Method for producing r-plane single crystal alpha alumina in massive form having substantially circular cross-section

Claim 1

In a process for the production of massive unicrystalline alpha alumina by (i) forming a melt of alumina by heating the same to form a melt at a temperature of at least 2040° C, (ii) inserting a seed rod of alpha alumina into the melt, (iii) maintaining an atmosphere over the melt which is substantially chemically inert to the melt, (iv) withdrawing the seed rod from the melt such that alumina material is solidified and crystallized on the seed rod to form a massive unicrystalline product of increasing length having a growth axis common with the longitudinal axis of the seed rod, the improvement for producing massive unicrystalline R-plane alpha alumina of substantially circular cross-section which comprises (a) eploying as the seed rod unicrystalline alpha alumina having an orientation such that its longitudinal axis is 571/2° from the C-axis of the seed crystal and is normal to an R-plane of said seed rod and (b) rotating said seed rod about its longitudinal axis at a rotation rate such that the solidified and crystallized alumina material upon withdrawal from the melt has an elongated end portion facet, the major axis of which, lying in a plane perpendicular to the longitudinal axis of the solidified and crystallized material, is substantially perpendicular to the plane defined by the C-axis and the longitudinal axis of the solidified and crystallized material.