Improved wideband coaxial methods for dielectric measurements on nitrogen ceramics

Abstract

Two methods of measuring the dielectric properties of materials - matched termination and coaxial line resonance- have been developed and used to study the properties of two groups of nitrogen ceramics, namely, silicon nitrides and oxynitride glasses. In these methods advantage is taken of the wide frequency range, from 500 MHz up to about 9 GHz, covered by a single apparatus - the General Radio slotted coaxial line. Previous measurements in this Department have indicated the difficulties in the determination of the loss tangent of low to medium loss samples (tan Ỏ ~ 10(^-3) - 10(^-2)). The two methods developed reduced these difficulties. The applicability of these methods was assessed using known materials including the high loss liquids water and chlorobenzene, medium loss solutions of chlorobenzene in cyclohexane and the low loss solids polymethyl methacrylate and polytetrafluoroethylene. The silicon nitride ceramics were in various degrees of nitridation given by the weight gain which ranged from 38% for partially-nitrided to a maximum of 63.2% for the fully-nitrided samples. The dielectric constant at 1 GHz increased from 4.51 for fully-nitrided to about 9.9 for the 38% weight gain samples. The fully-nitrided material has a loss factor of 7.6 x 10(^-3); this increased to 1.85 x l0(^-1)as the weight gain decreased to 38%. These values have been extrapolated to 'zero weight gain' and compared with pure silicon for which the dielectric constant is 11.7 and the loss factor approximately 0.2. The oxynitrides have dielectric constants between 6.5 and 7.5 depending on the cation present and the percentage of nitrogen substituted for oxygen in the glasses. Є' increased in the cation order Mg, Y, Ca and increased with increasing nitrogen substitution in each cation series. The loss factor, however, depends not only on these cation types but also on the other constituents of the glasses. The dielectric constant of both the silicon nitride ceramics and oxynitride glasses fitted the Jonscher universal law of dielectric response (Є' - Є) w(^n-1), where the exponent n - 1 for all the samples. Similarly, the loss factor for these materials showed a frequency dependence Є" w(^n-1), again with n approximately 1. This is a limiting case of dielectric behaviour corresponding to a frequency independent loss where most dipolar processes have been eliminated.