Low loss alumina dielectrics by aqueous tape casting: The influence of composition on the loss tangent

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO2, ranging from 0.05 wt.% to 5 wt.% (3 wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600 °C for 4 h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100 kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5 wt.% of TiO2 and 3 wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti4+) with high solubility in the Al2O3 crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO2 doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.