Microstructure and chemistry of second phases in MgO- and NiO-codoped alumina by analytical transmission electron microscopy

Abstract

Small amounts of additives can greatly affect the sintering of ceramic powders. Since the addition of small amounts of MgO (~ 0.25 wt%) to A1203 enables it to sinter close to theoretical density [1], the influence of small amounts of additives on the sintering of A1203 has been widely studied [2-4]. The additives form second phases which profoundly affect the sintering behaviour, i.e. reduce grain boundary mobility and inhibit exaggerated grain growth. MgO inhibits grain growth in fully dense alumina, and the degree of inhibition depends on the purity of the starting powder [5]. Studies have also been conducted concerning the influence of NiO on grain growth in alumina [4]. NiO has been reported to behave similarly to MgO [4, 6]. Sintering in MgOand FeO-codoped alumina has been studied by Zhao and Harmer [7] in order to investigate the role of multiple solid-solution additives in sintering. They observed that MgO inhibits grain growth strongly in very pure powders and FeO promotes grain growth more than densification in alumina. Therefore, FeO was not a favourable additive. In the study reported here, MgO and NiO were used as multiple solid-solution additives. The MgO-NiO system consists of a complete solid solution extending from the higher melting point of MgO to the lower melting point of NiO [8]. The purpose of this study was to investigate the microstructure and chemistry of second phases, segregated particles and crystalline defects in alumina codoped with MgO and NiO using analytical transmission electron microscopy (TEM). As a result, it became possible to infer the location of MgOand NiO-codopants and impurities during the sintering process. Alumina codoped with 0 .15wt% MgO and 0.10 wt % NiO was fabricated by hot pressing at 1480 °C and 27.58 MPa in vacuum. TEM samples were sectioned from the alumina using a low-speed diamond saw and mechanically ground to -300 ~m. Circular discs of 3 mm in diameter were core drilled from the ground sections, mechanically ground to -120/~m, then dimpled to -30/ ira . The samples were ion milled with 5 kV Ar ÷ ions at an incident angle of 12 ° until perforation was achieved. A light carbon film was evaporated on the samples to prevent charging in the electron microscope. The microstructure and chemistry of second phases, segregated particles and crystalline defects in the alumina were investigated by bright field image, convergent beam electron diffraction (CBED),