Orientation Effects on the High‐Temperature Morphological Evolution of Pore Channels in Sapphire

Abstract

Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphire substrates, using microfabrication techniques, ion-beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength () that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [11 00] and [112 0] directions yielded the smallest and largest pore spacings (-values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite-length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi-infinite channels. The pattern of channel evolution suggested two-fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six-fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.

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Surface-energy-anisotropy-induced orientation effects on Rayleigh instabilities in sapphire

“…However, it is not the variation in surface energy per se that matters, but the second derivative of the surface energy with orientation that is critical in dictating the perturbation energetics and kinetically dominant wavelengths [7]. Local [9][10][11][12], and on quantifying the effects of SEA on the morphological evolution of internal voids [9,[13][14][15][16][17][18][19][20][21][22][23][24][25] Glaeser [31] extended the analysis of Stölken and Glaeser [29] [32][33][34][35] to studies in which the equilibrium shape of both undoped and doped sapphire have been determined as a function of temperature [9-12, 22, 36].…”