differences between the processing techniques and the raw materials used in the previous studies. We have undertaken an extensive study to determine and to control the factors to reduce the loss tangent of AlN to match those of Al₂O₃ and BeO (tanδ=0.0005) without adversely affecting its thermal conductivity, dielectric strength, and mechanical strength.