Molecular Dynamics Computer Simulations of Calcium-Alumino-Silicate Intergranular Films between the basal and prism plan
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Molecular Dynamics Computer Simulations of Calcium-Alumino-Silicate Intergranular Films between the basal and prism planes of α-Al2O3 Stephen H. Garofalini and Shenghong Zhang Interfacial Molecular Science Laboratory Department of Ceramic and Materials Engineering Rutgers University
ABSTRACT Molecular dynamics computer simulations using a robust multibody potential were used to study the structure of the intergranular films (IGFs) formed between two different crystallographic orientations of α-Al2O3 crystals. The simulations show a localized ordering of the IGF at the interface of both the basal and prism planes caused by preferential adsorption of specific ions from the IGF onto the crystal planes. However, the results of the adsorption have significantly different effects on crystal growth of the specific orientations. The preferential adsorption of Ca ions from the IGF onto the (0001) surface inhibit growth in the direction. However, Ca does not affect adsorption of O and Al from the IGF onto the 1120 surface and potential growth of this orientation in
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the < 1120 > direction. The results are consistent with experimental data regarding anisotropic grain growth in this system and provide an atomistic view of this behavior.
INTRODUCTION Although the amorphous (glassy) intergranular film (IGF) occupies only a small volume percentage of the bulk polycrystalline ceramic, it can influence various mechanical, thermal, and chemical properties. However, the glassy nature of the IGF and its very thin width between crystals makes understanding the atomistic structure and the properties of these intergranular films experimentally difficult. A significant number of studies of IGFs have been performed on ceramic materials, with much emphasis on these films in nitrides 1-8. However, studies of the interface between silicate liquids and films with oxides, particularly alumina, have also been done9-15. Anisotropic grain growth in alumina has been reported to occur in the presence of specific concentrations of silica and/or calcia additions 10,16,17 Previous molecular dynamics (MD) simulations have been used to evaluate the interface structure and energy of silica and silicate IGFs in contact with the basal plane of α- Al2O3 18-20. These simulations employed multibody potentials that were stable for both
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α- Al2O3 as well as γ- Al2O3 21. The surface energies obtained from the latter simulations are consistent with earlier classical simulations 22, with relaxed surface energy for the Al terminated basal plane equal to 2.04 J/m2. More recent high level DFT calculations get a surface energy for this termination equal to 1.95 J/m2 23,24. The MD simulations 21 show a substantial relaxation inward of the terminating Al plane on the basal surface, similar to the DFT results 23,24. In the current work reported here, the interface structure of calcium aluminosilicate IGFs between an Al-terminated basal (0001) plane and an O-terminated prism 1120 plane was studied. In each case, the termination was the lowest energy
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