STUDY ON THE RELATION BETWEEN THE MICROSTRUCTURE AND PROPERTIES OF BORON MODIFITD CF BY STM

A PAN-based precursor carbon fiber (carbonized at 1300℃) was impregnated with different borides and heat-treated at 2500℃ under nitrogen atmosphere. The mechanical properties of the resulting graphite fibers have been measured and their microstructure investigated by X-ray diffraction (XRD) and scanning tunneling microscope (STM). It is shown that boron-doping improves the tensile modulus and strength of the fibers. This can be explained by the STM observations, which show smoother surface with fewer defects and improved crystallite orientation along the fiber axes for the boron-treated samples. The result of XRD shows that the boron-doping treatment increased the crystallite size.