Propriedades dielétricas de cerâmicas de CaCu3Ti4O12 com deficiência de CuO e TiO2 em função das diferentes rotas de síntese e sinterização

Abstract

CaCu3Ti4O12 (CCTO) ceramics have attracted increasing interest because of their extraordinary dielectric constant values (κ > 104 at 1 kHz). The structural, microstructural and dielectric characteristics of the CCTO are dependent on the synthesis and processing methods, being the subject of constant investigation. In this work the influence of some synthesis and sintering parameters on the dielectric properties of these CCTO ceramics was investigated. According to the coprecipitated synthesis method, ceramic powders of CaCuXTiYO12 (CCXTYO) were produced with stoichiometric variations associated with low CuO and TiO2 contents, where X|Y = 2.70|3.25; 2.80|3.50; 2.90|3.75; 2.95|3.87 e 3.0|4.0. The samples were exposed to different processing variables, being investigated the influence of these on the physical chemical characteristics of CCXTYO ceramics. A two-stage calcination route was carried out at 350 °C and 850 °C, at different heating rates, and with grinding inclusion in each. The sintering process included a first firing step, performed at 1100 °C, at a rate of 10 °C/min and in air; different landing times were included. A second, and subsequent step, was carried out at 1150 °C for 5 h. The variables and parameters of sintering and processing allowed to consolidate the maximization of the amount of CCTO phase in the microstructure and to promote grain growth to obtain CCXTYO ceramics with excellent dielectric properties. XRD patterns confirmed the majority presence of the CCTO phase in each compound, as well as discrete traces of secondary phases such as CuO, TiO2 and CaTiO3. Scanning Electron Microscopy (SEM) allowed evaluating the presence of the CuO and TiO2 crystalline phase in the grain boundaries, as well as the morphology, growth and evolution of the CCTO grains. In the frequency range 20 Hz to 5 MHz, at room temperature, measurements of impedance spectroscopy (IS) showed that the highest average grain size and composition (X|Y[2.95|3.87]PS1/PS2) exhibited the highest values of κ (3.5x104 , at 1 kHz). The ceramics with the narrower grain size distributions showed the lowest tanδ values (0.030, at 1 kHz). The dielectric measurements showed the existence of relaxation mechanisms, which were fitted by the Nyquist diagram, and modeled by equivalent circuits in parallel, in order to know the contributions of grain, grain boundary and electrode-sample interface. Different polarization mechanisms, especially Maxwell-Wagner, justify the dielectric behavior of this material, one of them originating in domain boundaries or any other type of internal barrier in semiconductor grains, and another generated in grain boundaries (explained by the IBLC model).