NOVELTY - Production of three-dimensional (3D) carbon fiber structures having an original carbon fiber (OCF) (1) covered with a network of secondary carbon fibers (SCF) (2) comprises: (a) thermally decomposing a carbonaceous gas in an oxidant-free atmosphere over the surface of heated original carbon fibers; and (b) recovering OCF with straight SCF grown as branches on OCF surfaces, hence realizing 3D carbon fibers.    USE - The three-dimensional carbon fiber structures are useful in adsorbents, catalyst supports, fuel cells, capacitors, medicine, refrigeration, and environmental control.    ADVANTAGE - The process provides carbon fiber materials with the increased surface area and improved micromechanical interaction with the matrix. The process for producing 3D carbon fibers by growing SCF and TCF on OCF is relatively simple, and it can be arranged in a continuous process. The thickness and the length of SCF and TCF can be easily controlled by the variation of the operational parameters, e.g. the temperature, the CG residence time, and the presence of additives. The process is not sensitive to the presence of small amounts of sulfur and other contaminants, thus inexpensive industrial grade CG can be used in the process. The growth of SCF and TCF does not produce any chemical changes in OCF. The 3D carbon fibers with straight and thick SCF can efficiently bind with the metal, ceramic, glass, and concrete matrices due to strong micromechanical interaction. The process allows to simultaneously cover OCF with a protective carbon coating, which can prevent any possible damage (both, chemical and mechanical) to the fiber during production of composite materials(particularly, corrosive attack by the ceramic and metal matrices).    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for an apparatus for producing 3D carbon fibers comprising: a first chamber (a) for growing SCF; means (b) for supplying OCF into the first chamber; means (c) for introducing carbonaceous gas (CG) into and out of the first chamber; means (d) for heating the OCF to a decomposition temperature for the CG; means (e) for controlling the temperature and the time of exposure of the heated OCF to the CG within the chamber until the desired SCF growth is realized and; means (f) for recovery of the SCF is provided.    DESCRIPTION OF DRAWING(S) - The figure shows a lateral cross-sectional view of the three-dimensional carbon fibers.    Original carbon fiber (1)    Secondary carbon filament (2)    Tertiary carbon filament. (3)