Based on these results, we propose

Based on these results, we propose a formation mechanism for the surface coating and the effect on the CSP. Fig. 8 shows a schematic diagram that depicts the mechanism. Uncoated BaTiO3 nanoparticles have a Ti-rich amorphous layer. The amorphous layer, which is hard to dissolve in the liquid phase, hinders the dissolution-precipitation process during the CSP. In this case, the density of the BaTiO3 ceramic is not high. When the BaTiO3 nanoparticles are coated by the BaTiO3 sol, the Ti-rich amorphous layer disappears and an ultrathin layer of BaTiO3 forms after calcination at 600 °C. The ultrathin layer can easily decompose and dissolve in the liquid phase, which enhances the dissolution-precipitation process during the CSP. The BaTiO3-coated nanoparticles promote close packing in the ceramic and thus enhance the density of the ceramic.