Ion Beam Mixing of Marker Atoms in Mo and Ru and Heat of Mixing
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The presence of other impurities was not experimental uncertainty. investigated. 15 For the 2 irradiation, we used 300 keV Kr ions with doses from 3 to 8 x 101 ions/cm . The irradiation was conducted at 77 and 300 K. The samples were analyzed by backscattering spectrometry (BS) at room temperature with 2 MeV He ions. The scattering angle was 1700 and the target was tilted to an angle of 10° with respect to the incident beam. To check if there were any significant amounts of mixing due to warming up of the samples to room temperature for BS analysis after irradiation at 77 K, we irradiated in a different system, Ru with a Au marker and Mo with a Pt marker at 7 K with in-situ BS analysis. For Ru with the Au marker, the result at 7 K (7.4 A /eV) was similar to that foid for a sample irradiated at 77 K and analyzed at room temperature (6.4 A /eV). For Mo with the Pt marker, the mixing results were slightly larger tgr the samples irradiated at 77 K and analyzed at room temperaltre (8.5 A /eV) than for the sample irradiated and analyzed at 7 K (6.0 A /eV). In any case, for both Mo and Ru samples irradiated at 77 K and analyzed at room temperature, the mixing results were equal, within the experimental uncertainties, to the results obtained from 7 K irradiation with in-situ BS. Thus the mixing due to the warming up of Ru and Mo samples from 77 K to room temperatue is insignificant and the present results are sufficient for the purpose of our experiment. RESULTS AND DISCUSSION Typical backscattering spectra taken before and after the irradiation are shown in Figure 1. All the backscattering signals from the marker elements were Gaussian before and after the irradiations. The increase in the energy variances of the marker signals due to the ion beam mixing was calculated from the expression 2
2 irr
2 unirr
(1) 2 2 where .irr and Qunirr are the measured energy variances of the signals from the irradiated and unirradiated samples. The standard deviation, (in units of length) for the broadening of the marker profile is given by , = Q/(N[e])
(2)
where N and [c] are the atomic number density and the stopping cross-section factor of the material for He backscattering from the marker atoms. The effective diffusion length Dt, was determined by analogy with diffusion theory, i.e. Dt= o2/2. For the discussion, Dt has been divided by the damage energy F obtained from the Monte Carlo computer simulation TRIM [131, and by the irraRiation dose, , to give the "mixing efficiency", Dt/qFD. The mixing results of markers in Mo and Ru matrices are shown in Figures 2 and 3. In these figures the abscissa is a scale for the heats of mixing for the matrix and marker atoms and which were obtained from values given by Miedema [14]. The ordinate shows the mixing efficiency, Dt/b•F. We estimate the experimental uncertainty to be about + 20%. The data are Rhown for the mixing results at both 77 and 300 K irradiation. In the Ru matrix, the difference in the mixing at the two temperatures is more pronounced than in Mo. In some cases, this diff
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