Silicon nitride having low dielectric loss

Claim 1

A sintered silicon nitride based ceramic dielectric of at least about 99% theoretical density suitable for use in high temperature radome applications and which consists of a sintering aid, a low dielectric loss promoter and the balance silicon nitride, containing from about 4% by weight to about 16% by weight of a sintering aid used to achieve said theoretical density, said sintering aid being selected from the group consisting of lanthanum oxide, yttrium oxide, aluminum oxide, lanthanum aluminate, yttrium aluminate and mixtures thereof, said ceramic improved by the presence therein of a low dielectric loss promoter at a level of from about 0.1% to about 2% by weight, said low dielectric loss promoter being selected from the group consisting of an oxide of nickel, cobalt, titanium, zirconium, vanadium, tin and mixtures and compounds thereof, said low dielectric loss promoter being effective to provide a low dielectric constant and a low loss tangent that remains essentially unchanged through the temperature range of from about 25° C. to about 1100° C., the dielectric constant being below about 8.0 of about 9.375 gigahertz throughout said temperature range, the loss tangent being less than about 0.003 throughout said temperature range.