This phenomenon could be attributed

This phenomenon could be attributed to agglomeration of the MgO particles when the MgO content f is larger than 0.125wt.%. One can see from SEM images and EDS results that MgO nanoparticles are evenly distributed in the grain boundaries (GBs) of CSFS in CSFS/0.075wt.%MgO and CSFS/0.125wt.%MgO composite samples (shown in Figure 1r and Figure 2, respectively). In contrast, as f increases from 0.125wt.% to 0.5 and 1wt.%, MgO particles are not evenly distributed and they are frequently agglomerated in GBs of CSFS matrix (as shown in Figure 2r), which causes decrease of the number of effective MgO particles that give rise to energy dependent carrier scattering, leading to reduction of λ, as we observed in Table 1.