▎ 摘　　要

NOVELTY - Composite material comprises a polymer material and reduced graphene oxide distributed within the polymer material. USE - The composite material is useful as field grading material that reduces electric field stress at a joint or termination of electric cables (all claimed), electric insulating material in medium or high voltage cable systems (cable insulation), and in machines bushings, generators, instrument transformers and capacitors. ADVANTAGE - The composite material: exhibits non-linear resistivity, a dielectric constant of at least 3, preferably 5-8, and a conductivity saturation of 1.00E-10 to 1.00E-8 (all claimed); prevents heat generation and failure at working voltage; exhibits good control of thermal runaway, capacitive field grading effect and larger breakdown strength; and allows adjusting of switching field and conductivity in field grading. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for: (1) reducing electric field stress at a joint or termination of an electric cable, comprising applying a field grading material having reduced graphene oxide distributed within the polymer material, to the joint or termination; (2) preparing a graphene oxide polymer material, comprising thermally reducing graphene oxide by increasing an ambient temperature of an atmosphere containing the graphene oxide to 70-160 degrees C, mixing the thermally-reduced graphene oxide with at least one polymer material and curing the mixture; and (3) the field grading material exhibiting non-linear resistivity, comprising the reduced graphene oxide distributed within the polymer material.