Crystallization of Glass Films on Single-Crystal MgO Substrates
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ABSTRACT Thin, calcium magnesium silicate glass films have been deposited onto (001)-oriented single-crystal MgO substrates by pulsed-laser deposition (PLD). The substrates were thinned to electron transparency and characterized in the transmission electron microscope (TEM) before and after the deposition and following different heat treatments. Energy Dispersive X-ray Spectroscopy (EDS) was used in the TEM to characterize the chemistry of the films. The heat treatments were performed both in situ and ex situ. Direct evidence for crystallization was obtained by in-situ experiments at 950'C. Subsequent heat treatments were performed for longer times in air between 950'C and 1 100'C. The crystallized phase was found to be monticellite (CaMgSiO 4 ) and the crystallites show moderate epitaxy with the underlying substrate. Films contaminated with aluminum resulted in the growth of magnesium aluminate spinel (MgAI20 4 ) epitaxially from the substrate. These observations of epitaxy indicated that crystallization initiated at the surface of MgO. INTRODUCTION Intergranular phases commonly observed in polycrystalline ceramics may affect the overall properties of the material. 1 By controlling the chemistry and crystallography of these phases, the properties may be suitably altered.2-4 In view of the complexities associated with devitrification studies of glassy phases in polycrystalline materials, novel experiments may be performed on single-crystal substrates. The thin-film approach described in this paper has been previously employed to study glass-crystalline interactions in the alumina-anorthite system successfully. 5 In that system, epitactic crystallization of anorthite phases indicated that the surface of alumina plays a major role during the devitrification process. Calcium magnesium silicate glass films are commonly observed in the intergranular regions of commercially prepared MgO compacts. 6 ,7 In the previous study involving interaction of liquid monticellite (CaMgSiO4 ) at the grain boundaries of MgO, the intergranular regions were found to comprise of crystalline monticellite (CaMgSiO 4 ) and a glassy phase rich in the impurities present in MgO. 8 TEM studies revealed that crystalline monticellite is invariably in contact with the MgO grains, suggesting that the crystallization process initiated at the surfaces of the MgO grains. In order to understand the role played by MgO during the devitrification process, single-crystal substrates were chosen for this study. EXPERIMENTAL Substrates Thin films of calcium magnesium silicate glass were grown by PLD onto specially prepared single-crystal (001) MgO substrates. The substrates were prepared from bulk by dimpling to about 15.tm followed by ion milling from the dimpled side at 180 with 5 kV Ar ions. Prior to the deposition all substrates were acid cleaned and annealed at 1350'C for 0.5 hour. The acid cleaning procedure that was adopted is described elsewhere. 9 After annealing, all the
633 Mat. Res. Soc. Symp. Proc. Vol. 321. @1994 Materials Research Society
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