Carbonization of polyacrylonitrile-based fibers under defined tensile load: Influence on shrinkage behavior, microstructure, and mechanical properties

Abstract

The influence of temperature and tensile mechanical load during carbonization of stabilized polyacrylonitrile fibers on the shrinkage behavior, the crystalline parameters and the mechanical properties of the resulting fibers is investigated. To this end, a newly developed experimental setup enables a defined heat treatment of fiber bundles up to a temperature of 1200 °C under control of tensile load. The attachment devices for the fiber bundles are located in the hot zone of the furnace. Additionally, carbonization treatments up to 1600 °C without application of load are performed.Three characteristic temperature regions of the carbonization process are identified.Between 250 °C and 450 °C the still ductile fibers contain rests of the polyacrylonitrile phase experiencing stabilization reactions. Stacked, heterocyclic planar structures grow due to molecular crosslinking. Between 450 °C and 1000 °C graphite-like turbostratic carbon is formed. Crystallites grow laterally, crystallinity increases, and the material becomes brittle. Above 1000 °C a pronounced increase in stacking height, lateral crystallite size and crystallinity occurs, while the interlayer distance decreases. Mechanical properties reach the values typical for commercial carbon fibers.Application of a tensile stress during carbonization influences fiber dimensions at temperatures between 250 °C and 450 °C, where the material shows ductile behavior. Shrinkage in fiber direction is hindered and radial shrinkage is favored. At temperatures above 1000 °C, crystalline parameters and mechanical properties of the resulting carbon fibers depend on the applied tensile stress. Carbonization without tensile stress results in significantly lower mechanical properties.The impact of heat treatment temperature and applied stress on carbon fiber properties demonstrate the possibility of a defined tailoring of fiber characteristics.