Effect of H_2O and CO_2 on Dielectric Properties of Single-Crystal Cordierite and Comparison with Polycrystalline Cordierite

The dielectric constants and dielectric loss values of two naturally occurring cordierite single crystals were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are as follows. Cordierite 1: κ^′_a= 5.653, tan δ= 0.009; κ^′_b= 6.106, tan δ= 0.003; κ^′_c= 5.658, tan δ= 0.010. Cordierite 2: κ^′_a= 5.765, tan δ= 0.0006; κ^′_b= 6.334, tan δ= 0.0007; κ^′_c= 5.807, tan δ= 0.0007. The agreement between measured dielectric polarizabilities as determined from the Clausius–Mosotti equation and those calculated from the sum of oxide polarizabilities according to α_D (mineral) =Σα_D (oxides) neglecting the channel H_2O and CO_2 for these two cordierite samples is ∼20%. Inclusion of the effects of water and CO_2 assuming mobile H_2O molecules with no correlation between their dipoles improves the agreement to ∼2%. Comparison of dielectric constants of manufactured cordierite ceramics shows a discrepancy between typical observed values of 4.5–5.0 and a theoretical value of 4.0 calculated from the Clausius–Mosotti equation. In certain cases, this discrepancy can be accounted for by the presence of high-κ^′ impurities, but in others there is no explanation for the differences