Análise da performance das propriedades dielétricas da matriz Li2LaNbTiO7 e seus compósitos e ligas sob variação de temperatura

Abstract

This study presents investigations of the dielectric properties of the ceramic matrix Li₂LaNbTiO₇ (LLN), with the addition of 5%, 20%, and 40% by weight of titanium dioxide (TiO₂). The samples were prepared using the solid-state reaction method, followed by calcination at a temperature of 1050°C. After calcination, the powders were pressed into cylindrical shapes and sintered at 1150°C. The resulting ceramic pieces were characterized for their structure and morphology using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), respectively. The XRD analysis confirmed the formation of the LLN matrix without the presence of spurious phases. In the composite analyses, the presence of new phases originating from the reaction of TiO₂ with the LLN matrix was observed. Dielectric characterization was carried out in the radiofrequency (RF) and microwave (MW) regions. A temperature-dependent study of the properties was conducted for potential applications in RF and MW devices. The study of the ceramics in the MW range suggests that the LLN-TiO₂ composites exhibit properties suitable for application as dielectric resonator antennas (DRA). Based on the results obtained using the Hakki-Coleman method, a dielectric permittivity (ε) of 15.10 was observed for the pure sample and 20.28 for the sample with a 40% addition of TiO₂, with dielectric loss tangents on the order of 10-3 for all samples. The resonance frequency temperature coefficient (τf) results showed that with the addition of 40% TiO₂, thermal stability was achieved with a value of τf=+7.33 ppm C-1. Numerical simulations were employed to evaluate the behavior of the LLN matrix as dielectric resonator antennas (DRA). The LLN matrix achieved a gain of 3.71 dBi, a bandwidth of 144 MHz, and a radiation efficiency exceeding 97%. In the RF region, the TCC study demonstrated that at 1 MHz, signal inversion occurs when analyzing the LLN and LLN5 samples, indicating that for a TiO₂ concentration between 0% and 5%, a composition with zero TCC can be identified—an ideal characteristic for applications in electronic devices requiring thermal stability. The AC conductivity study at different temperatures revealed a thermally activated conduction process, with activation energy (Ea) values ranging from 0.65 to 0.54 eV. Nyquist plots were fitted using an equivalent circuit model comprising two R-CPE associations related to grain and grain boundary effects.