Síntese e caracterização de compósitos mulita-fase vítrea usando resíduo do beneficiamento do caulim

Abstract

The processing of mullite-based composites is essential in materials science and engineering due to their distinctive properties and wide range of applications. However, significant challenges remain in the reuse of waste as sustainable raw materials, appropriate selection of additives, and application of new processing methodologies to optimize properties and reduce costs in obtaining these composites. This study is divided into four chapters, each addressing specific aspects of the research. Chapter 01 comprises a detailed introduction and comprehensive literature review on the subject. Chapter 02 investigates the effects of adding TiO2 on the physical, electrical, and dielectric properties of mullite-glass composites obtained by sintering mixtures of kaolinitic clay and kaolin waste with additions of up to 1% TiO2. Cylindrical pellets were obtained by uniaxial pressing at 40 MPa and sintered at 1400 °C. The obtained composites were characterized by X-ray diffraction (XRD/Rietveld), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), impedance spectroscopy (IS) (30°C and 400-800 °C). Porosity and apparent density were determined by the Archimedes’ principle. Electrical conductivity, dielectric constant, and dielectric loss depend heavily on microstructural characteristics such as glassy-phase and porosity and TiO2 addition. The activation energy (0.89 - 0.98 eV) for electrical conduction was lower than typical values found in the literature. The results provide evidence that the obtained mullite/glass composites are promising materials for electronics applications. Chapter 03 focuses on applying statistical tools to understand the influence of processing parameters on the physical properties of mullite/glass composites obtained using kaolin waste as the sole raw material. Additionally, it seeks to optimize the balance between cost and time in sintering processes. Cylindrical pellets were obtained following a full factorial 22 × 2 split-plot design. The influence of temperature, sintering time, and pressure on physical properties was evaluated. The structural and microstructural properties of the sintered samples were assessed by X-ray diffraction (XRD/Rietveld), scanning electron microscopy (SEM-FEG), and energy-dispersive spectroscopy (EDS). Porosity and apparent density were determined by the Archimedes’ principle. The estimated p-values indicate that the coefficients associated with applied pressure and its interaction with temperature and sintering time (γ and δ1) were statistically significant for porosity and apparent density. Higher pressure leads to lower apparent porosity in the analyzed system. The densification process was influenced by different sintering mechanisms depending on temperature (solid-phase sintering vs. liquid-phase sintering). Through the application of DoE with randomized restriction (split-plot design), it was possible to reduce furnace utilization by up to 80% in sintering processes. Finally, Chapter 04 presents the study's conclusions, a list of bibliographic references, and suggestions for future work, indicating areas that can be explored to deepen knowledge on the addressed topic.