| An engineering analysis of several plasma-sprayed oxide coatings (alumina, spinel, cordierite, and forsterite) has been performed with respect to both dielectric constant and dielectric loss measurements. Plasma-sprayed alumina coatings were prepared based upon differences in the particle size of the bulk powder. It is shown that the smaller sized particles produce coatings with the largest dielectric constant and dielectric loss. A pronounced dielectric dependence on particle size at lower frequencies is observed, which levels off as the frequency rises. It is suggested that the frequency dependence results principally from the $gamma$ - phase. Consistent with the above conclusion is the fact that the dielectric properties - frequency curves flatten out at low frequencies for plasma sprayed alumina specimens which have been annealed at 1200C to effect reversion to the $alpha$ - phase.;The dielectric constant for as-sprayed spinel varies from 11.4 at 100Hz to 8.4 at $10sp6$Hz. The dielectric loss varies from 0.20 at 100Hz to less than 0.03 at $10sp6$Hz. Annealed spinel (1200C for 48hrs) has a dielectric constant approximately one unit below the as-sprayed condition, but still exhibits a flat response with frequency.;The dielectric constant for as-sprayed cordierite varies from 7.4 at 100Hz to 7.1 at 10$sp{6}$Hz, with a respective dielectric loss of 0.12 to 0.07 over this same range. The dielectric constant of annealed cordierite, exhibits appreciable curvature with frequency, the dielectric constant ranging from 7.3 at 100Hz to 5.7 at 10$sp{6}$Hz. The dielectric loss versus frequency is flat, varying from only 0.07 to 0.05 over this same frequency range.;As-sprayed forsterite has a dielectric constant of 14 at 10$sp{6}$Hz and is essentially flat with frequency. The as-sprayed loss tangent varies from 0.40 at 100Hz to 0.27 at 10$sp{6}$Hz. Upon annealing, the dielectric constant of as-sprayed forsterite varies from 10 at 100Hz to 6.0 at 10$sp{6}$Hz, with an associated dielectric loss ranging from 0.12 at 100Hz to 0.09 at 10$sp{6}$Hz.;All annealed oxide coatings shows a strong temperature dependence for both the dielectric constant and loss tangent which is not observed for the as-sprayed condition. |
