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This study sheds light on the effect of alumina on dielectric constant and dielectric loss of novel CaCu3Ti4O12 composite ceramics. Alumina, at several concentrations, was deposited on the surface of CaCu3Ti4O12 particles via sol-gel technique. The dielectric constant significantly increased for all frequencies and the dielectric loss substantially decreased for low and intermediate frequencies. These observations were attributed to the change in characteristics of grains and grain boundaries. It was found that the insulating properties of the grain boundaries are improved following the addition of Al2O3. The relative density of CaCu3Ti4O12/Al2O3 composite ceramics decreased compared to the pure CaCu3Ti4O12 and the grain size was greatly changed with the alumina content affecting the dielectric properties. With the addition of alumina into CaCu3Ti4O12, tighter interfaces formed. The 6%- and 10%-alumina ceramics showed the minimum dielectric loss and the maximum dielectric constant, respectively. Both the dielectric constant and loss tangent decreased in the 20%-alumina ceramic due to the formation of CuO secondary phase. It was revealed that Al serves as an electron acceptor decreasing the electron concentration, if Al3+ ions substitute for Ti4+ ions, and as an electron donor increasing the electron concentration, if Al3+ ions substitute for Ca2+ ions. We established a processing-microstructure-properties paradigm which opens new avenues for novel applications of CaCu3Ti4O12/Al2O3 composite ceramics.

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