▎ 摘　　要

NOVELTY - The method involves providing a cathode structure having an outer surface. A sealing layer comprising 1-20 graphene sheets is deposited (303) directly on the outer surface of the cathode structure by co-depositing a dopant and carbon on the outer surface of the cathode structure to form a doped graphene sheet. The graphene sheets are conformed to the outer surface of the cathode structure. The cathode structure has a planar shape, a cylindrical shape, a helical shape or an irregular shape. The carrier substrate is a copper foil or a nickel foil. USE - Method for manufacturing thermionic cathode used in high power microwave device, compact electron accelerator, amplifier tube, klystron and medical x-ray imaging machine. ADVANTAGE - The graphene layer is doped using compounds similar or identical to the additive to improve the emission efficiency and lifetime of the thermionic cathode. The surface monolayer or multiple layers of graphene prevents the loss of the additive, which has low work function and provides photo-activating function. The lifetime is extended due to the monolayer or multiple layers of graphene. DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating a method for depositing a graphene layer on a substrate. Step for pre-treating a carrier substrate (301) Step for depositing a sealing layer comprising graphene sheets on the outer surface of the cathode structure (303) Step for applying a polymer film on the graphene layer (305) Step for removing the carrier substrate (307) Step for attaching the polymer film-graphene layer on the outer surface of the cathode structure (309)