Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al 2 O 3 -4 Wt Pct Nb 2 O 5 Ceramic Compoun
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Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al2O3-4 Wt Pct Nb2O5 Ceramic Compound J.L. SANTOS, R.L.S.B. MARC¸AL, P.R.R. JESUS, A.V. GOMES, E.P. LIMA Jr., S.N. MONTEIRO, J.B. DE CAMPOS, and L.H.L. LOURO Different amounts of LiF were added to an Al2O3-4 pct Nb2O5 basic ceramic, as sintering agent. Improved new ceramics were obtained with LiF concentrations varying from 0.25 to 1.50 wt pct and three sintering temperatures of 1573 K, 1623 K, and 1673 K (1300 °C, 1350 °C, and 1400 °C). The addition of 0.5 wt pct LiF yielded the highest densification, 94 pct of the theoretical density, in association with a sintering temperature of 1673 K (1400 °C). Based on X-ray diffraction (XRD), this improvement was due not only to the presence of transformed phases, more precisely Nb3O7F, but also to the absence of LiAl5O8. The preferential interaction of LiF with Nb2O5, instead of Al2O3, contributed to increase the alumina sintering ability by liquid phase formation. Scanning electron microscopy (SEM) results revealed well-connected grains and isolated pores, whereas the chemical composition analysis by energy dispersive energy (EDX) indicated a preferential interaction of fluorine with niobium, in agreement with the results of XRD. It was also observed from thermal analysis that the polyethylene glycol binder burnout temperature increased for all LiF concentrations. This may be related to the formation of hydrogen bridge bonds. DOI: 10.1007/s11661-017-4271-y Ó The Minerals, Metals & Materials Society and ASM International 2017
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INTRODUCTION
IN the ceramic processing of alumina (Al2O3), the use of relatively small concentrations of additives is relevant for microstructure control in association with properties improvement.[1,2] Among the several additives, those acting as sintering agents may contribute to reduce the hot-pressing temperature and increase the ceramic final density. This is the case of niobium oxide (Nb2O5), which was found to significantly reduce the Al2O3 sintering temperature.[3–10] Indeed, a temperature reduction from 1873 K to 1723 K (1600 °C to 1450 °C) was achieved with 4 wt pct addition of Nb2O5 due to the formation of liquid phase.[9] Furthermore, a specific technical consequence is Al2O3 embrittlement by grain boundary precipitation of AlNbO4. This provided an advantage for multilayered ballistic armors with a front Al2O3-4 wt pct Nb2O5, which J.L. SANTOS is with the Materials Science and Engineering Program, Military Institute of Engineering, Rio de Janeiro, Brazil and also with the Production Engineering Department, Esta´cio de Sa´ University, Rio de Janeiro, Brazil. Contact e-mail: [email protected] R.L.S.B. MARC¸AL, P.R.R. JESUS, A.V. GOMES, E.P. LIMA JR., S.N. MONTEIRO, and L.H.L. LOURO are with the Materials Science and Engineering Program, Military Institute of Engineering. J.B. DE CAMPOS is with the Mechanical Engineering Department, Rio de Janeiro State University, Rio de Janeiro, Brazil. Manuscript submitted January 9, 2017.
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