Orientation effects on the high-temperature morphological evolution of pore channels in sapphire

Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphir e 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 degrees C was monitored. In all cases, the channels broke up with a characteristic wavelength (lambda) that was much larger than that expected for a material with isotropic surfa ce energy, which reflected stabilization effects that were due to anisotrop y of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [1 (1) over bar 00] and [11 (2) over bar 0] directions yielded the smallest and largest pore spaci ngs (X-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 orienta tion, and the trend mirrors that observed for semi-infinite channels. The p attern of channel evolution suggested two fold rotational symmetry within t he basal plane, where, because of the nature of the experiment, apparent si x-fold symmetry is expected. Several factors that may contribute to or caus e an apparent or real loss of symmetry have been discussed.