Effective thermal conductivity of graphite materials with cracks


S. E. Pestchanyi and I. S. Landman


Forschungszentrum Karlsruhe, Institute for Pulsed Power and Microwave Technology, P.B. 3640, D-76021, Karlsruhe, Germany


In the reference design of ITER, carbon fibre composites (CFC) are foreseen as armour for the divertor strike zone due to their large thermal conductivity k at ITER normal operation temperatures T of 1000-1500K and due to high sublimation temperature of 4´103K, which is important to withstand not normal ITER events. However, recent experiments with CFC at T of (3-4)´103K [1] revealed an intolerable erosion rate despite of only a small decrease of measured final values of k, though actual thermal conductivity of carbon based materials at T>1.5´103K is almost unknown.

As demonstrated in [2], the main reason of the enhanced erosion at the extreme temperatures is a local surface overheating due to degradation of heat transport. Besides a natural inversely proportional temperature dependence of thermal conductivity of graphite grains, the significant decrease of k in the overheated surface layer is due to cracks at the interfaces between fibres and matrix of CFC and in the graphite matrix itself that appear in the course of heating.

The dependence of k on T and on the volumetric crack density N is modelled for macroscopic graphite samples using the three-dimensional thermomechanics code Pegasus 3D [3]. The graphite grains have anisotropic thermophysical properties. At the high heat loads typical of the divertor armour, thermostress due to the anisotropy is much larger than that due to the temperature gradient. The simulation demonstrates that the volumetric crack density both in fine grain graphites and in the CFC matrix depends mainly on the local sample temperature. The weak dependence of N on the temperature gradient allows calculation of effective thermal conductivity as function of T and N. The results obtained are used to explain the experiments with CFC and fine grain graphite [4] in which a clear energy threshold for the onset of brittle destruction was observed.




[1]        J. P. Bonal, C.H. Wu and D. Gosset, Simulation experimental investigation of plasma off-normal events on advanced silicon doped CFC-NS31. J. Nucl. Mater. 307-311 (2002) 100-105.

[2]        S. Pestchanyi and H. Wuerz, 3-D simulation of macroscopic erosion of CFC under ITER off-normal heat loads, to be published in Fusion Engineering and Design

[3]        S. Pestchanyi and H. Wuerz, Brittle destruction of carbon based materials under off-normal ITER-FEAT conditions. Physica scripta. T91, 84-89, 2001.

[4]        B. N. Bazylev, I.S. Landman, J. Linke, S. E. Pestchanyi, H. Wuerz, Energy Threshold of Brittle Destruction for Carbon Based Materials, these Proceedings