Explain the effects of solute/dopnats/impurities on the sintering behavior of ceramics. How could solutes push the pore separation region towards higher densities and larger grain sizes?

Figure 2. Grain size- density map for alumina. Depicted are the effects of raising surface diffusivity by a factor of 4; lowering lattice diffusivity by a factor of 2; and lowering grain boundary mobility by a factor of 34. This raises pore seperation region and lowers grain size trajectory (dotted lines), making it possible to sinter to complete density.