Anisotropic microstructure and properties of GNSs/MgO microwave-attenuating composite ceramics

Graphene nanosheets (GNSs)/MgO microwave attenuating composite ceramics with 0–15 vol% GNSs as an attenuating agent were prepared by hot-pressing, and the anisotropy of the microstructure, mechanical, thermal, conductive, dielectric, and microwave absorption properties was investigated. Although strengthening and toughening were obtained in both two directions, the strength and toughness for the 2 vol% sample were 34.6 MPa and 0.6 MPa m^1/2 higher in the in-plane direction than the through-thickness direction, respectively. With an increase in the GNSs content from 0 to 15 vol%, the room-temperature thermal conductivity decreased significantly from 55.8 W m⁻¹ K⁻¹ to 20.6 W m⁻¹ K⁻¹ in the through-thickness direction, while it decreased slightly and then remained at 51 W m⁻¹ K⁻¹ after 5 vol% in the in-plane direction. The in-plane direction also exhibited a slightly higher conductivity, as well as lower percolation threshold and critical exponent (2.71 vol% and 1.30 vs. 3.34 vol% and 1.51). Despite the substantially higher complex permittivity and dielectric loss in the X-band, the 2 vol % sample exhibited poorer microwave absorption properties (the minimum reflection loss was −10.5 dB at 8.3 GHz) in the in-plane direction than the through-thickness direction.