SYNTHESIS, PROPERTIES CaCu3Ti4O12 WITH COLOSSAL VALUE OF THE DIELECTRIC PERMITTIVITY

Abstract

Ceramic materials CaCu3Ti4O12 were synthesized by solid-phase reactions technique. The sequence of chemical reactions during the synthesis has been determined. Phase CaCu3Ti4O12 appears at 700 °C. At 800 – 900 °C, the intermediate phases CaTiO3, CuTiO3 and Ca3Ti2O7 are formed. Calcium and copper titanates, CaTiO3 and CuTiO3 interact to form CaCu3Ti4O12. Ca3Ti2O7 phase with pyrochlore structure is stable and prevent the formation of final product, CaCu3Ti4O12. A method for the synthesis of CaCu3Ti4O12 by solid-state reactions technique from previously synthesized CaTiO3 (at 1050 °С) and CuTiO3 (at 950 °С), taken in a molar ratio of 1:3, is proposed. This method give the possibility to avoid the appearance of an undesirable Ca3Ti2O7 phase with the pyrochlore structure and to reduce the content of free copper oxide to value less than 0.5 mol.%. In addition, instead of the copper oxide, which is usually used in solid-state reaction technique, the chemically more active form of the copper-containing reagent, CuCO3∙Cu(OН)2 were used. This reduce the synthesis time of the intermediate CuTiO3. The crystal structure, chemical composition, microstructure and electrophysical parameters of ceramics have been analyzed. The synthesized ceramics CaCu3Ti4O12 is cubic body-centered (space group Im-3) with the unit cell parameter a = 7.3932 Å, which agreed with the literature data. The calculated tolerance factor of CaCu3Ti4O12, t = 0.7626 is not sufficient for a stabilization of peroskite ABO3 structure; that is why the crystal structure of this compound contains 3 different cation sites: dodecahedral (Ca2+), octahedral (Ti4+), tetrahedral (Cu2+). At 1150 °C, the density of CaCu3Ti4O12 ceramic sintered has a maximum (90% of the theoretical density). At infra-low frequencies (10-3 Hz), the dielectric constant (e) reaches record values of 107, however, dielectric losses (tg d) up to 10 were observed. In the frequency range 10-3 - 105 Hz the value of ɛ exceeds 104; and at 105 Hz minimum of the dielectric losses (tg δ ~ 0.1) is observed. A comparative analysis of methods for the synthesis of CaCu3Ti4O12 shows that the synthesis conditions of material of the same chemical composition can be crucial in creating high dense ceramic with uniform grains, high dielectric constant and low dielectric losses in a wide frequency range.