Method for producing R-plane single crystal alpha alumina

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) forming a melt of alumina with at least one oxide selected from the group consisting of chromium oxide, iron oxide and magnesium oxide, the aggregate amount of selected oxide provided from about 90 ppm to 20,000 ppm in the aggregate of selected metal ion in the melt to provide a unicrystalline product of substantially circular cross-section, and (b) employing as the seed rod a seed rod of unicrystalline alpha alumina at a C-axis orientation of 571/2° having an orientation such that its growth axis is normal to an R-plane of said seed rod.