Investigação experimental sobre a estabilidade de fases em cerâmicas de Al2O3-Nb2O5-Y2O3

Abstract

Al2O3-Y2O¬ ceramics based on the YAG phase - Yttrium Aluminum Garnet (Y3Al5O12) have the potential for applications in aggressive environments and at high temperatures due to their corrosion resistance and creep resistance at temperatures above 1500ºC, while Al2O3-Nb2O5 ceramics they have attractive features for use as biomaterials and catalysts for oil refining. In addition, YNbO4-Nb2O5-Y2O3 ceramics have been considered for structural applications due to their mechanical properties at high temperatures. According to the phase diagram of the Al2O3-Y2O3 system, the following solid intermediate phases are presents: YAG, YAP and YAM. In the phase diagram of the Al2O3-Nb2O5 system, the existence of YNbO4 and AlNb11O29 phases is reported. However, information on the phase stability of Al2O3-Nb2O5-Y2O3 ceramics is limited in the literature. In this context, the present work aims at the experimental investigation of the phase stability in Al2O3-Nb2O5-Y2O3 ceramics. High-purity raw materials (> 99% -mass) of aluminum, niobium and yttrium oxides wehe used for preparing the Al2O3-Nb2O5-Y2O3 ceramics. Considering the invariant reactions and the biphasic regions of the corresponding available binary systems, the chemical compositions of the ternary ceramics were chosen aiming at the determination of three-phase regions. To homogenize, the powder mixtures weighing close to 1,5g were properly mixed for 60 minutes, in a SPEX mill using a WC-Co vial (80 mL) and balls (approximately 5 mm diameter). Then, the ground materials of Al2O3-Nb2O5-Y2O3 were compacted using an axial load near the 40 MPa, in order to prepare cylindrical samples with 10mm in diameter and 5mm in height. To reach the thermodynamic equilibrium of the Al2O3-Nb2O5-Y2O3 ceramics, the compacted samples were heat treated at 1300ºC for 72h and 150h (totaling 222 hours). The heat-treated samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS) techniques. From the X-ray diffraction results of the samples heat-treated for 72h, the following three-phase regions were confirmed, being only one of them biphasic: Y2O3+Y4Al2O9+Y3NbO7, Y3NbO7+Y3Al5O12+YNbO4, Y3NbO7+Y3Al5O12+YAlO3, YNbO4+Al2O3, YNbO4+AlNb11O29+AlNbO4, YNbO4+AlNb11O29+Al2O3∙9Nb2O5, YNbO4+Y3Al5O12+Al2O3 and YNbO4+Al2O3+AlNbO4. Samples heat-treated for 150h have confirmed the existence of three-phase fields already identified and to determine the others three-phase regions YNBO4+AlNb11O29+AlNb49O124 and YNBO4+AlNb49O124+Nb2O5.