In the case of the ZnO, polyetherimide, Mn2O3 and CoO composite (Figure 2c and h), the relative density was around 88%, which is noticeably lower than that of the first two cases. This can pertain to inhomogeneous mixing between the ceramic particles and the polymer phase, which it can form aggregation and hinder the ceramic phase densification. Hence, obtaining uniform dispersion of polymer and metal oxides, such as using cryogenic grinding, will be necessary to advance understanding on structure–property relationship of multiphase composites.