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Bi-2212 oxide superconductors have been synthesized by a diffusion process between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating layer. The substrate is formed into a shape of cylindrical rod 3 mm in diameter by CIP, and then sintered. The coating layer with Ag/sub 2/O addition is coated around the surface of the substrate rod. The heat treatment is performed at 820-860/spl deg/C to produce Bi-2212 diffusion phase. The Ag added to the coating layer enhances the diffusion reaction. The Bi-2212 diffusion layer, about 180 /spl mu/m in thickness, shows high density and oriented structure with plate-like grains. The transport I/sub c/ and J/sub c/ of the diffusion layer exceed 300 A and 2/spl times/10/sup 4/ A/cm/sup 2/, respectively, at 25 K and 0 T. The I/sub c/ decreases to about 100 A at 40 K and 0.5 T. Present investigation indicates that the bulk Bi-2212 oxide prepared by the diffusion process may be attractive for a current lead of cryocooler-cooled superconducting magnets.