Carbon nanofibers, method of producing carbon nanofibers, carbon fiber composite material using carbon nanofibers, and method of producing the carbon fiber composite material

1. A method of producing a carbon fiber composite material comprising:

grinding untreated carbon nanofibers produced by a vapor growth method, the untreated carbon nanofibers being ground at a peripheral speed of 50 to 200 m/s for 0.5 to 60 minutes by a rotary grinder so that the ground carbon nanofibers have a tap density of 1.5 to 10 times higher than a tap density of the untreated carbon nanofibers, wherein the fiber lengths of the untreated carbon nanofibers are reduced to only a small extent in the grinding;

mixing the carbon nanofibers into an elastomer, wherein specific portions of the elastomer are bonded to highly active sites of the carbon nanofibers through chemical interaction; and

uniformly dispersing the carbon nanofibers in the elastomer by applying a shear force to obtain a carbon fiber composite material.

2. The method of producing the carbon fiber composite material as defined in claim 1 ,

wherein the untreated carbon nanofibers are ground so that the ground carbon nanofibers have a specific surface area by nitrogen adsorption 1.1 to 5.0 times higher than a specific surface area by nitrogen adsorption of the untreated carbon nanofibers.

3. The method of producing the carbon fiber composite material as defined in claim 1 , further comprising:

heating the untreated carbon nanofibers at 2000 to 3200° C. before grinding.

4. The method of producing the carbon fiber composite material as defined in claim 1 , wherein the untreated carbon nanofibers have an average diameter of 70 to 100 nm.

5. The method of producing the carbon fiber composite material as defined in claim 1 , wherein the carbon nanofibers are dry-ground by utilizing impact and/or a shear force.