A method of producing carbon nanofibers is disclosed that substantially impacts the carbon nanofibers' chemical and physical properties. Such carbon nanofibers include a semi-graphitic carbon material characterized by wavy graphite planes ranging from 0.1 nm to 1 nm and oriented parallel to an axis of a respective carbon nanofiber, the semi-graphitic carbon material also being characterized by an inclusion of 4 to 10 atomic percent of nitrogen heteroatoms, the nitrogen heteroatoms including a combined percentage of quaternary and pyridinic nitrogen groups equal to or greater than 60% of the nitrogen heteroatoms. The method of manufacture includes, for example, preparing a Polyacrylonitrile (PAN) based precursor solution, providing the PAN-based precursor solution to a spinneret and then performing an electro-spinning operation on the PAN-based precursor solution to create the one or more PAN-based nanofibers. The electro-spinning operation includes passing the PAN-based precursor solution from the spinneret to a collector at a distance between 1 cm to 30 cm while providing an Alternating Current (AC) voltage between the spinneret and the collector, the AC voltage including a frequency ranging from 20 Hz to 100,000 Hz and either a Peak-to-Peak (P-P) voltage ranging from 100 V to 30,000 V or a Root-Mean-Square (RMS) voltage ranging from 100 V to 30,000 V. Afterwards, post-electro-spinning operations, stabilizing treatments and pyrolysis treatments are performed.

A carbon fiber manufacturing method includes joining first and second target fiber bundles with a joining fiber bundle, and carbonizing the joined bundles by feeding them through one or more carbonization furnaces. The joining includes forming an overlap between a first end of the joining fiber bundle and a second end of the first target fiber bundle and jetting a fluid to the overlap to form a first entangled portion, and forming an overlap between a second end of the joining fiber bundle and a first end of the second target fiber bundle and jetting a fluid to the overlap to form a second entangled portion. When the first and second entangled portions each have two or more entangling points with a tensile strength not less than 400 N, the relationship defined by the inequality is satisfied: 40>{L2/(L2−A)}×(S+13), where L2 is a length (mm) of an elongation section inside a first carbonization furnace upstream in a feeding direction of the fiber bundles, A is a maximum distance (mm) between an entangling point in the first entangled portion and an entangling point in the second entangled portion, and S is an elongation (%) of the joined fiber bundles fed through the carbonization furnace.

A drawing method is provided which enables a pressurized steam drawing of an acrylonitrile-based fiber bundle used as the precursor fiber of the carbon fiber bundle. In particular, a drawing method is provided which realizes a high processability when this treatment is conducted at a high draw ratio and high speed. This invention is a method for producing an acrylonitrile-based fiber bundle which includes the steps of spinning a spinning solution containing an acrylonitrile-based copolymer, and subjecting the fiber bundle to a pressurized steam drawing in a pressurized steam drawing apparatus (A) having at least two zones which are a preheating zone on the fiber bundle inlet side and a heating zone on the fiber bundle exit side, the two zones being separated by a seal member. The preheating zone is in a pressurized steam atmosphere at 0.05 to 0.35 MPa, the heating zone is in a pressurized steam atmosphere at 0.45 to 0.70 MPa, temperature difference ΔT1 in the preheating zone of the steam drawing apparatus in the fiber bundle-moving direction defined in the specification is up to 5° C., and temperature difference ΔT2 in the preheating zone of the steam drawing apparatus in the cross-sectional direction of the steam drawing apparatus defined in the specification is up to 5° C.

A carbon fiber bundle has a relationship among a crystallite size Lc (nm), a single-fiber compressive strength Fc (GPa) measured by a compressive fragmentation method of single-fiber composites, and an initial modulus E.sub.0 (GPa) in a resin-impregnated strand tensile test simultaneously satisfies formulas (1) to (3), and Lc is 4.00 nm or less:

Fc≥1.3×10/Lc−0.3  (1)

E.sub.0≤80×Lc+155  (2)

E.sub.0≥330  (3).

A method of preparing a carbon fiber including: preparing a precursor fiber for preparing a carbon fiber; and stabilizing the precursor fiber. The stabilization of the precursor fiber includes a first stabilization phase, a second stabilization phase, a third stabilization phase, and a fourth stabilization phase, which are set at four different temperatures between a temperature at which heat starts to be generated from the stabilization reaction of the precursor fiber and a temperature at which the generation of heat is maximized. Ozone gas is input while at least one phase of the third stabilization phase and the fourth stabilization phase is carried out.

A carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, and a single-fiber diameter of 6.0 μm or more, and having a residual twist count of 2 turns/m or more in a test in which one end is fixed end and another end is free end which is capable of rotation about the axis of a fiber bundle.

Provided are carbon fiber bundles which have high knot strength even if the single fiber fineness is large, and which have excellent handling properties and processability. The carbon fiber bundles have a single fiber fineness of 0.8-2.5 dtex, knot strength of 298 N/mm.sup.2 or greater. This method of producing carbon fibers having knot strength of 298 N/mm.sup.2 or greater involves a heat treatment step for heat treating, for 50-150 minutes, specific polyacrylonitrile-based precursor fiber bundles described in the description in an oxidizing atmosphere rising in temperature in the temperature range of 220-300 C.

A method of producing carbon fibers includes the step of providing polyacrylonitrile precursor polymer fiber filaments. The polyacrylonitrile precursor filaments include from 87-97 mole % acrylonitrile, and less than 0.5 mole % of accelerant functional groups. The filaments are no more than 3 deniers per filament. The polyacrylonitrile precursor fiber filaments can be arranged into tows of at least 150,000 deniers per inch width. The arranged polyacrylonitrile precursor fiber tows are stabilized by heating the tows in at least one oxidation zone containing oxygen gas and maintained at a first temperature T.sub.1 while stretching the tows at least 10% to yield a stabilized precursor fiber tow. The stabilized precursor fiber tows are carbonized by passing the stabilized precursor fiber tows through a carbonization zone. Carbon fibers produced by the process are also disclosed.

A carbon fiber includes single fibers having lengths of 10 cm or less, a fiber axis of each single fiber having an undulation width of 2.5 m or more when a part thereof between two points with a straight-line distance of 1 mm is observed from a lateral direction, and a coefficient of variation in the undulation width being 100% or less. A method of producing carbon fiber production includes performing stabilization treatment of a precursor fiber bundle for polyacrylonitrile based carbon fiber, performing pre-carbonization treatment, and performing carbonization treatment in this order, followed by cutting the resulting carbon fiber bundle in the form of continuous fiber to a single fiber length of 10 cm or less, the twist count of the fiber bundle during the carbonization treatment step being 16 turns/m or more.

An optimized process for the preparation of a spinning solution for the production of acrylic fiber precursors (PAN) of carbon fibers and an optimized process for the production of carbon fibers from said acrylic precursor (PAN), are described.