Thermally activated rotation of Ni1−xO particles within CaO grains

CaO/Ni1−xO powders in 4:1 molar ratio were sintered and then annealed at a homologous temperature (T/Tm) of 0.85 (1400°C) for 1–80 h and 0.74 (1200°C) for up to 300 h to study the orientation change of Ca-doped Ni1−xO particles within Ni-doped CaO grains by transmission electron microscopy. At 0.85Tm, the submicro-sized Ni1−xO particles (denoted as N) readily reached parallel epitaxy relationship, i.e. [100]N//[100]C, [010]N//[010]C, and occasionally [100]N//[100]C, [001]N//[011]C (denoted as 45°-off epitaxy) with respect to the host CaO grain (denoted as C). Larger particles require a longer annealing time (1 μm/10 h, 1.5 μm/20 h and 2 μm/80 h) to be in epitaxial orientation. By contrast, prolonged annealing at 0.74Tm failed to cause appreciable orientation change of the intragranular Ni1−xO particles regardless of the size of the particles. The parallel epitaxial Ni1−xO particles are more or less (111)N,C and (100)N,C faceted, whereas the 45°-off epitaxial particles are (100)N,C faceted. The effects of temperature, size and energy cusp on orientation change of the intragranular particles is in accordance with theoretical consideration of Brownian type rotation of the particle above a critical temperature for anchorage release at interface and a beneficial lower interfacial energy for the epitaxial relationship.