Control of Dielectric Constant and Loss in Alumina Ceramics

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Abstract
The dissipation factors and dielectric constants of alumina ceramics containing less than 100 ppm of impurities and of specimens doped with Si, Ti, Ca, Mg, Fe, and Cr ions were measured in the region 102 to 8.5 × 109 cps and 25° to 875° C. Multiple regression analysis of the data at 500° C and 106 cps showed a linear relation between impurity concentration and tan δ, with a correlation coefficient of 0.93. Si ions caused the greatest rise of tan δ, Mg and Ti were second, Ca third, and Cr and Fe had no significant influence. These effects diminished with rising frequency and became negligible in the microwave region. Activation energies of conduction for pure and doped alumina were estimated from measurements of δ at 105 cps and 500°C. Values between 1.2 and 1.6 ev were calculated for all compositions except the one containing Mg2+, for which 2.0 ev was obtained. At low frequencies, the dielectric constant (k′) rose exponentially with temperature, reflecting a similar rise in the number of free charge carriers contributing to interfacial polarization. At higher frequencies the temperature variation of k′ fell to a shallow positive slope of about 120 ppm per °C. This coefficient was not influenced by low concentrations of impurities but could be effectively compensated without excessive loss by additions of 10 to 20% SrTi03.