Determination of pipe diffusion coefficients in undoped and magnesia-doped sapphire (α-Al2O3):: A study based on annihilation of dislocation dipoles

The breakup of dislocation dipoles in plastically deformed samples of undoped and 30-ppm-MgO-doped sapphire (alpha-Al2O3) was monitored using conventional TEM techniques. Dislocation dipoles break up into prismatic dislocation loops in a sequential process during annealing; i.e., dislocation loops are pinched off at the end of a dislocation dipole. This pinch-off process is primarily controlled by pipe diffusion, and pipe diffusion coefficients at temperatures between 1300degrees and 1500degreesC were estimated by monitoring the kinetics of the dipole breakup process. We determined D-P(U) = 8.1((+9.1)(-4.3)) X 10(-3) exp [-(4.5 +/- 1.3eV)/kT)] m(2)/S for the undoped material. The pipe diffusion kinetics for the MgO-doped crystal was determined at 1250degrees and 1300degreesC and was about 6 times higher than for undoped sapphire. Finally, climb dissociation of the dislocations constituting the perfect dipoles in sapphire is common; annihilation of one set of partials can result in the formation of faulted dipoles, which can pinch off to form faulted dislocation loops. Du for faulted dipoles in the undoped material was determined at 1300degrees and 1350degreesC, and was about 4-10 times higher than for perfect dipoles.