April 23, 2021

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Tracer Diffusion Coefficients

     Ruediger Dieckmann is one of very few researchers the world who has the appropriate expertise and safety accreditation to use the radioactive isotopes required to measure tracer diffusion coefficients of ionically conducting glasses. He has more than 30 years of experience in performing radioactive tracer experiments in oxides, 10 of which in studying oxide glasses. Dieckmann’s students will measure the tracer diffusion coefficients of the mobile cations in the ion conducting glasses prepared at ISU.

      It is proposed to perform radioactive tracer diffusion measurements for network modifier ions in MGF glasses of different compositions at different temperatures. The glasses to be investigated are oxide and sulfide glasses containing Li, Na, and/or Ag as network modifiers. The cation of primary focus will be sodium. A radioactive isotope suitable for studying the tracer diffusion of Li does not exist. The tracer diffusion work will complement studies in the collaboration of the proposed program by other investigators, e.g., ionic conductivity (ISU), studies of the ion dynamics (Ilmenau and Munster), and relationship to structure (Chalmers, CMU, and Munster).

    The isotopes being considered are Na-22, Ag-110m and possibly Rb-83. The Cornell group has currently all the permits necessary to handle Na-22 and Ag-110m; Rb-83 could become included if necessary. Na-22 has a half-life of 2.6 years and emits β- (0.54 MeV) and γ-radiation (1.28 MeV) upon decay. Ag-110m has a half-life of 253 days and emits β- (0.085 and 0.53 MeV) and γ-radiation (0.66 and 0.89 MeV) upon decay Rb-83 has a half-life of 83 days and emits γ-radiation (most importantly at 0.521 and 0.53 MeV). It is planned to use the γ-radiation of 0.66 MeV of Ag-110 m, that of 1.28 MeV of Na-22 and possibly that of 0.521 MeV of Rb-83, for measuring residual radioactivity profiles to determine tracer diffusion coefficients.

      Radioactive tracer will be applied in the form of droplets of aqueous solutions (non-aqueous solutions in the case of the sulfide glasses) of isotope-containing salts on the surface of the glass samples. After carefully drying, the glass samples undergo a diffusion-anneal under pre-selected conditions, e.g., temperature and surrounding atmosphere. Oxide glasses will be diffusion-annealed in dry air while sulfur-containing glasses will be diffusion-annealed in dry nitrogen or argon containing a small amount of H2 to prevent sample oxidation.

      After diffusion-annealing, the initial residual radioactivity of a considered isotope will be measured. This is followed by a stepwise removal of thin glass layers by grinding, beginning at the surface of the sample where the tracer has been applied. After each material removal, the thickness of the removed layer and the remaining radioactivity are determined. The data obtained are then used to calculate a value for the tracer diffusion coefficient of the isotope i, Di*. If the diffusivity of i is constant everywhere in the sample, the tracer diffusion coefficient of i can be calculated by integrating the solution of Fick’s 2nd law for thin films.

       The Cornell group will also look into the possibility that water (a constant contaminate, especially for sulfide glasses) is taken up during diffusion annealing and influences the tracer diffusion in glass.

       For this purpose, the group will perform some tracer diffusion studies in moist environments, i.e., in gases saturated with water at a suitable temperature. If water is taken up from the environment during diffusion-annealing, it is expected that the diffusivity near the surface changes as observed in previous studies of the Cornell group in other silicate glasses. The equations to be used in such situations for analyzing residual radioactivity profiles are much more complicated than that shown above. They have been worked out for a two-layer situation in which the diffusion of an isotope can be described by one tracer diffusion coefficient for the near-surface region and another one for the underlying bulk. These solutions have been successfully used to analyze profiles for thesodium tracer diffusion in different glasses, Corning Code 1733, Type I silica, and model glasses of the type (CaO·Al 2O3)x(2SiO2)1−x. It is expected that they will also be useful for analyzing diffusion profiles in the glasses of interest for the proposed work if the incorporation of water should influence the diffusion of cations due to water-assisted structural changes in the near-surface region.

       While ionic conductivity measurements in sufficiently pure glasses of the types proposed to be studied probe the charge transport by all mobile ions, i.e., all the network modifying ions, a tracer diffusion experiment probes only the transport of one specific ion, in the case of the tracer diffusion of Na-22 the diffusion of Na. Therefore, ionic conductivity (ISU) and tracer diffusion studies (Cornell) complement each other. Taking properly into account the diffusion correlations present in all tracer diffusion experiments (Cornell), one can, by combining the results of ionic conductivity and of sodium and/or silver tracer diffusion experiments, deduce values for the diffusion and partial electrical conductivity of Li+ ions and for the mobility of these ions. This specific information is very important for modeling the overall charge transport in the glasses (Ilmenau, Munster) and also for linking the macroscopic charge transport (ISU) to detailed structural understanding of the MGFE (CMU, Chalmers, Munster). Cornell will support one graduate and two undergraduate students to conduct their research.