| Popis: |
By means of an electron microscope, the width of dislocation ribbons in a stress-annealed pyrolytic graphite and doped with iodine chloride (ICl) and bromine was investigated as a function of temperature in the range 100°K to 300°K. The impurity content amounted to 1·1 at % for ICl and 1·6 at % for bromine. Through measurements of the ribbons widths W in steps of 20°K, the average temperature coefficients 1 W · dW −dT were found to be 5 × 10−4, 3 × 10−4 and 1 × 10−4°K−1, for the original, ICl-doped and bromine-doped material respectively; this shows that the temperature dependence of the ribbon width in graphite is appreciably decreased by the presence of halogen impurities. The phenomenon is quantitatively analyzed in terms of the chemical interaction mechanism between stacking faults and impurities. A discussion of the viscous drag function of impurities for dislocations in relation to the internal friction is included. |
