Dewetting of Reactive CaMgSiO 4 Glass Films on Single-Crystal MgO(001) Substrate
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87 Mat. Res. Soc. Symp. Proc. Vol. 586 ©2000 Materials Research Society
RESULTS The cleavage steps on the MgO surface are clearly visible under the monticellite film in Figure 1(a). The film roughness measurements show that the as-deposited film of CaMgSiO 4 on the MgO(001) is essentially flat with small (-50 nm) undulations. After one annealing cycle at 1600'C for 5 min, these small undulations disappear and large irregularly shaped islands form which are -500 nm in diameter (see Figure l(b)). CaMgSiO 4 melts at -14850 C4 , so annealing a thin (9100 run) monticellite film of uniform thickness at 1600°C causes it to melt completely. On quenching to room temperature, molten silicate dewets the substrate. The irregularly shaped islands seen on Figure 1(b) are dewet droplets which, due to high quenching rate (1500'C in 10 see.), could not attain the expected spherical shape geometry5. These dewet islands are nearly
uniform in size and are not topographically related to the grains of the original CaMgSiO 4 film. It is believed that the liquid silicate wets the MgO(001) surface completely at elevated temperatures, and undergoes dewetting as the system is quenched from 1600'C to the glass transition temperature.
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Figure 1. Height-mode SPM images of the same surface area for the MgO(100)-CaMgSi04 system. (a) After film deposition. Small grains of silicate film do not obscure cleavage steps on the substrate surface; these steps are aligned top to bottom in the image. (b) After a cycle of annealing at 1650'C for 5 min. and quenching to room temperature. The melted film starts to form dewet islands at the surface. Thermal treatment of the dewet droplet at 1690°C for 15 min. in three 5-min. increments reveals a depression where the droplet had been located (Figure 2(a)). In contrast, the area of the substrate outside, but close to, the droplet is elevated above the surface level, forming a threshold-like structure. As could be seen in the image, the remnant of the silicate phase adheres to the well wall and protrudes out of the well. The shape of the well in the cross-section is asymmetric. An elevated threshold circles the boundary of the original droplet. The height of the threshold is -30 nm. Narrow ridges on the inside of the well were produced when the droplet stopped moving between heat-treatments 6. In agreement with the thermal etching data, acid treatment of the dewet droplet on the surface (see Figure 2(b)) reveals a similar depression
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underneath and an elevation (plateau) around the droplet circumference. Considering the fact that the original substrate was produced by cleaving the MgO crystal along the (001) plane, such depression-and-elevation structures cannot be related topographically to the cleavage geometry (see Figure 1(a)). Therefore, it must have been produced in the course of annealing/quenching through transfer of substrate material (MgO) from the glass to the air/ceramic/silicate triple7 junction, i.e.
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