Structural change of carbon‐fibres at high temperatures under load

Abstract

A B S T R A C T Creep tests were performed with carbon fibre-bundles (Toho Tenax) in a temperature range from 1500 to 1800 • C. The fibre-bundles were electrically heated in vacuum (pressure 10 −4 mbar) and tensile load was applied in a hydraulic testing machine. The creep parameters were obtained from varying temperature and loading conditions. Accompanying structural investigations were performed with X-rays (SAXS and WAXD). An increasing orientation of the graphene planes along the fibre (and thus the loading axis) could be observed with increasing temperature and load. No structural change and no creep, however, was observed for carbon fibres stabilized by an appropriate heat treatment. = interlayer-distance of graphene planes I(q, χ) = intensity-distribution L p = pore length q = absolute value of the scattering vector Q = activation energy for creep R g = radius of gyration R = gas constant, equals 8.314 J mol −1 K −1 T = temperaturė ε = strain rate θ = 1 2 of the scattering angle λ = X-ray wavelength σ = stress applied to the fibres χ = azimuthal angle I N T R O D U C T I O NCarbon fibres exhibit outstanding mechanical properties (high elastic modulus, high tensile strength and low weight), which are maintained up to high temperatures. Thus, these fibres are especially suited as reinforcing elements in composites for high-performance applications. 1 Carbon fibres are built up by hexagonal graphene layers forming small coherent units (crystallites separated by pores). 2 The crystallites with a size of some nanometers are preferentially oriented along the fibre axis, the degree of orientation being the most important parame-ter for the Young's modulus of the fibres. To determine this orientation and to gain information on the structure at the nanometer level, wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) are one of the most important techniques. 3-6 The degree of orientation and the interlayer distance can be determined from the distribution and the position of the 002-reflection in WAXD, 7,8 whereas SAXS can be used to quantify the mean orientation and the size of the pores. 9,10 For polyacrylonitrile-(PAN) based fibres, the most important parameter to control the structure is the final heat treatment. This heat treatment has been intensively discussed in the literature. 3,7,10,11 A high-temperature treatment changes the structure of the carbon fibres: the higher

High-temperature tensile cell forin situreal-time investigation of carbon fibre carbonization and graphitization processes

“…Tensile stress was applied during heating, and the thermomechanical properties and tensile behaviour of different fibre types were monitored as a function of temperature by stress-strain curves. The apparatus described by Rennhofer et al (2006Rennhofer et al ( , 2010Rennhofer et al ( , 2014 was also used to heat and stress fibre bundles (up to 1700 C) in vacuum, but, in addition, they also monitored microstructure evolution as a function of time at ultimate temperature using WAXD. A laboratory X-ray source was used in the initial report, requiring relatively long ($ 600 s) exposure times.…”