Mechanical and thermal properties of graphene nanosheets/magnesia composites

Highly dense graphene nanosheets (GNSs)/magnesia (MgO) composites were prepared by hot-pressing. The effect of GNSs content (0 vol%–7 vol%) on microstructural, mechanical and thermal properties was evaluated, and the mechanisms were identified. Incorporating GNSs inhibited the sintering and grain growth of MgO and produced a significant strengthening and toughening effect. With the increase in GNSs content, both flexural strength and fracture toughness increased and then decreased: the highest strength of 265 MPa and toughness of 3.3 MPa m^1/2 were obtained at 2 vol%, increasing by 37.3% and 32%, respectively, compared with monolithic MgO. Vickers hardness and modulus declined linearly. Thermal conductivity in the hot pressing direction decreased from 55.8 W/(m K) for MgO to 33.9 W/(m K) for 7 vol% GNSs/MgO. With the increase in temperature, both monolithic MgO and 2 vol% GNSs/MgO exhibited a decrease in thermal conductivity, which tended to be consistent and constant beyond 700 °C. The 2 vol% GNSs/MgO composite also showed nearly the same CTE as that of monolithic MgO.