Most sintered samples were studied by X-ray diffraction (XRD;
Model D-500, Siemens, Karlsruhe, Germany), with a scanning
step (2 ) of 0.05° and a scanning rate of 0.1°/s. Cross sections of
polished and etched sintered samples were observed under an
optical microscope (OM), and the grain sizes were measured by
the intercept method. Since, for the most part, the grains grow as
thin plates, the grain sizes reported here are the diameters, or
longer axes, of the grains. The samples were studied with field
emission scanning electron microscopy (FESEM) and energy
dispersive spectroscopy (EDS).