According to the sintering theory,

According to the sintering theory, the activation energy of sintering should be consistent with that of the apparent diffusion coefficient of the rate-limiting species of sintering. In sintering polycrystalline ceramics, it is common that both grain boundary and lattice diffusion of the rate-limiting species can simultaneously contribute to densification. Some reports [8,9] have suggested that the change of the relative contribution of Dl and Db (Dl is the lattice diffusion and Db is the grain boundary diffusion) due to the difference of the impurity level or grain size can affect not only the preexponential factor but also the activation energy of the apparent diffusion coefficient. However, the relative contribution of Dl and Db also is different at different temperatures even impurity level and grain size are the same because these two diffusivities have different temperature-dependence.