Journal of the American Ceramic Society. Vol. 75. No, 9. September l992

Effect of H2O and CO2 on Dielectric Properties of Single-Crystal Cordierite and Comparison with Polycrystalline Cordierite

Robert D. Shannon
Central Research and Development, Experimental Station 356/329
E. I. Du Pont de Nemours and Co., Wilmington, Delaware 19880—0356

Anthony N. Mariano

Carlisle, Massachusetts 01741

George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

The dielectric constants and dielectric loss values of two naturally occurring cordierite single crystals were deter mined at 1 MHz using a two-terminal method and empir ically determined edge corrections. The results are as fol lows. Cordierite 1: K'a = 5.653, tan 6 = 0.009; K'b= 6.106, tan 6 = 0.003; K'c= 5.658, tan 6 = 0.010. Cordierite 2:

K'a= 5.765, tan 6 = 0.0006; K"b= 6.334, tan 6 = 0.0007; K'c= 5.807, tan 6 = 0.0007. The agreement between mea sured dielectric polarizabilities as determined from the Clausius—Mosotti equation and those calculated from the sum of oxide polarizabilities according to aD(mineral)

= S aD(oxides) neglecting the channel H2O and CO2 for these two cordierite samples is ~20%. Inclusion of the effects of water and CO2 assuming mobile H20 molecules with no correlation between their dipoles improves the agreement to ~2%. Comparison of dielectric constants of manufactured cordierite ceramics shows a discrepancy be tween typical observed values of 4.5—5.0 and a theoretical value of 4.0 calculated from the Clausius—Mosotti equa tion. In certain cases, this discrepancy can be accounted for by the presence of high-k' impurities, but in others there is no explanation for the differences.