Abstract: Anomalous behavior in Fe+−, Cr+−, and Mn+−implanted Al2O3

Abstract

Stainless steel elements Fe, Cr, and Mn are possible sources of contamination to fusion reactor materials such as Al2O3 via sputtering processes or diffusion of metal atoms across common interfaces.This extended abstract summarizes preliminary results of ion beam and EPR studies of Fe+−, Cr+−, and Mn+−implanted single crystal Al2O3. Several unusual phenomena have been observed. These are: (1) proton bombardment alters the implanted Fe and Cr distributions in the Al2O3 near−surface region, (2) the usual Cr and Mn EPR spectra from diffused−in impurities in Al2O3 are not observed for Cr− and Mn−implanted Al2O3, and (3) background Fe contamination in unimplanted Al2O3 is converted to a paramagnetic charge state by proton bombardment, annealing to 1475°C, or by exposure to uv light. The following paragraphs briefly describe details of these experiments.Single crystals of Al2O3 (0.092×0.092×0.92 in.) were implanted at room temperature with 275 keV Fe+, Cr+, or Mn+ ions to fluences of 2×1015, 1016, and 1017 atoms/cm2. Special precautions were taken to avoid channeling and positive charge buildup during implantation. It was thought that Al2O3 surface charging could change the energy of the incoming ions and thus alter the distribution in depth of implanted Cr, Mn, or Fe. Most samples were first wrapped in Al foil which was silver painted to the Al2O3 edges. Windows were cut in the foil exposing ∠80% of the rectangular Al2O3 surfaces to the ion beam.Selected samples were then analyzed with EPR or the ion beam techniques of proton−induced characteristic x rays (PIX) and Rutherford backscattering (RBS). The PIX analysis was used, first of all, to establish the presence and approximate concentrations of the implanted metal atoms by detecting proton−excited characteristic x rays from Fe or Cr atoms. PIX and EPR data also revealed that all of the Al2O3 samples, prior to implantation, contained a large bulk impurity Fe concentration estimated at ∠1019−1020 atoms/cm3.RBS data using 1−MeV protons were used to record the profiles of the implanted Cr and Fe atoms. Each RBS analysis run used 5×1014 H+/cm2. However, it was observed that the Cr and Fe distributions changed with higher fluence proton bombardments. The peak of the Fe distribution decreased in intensity after 5×1015 H+/cm2 indicating a movement of Fe atoms into the Al2O3 bulk. In contrast, the Cr RBS peak increased and shifted to higher backscatteried energies indicating a motion of Cr atoms to the Al2O3 surface. This unusual Fe and Cr movement is not presently understood. EPR data were equally puzzling. First of all, no Cr EPR was observed in any of the three Cr+−implanted samples even after annealing to 1475°C. Also, the usual diffused−in spectra for Mn2+ and Mn4+ were not observed. These EPR results suggest that, for temperatures as high as 1475°C, implanted Fe, Cr, and Mn atoms do not behave as Fe, Cr, and Mn in doped diffused−in Al2O3. Continued study of implanted Al2O3, particularly the addition of He backscattering−channeling data, should provide a better understanding of the effects observed in these initial experiments.