Propriedades estruturais, elétricas e ópticas de CaCu3Ti4O12 (CCTO) e seu compósito BaTiO3(BTO)- CaCu3Ti4O12(CCTO) para uso em dispositivos de microondas e média-frequência

Abstract

Mechanical alloying has been used successfully to produce nanocrystalline powders of CaCu3Ti4012 (CCTO). The milled CCTO were studied by x-ray powder diffraction, infrared and Rarnan scattering spectroscopy. For two different milling procedures, CCTO was obtained after a couple of hours of milling (in average 30 hours of milling, depending in the reaction procedure). After 100 hours of milling the formation of CCTO was confirmed by x-ray powder diffraction in both procedures, with good stability. We also prepare the CCTO ceramic using the traditional procedure described in the literature, to compare with the milled ceramics. This milling process presents the advantage that melting is not necessary. The material can be compacted and transformed in solid cerarnic sarnples or used in others procedures of film preparation. The effect of the presence of CCTO (CaCu3Ti4012) in the dielectric and piezoelectric properties of the barium titanate (BTO-BaTi03) thick film was also studied. These films were prepared in two layers geometry using the screen printing technique on Al203 substrates. Mechanical alloying followed by the solid state procedure has been used successfully to produce powders of CCTO (CaCu3Ti4012) used in the films. The sarnples were studied using X-Ray diffraction, scanning electron microscopy (SEM), Rarnan and lnfrared spectroscopies, dielectric and loss measurements. We did a study of the dielectric permittivity and loss in the radio-frequency range (100Hz-10MHz), of the films. The role played by firing process in the film preparation and the crystallite size of CCTO and BTO in the dielectric constant and structural properties of the films are discussed. The traditional cerarnic procedure (solid state procedure) was used to produce bulk cerarnics of BTO (BaTi03), CCTO (CaCu3Ti4012) and (BTO)0.5:(CCTO)0.5 that were studied in the low frequency and microwave range of frequencies. The composite ceramic (BTO)0.5:(CCTO)0.5 is presenting a tendency to decrease the loss with frequency, which seems to be a mixing effect of the two phases. One can say that the mixture of BTO-CCTO ceramic is presenting quite distinct behaviour when one compares the low frequency and microwave regions of the spectra. In the low frequency region, the increase of the CCTO presence is increasing the dielectric constant (up to 1 kHz frequency). However in the microwave region one has the opposite behaviour. The presence of the CCTO phase is decreasing the K value and the loss factor. Considering the classic treatment applied for two or more phases present in a dielectric and the empirical logarithmic rule for the dielectric constant (K) and the dielectric constants (K1) of the individual phases oe conclude that in the microwave region of the spectra the value of the dielectric constant is in good agreement with the rule. However in the low frequency region of the spectra the deviation is quite strong. The measures in low temperatures show that, in the range of studied frequency, K is strongly dependent of the temperature and frequency. The bulk of the composite presents the same behaviour of BTO 100 and CCTO100, that it is of reducing K with the increase of the frequency and decrease of the temperature. Measurements of the performance of a planar antenna confirm the potential use of such materiaIs for small high dielectric planar antennas (HDA). These materials are also very promising for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the miniaturization of the devices is crucial.