▎ 摘　　要

Different amounts of graphene-wrapped magnesium oxide (G@MgO) powders are uniformly dispersed in poly(vinyl alcohol) (PVA) solution in different experiments to obtain solutions which are coagulated to obtain solid materials, which are then hot pressed at 413 K and 3 t of pressure to finally obtain 1 mm thick freestanding G@MgO/PVA composite sheets in which the constituents, namely, graphene and MgO (in the form of G@ MgO), are the nanofillers in PVA matrix. During synthesis of G@MgO powder, MgO nanoparticles are in situ wrapped by the graphene nanosheets as revealed by electron microscopy. Uniformity of G@MgO dispersion in PVA was confirmed by secondary electron micrographs and the consistency in Xray diffraction and Raman scattering data collected from different locations of the samples. Temperature (303-393 K) dependent complex permittivity of G@MgO/PVA composite sheets (including those prepared by casting) in low frequency (20 Hz to 2 MHz) and high frequency (i.e., X-band, 8.2-12.4 GHz) ranges are measured. In both frequency ranges, G@MgO/PVA composite sheets prepared by coagulation exhibited dielectric properties superior to those of PVA and G@ MgO/PVA composite sheets prepared by casting. A strong interfacial polarization is observed in coagulated and as-cast G@ MgO/PVA composite sheets. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both composite sheets' cases, while it is predominating in coagulated composite sheets due to the better network formation of the fillers in the coagulated samples than in the cast composite samples. The electromagnetic interference shielding effectiveness (EMI SE) values in the X-band frequency range (i.e., 8.2-12.4 GHz) of the G@MgO/PVA composite sheets prepared by coagulation are more than those prepared by casting for a particular weight fraction of G@MgO. At 393 K, for a particular G@MgO/PVA composite sheet prepared by coagulation, an excellent EMI SE of '-27.5 dB is measured. It is also experimentally elucidated that the absorption is the dominating mechanism for EMI SE in the prepared composite sheets.