18 R.H. DoremusThe fastest diffusing substance in alumina is hydrogen (H 2 ). Fast-diffusing cations are
sodium, copper, silver, with hydroniums (H 3 O+) the fastest of these monovalent cati-
ons. Many other di- and trivalent cations have diffusion coefficients intermediate
between these fast-diffusing ions and the slowest diffusers, the lattice elements alumi-
num 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 [41]. In only two studies [43, 44] is it likely true diffusion along grain
boundaries or dislocations was measured [41]. 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.
9 Chemical Properties
The decomposition of alumina at high temperatures can be deduced from its vapor
pressure; see Sect. 6.3 and Table 15.Fig. 4 Log diffusion coefficients vs. 10^4 /T for selected substances diffusing in alumina. From [41]