A. Marchand

Evolution des proprietes electroniques de carbones irradies par les neutrons

Abstract Two nuclear graphites, a well oriented graphite PGCCL and a pyrocarbon deposited at 2100°C were irradiated with neutron at 35°C. The electronic properties such as dia and paramagnetic susceptibilities, Hall effect, magnetoresistance, and electrical resistivity were measured, at room temperature, as a function of the irradiation dose. The studies show that the damage is less pronounced when the carbon is better ordered.

Proprietes electroniques et magnetiques de fibres de carbone evolution sous irradiation neutronique

Abstract Techniques developed for the measurement of the electrical resistivity and the diamagnetic anisotropy of carbon fibres, in the temperature range 77–300°K are described. Two types of fibres were studied: the Rigilor AC fibres treated at 1100°C and the Rigilor AG fibres treated at 2500°C. The measurements of electrical resistivity performed on single fibres show the presence of some correlation between this property and the cross-sectional area of the fibre. Measurements of diamagnetic susceptibility carried out on bundles of fibres demonstrate a large anisotropy of the fibres. The influence of small neutron irradiation doses on these properties has been studied at room temperature.

Recuit thermique de carbones irradies par les neutrons evolution des proprietes electroniques

Abstract In continuation of a study of the electronic properties of neutron irradiated carbons the resulting changes in their electronic properties (electrical resistivity, Hall effect, magnetoresistivity, magnetic susceptibility) were determined after various thermal anneals. Similarities in behaviour of PGCCL and graphite are observed. The pyrocarbon which is damaged most easily recovers its initial properties with the most difficulty, however, the previous damage does not affect its behaviour during graphitisation. Previously started measurements of the electron spin resonance have been completed.

Evolution de diverses classes de carbones par irradiation neutronique

Abstract We investigated in previous studies the electronic properties of three graphites and one pyrocarbon damaged by neutrons [1,2]. We have now extended this work to various other carbons and we present in this paper a unified presentation of the results and a quantitative evaluation of the damage in terms of positive hole concentration in the valence band vs irradiation dose. The materials tested and the electronic properties which were measured are listed in Table 1. Irradiations were performed at 35°C in conditions identical to those of previous studies [1,2] at the “Melusine” reactor of the Grenoble Nuclear Center. Doses D ranging from 1017 to 4 × 1020neutrons/cm2 were recorded. The variation of the electrical resistivity at room temperature is shown in Figs. 1 and 2; of the Hall coefficient in Table 2 and of the magnetic mean susceptibility in Fig. 5. Figure 4 presents the relative transverse magnetoresistance (i irradiated sample; o non irradiated). The paramagnetic susceptibility χp was measured directly by EPR and its variation at 298°K with the neutron dose as shown in Fig. 6. Using the data on variation of χpwith temperature, the paramagnetic susceptibility is split[6] into contributions of the “localized paramagnetic centers” (Fig. 7) and of the conduction carriers (Fig. 8). The latter, χdeloc (Fig. 8) increases more slowly than the dose, while χ,loc (Fig. 7) increases first very rapidly, then much more slowly at high irradiation dose, when some recombination of point defects takes place. At higher doses when the Hall coefficient begins to decrease with increase of the dose, the data may be used to calculate the number N of positive holes created by irradiation: Figure 9 shows that approx. 4 holes/cm2are produced by 1 neutron/.cm2. Figure 10 shows that χdeloc of carriers is roughly proportional to √N, as expected for the band structure of graphite. It is clear from Fig. 11 that Nloc increases proportionally to N at low irradiation doses, but slower than N at high doses (recombination of vacancies). Although the radiation-induced changes in electronic properties differ widely for different carbons, it is possible to find some regularities in these results. We find that, in a given “class of carbons,” each unirradiated sample j may be characterized by an “equivalent neutron dose” Δjp, Δjp would be the dose needed to alter a property P of the most perfect material of the class from its value to the value equal to the unirradiated sample j. Then all the variations of P with (D + Δjp) should be represented by a single curve for all the carbons of the same “class.” Such a parameter Δjp can be useful only if Δjp is independent from the nature of the property P: Δjp = Δj. Figure 12 shows the variation of the diamagnetic mean susceptibility −χ as a function of the “total dose” D + Δjp (D = real dose). There is indeed a single curve and the values of Δjp. (arbitrarily set at zero for pyrocarbon or for PGCCL) are listed in Table 3. In the same way Fig. 13 shows the “single curve” for the Hall coefficient A and Table 3 lists the corresponding values of ΔjA. It may be seen that Δj, is independent from the property P only for carbons belonging to the “class of graphites” or for those of the “class of pyrocarbons.” Papyex cannot be placed in any one of these classes, and there are not enough data for all the other carbons. For carbons in the same class, the value of Δj, can be interpreted in terms of defect concentration prior to irradiation. Finally, from the “single curves” of Figs. 12 and 13, an “irradiation path” for each class of carbons can be drawn by plotting −χ vs A. Figure 14 shows that the irradiation and annealing paths of graphites are the same.