Diffusion in Alumina
Experimental volume diffusion coefficients of substances in alumina are summarized in ref. 41. Values for the parameters D0 and Q (activation energy) from the Arrhenius equation:
D = D0 exp(— Q / RT) (11)
Table 19 Dielectric constant of sapphire as a function of temperature at frequencies from 103 to 109 Hz (from ). Orientation to c axis
The fastest diffusing substance in alumina is hydrogen (H2). Fast-diffusing cations are sodium, copper, silver, with hydroniums (H3O+) the fastest of these monovalent cations. Many other di – and trivalent cations have diffusion coefficients intermediate between these fast-diffusing ions and the slowest diffusers, the lattice elements aluminum and oxygen, which have about the same diffusion coefficients.
A number of experimenters have calculated diffusion coefficients D from “tails” on diffusion profiles in alumina, and attributed these D values to diffusion along dislocations, subboundaries, or grain boundaries. However, this attribution is doubtful in most cases, as discussed in . In only two studies [43, 44] is it likely true diffusion along grain boundaries or dislocations was measured . Mechanisms of diffusion in alumina are uncertain; a variety of charged defects have been suggested to control diffusion in alumina, but no interpretation is widely accepted because of discrepancies with experimental results. I have suggested that oxygen and aluminum diffusion in alumina results from transport of aluminum monoxide (AlO), and that AlO defects in the alumina structure are important in diffusion. These speculations have some support, but need more work to confirm them.