Investigação experimental sobre a estabilidade de fases em cerâmicas de Al2O3-Nb2O5-Ta2O5

Abstract

Alumina (Al2O3) is a material widely used in the production of advanced ceramics due to its high mechanical strength, while the Al2O3-Nb2O5 ceramics have been considered for use in supported catalysts, as well as those containing Ta2O5. However, information in the literature about phase transformations in Al2O3-Nb2O5-Ta2O5 ceramics is limited. Thus, the present work aimed at an experimental investigation on the phase stability of Al2O3-Nb2O5-Ta2O5 ceramics. Information on invariant reactions at 1300oC from the binary Al2O3-Nb2O5, Al2O3-Ta2O5 e Nb2O5 diagrams were adopted to determine the triphasic regions in this ternary phase diagram. High-purity raw materials (Al2O3, Nb2O5 and Ta2O5) were used to prepare the 64Al2O3-23.2Nb2O5-12.8Ta2O5, 35Al2O3-30Nb2O5-35Ta2O5, 15Al2O3-20Nb2O5-65Ta2O5, 22.6Al2O3-57.4Nb2O5-20Ta2O5, 25Al2O3-70Nb2O5-5Ta2O5, 3.5Al2O3-86.5Nb2O5-10Ta2O5 and 1Al2O3-89Nb2O5-10Ta2O5 ceramics (mol-%). Different powder mixtures were carefulness weighed and homogenized in a SPEX mill using WC-Co vial (80 mL) and Al2O3 balls with 10mm diameter. Following, the Al2O3-Nb2O5-Ta2O5 powder mixtures were compacted using an axial load close to 1 ton (approximately 125 MPa) to obtain the cylindrical samples with 10 mm diameter. Initially, the green compacted Al2O3-Nb2O5-Ta2O5 samples were heat-treated at 1300ºC for 72 h. To ensure that the equilibrium microstructures were reached, the green compacted of Al2O3-Nb2O5-Ta2O5 samples were heat treated at 1300 ºC for 222 h. X-ray diffractometry measurements were performed to identify the phases formed in heat-treated samples using the Rietveld method refining, together with scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses. Thus, the following two-phase and triphasic regions were confirmed for the isothermal section at 1300oC of the Al2O3-Nb2O5-Ta2O5 system: Al2O3+AlNbO4+AlTaO4, AlNbO4+AlTaO4+β–Ta2(1-x)Nb2xO5, AlTaO4+Ta2O5+β–Ta2(1-x)Nb2xO5, AlNbO4+AlNb11O29+α–Nb2(1-x)Ta2xO5 and AlNb49O124+Nb2O5+α–Nb2(1-x)Ta2xO5. Despite on some evidences on existence of the triphasic AlNbO4+α–Nb2(1-x)Ta2xO5+β–Ta2(1-x)Nb2xO5 and AlNb11O29+AlNb49O124+α–Nb2(1-x)Ta2xO5 regions, others studies will be required.