In order to find out the reason for

In order to find out the reason for the different TCC performance of the samples, TEM–EDS analysis has been conducted.Figure 3 shows the TEM images of the typical ceramic grains and EDS line-scan analysis results of the core–shell structure in the grains. The statistic results of dopants distribution in these samples are also illustrated in Fig. 3. It can be seen that, compared with A1-sample (by CS), the shell of A2-sample (by TSS) is thinner and it has a relatively higher intensity of dopants. It is because that during the first-stage sintering process, the kinetics for the doped elements diffusion is quite limited due to the very short holding time (0–10 min) at high temperature (11901C). Therefore, the shell region of TSS-sample is quite thin. This result is similar to others’ research, which reported that the fast-firing method was beneficial to control grain size, core–shell structure, and improvement of the TCC performance.8,9 Moreover, during the first-stage sintering, the samples achieved an intermediate density, and then during the second-stage sintering the samples reached full density without grain growth at a lower temperature, which is owing to the suppression of grain-boundary migration while keeping grain-boundary diffusion active.6 Similar results were also found in B1- and B2-samples. Therefore, the dopants in A2- and B2-sample (TSS-sample) were mainly concentrated in a very short distance from the grain boundary and the shells of TSS-samples were quite thinner than those of CS-samples. Additionally, by the present study, the finest grain size of the dielectric ceramic is as small as 100 nm (A2-sample) by TEM observation.