Characterisation of fine-grained oxide ceramics

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A range of high resolution techniques have been used to characterise the grain boundary segregation behaviour of rare earth (RE) doped (La, Gd, Eu and Yb) alumina and spinel. TEM based techniques (HR-TEM, HAADF STEM and EDS) have been used to study the structure and chemistry of grain boundaries. The use of a HAADF detector in STEM provides atomic number contrast and easy identification of heavy (RE) segregants. This has been used to produce high resolution RE elemental maps, showing the width of the segregated region to be less than the size of the electron probe (1 nm) for all boundaries studied. EDS showed that within a 1 nm thick boundary region (an upper limit) the RE cations would account for 10 (±2)% and 15 (±2)% of the cation total in alumina and spinel respectively. Preliminary results from ultra-high resolution STEM (probe size ∼0.1 nm) suggest that, in spinel, the segregated region is actually composed of a much thinner continuous monolayer of RE atoms at the grain boundary. This is consistent with HR-TEM, which showed spinel grain boundaries possessed minimal grain boundary structural disorder.AFM has been used to study the effect of RE grain boundary segregation on thermal grooving behaviour. The improvement in resolution that is achieved by operating in Tapping™ mode is shown to translate into an improved profile of the groove root. This has been used in conjunction with Electron Backscattered Diffraction (EBSD) to examine the relationship between grain boundary geometry and misorientation. The addition of RE dopants to alumina was found to significantly increase the size of grain boundary grooves. This can be attributed to the out-diffusion of RE segregants, an effect which compromises grain boundary energy calculations for materials with grain boundary segregation. AFM and EBSD are also used to relate anisotropic tribo-chemical polishing-wear with grain orientation.

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