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Electronic structure calculations of calcium silicate hydrates

Description: Many phases in the calcium-silicate-hydrate system can develop in cement exposed over long periods of time to temperatures above 25 C. As a consequence, chemical reactions involving these phases can affect the relative humidity and water chemistry of a radioactive waste repository that contains significant amounts of cement. In order to predict and simulate these chemical reactions, the authors are developing an internally consistent database of crystalline Ca-Si-hydrate structures. The results of first principles electronic structure calculations on two such phases, wollastonite (CaSiO{sub 3}) and xonotlite (Ca{sub 6}Si{sub 6}O{sub 17}(OH){sub 2}), are reported here. The calculated ground state properties are in very good agreement with experiment, providing equilibrium lattice parameters within about 1--1.4% of the experimentally reported values. The roles of the different types of oxygen atoms, which are fundamental to understanding the energetics of crystalline Ca-Si-hydrates are briefly discussed in terms of their electronic state densities. The good agreement with experiment for the lattice parameters and the consistency of the electronic density of states features for the two structures demonstrate the applicability of these electronic structure methods in calculating the fundamental properties of these phases.
Date: November 1995
Creator: Sterne, P. A. & Meike, A.
Partner: UNT Libraries Government Documents Department

Influence of stacking faults and alloy composition on irradiation induced amorphization of ZrCr{sub 2}, ZrFe{sub 2} and Zr{sub 3}(Fe{sub 1-x},Ni{sub x})

Description: These Zr-based intermetallics were irradiated with high-energy electrons at the ANL HVEM/Tandem facility. Although ZrCr{sub 2} and ZrFe{sub 2} have the same Laves phase C15 fcc crystal structure, their critical temperatures for amorphization under electron irradiation were 180 and 80 K, showing that Cr substitution for Fe in the sublattice had a marked effect on annealing. The low temperature dose to amorphization was higher in ZrFe{sub 2} than in ZrCr{sub 2} by a factor of two. Presence of a high density of stacking faults had a strong effect on amorphization in both compounds causing the critical temperature to be increased by 10-15 K. By contrast, addition of Ni to Zr{sub 3}(Fe{sub 1-x},Ni{sub x}) had no effect on amorphization behavior for x=0.1 and 0.5. These results are discussed in terms of current models of amorphization based on defect accumulation and attainment of a critical damage level, such as given by the Lindemann criterion.
Date: December 1995
Creator: Faldowski, J. A.; Motta, A. T.; Howe, L. M. & Okamoto, P. R.
Partner: UNT Libraries Government Documents Department