Influência da evolução de fases nas propriedades dielétricas de cerâmicas de CaCu3Ti4O12 sintetizadas em distintas atmosferas

Abstract

Dielectric ceramics based on CaCu3Ti4O12 (CCTO) are promising for energy storage devices due to their colossal dielectric constant values (> 104). This intriguing property results from the combination of intrinsic contributors, such as crystalline defects, and extrinsic contributors, such as the microstructure formed by semiconducting grains and electrically insulating grain boundaries. These contributions are extremely dependent on the synthesis and processing methodology of CCTO ceramics, as well as on the atmospheres used during their production, since they affect microstructural aspects, such as grain sizes, presence and distribution of secondary phases, and structural parameters such as oxygen vacancy formation. Thus, this work investigated the influence of crystalline defects and microstructure on the dielectric properties of CCTO ceramics. For this purpose, ceramics with stoichiometric variations, given by CaxCu3,0Ti4,0O12+δ (x = 1.0 or 1.1), were produced in distinct atmospheres. Ceramic powders were synthesized using the coprecipitation method, followed by the calcination step, performed for 6 hours at 850 °C, in atmospheric air or pure oxygen. Disk-shaped samples were sintered at two different temperatures, 1050 or 1100 °C, with a 2-hour dwell at the same atmospheres used during calcination. Subsequently, selected ceramics were subjected to heat treatment (post sintering) in pure helium atmosphere at 650 °C for 15 minutes, to evaluate the influence of this atmosphere on dielectric properties. Differential thermal analyses and thermogravimetric analyses allowed identification of the thermal processes of phase formation and decomposition. Structural analyses performed by X-ray diffraction on the calcined powders and sintered ceramics made it possible to evaluate the evolution of CCTO, CuO, TiO2 e CaTiO3 phase formation, quantified using the Rietveld method. Using scanning electron microscopy, micrographs of ceramic powders and fractured surfaces of sintered samples were obtained, allowing determination of the particle and grain size distributions, respectively. X-ray photoelectron spectroscopy (XPS) enabled identification of multiple valence states for Cu and Ti ions, correlating them with oxygen vacancy formation as a function of the atmospheres used during sample preparation. Positron annihilation lifetime spectroscopy (PALS) provided direct evidence of the presence of oxygen vacancy-type defects and the existence of subgrains within CCTO grains. Using impedance spectroscopy in the range of 20.0 Hz to 5.0 MHz and temperatures between 20 and 200 °C, values of dielectric constant (εᵣ) and loss factor (tanδ) were determined, as well as complex resistivity plots (ρ' × −ρ'') and electric modulus (M' × M''), and the equivalent circuits that allowed identification of three microstructural constituents, associated with grains, subgrain boundaries, and grain boundaries containing deep trap states. The activation energies calculated for samples without heat treatment after sintering varied between 0.316–0.769 eV for subgrains, 0.496–0.666 eV for grain boundaries, and 0.415–0.752 eV for deep trap states. The Ca1,1Cu3,0Ti4,0O12 sample, calcined at 850 °C and sintered at 1100 °C in oxygen, exhibited the highest dielectric constant value, 𝜀𝑟′ = 39,5×103, with tanδ = 0,23, at 1 kHz. The heat treatment in helium resulted in a reduction of 𝜀𝑟′ values and resistivities of the three identified microstructural constituents, attributed to additional oxygen vacancy formation, partial reduction of cations (Cu2⁺ → Cu⁺; Ti4+ → Ti3⁺), and structural reorganization which generated subgrain boundaries, promoting a decrease in Maxwell-Wagner interfacial polarization. The results demonstrate that, in addition to stoichiometric manipulation, atmospheric control during sintering and subsequent heat treatment is viable and fundamental for defect engineering and optimization of CCTO dielectric properties.