Method for producing high strength, melt spun carbon fibers

Hollow carbon fibers and carbon fibers having a generally C-shaped transverse cross-sectional area are produced by extruding a carbonaceous anisotropic liquid precursor through a spinneret having a capillary with a generally C-shaped cross-sectional area, into a fiber filament, controlling the viscosity of the molten precursor, the pressure of the molten precursor and the linear take-up speed of the filament to yield a fiber filament having a cross-sectional area shaped substantially like the shape of the cross-sectional area of the capillary and further having a line-origin microstructure, rendering the filament infusible, heating the filament in an inert pre-carbonizing environment at a temperature in the range of 600.degree. C. to 1000.degree. C. for 1 to 5 minutes, and heating the filament in an inert carbonizing environment at a temperature in the range of 1550.degree. C. to 1600.degree. C. for 5 to 10 minutes, to substantially increase the tensile strength of the filament. The carbon fiber filament so produced has a line-origin microstructure in which the origin line is located and shaped substantially as a line which constitutes the line formed by uniformly collapsing the perimeter of the transverse cross-sectional area of the fiber filament upon itself. The carbon fiber filament has a tensile strength greater than 200 ksi and as high as the 700 to 800 ksi range, yet a modulus of elasticity on the order of 25-35 msi. The top to bottom outside diameter of the fiber\u27s transverse cross-sectional area is on the order of 30 to 50 microns, and the wall thicknesses are on the order of 8 to 15 microns