Abstract: A process is disclosed for producing carbon fibers with modified surfaces by electrolytic treatment. In the process, electric current is passed between carbon fibers and a counter electrode in an electrolytic solution to which an aromatic compound having at least one hydroxyl group and/or amino group is added.

Abstract: A method for after-treatment of carbon fiber, which comprises by electrolytic oxidation treatment in an aqueous solution having an ammonium ion concentration of 0.2 to 4.0 mol/l and a pH of at least 7 using the carbon fiber as an anode, followed by treating the resulting fiber in water with an ultrasonic wave of at least 20 kHz frequency at an intensity satisfying the condition: ##EQU1## wherein, F is the frequency (kHz) and T is the treatment period (min) provided that T>0.1.

Abstract: A process for surface treatment of carbon fiber of which modulus is not lower than 30 ton/mm.sup.2, in two steps, which comprises the steps of subjecting a carbon fiber as an anode to a first electrolytic treatment in an aqueous solution of inorganic acidic electrolyte or of neutral salt electrolyte, and then subjecting the treated fiber to a second electrolytic treatment in an aqueous solution of ammonium salt of carbonic acid or of inorganic alkaline electrolyte.

Abstract: Disclosed are polyacrylonitrile carbon fibers excellent in substrate strength and adhesion to matrix resin wherein average nitrogen content in the whole carbon fibers which is measured by elementary analysis is 0.5-4.5% by weight and average nitrogen concentration (N/C atom number ratio) in the whole carbon fibers which is measured by elementary analysis and surface silicon concentration (Si.sub.2p /C.sub.1s atom number ratio), surface nitrogen concentration (N.sub.1s /C.sub.1s atom number ratio) and surface oxygen concentration (O.sub.1s /C.sub.1s atom number ratio) which are measured by X-ray photoelectron spectroscopy satisfy the following formulas:0.ltoreq.Si.sub.2p /C.sub.1s .ltoreq.0.020.67 N/C+0.08.ltoreq.O.sub.1s /C.sub.1s -2 Si.sub.2p /C.sub.1s .ltoreq.0.40.ltoreq.N.sub.1s /C.sub.1s -N/C.ltoreq.0.04and method for producing the carbon fibers.

Abstract: A method for detecting the surface characteristic of a carbon fiber, which comprises making the carbon fiber travel through an electrolyte solution, applying an electric potential in said solution between the carbon fiber serving as a working electrode and a counter electrode provided in the solution, said potential being varied continuously within a range of potential not higher than the standard electrode potential, and detecting continuously the resulting charnge of intensity of electric current, and a process for producing a carbon fiber giving a high interfacial bond strength between the fiber and matrix resin, which comprises making the carbon fiber have a constant surface characteristic by application of the above-mentioned detection method.

Abstract: An intermediate for composite materials which comprises a carbon fiber or a graphite fiber having a tensile strength of 300 to 600 kg/mm.sup.2 and a tensile modulus of 30 to 50 t/mm.sup.2 and giving an intensity of electric current per unit surface area as determined by a potential sweep method in the range of 0.1 to 0.4 .mu.A/cm.sup.2 and a matrix resin.